The interplay between space charge and recombination in conjugated polymer/molecule photocells

Abstract

We present an analytic description for the loss of photocurrent efficiency at moderate light intensities and demonstrate a simple technique for extracting the mobility of electrons in semiconducting polymer layers. The underlying theoretical analysis, which is based on a simple drift-recombination scheme, shows good agreement with the measured intensity-dependent photocurrent quantum efficiency over five orders of magnitude in intensity. The electron mobility extraction is demonstrated for pristine MEH-PPV. We use the combination of theoretical and experimental studies to discuss the role of recombination and space-charge effects in reducing photocurrent efficiency. We apply the analytical results to device design criteria and deduce that the minimum, low-field, mobility value of the slow carrier required to achieve close to ideal fill factor is $\sim {10}^{-2}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at AM1.5.