Bimodal Polarons and Hole Transport in Poly(3-hexylthiophene):Fullerene Blend Films

Abstract

The bimolecular recombination dynamics in blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) has been studied by transient absorption spectroscopy. On a microsecond time scale, two polaron bands were observed at 700 and 1000 nm and exhibited different bimolecular recombination dynamics. The 700-nm band decayed with a time-independent bimolecular recombination rate of 10⁻¹² cm³ s⁻¹. The activation energy was as small as ∼0.078 eV independently of the carrier density. On the other hand, the 1000-nm band decayed with a time-dependent bimolecular recombination rate, which varied from 10⁻¹² to 10⁻¹³ cm³ s⁻¹, depending on time or carrier density. The activation energy decreased exponentially from 0.178 to 0.097 eV with the increase in the carrier density. Therefore, we assigned the 700-nm band to freely mobile delocalized polarons in crystalline P3HT domains and the 1000-nm band to localized polarons trapped in relatively disordered P3HT domains. At a charge density of 10¹⁷ cm⁻³, which corresponds to 1 sun open-circuit condition, some localized polarons exhibited trap-free bimolecular recombination due to trap-filling. These findings suggest that not only delocalized polarons but also some localized polarons play a crucial role in the efficient hole transport in P3HT:PCBM solar cells.