Determining limitations of capacitance-voltage measurements of built-in voltage as an alternative to surface photovoltage for a-Si:H/c-Si heterojunctions
Strong band bending within crystalline silicon is important for improving carrier selectivity in the contact regions of silicon heterojunction solar cells. In this work, we compare the use of capacitance-voltage as an alternative to the surface photovoltage method for probing the built-in voltage associated with crystalline silicon band bending. The surface photovoltage technique is incapable of measuring structures with non-transparent contacts, while the capacitance-voltage technique shows saturation at high built-in voltages due to the formation of an inversion layer. The shift in built-in voltage and effect on open circuit voltage are demonstrated for variation across a range of amorphous Si p-doping levels. These experimental results are supported by simulation work, confirming the underlying band structure and carrier concentrations of photovoltaic Si heterojunctions and clarifying the limitations of capacitance measurements at high built-in voltages. Errors arising in capacitance-voltage measurements of these structures are explained and linked to deviations from the Mott-Schottky model approximations of a one-sided junction in full depletion.