Fabrication of Zn(O,S) thin films by ozone assisted photochemical deposition and its potential applications as buffer layers in kesterite-based thin film solar cells

Zn(O,S) thin films have attracted intensive attention for their potential application as Cd-free buffer layers in kesterite-based solar cells due to their desirable band alignment with Cu2ZnSnS4 (CZTS) or Cu2ZnSnSxSe4-x (CZTSSe) materials. In this work, Zn(O,S) thin films with controllable content of oxygen have been designed via ozone-assisted photochemical deposition processes. Results showed that the deposited Zn(O,S) thin films using various ozone flow rate are flat, compact and uniform with a thickness of ~ 40 nm, consisting of nanoparticles with a diameter of 30–50 nm. The introduction of ozone into synthesis processes can hardly impose any effect on the morphologies, but significantly modify and tune the oxygen contents in the deposited thin films. Moreover, optical absorption spectra indicate that the optical band-gap of Zn(O,S) films can be readily tuned from 3.3 to 3.7 eV by changing the ozone flow rates. Furthermore, it is confirmed that the solar cells with Zn(O,S)/CZTS structure has a higher conversion efficiency of 3.0% and a higher open circuit voltage of 0.516 V compared to those with ZnS/CZTS structure. Our research would open an easy and promising approach to prepare Cd-free buffer layer materials for kesterite-based thin film solar cells.