Charge separation and fullerene triplet formation in blend films of polyfluorene polymers with [6,6]-phenyl C61 butyric acid methyl ester

Abstract

Transient and steady state optical spectroscopies were used to study thin films made from a series of polyfluorene polymers blended with [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) in order to determine the influence of polymer ionisation potential on photoinduced charge separation. We find that the energy of the charge separated state ΔE_CS, given by the energy difference between the ionisation potential of the polymer and the electron affinity of the fullerene, must be smaller than a threshold value of about 1.6 eV for charge separation to occur. When ΔE_CS is greater than this threshold, PCBM triplet formation is observed in preference to charge pair generation. If ΔE_CS is similar to the threshold value, both PCBM triplet formation and charge separation occur in the blend film, with a tendency for charge separation to dominate over PCBM triplet formation as PCBM concentration increases. The mechanism of triplet formation is believed to be energy transfer to the PCBM singlet state followed by intersystem crossing. The threshold value of ΔE_CS is found to be similar to the PCBM singlet energy.