Thin films based on two-dimensional metal halide perovskites have achieved exceptional performance and stability in numerous optoelectronic device applications. Simple solution processing of the 2D perovskite provides opportunities for manufacturing devices at drastically lower cost compared to current commercial technologies. A key to high device performance is to align the 2D perovskite layers, during the solution processing, vertical to the electrodes to achieve efficient charge transport. However, it is yet to be understood how the counter-intuitive vertical orientations of 2D perovskite layers on substrates can be obtained. Here we report a formation mechanism of such vertically orientated 2D perovskite in which the nucleation and growth arise from the liquid–air interface. As a consequence, choice of substrates can be liberal from polymers to metal oxides depending on targeted application. We also demonstrate control over the degree of preferential orientation of the 2D perovskite layers and its drastic impact on device performance.

Metal halide perovskites have demonstrated strong potential for optoelectronic applications. Particularly, two-dimensional (2D) perovskites have emerged to be promising materials due to their tunable properties and superior stability compared to their three-dimensional counterparts. For high device performance, 2D perovskites need a vertical crystallographic orientation with respect to the electrodes to achieve efficient charge transport. However, the vertical orientation is difficult to achieve with various compositions due to a lack of understanding of the thin film nucleation and growth processes. Here we report a general crystallization mechanism for 2D perovskites, where solvent evaporation and crystal growth compete to influence the level of supersaturation and a low supersaturation is necessary to crystallize vertically oriented thin films starting from nucleation at the liquid–air interface. Factors influencing the supersaturation and crystallization dynamics, such as choices of organic spacers, solvents, and solvent drying rate, have a strong influence on the degree of crystallographic orientation. With this understanding of crystallization mechanism, we demonstrate direct crystallization of thin films with strong vertical orientation using three different organic spacers without any additives, and the vertically oriented 2D perovskites result in efficient and stable solar cell operation.

Impact of rationally selected additives in precursor solutions on the nucleation and growth of hybrid perovskite thin films is investigated.

We report on the thermal conductivities of two-dimensional metal halide perovskite films measured by time domain thermoreflectance. Depending on the molecular substructure of ammonium cations and owing to the weaker interactions in the layered structures, the thermal conductivities of our two-dimensional hybrid perovskites range from 0.10 to 0.19 W m–1 K–1, which is drastically lower than that of their three-dimensional counterparts. We use molecular dynamics simulations to show that the organic component induces a reduction of the stiffness and sound velocities along with giving rise to vibrational modes in the 5–15 THz range that are absent in the three-dimensional counterparts. By systematically studying eight different two-dimensional hybrid perovskites, we show that the thermal conductivities of our hybrid films do not depend on the thicknesses of the organic layers and instead are highly dependent on the relative orientation of the organic chains sandwiched between the inorganic constituents.

Halide dealloying in CsPbBrI 2 perovskite solar cells proves to be critical in achieving high performance. This dealloying occurs under steady illumination and is a reversible process, suggesting transitions between equilibrium states under dark and illuminated environments. Through decoupling charge collection in the electron-and hole-transporting layers, our photoluminescence, X-ray diffraction, and solar cell device characterization results suggest that the light-induced halide dealloying improves hole collection in solar cells, resulting in increased efficiency (9.2% ± 0.64 on average with a champion power conversion efficiency of 10.3%). Our results provide deeper insights on the impact of dealloying on perovskite solar cell performance and highlight the growing potential of all-inorganic perovskite solar cells.