Role of band alignment at the transparent front contact/emitter interface in the performance of wide bandgap thin film solar cells

Recent numerical modeling and experimental work have shown that appropriate conduction band alignment at the emitter/absorber interface is critically important for high efficiency CdTe solar cells. To date, however, the properties of the transparent conducting oxide (TCO)/emitter interface have not been thoroughly investigated. Here, we use numerical modeling to determine the role of the conduction band alignment at the TCO/emitter interface. Our findings are increasingly important to device performance as efforts are made to widen the bandgap of the absorber. We also investigate the relative importance of the barrier caused by poor front contact versus the barrier at the back contact. It is well known that a barrier at the back contact can reduce the open circuit voltage of the device and produce rollover in the current density-voltage but is not well appreciated that a barrier at the front of the device can play a completely analogous role. We also show that for optimum device performance at any absorber bandgap, the conduction band of the TCO must be no more than 0.3 eV below the conduction band of the emitter.