Transformation of carrier recombination mechanism as increasing the Germanium content in Cu2ZnGexSn1-xS4 single crystal prepared by molten salt method

Abstract

Return to the physics origin of Cu2ZnSnS4 type (CZTS-type, include CZGS, CZGTS, CZTSSe, etc.) material system, studying the influence of replacement elements on the optoelectronic properties by preparing high-quality single crystal materials to clarify the relationship between the key defect states of materials and carrier dynamics is the only way which must be passed to find efficient paths to improve the existing performance of CZTS-type optoelectronic devices. Herein, the optical behavior of high-quality Cu2ZnGexSn1-xS4 (CZGTS) single crystal prepared by the molten salt method and its relationship with Ge content were systematically studied using a photoluminescence (PL) and absorption tests at room temperature. Combined with the simple electrical tests, the transformation of carrier recombination mechanism as increasing the Germanium content in CZGTS single crystal before and after phase transition were reported for the first time. The transmission electron microscopy images, X-ray diffraction spectra and Raman scattering spectra of CZGTS were adopted to reveal the evolution mechanism of single crystal growth and phase transition caused by the change of Ge composition. The results show that the tunable blue-shift of PL peak emerges with the increase of Ge content and the excitation power dependence has a saturation phenomenon in the CZGTS single crystal particles. In consideration of the optical bandgap, PL peak position, PL intensity, carrier lifetime and the macroscopic resistivity, it is confirmed that the intrinsic PL behavior could be effectively tailored by different Ge amounts. Compared with the recombination PL of interband carrier (Ep > Eg) in Cu2ZnSnS4, the band tailed carriers (Ep < Eg) dominated by defect states would play a powerful role in CZGTS single crystal. Our work provides a strategy for tailoring the bandgap of CZGTS materials and a new perspective for understanding the influence of defect states on carrier dynamics.