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Abstract Cesium‐based quasi‐2D halide perovskites (HPs) offer promising functionalities and low‐temperature manufacturability, suited to stable tandem photovoltaics. 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 HPs are explored, thus identifying the chemical roles of the spacer cations on the synthesis and functionalities of the complex materials. Furthermore, a novel active machine learning algorithm leveraging a two‐stage decision‐making process, called gated Gaussian process Bayesian optimization is introduced, to navigate the discrete ternary chemical space defined with two distinctive spacer molecules. Through simultaneous optimization of photoluminescence intensity and stability that “tailors” the chemistry in the molecular space, a ternary‐compositional quasi‐2D HP film realizing excellent optoelectronic functionalities 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. 

Abstract Nanoscopic packing structures crucially determine the charge conduction and the consequent functionalities of organic semiconductors including bulk heterojunctions (BHJs), which are dependent on various processing parameters. Today's high‐performance colloidal quantum dot photovoltaics (CQDPVs) employ functional organic semiconductors as a hole transport layer (HTL). However, the processing of those films replicates a protocol dedicated to high‐performance organic PVs, and thus little is known about how to control the molecular packing structures to maximize the hole extraction function of the HTLs. Herein, it is uncovered that the random‐oriented, but closer‐packed BHJ crystallites, constructed by 1,2‐dichlorobenzene (o‐DCB) as a solvent, allow exceptional charge conduction vertically across the film and restrict diffusion‐driven charge transfer process, enabling ultrafast hole funneling from CQD to BHJ to be extracted. As a result, a power conversion efficiency of 13.66% with high photocurrent >34 mA cm⁻²is achieved by employingo‐DCB‐processed BHJ HTL, far exceeding the performance of the CQDPV solely employing neat polymer HTL. A charge conduction mechanism associated with the BHJ HTL structure suppressing the bimolecular recombination is proposed. This works not only suggests key principles to control the packing structures of organic HTLs but also opens a new avenue to boost optoelectronic performance.