We developed a technique allowing the direct observation of photoinduced charge-transfer states (CTSs)-the weakly coupled electron-hole pairs preceding the completely separated charges in organic photovoltaic (OPV) blends. Quadrature detection of the electron spin echo (ESE) signal enables the observation of an out-of-phase ESE signal of CTS. The out-of-phase Electron-Electron Double Resonance (ELDOR) allows measuring electron-hole distance distributions within CTS and its temporal evolution in the microsecond range. The technique was applied to OPV bulk heterojunction blends of different donor polymers, including the benchmark polymer P3HT and the high-performance polymer PCDTBT, with the fullerene PC61BM acceptor. The corresponding electron-hole distance distributions were obtained using the Tikhonov regularization. It was found that not only the dipolar interaction but also the exchange interaction contributes to the formation of the out-of-phase ELDOR signal. By varying the delay time after photoexcitation, we observed CTSs at different stages of charge separation. The initial distribution of the electron-hole distances for different blends correlates with their photoelectric conversion efficiency, with shorter average thermalization distances found for the blends of PC61BM with the less efficient regiorandom polymer P3HT. Spin-selective recombination of the CTS was unambiguously demonstrated for the blend of regioregular P3HT with PC61BM. It produces characteristic features in the out-of-phase ELDOR trace for small "dipolar" evolution times. These data allow us to estimate the CTS recombination rate for a certain distance between the electron and the hole within the CTS. The proposed method can be used to probe CTS in a variety of OPV active layer materials.

Out-of-phase ELDOR spectroscopy: A precise tool for investigating structure and dynamics of charge-transfer states in organic photovoltaic blends