Abstract
2D perovskites are a class of halide perovskites offering a pathway for realizing efficient and durable optoelectronic devices. However, the broad chemical phase space and lack of understanding of film formation have led to quasi-2D perovskite films with polydispersity in perovskite layer thicknesses, which have hindered device performance and stability. Here, a simple and scalable approach is reported, termed as the "phase-selective method", to fabricate 2D perovskite thin films with homogenous layer thickness (phase purity). The phase-selective method involves the dissolution of single-crystalline powders with a homogeneous perovskite layer thickness in desired solvents to fabricate thin films. In situ characterizations reveal the presence of sub-micrometer-sized seeds in solution that preserve the memory of the dissolved single crystals and dictate the nucleation and growth of grains with an identical thickness of the perovskite layers in thin films. Photovoltaic devices with a p-i-n architecture are fabricated with such films, which yield an efficiency of 17.1% enabled by an open-circuit voltage of 1.20 V,while preserving 97.5% of their peak performance after 800 h under illumination without any external thermal management.
Highlights
The 2D perovskites synthetized in this work consist of nanometer-thick layers of organicinorganic halide perovskite with the formula A’2An-1MnX3n+1 that are separated one from another by layers of large A’ organic cations.[25,26] A key challenge is to design a reliable, generally applicable and scalable process to synthetize phase-pure 2D perovskite thin-films dominantly formed by a homogeneous distribution of a single perovskite layer thickness, and with a perovskite layer thickness intermediate between the 2D perovskite n=1 and the 3D perovskite limiting cases.[24,27,28] the classical synthesis method typically yields a mixture of n-phase and 3DPKs.[23]
We anticipate that the discovery of a reliable synthesis method to fabricate phase pure 2D perovskite thin films will enable optoelectronic devices that concomitantly exhibit high efficiency, tunability, scalability, and long-term operational stability, as well as pave the path for new types of devices, which exploit the rich physics of 2D materials
We report on a phase-selective synthesis method involving a pre-crystallization step of 2D perovskite crystalline powders followed by single-step solution processing, to produce 2D perovskite thin-films principally formed from perovskite layers with a single-valued layer thickness, with good crystallinity and desired orientation suitable for fabricating optoelectronic devices
Summary
We performed an in-situ x-ray diffraction and absorbance of the thin-films during its growth, as illustrated, which has shown to be a powerful approach to identify the different phases (n values) during the thin-film formation.[23] These experiments were performed by spin casting the respective precursor solution on a glass microscope slide, which was kept at room temperature without annealing in order to slow down the kinetics of nucleation and film formation We note that this process yielded films like those obtained by post annealing (Supplementary Fig. 9). For film growths using the phase-selective method, the targeted n=3 2D perovskite phase formed relatively more quickly and continued growing over time as the amount of excess solution is consumed (Fig. 2 a,c) Based on these data, we infer that the phase-selective synthesis method bypasses the intermediate phase (absence of diffraction peak corresponding to intermediate phase complex), which promotes the formation of the desired layer thickness with negligible phase segregation. The films fabricated by this method resulted in photovoltaic devices with an efficiency of 17.1%, and enhanced stability
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