High-Performance Photodetectors with an Ultrahigh Photoswitching Ratio and a Very Fast Response Speed in Self-Powered Cu₂ZnSnS₄/CdS PN Heterojunctions

Cu₂ZnSnS₄ (CZTS) has been extended to the field of photodetection owing to its outstanding optoelectronic properties. However, the existence of the ineluctable defects in CZTS semiconductors affects and even determines the optoelectric processes including carrier generation, relaxation, transfer, and recombination. Moreover, photoresponse correlated to the defects in CZTS photodetectors has not well been documented and the possible physics mechanism is still unexplored. High-performance and self-powered PN heterojunction photodetectors are built from Cu₂ZnSnS₄ and CdS films. The devices exhibit a steady rectifying behavior and a prominent photovoltaic effect. The peak values of responsivity and detectivity are 220 mA W^–1 and 2.69 × 10¹⁰ Jones, respectively. A very fast response speed with rising and decay times of up to 18 and 19 μs and an ultrahigh photoswitching ratio beyond 10⁴ are demonstrated in these photodetectors. An abnormal dependence of the light response parameters on the incident power and temperature is found in these devices. This anomaly is explained by the formation of the defects and/or defect dipoles, which are evidenced by the temperature dependence of the photocurrent, the dependence of the capacitance on the bias voltage at different temperatures, and the derivative of capacitance with temperature.