Exciton Regeneration at Polymeric Semiconductor Heterojunctions

TL;DR

This study demonstrates that at polymeric semiconductor heterojunctions, excitons can be regenerated after charge separation through a process involving geminate pairs and exciplex formation, enhancing understanding of exciton dynamics.

Contribution

It reveals a new mechanism of exciton regeneration at heterojunctions, involving geminate pairs and endothermic back-transfer, supported by E-field-dependent photoluminescence spectroscopy.

Findings

Excitons can be regenerated after charge separation at heterojunctions.

Charge transfer produces geminate electron-hole pairs that can collapse into exciplexes.

Back-transfer to excitons is endothermic with an energy barrier of 100-200 meV.

Abstract

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation, and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1nm) which may collapse into an exciplex and then endothermically (E=100-200meV) back-transfer towards the exciton.