In this work, we describe details of a two-step deposition approach that enables the preparation of continuous and well-structured thin films of Cs 2 SnI 6 , which is a one-half Sndeficient 0-D perovskite derivative (i.e., the compound can also be written as CsSn 0.5 I 3 , with a structure consisting of isolated SnI 6 4− octahedra). The films were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UV−vis spectroscopy, photoluminescence (PL), photoelectron spectroscopy (UPS, IPES, XPS), and Hall effect measurements. UV−vis and PL measurements indicate that the obtained Cs 2 SnI 6 film is a semiconductor with a band gap of 1.6 eV. This band gap was further confirmed by the UPS and IPES spectra, which were well reproduced by the calculated density of states with the HSE hybrid functional. The Cs 2 SnI 6 films exhibited n-type conduction with a carrier density of 6(1) × 10 16 cm −3 and mobility of 2.9(3) cm 2 /V•s. While the computationally derived band structure for Cs 2 SnI 6 shows significant dispersion along several directions in the Brillouin zone near the band edges, the valence band is relatively flat along the Γ−X direction, indicative of a more limited hole minority carrier mobility compared to analogous values for the electrons. The ionization potential (IP) and electron affinity (EA) were determined to be 6.4 and 4.8 eV, respectively. The Cs 2 SnI 6 films show some enhanced stability under ambient air, compared to methylammonium lead(II) iodide perovskite films stored under similar conditions; however, the films do decompose slowly, yielding a CsI impurity. These findings are discussed in the context of suitability of Cs 2 SnI 6 for photovoltaic and related optoelectronic applications.

Chalcogenides such as CdTe, Cu­(In,Ga)­(S,Se)2 (CIGSSe), and Cu2ZnSn­(S,Se)4 (CZTSSe) have enabled remarkable advances in thin-film photovoltaic performance, but concerns remain regarding (i) the toxicity (CdTe) and (ii) scarcity (CIGSSe/CdTe) of the constituent elements and (iii) the unavoidable antisite disordering that limits further efficiency improvement (CZTSSe). In this work, we show that a different materials class, the BaCu2SnSe x S4–x (BCTSSe) system, offers a prospective path to circumvent difficulties (i–iii) and to target new environmentally friendly and earth-abundant absorbers. Antisite disordering and associated band tailing are discouraged in BCTSSe due to the distinct coordination environment of the large Ba2+ cation. Indeed, an abrupt absorption edge and sharp associated photoluminescence emission demonstrate a reduced impact of band tailing in BCTSSe relative to CZTSSe. Our combined experimental and computational studies of BCTSSe reveal that the compositions 0 ≤ x ≤ 4 exhibit a tunable nearly direct or direct bandgap in the 1.6–2 eV range, spanning relevant values for single- or multiple-junction photovoltaic applications. For the first time, a prototype BaCu2SnS4-based thin-film solar cell has been successfully demonstrated, yielding a power conversion efficiency of 1.6% (0.42 cm2 total area). The systematic experimental and theoretical investigations, combined with proof-of-principle device results, suggest promise for BaCu2SnSe x S4–x as a thin-film solar cell absorber.

IntroductionChalcogenide photovoltaic (PV) materials such as CdTe [1,2] and Cu(In,Ga)Se 2 (CIGSe) [3][4][5] have enabled remarkable progress in thin-film PV device performance, with each technology exceeding the 20% power conversion efficiency (PCE) barrier. However, two major concerns remain regarding these technologies-i.e., the negative environmental impacts of Cd Application of zinc-blende-related chalcogenide absorbers such as CdTe and Cu(In,Ga)Se 2 (CIGSe) has enabled remarkable advancement in laboratory-and commercial-scale thin-film photovoltaic performance; however concerns remain regarding the toxicity (CdTe) and scarcity (CIGSe/CdTe) of the constituent elements. Recently, kesterite-based Cu 2 ZnSn(S,Se) 4 (CZTSSe) materials have emerged as attractive non-toxic and earth-abundant absorber candidates. Despite the similarities between CZTSSe and CIGSe/CdTe, the record power conversion efficiency of CZTSSe solar cells (12.6%) remains significantly lower than that of CIGSe (22.6%) and CdTe (22.1%) devices, with the performance gap primarily being attributed to cationic disordering and associated band tailing. To capture the promise of kesterite-like materials as prospective "drop-in" earth-abundant replacements for closely-related CIGSe, current research has focused on several key directions to control disorder, including: (i) examination of the interaction between processing conditions and atomic site disorder, (ii) isoelectronic cation substitution to introduce ionic size mismatch, and (iii) structural diversification beyond the zinc-blendetype coordination environment. In this review, recent efforts targeting accurate identification and engineering of anti-site disorder in kesterite-based CZTSSe are considered. Lessons learned from CZTSSe are applied to other complex chalcogenide semiconductors, in an effort to develop promising pathways to avoid anti-site disordering and associated band tailing in future high-performance earth-abundant photovoltaic technologies.

In recent years, Cu ZnSn(S,Se) (CZTSSe) materials have enabled important progress in associated thin-film photovoltaic (PV) technology, while avoiding scarce and/or toxic metals; however, cationic disorder and associated band tailing fundamentally limit device performance. Cu BaSnS (CBTS) has recently been proposed as a prospective alternative large bandgap (~2 eV), environmentally friendly PV material, with ~2% power conversion efficiency (PCE) already demonstrated in corresponding devices. In this study, a two-step process (i.e., precursor sputter deposition followed by successive sulfurization/selenization) yields high-quality nominally pinhole-free films with large (>1 µm) grains of selenium-incorporated (x = 3) Cu BaSnS Se (CBTSSe) for high-efficiency PV devices. By incorporating Se in the sulfide film, absorber layers with 1.55 eV bandgap, ideal for single-junction PV, have been achieved within the CBTSSe trigonal structural family. The abrupt transition in quantum efficiency data for wavelengths above the absorption edge, coupled with a strong sharp photoluminescence feature, confirms the relative absence of band tailing in CBTSSe compared to CZTSSe. For the first time, by combining bandgap tuning with an air-annealing step, a CBTSSe-based PV device with 5.2% PCE (total area 0.425 cm ) is reported, >2.5× better than the previous champion pure sulfide device. These results suggest substantial promise for the emerging Se-rich Cu BaSnS Se family for high-efficiency and earth-abundant PV.

CH3NH3PbI3 films with micrometer grains and microsecond carrier lifetimes are prepared through additive engineering.