Herein, we reveal for the first time a comprehensive mechanism of poorly investigated electrochemical decomposition of CH3NH3PbI3 using a set of microscopy techniques (optical, AFM, PL) and ToF-SIMS. We demonstrate that applied electric bias induces the oxidation of I– to I2, which remains trapped in the film in the form of polyiodides, and hence, the process can be conceivably reversed by reduction. On the contrary, reduction of organic methylammonium cation produces volatile products, which leave the film and thus make the degradation irreversible. Our results lead to a paradigm change when considering design principles for improving the stability of complex lead halide materials as those featuring organic cations rather than halide anions as the most electric field-sensitive components. Suppressing the electrochemical degradation of complex lead halides represents a crucial challenge, which should be addressed in order to bring the operational stability of perovskite photovoltaics to commercially interesting benchmarks.

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We investigated the impact of a series of hole transport layer (HTL) materials such as Poly­(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), NiO x , poly­[bis­(4-phenyl)­(2,4,6-trimethylphenyl)­amine (PTAA), and polytriarylamine (PTA) on photostability of thin films and solar cells based on MAPbI3, Cs0.15FA0.85PbI3, Cs0.1MA0.15FA0.75PbI3, Cs0.1MA0.15FA0.75Pb­(Br0.15I0.85)3, and Cs0.15FA0.85Pb­(Br0.15I0.85)3 complex lead halides. Mixed halide perovskites showed reduced photostability in comparison with similar iodide-only compositions. In particular, we observed light-induced recrystallization of all perovskite films except MAPbI3 with the strongest effects revealed for Br-containing systems. Moreover, halide and β FAPbI3 phase segregations were also observed mostly in mixed-halide systems. Interestingly, coating perovskite films with the PCBM layer spectacularly suppressed light-induced growth of crystalline domains as well as segregation of Br-rich and I-rich phases or β FAPbI3. We strongly believe that all three effects are promoted by the light-induced formation of surface defects, which are healed by adjacent PCBM coating. While comparing different hole-transport materials, we found that NiO x and PEDOT:PSS are the least suitable HTLs because of their interfacial (photo)­chemical interactions with perovskite absorbers. On the contrary, polyarylamine-type HTLs PTA and PTAA form rather stable interfaces, which makes them the best candidates for durable p–i–n perovskite solar cells. Indeed, multilayered ITO/PTA­(A)/MAPbI3/PCBM stacks revealed no aging effects within 1000 h of continuous light soaking and delivered stable and high power conversion efficiencies in solar cells. The obtained results suggest that using polyarylamine-type HTLs and simple single-phase perovskite compositions pave a way for designing stable and efficient perovskite solar cells.

High-temperature crystal structure of the layered cuprates Ln 2 CuO 4 , Ln=Pr, Nd and Sm with tetragonal T'-structure was refined using X-ray powder diffraction data. Substantial anisotropy of the thermal expansion behavior was observed in their crystal structures with thermal expansion coefficients (TEC) along a-and c-axes changing from TEC(a)/TEC(c)≈1.37 (Pr)

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