Investigation of the Photocurrent in Bulk Heterojunction Solar Cells

TL;DR

This study examines the photocurrent behavior in bulk heterojunction P3HT:PCBM solar cells using pulsed measurements, identifying key voltage points and factors influencing device efficiency.

Contribution

It introduces a pulsed measurement technique to analyze photocurrent and clarifies the voltage conditions affecting charge dissociation and extraction in these solar cells.

Findings

Optimal symmetry point identified at 0.52--0.64 V

Voltage-independent photocurrent offset observed

Insights into device efficiency limitations gained

Abstract

We investigated the photocurrent in poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) solar cells by applying a pulsed measurement technique. For annealed samples, a point of optimal symmetry (POS) with a corresponding voltage $V_\text{POS}$ of 0.52--0.64 V could be determined. Based on macroscopic simulations and results from capacitance--voltage measurements, we identify this voltage with flat band conditions in the bulk of the cell, but not the built-in voltage as proposed by [Ooi et al., J. Mater. Chem. 18 (2008) 1644]. We calculated the field dependent polaron pair dissociation after Onsager--Braun and the voltage dependent extraction of charge carriers after Sokel and Hughes with respect to this point of symmetry. Our analysis allows to explain the experimental photocurrent in both forward and reverse directions. Also, we observed a voltage--independent offset of the photocurrent. As this offset is crucial for the device performance, we investigated its dependence on cathode material and thermal treatment. From our considerations we gain new insight into the photocurrent`s voltage dependence and the limitations of device efficiency.