Spatially resolved Fourier transform impedance spectroscopy: A technique to rapidly characterize interfaces, applied to a QD/SiC heterojunction

Abstract

We demonstrate a technique to quickly build and spatially map the frequency response of optoelectronic devices. The transfer function of a linear system is the Fourier transform of its impulse response. Such an impulse response is obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We introduce and apply Fourier transform impedance spectroscopy (FTIS) to a PbS colloidal quantum dot SiC heterojunction photodiode and validate the results using intensity-modulated photocurrent spectroscopy. Cutoff frequencies in the devices were as high as ∼10 kHz, showing their utility in advanced thin film and flexible electronics. The practical frequencies for FTIS lie in the mHz–kHz range, ideal for composite materials such as quantum dot films that are dominated by interfacial trap states. These can lead to characteristic lengths for charge collection ∼20–500 μm dominated by transmission line effects, rather than intrinsic diffusion and drift length scales, enabling extraction of interfacial capacitances and series/parallel resistances.