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

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.