Optoelectronic and material properties of solution processed Earth abundant Cu2BaSn S,Se 4 films for solar cell applications

Copper barium thioselenostannate, Cu2BaSnS4 xSex CBTSSe , absorbers employ low toxicity and abundant metals while offering low cost manufacturing options, controllable stoichiometry and band gap tunability from 2 eV at x 0 to 1.55 eV at x 3 . CBTSSe can therefore be considered a prospective candidate for maintaining or improving upon the advantages of already commercialized Cu In,Ga S,Se 2 CIGSSe and CdTe absorbers. In this study, we focus on solution deposited stoichiometric CBTSSe films with band gap of 1.59 eV x amp; 8776; 3 and explore the fundamental film properties. Temperature and excitation dependent photoluminescence studies reveal a dominant defect emission at 1.5 eV and a second deep defect feature at 1.15 eV. From time resolved terahertz measurements, we find a charge carrier electron and hole sum mobility of 140 cm2 Vs i.e., comparable to values in CIGSSe or Cu2ZnSnS4 xSex CZTSSe as well as a two component minority carrier lifetime. A longer lived lifetime component 2 ns arises from bulk recombination. However, strong recombination at the bare surface leads to a 50 ps lifetime, inferior to state of the art CIGSSe or CZTSSe absorbers. This recombination issue may worsen for CBTSSe CdS interfaces, due to a cliff like band alignment with 0.6 eV band offset, as revealed by ultraviolet photoemission spectroscopy. A low number of charge carriers within the absorber further contributes to a high series resistance. Employing these films, we also report the highest performance achieved from solution processed trigonal CBTSSe thin film photovoltaic devices, with open circuit voltage, short circuit current density, fill factor and efficiency of 470 mV, 14.3 mA cm2, 43.6 and 2.9 , respectively. The physical measurements provided on the stoichiometric solution processed CBTSSe absorber further point to critical areas for future improvement of CBTSSe and related photovoltaic cells in the quest for higher efficiency devices based on earth abundant metal