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 2006/12/10
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 2006/12/06
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.
Cap and Trade 2006/10/15
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 NOx and SOx. 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.
Solar Companies of Interest: Progress! 2006/07/16
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.
Wikipedia Solar Cell revisited 2006/04/23
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 2006/03/26
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?
Dye Sensitized Solar Cells 2006/03/19
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? 2006/03/08
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* 2006/03/04
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.
