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

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.
Peer Reviewed
Postprint (author's final draft)