All‐inorganic CsPbI3 perovskite has emerged as an important photovoltaic material due to its high thermal stability and suitable bandgap for tandem devices. Currently, the cell performance of CsPbI3 solar cells is mainly subject to a large open‐circuit voltage (VOC) deficit. Herein, a multifunctional room‐temperature molten salt, dimethylamine acetate (DMAAc) is demonstrated, which not only directly acts as a solvent for precursor solutions, but also regulates the phase conversion process of the CsPbI3 film for high‐efficiency photovoltaics. DMAAc can stabilize the DMAPbI3 structure and eliminate the Cs4PbI6 intermediate phase, which is easily spatially segregated. Meanwhile, a new homogeneous intermediate phase DMAPb(I,Ac)3 is formed, which finally affords high‐quality CsPbI3 films. With this approach, the charge capture activity of defects in the CsPbI3 film is significantly suppressed. Consequently, a VOC of 1.25 V and >21% power conversion efficiency are achieved, which is the record highest reported thus far. This intermediate phase‐regulation strategy is believed to be applicable to other perovskite material systems.

Black perovskite-phase cesium lead tri-iodide (CsPbI 3 ) has shown great potential in photochemical applications. However, colloidal CsPbI 3 still suffers from the existence of ligands attached to the surface of nanocrystals (NCs) and serious photoluminescence (PL) quenching. Here, we develop two different γ-phase CsPbI 3 (γ-CsPbI 3 ) NCs/ WS 2 heterostructures with two different two-dimensional morphologies of WS 2 (nanoplates and few-layered WS 2 nanosheets) to improve the photocatalytic property of γ-CsPbI 3 NCs. The large surface area and the presence of abundant functional groups on the surface of WS 2 enable a bonding interaction with precursors in the mixture solution, which can reduce the number of ligands and promote the formation and crystal quality of γ-CsPbI 3 NCs. Exhilaratingly, we find that γ-CsPbI 3 NCs fabricated with few-layered WS 2 nanosheets exhibit a significant enhancement in their photocatalytic property, which may result from the increased amount and improved crystal quality of γ-CsPbI 3 NCs and the superior carrier-transport property of few-layered WS 2 nanosheets, and these effects are beneficial for producing abundant hydroxyl radicals to completely degrade methylene blue (MB). The fresh γ-CsPbI 3 NCs/few-layered WS 2 nanosheets show a high photocatalytic degradation efficiency of nearly 100% in 30 min, and completely degrade MB into low-weight and low-toxicity inorganic molecules without any intermediate degradation products.

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