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  1. Abstract

    “Perovskite/carbon” interface is a bottle‐neck for hole‐conductor‐free, carbon‐electrode basing perovskite solar cells due to the energy mismatch and concentrated defects. In this article, in‐situ healing strategy is proposed by doping octylammonium iodide into carbon paste that used to prepare carbon‐electrode on perovskite layer. This strategy is found to strengthen interfacial contact and reduce interfacial defects on one hand, and slightly elevate the work function of the carbon‐electrode on other hand. Due to this effect, charge extraction is accelerated, while recombination is obviously reduced. Accordingly, power conversion efficiency of the hole‐conductor‐free, planar perovskite solar cells is upgraded by ≈50%, or from 11.65 (± 1.59) % to 17.97 (± 0.32) % (AM1.5G, 100 mW cm−2). The optimized device shows efficiency of 19.42% and open‐circuit voltage of 1.11 V. Meanwhile, moisture‐stability is tested by keeping the unsealed devices in closed chamber with relative humidity of 85%. The “in‐situ healing” strategy helps to obtain T80time of >450 h for the carbon‐electrode basing devices, which is four times of the reference ones. Thus, a kind of “internal encapsulation effect” has also been reached. The “in situ healing” strategy facilitates the fabrication of efficient and stable hole‐conductor‐free devices basing on carbon‐electrode.

     
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  2. Abstract

    Organic‐inorganic hybrid perovskite solar cells are susceptible to multiple influencing factors such as moisture, oxygen, heat stress, ion migration. Given the complex practical working conditions for solar cells, a fundamental question is how different failure mechanisms collaborate and substantially accelerate the device degradation. In this study, it is found that ion migration can accelerate the reaction between oxygen and methylammonium lead iodide perovskite in light conditions. This is suggested since regions with local electric fields suffer from more severe decomposition. Here it is reported that cesium ions (Cs+) incorporated in perovskite lattice, with a moderate doping concentration (e.g. 5%), can function as stabilizers to efficiently interrupt such a synergistic effect between oxygen induced degradation and ion migration while retaining the high performance of perovskite solar cells. Both experimental and theoretical results suggest that 5% Cs+ions incorporation simultaneously suppresses the formation of reactive superoxide ions () as well as ion migration in perovskites by forming additional energy barriers. This A‐site cations engineering is also a promising strategy to circumvent the detrimental effect of oxygen molecules in FA‐based perovskites, which is important for developing high‐efficiency perovskite solar cells with enhanced stability.

     
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  3. Abstract Fast reaction between organic salt and lead iodide always leads to small perovskite crystallites and concentrated defects. Here, polyacrylic acid is blended with organic salt, so as to regulate the crystallization in a two‐step growth method. It is observed that addition of polyacrylic acid retards aggregation and crystallization behavior of the organic salt, and slows down the reaction rate between organic salt and PbI 2 , by which “slow‐release effect” is defined. Such effect improves crystallization of perovskite. X‐ray diffraction study shows that, after addition of 2 m m polyacrylic acid, average crystallite size of perovskite increases from ≈40 to ≈90 nm, meanwhile, grain size increases. Thermal admittance spectroscopy study shows that trap density is reduced by nearly one order (especially for deep energy levels). Due to the improved crystallization and reduced trap density, charge recombination is obviously reduced, while lifetime of charge carriers in perovskite film and devices are prolonged, according to time‐resolved photoluminescence and transient photo‐voltage decay curve tests, respectively. Accordingly, power conversion efficiency of the device is promoted from 19.96 (±0.41)% to 21.84 (±0.25)% (with a champion efficiency of 22.31%), and further elevated to 24.19% after surface modification by octylammonium iodide. 
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  4. Van der Waals (vdW) epitaxial growth provides an efficient strategy to prepare heterostructures with atomically and electronically sharp interfaces. Herein, PbI2 was in situ thermally deposited onto exfoliated thin−layered CrOCl nanoflakes in high vacuum to fabricate vdW PbI2/CrOCl heterostructures. Optical microscopy, atomic force microscopy, X−ray diffraction, and temperature−dependent Raman spectroscopy were used to investigate the structural properties and phonon behaviors of the heterostructures. The morphology of PbI2 films on the CrOCl substrate obviously depended on the substrate temperature, changing from hemispherical granules to 2D nanoflakes with flat top surfaces. In addition, anomalous blueshift of the Ag1 and Au2 modes as the temperature increased in PbI2/CrOCl heterostructure was observed for the first time. Our results provide a novel material platform for the vdW heterostructure and a possible method for optimizing heterostructure growth behaviors. 
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  5. Crystallization of perovskite is monitored in carbon-electrode based, low-temperature, mesoscopic perovskite solar cells. Crystallographic and morphological properties of the perovskite are examined through changes in the film thickness of carbon-electrode or the volume of perovskite precursor. It is observed that, when a relatively thin carbon-electrode or large volume of perovskite precursor is used, perovskite crystallites mainly form on the device surface, leaving the bottom part of the device un-wetted. However, if a thicker carbon-electrode or less perovskite precursor is used, crystallization could be seen in the whole porous skeleton, and relative uniform distribution of perovskite crystallites is achieved. As such, uneven crystallization is observed. Such behavior is due to solvent evaporation on the surface, which facilitates nucleation processes on the surface, while retards crystallization on the bottom due to the Ostwald ripening effect. Charge transfer/recombination processes and photo-to-electric power conversion properties are studied. As expected, uneven crystallization results in retarded charge transfer and increased risk of recombination, and poor power conversion efficiency, for example, ∼3%. In contrast, uniform crystallization accelerates charge transfer and reduces recombination risk, and increases the efficiency to higher than 11% (AM1.5G, 100 mW/cm2). 
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  6. The modification by molybdenum trioxide (MoO3) buffer layer on the electronic structure between Co and black phosphorus (BP) was investigated with ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). It was found that the MoO3 buffer layer could effectively prevent the destruction of the outermost BP lattice during the Co deposition, with the symmetry of the lattice remaining maintained. There is a noticeable interfacial charge transfer in addition to the chemical reaction between Co and MoO3. The growth pattern of Co deposited onto the MoO3/BP film is the island growth mode. The observations reveal the significance of a MoO3 buffer layer on the electronic structure between Co and black phosphorus and provide help for the design of high-performance Co/BP-based spintronic devices. 
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