Spectroelectrochemical Investigation of the Optical Properties of a Carbazole-Based Emitter

Abstract

This study analyzes the optical properties and device stability of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) and 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN), representative carbazole-based thermally activated delayed fluorescence emitters, with a focus on their polaron states using spectroelectrochemical (SEC) analysis. The oxidation and reduction potentials of each material were determined by cyclic voltammetry. Furthermore, the absorption spectra obtained from SEC measurements are deconvoluted via Gaussian fitting to clearly distinguish the absorption characteristics of each material in both the neutral and polaron states. The results show that in their polaron states, both 4CzIPN and 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene exhibit enhanced absorption in the short- and long-wavelength regions compared with their neutral forms, resulting in spectral overlap with their photoluminescence spectra. Notably, stability evaluations using single-charge devices (electron- and hole-only devices) reveal a decrease of more than 10% in the photoluminescence intensity of the 4CzIPN-based hole-only device. This degradation is attributed to the absorption of excitation light (385 nm) by positive polarons, leading to the simultaneous generation of excitons from both the neutral molecules and positive polarons. Overall, the results demonstrate that SEC analysis enables the rapid assessment of emitter polaron characteristics prior to device fabrication and provides important guidelines for the design of highly efficient and stable thermally activated delayed fluorescence devices.