Substrate temperature optimization of pulsed-laser-deposited and in-situ Zn-supplemented-CZTS films and their integration into photovoltaic devices

Abstract

The pulsed laser deposition (PLD) technique was used to deposit CZTS thin films onto SLG/Mo substrates via the KrF-laser ablation of a composite target consisting of Cu2ZnSnS4 pellet onto which Zn strips were purposely affixed. The effect of the substrate temperature (Tsub) of the PLD-CZTS films on their structure and properties was systematically studied over the 25–500 °C temperature range. The Zn content of the films was found to increase mainly when Tsub is raised from 300 to 500 °C. While both XRD and Raman analyses confirmed that the films consist of the kësterite-single-phase of which crystallinity improves when Tsub is increased (from RT up to 400 °C), the near resonant Raman (at 325 nm) revealed the presence of ZnS phase at high Tsub (> 400 °C). The optical energy band gap (Eg) of the PLD-CZTS films was consistently found to decrease from 1.9 to 1.4 eV when Tsub is increased from RT to 500 °C. Our results pointed out the Tsub = 400 °C as the optimal deposition temperature that meets at best the properties required for the PLD-CZTS films for PV application. The post-annealing (in presence of S and Sn vapors at 560 °C) of the PLD-CZTS films has improved further their crystallinity and led to the formation of some ZnS secondary phase at their surface. By appropriately integrating these post-annealed films into SLG/Mo/CZTS/CdS/ZnO/ITO photovoltaic devices, we were able to demonstrate their photoconversion ability with a PCE of 3.3 % (Voc = 512 mV, Jsc = 12.5 mA/cm2 and a FF = 51.5 %). The analysis of their EQE spectrum suggests that the effective carrier collection length in the CZTS absorption layer needs to be extended further to achieve higher photoconversion efficiencies.