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Perovskite materials with excellent optical and electrical properties are promising for light‐emitting diodes. In the field of perovskite light‐emitting diodes (PeLEDs), organic materials additive engineering has been proved to be an effective scheme for enhancing efficiency and stability in PeLEDs. Most impressively, the reported external quantum efficiency of PeLEDs based on perovskite‐organic composite has reached over 20%. Herein, we will review the important progress of the organic materials' additive‐modified PeLEDs and discuss the remaining problems and challenges and the key research direction in the near future.

In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film‐growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities., Watching the defects: Defects play a pivotal role in the overall performance of perovskite solar cells. This Review focuses on central questions of “what defects exist in metal halide perovskites” and “how can one reduce detrimental defects towards high‐performance perovskite solar cells”.

Metal halide perovskite materials (MHPMs) have attracted significant attention because of their superior optoelectronic properties and versatile applications. The power conversion efficiency of MHPM solar cells (PSCs) has skyrocketed to 25.5 %. Although the performance of PSCs is already competitive, several important challenges still need to be solved to realize commercial applications. A thorough understanding of surface atomic structures and structure–property relationships is at the heart of these remaining issues. Scanning tunneling microscopy (STM) and spectroscopy (STS) can be used to characterize the surface properties of MHPMs, which can offer crucial insights into MHPMs at the atomic scale. This Review summarizes recent progress in STM and STS studies on MHPMs, with a focus on the surface properties. We provide understanding from the comparative perspective of several different MHPMs. We also highlight a series of novel phenomena observed by STM and STS. Finally, we outline a few research topics of primary importance for future studies.

Since a hybrid perovskite‐based solar cell was reported for the first time, its efficiency and stability were boosted to remarkably high levels. With record efficiencies beyond 22 % and record stability up to 10 000 h under working conditions, perovskite solar cells are a promising candidate for further commercial applications. Among the myriads of fabrication methods to deposit perovskite films, we have chosen a unique technique that only needs methylamine gas (CH3NH2) to convert the inorganic film. In contrast to all other fabrication methods, this is based on room temperature and exposure to an organic gas exposure at ambient pressure, which can be easily employed for large‐area and fast coatings. In this Review, the advantages and characteristics of methylamine gas applications will be discussed. First, we will evaluate the different techniques to convert an inorganic film into a perovskite with an emphasis on the chemical interactions. Then, we will summarize the applications of methylamine gas to “heal” and improve the perovskite films prepared by other methods.

ABX(3) type metal halide perovskite solar cells (PSCs) have shown efficiencies over 25%, rocketing toward their theoretical limit. To gain the full potential of PSCs relies on the understanding of the device working mechanisms and recombination, the material quality, and the match of energy levels in the device stacks. In this review, the importance of designing PSCs from the viewpoint of surface/interface science studies is presented. For this purpose, recent case studies are discussed to demonstrate how probing of local heterogeneities (e.g., grains, grain boundaries, atomic structure, etc.) in perovskites by surface science techniques can help correlate material properties and PSC device performance. At the solar cell device level with active areas larger than millimeter scale, the ensemble average measurement techniques can characterize the overall average properties of perovskite films as well as their adjacent layers and provide clues to understand better the solar cell parameters. How generation and healing of electronic defects in perovskite films limit the device efficiency, reproducibility, and stability, and induce the time-dependent transient behavior in the current-voltage curves are also the central focus of this review. On the basis of these studies, strategies to further improve efficiency and stability, as well as reducing hysteresis are presented.

A large number of derivative phases in inorganic perovskites are reported with special structures and extraordinary performances in photoelectronic device applications. The reverse phase transition between derivative phases and perovskites usually induces recrystallization or forms mixed components. In this work, derivative phase‐induced growth of the CsPbBr₃ micro–nanowire (MW) array by utilizing phase transition of the 2D CsPb₂Br₅ phase is reported. Owing to its layered structure and phase transition, annealing of CsPb₂Br₅ at a temperature of 550℃ combined with solvent quenching leads to a templating effect to form a high‐quality CsBr MW array. Subsequent PbBr₂ deposition and the second annealing are employed to form aligned CsPbBr₃ MW arrays. Based on this method, a CsPbBr₃ MW array‐based photodetector is fabricated. The large grain size, less grain boundaries, and lower surface potential of the CsPbBr₃ MW array lead to high device performance with a responsivity of 7.66 A W⁻¹, a detectivity of ≈10¹² Jones, and long‐term operational stability over 1900 min.

Solar cells based on metal halide perovskites have emerged as a promising low-cost photovoltaic technology. In contrast to inert atmospheres where most of the lab-scale devices are made to date, large-area low-cost production of perovskite solar cells often involves processing of perovskites in various atmospheres including ambient air, nitrogen, and/or vacuum. Herein, the impact of atmosphere on the energy levels of methylammonium lead halide perovskite films is systematically investigated. The atmosphere is varied to simulate the typical fabrication process. Through a comprehensive analysis combining the Fermi level evolution, surface photovoltage, photoluminescence properties, photovoltaic performance, and device simulation, an overall landscape of the energy diagram of the perovskite layer is able to be determined. The findings have direct implications for real-world devices under typical atmospheres, and provide insights into the fabrication-process design and optimization. Furthermore, a universal Fermi level shift under vacuum for lead halide-based perovskites revealed in this study, urges a refreshed view on the energetics studies conducted without considering the atmospheric effect.

Inorganic perovskite solar cells (PSCs) have attracted enormous attention during the past 5 years. Many advanced strategies and techniques have been developed for fabricating inorganic PSCs with improved efficiency and stability to realize commercial applications. CsPbBr3 is one of the representative materials of inorganic perovskites and has demonstrated excellent stability against thermal and high humidity environmental conditions. The power conversion efficiency of CsPbBr3-based PSCs has increased significantly from 5.95% in 2015 to 10.91%, and the storage stability under moisture (approximate to 80% relative humidity) and heat (approximate to 80 degrees C) is more than 2000 h. The outstanding performance of CsPbBr3 PSCs shows great potential in light-to-electricity conversion applications. In this review, recent developments of CsPbBr3-based PSCs including the physico-chemical as well as optoelectronic properties, processing techniques for fabricating CsPbBr3 films, derivative phase structures, efficiency, and stability of devices are reviewed and discussed. Finally, the challenges and outlook of CsPbBr3 PSCs for future research directions are outlined.

Electron transport layer (ETL) is a functional layer of great significance for boosting the power conversion efficiency (PCE) of perovskite solar cells (PSCs). To date, it is still a challenge to simultaneously reduce the surface defects and improve the crystallinity in ETLs during their low-temperature processing. Here, a novel strategy for the mediation of in situ regrowth of SnO2 nanocrystal ETLs is reported: introduction of controlled trace amounts of surface absorbed water on the fluorinated tin oxide (FTO) or indium-tin oxide (ITO) surfaces of the substrates using ultraviolet ozone (UVO) pretreatment. The optimum amount of adsorbed water plays a key role in balancing the hydrolysis-condensation reactions during the structural evolution of SnO2 thin films. This new approach results in a full-coverage SnO2 ETL with a desirable morphology and crystallinity for superior optical and electrical properties, as compared to the control SnO2 ETL without the UVO pretreatment. Finally, the rigid and flexible PSC devices based on the new SnO2 ETLs yield high PCEs of up to 20.5% and 17.5%, respectively.

An improved understanding of catalyzed reactions at anatomic level is of great importance for tailoring processes inthe chemical industry. Catalyzed reactions are typically con-ducted near or above atmospheric pressure and at elevatedtemperatures on nanometer-sized crystallites deposited onporous oxide materials.

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost-effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO2 are systematically studied. Ar and O-2 are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO2 films. With the optimized structure, SnO2 films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO2 electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T-80 operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO2 film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm(2) fabricated on 5 x 5 cm(2) substrates (geometry fill factor = 91%), and a T-80 operational lifetime of 515 h.

In this work all-inorganic perovskite CsPbIBr2 are doped with Mn to compensate their shortcomings in band structure for the application of perovskite solar cells (PSCs). The novel Mn-doped all-inorganic perovskites, CsPb1-xMnxI1+2xBr2-2x, are prepared in ambient atmosphere. As the concentration of Mn2+ ions increases, the bandgaps of CsPb1-xMnxI1+2xBr2-2x decrease from 1.89 to 1.75 eV. Additionally, when the concentration of Mn dopants is appropriate, this novel Mn-doped all-inorganic perovskite film shows better crystallinity and morphology than its undoped counterpart. These advantages alleviate the energy loss in hole transfer and facilitate the charge-transfer in perovskites, therefore, PSCs based on these novel CsPb1-xMnxI1+2xBr2-2x perovskite films display better photovoltaic performance than the undoped CsPbIBr2 perovskite films. The reference CsPbIBr2 cell reaches a power conversion efficiency (PCE) of 6.14%, comparable with the previous reports. The CsPb1-xMnxI1+2xBr2-2x cells reach the highest PCE of 7.36% (when x= 0.005), an increase of 19.9% in PCE. Furthermore, the encapsulated CsPb0.995Mn0.005I1.01Br1.99 cells exhibit good stability in ambient atmosphere. The storage stability measurements on the encapsulated PSCs reveal that PCE is dropped by only 8% of the initial value after >300 h in ambient. Such improved efficiency and stability are achieved using low-cost carbon electrodes (without expensive hole transport materials and Au electrodes).

Mixed cation hybrid perovskites such as CsxFA1−xPbI3 are promising materials for solar cell applications, due to their excellent photoelectronic properties and improved stability. Although power conversion efficiencies (PCEs) as high as 18.16% have been reported, devices are mostly processed by the anti-solvent method, which is difficult for further scaling-up. Here, a method to fabricate CsxFA1−xPbI3 by performing Cs cation exchange on hybrid chemical vapor deposition grown FAPbI3 with the Cs+ ratio adjustable from 0 to 24% is reported. The champion perovskite module based on Cs0.07FA0.93PbI3 with an active area of 12.0 cm2 shows a module PCE of 14.6% and PCE loss/area of 0.17% cm−2, demonstrating the significant advantage of this method toward scaling-up. This in-depth study shows that when the perovskite films prepared by this method contain 6.6% Cs+ in bulk and 15.0% at the surface, that is, Cs0.07FA0.93PbI3, solar cell devices show not only significantly increased PCEs but also substantially improved stability, due to favorable energy level alignment with TiO2 electron transport layer and spiro-MeOTAD hole transport layer, increased grain size, and improved perovskite phase stability.

2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) hole transport material (HTM) is a milestone in the history of perovskite solar cells (PSCs). Proper choice of HTMs is key factor for efficient charge extraction and stability in solar cells. Spiro-MeOTAD is proven to be the most suitable HTM for testing PSCs due to its facile implementation and high performance. Similarly, spiro-MeOTAD is receiving attention in other applications other than in solar cells due to its desirable properties. However, spiro-MeOTAD is under debate regarding the topics of cost-performance, long-term stability, degradation issues (induced by temperature, additives, film quality, and environmental conditions), coating technologies compatibility, reliance on additives, and hysteresis. In this review, the advent of spiro-MeOTAD, and related aforementioned issues about spiro-MeOTAD are discussed. In addition, spiro-MeOTAD properties, alternative and new additives, other applications, and new HTMs that is comparable or outperforms spiro-MeOTAD in PSCs are summarized. In the outlook, the future research directions based on reported results that warrant further investigations are outlined.

Semitransparent perovskite solar cells (st-PSCs) have received remarkable interest in recent years because of their great potential in applications for solar window, tandem solar cells, and flexible photovoltaics. However, all reported st-PSCs require expensive transparent conducting oxides (TCOs) or metal-based thin films made by vacuum deposition, which is not cost effective for large-scale fabrication: the cost of TCOs is estimated to occupy ≈75% of the manufacturing cost of PSCs. To address this critical challenge, this study reports a low-temperature and vacuum-free strategy for the fabrication of highly efficient TCO-free st-PSCs. The TCO-free st-PSC on glass exhibits 13.9% power conversion efficiency (PCE), and the four-terminal tandem cell made with the st-PSC top cell and c-Si bottom cell shows an overall PCE of 19.2%. Due to the low processing temperature, the fabrication of flexible st-PSCs is demonstrated on polyethylene terephthalate and polyimide, which show excellent stability under repeated bending or even crumbing.

This study reports two new, simple and cost-effective hole transporting materials for perovskite solar cells. These novel structures namely N4,N4,N4′″,N4′″-tetrakis(4-methoxyphenyl)-[1,1′:4′,1″:4″,1′″-quaterphenyl]-4,4′″-diamine (TPA-BP-TPA), and (E)-4′,4′″-(ethene-1,2-diyl)bis(N,N-bis(4-methoxyphenyl)-[1″,1′″-biphenyl]-4-amine) (TPA-BPV-TPA) are based on linear π-conjugated linkers and triphenylamine endcappers. These materials possess good solubility and appropriate highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Upon testing them as hole transporting materials in perovskite solar cells, in particular, the device with TPA-BPV-TPA exhibits a higher power conversion efficiency (PCE) of 16.42%, which is almost equivalent to the PCE using the conventional expensive 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)-9,9′-spirbiuorene (SPIRO-OMeTAD) compound under similar conditions. Additionally, the device stability measured using this newly developed low-cost compound retains almost 87% of the initial performance after 10 days compared to standard SPIRO-OMeTAD-based devices. From this outstanding outcome it is revealed that simple triphenylamine-based hole-transporting materials with various kinds of π-conjugated linkers can pave the way for developing a new generation of simple hole-transporting materials for low-cost perovskite solar cells.

Silver is a low-cost candidate electrode material for perovskite solar cells. However, in such cells the silver electrodes turn yellow within days of device fabrication. The color change is also accompanied by a dramatic decrease in the power conversion efficiency when compared to otherwise identical devices using gold electrodes. Here, it is shown that the color change results from silver oxidation to silver iodide, due to a reaction with iodine in methyl ammonium lead perovskite. The change in X-ray diffraction and X-ray photo­electron spectroscopy is discussed. Exposure to air accelerates corrosion of the Ag electrodes when compared to dry nitrogen gas exposure. However, iodine not reacted with silver is observed by X-ray photoelectron spectroscopy even for the perovskite solar cell kept in dry nitrogen gas. It is proposed that silver iodide is formed when methyl ammonium iodide migration is facilitated by the small pinholes in the hole transport layer spiro-MeOTAD.