A main concern of the promising DMF‐based Cu2ZnSn(Sx,Se1‐x)4 (CZTSSe) solar cells lies in the absence of a large‐grain spanning structure, which is a key factor for high open‐circuit voltage (Voc) and power conversion efficiency (PCE). A new strategy to achieve CZTSSe large‐grain spanning monolayer is proposed, by taking advantage of the synergistic optimization with a Cu2+ plus Sn2+ redox system and pre‐annealing temperatures. A series of structural, morphological, electrical, and photoelectric characterizations are employed to study the effects of the pre‐annealing temperatures on absorber qualities, and an optimized temperature of 430 ℃ is determined. The growth mechanism of the large‐grain spanning monolayer and the effect of redox reaction rate are carefully investigated. Three types of absorber growth mechanisms and a concept of critical temperature are proposed. The devices based on this large‐grain spanning monolayer suppress the recombination of carriers at crystal boundaries and interfaces. The champion device exhibits a high Voc (>500 mV) and PCE of 11.76%, which are both the maximum values among DMF‐based solar cells at the current stage.
Abundant intrinsic defects and defect clusters in Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells lead to severe nonradiative recombination and limited photoelectric performance. Therefore, developing effective method to suppress the detrimental defects is the key to achieve high-efficiency solar cell. Herein, a convenient two-step cooling strategy in selenization process is reported to suppress the Cu Zn and Sn Zn defects and defect clusters synergistically. The results show that rapid cooling during section from selenization temperature to turning temperature can inhibit the volatilization of Sn and restrain the corresponding Sn-related defects, while slow cooling during the subsequent temperature section can reduce the degree of Cu-Zn disorder. Benefitting from the synergistic effect of two-step cooling, a significantly lowered concentration of Sn Zn and Cu Zn defect and their defect clusters [2Cu Zn +Sn Zn ] in absorber is observed, meanwhile, a reduced band tailing effect and promoted carrier collection efficiency of the photovoltaic device is obtained. Finally, a device with improved open-circuit voltage (V oc ) of 505.5 mV and efficiency of 12.87% is achieved. This study demonstrates the impact of cooling process on defects controlling for the first time and provides a simple and effective new strategy for intrinsic defect control, which may be universal in other inorganic thin film solar cells.
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