In the past decade, three-dimensional (3D) perovskites have been under intense study. However, these devices always suffer from severe performance degradation. Recently, two-dimensional (2D) perovskites have attracted increasing attention due to their excellent environmental stability, but their low power conversion efficiency (PCE) limits their application. Therefore, many strategies have been applied to manipulate the crystallization and suppress the defects and low-n value phases. Additive engineering, as one of the most effective methods to achieve the above-mentioned goals, has been extensively studied. In this contribution, thiourea (TU) and methylammonium chloride (MACl) are successfully employed to contribute synergistically to optimize the crystallization process, leading to an enlarged grain size, smooth and dense surface morphology, and suppressed distribution of n values for improved phase purity. As a result, the optimized inverted Dion−Jacobson (DJ) 2D perovskite solar cell (PSC) device delivers an elevated PCE of 12.16% with a significantly improved short-circuit current density (J SC ) of 18.78 mA cm −2 and fill factor (FF) of 62.70%. In addition, the optimized devices show good environmental stability when exposed to ambient air.

The fabrication of organic solar cells (OSCs) by a layer‐by‐layer (LBL) method has attracted growing attention in recent years. As already known, the pre‐aggregates of conjugated polymers in solution have a profound impact on their microstructure morphology in films. Herein, by simply controlling the solution temperature and annealing processes, the pre‐aggregation behavior of D18 polymer in solution can be fine‐tuned and the microstructure of D18 bottom layer is well manipulated. The optimized D18 bottom layer can effectively regulate L8‐BO upper‐layer‐forming suitable networks for efficient charge transportation. In addition, a vertical phase separation with a special D/D:A/A structure (P‐i‐N‐type component distribution) is also formed. As a result, compared to the 16.43% power conversion efficiency (PCE) of the bulk heterojunction devices, such control enables bilayer OSC devices based on the polymer D18 and L8‐BO to deliver an enhanced PCE of 18.02% with simultaneously improved short‐circuit current density, open‐circuit voltage, and fill factor. It is also demonstrated in these results that the LBL deposition process utilizing the pre‐aggregation of polymer and its fiber‐network‐forming ability is a very promising approach to improve charge dynamics, suppress carrier recombination, and fabricate highly efficient OSCs.

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