Transparent Conducting Oxides
The collaboration between Ryan O’Hayre and David Ginley at the National Renewable Energy Laboratory (NREL) incorporates students into a wonderful workgroup at NREL introducing them to cutting edge material science in photovoltaic technology.
Transparent conducting oxides (TCO) are of special importance to solar cells; letting light into the solar cell to convert light into energy while acting as collectors for converted energy. TCOs can be made out of several materials, but they must be transparent like glass but also conductive like metals. The two figures below demonstrate the transparency of gallium doped zinc oxide (on the left with the NREL logo) and niobium doped titanium oxide (on the right).
TCOs are not exclusive to solar cells but are used in display technology, low emissivity windows, and electrochromic devices [1-4]. As seen in the next figure , TCOs are currently being developed for the use as a transparent thin film transistor in transparent displays.
Commercially, the TCO market is dominated by aluminum-doped ZnO and tin-doped indium oxide. Indium tin oxide (ITO) offers one of the best combinations of high optical transparency and high electrical conductivity. However, improved TCO materials will be needed to meet future technologies. The classic phase space of transparent conducting oxides includes zinc oxide (ZnO), indium oxide (In2O3), tin oxide (SnO2), cadmium oxide (CdO), and gallium oxide (Ga2O3). The potential exists for unique mixtures of these compounds. NREL has the ability to efficiently probe this vast phase space using combinatorial approaches, generating an array of compositions on a single sample by simultaneously sputtering from multiple targets. New TCO materials, such as amorphous indium zinc oxide (IZO), are being explored using this sputtering approach to deposition.
The collaboration between Ryan O’Hayre and David Ginley at NREL is also interested in exploring solution deposition methods for TCOs in an effort to reduce manufacturing costs. Liquid precursor deposition methods (including spin coating, spray deposition, and inkjet printing) are attractive because the equipment required is simple and inexpensive, the methods are scalable to large substrate sizes, and they are carried out under atmospheric conditions. With the appropriate inks, all the features of a solar cell (including the absorber, dopants, transparent conductor, and front and rear metal contacts) could be sprayed or directly printed. Collaborative research efforts are underway developing precursors and the methods for the solution deposition of TCOs.
The photovoltaic group at NREL encompasses many different solar cell technologies: amorphous silicon (a-Si), crystalline silicon (x-Si), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and countless organic based solar cells. Below is a picture of a CdTe solar cell broken down into the individual layers (on the left), and an actual installation of First Solar CdTe solar cells (on the right).
Industrial processing technology as well as countless characterization instruments support all of this research. The processing technology is used to directly implement new processes straight to industry, providing experience to researchers who wish to transfer their efforts to industry. At the same time all the characterization instruments help develop and tune new technologies. Some of the characterization instruments at NREL are: x-ray diffraction (XRD), scanning electron microscopy (SEM), tunneling electron microscopy (TEM), Auger spectroscopy, Raman spectroscopy, ellipsometry, secondary ion mass spectroscopy (SIMS). For other facilities at NREL and CSM, please check the facilities link.
NREL supports research in photovoltaics as well as other renewable fields such as biomass, geothermal, hydrogen and fuel cells, wind technology, basic sciences, and countless efforts to increase the efficiency of buildings and electric infrastructure systems.
- R. G. Gordon, “Criteria for Choosing Transparent Conductors,” MRS Bulletin, vol. 25, pp. 52-57, August 2000.
- E. Fortunato, D. S. Ginley, H. Hosono, and D. C. Paine, “Transparent Conducting Oxides for Photovoltaics,” MRS Bulletin, vol. 32, pp. 242-247, 2007.
- T. J. Coutts, D. L. Young, and X. Li, “Characterization of Transparent Conducting Oxides,” MRS Bulletin, vol. 25, pp. 58-65, August 2000.
- B. G. Lewis and D. C. Paine, “Applications and Processing of Transparent Conducting Oxides,” MRS Bulletin, vol. 25, pp. 22-27, August 2000.
- K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, vol. 432, pp. 488-492, 2004.