Department of Physics, Engineering Physics & Astronomy

Queen's University
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Department of Physics, Engineering Physics & Astronomy
Department of Physics, Engineering Physics & Astronomy

The Dependence of Charge Recombination Dynamics on the Solid-State Microstructure in Neat, Semicrystalline Polymeric Semiconductors

Carlos Silva
Université​ de Montréal

Friday, October 19, 2012
1:30 PM @ Stirling A

Abstract:

Optimal implementation of conjugated polymers in organic semiconductor devices such as solar cells requires an understanding of the relationship between solid-state microstructure and electronic properties. For example, charge generation and recombination dynamics are very sensitive to thin-film microstructure. Here, we investigate the dynamics charge recombination in the neat polymeric semiconductor regioregular poly(3-hexylthiophene) (P3HT) by means of time-resolved photoluminescence (PL) spectroscopy as a function of the solid-state microstructure, imposed by the molecular weight of the polymer and the thin-film processing route. At 10 K, we observe delayed PL that originates from recombination of long-lived geminate-polaron pairs. The delayed PL decays over microsecond timescales, which is orders of magnitude longer than the exciton lifetime. We find that > 36% of the total PL in solution-processed films of high-molecular-weight (331 kg/mol) material, which features a two-phase semicrystalline microstructure of alternating chain-entangled amorphous phases and crystalline lamella. In this microstructure, both the yield and the decay dynamics depend upon molecular weight. However, in films with dominant one-phase chain-extended microstructures, delayed PL is strongly suppressed. We thus find that a two-phase morphology leads to a higher yield of delayed PL with a higher distribution of decay rates than one-phase, chain-extended morphologies. By invoking a kinetic model including interconversion of excitons and geminate polarons, we conclude that geminate electron-hole pairs are on average further apart in two-phase morphologies, and that these microstructures result in a higher yield of geminate pairs that do not recombine within the timeframe of our experiment.