Fill Factor Losses in Cu sub(2)ZnSn(S sub(x)Se sub(1-x)) sub(4) Solar Cells: Insights from Physical and Electrical Characterization of Devices and Exfoliated Films

Besides the open circuit voltage (V sub(OC)) deficit, fill factor (FF) is the second most significant parameter deficit for earth-abundant kesterite solar cell technology. Here, various pathways for FF loss are discussed, with focus on the series resistance issue and its various contributing factors. Electrical and physical characterizations of the full range of bandgap (E sub(g) = 1.0-1.5 eV) Cu sub(2)ZnSn(S sub(x)Se sub(1-x)) sub(4) (CZTSSe) devices, as well as bare and exfoliated films with various S/(S + Se) ratios, are performed. High intensity Suns-V sub(OC) measurement indicates a nonohmic junction developing in high bandgap CZTSSe. Grazing incidence X-ray diffraction, Raman mapping, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy indicate the formation of Sn(S,Se) sub(2), Mo(S,Se) sub(2), and Zn(S,Se) at the high bandgap CZTSSe/Mo interface, contributing to the increased series resistance (R sub(S)) and nonohmic back contact characteristics. This study offers some clues as to why the record-CZTSSe solar cells occur within a bandgap range centered around 1.15 eV and offers some direction for further optimization. Fill factor (FF) losses are the second most significant barrier for Cu sub(2)ZnSn(S sub(x)Se sub(1-x)) sub(4) (CZTSSe) solar cells, after the well-known open circuit voltage deficit (V sub(OC,def)). The sources of FF losses, in particular the series resistance in full bandgap-range CZTSSe, are investigated. CZTSSe with high S/(S + Se) ratios are found to develop nonohmic back contact at the CZTSSe/Mo interface.