Reaction Pathway for Efficient Cu₂ZnSnSe₄ Solar Cells from Alloyed CuSn Precursor via a Cu‐Rich Selenization Stage

Abstract

The selenization of stacked elemental metallic layers (CuSn–Zn) is a commonly reported approach in kesterite Cu2ZnSnSe4 (CZTSe) processing. CZTSe formation via this approach usually involves a reaction route containing binary selenides, such as SnSe2−x. The high volatility of these phases at the necessary annealing temperatures (500–550 °C) makes this reaction pathway prone to Sn loss, which makes it challenging to control the composition and quality of the grown material. Herein, an approach based on stacked elemental and alloyed precursors is reported, and the benefits of using a Zn/CuSn/Zn configuration are discussed. The absence of nonalloyed elemental Sn helps in suppressing the formation and subsequent evaporation of SnSe2−x phases, preventing Sn loss from the film during selenization. This reaction pathway involves a process scheme which 1) starts with the growth of CZTSe in a “Cu‐rich” environment, 2) includes a shift of the composition by supply of SnSe2−x vapor, and 3) terminates in the “Cu‐poor” regime, leading to device efficiencies above 10%. This composition shift in the presented process appears similar to the final stage of the commonly known CIGSe three‐stage coevaporation.

Vapor-Phase Incorporation of Ge in CZTSe Absorbers for Improved Stability of High-Efficiency Kesterite Solar Cells

“…The amount and purities of the added reactants were 210 ± 2 mg of Se pellets (Mateck, 5N purity), 37.0 ± 0.4 mg of Sn wire (AlfaAesar, 4N purity, i.e., 10.0 ± 0.1 cm wire with 0.25 mm diameter) and, depending on the desired Ge addition, between 0 mg (reference process) and 110 ± 5 mg of Ge flakes (AlfaAesar, 5N purity). The reference process for synthesizing Ge-free absorbers is described in more detail elsewhere [11][12][13]. In brief, the Cu-rich and alloyed precursor layer (Zn/Cu-Sn/Zn) is annealed in presence of elemental Sn wire and Se pellets in a semi-closed graphite box.…”

“…In brief, the Ge is incorporated into the already-formed kesterite-absorber layer via the vapor phase during the high-temperature phase of the selenization process. During this step the composition of the layer is usually shifted from Cu-rich to Cu-poor [11,12]. Since the main kesterite phase formation route is unaltered, the process offers high reproducibility in terms of layer morphology and targeted final-layer composition.…”

Defect Control for 12.5% Efficiency Cu2ZnSnSe4 Kesterite Thin‐Film Solar Cells by Engineering of Local Chemical Environment

“…The group from Carl von Ossietzky University of Oldenburg also reported over 11% efficiency CZTSe solar cells based on Zn/Cu-Sn/Zn precursors with Sn-poor composition. [15,50,51] Their unique alloyed precursor with Zn on the top and unique growth process of shifting composition from Cu-rich (or Sn-poor) to Cu-poor (or Sn-sufficient) regime at high temperature provides a proper Zn-rich (relative to Sn) local chemical environment during the synthesis of CZTSe, [51] which may be attributable for their high-performance devices. However, with such growth process, precisely controlling the amount of Sn supply via SnSe 2−x vapor could be a challenge as over-supply of Sn may deteriorate the surface of CZTSe absorber by forming Sn Zn related defects and/or SnSe 2−x secondary phase.…”

Tuning of Precursor Composition and Formation Pathway of Kesterite Absorbers Using an In‐Process Composition Shift: A Path toward Higher Efficiencies?

“…The increase in the amount of Sn for the reference sample (i.e., 0 s) is found to be approximately 30% (from Cu/Sn = 2.4 to Cu/Sn = 1.89) and matches with the previously reported results. [ 11 ] As reported there, the expected absence of elemental Sn helps to avoid the formation of volatile Sn‐chalcogenides and promotes the kesterite formation mostly via ternary phase. The composition shift for the sample with additional Sn amount (324 s of sputtering, red circles) shows a reduction in Sn content by 13% (Cu/Sn = 1.4 toward Cu/Sn = 1.68), which occurs by evaporation of volatile Sn selenides.…”

“…In this study, we are exploring the possibilities of our previously reported growth process [ 11 ] and present the resulting device properties in composition maps of the as‐grown precursor and final absorber. Various degrees of in‐process composition shifts based on different starting Sn amounts in the precursor are investigated.…”