Measurement of contact surface photo-voltage from forward bias C-V characteristics of P3HT:PCBM based BHJ solar cells

- Surface photovoltage (SPV) and associated carrier accumulation at the contact are extracted from C-V characteristics.
- Two architectures of P3HT:PCBM bulk heterojunction viz. conventional and inverted are compared.
- Light intensity dependence of SPV and carrier accumulation obtained.
- Inverted structure with higher efficiency shows less SPV indicating smaller contact losses.

The surface photovoltage (SPV) and carrier accumulation in open circuit conditions are sensitive indicators of the contact quality and band bending. However, these are not accessible directly in experiments. We analyze the capacitance-voltage (C-V) characteristics in P3HT:PCBM based bulk heterojunction to investigate SPV for both conventional (transparent anode) and inverted (transparent cathode) architectures. The C-V characteristics under dark conditions are analyzed using drift-diffusion model to extract built-in voltage, while under white light illumination it yields SPV. The SPV and the corresponding carrier density at the anode are quantified as a function of light intensity for conventional structure and compared with those of inverted structure to distinguish between the mechanisms in the two cases. The inverted structure with higher open circuit voltage is marked with less SPV compared to the conventional one. The technique of obtaining SPV as described here and the underlying mechanism can be used to suggest modifications in device architecture.