Tunable Exciton Dissociation and Luminescence Quantum Yield at a Wide Band Gap Nanocrystal/Quasi-Ordered Regioregular Polythiophene interface

Abstract

A comprehensive understanding of the effect of polymer chain aggregation-induced molecular ordering and the resulting formation of lower excited energy structures in a conjugated polymer on exciton dissociation and recombination at the interface with a wide-bandgap semiconductor is provided through correlation between structural arrangement of the polymer chains and the consequent electrical and optoelectronic properties. A vertical diode-type photovoltaic test probe is combined with a field effect current modulating device and various spectroscopic techniques to isolate the interfacial properties from the bulk properties. Enhanced energy migration in the quasi-ordered (poly(3-hexylthiophene)) (P3HT) film, processed through vibration-induced aggregation of polymer chains in solution state, is attributed to the presence of the aggregation induced interchain species in which excitons are allowed to migrate through low barrier energy sites, enabling efficient iso-energetic charge transfer followed by the downhill energy transfer. We discovered that formation of nonemissive excitons that reduces the photoluminescence quantum yield in the P3HT film deactivates exciton dissociation at the donor (P3HT) close to the acceptor (ZnO) as well as in the P3HT far away from the ZnO. In other words, exciton deactivation in its film state arising from the quasi-ordered structural arrangement of polymer chains in solution is retained at the donor/acceptor interface as well as in the bulk P3HT. Effect of change in the highest occupied molecular orbital level and the resulting energy band bending at the P3HT/ZnO interface on exciton dissociation is also discussed in relation to the presence of vibration-induced aggregates in the P3HT film.