The thermal annealing effects on the photocarrier dynamics in thin films of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) on quartz substrates are investigated. Both the photocarrier generation via charge-transfer state and the charge transport in PCBM films are studied by time-resolved photoconductance (PC) measurement. More than 90% of external photocarrier generation efficiency relative to the non-annealed film is reduced upon annealing of the PCBM film at high temperatures. Excitation light power dependence of PC indicates that the photocarrier in the non-annealed film is generated by a one-photon process, while photons of more than one are required in highly annealed films. The multiphotonic process for carrier generation and the substantial reduction of carrier density caused by the thermal treatment are associated with trap formation. The density of photoinjected carrier is also affected by applying external magnetic field. The observed positive magnetophotoconductance (MPC) in non-annealed film is understood in terms of the incoherent spin conversion in nongeminate electron–hole (e–h) pairs with a selective recombination from singlet e–h pairs. In highly annealed film, the broad magnetic field dependence of MPC with negative phase is suggested to originate from the detrapping of trapped carrier assisted by collision with the triplet exciton.
2013 The Chemical Society of Japan