Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/Fullerene Blend Films

Bimolecular recombination, an important loss mechanism in organic solar cells, has been investigated using transient absorption spectroscopy for poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films and analogues of these components. Data are analyzed as a function of blend composition, postdeposition thermal annealing, and excitation density. Comparison of transient spectra for P3HT:PCBM films with analogous films employing P3HS and PC70BM allows the assignment of the photoinduced absorption features. These decay dynamics are analyzed on the nanosecond to millisecond time scales and are shown to be in excellent agreement with a bimolecular recombination model in the presence of an exponential distribution of localized (trap) states. Thermal annealing results in an acceleration of these decay dynamics, which is assigned to a reduction in the depth of the trap states and correlated with an increase in film crystallinity. The decay dynamics are analyzed to obtain an effective recombination coefficient that is charge density dependent at low polaron densities, but becomes independent of charge density at high charge carrier densities (>1018 cm−3). This transition is assigned to trap filling, with the recombination coefficient measured at high charge density (k = 3 × 10−12 cm3 s−1) corresponding to the trap-free limit. From transient spectroscopic behavior we estimate the density of intraband trap states in unannealed P3HT:PCBM blend films to be ∼7 × 1017 cm−3.