Photogeneration and Ultrafast Dynamics of Excitons and Charges in P3HT/PCBM Blends

Abstract

The photogeneration quantum yield and dynamics of charge carriers and excitons in thin films of neat regioregular poly(3-hexylthiophene) (P3HT) and blends with [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) were studied with ultrafast optical pump−probe spectroscopy. In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to 0.15 per absorbed photon. The remaining fraction of absorbed photons leads to formation of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial value on a time scale of 100 ps followed by decay to a no longer observable yield after 1 ns. Addition of 50% PCBM by weight leads to ultrafast (<200 fs) formation of charge pairs with a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an order of magnitude faster than in neat P3HT, which is expected to be at least in part due to interfacial exciton dissociation into charge carriers. The yield of charges in the blend has decayed to about half its initial value after 100 ps, while no further decay is observed within 1 ns. The small fraction (∼1%) of excitons in neat P3HT that is probed by photoluminescence measurements has a lifetime of 660 ps, which significantly exceeds the 200 ps lifetime of nonfluorescent excitons that are probed by transient absorption measurements. The nonfluorescent excitons have a diffusion coefficient of about 2 × 10 −4 cm 2 /s, which is an order of magnitude smaller than reported values for fluorescent excitons. The interaction radius for second-order decay of photoexcitations is as large as 8−17 nm, in agreement with an earlier result in the literature.