Layer‐by‐Layer‐Processed Organic Solar Cells with 18.02% Efficiency Enabled by Regulating the Aggregation of Bottom Polymers

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.