Xiao Zhang

Quasi‐2D lead‐halide perovskites consist of conducting inorganic layers with tunable thickness (n) separated by large organic spacer cations. Typically, domains with different n and bandgaps are formed within a single film. Here, the crystallization of the films is tuned by mixing Dion‐Jacobson (DJ) with Ruddlesden‐Popper (RP) spacer cations. Compared to the quasi‐2D perovskite film based on solely the DJ type spacer 1,4‐phenylenedimethylammonium (PDMA), a film with less defects and more vertically aligned crystallization is achieved by addition of the RP type spacer propylammonium (PA). As the film structure plays an important role in the photophysics, time‐resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) are used to investigate the impact of mixing these spacer cations on the dynamics of hot carrier cooling, the occurrence and directionality of energy or electron transfer between the different domains, and the exciton and charge carrier dynamics. Exciton transfer from low‐n to high‐n domains occurs at a favorable faster rate for the PDMA‐based film (0.0640 ps⁻¹) compared to the PA‐based film (0.0365 ps⁻¹), while the mixed spacer film demonstrates intermediate behavior (0.0473 ps⁻¹). This study facilitates the design of advanced materials with optimized photophysical characteristics for a next generation of optoelectronic devices.

Quasi-2D perovskites have attracted attention as potential solar energy absorber materials due to their balanced efficiency and stability and their unique quantum-well structures. In order to facilitate directional excitons and charge carrier transport and preferential energy transfer landscape in photovoltaic thin films, the phase distribution formed by different types of microstructural domains should be regulated. In this work, the Dion–Jacobson-type spacer 1,4-phenylenedimethanammonium (PDMA) was used, and different strategies were pursued to control the phase distribution in formamidinium-based (FA) quasi-2D perovskites based on the composition of (PDMA)FA₄Pb₅I₁₆. In general, doping with FACl modulated the crystallization kinetics, forming 2D low-n crystals on the top surface or a reversed-gradient phase distribution, depending on whether excess or substitutional doping was employed. Alternatively, mixing with a Ruddlesden–Popper spacer helped bridging to adjacent octahedra in pure PDMA-based perovskites and improved crystallization, while regulating the quantum-well structures to give a normal-gradient phase distribution, where 2D domains resided on the bottom side. By combining FACl doping and spacer mixing, the film showed both a reversed-gradient phase distribution and larger vertically aligned grains. This work contributes to the knowledge of how to manipulate and regulate the phase distribution in FA-based quasi-2D perovskites and further paves the way for fabricating corresponding devices with high efficiency and stability.

Lead halide perovskites with mixed cations/anions often suffer from phase segregation, which is detrimental to device efficiency and their long‐term stability. During perovskite film growth, the gel stage (in between liquid and crystalline) correlates to phase segregation, which has been rarely explored. Herein, cation diffusion kinetics are systematically investigated at the gel stage to develop a diffusion model obeying Fick's second law. Taking 2D layered perovskite as an example, theoretical and experimental results reveal the impact of diffusion coefficient, temperature, and gel duration on the film growth and phase formation. A homogenous 2D perovskite thin film was then fabricated without significant phase segregation. This in‐depth understanding of gel stage and relevant cation diffusion kinetics would further guide the design and processing of halide perovskites with mixed composition to meet requirements for optoelectronic applications.