The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10. 1002/aenm.201700414. used mesoporous TiO 2 ESLs, but the complex synthesis procedure and requirement of sintering at temperatures above 450 °C make the process undesirable for large-scale roll-to-roll manufacturing. [3] Therefore, significant efforts have been made on developing efficient low-temperature planar PVSCs. [23][24][25][26][27] For PVSCs with a regular configuration, the more popular low-temperature processed ESL materials are transition metal oxides such as TiO 2 , [26,28] ZnO, [29,30] SnO 2 , [31,32] and Zn 2 SnO 4 . [23,24] Due to its relatively simple synthesis process, chemical stability, [33][34][35] and high electron mobility, [33,36,37] SnO 2 ESL has shown great promise in making efficient planar PVSCs. For example, Ke et al. first fabricated PVSCs with PCE over 16% using a low-temperature solution processed SnO 2 ESL. [31] Since then, many other groups have also reported efficient PVSCs using low-temperature processed ESLs. [23,33,37] Recently, Gratzel and co-workers reported planar PVSCs with efficiencies approaching 21% using SnO 2 ESLs synthesized by low-temperature chemical bath deposition. [32] Low-temperature SnO 2 ESLs have been deposited by various methods including spin-coating, [31] dual-fuel combustion, [38] chemical-bath deposition, [32] and atomic-layer deposition (ALD). [34,39] ALD produces the most compact thin films compared to the other methods mentioned. However, PVSCs using ALD SnO 2 ESLs often show current-density-voltage (J-V) Adv. Energy

E. H. (2022). Regulating surface potential maximizes voltage in all-perovskite tandems. Nature.

We show that the cooperation of lead thiocyanate additive and a solvent annealing process can effectively increase the grain size of mixedcation lead mixed-halide perovskite thin films while avoiding excess lead iodide formation. As a result, the average grain size of the wide-bandgap mixed-cation lead perovskite thin films increases from 66 ± 24 to 1036 ± 111 nm, and the mean carrier lifetime shows a more than 3-fold increase, from 330 ns to over 1000 ns. Consequently, the average open-circuit voltage of wide-bandgap perovskite solar cells increases by 80 (70) mV, and the average power conversion efficiency (PCE) increases from 13.44 ± 0.48 (11.75 ± 0.34) to 17.68 ± 0.36 (15.58 ± 0.55)% when measured under reverse (forward) voltage scans. The best-performing wide-bandgap perovskite solar cell, with a bandgap of 1.75 eV, achieves a stabilized PCE of 17.18%.

Tin oxide (SnO 2 ) electron selective layers (ESLs) processed by low-temperature plasma-enhanced atomic layer deposition (PEALD) hold promise for fabricating lightweight and efficient flexible lead halide perovskite solar cells (PVSCs). However, the as-synthesized SnO 2 ESLs typically lead to flexible PVSCs with lower open-circuit voltage (V OC ) and fill factor (FF) as well as a higher degree of current density−voltage (J−V) hysteresis, compared to PVSCs fabricated on rigid substrates. Here, we report that facile water vapor treatment of PEALD-synthesized SnO 2 ESLs can effectively improve the V OC and FF while reducing the degree of J−V hysteresis. The improvement in device performance is mainly attributed to the improved conductivity and electrical mobility of SnO 2 ESLs enabled by water vapor treatment. With such treatment, our best flexible PVSC fabricated on a commercial substrate shows a power conversion efficiency of 18.36 (17.12)% when measured under a reverse (forward) voltage scan and a stabilized efficiency of 17.08%, which is the highest reported efficiency for flexible PVSCs with the regular structure.

Surface treatment using large alkyl/aryl ammonium cations has demonstrated reduced open-circuit voltage (V OC ) deficits in perovskite solar cells (PSCs), but the origin of the improvements has been vaguely attributed to defect passivation. Here, we combine microscopic probing of the local electrical properties, thermal admittance spectroscopic analysis, and firstprinciples calculations to elucidate the critical role of arylammonium interface layers in suppressing ion migration in wide-bandgap (WBG) PSCs. Our results reveal that arylammonium surface treatment using phenethylammonium iodide increases the activation energy barrier for ion migration on the surface, which suppresses the accumulation of charge defects at surface and grain boundaries, leading to a reduced dark saturation current density in WBG PSCs. With device optimization, our champion 1.73 eV PSC delivers a power conversion efficiency of 19.07% with a V OC of 1.25 V, achieving a V OC deficit of 0.48 V.

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Organic-inorganic metal halide perovskite single-junction solar cells have attracted great attention in the past few years due to a high record power conversion efficiency (PCE) of 23.7% and low-cost fabrication processes. Beyond single-junction devices, low-temperature solution processability, and bandgap tunability make the metal halide perovskites ideal candidates for fabricating tandem solar cells. Tandem solar cells combining a wide-bandgap perovskite top cell and a low-bandgap bottom cell based on mixed tin (Sn)-lead (Pb) perovskite or a dissimilar material such as silicon (Si) or copper indium gallium selenide (CIGS) offer an extraordinary opportunity to achieve PCEs higher than Shockley-Queisser (SQ) radiative efficiency limits (∼33%) for single-junction cells. In this review, we will summarize recent research progress on the fabrication of wide-(1.7 to 1.9 eV) and low-bandgap (1.1 to 1.3 eV) perovskite singlejunction cells and their applications in tandem cells. Key challenges and issues in wide-and lowbandgap single-junction cells will be discussed. We will survey current state-of-the-art perovskite tandem cells and discuss the limitations and challenges for perovskite tandem cells. Lastly, we conclude with an outlook for the future development of perovskite tandem solar cells.

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