Cu₂PdSnSe₄ and Cu₂PdSn(S,Se)₄ Palladium-Substituted Kesterite Nanomaterials for Thin-Film Solar Cells

Kesterites are being studied intensively as sustainable absorber materials for solar cell development. However, elements such as Zn and Cu exhibit antisite defects that function as charge traps and recombination centers that affect the light absorption and carrier transport efficiencies of kesterite solar cells. The substitution of Zn or Cu with other metals is one of the strategies used to improve the photovoltaic performance of kesterites. This study focuses on the preparation and photovoltaics of Cu₂PdSnSe₄ (CPTSe) and Cu₂PdSn(S,Se)₄ (CPTSSe) kesterite nanoparticles (containing Pd instead of Zn) by a modified solvothermal (polyol) microwave synthesis method. The nanomaterials exhibited a tetragonal kesterite crystal structure with polydispersed morphology and average crystallite sizes of 22 and 17 nm for CPTSe and CPTSSe, respectively. DAMMIF ab initio analysis of the small-angle X-ray scattering data determined the shape of CPTSe and CPTSSe nanomaterials to be ellipsoidal. Ultraviolet-visible (UV-vis) spectroscopy revealed red-shift absorption properties, with bandgap energy values of 1.13 eV (CPTSe) and 1.20 eV (CPTSSe), thereby making them suitable light absorber materials for photovoltaic applications. Photoluminescence spectroscopy characterization confirmed the attenuation of defect concentrations in CPTSe and CPTSSe compared to the Zn analogue, which positively impacts the charge-carrier transport and recombination properties. A preliminary test of the materials in superstrate photovoltaic cell devices yielded power conversion efficiency values of 1.32% (CPTSe) and 3.5% (CPTSSe). The CPTSe- and CPTSSe-based photovoltaic devices maintained ∼70% mean open-circuit voltage (Voc), which is a significant improvement over the ∼20% V _oc retained by Zn-based kesterites after 24 days.