Ultrafast relaxation dynamics in a polymer: fullerene blend for organic photovoltaics probed by two-dimensional electronic spectroscopy

Ultrafast charge transfer from a photoexcited donor to an acceptor moiety is at the heart of the energy conversion in organic photovoltaics (OPVs). Efficient charge transfer on ultrafast, sub-100-fs timescales has been reported in many OPV materials. Yet at present, the elementary mechanisms underlying this process in OPV materials, in particular the role of coupled electronic and nuclear motion for the transfer dynamics and yield, are still unclear. Here, we use ultrafast two-dimensional electronic spectroscopy (2DES) to investigate vibronic couplings in the initial, light-induced charge separation dynamics in a blend of poly-3-hexyl-thiophene (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), a prototypical OPV system. At early times, we observe a distinct breakup of the unstructured linear spectrum into a series of well-resolved vibronic resonances. A comparison to 2DES spectra of pure P3HT suggests that these resonances arise from the vibronic coupling between donor states of the polymer and charge-separated states involving the PCBM acceptors. We identify new, short-lived diagonal peaks, decaying substantially within only about 20–30 fs and lacking a well-resolved cross-peak structure. We argue that these unexpected dynamics likely arise from strong anharmonic couplings to several vibrational modes. One possibility to explain the rapid decay of the blend peaks would be passing of the photoexcited wavepacket through a conical intersection. Our results suggest that nonadiabatic dynamics on multidimensional potential energy surfaces (PESs) might be highly relevant for the initial steps of light-induced charge separation in organic materials. Since theoretical investigations of vibronically-assisted dynamics in such complex organic systems are just emerging, we hope that our results will stimulate further experimental and theoretical work on the role of such dynamics in artificial energy conversion materials. To this end, coherent multidimensional spectroscopy might be a key experimental tool.