Heliotactic Press

Interdisciplinary exploration of solar energy conversion, photovoltaics, and integrative design, and scientific philosophy.

Are you Sustainable? 2007/03/31

Filed under: American Chemical Society,environmentally aware materials science,indium,photovoltaics,solar cells,sustainability,third generation — nanomech @ 03:51

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.

 

Case Study: Indium and Tin 2006/06/17

Filed under: environmentally aware materials science,indium,ITO,tin — nanomech @ 06:38

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.

 

Transparent Conductive Oxides 2006/05/27

Filed under: indium,sheet resistance,tin,zinc — nanomech @ 14:25

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 (Rs, in units of Ω per arbitrary square area). In this case,

R = Rs x square area, and
Rs = 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.

 

 
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