Photoexcited Electron and Hole Polaron Formation in CdS Single Crystals Revealed by Femtosecond Time-Resolved IR Spectroscopy

Photoexcitation polarons in semiconductor materials play a crucial role in various fields, such as enhancing electrical conductivity, influencing chemical selectivity, and improving reaction efficiency in semiconductor photocatalysis. To comprehensively understand the properties of polarons, time-resolved mid-infrared, near-infrared, and visible transient absorption spectroscopies were employed to investigate the photogenerated carriers in cadmium sulfide (CdS) single crystals. We have determined the midgap energy levels of defect states by transient infrared absorption-excitation energy scanning spectra. Furthermore, we observed the formation of both the electron and hole polarons, where the electron polarons are coupled to the longitudinal acoustic phonons, while the hole polarons are coupled to both optical and longitudinal acoustic phonons. The photogenerated free electrons were trapped by the S-vacancies at 0.14 eV below the minimum of the conduction band, leading to the formation of electron polarons with a trapping time constant of 4.5 ps, while hole polarons are formed at Cd vacancies located at 0.04 eV above the maximum of the valence band with a time constant of 2.9 ps. Our determined midgap energy levels of the trapped states together with dynamical properties of electron and hole polarons provide insights into optimizing the photocatalytic property of CdS.