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Incorporating substitutional elements from groups I, II and IV is an important approach to improve the quality of kesterite thin films. Among the elements, Ag and Ge are most effective in promoting the performance of Cu2ZnSn(S,Se)4 solar cells. Although several theoretical and experimental studies have been conducted on the Ag/Ge-alloyed kesterite, there are significant differences in the reported morphological characteristics, photovoltaic performance and even the optimal Ag/Ge substitution ratio due to different preparation processes utilized, which leads to difficulties in comparing the Ag-incorporated and Ge-incorporated kesterite solar cells. Currently, there is little literature on the comparative study concerned. Herein, we fabricated Ag-alloyed and Ge-alloyed kesterite solar cells based on the same solution method and process conditions, and laterally compared the effects of Ag substitution and Ge substitution on the phase compositions, morphologies, optical bandgaps, minority carrier lifetime and device performance of kesterite Cu2ZnSn(S,Se)4 thin films. [ABSTRACT FROM AUTHOR]

Cation doping is an important approach to improve the performance of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film solar cells. Since different cation dopant has different effects on the defects and crystallization of CZTSSe absorber, it is expected to achieve better device performance via the synergistic effects of double cation co-doping compared with the single cation doping. Herein, we investigated the effects of Ag/Ge co-doping on CZTSSe solar cells by doping Ge in an already optimized Ag-alloyed CZTSSe (ACZTSSe). It is found that incorporating a small amount of Ge can further improve the crystallinity and minority carrier lifetime of ACZTSSe. The low-temperature photoluminescence results reveal that the Ge doping primarily suppressed the Sn Zn defects in ACZTSSe, thereby further boosting the device performance. However, higher Ge doping concentration can lead to a discontinuous surface morphology, which is believed to be related with the relatively low melting temperature and double-layered structure of ACZTSSe absorber. The deteriorated surface morphology results in photocurrent loss in the near-infrared region and decreased fill factor. As the Ge doping ratio increases, there is a balance between the defects and morphology. A champion power conversion efficiency (PCE), 11.4%, was achieved for the Ag/Ge co-doped CZTSSe solar cells at Ge substitution rate of 3%. Compared with the single Ag doping, the average PCE was further promoted by 0.5% due to Ag/Ge co-doping. • Ge doping can further improve the crystallinity and TRPL lifetime of ACZTSSe. • Ge doping primarily suppresses the Sn Zn defects of ACZTSSe. • Ge doping in ACZTSSe leads to discontinuous surface morphology. • Better device performance was achieved by Ag/Ge co-doping. [ABSTRACT FROM AUTHOR]

Large open-circuit voltage deficit is still the main factor restricting the performance of kesterite Cu 2 ZnSn(S,Se) 4 thin film solar cells. The high concentration of Cu Zn defects and [2Cu Zn + Sn Zn ] defect clusters are believed to be the significant origins of severe band tailing of the kesterite absorber. A promising way to improve the open-circuit voltage and power conversion efficiency of the device is substituting Ag+ with a larger ionic radius for Cu+ to suppress the defects and construct a front-graded bandgap. However, the Ag doping in kesterite can significantly lower the melting temperature, which results in difficulties in forming an Ag grading distribution across the absorber due to the strong thermal diffusion. Herein, we innovatively introduce a ZnO insert layer in the precursor film, which is expected to slow down the Ag-Cu interdiffusion during the selenization, thereby forming a desired front-graded bandgap. The Ag grading distribution in absorbers is regulated by the thickness of ZnO insert layer. The effects of ZnO insert layer on the phase compositions, morphologies, optical properties, and photovoltaic performance of Ag-doped kesterite absorbers are investigated. [ABSTRACT FROM AUTHOR]