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.
