Charge carrier mobility in regioregular poly(3-hexylthiophene) probed by transient conductivity techniques: A comparative study

TLDR

The study employed time‑of‑flight and charge‑carrier‑extraction‑by‑linearly‑increasing‑voltage techniques on purified regioregular poly(3‑hexylthiophene) to measure the temperature and electric‑field dependence of charge‑carrier mobility. Above 130 K the two methods agree, showing that the usual Poole–Frenkel field dependence of mobility weakens around 250–270 K and turns negative at higher temperatures due to positional disorder, and that the charge‑generation mode has little effect except below 110 K where photogenerated carriers exhibit greater transit‑time dispersion.

Abstract

The temperature and electric-field dependence of charge carrier mobility has been studied by a conventional time-of-flight technique in chemically purified, low dark conductivity samples of regioregular poly(3-hexylthiophene). Subsequently, the mobility of doping-induced charge carriers has been determined using the technique of charge carrier extraction by linearly increasing voltage in the same samples exposed to air. The charge carrier mobility determined by both experimental techniques correspond well to each other at temperatures above $130\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that these experimental techniques are mutually consistent. The study clearly shows that the typical $\mathrm{log}\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\ensuremath{\propto}\ensuremath{\beta}{E}^{1∕2}$, $\ensuremath{\beta}>0$ Poole--Frenkel-like electric-field dependence of the charge carrier mobility diminishes at temperatures around $250--270\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $\ensuremath{\beta}$ becomes negative at higher temperatures. Such negative electric-field dependence of mobility observed by both experimental techniques is attributed to positional disorder in a random-organic dielectric and analyzed in the framework of the disorder formalism. Finally, the overall agreement indicates that the mode of charge generation has negligible effect on the temperature- and electric-field dependence of mobility except at the lowest temperatures $(<110\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, where transit time dispersion of the photogenerated charge carriers probed by the ToF technique is more pronounced.