Thermal effect on CZTS solar cells in different process of ZnO/ITO window layer fabrication

Abstract

In this work, a window layer consisting of zinc oxide (ZnO) and indium‑tin-oxide (ITO) thin films was prepared by sputtering under different heat treatment processes (in-situ or post annealing) to study the effect on structural ordering and performance of Cu2ZnSnS4 (CZTS) solar cells. It is found that all characteristic performance parameters including open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and efficiency of CZTS solar cells were improved for the window layer with ITO thickness = 350 nm post-annealed at lower temperature. Specifically, Voc of the CZTS device was increased by 57 mV when the window layer was post-annealed at 200 °C (Voc = 585 mV) compared to the device with window layer made at 300 °C (Voc = 528 mV). We further reduced the thickness of the window layer with ITO thickness = 140 nm which was deposited in an in-situ annealing process at 220 °C. Compared to the 350 nm thick film made by post-annealing, it is found that the thin film made by in-situ annealing help to further boost the Voc (equals 635 mV) and thus efficiency (3.20%) of CZTS solar cells. Study of the evolution of full width half maximum (FWHM) and intensity ratio (Q) of major Raman scattering peaks of CZTS indicates a much reduced structure disorder in the CZTS material with ZnO/ITO window layer annealed at low temperature. Further analysis of the evolution of diode ideality factor (A), reverse saturation current density (J0) and Urbach energy (Eu) of these devices revealed that the quality of bulk CZTS and/or CZTS/CdS heterojunction were more degraded at higher temperature. However, the in-situ annealing provided extra benefits to device performance which is possibly due to modification of interfacial properties of CZTS/CdS and/or CdS/ZnO.