Nanomech in Photovoltaics

What to do with solar in the economic turndown

2009-03-21T15:32:00.005+01:00

As you may have noted, I am in the process of developing a new blog (Heliotactic.org). The reasons? First, the Sun allows for a plethora of possibilities, and I wanted to work with a bigger canvas! Second, I feel the need to open up the blog to entries from guests, to create a diverse perspective of all things tied to solar (including energy efficiency, green roofs, passive solar design, energy recovery and cogeneration). And third, and most frankly, PV is the most expensive solar investment for the individual. In this economic depression, we need to know what technologies are affordable and offer the highest rewards for the initial investment. I’ve been told again and again that solar hot water is the most obvious, no-brainer tactic in the solar arsenal. It’s cheap (< $6000 for everything), it’s easy, and by replacing/complementing your electric or gas (or fuel oil) water tank (with federal and state incentives), payback is often less than 5 years.

My recent experiences have included teaching solar energy conversion, developing tools for solar resource assessment, and leading a great team to design, build, and operate a solar-powered house (

Solar Jobs = Green Collar Jobs!

2008-12-28T16:13:00.004+01:00

As a researcher and instructor dealing with solar energy conversion, I am acutely aware of the immediate need (or ASAP) for a smart, flexible labor force--capable and trained to install and maintain our new solar technologies. Solar energy will be the heart of the new green collar job sector, as we will need to deploy PV and solar hot water technologies to residential and commercial buildings for a carbon-constrained future.

Analogy:
I want to use the familiar example of technologies for indoor air quality and thermal comfort: HVAC systems (Heating, Ventilation, and Air Conditioning). Think about how many air conditioning units are now an integral part of buildings in the country. Consider the labor force that is required for AC/heating installation, duct installation, monitoring and control systems (e.g. thermostats), and maintenance or repairs (hint: it is a huge industry). Now think about how little you think about these systems (because they just work). There is similar (perhaps even greater) potential for green collar jobs--earning a paycheck and helping society and the environment!

The Very Near Future:
Green collar jobs for solar technologies are here! Training is in full gear in states like California, New Jersey, and Florida, and is ramping up in Wisconsin and Pennsylvania. At Penn State, we are already working on a training course for PV installation, as well as an upper level college course in solar energy technology design.

Additional reading: NYT article on PV installers as the new wave of green collar jobs.

Solar technologies are really diverse

2008-12-25T16:38:00.001+01:00

In preparing for my annual Spring course “Design of Solar Energy Conversion Systems”, I am reminded of just how many diverse technologies can be derived from our nearest large-scale fusion reactor. I will make exceptions to the obvious: horticulture and wind energy are derived from the sun too.

Here are some ideas beyond PV and concentrating PV (CPV):

1. Passive/Active Solar Water Heating Systems (in your showers, dishwashers, heating your floors)

2. Commercial/Distributed Space Heating Systems (using Solar Walls, Phase Change materials, Pebble-bed hot air storage).

3. Solar Cooling (Yes! you can cool with the sun and heat pumps, dessicants, refrigeration cycles).

4. Solar Industrial Process Heat and Solar Ponds (Do you own a mine or a refinery? Look into ways that you could dramatically reduce your energy bills!)

5. Solar Thermal Power Systems (Also called Concentrating Solar Power--CSP--this is the technology with the best odds at being the next wave of electric power from the sun).

6. Don’t forget solar chemistry (not just growing plants) to make hydrogen and other fuels!

Solar is very close to breaking out. Why not invest in solar tech?

Educational Links on Photovoltaics and Solar Energy

2008-11-09T15:44:00.004+01:00

Where would be the best place to get an update of solar energy conversion, and photovoltaics in particular? That would be in a classroom, where you can ask questions and sort through the multiple topics of materials, sources of photovoltaic action (drift, diffusion, electrokinetic phenomena), and the difference between a cell, module, and an array. You would also be able to see that PV is only a tiny segment of an otherwise broad portfolio of technologies to make use of the sun for heating, cooling, making chemicals, making electricity from turbines, and so on. I offer two core courses at Penn State that deal with these subjects, but obviously there is a larger audience out there that would like information. Thankfully, we will be producing a web-based course dealing with photovoltaics, but that will be about a year off.

Therefore, I would recommend two web-based books for the curious, right now! The first is an educational project that began as an international collaboration between the University of Delaware and the University of New South Wales, funded by an IGERT grant. The site is called Photovoltaics: Devices, Systems and Applications CD-ROM, and the authors are Christiana Honsberg and Stuart Bowden. This includes interactive diagrams, movie clips of the silicon manufacture process, and a good review of solar energy. You will need to download Shockwave from Adobe. Up until recently, the Shockwave addition did not work for Macintosh systems, so I was more hesitant at recommending the site. But now: go for it! You will be busy for weeks. Note that the site is dedicated to silicon devices, and will not provide a comprehensive description of thin film PV devices and the principles of operation. That being said, the site is a gem.

The second book is not as web savvy, but does contain fantastic fundamental information on solar energy conversion. The resource is Power from the Sun by by William B. Stine and Michael Geyer, at California State Polytechnic University in the USA and IEA SolarPACES in Spain. This text is more like the classic paper text by John Duffie and William Beckman: Solar Engineering of Thermal Processes, in which multiple solar energy conversion technologies are described.

There you go, solar energy enthusiasts! Go to school and get informed on solar energy. But if you are tied up with other things (like life), in the mean time do some winter reading and find out how much potential solar energy has as a sustainable technology!

More Photovoltaics to come...

2008-08-31T15:43:00.002+02:00

It looks like there is interest in the principles of photovoltaics. I will be reviewing and revising my older posts on the subject in the next few days. Come back shortly for more!

Photovoltaics: Levels of Irradiance

2008-08-02T18:54:00.008+02:00

Let’s talk about light interacting with a semiconductor to yield electricity. Today’s topic is to distinguish between low levels of irradiance and high levels of irradiance. Effectively, we are asking for an estimate of the concentration of photons being delivered from a high energy source to a low energy absorber/collector.

When we say low levels of irradiance, we are estimating a scale of light concentration that is typical of the diffuse and direct component of unconcentrated “global” or “total” solar radiation, or the light from a standard incandescent lamp or fluorescent lamp. This could be anywhere <1000 mW/cm², or 10x the sun’s concentration (remember, this is just a crude scale, not a hard and fast rule--don’t take this back to your classes). The standard for testing solar cells inside the earth’s atmosphere is called Air Mass 1.5 Global (AM 1.5G), because the light from the sun passes through 1.5 lengths of a generic Earth’s atmosphere to generate a convenient irradiance of ~ 100 mW/cm². Low levels of light such as this provide a sufficient number of photons (packets of light) to excite the electrons into an unoccupied level of energy (the conduction band). However, the population distribution of the majority carriers does not change significantly. That’s okay: the key player in a photovoltaic absorber is the minority carrier (n-type semiconductor: a hole; p-type semiconductor: an electron), and the population of minority carriers does change significantly with light absorption. Minority carrier transport gets the job done, in fact, because they are the limiting rate in the absorber reactor. You can find out more about charge carriers and carrier transport in the Photovoltaics CDROM from Honsberg and Bowden, Chapter 3 (although it doesn’t work completely for Macs, sadly)

What is high irradiance? You’ve heard the warnings about strong lasers pointing into others’ eyes? A laser is a coherent, collimated light source (the photons’ waves are in phase and heading the same direction), such that the photons can be very concentrated. If sufficient numbers of photons are absorbed by a semiconductor, the population of photoexcited charge carriers can be much greater than the majority carriers, and there a population inversion occurs, leading to stimulated emission (Light Amplification by Stimulated Emission of Radiation).

The photons from light bulbs and suns are neither coherent nor collimated, although they can be concentrated significantly to potentially cause a population inversion and stimulated emission (yes, there is the possibility for a solar laser). However, before that stage there are other phenomena that occur, making it a bit more complicated.

Concentrating cells allow an increased flux of photons to the smaller receiver/absorber using a larger aperture to collect the solar light. The geometric concentration ratio is the ratio of the area of an aperture to that of the absorber (C=A_apt/A_abs).^1,2 For a perfect concentrator (as a point on the surface of Earth), the radiation from the Sun on the aperture-receiver assembly is only a fraction of the total radiation emitted by the Sun, given a half-angle subtended by the Sun of 0.27°. Assuming a blackbody, the absorber would have a maximum theoretical concentration ratio of 45,000 (for a circular concentrator) or 212 (for a linear,trough concentrator).¹ The higher the concentration,the higher the photon flux (including increasing temperature),and the more precise the optics of the collector must be to deliver. This is an extreme energy flux for any semiconductor. Under high illumination levels, one will observe a decrease in minority carrier lifetimes and related diffusion path lengths. However, 45.6% of the suns power is contained in the infrared band (the part that makes things "hot"). Thermally, an imaging concentrator (C>> 10; analogous to camera lenses) can produce temperatures from 500 to 1500 °C at the absorber.² This increased temperature can be used to drive thermal work (steam generation) or thermophotoelectrochemical reactions for concentrating solar power (CSP, not to be confused with CPV), but is not necessarily good for photovoltaic performance. High temperatures tend to decrease the efficiency of a photovoltaic device. In particular, this is why members of the microelectronics industry are getting into the concentrating photovoltaics field (CPV)--they know how to cool superhot microelectronics, and will do the same with CPV devices.

It is so interesting to see how this is all a great spread of possibilities that one can derive from our nearest fusion reactor!

Text sources:
1. Rabl, A. Active Solar Collectors and Their Applications. 1985 Oxford University Press, New York

2. Duﬃe, J. A.; Beckman, W. A. Solar Engineering of Thermal Processes. (3rd Ed.) 2006 John Wiley & Sons Inc, Hoboken, NJ, USA.

3. Andreev, V. M.; Grilikhes, V. A.; Rumyantsev, V. D. Photovoltaic Conversion of Concentrated Sunlight. 1997, John Wiley & Sons Ltd, Chichester, England.

Surfing more and more photovoltaics!

2008-07-23T19:36:00.005+02:00

In just a few years since returning from France in 2006, I have noticed some significant improvements in the world of PV within the United States. In fact, it seems that there is a wave of solar development and deployment that is rolling across the country!

Let me preface this glowing remark by commenting that not all was so great even two or three years ago. I had been working for a year in a laboratory in France that specialized in basic research for silicon and eta-cell (extremely thin absorber) thin film photovoltaic devices. While there, I was working with members of industry, the French government and power company, and the French national lab system. It seemed that there was a great vertical integration of research, industry, and deployment in France (and even more occurring in Germany). It was therefore a bit of a let down to return and learn how far behind the US was in terms of this integration. Yes, there are two major centers for research in Colorado (NREL) and Florida (FSEC), but as a national whole, the system seemed a bit worn, frumpy, and patchwork in nature. In truth, the USA went through about a 25 year period where not much was visible at all in solar research. The funding had dried up, leaving room only for the biggest four or five names in materials research and computer simulation (who supplemented their funding with studies in refrigeration). Now, many of the notable solar researchers are either retired scientists, microelectronics specialists, or emeritus professors.

However, in the two years since I returned there has been a dramatic bootstrapping occurrence. Just as we are looking to “next generation” PV technology, so are we seeing “next generation” researchers, educators, and industrial developments! Gunther Portfolio is a great blog for keeping us informed about developments for investing, and SolarBuzz and PVNews/Greentech Media also have regular installments of more and more PV industry growth.

In education, Penn State launched a new Spring 2008 course from the Dept. of Energy & Mineral Engineering, focused on solar energy conversion (with emphasis on photovoltaic conversion). Penn State also has plans to develop another more hands-on course in photovoltaics for extended education in the near future. Prof. Tonio Buonassisi of MIT has also announced a course in photovoltaics set for this Fall 2008 semester. The students have spoken, and they want more information on the current state of the art in solar and photovoltaics!

In the federal government realm, we are still sadly lacking a signal to encourage PV via incentives. The residential tax credit is slated to expire at the end of this year (following an extension). You will find much better luck for incentives on a state by state basis (see DSIRE). However, we did just receive a call to action by former Vice President Al Gore that may put more senators and representatives “in the mood” for renewable electricity generation. Also, the Solar Decathlon is to continue until 2015, the projected year for levelized cost of electricity generation from PV to be competitive with coal-fired electricity generation. The sponsor (DOE/NREL) projects half a billion visitors to the Mall area over a three-week period in September 2009, and anticipates global exposure to the Solar Decathlon concept to over one billion people. The Solar Decathlon is also exerting a viral effect on solar engineering and design, as it is inspiring similar competitions globally. Even now, a Solar Decathlon Europe is planned for 2010 in Madrid, Spain. The city of Beijing will be holding the 2009 Delta Cup – International Solar Building Design Competition, where the winning homes will be deployed in the earthquake-hit areas of Sichuan.

Keep up the good work, solar community! Let’s continue to work together to provide more information and more incentive for the broad public to adopt solar renewable energy. Of course, if a major component of that is photovoltaics, I would be pretty ecstatic!

What is disruptive technology?

2008-07-02T21:10:00.003+02:00

Quick question: would you interpret quantum dots as disruptive technology for light absorbing solar energy, or concentrating solar power (CSP)? One is a fairly recent topic in the photovoltaic world, and the other has been around for over one hundred years.

A quantum dot is a nanoparticle in which the excited states (high energy electrons and holes) are "confined" by the very small dimensions of the particle. This leads to increased energy in the excited states (no where to go but up in energy), and has resulted in many new technologies. One proposed technology would use quantum dots as light absorbers for a photovoltaic effect, where one could collect mulitiple electrons (increased photocurrent) or very high energy electrons (increased photovoltage). The up side is that quantum dots sound sooo cool, why not make them into PV devices? The down side is that the rates of charge carrier extraction (collecting the electrons to do work) are still way too high to get much efficiency out of them. A lot of research needs to occur before you start seeing purely quantum dot PV. The disruption appears to be far away.

On the other side, if you concentrate the sun's power, you can use it effectively for multiple applications, and often you don't need radical new technologies. Rather, a combination of straight forward technologies in a new way may lead to something disruptive. You can concentrate the sun's visible light (48% of the suns power, or 656 W/m2) for photovoltaics, OR you can concentrate the sun's infrared light (45.6% of the total power, or 623 W/m2) and use the thermal heat to do work! Either way, by concentrating you take a diffuse source and, well, concentrate it. Certainly, you would need to cool a PV collector, but what about a thermal collector powering a turbine to generate electricity? In 1878, a solar power collector was exhibited at the World's Fair in Paris, France. Between 1907 and 1913, an American engineer (F. Shuman) developed solar powered hydraulic pumps with a concentration ratio of about 4.5:1.¹

And the kicker, CSP is getting closer and closer to being the first economically viable solar technology--opening the doors to the following technologies? Is this disruption, by opening the possibilities of solar power beyond the single junction photovoltaic device?

1. D. Y. Goswami, F. Kreith, and J. F. Kreider Principles of Solar Engineering 2nd Ed. (2000) Taylor & Francis, Philadelphia, PA.

Natural Fusion recap

2008-04-19T15:25:00.004+02:00

It has been a very busy semester at Penn State. I've served as the faculty director for the Solar Decathlon 2009 effort (Natural Fusion project), I'm developing a course in solar energy conversion, I'm establishing my materials research laboratory, and I'm the outreach and recruiting coordinator for my department. Even so, one of the fun aspects of my job is that so many things overlap each other, and there seems to be an unusually high rate of "moments of synchronicity". The class overlaps with the project, and the project helps with recruiting, and I've gotten to know more people on campus than I ever would have hoped for in my first year at University Park. What a blast!

In the Natural Fusion project, I have 15 amazing students serving as project managers from multiple colleges. They all have the vision and energy to turn this competition into a brilliant learning experience for integrated design, green building, and entrepreneurship. We also already have a team of over 100 (!) students that are helping in our design and marketing process.

I am also fortunate to have two other experienced faculty members deeply involved in the mentoring experience, and several more faculty available for support in the future. The team has been out to several industries seeking support in mentoring, materials, and direct funding--and things are looking very good. Even in the first four months, it appears that we will be testing and deploying many new technologies in photovoltaics and energy efficient materials. It's not without its stressful periods, but I feel that we have a really great thing going that will be both memorable and valuable to all of our futures.

Solar Decathlon...ho!

2007-12-11T22:44:00.000+01:00

The Solar Decathlon is a progressive competition, offered to selected universities across the nation and outside of the USA every other year, in which students from multiple disciplines design and build a home completely powered by the sun. The focus of the competition is to combine BIPV (Building Integrated PhotoVoltaics) with new energy efficient architecture and its engineering systems. The competition was initiated in 2002 by NREL/DOE in conjunction with major sponsorship by British Petroleum, and the official two-year cycle was continued as of 2005 (SD2005).

The Decathlon operates within the general goals of the Solar America Initiative of the DOE, to make photovoltaic solar energy cost-competitive with conventional energy forms by 2015 (levelized costs of $0.10/kWh for PV). A major focus is to encourage relations between Academia and Industry for integrated design of photovoltaics within standard building practices. However, it includes the incorporation of the project into the curriculum of the students, as well as their involvement with industry. The Decathlon is projected to continue until 2015.

The winner of SD2007 was: Technische Universität Darmstadt. That’s correct; Germany won the USA solar home competition on their first try (why shouldn’t the biggest PV player in Europe be a strong competitor?). Perhaps this was an appropriate challenge to wake up integrated PV education in the states.

My own new home, Penn State University, took 4th place on their first attempt, Morningstar Pennsylvania. We’re looking forward to the opportunity to return and improve upon that standing in SD2009. It’s a great opportunity for students and faculty alike, and all products displayed in the Solar Decathlon homes are commercially available, which will make the project pretty interesting as the competitions progress to 2015.

Goals in Interdisciplinary Research

2007-09-17T23:04:00.000+02:00

In today’s research society, there is value in we. I don’t really know that this premise has changed over the years, but the message seemed to have been lost or mixed up in the pressures for making an independent name of your research in university life. Young researchers are fed information from senior researchers that they need to stay focused—and maybe it gets misinterpreted as staying isolated.

We’ve been told that “once upon a time”, someone starting out into the academic world was open to develop one’s personal, independent ideas. Funding was talked about as plentiful (or at least more probable to acquire by writing a grant proposal than today). But now we know, those of us trying to break upward into a stable research program. It’s just not a good strategy for a newcomer in grant writing and fund-seeking. Today’s research is cut-throat competitive, and even more so if you try to go it on your own. Working alone is an invitation to blow out your tire before you even get rolling.

You can’t know everything, even regarding a particular subject like solar energy (especially with solar energy). Help from others is needed to strengthen your research. It is important to build a network of skeptical, critical thinking colleagues who can look at your goals from unusual angles. You want a collective of shared interests, because there is power in numbers. They have the same urgent goals for support as you do.

So how does one make unique contributions while maintaining a source of funding? Work in bigger circles. Be open to defining your colleagues by a broader set of criteria. Communicate outside of your discipline and be positive of your own abilities.

It’s scary to look out across that void between disciplines, to reach out and communicate with someone you don’t know when you’re not even remotely an expert. But in order to support modern research, we need to span that void as another form of exploration. Because it very possible that we’re not even aware of the potential from the expert on the other side.

On a road to somewhere!

2007-08-18T23:03:00.000+02:00

Greetings all. My delay in contributing to these posts was for a very good reason. After many years of graduate school, and after experiencing the transient life of a postdoc, moving from Wisconsin to France and then back to Wisconsin for positions as a research scientist, I believe I will be staying put for a while.

We're in the process of relocating the whole family to State College, Pennsylvania for my new position as an assistant professor at Penn State, in the Department of Energy and Mineral Engineering. I will be pursuing my dream of environmentally aware materials science in the pursuit of new photovoltaic devices. I admit, I'm excited and terribly nervous at the same time. I plan to work hard and make progress in my research, and in extending my network of connections with academia, government, and industry. I also really want to be a good mentor to both undergraduates and graduate students. So much of this, you just have to do it rather than make the perfect plan. The system is dynamic and fun, and more like surfing than following a recipe.

So wish me luck, and keep an eye out for new posts from the bench of the new nanomech professor!

PV Documetaries: The Power of the Sun vs. Saved by the Sun

2007-05-13T19:35:00.000+02:00

When making a science documentary, shouldn't a good science background be involved in the production?

Please note: The solar power of today is not the solar power of the 1970s (or the 80s, or the 90s...)!

I have viewed the two recent big documentaries on solar power: The Power of the Sun and Saved by the Sun. In comparison, my summary argument: a scientific background goes a long, long way to gelling the message of a science documentary.

For those short on time: watch The Power of the Sun and show it to all of your friends. You can purchase the DVD for US$10 +shipping (half price if you are a teacher!). It provides the scientific background of silicon photovoltaics (PV), as well as a future for solar power. The latter documentary is a highly-diluted and out-dated science commentary with a generous mix of 60s and 70s nostalgia to mask the lack of content and vision.

Both films are produced in the US, and deal with the recent boost in development and financial interest in solar power. The executive producer of The Power of the Sun was Nobel Laureate Walter Kohn, while Saved by the Sun was produced by Steven Latham, Larry Klein, and Evan I. Schwartz (scientific backgrounds unknown) under the NOVA series of PBS television.

While the NOVA special has some value in getting people aware of commercially available PV, there is a distinct lack of presentation of the science and the broad range of industries already involved in major PV businesses. They don't give credit where it is due, and misplace other credits by a lack of depth of inquiry. Instead of talking about the fact that PV in Japan is now unsubsidized, and many new Japanese homes have PV funding incorporated into home loans, the film implied how "curious" the Germans are for creating a heavily subsidized solar market and entire farms of PV. Skeptical economists are called in to reassure us that solar energy is still a long way off folks. That could never happen in the USA, economic analysts scoff. Except that it is happening in the rest of the world, and we've woven ourselves into a global economy. A market with a 37% cumulative growth rate (meaning a doubling time of 2.2 years), and a 2006 peak power output of ~2.5 GW (Gigawatts) is not an economic fringe commodity.

Following this disjoint, Prof. Nate Lewis of Cal Tech is interviewed for his contributions to photoelectrochemistry (which are very significant). However, instead of going into the new chemistry his group works on, he is essentially given credit for recently creating dye-sensitized solar cells (that were first developed by Brian O'Regan and Michael Grätzel in Switzerland and reported in Nature in 1991). Couldn't the filmmakers have asked Nate, so how long has this been around?

Add on top of all of this, a penchant for 1960s and 1970s counter cultural pop tunes including the word "sun" or "sunshine", and you have officially alienated the new generation of PV consumers. Thank you NOVA and PBS.

Prof. Kohn had informed us at the American Chemical Society's viewing of The Power of the Sun, that PBS turned down the opportunity to show this film. It was deemed too controversial, due to a comment by narrator John Cleese that the world was "dangerously dependent on fossil fuels, even addicted to them", and due to a suggestion that fossil fuel combustion may even be linked to "global warming". Curiously, one G. W. Bush did see The Power of the Sun in a personal viewing, and something of that phrase slipped into one of his speeches.

Hmm, I guess good science does have an influence on policy.

* Plot developed by JRSB from data by PVNews and the Prometheus Institute for Sustainable Development.

A change of scenery...

2007-04-03T01:23:00.000+02:00

As a curious turn of events, I will be cross-posting my future blog entries at Nature Network.

I heard about this new science-based social network being established at the American Chemical Society conference in Chicago (Mar. 28, 2007), and I was determined to give it a try. I've been a member of the Connotea online publication reference program for a year now (approximately as long as it's been in public existence I believe). And while I admit that I haven't observed a major shift in the physical science-based links at Connotea, I am sure that social networks, tagging, and other aspects of Web 2.0 design will percolate through the science community soon enough.

So please, feel free to enjoy the site here. But with all good luck, Nanomech in Photovoltaics may see a migration in the future solely to Nature Network.

The Nature of David Suzuki

2007-03-31T18:20:00.000+02:00

Last week I had the privilege of listening to a special lecture by Professor D. Suzuki for the Distinguished Lecturer series from the University of Wisconsin. Prof. Suzuki is a geneticist and ecologist from the University of British Columbia, and is very well know for his hosting of the television science show The Nature of Things, on the Candian Broadcasting Corporation (CBC) since the late-1970s. He is also renowned for his strong support of the environmental movement and his activism toward influencing governmental policy regarding the environment. I can still remember watching his show in the 80s while living in North Dakota (as we received the CBC in our television line-up). He really did inspire a love for nature and science for me.

That evening, he did not disappoint (a few summary points that stuck with me):

ECO is derived from the Greek oikos, meaning “home”

Hence:
ecology is the study of the home

and

economy is the management of the home

“It’s time to put the ‘eco-’ back in economics.“ Exponential constant growth is unsustainable, and the ecology should guide the economy--not the reverse.

People used to say think globally, and act locally. But "thinking globally" is too overwhelming, and people just throw up their hands and say, 'Well, there's nothing I can do about it. The problem is just too big.' Instead, we should really do as David Barry says: 'think locally, and act locally', because then the problem becomes more tangible, and people feel less intimidated by the prospect of bringing about change.

MY THOUGHTS: His suggestion to link the schools of economy with those of environmental studies and ecology really hit home. In that system, I believe there is a natural opportunity for linking materials science and technology into the process. In such a way, the materials produced are guided by with environmentally aware design and marketing of that product to an economy that understands the concept of a limited reservoir of energy, water, and materials on Earth's accessible crust. In an educational sense, this means incorporating coursework in ecology and geoscience into the fields of economics and materials science. I have a strong hunch that environmental engineering will be a passing term on the way to the next generation of modern society. Very soon, ALL engineers and scientists will be required to be environmental engineers in the context of their own discipline, just out of the influence of limited reserves.

Please note: Prof. Suzuki has a foundation to address sustainability and global climate change: the David Suzuki Foundation. This site is a wonderful tool for education on issues of sustainability. The site also contains simple, easy personal changes that will help diffuse the footprint of modern human society on Earth.

* Image copyright Ronica Skarphol Brownson (2006)

Are you Sustainable?

2007-03-31T10:51:00.000+02:00

The looming question of sustainable practices in chemistry and materials was a central topic at the American Chemical Society this week in Chicago. There were several symposia related to chemical education of sustainability, sustainability in water resources, and (my particular favorite): sustainability and energy. The 2007 ACS president, Dr. Katie Hunt, has made sustainability one of her core issues, and you can hear (or read) all about her in this interview on Science and Society.

Prof. Art Nozik of Center for Basic Sciences at the National Renewable Energy Laboratory (NREL) arranged a top notch session on Realizing the Full Potential of Solar Energy Conversion through Basic Research in Chemistry and Biochemistry on Tuesday (Mar. 26, 2007), with speakers Nathan Lewis, Michael Graetzel (of the dye-sensitized solar cell), A. Paul Alivisatos, and A. Nozik himself (speaking on quantum dots and multiple exciton generation from high energy photons). Prof. Nathan Lewis has presented this data to President Clinton in the past, and his talk on alternative energy was shocking, alarming, and invigorating all at once. In short, the only source of power that we have enough supply for is : solar. We don’t have enough wind, wave, geothermal, nuclear, biomass, etc. in our resources to cut our CO₂ levels and to create enough energy for only 2x the amount required to feed every human by 2050. You can find a link for the talk here.

Michael Graetzel’s talk was very interesting, and I’m delighted to hear progress has been made on dye stabiliy in UV, and new electrolytes have been developed using ionic liquids that remove the sealing problem encountered in acetonitrile-based electrolytes. In Graetzel’s words, dye-sensitized cells can be made now to withstand a 20 year life cycle (estimated), and have maximum performaces at 11% efficiency. Not too bad for an inexpensive alternative!

In addition, we were treated to a wonderful movie produced by Nobel Laureate Walter Kohn (UCSB) called The Power of the Sun. The short film is narrated by John Cleese, and can be obtained for only $10 from the University of California Santa Barbara website. The package includes an educational film for students as well. This film would be appropriate for high school science classes through college or university, and could be a very useful as an educational tool. It could be combined in an educational section on energy, or solar power, and the website has additional supplemental educational materials online.

I was disappointed in most of the other talks outside of the sustainablity symposia. Often the researcher/presenter did not gear the presentation toward a more general science audience. Hence, the context of the study was lost to the outside listener, and the importance that a study may have to a peripheral research topic.

For all of the hot talk about the importance of solar energy and the importance of third generation PV technologies, almost no mention was given of studying the interface between quantum dots and the electron/hole collectors necessary for doing work as a third generation photovoltaic cell. Considering that the interface is where the electron transfer occurs (aka: "chemistry"), I was quite surprised at the vacancy in that subsection of research.

The elephants of new PV technology were also in the room: the toxic heavy metal cadmium used in new solar materials (CdSe, CdS, CdTe by A. Paul Alivisatos), and the proposed superiority of CIGS (copper indium gallium selenide) PV cells, despite the very relevant indium shortages from limited supplies and competitive markets in flat panel displays. I felt these topics were not properly addressed, or maybe the main scientists are just not aware of the environmental implications of their research. We should present these materials issues to international audiences such as the ACS conference--as they are being developed--to create an environmental and ecological awareness of the most probable impact of our materials research should they be implemented on a national or global scale.

However, the meeting was indeed a recharging event for me. I left with a lot of positive momentum from the discussions on sustainability and the surrounding research that photovoltaic solar cell materials research. Most definitely PV is a strong route of scientific pursuit, and has many opportunities for new lines of research. If Prof. Nathan Lewis is correct, it will become one of the largest industries of our generation, and we should need a considerable amount of minds working toward sustainable solutions.

Environmental Chemistry in Review

2007-03-05T01:30:00.000+01:00

I was recently reading a the introductory statements in an older issue of the American Chemical Society’s journal Chemical Reviews. The issue was devoted to Environmental Chemistry, and the guest editor was Prof. István T. Horváth, currently a professor at the Institute of Chemistry at Eötvös University, in Budapest, Hungary (formerly a senior staff chemist at the Exxon Research and Engineering Company). I admit, I was originally looking for an article on dye-sensitized solar cells, but this introduction has an outstanding comment on ethical behavior in materials development. Something to mull over:

*Introduction: Chemists should be aware of the environmental implication of their chemistry.

“I hope that dedicating an issue of Chemical Reviews to environmental chemistry will increase environmental awareness among chemists. For example, it is no longer sufficient to make “marvelous” new molecules solely on the basis of their marketable properties. Although marketability is an appropriate goal, we, as scientists, must also be concerned with our creations’ potentials for environmental impact. At the same time, we should constantly tighten our scientific standards for generating experimental data, so that any conclusions drawn from such are and will be unambiguous ...

It is in our interest, indeed, in the interest of all of society, to remain vigilant to the impacts of chemicals on the environment. We would strive to keep environmentally acceptable processes alive and minimize our activities that involve unmanageable environmental risks.“

I find it interesting that over 10 years after his comments, we are only beginning to realize the huge environmental influence that chemists and materials scientists hold in their hands when they introduce ”marvelous“ new materials (including photocatalytic nanoparticles, quantum well lasers, ultracapacitors, and carbon nanotubes). Once a material is introduced and developed on a global scale, the waste component arises for material disposal, followed by issues of materials fate in the environment. As scientists and engineers, we have a responsibility to remain aware of the global environment when we make new materials for society.

*From Chemical Reviews, 1995 (95)1, pg. 1

New Member of the Sidebar Community

2006-12-19T18:57:00.000+01:00

After a break during the holidays, I'd like to point out that there is a new sidebar member reporting on photovoltaics in industry that all can access from the Nanomech in PV site. GUNTHER Portfolio is a blog focused on reports from industry regarding international photovoltaic technology, companies in the solar industry, and marketing. Ed Gunther presents a much needed layman's/businessman's portfolio of the rapidly developing solar energy industry, and his blog is worth taking a read. Enjoy.

2006 US Solar Energy Report and Silicon Report

2006-12-10T08:20:00.000+01:00

OK, for those of you interested in the bottom line of solar production this year (and not just photovoltaics), please look at the free download from the Prometheus Institute. Consider that photovoltaics have seen 20% growth in installed modules over the past year in the US alone--not too bad!

But wait! That's not all; for those of your interested in the progress of the global purified silicon shortage, and how soon we should expect to overcome the shortage, read the report on silicon.

Note that I've replaced Monkeysign's blog link in the sidebar with that of the Prometheus Institute. If you have the funds available and are interested in the progress of the solar industry, I would recommend getting a subscription to their monthly report, PVNews. This is a gem of a trade journal, and digests important progress into manageable numbers for discussions with friends and colleagues.

Sad News: Monkeysign's Blog is Unavailable

2006-12-06T17:14:00.000+01:00

Monkeysign, where are you?

"Monkeysign" is (was?) the moniker of a science blogger who consistently reported on the progress of the global photovoltaics industry. Unfortunately, the blog appears to be gone for now. Hence, the link in the sidebar leads to nowhere.

The user tag apparently came from a description of the ampersand in an email address as a "monkeysign" from a friend. It was funny, yet distinct, and it easily set him or her apart from the rest of the crowd (not to mention the excellent news updates). This is unfortunate news for those of us who enjoyed reading it, and especially for those who did not have the chance to see Monkeysign's blog. My thanks to Eugene H. for notifying me of the situation.

Postdoctoral Research Aspirations

2006-10-30T18:10:00.000+01:00

In the previous post, I was asked by reader and fellow scientist Riverie to comment on my experience in finding and securing postdoctoral research scientist positions. Submitted under the category of scientific philosophy, I have chosen the following response as a separate entry.

Like most employment successes, my postdoctoral opportunities have been initiated through positive connections between my former advisor (or myself) and interested parties. My credibility for scientific research improves with each year of additional research (improved network and letters of recommendation) and with successive publications (improved CV). In my experience, one improves the odds of selection for an initial postdoctoral position by searching within the known network of one's advisor(s), and by always requesting the advisor to make initial inquiry contact regarding a position. With successive years, you should be developing your own network, but this process takes some time to “seed”.

Although I have a second position now, I can attest to the difficulty of a "cold call" to another professor (especially outside of their field) expressing your interest in a research position. Professors have exceedingly busy schedules, and often don't have the time to confirm your credentials without some advanced confirmation from a peer, expressing that a candidate is worthy of consideration. Given the inadequate time allotted to the selection process by the professor in need, the risk for selecting a poor candidate is high and one must expect them to be very conservative in drawing their short list. Even so, if the prospective position has no funding and you have no funding, there can be no opportunity, and professors should inform the advisor right away of such a limitation.

If you are selected as a candidate and if it is possible, visit the lab. This was not possible for me to do for my experience in France, so it was extremely fortunate that my postdoc was both highly successful in research and that I had an excellent working environment with the group and group leader. I know the lab that I am currently working in, so I was certain I would be able to adjust and work efficiently.

The truth is, you need to be at the right place and time, with the additional nod from a peer, to garner consideration for a position. As much as they may be though of in terms of inexpensive labor from the point of view of the graduate student, a postdoctoral researcher is substantially more expensive to support and is required to accomplish much more than a student. There are also expectations from the postdoctoral researcher that some mentoring will occur, as an apprenticeship to managing your own future research team (in academia or business). The collaboration is an additional time investment for the professor, and can be looked upon as a means for the postdoctoral researcher to become more efficient at what she/he already does well. So, one should be able to make a good case for an independent work initiative, and expect to perform the majority of the assigned research on your own (with critiques along the way).

Once you are in the research position, expect that a mentoring professor is there to correct your form; you must be open to listen to what they suggest, even if you want to stubbornly adhere to your own hunch. On the other hand, you should continue to express your own opinions regarding the research to spur scientific discussion. If you don’t have the data to back up your claims, be prepared to listen to suggestions and go back to the bench to find that data that clarifies the discussion. Often, you will be surprised by the results of the compromise.

Finally, never underestimate the network of people that are in the surrounding group and laboratories. These people will all be potential connections. Keep your mind open to interdisciplinary connections, even if the moment is not ripe now. And remember that you are the transient in the laboratory; more so than the graduate students, even. Respect that the laboratory and office is your temporary home, and keep the peace. A postdoctoral position is not the place for the great revolt against the machine, and you are expected to behave according to the internal rules of the lab (provided they do not breach ethical standards of scientific behavior).

Bearing all that in mind, good luck!

Cap and Trade

2006-10-16T04:59:00.000+02:00

California led the way in environmental action once again by signing into law policies (AB32) for greatly reducing climate-affecting greenhouse gases (such as CO₂ from fossil fuel combustion). National Public Radio has an interesting debate on the subject. The name of the game is cap and trade, the very same type of policy that we used in the 1980s to reduce acid rain-causing gases such as NO_x and SO_x. Essentially, the California government will set caps (upper production limits) on the levels of greenhouse gas emissions, and will trade emission levels (higher is some parts of the state, lower levels in other parts) among the state to achieve a net level of emission that meets the state cap level.

Fossil fuels are already heavily subsidized. We need to ask, is there a better way to use our tax dollars that also reduces greenhouse gas emissions and aids reduction in major climate changing forces? In making a comparison of solar and fossil fuel energy, we rarely admit that fossil fuels are hugely subsidized. On top of that, consider the prospects for major coal power plants. What is the one resource that the US has a temporary plentiful abundance of? Coal. What is one of the worst combustion sources for greenhouse gas production? That same material: coal. Texas is planning three new enormous coal power plants, set to make it one of the US's worst states for greenhouse gas emitters. Couldn't we be shifting our subsidies to crop damage, storm recovery, and renewable energy plans?

The NPR show makes a good point that jobs for electricians installing solar modules, or plumbers installing solar water heating modules cannot be subcontracted out to foreign countries for lower rates. This is something that requires American labor resources. As part of my research solar electrical materials in academia, I am aware of the need to know about the ground-level labor that is required for device implementation, and the industrial interest in these new technologies. From my own contacts in the renewable energy sector, I am finding that here in Wisconsin, USA, there is a very progressive interest in building the core structure for solar and wind energy use. There are more and more meetings for large industry, not to dream about solar energy, but to actively plan for the next wave of labor and materials that will be set in place to institute solar module installation in homes across Wisconsin. There are jobs in converting our economy to renewable energy.

Transitions in Perspective

2006-10-01T13:29:00.000+02:00

I have recently returned to the States from a fantastic postdoctoral research experience just outside of Paris, France. There, I was able not only to work on eta-solar cell materials research, but also to learn another culture and language, and to get outside of my comfort zone. The discussions that I have had in the Lévy-Clément research group the past year have been nothing short of brilliant, and I thank all of the researchers sincerely for their friendship and patience to enter into the "what is fire" discussions of PV.

Now, I have returned to Madison, Wisconsin to apply many of the ideas that I'd developed in France in another round of postdoctoral photovoltaic materials research. I find that I am not alone in my interests, as a PhD graduate student and a senior undergraduate in chemical engineering have begun participation in my research projects. It is always exciting to see that fresh interest in the strange but fun subject of photovoltaics. Onward! to la nouvelle vague.

Environmental Technology: What is it?

2006-08-12T10:01:00.000+02:00

I pursue my research interests in solar cells because I find that science is always more interesting at the interface of things (both literally and figuratively). The juxtaposition of materials development, earth science and environmental awareness is termed environmental technology. Because I have developed the building blocks from my multidisciplinary background into unique tools to fuel the opening field of environmental technology, I believe I am well positioned to have a little discussion about the subject.

My research deals with new designs and materials for non-silicon solar cells, and my background is that of a non-traditional environmental chemist and mineralogist (geoscientist). How is it that I find myself extending my education to non-traditional applications? One could say that both geology and environmental chemistry are already interdisciplinary fields of study, and to add another layer of interest only makes the process more interesting. However, the overlay of that additional element may act as a lens to help to focus a field onto those areas that are significantly helpful to mankind and those which may also stimulate the new post-fossil fuel economy. Yes, I believe there are ethical and financial incentives for pursuing environmental technology. But what is it?

Environmental technology is a subfield of research in environmental science concerned with topics of sustainable development in terms of energy, water quality, air quality, and waste treatment. Environmental technology is not necessarily a new topic, but I would have to say it has not yet come into its "own" in the larger umbrellas of environmental science or civil and environmental engineering. Past research in environmental technology has been strongly related to water remediation and waste treatment, and hence has had a close association with civil and environmental engineering. Currently, research has included alternative energy materials development, such as ultracapacitors, inorganic fuel cell membranes, and yes even solar cell development.

There is also a development to fold in the principles of green chemistry developed by chemists (largely organic), with the environmental technology goal of sustainable chemical development "upstream". The metaphor being a river in which any waste products are minimized and with the goal of environmental premediation (including the 12 Principles of Green Chemistry^*). Normally, waste is simply released into the river or sewer system and subsequently remediated, or treated after the fact, "downstream" from the point source of pollution. This goal of materials premediation will require new technologies that adapt learning from the earth sciences and environmental sciences for the common goal of sustainable resource development and low enviromental impact.

I have mixed scientific disciplines before and found that it can reveal amazing new discoveries. For example, if you overlay interests in geochemistry and mineralogy with genetics and microbiology, you have a curious field called geomicrobiology, where scientists have found fascinating results about microbial influences on the chemistry of mines, their survival near toxic "black smokers" in the ocean, and their influence on arsenic removal from groundwater by precipitating nanoparticles. In a similar fashion, if you combine interests in materials science with environmental chemistry and earth science, you have a burgeoning field called environmental technology with much potential for the new science economy.

Note: links of interest and examples of Environmental Technology

Green Pages: The Global Directory for Environmental Technology

Environmental Science and Technology at PNNL

Environmental Technology with Ceramic Membranes (University of Wisonsin--Madison)

National Environmental Technology Institute (University of Massachusetts--Amherst)

* The 12 Principles of Green Chemistry were first published by Paul Anastas and John Warner in Green Chemistry: Theory and Practice (Oxford University Press: New York) 1998.

Modes of Scientific Reason

2006-07-29T17:33:00.000+02:00

How should scientific research and reasoning be carried out? Are there universally- defined rules inherent to scientific discovery, or is it more like modern philosopher Paul Feyerabend¹ suggests, as an anarchistic realm punctuated by domains of apparent organization, each with a limited range of usefulness to scientific discovery?

One of the beauties I've found in doing research in another country is that, if you listen closely to the talk of your foreign colleagues, you may find the underlying principles of research are going to be defined differently than your own (even alien to your own scientific reasoning). I had this very experience in France, when my colleague was speaking of a mechanism of a chemical reaction and told me that another scientist had "proven" the mechanism was correct. Stop the press..."Wait, you can't say something is proven in fields outside of mathematics! Everybody knows that you can only disprove a hypothesis, and then only demonstrate strong support for a hypothesis that has not been struck down yet. An experiment has to be falsifiable, but cannot be proven." In fact, my music-teacher wife still remembers the day, in 11th grade Advanced Biology, when her teacher (Mr. Hjelle, pronounced "Jelly"... yes really) told her that there is no such thing as scientific fact; everything is theory, because we cannot prove anything beyond the shadow of a doubt. We can disprove, but to say we've "proven" something is a very arrogant and dismissive view upon scientific research.

The response of my colleague to me was that, in France, it is quite common to refer to something as being proven, as everybody knows that proof is essentially a shorthand or map to express the conditions of the regularities in an extremely well defined experimental setting. In her view, with sufficient support, one can prove something (as an asymptotic narrowing to an accurate description of a regular event). And beyond a certain point, it becomes ridiculous to search for obscure, exhaustive hypotheses to test to disprove something that has been found to be robust from experimentation.

My colleague is the senior researcher in our group, and I admire her scientific rigor and conservative approach in assessing and presenting scientific data to the public. At the time, we had been working together for over nine months. Through her actions, my colleague has repeatedly demonstrated her abilities as a rock-solid scientist. In other words, she is so effective with her approach to science that I have no reason to suspect that her mode of reasoning is any different from my own (which I had learned from my mentors in the USA). Yet it most certainly is different, and the months following have been challenging and enlightening in terms of expanding my approach to scientific discovery. So where was the source of this schism between each of our well-accepted, yet different philosophies of science? Can there be an optimal or universal method of scientific reasoning in scientific discovery, or is there "more than one way to get there"?

As an illustration, if we assume two laboratories on separate continents have expert researchers, well accepted by peer-review and having strong experimental results in the same field, are the two researchers able to arrive at similar results if they have separate scientific philosophies? In other words, as an expert scientist, is your learned approach to scientific reasoning that much better than another's?

I'll take the proposition one step further: assume one researcher has an outstanding pedigree in a field of research (say a chemist, like Marie Curie), while another is a bright, talented, very well-read, but self-taught inquirer (let's say a bookbinder-turned-scientist, like Michael Faraday). The former has been trained to use a rather robust yet strict set of principles based on her own mentor's rubrics to arrive at well reasoned results, while the latter uses an assembly of methods that he has developed on his own thorough experience. The methods of the latter (self-discovered) are a combination of very systematic experiments (not unlike those of the former) and some rather haphazard modes of experimentation and "exploration", that do not have a strict rules of progress, but tend to produce positive results nonetheless. Upon arriving at a successfully scientific discovery, both can communicate the results quite well, and have papers accepted into highly regarded journals in the literature. Now, which researcher was the better scientist? It would be an extreme act of prejudice to say the pedigreed scientist was better because of formal qualifications, given the assumption that the self-trained researcher in fact can have the same qualities of insight, rigor, communication and perseverance. But it would be equally biased to place an overwhelming romantic support for the self-taught individual as an underdog of research (something like Good Will Hunting). The fact is, in my illustration I assume they are both excellent scientists, and deserving of praise.

In considering the history of science in France, Germany, Austria/Hungary, England and America and the philosophies of science that developed in each country (perhaps another post), the answer is that yes, there is a difference in our scientific training and our scientific philosophies from France to the USA. Without really knowing it at the time, I had tripped over a break between a few schools of scientific reasoning. In my subsequent investigation, I found that France is influenced by the two systems of inductive reasoning inherited from Pierre-Simon Laplace, and more recent and practical conventionalism favored by Henri Poincaré^{2, 3} and Pierre Duhem³ (i.e. a hypothesis is neither truly verifiable nor falsifiable, but serves to make generalizations and predictions beyond experience). In contrast, the principle of falsifiability, argued by Karl Popper is in opposition to inductive reasoning. Falsifiability has been eagerly accepted in the USA as a core scientific reasoning tenet to separate "science" from "non-science". Arguably, falsifiability is not really the only manner in which experimentation and inquiry occurs. And even though several modern philosophers have argued against falsifiability as a basis for scientific reasoning and have subsequently suggested alternatives that are more closely related to science in practice, many American scientists have clung to Popper's criterion. To my honest surprise, I also found myself unquestioningly in lock-step with it in the earlier conversation with my colleague.

This led me to think, perhaps there are more important questions than asking why there are different modes of scientific reasoning; such as why was I conditioned to think that there is a fixed and ordained philosophy of science? In essence, I have trained for 15 years within the shadow of a doctrine of scientific reasoning, and in the process it has been necessary to condition (and some would argue brainwash) myself into that doctrine to pass through the hoops of academia. It is the consequence and necessity of a doctorate in science. One of the components in my education that I sought out on my own was information in the history and philosophy of science--something to add awareness to your our scientific preconceptions. As I walk away from the training of the USA and into the sphere of another doctrine of scientific reasoning, I have discovered new tools and perspectives that I believe will make me a better scientist and mentor. Not the least important result of this call to order is the reminder to encourage my future students to read as much as they can in the philosophy of scientific reasoning and the philosophy of science.

One can also find these fixed vantage points in scientific reasoning across various fields of research. In fact, I propose that discovering these differences through interdisciplinary research can hold some of the tools to make scientific breakthoughs. It's the moment of "Huh, I never thought of it that way..." And why?, not because one doesn't have the cognitive skills and imagination to arrive at a certain line of reasoning, but rather because one was not introduced to the extent of using an alternative scientific reasoning by training. Once you've defined a point of view, pick another one and see if you like the scenery. There's plenty more where that came from!

References and suggested readings:
1. Feyerabend, P. 1975 Against Method: Outline of an Anarchistic Theory of Knowledge. London: Verso.
Discussion of Feyerabend's "Against Method" by Paul Newall at the Galilean Library
2. Jules Henri Poincaré (1854-1912). Mauro Murzi, Internet Encyclopedia of Philosophy, July 29, 2006. (including a summary of his perspectives on Conventionalism, science and hypothesis)
3. Howard, D. 2005 Physics Today, December, 34.

Solar Companies of Interest: Progress!

2006-07-16T16:43:00.000+02:00

After hearing Dr. Richard Swanson give an expert talk at the Palo Alto Research Center regarding progress in cost and performance for single crystal silicon solar cells, I was both intrigued, better informed, and somewhat challenged for my own research interests.

Dr. Swanson is the president and CTO of solar cell and solar module manufacturer SunPower, based in San Jose, California. He gave a general briefing of single crystal silicon solar cell development in the PV industry from the late 1970s until now. The presentation was both a reality check and a highly optimistic outlook on silicon solar technology. Please take the time to listen to or watch the presentation (about 50 minutes) if you have interests in solar cell development for the masses (like solar cell integration into rooftop shingles in your house). What an interesting researcher, company, and technology overview.

My summary and thoughts:

Learning Curve
Single crystal solar cells are the most efficient photovoltaics in the silicon family, but had previously been regarded as too expensive for practical use. Not any longer, though, says Dr. Swanson. Improvements through production cost reductions and increases in solar energy conversion efficiency have progressed at a measurable rate (a learning curve) such that material energy costs were found to be $3/Watt in 2002 (to make a complete PV module for use)--as compared to around $30/Watt in the early 1970s. A large part of this cost reduction was due to the huge increase in silicon demand for the microchip industry, and presently the PV industry is about to overtake the microchip industry in total demand for the polycrystalline silicon.

Silicon Shortage and China
Also, there is a shortage of silicon purification plants right now, making the cost a little over $2/Watt. Silicon plants cost hundreds of millions to build and take three years just to turn them "on". China is currently building three of these enormous plants to make up for the new demand from both PV and microelectronics, and Dr. Swanson expects China to quickly become the epicenter of PV in the world. "The interest in photovoltaics in China is nothing but phenomenal." The leading Chinese solar cell company is called Suntech and is publicly traded on the NYSE (and the main owner is now the richest man in China). Given the emerging new silicon production plants, Dr. Swanson expects costs to be below $1/Watt by 2012. In his opinion, that is the point at which USA governmental financial subsidizing of solar cell module costs for residential applications will no longer be necessary. A light at the end of the tunnel!

Embedded Energy
Also, the "embedded energy", or the energy costs needed to purify the silicon precursor, make a silicon solar cell, the glass in the module, etc. requires approximately three years to be recovered as an equivalent from the sun. Given his estimates, by 2012 that will be reduced to 1.5 years to recover the energy. Now, there's a tidbit that I'm happy to have a number for in terms of environmental consequences. Consider, the initial energy required to make the complete solar cell did not come from the sun, but from fossil fuels. But for every Watt you gather from your own solar cell modules (after collecting the embedded energy), your own demands of a fossil fuel power plant is reduced.

Necessary Breakthrough
For me, the interesting and challenging factor stated was that a major technological breakthrough is necessary in the years following 2012 to lower costs to the expected value of less than $0.65/Watt twenty years from now. This would be a point at which solar cell power plants could conceivably be manufactured and put out in the desert to return a profit. His words were that the industry has no idea of how they will get to that point as of yet. So there's the challenge for all of the up-and-coming scientists and engineers dealing in photovoltaic research for areas other than silicon. The gauntlet has been tossed, we all have less than twenty years to come up with a brilliant disruption in solar cell technology (silicon or otherwise).

Thin-Film Successes
At the last minute of questions following Dr. Swanson's presentation, a member of the audience asked about thin-film technologies (and other new solar technologies) from established new companies such as Nanosolar, based in Palo Alto, California (the city site of the talk, in fact). Nanosolar uses a non-silicon thin-film assembly called CIGS (copper-indium-gallium-diselenide). Normally CIGS is deposited in high vacuum, and was viewed as too costly for commercial production. Nanosolar appear to have developed an "ink" containing nanoparticles of CIGS material which can be roll-to-roll printed without a high vacuum (much less expensive). Dr. Swanson's response was that the industry such as Nanosolar will have to beat the learning curve rate of silicon wafers to become a new competitive market, but that companies like Nanosolar appear to have a lot of potential for the future.

Additional materials side notes:
When solar cells become incorporated into industrial power plants, new high performance power storage materials (batteries, capacitors, etc.) will be essential for PV success. So go out and build a better battery too, my materials research colleagues.

Inverter: the "part" in the solar cell system in your house that transforms the direct current (DC) power from your PV into alternating current (AC) that you can use and feed into the electricity grid. This is the single part most likely to break over the course of 5-10 years. Want to work on a side technology? Work on the materials for electrical inverters.

Thanks so much to Sarah W for pointing me to this link, and the Palo Alto Research Center (PARC) (a subsidiary of Xerox Corporation) for making this talk available on the web! A fascinating way to spend a sunday afternoon in the Parisian heat.

Case Study: Indium and Tin

2006-06-17T13:38:00.000+02:00

Where does any raw material come from? Meaning, where on the surface of this planet can we find an ore body to mine? Once you know where a ore body is, how much can you take out of the earth and still have the product be economically viable (considering clean-as-you go mining)? Once you get a metal ore out of the ground (you must dig a mine), think about how it is processed and refined, and then think of the environmental impact of the purified substance once it has been liberated and refined from the raw ore. What do you do with the raw material when you have it? What happens to the the purified material/chemical when you don't want it any more? How much energy is required to recycle it? What is the fate of the material when you release it back into the environment? This is a cradle-to-grave study, and is a core principle in environmentally aware materials science.

We don't need to just think in an abstract way about the global ore reserves of a mineral or element, there are information resources on-line at our web fingertips via the USGS (United States Geological Survey). As an aside, I truly enjoy the transparency of operation, open access to information, and the hands-on attitude of the USGS. Sure, it may be a bloated government operation, but it's one that probably does far more positive action for the improved quality of life of Americans than several other, more aggressive modern mega-gov-departments.

One of the sub-disciplines is in geology is economic geology. An economic geologist is responsible to find an ore body (using scientific principles of ore formation); estimate the size, purity (for valued minerals), and accessibility of the ore body via prospecting (drilling, satellite imagery, GIS global positioning information, and a battery of geophysical tests); and then report the findings and estimates to the public or to an interested private company. In our case, the USGS has done this sort of work for the public and reported annual estimates of major minerals of interest. If you visit the Commodity Statistics and Information page, you'll find a virtual treasure trove (literally and figuratively) of information on mined materials on a global scale.

As two examples, I will use the elements "Indium" that is used in ITO (Indium Tin Oxide), and "Tin" used in ITO and fluorine-doped tin oxide (FTO, or SnO₂:F). Both are used with Transparent Conductive Oxides (TCOs) that are popular right now for the LCD industry, and some use is being channelled to new solar cell designs.

If I look down the USGS list to find Indium, I see a whole host of information summarized just in the introduction. Wow, that's great! We need to mine zinc sulfide ore to get indium, so in order to get more indium there needs to be more demand for zinc. But wait, there's more! In the Indium 2006 Mineral Commodities Summary we get access to a full PDF of data. What do we find out? Well, indium use grew by 15% from 2004 to 2005, we don't have any of our own on reserve (stockpile), and we import the majority of our indium from China, Canada, and Japan (from highest to lowest). Also, only 15% of all ITO goes into actual products. Due to difficulties, length of processing, and high costs, the rest of the unused ITO is scrapped and not recycled at all.

OK, now compare Indium to the element found in TCOs, Tin. Fluorine-doped tin oxide (SnO₂:F) has many of the same uses as ITO, but costs less and is often more useful in solar cell research (because indium tends to diffuse into the light absorbing materials with thermal annealing). In the Tin 2006 Mineral Commodities Summary, we again get some juicy information. We see that, again, we don't mine tin much at all inside the US, but the countries that we do import it from are much more diversified. The applications of tin itself are much more diversified, and it is not nearly the trace metal resource that indium is (right up there with silver). While tin is not an abundant element, we do have tin stockpiled, and tin recycling is around 61% as of 2005. No words about SnO₂:F recycling, though--my suspicion is that it is just as difficult to recycle as ITO thin films.

Which of the two would you choose to work with?

I will make one additional note on the environmental impact of both of these materials in their oxide state as thin films. Both tin oxide and indium oxide are pretty stable at Earth's surface, relatively inert (they don't dissolve easily), and have not been found to cause dramatic toxicity problems as oxides. Given the small amount of information at hand regarding toxicity, if you don't recycle either of these, you are unlikely to significantly alter the environment at the waste repository. So it's more a choice of economics and reserve materials at the moment. Phew, at least something that we make for new technologies is useful and not horribly toxic!

But you know what? Sometimes even oxides are altered, and then there may be a toxicity risk in the dissolved species. This is the true weak spot in the case study, where we assume the small amount of research on tin oxide and indium oxide is sufficient to assure environmental fate. Once upon a time, not so long ago, in a county not far from your home, scientists perceived PCBs (polychlorinated biphenyls, used as electrical insulators, liquid coolants, and fire retardants) as "safe" (on the basis of a small amount of information). They are quite inert and not many people had researched their toxicity, until after they were widely distributed in our everyday technologies--and then it became apparent that things were not so safe with PCBs. In the organic chemistry laboratories, you also tended to wash your hands with benzene then, but that's another story.

In environmental materials science, we need to know the extremes at which our materials break down, and if those conditions exist in the outside environment. Research on the chemical stability of these films, and the fate of dissolved tin and indium species would be handy information. A classic euphemism in materials science was make it, then break it. This used to be used for building materials like cement, steel, aluminum alloys, and brass. We now need to adapt it to a new wave of materials science that also includes molecular compounds and nanoparticles and thin films.

Environmentally Aware Materials Science

2006-06-17T13:36:00.000+02:00

Form and Function have new roles in materials science when viewed through different watch glasses. Environmental consistency or awareness is one of those lenses through which I inspect those roles. Is improved Function (e.g. a new "wow" compound or nanostructure) worth application if the materials used disruptively change the outdoor environment and human/animal physiology? Is better Form (on the small scale a core element or molecule, or on the large scale a wind or solar station) worthy of development if the supplier cannot meet demand for a growing industry?

Let's view the materials in advanced photovoltaic design from an environmental perspective. Not necessarily as a pejorative concept of "bad for the environment", so before passing this post off as a tree hugger's celebration, read on. The environment is more than an idealistic biological greenspace--it is also the raw minerals from the crust that drive our technologies, the water we cannot live without and the air that we breath. With all our metals and semiconductors used in industry, one must "dig up" the raw ore, then separate the ore into constituents, and finally refine the element or salt before research and industry can use it and convert it into a specific chemical compound. This takes energy, land space, water, and green space. But if you want your next i-pod, blackberry, laptop, television, hybrid car, electric car, solar cell, or wind farm; you need to dig into the dirt to get the materials to make them function with such an appealing form. As a formally trained geologist and an environmental materials scientist (and the grandson of a mining engineer), these reflections are smack in the middle of my think-space. As such, I am constantly reminded of the links tying technologies to geology and environmental chemistry.

I encourage all materials researchers to use tools of environmental awareness to assess the practicality of going down each road of materials research. If the impact of our research is a global technology that everyone will want--can everyone get access to the materials to produce it, or will it be a short term solution that will be isolated to the very rich? Will the use of that material require an energy intensive processing? What will the emissions be from refining, and how easily is the product recycled? Finally, what is the fate of the material when it is reintroduced into the environment in a waste repository, and what is the toxicity of the altered chemicals in the environment. This is termed a cradle-to-grave perspective, and while I don't really enjoy the anthropomorphic connotation, it is a fairly new concept to advanced materials research.

Summary
Consider a material used for technology. The following factors are valid in a global economy of technological development:
1. Raw resources (local or imported)
2. Energy use from refining
3. Gas and particulate emissions from refining
4. Recyclable?
5. Energy use from recycling
6. Toxicity
7. Fate of the material as it is exposed to the environment

Transparent Conductive Oxides

2006-05-27T21:25:00.000+02:00

Ok, so here's a trick: make a solid inorganic material that is transparent to visible light like SiO₂ glass, but also make that material conductive to electricity like a metal. The resulting films belong to a class of materials called "transparent conductive oxides" (frequently abbreviated TCO) and they have become useful tools for developing third generation solar cells. True, they are actually thin films deposited on regular glass--but what an interesting component in advanced photovoltaics.

As I have described here and there, new sensitized solar cells use materials that are sandwiched between two conductive plates. But if both of the plates were metal, very little sunlight would be able to penetrate to the light absorbing sensitizer material, right? Hence, the need for a window that permits light transmission and electrical conductivity.

In fact, TCOs are not new to industry. ITO (Indium Tin Oxide, 80-90% indium oxide with a minor amount of tin oxide) has been very popular for LCD flat panel displays. They generally have a slightly yellowed appearance and have also been used as infrared-reflective coatings on windows. Hmm, take a moment to reflect upon Indium. Where does it come from? Do we have a lot of it on Earth? More on that next post...moving on.

Fluorine-doped tin oxide (SnO₂:F) is becoming more popular in research-based sensitized photovoltaics. Although the conductivity performance can be slightly lower than ITO, SnO₂:F is generally less expensive in materials cost and manufacturing, and avoids problems with indium diffusion into the n-type TiO₂ or ZnO nanostructured film following annealing treatments.

A third substitute that may develop with time is aluminum doped ZnO (ZnO:Al). Again, the performance is lower, but the materials costs are also much lower. It should be noted that the "doping" levels in these oxides can be quite high, up to 10% of the material's mass, so we're really talking minor elemental contributions rather than trace additions to shift the behavior of the original metal oxide.

Side note: After finishing this blog, I came across yet another interesting variety; by combining zinc oxide and tin oxide you can make Zinc Tin Oxide (ZTO). Hmmm...

Performance of a conductor can be described in terms of resistivity (rho, ρ : the inverse of charge conductivity of a material in units of Ω•cm), an inherent property of the material that describes its electrical resistance. Resistance of a material can be described as

R= ρ x L/A (in units of Ohms, Ω).
But these are thin films of materials; instead of being described in terms of bulk resisivity, they are characterized in terms of a sheet resistance (R_s, in units of Ω per arbitrary square area). In this case,

R = R_s x square area, and
R_s = ρ/thickness of the film.
Most test films in laboratories use a sheet resistance less than 10 Ω/sq. The lower the better, and in general the sheet resistance is decreased with thinner films (as the equation demonstrates). Once you can make a TCO though, there is usually a critical threshold to reducing the film thickness (and hence the sheet resistance) that is dependent on the method of deposition (spray pyrolysis, chemical vapor deposition, sputtering, etc.) and the quality of the material deposited. For a give method, there are problems in film thickness uniformity, material crystallinity, and complete coverage of the substrate surface below the respective critical threshold.

Given that TCOs are often the "starting point" for assembling newly developing solar cells, I think we should pay attention to their development in the future. In my next post, I will address how perspectives from a geological and environmental perspective can shift our proposed "optimal choice" of materials development in TCOs. Within the context of an environmentally aware materials research project, our options are shaped not only by materials performance, but also by ore availability, costs of processing and refining (in terms of expended energy and CO₂ emissions), and materials toxicity and chemical fate during recycling or disposal.

Thinking Digital

2006-05-06T13:08:00.000+02:00

We live in a digital age...but what does that mean? I don't believe people are aware of the full extent that the coined term implies. And hey, that's okay--there's enough stuff out there to worry about in practical daily life before asking a pointed What is Fire? type of question. People are happy to use their tiny mobile phones and without even needing to ponder how central those magical boxes are to so many varieties of energy transfer. Energy transferrals which all achieve a common goal of information exchange. We can propose that "digital" implies microchips processing electrical pulses of a set voltage, antennae passing along microwave bursts, and fiber optics passing along pulses of light. All three passing along discrete "bits" of information just like Morse code. But what happens if we expand our sphere of thought, and change our goal from information exchange to something else? Hmm...first let's settle on what it means to be thinking digital.

From classic studies of information transfer and exhange two terms emerged: analog and digital.^[1] In an analog system, you vary the intensity of the signal along with its position in space/time continuously. Imagine that you have an amplifier and speaker system hooked up to a record player (phonograph) and a compact disc player. Imagine the zig-zag grooves in a vinyl record (I'm imagining Zappa in particular). With the aid of a needle on a phonograph and the right speed of rotation (in the right direction too--don't want any subversive messages from those albums just yet), the signal translates as an analog stream of information that you hear directly as sound. But in a (binary) digital system, you form the signal into discrete ON/OFF elements and place them at discrete periodic places in space/time. Now imagine the dots and dashes ("pits") etched into a compact disc. With the aid of a laser and a microchip (converting the bit coding given a known rate of data sampling), the signal translates as audio pulses of information that you recognize as sound.

But why don't we hear the pulses? Because, if you squeeze the pulses together close enough--the two signals are equivalent. "Peaches en Regalia" will have the same information content for both players (admittedly, without the romance of vinyl white noise). Your ears hearing the pulses will tolerate the steps in exactly the same way that your eyes tolerate the watching rapidly changing frames in the movie theatre as continuous motion. In other words, by reducing the spacing of discrete pulses beyond a certain threshold, the aural neurons in your ear cannot tell the difference between a digital source and a continuous analog source.

But almost more important than that good quality sound or picture is the fact that the dots on a compact disc are not truly discrete! Whoa, take a minute to breathe as I just changed the perspective. Are the raised areas ("pits") on a compact disc really square, with sharp changes to ON and OFF? Well, no--but the laser reading the pits will tolerate the fuzzy edges of the steps as an acceptable level of "noise". By making the intensity of the CD pulses above a certain threshold (deep pits), and by keeping the size and spacing of dots larger than a second threshold (broad, well spaced pits) to separate the fuzzy edges, the photodiode in your compact disc player also cannot tell the difference between a continuous analog and a true digital source either. This process even has a name and a formula, the Nyquist-Shannon sampling theorem: for a band-limited signal the sampling frequency must be greater than twice the input signal bandwidth in order to reconstruct the original perfectly from the sampled version. In other words, we can make perfect systems out of imperfect parts as long as we scale down to a certain threshold.

And that brings us to considering another form of "digital". Digital materials assembly; used in the sense that discrete units of a compound material (primary clusters of atoms as “bits”, to extend the analogy) are self-assembled to form a new product having unique collective attributes distinct from those of the original “bit”. Something like this is being investigated on macroscopic scales at SQUID labs, and it would be really groovy if one could assemble nanoscale digital materials. The reason being, you can pack a whole lot of signal into an itty-bitty volume. The main contribution to the signal in many nanoscale materials is at the surface, and when you shrink something down to the size of molecules (<5nm), the material is almost all surface.

But wait a second, nanoscale digital materials? That rings a bell--what about DNA? Yes, DNA is an old-school nanoscale digital material (like prehistoric...like Precambrian) using four bits (the molecules Adenine and Thymine, Guanine and Cytosine) that link together in two types of pairings (A with T and G with C) in a double chain of bit-pairs with periodic spacings. Okay, so we have a "proof of concept" for nanoscale digital materials that has been road-tested for around 3.5 billion years. I think we can step up to the plate and work around this concept in research now.

This change in perspective from "analog" to a “bit-wise” assembly of materials allows the researcher to apply concepts from the Digital Revolution to materials design. From this perspective, the researcher is aware that all of the nanomaterial parts being used to assemble a new structure are imperfect and “analog” in form (just like the pits in a compact disc, the primary clusters have unfulfilled surface bonds, and distorted structures). But the nanomaterial parts are also very much digital, and the goal would be for those particles to self-assemble into a new material with unique properties different from the individual unit.

As it turns out, we have another tangible example of that as well. These materials are called photonic crystals, and the most prevalent example is the naturally occurring gemstone: opal (although researchers are actively pursuing other synthetic materials and routes of assembly). Opal is a naturally occurring colloidal crystal composed of SiO₂ (silica) spheres (hundreds of nm in diameter) packed together in an ordered 3-D array, and surrounded by molecular water. By itself, silica is optically transparent and is the basis for ordinary window glass. But packed together in hydrated spheres, the path of light is affected and distorted to reveal an unusual pastel rainbow play-of-colors. Even a thin film of this nanostructured material can be used to change the way that light propagates--and it's nothing like glass.

So let's return to expanding our sphere of thought about digital applications. What other goals can we achieve from nanoscale digital materials other than information transfer? Perhaps we can assemble a material of nanoscale proportions to efficiently extract electrons and holes from a light absorbing matrix and effectively shuttle them to ohmic contacts, as in a photovoltaic device? We could envision getting several-times the proverbial bang for an invested buck in these systems. Ideally, one can foresee increasing the surface-charge area, increasing the efficiency of charge separation, while also reducing the volume of material that we need to use to construct a solar cell. Maybe the same principles could be translated into building better energy storage devices (e.g. batteries and ultracapacitors) as well. It often strikes me that those would be some pretty cool advanced photovoltaics and battery technologies--kind of like switching from a coiled-filament light bulb in a flashligt to using an intensely bright laser in a pointer. To put things in perspective, would you have thought 50 years ago when the first coherent light systems were being made in research labs, that the results would lead to the pocket laser diodes today? What are the possibilities, and where could "thinking digital" take us in the next 50 years of solar cell and battery research?

Note: many of the links provided come from the HyperPhysics site at the Dept. of Physics and Astronomy at Georgia State University. HyperPhysics is a robust science education source for physical phenomena and their applications in our lives.

[1] Claude Shannon. A mathematical theory of communication. The Bell System Technical Journal, 27:370–423, 623–656, July, October 1948.

Wikipedia Solar Cell revisited

2006-04-23T13:18:00.000+02:00

After some deliberation and slow integration as a registered Wikipedian into the Solar Cell discussion page (at Talk:Solar Cell), I have finally submitted a major revision to the Wikipedia Solar Cell page. I began my user integration by making small edits, and adding commentary to the Talk:Solar Cell discussion regarding the development of the wiki page. Eventually I proposed a "spring cleaning" of the Wikipedia site with specific goals to remove redundant text, reduce commercial and personal plugs for specific technologies, and to add a more general perspective on the "science" of solar cells.

Then I waited--for about two weeks. I received a lot of encouragement to "go ahead" from the editing community on the Talk:Solar Cell page and to my Wikipedia user site. As in all things, one needs to establish a line of credibility before just announcing that their information is the correct scientific explanation. And finally, today, I pulled together all of my edits and reorganization plans and submitted the major revision, with an explanatory submission in the Talk:Solar Cell page just in case specific edits needed debate. In fact, I didn't actually alter the viable content in the wiki very much at all. I simply added a few extra comments in the Theory and Light Absorbing Materials subsections--the rest was just moving the chairs around the room and throwing out the broken ones.

What did I find from this experience? I had reduced the size of the bloated wiki from 49 kilobytes to: 40 kilobytes. Not such a grand change all in all. The suggested size for a Wikipedia article is below 32 kilobytes, but there is a lot of interesting scientific information specifically addressing this fascinating, lively, and changing technology. We'll see how well the changes are accepted (and modified upon) within the next few weeks. It doesn't take long for rapid changes in Wikipedia once the ball starts rolling. The experience also made me dig deeper into my understanding of photovoltaic processes and the calculations involved for energy conversion efficiency. So the process became a wonderful learning experience as well as fullfilling to present an interesting science topic in a good light.

Dawn of the Solar Era: A Wake-Up Call

2006-03-26T16:48:00.000+02:00

An interesting solar cell "pep-talk" article from Francis de Winter and Ronald Swenson in Solar Today. It presents a commentary on the connection between the geologists whose job it was (and still is) to estimate the reserve quantity for oil and natural gas reservoirs, how they opened up the reserve information to the public, and how this information dramatically influenced the solar drive of the 1970s (and the current drive today). There are some interesting historical personages presented (including Dr. Hubbert from Shell Oil) and a lobby for why solar power is a better choice than petroleum alternatives (e.g. biodiesel) or nuclear energy.

The Global Hubbert Peak (formerly just the "Hubbert Peak" because it was derived to address American-accessible reserves) is a theory predicting the date at which our world-wide petroleum demands will exceed the supply (American = 1971; World = ~2000). Like Moore's Law, this is an estimated predictive model based on empirical evidence of resource reserves, technological advancement, and materials consumption. Similar to Moore's Law, the evidence appears to be following the model.

I leave the more in-depth searches for explanations of the Global Hubbert Peak to the reader. This is a phrase like "global warming", and the topic is more complicated than I want to address in this blog. I will however, stake my opinion as a former geologist that the principle of limited reserves of oil in the ground is very very real, and we will hit an impasse between supply and demand relatively soon (if not already). Ask yourself, why are the major oil companies also the largest investors in solar panel technology?

"P-N Junction Junction, what's your function?"

2006-03-25T12:00:00.000+01:00

Ok, if you remember Schoolhouse Rock you'll have a certain groovy tune in your head for the rest of the day. I think it's best to take a moment and lay down some basic descriptions and definitions of the parts of a p-n junction for those of us who want to know about currently available PV technologies. I mention these topics in passing in the other entries, and here's where we can establish a baseline.

There is a lot of solid, scientifically validated information out on the web dealing with classic semiconductor physics. Just doing a search on "p-n junction" yields pages of websites with information, educational tools, java applications, etc. I recommend the simple yet clear descriptions at the Georgia State science site: HyperPhysics; especially the section on Semiconductor Concepts. This is a silicon-dedicated site, but the physics apply to other materials as well.

And if you really want to get a deep abstract interpretation of these things, read more on the double layer of plasma physics. Note: I use the basis of plasma physics rather than the basis of electrochemistry in this respect, because the definition of the double layer model in electrochemistry assumes a fixed surface charge or surface charge layer. This model works well enough for an oxide particle in aqueous solution, but is limiting in trying to explain mirrored effects of charge attenuation for solid/solid semiconductor interactions.

Bulk semiconductor materials:
p-type: a semiconductor that contains impurities (adding atoms, missing atoms, replacing atoms) such that the major charge carriers are positively charged electron holes. Given that the majority carriers are so prevalent relative to the electrons, if you "add" more holes from an outside source, they will be able to diffuse freely through the p-type material with a very low probability of recombining with an electron (i.e. "hole transparent").

i-type: an "intrinsic" semiconductor. This material contains no impurities and the populations of electrons and electron holes are the same. Electron-hole pairs (termed excitons) recombine easily because they are oppositely charged and opposites attract.

n-type: a semiconductor that contains impurities (adding atoms, missing atoms, replacing atoms) such that the major charge carriers are negatively charged electrons. Given that the majority carriers are so prevalent relative to the holes, if you "add" more electrons from an outside source, they will be able to diffuse freely through the n-type material with a very low probability of recombining with a hole (i.e. "electron transparent").

Junction (read "interface"):
Placing two materials in atomically-close contact such that their physical properties at the interface are very different from that of the bulk material. Hence, the term junction is a materials science vehicle for talking about an electrochemical interface between solids.

Electrochemical Potential
Two things to keep in mind: each material is a reservoir for a certain dominant species of charge carriers, and the respective charge carriers are oppositely charged on either side of the junction. So you have two potentials that make the holes and electrons "want" to drift and diffuse into each other. First is an electrostatic potential ( "opposites attract", so there is electrostatic drift of charges). Second is a high concentration gradient at the interface that drives one to pour into the other like two waterfalls (the diffusion from high chemical potentials). In a p-n junction the dominant component is the electrostatic potential. This is not true for a vast majority of electrochemical interfaces (electrochemical ultracapacitors, plant and algal photosystems, neuronal charge transfer) where chemical potential gradients prevail.

Space Charge
An excess or deficiency of electrons/holes/ions that build up in one region of a material (such as at the interface or junction). The layer of this excess/deficiency found at the interface of a junction is called the space charge layer (or double layer). In a p-n junction, this is best described as a deficiency. When a p-type semiconductor is connected to an n-type semiconductor via a junction, an electrochemical driving force or potential causes the charges to move toward one another and recombine, making a neutral zone called the depletion region.

Depletion Region
Another way to talk about the space charge layer at the junction (if we can talk about holes with respect to electrons, we can talk about depletion regions with respect to space charge layers). Opposites attract, right? Right--and then they recombine and there are no charges. As an analogy, imagine if you were to roll hot coals and ice cubes into one another--in the end you would have neither hot coals nor ice cubes (in fact, you get low energy mud). The same thing occurs for electron-hole pairs. They recombine and form a layer of no charge, which inhibits an more drift of electrons and holes across the junction. In reality this conjugate pouring tapers off (attenuates) and each material is left with a deficiency of electrons or holes (a space charge layer) that would not have existed without the junction.

Electric Field
Where you have a junction with a p-type and an n-type material (in the dark), a space charge layer (the insulating depletion region) rapidly develops. In response to this insulating layer, an electrostatic potential is maintained across the junction (0.6-0.7 V in silicon). This works just like a dam to hold back the waterfall event. When light of the appropriate energy level hits the p-n junction and is absorbed, electron-hole pairs are generated. What happens to these pairs? Well, if they have nowhere to go (no potential to separate them) they just recombine. But because the electric field is present, the charges feel a pull to pour over the "dam" and flow down the circuit.

The electric field from this potential is a cornerstone of First Generation Photovoltaics. In that particular technology, if you don't have a "field" you cannot harvest the photovoltaic effect, because there is no driving force to separate photogenerated charges. But remember that the full scope of driving forces are described as the electrochemical potential, not just the electrostatic potential. Newer devices like Advanced Photovoltaics (and very efficient organisms called plants, algae, and photosynthetic bacteria) take advantage of chemical potentials as well.

So, do we have to define a solar cell as a p-n junction? NO, but they are really common in today's technology.

Dye Sensitized Solar Cells

2006-03-19T10:11:00.000+01:00

Dye sensitized solar cells (or DSSC) are one of the earliest varieties of non-p-n junction solar cells developed. This is also one of the earliest and most well publicized varieties of the advanced solar cell varieties. As a historical note, the concept for these cells was inspired by some of the most ancient photovoltaic devices on the planet: bacteria, algae and plants, via photosynthesis.

The current systems are based on complex surface interactions between a mesoporous titanium dioxide (TiO₂) thin film, a monolayer of light-absorbing organometallic dye, and a surrounding electrolyte material that completes the charge transfer system.

The dye absorbs incident photons and uses that energy to make electrochemical charge carriers (e.g.: electrons and holes). The TiO₂ and the electrolyte then selectively separate the electrochemical charge carriers, and these carriers diffuse off (largely due to chemical potential) to the positive and negative electrical contacts.

One of the most important contributions of DSSC to the field of photovoltaics was to raise a simple question. Do you need a p-n junction, and the electrostatic field that forms as a result of a p-n junction, to separate charge carriers? As it turns out: no, you don't.

Ok, if you don't need an electrostatic field to separate photogenerated charge carriers, then how does the device work? The results of the dye sensitized cell raised this new question. The data gathered from DSSCs has opened up many fundamental questions as to the general science of a photovoltaic device, and a lot of very interesting research is being pursued in this vein now. I will try to present more on these recent findings in the future, as there doesn't seem to be any one source of information to tie the results together. I believe that these results will also be vital in addressing the important parameters for ETA solar cells, organic polymer solar cells, and third generation photovoltaics in general. We shall see...

Can we talk general photovoltaics?

2006-03-08T22:48:00.000+01:00

Traditional Si wafer photovotaic technology (remember, this specific solar cell variety is from the First Generation of solar cells) has been around long enough for many people to accept the specific case of the p-n junction as canon. In response to a sense that this photovoltaic technology is fixed, there has been a cultural softening of the more general and fundamental principles of photovoltaics.

This is most evident for the open access, public sites that publish photovoltaic information. Specifically, I'm writing about Wikipedia. I've frequently made edits in the Wikipedia entry for solar cells and photovoltaic cells (such that the definition of PV does not equate to a specific case of the p-n junction) only to have the edits quickly shifted back to very rudimentary and often very commercial definitions of solar cells. The fact that my edits were altered is not surprising, this is an open access encyclopedia. What is disappointing is the distinct re-editing to serve commercial goals rather than scientific information. The analogy is to state that a "book" is a bound story contained in text printed on paper, despite the fact that books can be made now in audio and digital formats and despite the fact that text printed on paper (analogous to a p-n juction) can be generalized to many other applications that do not fall under the description of a "book".

Almost worse than the limited view of photovoltaic technology is the mess of text and poor graphics that follows the introduction of the solar cell link. To coin a word used in computer programming, the entry is a kludge (meaning it is a rough workaround rather than a well designed product), and needs many more expert eyes scanning and refinishing it.

Where are the modern photovoltaic scientists, from whom I have read many many articles? Given the need for real CO₂-free energy sources, I urge the move to step out from closed journal sources and present peer-reviewed science in a fashion that the public can understand! Currently, the state of science with respect to photovoltaics is a disgrace at wikipedia. Materials scientists are dropping the proverbial ball for future developments in photovoltaics by not exploiting the new web resources such as Wikipedia. You want to see a clean, mean site devoted to new technology? Look up fuel cells on Wikipedia. It is no wonder that our man on the street believes solar cell technology to be no different now than from the 1970s, yet believes that fuel cell technology will be available to the public within the next few years. We already have had solar cells for thirty years! Solar cells (yes, silicon wafer technology) can pay for themselves well within their life-cycles, but the general public is not of this opinion. Why? Poor public relations.

Now is the time for open access education to benefit solar cell technology. One of the first places we can start is to aggressively moderate Wikipedia to represent the science of photovoltaics, rather than what the old text books regard as canon. We can do better.

(and with that, I'm going back to give the solar cell edit another try...)

Variations in ETA-Cell Models: Photocapacitors*

2006-03-04T17:51:00.000+01:00

Currently, ETA stands for Extremely Thin Absorber, and an ETA solar cell is an advanced photovoltaic design where a porous or structured material (2-10 micrometers in thickness) is coated with layer of a light-absorbing inorganic semiconductor.

The ETA-cells in the literature have two distinct conformations of materials sandwiched between two conductive contact plates (ohmic contacts). One conformation is a three-component system: a ZnO nanowire matrix, coated by a metal chalcogenide (MC; typically metal sulfides, metal tellurides, or metal selenides) absorber and a wide band gap p-type material (CuSCN; copper thiocyanate)¹. The other is a binary matrix of mesoporous TiO₂ (anatase) filled with a high purity small band gap p-type material (generates electron-hole pairs and is a majority hole carrier). Neither one of these systems truly fulfills the requirements of a p-n junction: the ZnO system is more of a non-traditional p-i-n heterojunction (where i stands for "intrinsic"); and the characteristic lengths in the TiO₂ system are too small for band bending, so there should be no space charge layer characteristic of a p-n junction. We are led to ask, how small is small?, or at what scale does a material change its properties? Perhaps a more appropriate model can be applied to both of these ETA systems using a single conceptual structure: a photo-induced capacitor or photocapacitor².

The traditional capacitor consists of a material sandwiched between two conducting plates. The capacitance (C) for two parallel plates is proportional to the surface area (A) and inversely proportional to the distance between the two plates (d), both related by the dielectric constant of the absorber:

C = εA/d.
A photocapacitor should behave in a more unique manner, in that the charges are selectively shuttled to the conductive contacts (by diffusion) inside their respective majority carrier material. In a sense, these materials function as selective conductors. For example, both ZnO and TiO₂ are n-type materials (negative charge carriers are dominant) due to oxygen vacancies, and selectively admit electrons into their structure. The electrons cannot occupy filled levels and must follow a diffusion path (rather than electrostatic drift) to a back contact or recombine at the surface. Similar processes are expected to occur for holes in p-type materials (positve charge carriers are dominant), reducing the probability of surface recombination with an oppositely charged carrier. Although conductive, each contact also has a different work function that may drive carrier migration. With this type of definition, we can take advantage of all of these properties in describing the two ETA solar cell designs.

What model could a nanowire ZnO/MC/CuSCN interface belong to? We can agree that the charge carriers are generated specifically by photons absorbed in the intermediate layer (the MC), as the band gap is too large in the other materials to absorb photons. For a simple case, the MC can be an intrinsic material, having an equal number of electrons as holes in the unperturbed state. Outer space charge layers may develop in each of the majority carrier materials—as would normally occur in a p-n junction—but this exchange would be effectively balanced in the intrinsic material as a zero net change. In this way, we have simulated an electrostatic separation of charge on a sub-micron scale in the classical capacitor sense (figure below). Given the classic plate model, the capacitance of this structure is increased by reducing the thickness of the intrinsic (“dielectric”) material to tens of nanometers. The surface area is higher too, also increasing the capacitance. The electrostatic capacitance is the initial driving potential for charge carrier separation in the MC absorber, and the selective shuttling into unfilled bands allows the majority carriers to diffuse into the structure.

Now what model could be appropriate for the mesoporous TiO₂/MC ETA solar cell? In this case, the two parallel conductive plates are the TCO and metal back contacts. The internal “dielectric” between the conductive plates is a uniformly heterogeneous mixture. This structure most closely resembles the structure of supercapacitors or ultracapacitors in charge storage materials. The enhancement factor comes from a dramatically enhanced surface area (A) in the materials (while the inter-plate distance, d, is relatively large). The materials can be more effectively conceived as a “solution of surface” in this case (or a filled sponge). Charge carriers are generated in the absorber/p-type material and are selectively separated by diffusion. The difference in work functions of the two contacts (Pt/Au = 6.35/5.1 eV, SnO2 ~ 4.3 eV) could be another driving force in charge separation to aid the directionality of charge carrier separation .

Finally how can quantum dots (QD) affect the mesoporous TiO₂ ETA solar cell? QDs have an unusual result of strong confinement, in that hot charge carriers (e.g. electrons generated well above the lowest conduction band) have been found to undergo impact ionization (a reverse Auger process) to produce multiple lower-energy carriers, rather than being lost to thermalization (as in unconfined photovoltaic systems).³ In this sense, QDs can act as islands of MEG (multiple exciton generation) at the interface of TiO₂ and the p-type material, increasing the charge carrier density in these ETA-cells. It also seems to suggest than a full “monolayer” coverage of QDs across the entire interface (a highly improbable condition) would not be as effective due to a loss in dimensionality and higher likelihood of charge carrier recombination. Researchers at NREL (Golden, CO) have recently found conclusive evidence of MEG in PbSe and PbS QDs.⁴ This suggests a very promising future for inorganic photovoltaics, even though much work needs to be performed on the raw materials before these systems can be optimized and used commercially. However, with the appropriate model to guide us, we will be able to refine our selection of materials and rapidly move ETA solar cell technology into the forefront of the new generation of advanced photovoltaics.

1) Lévy-Clément, C.; Tena-Zaera, R.; Ryan, M.; Katty, A.; Hodes, G. Advanced Materials 2005 17, 1512.
2) Miyasaka and Murakami submitted a similar term for a hybrid dye-sensitized cell and electrochemical capacitor. Appl. Phys. Lett. 2004 85, 3932. Just to be clear, this is not the same concept.
3) Nozick, A. J. Annu. Rev. Phys. Chem. 2001 52, 193.
4) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.;Milic, O. I.; Nozick, A. J.; Shabaev, A.; Efros, A. J. Nano Lett. 2005 5(5), 865.

Inorganic Photovoltaic Nanocomposites

2006-03-04T17:13:00.000+01:00

The demand for energy is only growing. High growth markets such as India and China will soon place enormous pressures on the US for petrochemical resource competition. I've worked several years as a materials scientist in nanoparticle synthesis, and due to my background in environmental chemistry and geology I am very familiar with the limits of our current energy resources. As unsustainable energy reserves are depleted, an environmentally-aware materials scientist must enter new frontiers developing composite systems on the nanoscale that address energy collection and storage. We need an rapid shift in materials development to renewable energy sources and energy storage systems that utilize the advances from nanomaterials research. This is also fulfilling a goal of environmental premediation, where we engineer new technologies that minimize our impact on the ecosystem. Impact minimization can include eliminating CO₂ emissions (such as in photovoltaic energy collection) or altering the materials used that may present a toxic impact in our air and water supplies. Materials that bring people closer to energy independence and cleaner energy usage will drive the future technology markets, and American universities should be at the front of these researches.

My primary research interest is in the synthesis, assembly, and characterization of inorganic composite photovoltaic systems based on mesoporous TiO₂. This is a type of "sensitized solar cell" (an advanced photovoltaic), where a sponge-like substrate is coated with a light-sensitive material and sandwiched between an electrolyte and two electrodes (ohmic contacts). The principles for sensitized solar cells are based on pioneering research done by Grätzel, O’Regan, and others with dye-sensitized solar cells.^1,2 The dyes have been Ruthenium-centered organometallics that bind to the TiO₂ surface. While dye-sensitized photovoltaics have begun to show some success in industry, an efficient inorganic composite that replaces the organic dye is also being pursued. This is in part simply scientific curiousity, but is also due to an apparent plateau in progress using organometallic dyes and sealing difficulties of the organic solvents/electrolytes at elevated temperatures.² In my post-doctoral research we are working on an inorganic composite system called the ETA solar cell; or Extremely Thin Absorber solar cell.

As displayed in the cartoon, an ETA-cell can be designed as two wide band gap materials (n-type and p-type) combined with a 30-50 nm thin film layer of light absorbing semiconductor or "sensitizer" (energy gap: E_g = 1.1-1.8 eV) sandwiched between, creating a nanostructured heterojunction. There is also a second general conformation of ETA-cell that uses the light absorbing material to serve as the sensitizer and the p-type layer. The two wide band gap materials provide an electrochemical potential difference to help the separation of photogenerated electron-hole pairs (excitons) from in the light absorbing material.

Separation is also aided by carrier-selective removal of electrons/holes into their respective transparent media. TiO₂ (anatase) is an ideal n-type wide band gap material (E_g = 3.2 eV), and is a well known component in dye-sensitized solar cells (effectively transparent to electrons). ZnO is also being studied as an n-type wide band gap material, and shows promise as a nanowire substrate.³ CuSCN (copper thiocyanate; E_g = 3.4 eV) is a p-type wide band gap material currently being studied with both n-type materials.^3,4 A p-type semiconductor is effectively transparent to holes, hence the positively charged "bubbles" (marking the absence of electrons) are free to diffuse though it. Sensitizing (light-absorbing) materials are deposited within the TiO₂ pores, and a layer of CuSCN closes the junction as a wide band gap p-type semiconductor. Contacts are made using conductive glass (SnO₂:F, 10 Ω/square) and Pt or Au to create a complete inorganic solar cell.

Within a 10 μm film of mesoporous titania, there is a substantially large increase in surface area-dependent photocurrent, due to surface areas >200 m²/g (as compared to ~10 m²/g for a float glass surface). This is crucial, as the functional portion of the light-absorbing material (dye or inorganic semiconductor) is the monolayer in direct contact with the titania surface. In the case of mesoporous cells this high surface area also increases the number of internal reflections of light within the device. This is a light-trapping effect that increases the path-length for photon absorbance, and with it the probability of charge carrier generation.

The photovoltaic systems I study have close relationships to electrochemical energy storage systems such as batteries and ultracapacitors. Hence, results in my own area of study extend into applications for other materials systems for energy storage. With respect to environmental studies, the nanoparticulate suspensions and electrodes that serve as components of assembly to this technology development can also be used to study environmental impacts of high surface area metal oxides and metal chalcogenides in the environment, or can be developed as high surface area environmental probes.

1) Oregan, B.; Gratzel, M. Nature 1991 353, 737.
2) Tributsch, H. Coord. Chem. Rev. 2004 248, 1511.
3) Lévy-Clément, C.; Tena-Zaera, R.; Ryan, M.; Katty, A.; Hodes, G. Adv. Mater. 2005 17, 1512.
4) Könenkamp, R.; Ernst, K.; Fischer, Ch.-H; Lux-Steiner, M. C.; Rost, C. Phys. Stat. Sol. (a) 2000 182, 151.

Why should an environmental scientist be performing materials development?

2006-03-04T14:02:00.000+01:00

Environmental science is fairly well devoted to sampling from natural open systems (rivers, lakes, mines, groundwaters, and air), characterizing the samples by various analytical methods, and applying the data towards aiding local, state and federal policy changes in environmental regulation. There may be a way to expand the influence of environmental science into practical applications and new technologies. Perhaps we can change the source of the pollutants by working directly with the materials scientists and chemists responsible for generating new technologies. I feel there is a niche for environmental scientists to merge with the materials sector and help guide the development of materials that may have a minimum impact on the environment. This builds upon my philosophy of environmental premediation, where the environmental scientist actively pursues materials research with the goal of establishing a link between environmental awareness and engineering new technologies.

This shift would be analogous to the changes that mining industry took in the USA and Canada during the past century. Mining engineers communicated directly with geoscientists to find out what steps could be taken (e.g. microbial bioremediation, clay linings for tailing ponds, and replanting trees and brush) to minimize harsh impacts of cyanide disposal, surface water acidification, and erosion run-off that contaminated drinking waters and destroyed local ecologies. Note: these steps were performed in order to avoid heavy fines when the mines closed down, so the people who influence policy change do have a critical complementary role. The industrial policy of "clean as you go" was developed to minimize costs, which had the added benefit of greatly reducing the impact of mining on North American locales. Mining is never a happy shiny green experience, but it is distinctly better than 100 years ago, because of the communication between environmentally aware scientists and savvy materials engineers.

From my own experience as a materials researcher in an environmental chemistry program, I am very conscious of the residues of past materials developments, and how they impact our natural water resources. An excellent example of these impacts can be seen in the successful synthesis of high dielectric, flame retardant materials such as PCBs (polychlorinated biphenyls) that contaminated our water sheds in the past generation, inducing problems with immune, endocrine, neurological and reproductive systems as well as being carcinogenic; only to be replaced by the successive generation of chemically similar PBDEs (polybrominated diphenyl ethers) in computer devices and flame retardant furniture. One of the impacts of this type of materials development, one without an equal awareness of materials fate, is huge economic loss from clean-up fees, or corporate and personal taxation for land maintenance. Materials development cannot be pursued without ecological awareness, and environmental science programs can set an excellent core value for green materials engineering. Developing a strong awareness of the fate of materials and a strategy for creating alternatives (direct lines of communication between the developers and those whose monitor the ramifications of those developments) are key elements to be taught to the future students of materials science.

Clean accessible water, easily accessed energy, and a multitude of energy storage systems will drive future markets. I project that green technologies, motivated by environmental premediation (the focus of designing technologies that minimize our footprint on the environment) will be the impetuses for the next wave of materials development.

Why should we invest our scientific knowledge in solar cells?

2006-03-04T13:55:00.000+01:00

Given America’s increasing need for efficient green energy generation and storage, we must maintain our innovative edge for materials development of photovoltaic devices using the power of the small, via nanostructured systems. I'm writing about a fresh view in environmental materials engineering: environmental premediation . An environmental materials area of research will complement the demand for CO₂-free energy systems and energy efficient technologies in our country. The advances in Third Generation photovoltaics (including inorganic-sensitized solar cells, and dye-sensitized solar cells) have provided us with new alternatives for reduced-cost solar energy conversion. And now photovoltaic cells using quantum dots as light absorbing materials are on the verge of major breakthroughs. In fact, recent results from NREL (US Dept. of Energy’s National Renewable Energy Laboratory) have demonstrated that solid-state photovoltaic composites based on light-absorbing quantum dots show promise to provide up to 65% energy conversion efficiency (3x that of current Si cell limits). The future is wide open for discovering energy solutions using the tools of nanotechnology in materials science.

1) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.;Milic, O. I.; Nozick, A. J.; Shabaev, A.; Efros, A. J. Nano Lett. 2005 5(5), 865.

What are the successive generations of photovoltaics (PV)?

2006-02-18T15:32:00.000+01:00

I have begun this blog as an interdisciplinary exploration of nanomaterials and nanomaterial interactions involved in the synthesis and assembly of Third Generation photovoltaics. So what are the Three Gs (aside from a favorite tequila maker)?

The following are my own interpretations of the three generations of solar cell technologies that have taken root and now have been co-evolving over the past several decades. No doubt, if you have an financial or intellectual investment in one of the three, you will have your own well prepared ideas of what your technology represents. I humbly apologize if I have horribly maligned the status of your research by presenting my interpretations, and invite all to read more about specific technologies at the DOE web site also on the sidebar links.

First Generation: This term refers to the classic p-n junction photovoltaic. Typically, this is made from Silicon (multicrystalline and single crystalline) doped with other elements to make them preferentially positive (p) or negative (n) with respect to electronic charge carriers. However, in the past these devices were made from other materials like Germanium as well.

Second Generation: Thin films of photon-absorbers and layered stacks of thin films. These families of devices are working toward the purest, most efficient capture of light and conversion to electricity. They are elegant and artful, and also delicate and difficult to scale up to industrial levels of production. If the First Gen cells can be viewed as analogous to Microsoft (in that they work, but not optimally), then these varieties are surely the Macintosh-version of solar cells. The materials used in these cells are often designer semiconductor films, and can combine multiple light absorbing materials in a "stack" of films, with each absorbing a slightly different range of light wavelengths than the one below it. While they have been made in the lab and successfully applied in space technologies, they have not gained public attention in the way that Si has. They may prove to be much more efficient in solar energy conversion, but they are much more expensive than a simple silicon wafer.

Third Generation: The wild west! An open territory characterized by monsters of photovoltaic science. This can include dye-sensitized cells, polymer-fullerene cells, ETA cells (for Extremely Thin Absorber, not the radical Basque resistance). Most of the time these are broadly referred to them as "non"-p-n junction photovoltaics, because the old models of p-n semiconductors required a mesoscopic electric field to induce charge carrier "drift", and these devices are a bit more intricate. Some parts of this research are avoided by traditional PV materials scientists and physicists because there is a lot to be learned about the new rules of photovoltaics here that is closer to chemistry.