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Halide perovskite solar cells have seen dramatic progress in performance over the past several years. Certified efficiencies of inverted structure (p-i-n) devices have now exceeded 20%. In these p-i-n devices, fullerene compounds are the most popular electron-transfer materials. However, the full function of fullerenes in perovskite solar cells is still under investigation, and the mechanism of photocurrent hysteresis suppression by fullerene remains unclear. In previous reports, thick fullerene layers (>20 nm) were necessary to fully cover the perovskite film surface to make good contact with perovskite film and avoid large leakage currents. In addition, the solution-processed fullerene layer has been broadly thought to infiltrate into the perovskite film to passivate traps on grain boundary surfaces, causing suppressed photocurrent hysteresis. In this work, we demonstrate an efficient perovskite photovoltaic device with only 1 nm C deposited by vapor deposition as the electron-selective material. Utilizing a combination of fluorescence microscopy and impedance spectroscopy, we show that the ultrathin C predominately acts to extract electrons from the perovskite film while concomitantly suppressing the photocurrent hysteresis by reducing space charge accumulation at the interface. This work ultimately helps to clarify the dominant role of fullerenes in perovskite solar cells while simplifying perovskite solar cell design to reduce manufacturing costs.

1028 wileyonlinelibrary.com COMMUNICATION the ligand structures to affect aggregation, crystal structure, and intermolecular proximities. However, once molecules are designed and integrated into optoelectronic devices, their performance typically suffers from arbitrary energy level alignments, resulting in lower-than-ideal open-circuit voltages, low carrier mobilities and diffusion lengths, and limited absorbance past 1000 nm. In this work, we synthesize a new series of heptamethine salts [ 12 ] with the highest occupied molecular orbital (HOMO) levels that can be tuned by varying the anion electronegativity. [ 13 ] These organic salts are used in photovoltaic and photodetector cells to demonstrate photoresponse at deep NIR wavelengths and open-circuit voltages nearing their excitonic limit. Using optical modeling and open-circuit voltage tuning [ 14,15 ] we identify limiting factors for performance and strategies for performance enhancement.Heptamethineindolium, λ max = 1024 nm) coordinated with the counterions tetrafl uoroborate (BF 4 − ) and tetrakis(pentafl uorophenyl)borate (TPFB − ) are shown in Figure 1 a. We focus on these molecules for their absorption ranges that extend to 1400 and 1600 nm for cations 1 and 2, respectively, (Figure 1 b). Figure 1 c shows a summary of the m / z synthesis verifi cation for the cation and anion masses. In previous studies, weakly coordinating anions like TPFB have been shown to modulate the frontier energy levels of organic cations used as donors in photovoltaic confi gurations, thereby increasing the open circuit voltage ( V OC ) with little or no impact on the bandgap or absorption range. [ 13,16 ] Solar cell devices with the structure indium tin oxide (ITO)/10 nm MoO 3 / t nm salt/40 nm C 60 /7.5 nm bathocuproine (BCP)/80 nm Ag were prepared using the four salts as a function of thickness ( Figure 2 a). Donor layers of each organic salt were spin-coated from N,N -dimethylformamide under nitrogen while other layers were thermally deposited in vacuum. The thickness for each salt was controlled by varying the solution concentration. For comparison purposes, the J -V and EQE for devices with similar salt thicknesses (12 ± 1 nm) are plotted in Figure 2 b,c and average performance metrics are shown in Table 1 . The fi ll factors (FF) for these devices, 0.3-0.5, are slightly lower than our previous demonstrations with larger bandgap organic salts due to decreased shunt resistances from the lower bandgap and series resistance from a potential interface barrier between the donor and MoO 3 . The exchange of BF 4 for TPFB nearly doubles the V OC from 0.13 to 0.33 V for cation 1 and 0.17 to 0.25 V for cation 2. This enhancement in

Perovskite semiconductors have emerged as competitive candidates for photovoltaic applications due to their exceptional optoelectronic properties. However, the impact of moisture instability on perovskite films is still a key challenge for perovskite devices. While substantial effort is focused on preventing moisture interaction during the fabrication process, it is demonstrated that low moisture sensitivity, enhanced crystallization, and high performance can actually be achieved by exposure to high water content (up to 25 vol%) during fabrication with an aqueous‐containing perovskite precursor. The perovskite solar cells fabricated by this aqueous method show good reproducibility of high efficiency with average power conversion efficiency (PCE) of 18.7% and champion PCE of 20.1% under solar simulation. This study shows that water–perovskite interactions do not necessarily negatively impact the perovskite film preparation process even at the highest efficiencies and that exposure to high contents of water can actually enable humidity tolerance during fabrication in air.