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In recent years, the primary reasons for low efficiency Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells have been attributed to Sn Zn defects and related defect clusters, as well as the balance of absorption layer bandgap for short-circuit current density (J sc) and open-circuit voltage (V oc). Ge gradient has been theoretically proven to be an effective strategy to change traditional flat bandgap structure and suppress Sn Zn defects and related defect clusters for improving the device performance of CZTSSe solar cells. However, the potential of Ge gradient has not been fully verified. In this work, Ge doping effectively reduces the formation of Sn Zn defects and [2Cu Zn + Sn Zn ] defect clusters in the CZTSSe absorption layer. In addition, it can be found that the construction of the V-type bandgap is the best solution for balancing between V oc and J sc. By using the bandgap grading strategy, the disparity between the recombination barrier of the junction and the optical bandgap's minimum value is widened, thereby obtaining a large V oc. The V-type doped device has the highest efficiency when V oc is 0.45 V, which can reach 8.03%. This Ge graded substitution method provides an alternative absorption layer structure for improving V oc and the preparation of future high-efficiency kesterite photovoltaic devices. [ABSTRACT FROM AUTHOR]

(Na 0.1 Cu 0.9) 2 ZnSn(S,Se) 4 thin films with a single kesterite phase were synthesized using a sol-gel spin-coating method accompanied by rapid post-annealing. In this study, we investigated the effect of selenization time on the crystal quality and photoelectric performance of the (Na 0.1 Cu 0.9) 2 ZnSn(S,Se) 4 films. It was found that the crystallinity and morphology of the films was enhanced, and some of bigger Se substituted for the S site in (Na 0.1 Cu 0.9) 2 ZnSn(S,Se) 4 with increasing the selenization time. The bandgap of the film can be regulated from 1.04 eV to 0.99 eV by varying the selenization time. In addition, all films showed p-type conductive characteristics, and films with optimal electrical performance could be obtained by optimizing the selenization time. Finally, the (Na 0.1 Cu 0.9) 2 ZnSn(S,Se) 4 thin film with the best crystal quality and optical-electrical characteristics was obtained at an optimized selenization time of 15 min. A high power conversion efficiency (PCE) of 3.92% was obtained for the (Na 0.1 Cu 0.9) 2 ZnSn(S,Se) 4 device, which is 42% higher compared to that of the undoped Cu 2 ZnSn(S,Se) 4 (CZTSSe) device. [ABSTRACT FROM AUTHOR]

We have synthesized Cu 2 Mg x Zn 1–x Sn(S,Se) 4 (0 ≤ x ≤ 0.6) thin films by a facile sol-gel method, and studied the influence of Mg concentration on the crystal structure, surface morphology and photoelectric performance of Cu 2 Mg x Zn 1–x Sn(S,Se) 4 thin films systematically. It was shown that the smaller Zn 2+ in Kesterite phase Cu 2 ZnSn(S,Se) 4 will be replaced by larger Mg 2+ , forming uniform pure phase Cu 2 Mg x Zn 1–x Sn(S,Se) 4 . The band gap of Cu 2 Mg x Zn 1–x Sn(S,Se) 4 films can be adjusted from 1.12 to 0.88 eV as the x value changes from 0 to 0.6. Furthermore, the Cu 2 Mg x Zn 1–x Sn(S,Se) 4 thin films with large grain size, smooth surface and less grain boundaries was obtained at an optimized condition of x = 0.2. The carrier concentration of Cu 2 Mg x Zn 1–x Sn(S,Se) 4 thin film reaches the maximum 6.47 × 10 18 cm −3 at x = 0.2, which is a potential material to be the absorption layer of high efficiency solar cells. [ABSTRACT FROM AUTHOR]