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Understanding the degradation kinetics of perovskite materials in operating perovskite solar cells (PSCs) is critical for developing methods to further enhance their device stability. However, it remains a challenge to capture the details and characteristics of the perovskite degradation process inside operating PSCs under different environmental conditions in real‐time at the microscopic scale. Herein, a novel, nondestructive real‐time in situ monitoring method based on a tapered seven‐core fiber optic sensor is demonstrated. The fiber‐optic sensor can be facilely inserted near the surface of the perovskite layer of a working PSCs to monitor its degradation kinetics without interfering with the performance of the device. Stable and reproducible correlations between real‐time material degradation and optical wavelength response under different humidity and temperature conditions are observed and quantified. This new measurement tool allows real‐time access to the detailed degradation evolution of PSCs, which not only guides the design of more stable devices but also offers the possibility to monitor the health status of PSCs in field applications. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Formamidinium (FA)‐based Sn‐Pb perovskite solar cells (FAPb0.5Sn0.5I3 PSCs) with ideal bandgap and impressive thermal stability have caught enormous attention recently. However, it still suffers from the challenge of realizing high efficiency due to the surface imperfections of the transport materials and the energy‐level mismatch between functional contacts. Herein, it is demonstrated that the modification on buried interface with alkali metal salts is a viable strategy to alleviate these issues. We systematically investigate the role of three alkali metal bromide salts (NaBr, KBr, CsBr) by burying them between the NiOx hole transport layer (HTL) and the perovskite light‐absorbing layer, which can effectively passivate interface defects, improve energy‐level matching and release the internal residual strain in perovskite layers. The device with CsBr buffer layer exhibits the best power conversion efficiency (PCE) approaching 20%, which is one of the highest efficiencies for FA‐based Sn‐Pb PSCs employing NiOx HTLs. Impressively, the long‐term storage stability of the unencapsulated device is also greatly boosted. Our work provides an efficient strategy to prepare desired FA‐based ideal‐bandgap Sn‐Pb PSCs which could be applied in tandem solar cells. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid perovskite solar cells (PSCs) are regarded as a promising next‐generation photovoltaic technology. However, poor device stability limits their commercialization. Carbon‐based inorganic PSCs (C‐IPSCs) can meet stability challenge owing to the absence of organic components. Meanwhile, the hydrophobic carbon materials as hole transport layers and back electrodes simplify fabrication process, decrease costs, and protect devices from moisture erosion. Since the first attempt for C‐IPSCs in 2016, a series of strategies have been proposed to improve device performance, including the fabrication technique optimization, solvent engineering, composition engineering, interface engineering, charge transport layer optimization, and so on, and the power conversion efficiency of C‐IPSCs are rapidly increased from initial 5% to exceeding 15%. In this review, the recent progress of C‐IPSCs is summarized, the existing challenges in this field are discussed, followed by a prospect for future development. [ABSTRACT FROM AUTHOR]

Low band gap tin‐lead perovskite solar cells (Sn−Pb PSCs) are expected to achieve higher efficiencies than Pb‐PSCs and regarded as key components of tandem PSCs. However, the realization of high efficiency is challenged by the instability of Sn2+ and the imperfections at the charge transfer interfaces. Here, we demonstrate an efficient ideal band gap formamidinium (FA)‐based Sn−Pb (FAPb0.5Sn0.5I3) PSC, by manipulating the buried NiOx/perovskite interface with 4‐hydroxyphenethyl ammonium halide (OH‐PEAX, X=Cl−, Br−, or I−) interlayer, which exhibits fascinating functions of reducing the surface defects of the NiOx hole transport layer (HTL), enhancing the perovskite film quality, and improving both the energy level matching and physical contact at the interface. The effects of different halide anions have been elaborated and a 20.53 % efficiency is obtained with OH‐PEABr, which is the highest one for FA‐based Sn−Pb PSCs using NiOx HTLs. Moreover, the device stability is also boosted. [ABSTRACT FROM AUTHOR]

With the advantages of narrow bandgap and excellent thermal stability, fomamidinium lead triiodide (FAPbI3) perovskite holds the promise to boost the power conversion efficiency (PCE) of perovskite solar cells (PSCs) to over 25%. However, such a promise is blurred by the poor structural stability of the black α‐phase FAPbI3, as it can spontaneously transform into photoinactive δ‐phase at room temperature and this process can be accelerated by the ambient moisture. The incorporation of small ions such as cesium (Cs+), methylammonium (MA), and bromide (Br−) into the perovskite lattice is proven to be successful in stabilizing the α‐phase FAPbI3; however, the resultant mixed perovskites suffer the phase segregation problem, which inevitably undermines the long‐term stability of the corresponding PSCs. Therefore, continuous efforts are made to realize stable and pure‐phase α‐FAPbI3 perovskites. Herein, the recent progress on the development of efficient and stable FAPbI3 PSCs is summarized with a focus on the different phase stabilization strategies. In addition, the challenges and possible directions for future study are proposed. [ABSTRACT FROM AUTHOR]

Carbon‐based inorganic perovskite solar cells (C‐PSCs) have attracted intensive attention owing to their low cost and superior thermal stability. However, the bulk defects in perovskites and interfacial energy level mismatch seriously undermine their performance. To overcome these issues, a multifunctional dual‐interface engineering is proposed with a focus on low‐temperature CsPbI2Br C‐PSCs, where the potassium trifluoroacetate (KTFA) and the 4‐trifluorophenyl methylammonium bromide (CF3PMABr) are introduced beneath and on top of the perovskite layer, respectively. It is found that TFA‐ ions locate at the SnO2/CsPbI2Br interface, whereas a small amount of K+ ions diffuse into perovskite lattice to participate in nucleation and crystallization, resulting in more favored interfacial energy level alignment, improved film quality, passivated interfacial defects, released interfacial strain, as well as suppressed charge recombination and ion migration. Meanwhile, the CF3PMABr passivates I/Br vacancies and forms 2D perovskite capping layer to facilitate hole extraction at the CsPbI2Br/carbon interface. As a result, a remarkable power conversion efficiency (PCE) of 14.05% with an open‐circuit voltage of 1.273 V is achieved. To the best of the authors' knowledge, it is currently the highest PCE reported for low‐temperature CsPbI2Br C‐PSCs. Furthermore, the nonencapsulated device exhibits improved moisture, thermal, and illumination stability in ambient air. [ABSTRACT FROM AUTHOR]

Solution‐processed nickel oxide nanocrystals (NiOx NCs) ink can be facilely applied to deposit NiOx thin films as the hole transport layer (HTL) for perovskite solar cells (PSCs). Both the efficiency and stability of the corresponding PSCs depend significantly on the size and the energy levels of the as‐synthesized NiOx NCs; however, previous studies have shown that these two aspects can be hardly controlled synchronously to maximize the device performance. Herein, a novel synthesis of highly dispersed NiOx NCs is demonstrated by employing tetraalkylammonium hydroxides (TAAOHs, alkyl = methyl, ethyl, propyl, butyl) as precipitating bases, where the varied alkyl chain lengths of TAAOHs enable the size control of the NiOx NCs and the subsequent altering of their Ni3+ contents, leading to tunable energy levels of the NiOx thin films. With the longest butyl chain, the smallest crystal size and the optimal energy level alignment at the NiOx/perovskite interface are achieved. After further passivating the detrimental Ni3+ species on the surface of NiOx HTL, a remarkable power conversion efficiency (PCE) approaching 23% is obtained, which is one of the highest PCEs reported for NiOx‐based inverted PSCs. Furthermore, the unencapsulated device exhibits excellent ultraviolet stability, which maintains ≈87% of its PCE after 200 h exposure. [ABSTRACT FROM AUTHOR]

The realization of highly efficient perovskite solar cells (PSCs) in ambient air is considered to be advantageous for low‐cost commercial manufacturing. However, it is fundamentally difficult to achieve comparable device performance to that obtained in an inert atmosphere, especially when the ambient humidity is high. Here, an effective precursor engineering that simultaneously employs non‐halide lead acetate and lead thiocyanate lead sources for fabricating high‐quality methylammonium lead iodide perovskite films in ambient air with enhanced moisture tolerance, is reported. The presence of Ac– and SCN– ions not only enables the facile formation of homogeneous and highly crystalized perovskite films, but also directs the uniform growth of the crystals along the (110) direction. Accordingly, a 20.55% efficiency is demonstrated, one of the best results for air‐processed MAPbI3 PSCs, which is also the highest value achieved with non‐halide lead sources. Furthermore, the unencapsulated device shows fivefold prolonged air stability (3600 h) compared to the conventional PbI2‐based PSC. Together with the use of non‐toxic antisolvent, this strategy is fully compatible with ambient air operation and thus of great potential for practical applications. [ABSTRACT FROM AUTHOR]

Tin–lead (Sn–Pb) binary low‐bandgap perovskites are more environmentally friendly than conventional Pb‐based perovskites and promise to deliver high photovoltaic performance by constructing tandem solar cells. However, the energy‐level mismatch between functional layers and tremendous trap states in perovskite films make it challenging to reduce the high open‐circuit voltage (Voc) loss in Sn–Pb binary perovskite solar cells (PSCs). Herein, energy loss reduction at the hole collection interface in Sn–Pb binary PSCs is demonstrated using nickel oxide (NiOx) as the hole transport material (HTM) with optimal poly[(9,9‐bis(3′‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctyfluorene)] (PFN) modification, which enables a significantly enhanced Voc compared to the traditional poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)‐based devices. The NiOx/PFN bilayer has a downward‐shifted valence band compared to PEDOT:PSS, providing well‐matched energy‐level alignment with the perovskite material, resulting in more fluent charge transfer and reduced Voc losses. The optimized device has a high Voc of 0.88 V and an efficiency of 19.80%, surpassing the previous results reported for NiOx‐based Sn–Pb PSCs. Moreover, the robust NiOx/PFN substrate and the high‐quality perovskite film grown on it make the device less vulnerable to ambient exposure. This work highlights the significance of ideal hole conductors and interface engineering in efficient and stable Sn–Pb low‐bandgap PSCs. [ABSTRACT FROM AUTHOR]

Electron transport materials (ETMs) play a significant role in perovskite solar cells (PSCs). However, conventional solution processable organic ETMs are mainly restricted to fullerene derivatives and it is challenging to obtain nonfullerene ETMs with satisfactory properties. In this work, a new organic semiconductor SPS‐4F is synthesized by utilizing the classical spiro[fluorine‐9′9‐thioxanthene] unit to construct a π‐extended core. Although spiro is normally used in hole transport materials, the new spiro derivative SPS‐4F is successfully used as an ETM in inverted PSCs with power conversion efficiency over 20%. In addition, SPS‐4F can strongly coordinate with MAPbI3 perovskite and lead to efficient surface trap passivation. The resultant PSCs exhibit excellent stability in air because of the hydrophobic property of SPS‐4F. This work opens up opportunities to obtain a new family of ETMs based on spiro and paves a way to the fabrication of high‐performance PSCs with low cost. [ABSTRACT FROM AUTHOR]

Tin‐based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin‐based PSCs because Sn2+ in perovskites can be easily oxidized to Sn4+ upon air exposure. Here we report the fabrication of air‐stable FASnI3 solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl2. The interaction between the sulfonate group and the Sn2+ ion enables the in situ encapsulation of the perovskite grains with a SnCl2–additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin‐based PSCs. [ABSTRACT FROM AUTHOR]

Electron Transport Materials: -Extended Spiro Core-Based Nonfullerene Electron-Transporting Material for High-Performance Perovskite Solar Cells (Adv. Funct. Keywords: electron transport materials; nonfullerenes; passivation; perovskite solar cells; spiro derivatives EN electron transport materials nonfullerenes passivation perovskite solar cells spiro derivatives 1 1 1 06/22/20 20200618 NES 200618 In article number 2001073, Hong Meng, Feng Yan, and co-workers design, synthesize, and successfully use a non-fullerene electron transport material based on a new spiro derivative, SPS-4F, in perovskite solar cells, leading to high efficiency as well as good stability of the devices. Electron transport materials, nonfullerenes, passivation, perovskite solar cells, spiro derivatives. [Extracted from the article]

Organic–inorganic hybrid perovskites have emerged as promising functional materials for high‐performance photodetectors. However, the toxicity of Pb and the lack of internal gain mechanism in typical perovskites significantly hinder their practical applications. Herein, a low‐voltage and high‐performance photodetector based on a single layer of lead‐free Sn‐based perovskite film is reported. The device shows broadband response from ultraviolet to near‐infrared light with a responsivity up to 105 A W−1 and a high gain at a low operating voltage. The outstanding performance is attributed to the high hole mobility, p‐doping nature, and excellent optoelectronic properties of the Sn‐based perovskite. Moreover, the device is assembled on a flexible substrate and demonstrates both high sensitivity and good bending stability. This work demonstrates a route for realizing nontoxic, low‐cost, and high‐performance perovskite photodetectors with a simple device structure. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Formamidinium (FA)‐based Sn‐Pb perovskite solar cells (FAPb0.5Sn0.5I3 PSCs) with ideal bandgap and impressive thermal stability have caught enormous attention recently. However, it still suffers from the challenge of realizing high efficiency due to the surface imperfections of the transport materials and the energy‐level mismatch between functional contacts. Herein, it is demonstrated that the modification on buried interface with alkali metal salts is a viable strategy to alleviate these issues. We systematically investigate the role of three alkali metal bromide salts (NaBr, KBr, CsBr) by burying them between the NiOx hole transport layer (HTL) and the perovskite light‐absorbing layer, which can effectively passivate interface defects, improve energy‐level matching and release the internal residual strain in perovskite layers. The device with CsBr buffer layer exhibits the best power conversion efficiency (PCE) approaching 20%, which is one of the highest efficiencies for FA‐based Sn‐Pb PSCs employing NiOx HTLs. Impressively, the long‐term storage stability of the unencapsulated device is also greatly boosted. Our work provides an efficient strategy to prepare desired FA‐based ideal‐bandgap Sn‐Pb PSCs which could be applied in tandem solar cells. [ABSTRACT FROM AUTHOR]