Abstract
In this study, Cu-Zn-Sn (CZT) precursor film was deposited on a Mo-coated soda-lime glass substrate by a DC-magnetron sputter with alloy Cu-Zn-Sn and Zn targets. As-prepared CZT precursor film was moved to a tube furnace for selenization and post sulfurization (sulfurization after selenization, SAS) process. Experimentally, CZT precursor film was first selenized at 475 ℃ in H2Se/Ar atmosphere for 15 minutes, followed by sulfurization at 495 ℃ in H2S/Ar atmosphere for 15 minutes to form Cu2ZnSn(S, Se)4 (CZTSSe) thin film. In order to understand the role of selenization and sulfurization in the SAS process, the study is divided into two parts. The first part is focused on the grain growth of CZT precursor film at different selenization temperatures. Surface morphology, thickness and element distribution were examined by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). The crystallography and phase information of the samples were studied by glazing angle X-ray diffraction (GIXRD) and Raman spectroscopy. Based on the experimental results, a plausible growth model of Cu2ZnSnSe4 (CZTSe) thin film was proposed. The CZTSe device properties were analyzed by I-V and external quantum efficiency measument systems. Among all samples, precursor film selenized at 475 ℃, which was called CZTSe-475, had the highest conversion efficiency of 2.1%, open circuit voltage of 0.27 V and short circuit current density of 22.6 mA/cm¬2 under AM 1.5G illumination. In the second part, a selenized sample treated with an additional sulfurization (SAS) process (device named as CZTSSe-7.5) had the conversion efficiency of 7.5%, open circuit voltage of 0.39 V and short circuit current density of 33.1 mA/cm¬2 under AM 1.5G illumination. Compared the CZTSe-475 device with the CZTSSe-7.5 one under a negative applied voltage (e.g., -0.5 V), the ratio of EQE (-0.5 V)/EQE (0 V) of the CZTSe-475 device increased in the short wavelength region (400－600 nm), but the ratio had no change in the CZTSSe-7.5 device. The result suggests that the defect density in the CZTSSe-7.5 device would be smaller than that in the CZTSe-475 device, which implies that the effect of the SAS process was passivation for the thin film. Moreover, conductive atomic force microscopy (CAFM) shows that local current appeared to exist near the grain boundaries (GBs). Photoluminescence study exhibits that both VCu and ZnCu defects existed in CZTSe-475 and CZTSSe-7.5 thin films. On the other hand, ZnSn defect, which might decrease the conversion efficiency of the device, was thought to exist only in CZTSSe-7.5 thin film.