Role of Deep Defect States in Anomalous Photovoltage Relaxation of Polycrystalline Silver-Doped Cadmium Telluride Thin Films

The present work investigates the long-term relaxation behavior of anomalous photovoltage and photoelectret voltage in polycrystalline CdTe:Ag thin films prepared by oblique thermal evaporation. The relaxation kinetics were studied at room temperature under controlled photopolarization conditions. Experimental results reveal a pronounced two-stage relaxation behavior, consisting of a fast initial component with characteristic times of 2–4 min and a slow long-term component extending up to 25–45 min. The short-term relaxation is attributed to shallow trapping centers located near the band edges, while the long-term relaxation is governed by deep trapping centers associated with impurity–defect complexes involving silver atoms and cadmium vacancies. The observed metastable photoelectric states are explained within a bulk trapping model combined with near-surface band bending effects, which lead to asymmetric photocarrier separation and long-lived charge retention. The obtained results provide new insight into defect-controlled relaxation processes in CdTe:Ag thin films and demonstrate the crucial role of deep trapping centers in the formation and stability of photoinduced electric states. These findings are relevant for optimizing the photoelectric performance and long-term stability of CdTe-based photovoltaic and optoelectronic devices.