Non-resonant phase sensitive approach for time resolved microwave conductivity in photoactive thin films

Abstract

Time-resolved microwave conductivity (TRMC) is a contactless technique utilized for the investigation of carrier density, transport properties, trapping phenomena, and recombination parameters in charge transport materials. Traditional TRMC methods rely on resonant cavities or resonators, which impose limitations on the frequency range and accuracy of measurements. In this study, we introduce an innovative approach that employs a non-resonant coplanar transmission line and a microwave interferometric detection scheme to investigate the phase-dependent complex microwave conductivity. Additionally, we demonstrate unique calibration techniques for determining the absolute complex microwave conductivity by combining transient photoconductivity (TPC) and electron spin resonance (ESR) as complementary methods. By utilizing a phase-sensitive microwave interferometer, our detection scheme significantly enhances measurement sensitivity and eliminates the need for a resonant cavity. This broadband detection system enables direct measurement of phase-dependent changes in film conductivity (Δσ). Moreover, it allows us to measure subtle variations in sample photoconductivity upon optical excitation and accommodates greatly restricted volumes (∼nL) consistent with typical device sizes. Here we demonstrate the utility of this technique on a series of poly(3-hexylthiophene) (P3HT) and the electron acceptor [6,6]-phenylC61-butyric acid methyl ester (PCBM) thin films with varying concentrations of PCBM and film thickness.