Photophysics of Charge Generation in Organic Photovoltaic Materials: Kinetic Studies of Geminate and Free Polarons in a Model Donor/Acceptor System

Abstract

Photoinduced charge generation is studied in a model donor/acceptor blended system, consisting of bulk-heterojunction blends of the conjugated polymer regioregular poly(3-hexylthiophene) (P3HT) and the dye molecule phthalocyanine (PC). Excitation at 780 nm, into the PC NIR absorption band, creates excitons in PC domains without populating the P3HT excited state. The kinetics of geminate polaron pairs (GPP) and free polarons can then be followed without the interference from the signatures of directly created polarons, energy transfer, or significant exciton migration. The donor/acceptor film morphology has a significant impact on the ratio of free polarons and GPP that are formed: increased phase segregation leads to a pronounced increase on the fraction of free polarons formed directly. Unlike the free polaron population, GPPs are affected by fast recombination directly to the ground state, occurring with a time constant of 170 ps. Both GPP and free polarons are formed at the P3HT/PC interfaces. Free polarons are, however, able to migrate from these interfaces into the bulk, quasi-crystalline P3HT regions. Finally, the results obtained with our model system were compared with the photoinduced charge generation in P3HT/PCBM 1:1 blend films. Indication for free polaron formation yield following electron transfer of ∼100% was obtained; no GPP formation was observed. This is in agreement with previous studies of both the photophysics and OPV device characterization based on these blends.