Time-Resolved EPR of Photoinduced Excited States in a Semiconducting Polymer/PCBM Blend

We studied by means of electron spin resonance (EPR) techniques the photoinduced processes in a poly[(9,9-bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]-thiadiazol-4,8-diyl) (F8BT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C-61 (PCBM) blend. Using light-induced EPR (LEPR), we observed the generation of charge-separated states in the blend upon visible light excitation, and we detected the formation of the PCBM excited triplet state by using time-resolved EPR (TREPR) with microsecond time resolution. From spectral simulation of the TREPR spectrum we were able to identify a double pathway for the PCBM triplet generation: the normal intersystem crossing from PCBM excited singlet state (either directly produced by light absorption or populated by singlet-singlet energy transfer from the polymer) and a generation pathway via polaron pairs recombination. The experimental evidence of this process allows to assign the relative order of the energies of the polaron pairs and the triplet states and provides a more complete description of the photophysical processes taking place in this photoactive blend