Enhanced Crystalline Phase Purity of CH₃NH₃PbI_3–<i>x</i>Cl<i>_x</i> Film for High-Efficiency Hysteresis-Free Perovskite Solar Cells

Despite rapid successful developments toward promising perovskite solar cells (PSCs) efficiency, they often suffer significant hysteresis effects. Using synchrotron-based grazing incidence X-ray diffraction (GIXRD) with different probing depths by varying the incident angle, we found that the perovskite films consist of dual phases with a parent phase dominant in the interior and a child phase with a smaller (110) interplanar space (<i>d</i>₍₁₁₀₎) after rapid thermal annealing (RTA), which is a widely used post treatment to improve the crystallization of solution-processed perovskite films for high-performance planar PSCs. In particular, the child phase composition gradually increases with decreasing depth till it becomes the majority on the surface, which might be one of the key factors related to hysteresis in fabricated PSCs. We further improve the crystalline phase purity of the solution-processed CH₃NH₃PbI_3–<i>x</i>Cl<i>_x</i> perovskite film (referred as <i>g</i>-perovskite) by using a facile gradient thermal annealing (GTA), which shows a uniformly distributed phase structure in pinhole-free morphology with less undercoordinated Pb and I ions determined by synchrotron-based GIXRD, grazing incidence small-angle X-ray scattering, scanning electron microscopy, and X-ray photoelectron spectroscopy. Regardless of device structures (conventional and inverted types), the planar heterojunction PSCs employing CH₃NH₃PbI_3–<i>x</i>Cl<i>_x</i> <i>g</i>-perovskite films exhibit negligible hysteresis with a champion power conversion efficiency of 17.04% for TiO₂-based conventional planar PSCs and 14.83% for poly­(3,4-ethylenedioxythiophene:poly­(styrenesulfonate) (PEDOT:PSS)-based inverted planar PSCs. Our results indicate that the crystalline phase purity in CH₃NH₃PbI_3–<i>x</i>Cl<i>_x</i> perovskite film, especially in the surface region, plays a crucial role in determining the hysteresis effect and device performance

<i>In Situ</i> Observation of Thermal Proton Transport through Graphene Layers

Protons can penetrate through single-layer graphene, but thicker graphene layers (more than 2 layers), which possess more compact electron density, are thought to be unfavorable for penetration by protons at room temperature and elevated temperatures. In this work, we developed an <i>in situ</i> subsecond time-resolved grazing-incidence X-ray diffraction technique, which fully realizes the real-time observation of the thermal proton interaction with the graphene layers at high temperature. By following the evolution of interlayer structure during the protonation process, we demonstrated that thermal protons can transport through multilayer graphene (more than 8 layers) on nickel foil at 900 °C. In comparison, under the same conditions, the multilayer graphenes are impermeable to argon, nitrogen, helium, and their derived ions. Complementary <i>in situ</i> transport measurements simultaneously verify the penetration phenomenon at high temperature. Moreover, the direct transport of protons through graphene is regarded as the dominant contribution to the penetration phenomenon. The thermal activation, weak interlayer interaction between layers, and the affinity of the nickel catalyst may all contribute to the proton transport. We believe that this method could become one of the established approaches for the characterization of the ions intercalated with 2D materials <i>in situ</i> and in real-time