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I2–II–IV–VI4 (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se): Chalcogenides for Thin-Film Photovoltaics
journal contribution
posted on 2017-08-21, 00:00 authored by Tong Zhu, William P. Huhn, Garrett C. Wessler, Donghyeop Shin, Bayrammurad Saparov, David B. Mitzi, Volker BlumRecent
work has identified a non-zinc-blende-type quaternary semiconductor,
Cu2BaSnS4–xSex (CBTSSe), as a promising candidate for thin-film
photovoltaics (PVs). CBTSSe circumvents difficulties of competing
PV materials regarding (i) toxicity (e.g., CdTe), (ii) scarcity of
constituent elements (e.g., Cu(In,Ga)(S,Se)2/CdTe), and
(iii) unavoidable antisite disordering that limits further efficiency
improvement (e.g., in Cu2ZnSnS4–xSex). In this work, we build on
the CBTSSe paradigm by computationally scanning for further improved,
earth-abundant and environmentally friendly thin-film PV materials
among the 16 quaternary systems I2–II–IV–VI4 (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se). The band
structures, band gaps, and optical absorption properties are predicted
by hybrid density-functional theory calculations. We find that the
Ag-containing compounds (which belong to space groups I222 or I4̅2m) show indirect
band gaps. In contrast, the Cu-containing compounds (which belong
to space group P31/P32 and Ama2) show direct or nearly direct band
gaps. In addition to the previously considered Cu2BaSnS4–xSex system,
two compounds not yet considered for PV applications, Cu2BaGeSe4 (P31) and Cu2SrSnSe4 (Ama2), show predicted quasi-direct/direct
band gaps of 1.60 and 1.46 eV, respectively, and are therefore most
promising with respect to thin-film PV application (both single- and
multijunction). A Cu2BaGeSe4 sample, prepared
by solid-state reaction, exhibits the expected P31 structure type. Diffuse reflectance and photoluminescence
spectrometry measurements yield an experimental band gap of 1.91(5)
eV for Cu2BaGeSe4, a value slightly smaller
than that for Cu2BaSnS4.