Optical optimization of high resistance transparent layers in thin film cadmium telluride solar cells

Thin film photovoltaic devices are multilayer opto-electrical structures in which light interference occurs. Light reflection at the interfaces and absorption within the window layers reduces transmission and, ultimately, the conversion efficiency of photovoltaic devices. Optical reflection losses can be reduced by adjusting the layer thicknesses to achieve destructive interference within the structure of the cell. The light transmission to the CdTe absorber of a CdS/CdTe cell on a fluorine doped tin oxide transparent conductor has been modeled using the transfer matrix method. The interference effect in the CdS layer and high resistance transparent buffer layers (SnO2 and ZnO) has been investigated. The modeling shows that due to relatively high absorption within the SnO2 layer, there are modest benefits to engineering anti-reflection interference in the stack. However, a ZnO buffer layer has limited absorption and interference can be exploited to provide useful anti-reflection effects. Optical modeling and optimization shows that for a 50 nm CdS layer, a maximum transmission of 78.5% is possible using ZnO as a buffer layer at 58 nm thickness, and 78.0% for a SnO2 buffer layer at a thickness of 48 nm.