Photogeneration and recombination of charge carrier pairs and free charge carriers in polymer/fullerene bulk heterojunction films

Photo-generation and recombination of free charge carriers in poly-3 (hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blend films has been studied at different PCBM concentrations by means of fluorescence spectroscopy and transient photocurrent methods. We show that more than 80% of excitons form charge transfer (CT) states at PCBM concentrations above 4%. Efficiency of the CT state dissociation into free charge carries strongly depends on the PCBM concentration; the dissociation efficiency increases more than 30 times when PCBM concentration increases from 1 to 32%. We attribute the strong concentration dependence to formation of PCBM clusters facilitating electron migration and/or delocalization. Reduced charge carrier recombination coefficient has also been observed at high PCBM concentrations. We suggest that this may be partly caused by the reduced stability of reformed Coulombicaly bound charge pairs.