Layered 2D perovskites are making inroads as materials for photovoltaics and light emitting diodes, but their photophysics is still lively debated. Although their large exciton binding energies should hinder charge separation, significant evidence has been uncovered for an abundance of free carriers among optical excitations. Several explanations have been proposed, like exciton dissociation at grain boundaries or polaron formation, without clarifying yet if excitons form and then dissociate, or if the formation is prevented by competing relaxation processes. Here we address exciton stability in layered Ruddlesden-Popper PEA2PbI4 (PEA stands for phenethylammonium) both in form of thin film and single crystal, by resonant injection of cold excitons, whose dissociation is then probed with femtosecond differential transmission. We show the intrinsic nature of exciton dissociation in 2D layered perovskites, demonstrating that both 2D and 3D perovskites are free carrier semiconductors and their photophysics is described by a unique and universal framework.

The rate of light emission per unit carrier concentration, or radiative rate, is a fundamental semiconductor parameter that determines the limit photoconversion efficiency of solar cells. In hybrid perovskites, a...

Inorganic metal halide perovskites such as CsPbI3 are promising for high‐performance, reproducible and robust solar cells. However, inorganic perovskites are sensitive to humidity, which causes the transformation from the black phase to the yellow δ, non‐perovskite phase. Such phase instability has been a significant challenge to long‐term operational stability. Here, we report a surface dimensionality reduction strategy, using 2‐(4‐aminophenyl) ethylamine cation to construct a Dion‐Jacobson 2D phase that covers the surface of the 3D inorganic perovskite structure. The Dion‐Jacobson layer mainly grew at the grain boundaries of the perovskite, effectively passivating surface defects and providing favourable interfacial charge transfer. The resulting inorganic perovskite films exhibited excellent humidity resistance when submerged in an aqueous solution (Isopropanol: Water = 4: 1v/v) and exposed to a 50% humidity air atmosphere. The DJ 2D/3D inorganic perovskite solar cell (PSC) achieved a power conversion efficiency (PCE) of 19.5% with a Voc of 1.197 eV. It retained 83% of initial PCE after 1260 h of maximum power point tracking under 1.2 sun illumination. Our work demonstrates an effective way for stabilising efficient inorganic perovskite solar cells.This article is protected by copyright. All rights reserved

Stability issues could refrain from commercializing lead halide perovskite solar cells (PSCs) despite having comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long‐term stability is gaining incremental importance. Excess lead iodide (PbI2) causes perovskite degradation, although it aids in crystal growth and defect passivation. Herein, we synthesized functionalized oxo‐graphene nanosheets (Dec‐oxoG NSs) to effectively manage the excess PbI2. Dec‐oxoG NSs provide anchoring sites to bind the excess PbI2 and passivate perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free electrons. The inclusion of Dec‐oxoG NSs leads to a PCE of 23.7% in inverted (p‐i‐n) PSCs. Devices retain 93.8% of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one‐sun illumination, and exhibit high stability under thermal and ambient conditions.