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Finding sustainable and renewable energy to replace traditional fossil fuel is critical for reducing greenhouse gas emission and avoiding environment pollution. Solar cells that convert energy of sunlight into electricity offer a viable route for solving this issue. At present, halide perovskites are the most potential candidate materials for solar cell with considerable power conversion efficiency, whereas their stability remains a challenge. In this work, we summarize four different key factors that influence the stability of halide perovskites: (a) effect of environmental moisture on the degradation of halide perovskites. The performance of halide perovskite solar cells is reduced due to hydrated crystal hinders the diffusion of photo‐generated carriers, which can be solved by materials encapsulation technique; (b) photo‐induced instability. Through uncovering the underlying physical mechanism, we note that materials engineering or novel device structure can extend the working life of halide perovskites under continuous light exposure; (c) thermal stability. Halide perovskites are rapidly degraded into PbI2 and volatile substances as heating due to lower formation energy, whereas hybrid perovskite is little changed; (d) electric field effect in the degradation of halide perovskites. The electric field impacts significantly on the carrier separation, changes direction of photo‐induced currents and generates switchable photovoltaic effect. For each key factor, we have shown in detail the underlying physical mechanisms and discussed the strategies to overcome this stability difficulty. We expect this review from both theoretical and experimental points of view can be beneficial for development of perovskite solar cell materials and promotes practical applications.

Optoelectronic devices based on metal halide perovskites, including solar cells and light-emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic-inorganic hybrid perovskite materials can enable high-performance, solution-processed field-effect transistors (FETs) for next-generation, low-cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single-crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source-drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such "ideal" interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom-contact, bottom-gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single-crystal FETs with high mobility of up to ≈15 cm2 V-1 s-1 at 80 K. This work addresses one of the key challenges toward the realization of high-performance solution-processed perovskite FETs.

Based on density functional theory calculations, we have studied the electronic structures and magnetic properties of passivated AlN nanowires (NWs) with Mg dopants. The calculated results show that double Mg atom doped passivated AlN NWs display ferromagnetic properties, and the total magnetic moment is 1.80 µB per 96-atom unit cell excluding the pseudohydrogen atoms. However, a couple of Mg atom doped bare AlN nanowire unit cells display anti-ferromagnetic (AFM) properties. Unequal properties of magnetic coupling in different Mg-doped AlN structures are due to the different localization and overlapping of impurity wave functions. It is also found that the ferromagnetic stability and Curie temperature of the passivated AlN NWs are much higher than those of bulk structures.

International audience; In the past decade, controlled/living radical polymerization (CRP) processes have seen a considerable surge of interest owing, in part, to their relevance to the accessibility of a variety of well‐defined polymer structures (e.g. predetermined molecular mass, narrow molecular weight distribution).1 We have been interested in the one‐electron reactivity of transition‐metal complexes and its relevance in CRP.2 One way in which transition‐metal complexes can be used to control radical polymerization is through a reversible deactivation. The growing radical chain is trapped by formation of a metal–carbon bond to yield a metal‐capped polymer chain, which is a dormant organometallic species (Figure 1). We refer to this particular control mechanism as “organometallic radical polymerization” (OMRP).2 One of the outstanding challenges in this area is the possibility to control the polymerization of less reactive monomers (e.g. vinyl chloride, vinylidene dichloride, vinyl acetate), for which activation is made difficult by the relatively strong bonds established with common radical traps.

Five novel vanadium(III) complexes [PhN = C(R-2)CHC(R-1)O]VCl2(THF)(2) (4a: R-1 = Ph, R-2 = CF3; 4b: R-1 =t-Bu, R-2 = CF3; 4c: R-1 = CF3, R-2 = CH3; 4d: R-1 = Ph, R-2 = CH3; 4e: R-1 = Ph, R-2 = H) have been synthesized and characterized. On activation with Et2AlCl, all the complexes, in the presence of ethyl trichloroacetate (ETA) as a promoter, are highly active precatalysts for ethylene polymerization, and produce high molecular weight and linear polymers. Catalyst activities more than 16.8 kg PE/mmolv h bar and weight-average molecular weights higher than 173 kg/ mol were observed under mild conditions.

The copolymerizations of ethylene and cyclopentene with bis(β-enamino-ketonato) titanium complexes ([(Ph)NC(R2)CHC(R 1 )O] 2 TiCl 2 ; R 1 = CF 3 and R 2 = CH 3 for 1a, R 1 = Ph and R 2 = CF 3 for 1b; and R 1 = t-Bu and R 2 = CF 3 for 1c} activated with modified methylaluminoxane (MMAO) as a cocatalyst were investigated. High-molecular-weight copolymers with cis-1,2-cyclopentene units were obtained. The catalyst activity, cyclopentene incorporation, polymer molecular weight, and polydispersity could be controlled over a wide range through the variation of the catalyst structure and reaction parameters, such as the Al/Ti molar ratio, cyclopentene feed concentration, and polymerization reaction temperature. The complex 1b/MMAO catalyst system exhibited the characteristics of a quasi-living ethylene polymerization and an ethylene-cyclopentene copolymerization and allowed the synthesis of polyethylene-block-poly(ethylene-co-cyclopentene) diblock copolymer.

Ethylene-propylene copolymerization, using [(Ph)NC(R-2)CHC(R-1)O](2)TiCl2 (R-1 = CF3, Ph, or t-Bu; R-2 = CH3 or CF3) titanium complexes activated with modified methylaluminoxane as a cocatalyst, was investigated. High-molecular-weight ethylene-propylene copolymers with relatively narrow molecular weight distributions and a broad range of chemical compositions were obtained. Substituents R-1 and R-2 influenced the copolymerization behavior, including the copolymerization activity, methylene sequence distribution, molecular weight, and polydispersity. With small steric hindrance at R-1 and R-2, one complex (R-1 = CF3; R-2 = CH3) displayed high catalytic activity and produced copolymers with high propylene incorporation but low molecular weight. The microstructures of the copolymers were analyzed with C-13 NMR to determine the methylene sequence distribution and number-average sequence lengths of uninterrupted methylene carbons.