Rational design of Cu2ZnSn(S,Se)4 thin film photovoltaics for adopting RF-sputtered Zn(O,S) as an environmental-benign buffer layer

RF-Sputtered Zn(O,S) films from a single Zn(O,S) ceramic target having a [S]/([O]+[S]) ratio of 0.25 was assessed as a buffer layer in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaics for the first time. It was found that the composition of sputtered Zn(O,S) films does not necessarily resemble the [S]/([O]+[S]) target compositional ratio. A further structural investigation suggested that the Zn(O,S) film crystallizes as a hexagonal wurtzite structure, denoting a predominant hexagonal wurtzite ZnO structure character over cubic sphalerite ZnS one. The implementation of sputtered Zn(O,S) films as a buffer layer in kesterite photovoltaic yields poor performance when the absorber was a pure sulfide-kesterite or Cu2ZnSnS4 (CZTS) which is due to a non-ideal band discontinuity, verified by Kelvin Probe (KP) and APS (Ambient Pressure Photoemission Spectroscopy) for ionization energy. To maximize the photovoltaic performance, the kesterite absorbers with various ([S]/[S+Se]) ratios were deliberately prepared from sulfoselenization of dimethyl sulfoxide ink-based metal salt precursors, targeting an optimized kesterite/Zn(O,S) band alignment. Complementary analysis using Grazing Indience X-ray diffraction, TEM, SEM-EDX and Raman spectroscopy reveals that the as-sulfoselenized films crystallize as a kesterite phase, but also exhibit non-uniform elemental Cu, Zn, Sn and chalcogen distributions. Cu and Sn depletions are found near the kesterite film surface whereas Zn-rich area can be identified on the film surface. The sulfoselenization also yields the kesterite films with S-rich surface, along with Se enrichment in the bulk film. KPM results show that the conduction band minimum of sulfoselenized kesterite films changes as a function of ([S]/[S+Se]) ratio, modifying the absorber/Zn(O,S) conduction band offset. The photovoltaic device with a sputtered Zn(O,S) buffer layer delivers the highest power conversion efficiency when an absorber with ([S]/[S+Se]) ratios of 0.5 was employed, demonstrating an efficiency of 5.16 % with a short circuit current of 28.69 mA/cm2, an open circuit voltage of 398.15 mV and a fill factor of 45.15 %.