Extraction of Front- and Rear-Interface Recombination in Silicon Double-Heterojunction Solar Cells by Reverse Bias Transients

We present a method, based upon reverse-recovery (RR) transient measurements, for determining the interface recombination parameters of double-sided heterojunction solar cells. A physics-based model is developed, and normalized parameters are used to provide results that can be scaled to arbitrary wafer thickness and minority-carrier diffusion coefficient. In the case of dominant recombination at only one interface, interface recombination velocity can be extracted directly from RR times. In devices with significant recombination at both interfaces, numerical modeling must be used. The effects of minority-carrier current spreading in small devices can be corrected for analytically. The results are then applied to both PEDOT/n-Si and PEDOT/n-Si/TiO2 heterojunction cells. We find that the PEDOT/n-Si interface, despite favorable band offsets and a significant built-in voltage, is not an ideal hole injector because of recombination at the PEDOT/n-Si interface. We also find that the effective surface recombination velocity at the Si-TiO2 interface in a metallized device is 330 cm/s, confirming that the interface has a low defect density. Finally, we reflect on the significance of these results for the further development of silicon heterojunction cells.