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

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 88K (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 changing the wavelength from the visible to the NW. This is attributed to recombination of CT states to triplet level of P3HT. However, as the yields for P3HT:PCBM and P3HT:bisPCBM are comparable on visible excitation, we conclude that for the latter blend formation of mobile charge carrier occurs primarily via a thermally nonrelaxed, hot CT state. This observation indicates that the excess energy involved in the exciton dissociation process is indeed important to avoid recombination to the triplet level and to achieve higher yields of charge carrier generation. On the basis of these findings, we suggest that the excess energy can be small as long as the triplet level of the polymer is located energetically higher than the CT state. This insight is of particular interest for the rational design of novel polymer/fullerene systems to achieve higher power conversion efficiencies.