Co-Solvent Engineering for Solution-Processed CZTSSe on a Transparent Substrate

Recent breakthroughs in efficiencies of up to 14.9%¹ for Cu₂ZnSn(S,Se)₄ (CZTSSe) solar devices have reignited interest in CZTSSe as an inorganic thin-film solar absorber. Traditionally, Mo-coated soda lime glass (SLG) substrates have been the standard for chalcogenide thin-film photovoltaics (PVs). Such substrates, however, can hinder the applications of thin-film PV, namely, semi-transparent and bifacial irradiation for building integrated PV applications. In this study, we explore the deposition of CZTSSe directly onto fluorine-doped tin oxide (FTO) by solution-based methods. The precursor solutions containing copper(II) chloride dihydrate, tin(II) chloride, zinc chloride and thiourea are formulated by employing mixtures of dimethyl formamide (DMF) and isopropanol (IPA).^2,3 We also investigate the effect of adopting a one- or two-step heat treatment process at each layer during spin coating, prior to the reactive annealing step in the presence of Se. The effect of solvent composition and adopting an additional heat treatment step have been studied here by exploring the thin-film crystallinity, composition, secondary phase formation, and device performance, featuring CdS as buffer layer. Our data shows that solutions with a DMF:IPA mixture of 1:3 result in the most homogeneous grain growth and suppression of secondary phases, with a champion front illumination power conversion efficiency of 5.2%. We rationalise device performance in terms of the topography and surface electronic properties, as probed by energy-filtered photoemission electron microscopy.^4,5<br/><br/>1. NREL, Best Research-Cell Efficiency Chart, 2024<br/>2. D. Tiwari, T. Koehler, X. Lin, R. Harniman, I. Griffiths, L. Wang, D. Cherns, R. Klenk, David J. Fermin, <i>Chem. Mater.</i> 2016, <b>28</b>, 4991−4997<br/>3. R. E. Agbeneyke, J. Keynon, A. Sheppard, N. Benhaddou, N. Fleck, M. A. Alkhalifah, D. Tiwari, J. W. Bowers, D. J. Fermin, <i>in progress</i>, 2024<br/>4. D. Tiwari, M. Cattelan, R. L. Harniman, A. Sarua, A. Abbas, J. W. Bowers, N. A. Fox, D. J. Fermin, <i>iScience</i>, 2018, <b>9</b>, 36-46<br/>5. D. Tiwari, M. Cattelan, R. L. Harniman, A. Sarua, N. A. Fox, T. Koehler, R. Klenk, D. J. Fermin, <i>ACS Energy Lett.</i>, 2018, <b>3</b>, 2977-2982