Charge blocking layers in thin-film/amorphous photovoltaics

The open circuit voltage of solar cells is a critical property which, in many thin film devices, is found to be much lower than the theoretical limit associated with the material's absorption energy gap. A known method for improving the open circuit voltage is that of adding charge blocking layers (a.k.a. charge selective or charge transport layers). However, such interfacial layers are often found to decrease the fill factor or even cause undesired S-shape in J-V curves. In the current work, we revisit the blocking-layer method through detailed device simulations, explain how such layers enable significant Voc increase (even above the built-in voltage), and show that the layers' resistance is responsible for the problematic behavior observed. Having identified the source of reduced fill factor, we suggest methods of reducing the resistivity and recovering the fill factor.