Investigation of combinatorial coevaporated thin film Cu2ZnSnS4 (II): Beneficial cation arrangement in Cu-rich growth

Abstract

Cu2ZnSn(S,Se)4 (CZTSSe) is an earth-abundant semiconductor with potential for economical photovoltaic power generation at terawatt scales. In this work, we use Raman scattering to investigate phase coexistence in combinatorial CZTS thin films grown at 325 or 470 °C. The surface of the samples grown at 325 °C is rough except for a prominent specularly reflective band near and along the ZnS-Cu2SnS3 (CTS) tie line in the Cu-Zn-Sn-S quaternary phase diagram. All structurally incoherent secondary phases (SnS2, CuS) exist only as surface phases or are embedded as separate grains, whereas the structurally coherent secondary phase CTS coexists with CZTS in the dense underlying film. In films grown at 325 °C, which are kinetically trapped by the low growth temperature, a change is observed in Cu and Sn site occupancy, evidenced by the shift from cubic-CTS in the Cu-rich region (Cu/Sn > 2) to more tetragonal-CTS in the Sn-rich region (Cu/Sn < 2). For CZTS samples grown at 470 °C, CTS is not observed and regions grown under excess Sn flux are more disordered than Cu-rich regions evidenced by broader CZTS A mode peaks. Therefore, increasing Sn chemical potential results in more CZTS lattice disorder, suggesting, with other evidence, the formation of Sn antisite defects. In contrast, the CZTS A mode breadth is insensitive to Zn richness suggesting that excess Zn does not induce significant disorder within the CZTS lattice. We postulate that initially growing CZTS films Cu-rich (Cu/Sn > 2) results in higher cation ordering meaning fewer antisite defects.

Photovoltaic materials: Cu2ZnSnS4 (CZTS) nanocrystals synthesized via industrially scalable, green, one‐step mechanochemical process

“…The values are in a good agreement with the experimental data reported previously for CZTS phase. 16,36,37 Small amounts of exsolved Cu 2-x S phases (≈470 cm −1 ) typically formed as an impurity during the fabrication of CZTS may be present in the assynthesized powders (Figure 4A,C).…”

Properties of Cu2ZnSnS4Films by Sulfurization of CuS-SnS-ZnS Precursors Using Ditert-butylsulfide at Atmospheric Pressure

“…• C. 15,16 It can be depicted as follows: In Fig. 1, there are weak (111) SnS and (311) SnS signatures in the CZTS films sulfurized at 500 and 550…”

“…Specifically, it follows the reported reaction pathways of forming the CZTS phase from the binary sulfides and the potential ternary phases created at low temperatures, as described by the sequential reactions: 15,16 2CuS + SnS + ZnS → Cu 2 SnS 3 + ZnS → Cu 2 ZnSnS 4 [1] E. A. Lund et al 16 showed the CuS-ZnS-SnS pseudo-ternary phase diagram (at 325…”

“…1, since there are only XRD peaks of CuS, SnS and ZnS binary phases in the as-deposited films, it is reasonable to assert that the formation of the CZTS phase in the DTBS sulfurized films at low temperatures primarily follows the sequential reactions 1 and CuS-ZnS-SnS system. 16 It is known that very close interplanar spacing occur in certain planes of different binary sulfides such as d(102) CuS ∼ d(111) ZnS and d(103) CuS ∼ d(111) SnS . One can hardly distinguish whether there is one or two different binary sulfides in the film based on the θ-2θ XRD data of a DTBS-sulfurized CZTS film (Fig.…”

Optimization of Cu2ZnSnS4 Absorber Layers via Sulfurization of Electrodeposited Cu-Zn-Sn Precursors for Photovoltaic Applications

“…[20,39,[43][44][45]. The Sn-S subsystem includes three possible phases: SnS, Sn 2 S 3 , SnS 2 ; all these phases can be present between room temperature and 600 °C [46][47][48].…”