Highly-Efficient Charge Separation and Polaron Delocalization in Polymer-Fullerene Bulk-Heterojunctions: A Comparative Multi-Frequency EPR & DFT Study

摘要

The ongoing depletion of fossil fuels has led to an intensive search foradditional renewable energy sources. Solar-based technologies could providesufficient energy to satisfy the global economic demands in the near future.Photovoltaic (PV) cells are the most promising man-made devices for directsolar energy utilization. Understanding the charge separation and chargetransport in PV materials at a molecular level is crucial for improving theefficiency of the solar cells. Here, we use light-induced EPR spectroscopycombined with DFT calculations to study the electronic structure of chargeseparated states in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerenederivatives (C60-PCBM and C70-PCBM). Solar cells made with the same compositesas active layers show power conversion efficiencies of 3.3% (P3HT), 6.1%(PCDTBT), and 7.3% (PTB7), respectively. Under illumination of thesecomposites, two paramagnetic species are formed due to photo-induced electrontransfer between the conjugated polymer and the fullerene. They are thepositive, P+, and negative, P-, polarons on the polymer backbone and fullerenecage, respectively, and correspond to radical cations and radical anions. Usingthe high spectral resolution of high-frequency EPR (130 GHz), the EPR spectraof these species were resolved and principal components of the g-tensors wereassigned. Light-induced pulsed ENDOR spectroscopy allowed the determination of1H hyperfine coupling constants of photogenerated positive and negativepolarons. The experimental results obtained for the different polymer-fullerenecomposites have been compared with DFT calculations, revealing that in allthree systems the positive polaron is distributed over distances of 40-60 A onthe polymer chain. This corresponds to about 15 thiophene units for P3HT,approximately three units PCDTBT, and about three to four units for PTB7. Nospin density...