VIS/NIR Photocurrent Spectroscopy on Organic Semiconductors by Lock-in Amplification

Abstract

Photocurrent spectroscopy provides a nondestructive method to study optoelectronic properties of semiconducting materials based on the photo-absorption induced current. This research reports of the successful development of a novel photocurrent spectroscopy setup that uses lock-in amplification to measure currents down to 0.1 pA with a spectral resolution of 0.63 nm. The setup has been applied to study three small molecule organic semiconductors of various morphology. Measurements on rubrene single crystals indicate an oxidation triggered antibatic photocurrent response with respect to the spectral absorption. Rubrene thin film samples revealed an impact of the film thickness on photo-conductive behavior up to a thickness of 52 nm. A comparative study has been conducted on different morphologies of the novel organic semiconductor TIPS pentacene. The TIPS pentacene inkjet-printed crystal was studied for the first time. These crystals exhibit a photocurrent response and bandgap equivalent to single crystalline samples. Finally, this research reports the first measurements on TIPG pentacene. A pentacene derivative similar to TIPS pentacene with substituted germanium (Ge). The Ge substitution causes a slight blueshift but no further significant change in the photo-conductive response of the material.