In situ process monitoring during multistage coevaporation of Cu2ZnSnS4 thin films

Abstract

A multistage coevaporation process for the direct growth of Cu2ZnSnS4 (CZTS) thin films without additional atmospheric sulfurization was investigated. To obtain reproducible CZTS films, in situ process monitoring of the film growth was developed by measuring the apparent substrate temperature (Tpyro) using a pyrometer. After CZTS depositions terminated at various endpoints, ex situ characterization of the film properties was performed to clarify the growth mechanism of the films. The results provided clear evidence that CZTS phase formation was significantly delayed via re-evaporation of Sn–S-based compounds in the early part of the first stage, leading to the initial formation of a dominant (CuS + ZnS) structure that coexisted with a small amount of CZTS. CZTS phase formation was then facilitated by the (CuS + ZnS) precursor via a Cu-rich to Cu-poor sequence with an apparent variation in Tpyro during the second stage, and the slightly segregated CuS phase was nearly consumed under (Zn + Sn + S) fluxes. Consequently, CZTS thin films containing close-packed grains with a single kesterite structure were successfully grown under excess Sn and S fluxes, even at moderate Tsub below 500 °C. The best solar cell with a Glass/Mo/CZTS[Cu/(Zn + Sn) = 0.71, Zn/Sn = 1.6]/CdS/ZnO:Ga structure and a NaF precursor layer yielded an active area (0.170 cm2) efficiency of 3.84% (Voc = 567 mV, Jsc = 11.3 mA/cm2, and FF = 0.603). The diode properties under dark and light conditions were also evaluated.