On the energetics of bound charge-transfer states in organic photovoltaics

A comprehensive understanding of the charge generation mechanism in organic solar cells is critical for further improvement of device performance. Currently, the origin and magnitude of the coulombic binding energy of the charge-transfer state (CTS), an intermediate state which is fundamental for the charge separation process, are still under debate. Here, we propose a new approach for determining the dissociation energy of localised CTSs for a range of devices with different alignments of molecular energy levels (tuned by chemical modifications of fullerene) and disorder (adjusted by the blend composition) using temperature-dependent pump-push photocurrent spectroscopy. We observe that the dissociation of localised CTSs from initial excitation is a temperature-dependent process, and we determined the binding energy of these CTSs by measuring a single activation energy over a wide temperature range. We propose a simple qualitative picture to explain the observation, based on the split between the bound CTSs and free charges. In all the material systems studied here, the activation energy falls within the range of 90 +/- 50 meV (corresponding to similar to 1 nm separation of an electron-hole pair). Surprisingly, the binding energy does not depend on the material composition or the driving energy (similar to 150 meV variation) for charge separation. In contrast, the number of formed bound states and their following recombination dynamics are material-and nanomorphology-sensitive. Such observations in the studied benchmark polymer: fullerene systems reveal unexpected similarities in the energetics of CTSs formed in different electronic environments. This makes our results of general importance for understanding the photophysics at the heterojunction interface and for further development of organic photovoltaics.