Tailoring Molecular Space to Navigate Phase Complexity in Cs-based Quasi-2D Perovskites via Gated-Gaussian-Driven High-Throughput Discovery

Cesium-based quasi-two-dimensional (2D) halide perovskites (HPs) offer promising functionalities and low-temperature manufacturability, which are suited to stable tandem photovoltaics (PVs). However, the chemical interplays between the molecular spacers and the inorganic building blocks during crystallization cause substantial phase complexities in the resulting matrices. To successfully optimize and implement the quasi-2D HP functionalities, a systematic understanding of spacer chemistry, along with the seamless navigation of the inherently discrete molecular space, is necessary. Herein, by utilizing high-throughput automated experimentation, the phase complexities in the molecular space of quasi-2D HP films are explored, thus identifying the chemical roles of the spacer cations on the synthesis and functionalities of the complex materials. Furthermore, we introduce a novel active machine learning algorithm leveraging a two-stage decision-making process, called gated Gaussian process Bayesian optimization (Gated-GPBO), to navigate the discrete ternary chemical space defined with two distinctive spacer molecules, 1,4-butanediammonium and phenethylammonium. Through simultaneous optimization of photoluminescence intensity and stability of quasi-2D HPs which ‘tailors’ the chemistry in the molecular space, a ternary-compositional quasi-2D HP film realizing excellent optoelectronic functionalities, as well as the high PV performance is demonstrated. This work not only provides a pathway for the rational and bespoke design of complex HP materials but also sets the stage for accelerated materials discovery in other multifunctional systems.