Polarons, Bipolarons, and Electrical Properties of Crystalline Conducting Polymers

Abstract

Abstract In this review article, we focus on the electrical conductivity (σ), the charge number densities (n), and the carrier mobilities (μ) in crystalline conducting polymers doped by various methods, including doping with an ionic-liquid-gated transistor (ILGT) configuration and H2SO4 treatment. Electrical properties of crystalline conducting polymer poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) were investigated with a combination of ILGT configuration doping and operando Raman spectroscopy. The type of carrier, a positive polaron or a positive bipolaron, was determined with Raman spectroscopy. The values of σ and μ were determined as a function of n. The highest mobility of positive bipolarons was 1.2 cm2 V−1 s−1 at n = 6.2 × 1020 cm−3. The electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) was increased upon H2SO4 treatment. The H2SO4 treatment caused anion exchange, resulting in crystallization of the doped PEDOT. The charge density was 7.3 × 1021 cm−3. The mobility of positive bipolarons was 1.9 cm2 V−1 s−1. In heavily doped crystalline PBTTT and PEDOT, spinless positive bipolarons existed stably, and were highly mobile.