Increasing short-circuit current in silicon heterojunction solar cells

Silicon heterojunction solar cells are emerging photovoltaic devices that have gained particular interest for their very high open-circuit voltages. Unfortunately, the amorphous silicon layers that both passivate the crystalline silicon surfaces and act as emitter and back-surface field introduce new design constraints that often reduce the short-circuit current and fill factor compared to diffused junction solar cells. Here, we investigate the roles of the front and back transparent conductive oxide films, as well as the front amorphous silicon stack, in current generation. Decreasing the doping density of the indium tin oxide (ITO) films at the front of the cells trades optical losses for electrical losses as parasitic, long-wavelength absorption is reduced but film resistance is increased. However, high currents can be obtained while retaining respectable fill factors with proper ITO and metallization combinations. The rear ITO doping may be tuned to promote long wavelength transparency while avoiding fill factor losses due to contact resistance. The p-type amorphous silicon film that forms the emitter is found to be electrically dead in the sense that no light absorbed in this layer contributes to the current. Reducing the layer thickness to improve current generation comes at a price, however, as fill factor also falls. 2 × 2 cm² screen-printed silicon heterojunction solar cells fabricated with these considerations in mind exhibit short-circuit densities above 38 mA/cm², open-circuit voltages over 725 mV, and efficiencies as high as 20.8%.