Modulated-photocurrent spectroscopy of single-crystal organic semiconductor rubrene with pristine and trap-dominated surfaces

Abstract

High-performance, benchmark organic semiconductor rubrene is investigated by modulated-photocurrent (MPC) spectroscopy. In this technique, periodically modulated and steady-state photocarrier populations are simultaneously generated in the sample by two independent light beams, with the resultant photocurrent measured by a lock-in amplifier. The technique allows identifying a bandlike carrier motion (as opposed to incoherent hopping) as the dominant charge transport mechanism in this highly crystalline molecular semiconductor, in agreement with prior studies, yet without the need for fabrication of complex transistor devices. Moreover, MPC spectroscopy is used to determine the important parameters, such as the density of states for traps, their carrier capture coefficient, and attempt to escape frequency, as well as the quantum efficiency of photocarrier generation and the photocarrier mobility. The MPC spectroscopy is also shown to be able to discriminate charge transport at the surface of the crystal from that occurring in the bulk. The wealth of the thus obtained information is essential for our better understanding of the microscopic mechanisms governing photoconductivity in this class of materials.