Growth and Photovoltaic Device Application of Cu2BaGe1–xSnxSe4 Films Prepared by Selenization of Sequentially Deposited Precursors

Toward suppressing the formation of anti-site defects and related defect clusters in kesterite Cu2ZnSnS4–xSex (CZTS) absorber films, Cu2–II–IV–X4 (II = Sr, Ba; IV = Ge, Sn; X = S, Se) compounds have recently been introduced. Cu2BaGe1–xSnxSe4 (CBGTSe) is one of the materials that belongs to this chalcogenide family, with a tunable band gap (1.6 eV ≤ Eg ≤ 1.9 eV, based on the x value) having been demonstrated for bulk samples. In this report, we demonstrate the deposition of CBGTSe films and report the first solar cells based on CBGTSe as a light absorber. CBGTSe films have been fabricated from selenization of Cu–Ba–Ge–Sn precursor layers sequentially deposited using a combination of vacuum-based sputtering (for Cu, Sn, and Ge) and evaporation (for Ba) techniques. We observe that the films prepared by direct selenization of as-deposited precursors exhibit blisters, originating from partial delamination among precursor layers during the selenization process. Introducing a vacuum pre-annealing step mitigates this issue and enables the formation of high-quality CBGTSe films. Also, ex situ film growth studies show that the Cu content correlates with the formation of a Cu-rich intermediate phase and plays an important role in the formation of single-phase CBGTSe films with large grain sizes. Finally, our first prototype CBGTSe solar cells exhibit a maximum power conversion efficiency of 3.06%.