Electronic Excitations and Insulator-Metal Transition in Poly(3-hexylthiophene) Organic Field-Effect Transistors

TL;DR

This paper combines theoretical modeling and experimental spectroscopy to investigate charge injection and the insulator-metal transition in P3HT organic transistors, identifying bipolarons as key localized states and approaching conditions for a first-order transition.

Contribution

It provides a comprehensive analysis of charge injection mechanisms and the nature of localized states in doped P3HT, proposing bipolarons as the dominant charge carriers and linking experimental data with transition scenarios.

Findings

Charge-induced localized states are bipolarons.

Highest doping levels approach a first-order transition.

Theoretical predictions align with infrared spectroscopy data.

Abstract

We carry out a comprehensive theoretical and experimental study of charge injection in Poly(3-hexylthiophene) (P3HT) to determine the most likely scenario for metal-insulator transition in this system. We calculate the optical absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. We also analyze the electronic excitations for three possible scenarios under which a first-- or a second--order metal--insulator transition can occur in doped P3HT. These theoretical scenarios are compared with data from infrared absorption spectroscopy on P3HT thin film field-effect transistors (FET). Our measurements and theoretical predictions suggest that charge-induced localized states in P3HT FETs are bipolarons and that the highest doping level achieved in our experiments approaches that required for a first-order metal--insulator transition.