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.

Crystallographic orientations play a crucial part in optoelectronic properties of crystalline materials. This paper reported significant enhancement of optoelectronic properties in γ-phase copper silver iodide (γ-Cu x Ag 1−x I, 0.6 ≤ x ≤ 0.9) films compared with Cu-poor (0.1 ≤ x ≤ 0.4) counterparts due to highly (111)-preferred orientation and larger grains. Transient surface photovoltage spectroscopy reveals that Cu x Ag 1−x I (0.6 ≤ x ≤ 0.9) exhibits faster transferring rate and slower recombination rate of charge carriers than Cu-poor ones. The preferred orientation significantly influenced the photoelectric properties by reducing photocarrier recombination. In X-ray diffraction patterns and Rietveld results, it was displayed that Cu x Ag 1−x I (x = 0.1−0.9) films exhibit (111)-preferred orientation and higher crystallinity when Cu doping content increased. Scanning electron microscope electron channeling contrast imaging analysis demonstrated (111) with most grains with larger size in Cu-rich films compared with Cu-poor films that were grown in the step flow growth mode from (111)-preferred prisms defined as basic units. This paper may provide a valuable basis for Cu x Ag 1−x I with tunable band gaps, prepared via a simple route of a direct metal surface elemental reaction, applied in the hole transport layer or buffer layer in thin-film solar cells to achieve higher efficiencies.