Energetics of Excited States in the Conjugated Polymer Poly(3-hexylthiophene)

TL;DR

This study investigates the energetics of excited states in the conjugated polymer P3HT, revealing key energy levels involved in charge generation and transport, which are crucial for optimizing organic opto-electronic devices.

Contribution

The paper introduces a novel approach to analyze excited states in conjugated polymers, providing direct measurements of energy levels relevant to photogeneration and charge transport.

Findings

Transport gap of 2.6 eV determined by photoemission spectroscopy.

Photogenerated singlet exciton requires 0.7 eV to overcome binding energy.

Intermediate charge transfer state is 0.3 eV above the singlet exciton.

Abstract

There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimisations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene) (P3HT), we demonstrate the general applicability of our novel approach. From photoemission spectroscopy of occupied and unoccupied states we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.