Mechanism of Mobile Charge Carrier Generation in Blends of Conjugated Polymers and Fullerenes: Significance of Charge Delocalization and Excess Free Energy

Abstract

Despite significant improvement in power conversion efficiencies of bulk-heterojunction solar cells, the mechanism of mobile charge carrier generation is still under debate. The time-resolved microwave conductivity technique is used to investigate the mobile charge carrier generation in blends of P3HT with monoPCBM and bisPCBM by varying the excitation wavelength from the visible to NIR and the temperature from 88 to 300 K. NIR excitation corresponds to the transition of an electron from the HOMO of the P3HT directly to the LUMO of the fullerene forming the charge transfer band (CT). From the results it is inferred that the binding energy between the electron and hole in the CT state is smaller than thermal energy at 88 K (7.8 meV) that is in large contrast to previously reported values of 0.3 eV. This is ascribed to efficient charge delocalization, which increases the mean distance between the electron and hole at the interface. For P3HT:bisPCBM, the yield of charge carries decreases by a factor of 3 on ...