The mixed caesium and formamidinium lead triiodide perovskite system (Cs1-xFAxPbI3) in the form of quantum dots (QDs) offers a new pathway towards stable perovskite-based photovoltaics and optoelectronics. However, it remains challenging to synthesize such multinary QDs with desirable properties for high-performance QD solar cells (QDSCs). Here we report an effective ligand-assisted cation exchange strategy that enables controllable synthesis of Cs1-xFAxPbI3 QDs across the whole composition range (x: 0-1), which is inaccessible in large-grain polycrystalline thin films. The surface ligands play a key role in driving the cross-exchange of cations for the rapid formation of Cs1-xFAxPbI3 QDs with suppressed defect density. The hero Cs0.5FA0.5PbI3 QDSC achieves a certified record power conversion efficiency (PCE) of 16.6% with negligible hysteresis. We further demonstrate that QD devices exhibit substantially enhanced photostability compared to their thin film counterparts because of the suppressed phase segregation, retaining 94% of the original PCE under continuous 1-sun illumination for 600 hours.

Lead halide perovskites have witnessed significant progress in low-cost and high-efficiency photovoltaics, with a rapid increase in photovoltaic efficiencies from 3.8% to a certified record of 25.2% in the past decade. [1-4] However, the viability and practical scale-up implementation are limited by the stability and toxicity of the lead halide perovskites. [5,6] To circumvent these two

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The phenethylammonium cation significantly promotes the formation of fully-covered thin-films of hybrid bismuth organohalides with low surface roughness and excellent stability.

Charge carrier transport in materials is of essential importance for photovoltaic and photonic applications. Here, the authors demonstrate a controllable acceleration or deceleration of charge carrier transport in specially structured metal‐alloy perovskite (MACs)PbI3 (MA= CH3NH3) single‐crystals with a gradient composition of CsPbI3/(MA1−xCsx)PbI3/MAPbI3. Depending on the Cs‐cation distribution in the structure and therefore the energy band alignment, two different effects are demonstrated: i) significant acceleration of electron transport across the depth driven by the gradient band alignment and suppression of electron–hole recombination, benefiting for photovoltaic and detector applications; and ii) decelerated electron transport and thus improved radiative carrier recombination and emission efficiency, highly beneficial for light and display applications. At the same time, the top Cs‐layer results in hole localization in the top layer and surface passivation. This controllable acceleration and deceleration of electron transport is critical for various applications in which efficient electron–hole separation and suppressed nonradiative electron–hole recombination is demanded.