Charge Collection in Bulk Heterojunction Organic Photovoltaic Devices: An Impedance Spectroscopy Study

Through thickness and applied bias variation, charge collection in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaic (OPV) devices was investigated with impedance spectroscopy. An equivalent circuit model incorporating chemical capacitance (Cμ), recombination resistance (R₂), and transport resistance (R₁) was used to analyze the results. Insufficient carrier extraction, exhibiting diffusion transport characteristics at high frequencies, was found in devices with a thick active layer. These devices also display a higher chemical capacitance, indicating greater carrier accumulation, and a lower recombination resistance, signaling increased bimolecular recombination. Increasing internal field with negative applied bias enhances carrier collection by reducing carrier accumulation and recombination. Moreover, we showed explicitly that charge collection can be quantified by (R₂/R₁)½, which is proportional to device fill factor. These results demonstrate that impedance spectroscopy is an effective tool for investigating charge collection in OPV devices.