Trigonal Cu₂-II-Sn-VI₄ (II = Ba, Sr and VI = S, Se) quaternary compounds for earth-abundant photovoltaics

Abstract

We propose trigonal Cu2-II-Sn-VI4 (II = Ba, Sr and VI = S, Se) quaternary compounds for earth-abundant solar cell applications. Through density functional theory calculations, we show that these compounds exhibit similar electronic and optical properties to kesterite Cu2ZnSnS4 (CZTS): high optical absorption with band gaps suitable for efficient single-junction solar cell applications. However, the trigonal Cu2-II-Sn-VI4 compounds exhibit defect properties more suitable for photovoltaic applications than those of CZTS. In CZTS, the dominant defects are the deep acceptors, Cu substitutions on Zn sites, which cause non-radiative recombination and limit the open-circuit voltages of CZTS solar cells. On the contrary, the dominant defects in trigonal Cu2-II-Sn-VI4 are the shallow acceptors, Cu vacancies, similar to those in CuInSe2. Our results suggest that the trigonal Cu2-II-Sn-VI4 quaternary compounds could be promising candidates for efficient earth-abundant thin-film solar cell and photoeletrochemical water-splitting applications.

Earth‐Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu2BaSn(S,Se)4 Absorber

“…Moreover, these compounds exhibit a tunable bandgap in the range of 1.5–2.0 eV, spanning relevant values for single‐ or multiple‐junction photovoltaic or photocatalytic applications . According to advanced theoretical studies on trigonal CBTSSe compounds, the dominant point defect in this system is the copper vacancy ( V Cu ) as a shallow acceptor, similar to the situation in high‐performance CIGSSe, with other acceptor and donor defects having higher formation energies and therefore reduced probability of occurrence . Specifically, the formation energies of Cu‐Ba and Cu–Sn antisite defects are 1.0 eV higher than that of V Cu , due to large ionic size/charge mismatch among constituents and distinct coordination environment of the large Ba 2+ cation.…”

“…Additionally, the full‐width‐at‐half‐maximum (FWHM) of the PL peak for Cu 2 BaSnS 4− x Se x with x = 3 (≈60 nm) is much smaller compared to the FWHM for Cu(In,Ga)(S,Se) 2 (≈100 nm) and Cu 2 (Zn,Sn)(S,Se) 4 (≈190 nm) reported in the literature . Presumably, the very different coordination environments around Cu + , Ba 2+ , and Sn 4+ discourage the formation of Cu‐Ba and Sn‐Ba antisite disordering, leading to the observed suppression of band tailing …”

“…Most recently, Cu 2 BaSnS 4− x Se x (CBTSSe) materials with a trigonal structure (space group P 3 1 ) and composed of only earth‐abundant metals (i.e., the abundances of Cu (47 mg kg −1 ), Ba (650 mg kg −1 ), and Sn (2.5 mg kg −1 ) in the earth's crust are >25× larger than that of In (0.1 mg kg −1 )) have been proposed and demonstrated as emerging PV absorbers to address these issues, employing ionic size mismatch coupled with coordination discrimination to prevent detrimental formation of antisite disorder and associated band tailing . Moreover, these compounds exhibit a tunable bandgap in the range of 1.5–2.0 eV, spanning relevant values for single‐ or multiple‐junction photovoltaic or photocatalytic applications . According to advanced theoretical studies on trigonal CBTSSe compounds, the dominant point defect in this system is the copper vacancy ( V Cu ) as a shallow acceptor, similar to the situation in high‐performance CIGSSe, with other acceptor and donor defects having higher formation energies and therefore reduced probability of occurrence .…”

Solar Energy Capture Materials

“…This suggest that it shall be possible to grow trigonal Cu2CaSnS4 with much less native defects compared to the other tetragonal Cu2XSnS4 compounds. Moreover, also the compounds with the much larger Sr or Ba elements should discourage the formation of (Cu − + Cu + ) antisite disorder, but also due to trigonal structure [96].…”

“…Zhong et al[94] found that stannite Cu2MgSnS4 (and Cu2MgSnSe4) are thermodynamic stable, but, similar to Wang et al, that kesterite and stannite phases of Cu2CaSnS4 (and Cu2CaSnSe4) are unstable with respect to competing compounds. Recent developments of devices based on emerging Cu2BaSnS4 (or Cu2SrSnS4) compound and anion S/Se alloying yield in early attempts a solar cell efficiency of 1.6%[95].Experimental and theoretical characterization reveals that these compounds with large group-II elements crystallize in trigonal or orthorhombic crystalline structure[95,96]. XS], or similar mixture of compounds.…”