Photoluminescence of P3HT:PCBM bulk heterojunction thin films and effect of external electric field

Steady-state and time-resolved photoluminescence (PL) and electrophotoluminescence (E-PL) have been recorded for bulk heterojunction samples of thin films of regio-regular poly(3-hexylthiophene) (P3HT) and methyl [6,6]-phenyl-C61-butanoate (PCBM). The PL spectra of the bulk heterojunction samples showed a monotonic decrease of PL quantum yield with increased PCBM content in the film. The PL decay was measured with both a femtosecond up-conversion technique and a time-correlated single-photon-counting technique. The mechanism of PL quenching deviated from a simple Stern-Volmer equation, explained in terms of an apparent static quenching with a sphere-of-action model and dynamic quenching. With increased content of PCBM, the static quenching played a dominant role. Upon application of an electric field, the E-PL spectra of both P3HT and bulk heterojunction samples showed an enhanced quantum yield and incremented lifetime of PL of P3HT. The long-range electron transfer from P3HT to PCBM was retarded upon application of an electric field; the field-induced change increased with increasing PCBM content.