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 (T-sub) of the PLD-CZTS films on their structure and properties was systematically studied over the 25-500 degrees C temperature range. The Zn content of the films was found to increase mainly when T-sub is raised from 300 to 500 degrees C. While both XRD and Raman analyses confirmed that the films consist of the kesterite-single-phase of which crystallinity improves when T-sub is increased (from RT up to 400 degrees C), the near resonant Raman (at 325 nm) revealed the presence of ZnS phase at high T-sub (> 400 degrees 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 T-sub is increased from RT to 500 degrees C. Our results pointed out the T-sub = 400 degrees 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 degrees 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 % (V-oc = 512 mV, J(sc) = 12.5 mA/cm(2) 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

In this study, CZTS thin films were deposited on SLG/Mo substrates using pulsed laser deposition (PLD) technique, with the optimal substrate temperature found to be 400 degrees C for achieving the desired properties for PV application. Increasing the substrate temperature led to an increase in zinc content and a decrease in optical energy band gap of the films. Post-annealing of the films further improved crystallinity and led to the formation of a ZnS secondary phase, resulting in a photoconversion efficiency of 3.3% in photovoltaic devices.