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  Suresh, S.; Abou-Ras, D.; Chowdhury, T.H.; Uhl, A.R.: Over 13% Efficient, Ambient Air-Processed CuIn(S,Se)₂ Solar Cells via Compositional Engineering of Molecular Inks. Solar RRL 7 (2023), p. 2300437/1-7

10.1002/solr.202300437
Open Access Version

Abstract:
A dimethylformamide (DMF) and thiourea (TU)-based ink deposition route is used to fabricate narrow bandgap (≈1.0 eV) CuIn(S,Se)2 (CISSe) films with Cu-poor ([Cu]/[In] = 0.85), stoichiometric ([Cu]/[In] = 1.0), and Cu-rich ([Cu]/[In] = 1.15) compositions for photovoltaic applications. Characterization of KCN- or (NH4)2S-treated Cu-rich absorber films using X-ray diffraction and scanning electron microscopy confirms the removal of copper-selenide phases from the film surface, while electron backscatter diffraction measurements and depth-dependent energy-dispersive X-ray spectroscopy indicate remnant copper-selenides in the absorber layer bulk. Contrary to best practice for vacuum-processed cells, optimum [Cu]/[In] ratios appear to be stoichiometric, rather than Cu-poor, in DMF–TU-based CISSe devices. Accordingly, stoichiometric film compositions yield large-grained (≈2 μm) absorber layers with smooth absorber surfaces (root mean square roughness <20 nm) and active area device efficiencies of 13.2% (without antireflective coating). Notably, these devices reach 70.0% of the Shockley–Queisser limit open-circuit voltage (i.e., 526 mV at Eg of 1.01 eV), which is among the highest for ink-based CISSe devices.