Vertical Component Distributions in Organic Solar Cells Controlled by Photocrosslinking and Layer‐by‐Layer Deposition

Abstract The vertical component distribution is investigated in bulk‐heterojunction (BHJ) type organic solar cells (OSCs) by combining photocrosslinking of donor polymers with layer‐by‐layer (LbL) deposition of acceptor molecules. Different concentrations of a tetradiazirine photocrosslinker controlled the crosslinker density of the polymer films, which in turn influenced the permeation behavior of acceptor molecules during LbL deposition. Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS), X‐ray photoelectron spectroscopy (XPS), and grazing incidence wide‐angle X‐ray scattering (GIWAXS) analyses revealed the effect of crosslinker density on the vertical distribution of donor and acceptor materials. Increasing crosslinker density during LbL processing produces distinct bilayer‐like structures, with each layer having different component ratios. OSC performance is optimized at lower crosslink densities with the uniformly mixed structure, while higher densities reduce the donor‐acceptor interface, thereby decreasing power conversion efficiency from 12.6% (0.3 wt.%) to 4.48% (2.0 wt.%). These findings challenge the previous assumption that molecular permeation during LbL deposition naturally results in continuous component gradients or p‐i‐n structures, which are proposed as an advantage of the LbL method over traditional BHJ structures.