Optimizing the Ratio of Sn⁴⁺ and Sn²⁺ in Cu₂ZnSn(S,Se)₄ Precursor Solution via Air Environment for Highly Efficient Solar Cells

Abstract

The use of different Sn valence states (such as Sn4+ and Sn2+) in the Cu2ZnSn(S,Se)4 (CZTSSe) precursor solution is especially important for the quality of the subsequent growth of the CZTSSe films. The latest study has found that replacing SnCl2·2H2O with anhydrous SnCl4 can remarkably improve the performance of CZTSSe solar cells, but it needs to be operated in the glovebox. Herein, for the precursor solution, SnCl4·5H2O powder is used instead of anhydrous SnCl4 in air environment, and the proportion of Sn4+ and Sn2+ precursor solutions is further systematically studied. When the ratio of Sn4+ to Sn2+ is 1:1, a uniform, compact, and noncracking CZTSSe thin film is obtained, effectively alleviating the interface recombination and reducing the concentration of deep‐level defects. In particular, the concentration of CuZn antisite defects is decreased by an order of magnitude, and the carrier recombination and band tail effect are alleviated. When JSC is maintained, VOC and FF are considerably improved. Finally, CZTSSe thin‐film solar cells are fabricated with an efficiency of over 11%. Herein, the feasibility of controlling the ratio of Sn4+ to Sn2+ in the CZTSSe precursor solution for higher efficiency of CZTSSe thin‐film solar cells is demonstrated.

Grain Growth Mechanism of CZTSSe Films Controlled by the Evaporation Area of Se

“…[10] Despite this, the efficiency of solutionprocessed Cu 2 ZnSn(S, Se) 4 solar cells still remains much lower than the detailed balance limiting efficiency of 32%, which is hindered by the severe nonradiative recombination and large Voc def resulting from the imperfect crystal quality. [11][12][13][14][15] As the most important process affecting the crystallinity of thin CZTSSe films, selenization needs to be further studied. In general, solid Se particles are widely used as Se source for the postselenization of CZTSSe because of the low toxicity and relatively higher safety.…”

Ag, Ti dual-cation substitution in Cu2ZnSn(S,Se)4 induced growth promotion and defect suppression for high-efficiency solar cells

“…For the kesterite structured CZTSSe system, the main reasons accounting for the severe V OC decit (V OC,def ) include the band tail states generated by the high concentration of deep-level defects and defect clusters 4,6 (e.g., Cu Zn , Cu Sn , and [2Cu Zn + Sn Zn ]), the formation of secondary phases 7,8 (e.g., CuS, SnS 2 and ZnS) during the crystal growth process, and the mismatching energy band alignments between various functional layers. [9][10][11] Several approaches have been attempted to mitigate these issues, including cationic substitution, 12,13 grain growth optimization, [14][15][16] heterojunction post-annealing, 17 graded bandgap engineering, 18 and so on.…”

“…, CuS, SnS 2 and ZnS) during the crystal growth process, and the mismatching energy band alignments between various functional layers. 9–11 Several approaches have been attempted to mitigate these issues, including cationic substitution, 12,13 grain growth optimization, 14–16 heterojunction post-annealing, 17 graded bandgap engineering, 18 and so on.…”

Progress and prospectives of solution-processed kesterite absorbers for photovoltaic applications

“…8f), which reduces the concentration of deep defects, fortunately, achieving efficiency of over 11%. 34 In addition, Zhao et al reported a simple and environmentally friendly additive strategy by adding water additives, which can improve the homogeneity and thermogravimetric characteristics of the precursor solution by adjusting the particle size and coordination behavior of the precursor solution, achieving a certified CZTSSe device with a PCE of 12.07%. 140 In 2022, an efficiency of 12.87% was achieved using a two-step cooling strategy, which can suppress the Cu Zn and Sn Zn defects and defect clusters synergistically.…”

“…Currently, various environmentally friendly, safe, stable and high-efficiency solvent systems have been successfully developed, including dimethyl sulfoxide (DMSO), N , N -dimethylformamide (DMF), ethylene glycol methyl ether (EGME), thioglycolic acid and ammonia (TGA), and ethylenediamine/ethanedithiol system (EN/EDT). 31–35 Both vacuum method and solution processes have resulted in significant improvements in CZTSSe device performance, 36–39 and the development of PCE evolution is shown in Fig. 1.…”