Growth and Photovoltaic Device Application of Cu₂BaGe_1–xSn_xSe₄ Films Prepared by Selenization of Sequentially Deposited Precursors

Abstract

Toward suppressing the formation of anti-site defects and related defect clusters in kesterite Cu 2 ZnSnS 4−x Se x (CZTS) absorber films, Cu 2 −II−IV−X 4 (II = Sr, Ba; IV = Ge, Sn; X = S, Se) compounds have recently been introduced. Cu 2 BaGe 1−x Sn x Se 4 (CBGTSe) is one of the materials that belongs to this chalcogenide family, with a tunable band gap (1.6 eV ≤ E g ≤ 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%. KEYWORDS: Cu 2 BaGe 1−x Sn x Se 4 , CBGTSe, chalcogenide, thin film, solar cell

Ag Alloying in Cu2−yAgyBa(Ge,Sn)Se4 Films and Photovoltaic Devices

“…It is noteworthy that such improvement with annealing has also been reported for other copper chalcogenidebased solar cells-that is, CZTS, [40][41][42] CBTSSe, [8] CBGSe, [10] and CBGTSe. [11] The improvement has been attributed to modification of heterojunction properties [40,41] and the passivation of grain boundaries by oxides (e.g., SnO x ). [8,42] J-V and EQE curves for the best-performing postannealed ACBGTSe solar cells are shown in Figure 6, with the corresponding solar cell performance parameters summarized in Table 1.…”

Spiers Memorial Lecture: Next generation chalcogenide-based absorbers for thin-film solar cells

“…CBGTSe films ( x = 0.5–0.6) have been prepared by selenizing Cu–Ba–Ge–Sn precursor multilayers that were sequentially deposited using vacuum-based techniques, including both sputtering (for Cu, Ge and Sn) and evaporation (for Ba). 137 Vacuum-deposition of related Cu 2 –II–IV–X 4 films ( i.e. , CBTS, 106,132,138 CBTSSe, 105,139,140 and CSTS 141 ) to yield functioning solar cells has only been demonstrated using the co-sputtering approach.…”

“…, Ba). 142 Kim and Mitzi’s study 137 showed that such issues can be prevented using a high temperature vacuum pre-annealing step to partially homogenize the metallic precursor layer. The CBGTSe films exhibit a PL peak centered at 1.67 eV at room temperature, which is at a lower energy than for CBGSe (1.96 eV), reflecting an ∼0.3 eV bandgap reduction via partial substitution of Ge with Sn.…”

“…CBGTSe lms (x ¼ 0.5-0.6) have been prepared by selenizing Cu-Ba-Ge-Sn precursor multilayers that were sequentially deposited using vacuumbased techniques, including both sputtering (for Cu, Ge and Sn) and evaporation (for Ba). 137 Vacuum-deposition of related Cu 2 -II-IV-X 4 lms (i.e., CBTS, 106,132,138 CBTSSe, 105,139,140 and CSTS 141 ) to yield functioning solar cells has only been demonstrated using the co-sputtering approach. The challenges associated with high quality lm growth from sequential deposition of elemental layers have been attributed in part to issues related to the alkaline earth element (e.g., Ba).…”

“…The challenges associated with high quality lm growth from sequential deposition of elemental layers have been attributed in part to issues related to the alkaline earth element (e.g., Ba). 142 Kim and Mitzi's study 137 showed that such issues can be prevented using a high temperature vacuum pre-annealing step to partially homogenize the metallic precursor layer. The CBGTSe lms exhibit a PL peak centered at 1.67 eV at room temperature, which is at a lower energy than for CBGSe (1.96 eV), reecting an $0.3 eV bandgap reduction via partial substitution of Ge with Sn.…”