Conventional wisdom is that the MAI and FAI are stable in the solution, but actually they are not. We demonstrated that the MAI first deprotonated to form methylamine (MA), and then MA reacted with FAI to form two condensation products N-methyl FAI and N, N 0 -dimethyl FAI. Moreover, triethyl borate was introduced to stabilize the perovskite precursor solution, which significantly reduced the impure phase in the perovskite film and enhanced the device performance and reproducibility.

Inserting an ultrathin low-conductivity interlayer between the absorber and transport layer has emerged as an important strategy for reducing surface recombination in the best perovskite solar cells. However, a challenge with this approach is a trade-off between the open-circuit voltage ( V oc ) and the fill factor (FF). Here, we overcame this challenge by introducing a thick (about 100 nanometers) insulator layer with random nanoscale openings. We performed drift-diffusion simulations for cells with this porous insulator contact (PIC) and realized it using a solution process by controlling the growth mode of alumina nanoplates. Leveraging a PIC with an approximately 25% reduced contact area, we achieved an efficiency of up to 25.5% (certified steady-state efficiency 24.7%) in p-i-n devices. The product of V oc × FF was 87.9% of the Shockley-Queisser limit. The surface recombination velocity at the p-type contact was reduced from 64.2 to 9.2 centimeters per second. The bulk recombination lifetime was increased from 1.2 to 6.0 microseconds because of improvements in the perovskite crystallinity. The improved wettability of the perovskite precursor solution allowed us to demonstrate a 23.3% efficient 1-square-centimeter p-i-n cell. We demonstrate here its broad applicability for different p-type contacts and perovskite compositions.

Inorganic CsPbI 3 is promising to enhance the thermal stability of perovskite solar cells. The dimethylamine iodide (DMAI) derived method is currently the most efficient way to achieve high efficiency, but the effect of DMAI has not been fully explained. Herein, the chemical composition and phase evolution of the mixed DMAI/CsPbI 3 layer during thermal treatment has been studied. The results demonstrate that, with the common DMAI/CsI/PbI 2 recipe in DMSO solvent, a mixed perovskite DMA 0.15 Cs 0.85 PbI 3 is first formed through a solid reaction between DMAPbI 3 and Cs 4 PbI 6 . Further thermal treatment will transform the mixed perovskite phase directly to γ-CsPbI 3 and then spontaneously convert to δ-CsPbI 3 . It has been also demonstrated that the DMA 0.15 Cs 0.85 PbI 3 phase is thermodynamically stable and shows a bandgap of 1.67 eV, which is narrower than 1.73 eV of γ-CsPbI 3 . The device efficiency of the mixed DMA 0.15 Cs 0.85 PbI 3 perovskite is therefore highly improved in comparison with the pure inorganic γ-CsPbI 3 perovskite.

An easy and scalable methylamine (MA) gas healing method was realized for inorganic cesium-based perovskite (CsPbX 3 )l ayers by incorporating ac ertain amount of MAX (X = IorBr) initiators into the raw film. It was found that the excess MAX accelerated the absorption of the MA gas into the CsPbX 3 film and quickly turned it into al iquid intermediate phase.T hrough the healing process,ahighly uniform and highly crystalline CsPbX 3 film with enhanced photovoltaic performance was obtained. Moreover,t he chemical interactions between as eries of halides and MA gas molecules were studied, and the results could offer guidance in further optimizations of the healing strategy.Inthe past decade,organic-inorganic lead halide perovskite solar cells (PSCs) have witnessed rapid development. [1] The power conversion efficiency(PCE) of PSCs has been rapidly increased to up to 23.7 %. [2] Not only small-area laboratory devices,but also modules with up to 277 cm 2 in size have been reported with high PCEs of more than 17 %. [3] Currently,the stability of these organic-inorganic hybrid perovskite materials is considered to be the biggest challenge for their future commercial utilization. [4] One promising direction to address this issue is to replace the organic cations with inorganic cations such as Cs + to form all-inorganic perovskite materials. [5] All-inorganic perovskite materials CsPbX 3 (X = I, Br) have aband gap ranging from 1.72 eV for CsPbI 3 to 2.3 eV for CsPbBr 3 . [6] Among them, the cesium/lead mixed-halide perovskite CsPbI 2 Br has attracted greatest attention because it provides the best balance between band gap and phase stability. [7] Thes calable preparation of inorganic PSCs is undoubtedly another urgent issue.Although many methods have been developed for organic-inorganic perovskite films,barely any of them can be directly translated to the preparation of allinorganic perovskite layers. [8] Fori nstance,t he use of hydriodic acid (HI) additive in the precursor solution was confirmed to enable the formation of the hybrid mixed-cation perovskite phase Cs x DMA 1Àx PbI 3 but not that of the inorganic CsPbX 3 . [9] To obtain high-quality perovskite films,there are two general approaches:1 )Controlling the film crystallization and growth process during film deposition;a nd 2) introducing an additional post-treatment process to improve the film quality.Foro rganic-inorganic perovskite films,t he methylamine (MA) gas healing method has been widely studied in the past three years and has exhibited great compatibility with commercial film-making equipment. [10] Thef ormation of al iquid intermediate phase (normally MAPbI 3 ·x MA) plays acritical role in the MA gas healing process. [10a-c,11] We found that, quite differently,the previously used MA molecules can hardly break the ···Cs À X À Pb··· chemical bonds in the inorganic CsPbX 3 perovskite phase to form al iquid intermediate phase.T os olve this problem, herein, an excess of am ethylammonium halide (MAX) was introduced to the CsPbX 3 initial films to form mix...

Organic–inorganic hybrid perovskites (OIHPs) are one of the hottest fields on account of their immense potential for photovoltaics. As one of the most promising OIHPs, formamidinium (FA)‐based perovskites have been developed very fast in the past few years. The power conversion efficiency (PCE) has reached certified 24.2%, which is comparable with that of monocrystalline silicon solar cells. However, the easy formation of nonperovskite δ‐phase formamidinium lead triiodide (FAPbI3) at a low temperature needs to be solved when fabricating a high‐quality light absorber layer. Several strategies have been used to avoid the formation of δ‐phase FAPbI3 and improve phase stability in recent years such as tolerance factor adjustment, dimensional engineering, addictive processing, interfacial modification, defects passivation, and in situ growth. These approaches can enhance the phase stability to some extent; however, their contribution to long‐term stability and especially their real mechanism is still unknown. Herein, the relationships among the tolerance factors, the structure of FAPbI3, and the phase transition phenomenon are summarized. In addition, various methodologies and potential mechanisms for stabilizing α‐phase FAPbI3 at room temperature (RT) are discussed. In conclusion, a series of challenges in the popular processings of perovskite solar cells and their corresponding solutions that help achieve commercialization faster are summarized.

Interface engineering is of great concern in photovoltaic devices. For the solution‐processed perovskite solar cells, the modification of the bottom surface of the perovskite layer is a challenge due to solvent incompatibility. Herein, a Cl‐containing tin‐based electron transport layer; SnOx‐Cl, is designed to realize an in situ, spontaneous ion‐exchange reaction at the interface of SnOx‐Cl/MAPbI3. The interfacial ion rearrangement not only effectively passivates the physical contact defects, but, at the same time, the diffusion of Cl ions in the perovskite film also causes longitudinal grain growth and further reduces the grain boundary density. As a result, an efficiency of 20.32% is achieved with an extremely high open‐circuit voltage of 1.19 V. This versatile design of the underlying carrier transport layer provides a new way to improve the performance of perovskite solar cells and other optoelectronic devices.

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The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FAxCs1−xPbI3 has shown great promise owing to its improved thermal and moisture stability. However, the study of FAxCs1−xPbI3 is concentrated on formamidine (FA)‐rich perovskite, whereas cesium (Cs)‐rich FAxCs1−xPbI3 perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs4PbI6‐mediated method is developed to synthesize Cs‐rich FAxCs1−xPbI3 perovskites. It is demonstrated that Cs4PbI6 intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA‐rich perovskite phase to finally form the Cs‐rich FAxCs1−xPbI3 perovskite. The results indicate that ≈15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV‐irradiation stability. The FA0.15Cs0.85PbI3 perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs‐based perovskites for efficient and stable solar cells.