2D Phase Formation on 3D Perovskite: Insights from Molecular Stiffness.

Several studies have demonstrated that low-dimensional structures (e.g., two-dimensional (2D)) associated with three-dimensional (3D) perovskite films enhance the efficiency and stability of perovskite solar cells. Here, we aim to track the formation sites of the 2D phase on top of the 3D perovskite and to establish correlations between molecular stiffness and steric hindrance of the organic cations and their influence on the formation and crystallization of 2D/3D. Using cathodoluminescence combined with a scanning electron microscopy technique, we verified that the formation of the 2D phase occurs preferentially on the grain boundaries of the 3D perovskite. This helps explain some passivation mechanisms conferred by the 2D phase on 3D perovskite films. Furthermore, by employing in situ grazing-incidence wide-angle X-ray scattering, we monitored the formation and crystallization of the 2D/3D perovskite using three cations with varying molecular stiffness. In this series of molecules, the formation and crystallization of the 2D phase are found to be dependent on both steric hindrance around the ammonium group and molecular stiffness. Finally, we employed a 2D/3D perovskite heterointerface in a solar cell. The presence of the 2D phase, particularly those formed from flexible cations, resulted in a maximum power conversion efficiency of 21.5%. This study provides insight into critical aspects related to how bulky organic cations' stiffness and steric hindrance influence the formation, crystallization, and distribution of 2D perovskite phases.

Tiny spots to light the future: advances in synthesis, properties, and application of perovskite nanocrystals in solar cells.

Perovskites are in the hotspot of material science and technology. Outstanding properties have been discovered, fundamental mechanisms of defect formation and degradation elucidated, and applications in a wide variety of optoelectronic devices demonstrated. Advances through adjusting the bulk-perovskite composition, as well as the integration of layered and nanostructured perovskites in the devices, allowed improvement in performance and stability. Recently, efforts have been devoted to investigating the effects of quantum confinement in perovskite nanocrystals (PNCs) aiming to fabricate optoelectronic devices based solely on these nanoparticles. In general, the applications are focused on light-emitting diodes, especially because of the high color purity and high fluorescence quantum yield obtained in PNCs. Likewise, they present important characteristics featured for photovoltaic applications, highlighting the possibility of stabilizing photoactive phases that are unstable in their bulk analog, the fine control of the bandgap through size change, low defect density, and compatibility with large-scale deposition techniques. Despite the progress made in the last years towards the improvement in the performance and stability of PNCs-based solar cells, their efficiency is still much lower than that obtained with bulk perovskite, and discussions about upscaling of this technology are scarce. In light of this, we address in this review recent routes towards efficiency improvement and the up-scaling of PNC solar cells, emphasizing synthesis management and strategies for solar cell fabrication.

Improving the Stability and Efficiency of Perovskite Solar Cells by a Bidentate Anilinium Salt.

We report improvement in the perovskite solar cell efficiency and stability after passivation with an organic molecule decorated with two anilinium cations. We compare this salt with its neutral analog and found that the change in the electron density distribution upon protonation and the presence of the halide anion are key to explaining the better passivation ability of the salt. In addition, we show that the counteranion has a significant impact on the performance of the device.