Origin of negative and excess capacitances in carrier selective contact-based silicon heterojunction solar cells: role of back contact in conductivity modulation

We have investigated the effect of back contact on the evolution of capacitance-voltage (C-V) characteristics in molybdenum oxide (MoO (x) ) based Ag/ITO/MoO (x) /n-Si/LiF (x) /Al carrier selective contact (CSC) solar cells. From MoO (x) cell without i-a-Si:H back surface passivation layer, the negative capacitance (NC) is observed from low to high (100 Hz-500 kHz) frequency in forward bias C-V characteristics, whereas, with the i-a-Si:H layer, the positive excess capacitance is observed from 100 Hz to 50 kHz. The NC from a device is explained based on the bulk conductivity modulation in the c-Si wafer and mismatch of capture/emission dynamics of injected charge carriers at the back contact. With an additional back i-a-Si:H layer: (a) injection/extraction of majority/minority carriers is restricted, and the conductivity modulation is decreased, and (b) slowing down the movement of charge carriers increases the probability of carrier capture in the defect centers, which is reflected with the excess capacitance from a device. The carrier dynamics of the CSC cells were also analyzed under the light and the bias voltage to understand the C-V characteristics.