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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]

Crystallization growth occupies a crucially important role in terms of impacting the film microstructure and device characteristics of the kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cell. Previous studies frequently observed that the selenized CZTSSe film consists of a multiple-layer structure with a large-grained top layer and a fine-grained bottom layer. Herein, a novel strategy is proposed for adjusting spin-coated layers and conducting two-step selenization to promote the fine-grain layers growth of Bi-doped CZTSSe (CZTBiSSe) films for high-performance solar cells. This study systematically investigates the effects of spin-coated layers and the selenization process on growth kinetics of the CZTBiSSe films. By comparison, the optimal spin-coated layer can be identified as "7 + 3", the CZTBiSSe film has a bilayer large grain structure from up to down, which reduces the grain boundaries, thus reducing the charge recombination loss, and is more conducive to charge carrier transfer. Finally, a CZTBiSSe solar cell with the highest efficiency of 7.27 % was obtained, which increased by 36 % more than one prepared using traditional methods (5.34 %). [ABSTRACT FROM AUTHOR]

We fabricated high-quality n-type Ag 2 ZnSnSe 4 (AZTSe) film with kesterite structure by using a simple solution method. A Cu 2 ZnSnSe 4 (CZTSe)/AZTSe-based solar cell was designed and prepared by inserting AZTSe layer between CZTSe and CdS of the traditional CZTSe-based solar cell. Compared with the traditional device, an increase from 337 to 432 mV in open circuit voltage (V oc) and an accompanying rise from 3.40% to 4.72% in power conversion efficiency (PCE) were observed. To well understand the PCE improvement of the CZTSe/AZTSe-based solar cells, we calculated the band alignments of CZTSe/AZTSe and CZTSe/CdS heterojunctions using first-principles calculations, demonstrating that the CZTSe/AZTSe and CZTSe/CdS interfaces have type-II and type-I band alignments, respectively. Moreover, the band offset of AZTSe/CdS is lager than the one of CZTSe/CdS. Combined with the calculation results, the mechanism of influence of the AZTSe on the PCE improvement is discussed in detail. Our conclusions show that the addition of the AZTSe layer is a potentially applicable method to obtain CZTSe-based solar cells with higher V oc and PCE. [Display omitted] [ABSTRACT FROM AUTHOR]