This talk will follow the emergence of several promising thin-film solar energy conversion materials based on earth-abundant Cu2ZnSn(S,Se)4 (CZTS), Cu2BaSn(S,Se)4 (CBTS) and related systems, having a I2–II–IV–VI4 stoichiometry and an interconnected network of metal (I, IV) chalcogenide (VI) tetrahedra. Simple solution- and vacuum-based film deposition approaches have enabled fabrication of relatively high-performance absorber layers, with resulting photovoltaic (PV) sunlight-to-electricity power conversion efficiencies exceeding 12% for CZTS  and 5% for CBTS . CBTS has also recently been incorporated into photoelectrochemical (PEC) cells, yielding a stable (over 10 hr) 12 mA/cm2 photocurrent at 0 V/RHE . In CBTS, the much larger Ba ion occupies a site that has 8-fold coordination rather than 4-fold (as for Cu, Zn and Sn), reducing the probability of anti-site disorder and associated band tailing (relative to CZTS), which can limit device performance. A similar arrangement can be found within the broader family of I2–II–IV–VI4 (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se) compounds, which fall into five distinct structural types . Computational and preliminary experimental assessment reveals several family members, beyond CBTS, that may be interesting for PV/PEC. Although at an early stage of development, CBTS and the broader concept of employing atomic size and coordinatlion discrepancy for limiting anti-site disorder may offer a pathway for overcoming performance issues encountered within complex multinary chalcogenide semiconductors.
This work was supported by NSF CBET-1511737.
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