(581c) Nanoparticle-Based Growth of Chalcogenide Perovskites for Photovoltaics

Chalcogenide chalcopyrite CuIn_1-xGa_XSe₂ (CIGS) and kesterite Cu₂ZnSn(S/Se)₄ (CZTS/Se) thin-film solar cells have recently reported efficiencies of 22.6% and 12.6%, respectively.^1,2 Large-grain absorber growth from the reactive sintering of nanoparticle precursor films in chalcogen atmosphere has been demonstrated as a promising low-cost and scalable solution-based fabrication technique for both materials. However, the major limiting factor of CZTS/Se cells is the loss of charge carriers via recombination during charge transportation and collection. CIGS cells suffer from efficiency decreases due to midgap defect states, dangling bonds, and recombination at interfaces, which is particularly detrimental in nanoparticle-growth based films. A defect tolerant material with efficient charge transport is sought to move thin-film absorber efficiency closer to the Shockley-Queisser limit of 33.7%.

Perovskite (ABX₃) semi-conductors are predicted to have exceptional charge transport due to the framework of corner-sharing [B-X₆]^n- octahedra. Organic-inorganic halide perovskite cells (e.g. CH₃NH₃PbI₃) have reached efficiencies of 25.2%,³ and shown ideal properties such as long electron-hole diffusion lengths, electronically benign grain boundaries, and shallow dominant point defects. However, these organic-inorganic halide perovskites suffer from instability issues under ambient conditions as exposure to heat or moisture will initiate diffusion of the organic cation and breakdown of the crystal structure.

Several chalcogenide perovskites (BaZrS₃, BaZrSe₃, SrZrSe₃, BaHfSe₃, SrHfS₃, SrHfSe3, CaTiS₃, BaHfS₃) have recently been proposed as viable solar absorber candidates via computational screening.⁴ Chalcogenide perovskites exhibit much greater stability than organic-inorganic halide perovskites based on the greater interaction between A²⁺ and [BX₆]^8- in the former compared to A⁺ and [BX₆]^4- in the latter. Reports on the syntheses of these materials are limited (BaZrS₃, SrHfS₃, BaHfS₃) and have only been realized using solid-state methods. The further development of research and commercial applications of these materials depends heavily on the discovery of simple, scalable solution-based syntheses.

Here, we present the straight-forward aqueous synthesis of oxide perovskite nanocrystals [ABO₃, where A = Ba, Sr and B = Zr, Hf, Sn] and investigate their subsequent reactive annealing to form corresponding thin-film chalcogenide perovskites [AB(S,Se)₃]. X-ray diffraction and electron microscopy is used to characterize phase transitions and grain growth during reactive annealing. Optoelectronic characterization, including photoluminescence, absorption coefficient, and carrier lifetimes, is used to characterize the resulting films. Stability of the films was measured by repeating optoelectronic measurements after prolonged exposure to ambient conditions. This work demonstrates the feasibility of producing stable chalcogenide perovskite thin-films with favorable optoelectronic properties from solution-based nanoparticle growth techniques.

1. Wang, W. et al. Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Adv. Energy Mater. 4, 1–5 (2014).
2. Jackson, P. et al. Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%. Phys. Status Solidi - Rapid Res. Lett. 10, 583–586 (2016).
3. Champion Module Efficiencies. National Renewable Energy Laboratory (2020). Available at: https://www.nrel.gov/pv/module-efficiency.html.
4. Sun, Y.-Y., Agiorgousis, M. L., Zhang, P. & Zhang, S. Chalcogenide Perovskites for Photovoltaics. Nano Lett. 15, 581–585 (2015).