Analysis of the ac response of an organic bulk-heterojunction solar cell based on AnE-PVstat:PCBM

• The cole–cole impedance plots show clearly the presence of two semicircular arcs pointing to the origin.
• We modeled the impedance behavior using an equivalent circuit model incorporating chemical capacitance (Cμ), recombination resistance (Rrec), transport resistance (Rt) and contact electrical resistance (Rco).
• As an originality, we have taken Rco as negative values in the bipolar regime to reproduce theoretically the small arc shown in the cole–cole plots.
• We found that the recombination time τrec decreases with increasing bias voltages. This result is in agreement with the direct Langevin-type bimolecular recombination.
• We also determined the diffusion time (τdif) and the diffusion length (Ln) and other parameters. Average mobility of global carriers for the device is around 4 10−3 cm2 V−1 s−1 which is in good agreement with that derived using PCBM electron-only devices.

We report an analysis of the ac response of a bulk-heterojunction solar cell in a standard architecture made of an anthracene containing poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene)/1-(3 methoxy carbonyl) propyl-1-phenyl[6,6] and C60 (AnE-PVstat:PCBM) blend, these were made in detail at room temperature. Impedance spectroscopy is showing “backwards” arcs in Cole–Cole figures at low frequencies that are interpreted through equivalent circuit using negative resistances for electrical contacts. This equivalent circuit model incorporated chemical capacitance (Cμ), recombination resistance (Rrec), transport resistance (Rt) and contact electrical resistance (Rco). We have taken Rco as negative values in the bipolar regime to theoretically reproduce the small arc shown in the Nyquist plots. In addition, negative capacitance (NC) was observed under positive dc biases in the bipolar regime. The recombination time τrecτrecdecreases with increasing bias voltages. This result is in agreement with the direct Langevin-type bimolecular recombination. We determined the diffusion time (τdifτdif) and the diffusion length (Ln) and other parameters. Average mobility of global carriers for the device is around 4 10−3 cm2 V−1 s−1 which is in good agreement with that derived using PCBM electron-only devices.