Concentration-Dependent Hole Mobility and Recombination Coefficient in Bulk Heterojunctions Determined from Transient Absorption Spectroscopy

A simple analytical function, based on the multiple trapping model, is used to describe the bimolecular recombination of charge carriers in a bulk heterojunction (BHJ) film in the presence of an exponential energetic tail of localized hole “trap” states. The function is used to fit charge carrier decay data from an unannealed P3HT/PCBM film measured by transient absorption. The analysis assumes that only free holes participate in recombination and transport. This implies an effective recombination rate coefficient which varies with the ratio of free to trapped holes. The fit parameters yield a bimolecular recombination constant for free holes with free electrons ( k 0 = 3.4 × 10 −12 cm 3 s −1 ) and information about the distribution of trap states (trap distribution parameter β = 0.29). Assuming the Langevin recombination limit, the analysis yields a concentration-dependent effective hole mobility saturating at μ 0 ≈ 7 × 10 −2 cm 2 V −1 s −1 . This approach should be useful to compare BHJs in a consistent and meaningful manner.