(536d) Sputter Deposition of Semi-Crystalline Tin Dioxide Films for CIGS Solar Cells

Tin dioxide (SnO₂) is emerging as an important material for use in copper indium gallium diselenide (CIGS) based solar cells. Amorphous SnO₂ may be used as a glass overlayer for the entire device and protecting it against water permeation. SnO₂ is also a viable semiconductor candidate to replace the wide band gap zinc oxide (ZnO) window layer to improve long-term device reliability. The film properties required by these two applications are different. Amorphous films have superior water permeation resistance while polycrystalline films generally have better charge carrier transport properties. Thus, it is important to understand how to tune the structure of SnO₂ films between amorphous and polycrystalline. Using X-ray diffraction and Hall-effect measurements, we have studied the structure and electrical properties of SnO₂ films deposited by radio frequency (RF) magnetron sputtering as a function of deposition temperature, sputtering power, feed gas composition and film thickness.  Films deposited at room temperature and thinner than 200 nm tend to be amorphous. Film crystallinity increases with film thickness and deposition temperature but is not affected significantly by sputtering power. Films with resistivities ranging between 20 mΩ cm to 800 mΩ cm are deposited. The films are n-type with carrier concentrations in the 3´10¹⁸ cm^-3 to 3´10²⁰ cm^-3 range. Carrier concentration decreases with oxygen concentration in the feed gas. Electron mobilities range from 1 to 10 cm²/V s and increase with increasing film thickness, oxygen addition to the feed gas and film crystallinity. Electron mobilities in the 1-3 cm²/V s range are obtained even in amorphous films. When we apply these differently deposited SnO₂ layers for the CIGS solar cell damp-heat performance studies. Approximately 0.2 micron and thicker amorphous tin dioxide layers deposited on top of the completed CIGS solar cells can significantly increase the device lifetime by forming a barrier against water diffusion. The SnO₂ overlayer protect the ~ 93 % of the initial solar cell efficiency even after 150 hours of damp-heat treatment (85 °C/85 % RH) when the solar cells without the SnO₂ layer lost nearly 80 % of their initial efficiency within 24 hours of commencing the test. The replacement of water sensitive and water permeable ZnO window layer of CIGS solar cells is provided with an amorphous or polymorphous (mixture of amorphous and polycrystalline material) tin dioxide layer. Using identical CIGS layer and fabrication, solar cells made with ZnO or SnO₂ window layers give similar overall power conversion efficiencies. We demonstrate an 8.3 % efficient CIGS solar cell with a SnO₂ window layer. Same solar cell fabrication process and CIGS film with ZnO window layer resulted in 8.7 % overall power conversion efficiency. The open circuit voltages of the two cells were the same indicating that the band alignment with the SnO₂ film is suitable for CIGS.