We use a layered solution crystal growth technique to synthesize high-quality single crystals of phenylalkylammonium lead iodide organic/inorganic hybrid compounds. Single-crystal X-ray diffraction reveals low-dimensional structures consisting of inorganic sheets separated by bilayers of the organic cations. The shortest alkyls yield 2-dimensional structures consisting of inorganic sheets of corner-sharing PbI 6 -octahedra.However, the longer alkyls induce both corner-and face-sharing of the PbI 6 -octahedra, and form new compounds. Density Functional Theory calculations including spin-orbit coupling show quantum confinement in two dimensions for the shorter alkyls, and in one dimension for the longer alkyls, respectively. The face-sharing PbI 6 -octahedra create a confinement leading to effectively one-dimensional behavior. These confinement effects are responsible for the observed peak shifts in photoluminescence for the different phenylalkylammonium lead iodide hybrids. Our results show how the connectivity of the octahedra leads to confinement effects that directly tune the optical band gap.

Two-dimensional metal halide perovskites of Ruddlesden-Popper type have recently moved into the centre of attention of perovskite research due to their potential for light generation and for stabilisation of their 3D counterparts. It has become widespread in the field to attribute broad luminescence with a large Stokes shift to self-trapped excitons, forming due to strong carrier-phonon interactions in these compounds. Contrarily, by investigating the behaviour of two types of lead-iodide based single crystals, we here highlight the extrinsic origin of their broad band emission. As shown by below-gap excitation, in-gap states in the crystal bulk are responsible for the broad emission. With this insight, we further the understanding of the emission properties of low-dimensional perovskites and question the generality of the attribution of broad band emission in metal halide perovskite and related compounds to self-trapped excitons.

Layered halide perovskites have recently shown extraordinary potential for low‐cost solution‐processable optoelectronic applications because of their superior moisture stability over their 3D counterparts. However, few studies have investigated the effect of light on layered hybrid perovskites. Here, the mechanically exfoliated nanoflakes of the 2D perovskite (PEA)2PbI4 (PEA, 2‐phenylethylammonium) are used as a model to investigate their intrinsic photostability. The light‐induced degradation of the flakes is investigated by using in situ techniques including confocal laser scanning microscopy, wide‐field fluorescence microscopy, and atomic force microscopy. Under resonant photoexcitation, (PEA)2PbI4 degrades to PbI2. It is clearly shown that this process is initiated at the crystal edges and from the surface. As a consequence, the photoluminescence of (PEA)2PbI4 is progressively quenched by surface traps. Importantly, the light‐induced degradation can be suppressed by encapsulation using hexagonal boron nitride (hBN) flakes and/or polycarbonates. This report sheds light on a specific mechanism of light‐induced degradation in layered perovskites and proposes a new encapsulation method to improve their photostability.

The higher dielectric constant and electron donating properties of PTEG-1 electron extraction layer synergistically helps to eliminate the light soaking effect and enhance device performance.