Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells.

Abstract

This article reports a purely experiment-based method to evaluate the time-dependent charge carrier mobilities in thin-film organic solar cells (OSCs) using simultaneous charge extraction by linearly increasing the voltage (CELIV) and time-resolved microwave conductivity (TRMC) measurements. This method enables the separate measurement of electron mobility (μe) and hole mobility (μh) in a metal-insulator-semiconductor (MIS) device. A slope-injection-restoration voltage profile for MIS-CELIV is also proposed to accurately determine the charge densities. The dynamic behavior of μe and μh is examined in five bulk heterojunction (BHJ) OSCs of polymer:fullerene (P3HT:PCBM and PffBT4T:PCBM) and polymer:nonfullerene acceptor (PM6:ITIC, PM6:IT4F, and PM6:Y6). While the former exhibits fast decays of μh and μe, the latter, in particular, PM6:IT4F and PM6:Y6, exhibits slow decays. Notably, the high-performing PM6:Y6 demonstrates both a balanced mobility (μe/μh) of 1.0-1.1 within 30 μs and relatively large CELIV-TRMC mobility values among the five BHJs. The results exhibit reasonable consistency with a high fill factor. The proposed new CELIV-TRMC technique offers a path toward a comprehensive understanding of dynamic mobility and its correlation with the OSC performance.