A simple method to measure intermolecular charge-transfer absorption of organic films

Abstract

Abstract Intermolecular charge transfer (CT) complexes of organic molecules are actively researched due to their wide use and strong influence on organic optoelectronic device performance. In previous studies, exciplexes, excited-state intermolecular CT complexes, failed to show absorption in ultraviolet-visible-near-infrared absorption spectroscopy measurements. Recently, however, direct exciplex CT absorption from the ground state of donor-acceptor mixed films was detected in the sub-bandgap region by Fourier transform photocurrent spectroscopy (FTPS) and photothermal deflection spectroscopy (PDS) measurements. Here, we report a simple method to measure intermolecular CT absorption that combines simple reflection–transmission and ellipsometry measurements of thick films to determine the extinction coefficient for the small-molecular CT state. The extinction coefficients extracted from different film thicknesses were consistent, up to 10−4 in the sub-bandgap wavelength region, comparable to that obtained with PDS executed on thin films.