Enhancing inter-domain connectivity by reducing fractal dimensions: the key to passivating deep traps in organic photovoltaics†

The detrimental impact of non-geminate recombination on high-performance organic photovoltaics has been recognised and primarily attributed to bimolecular recombination. However, the recent surge in Y-series acceptor-based systems has drawn attention to deep-trap-assisted monomolecular recombination. This study reveals the morphological origin of deep traps in the prototypical PM6:Y6 system, identifying isolated crystalline and amorphous Y6 domains as key contributors. The findings underscore the importance of improving inter-acceptor domain connectivity for effective trap passivation. For the first time, we have pinpointed a crucial metric for inversely quantifying the inter-acceptor domain connectivity: the crystalline domain fractal dimension (D_f). Due to the self-similar nature of fractal structures, the fractal dimension propagates across multi-length scales and can be controlled by tuning local intermolecular aggregation motifs. Remarkably, combining diiodide benzene (DIB) as the additive and layer-by-layer (LBL) processing effectively promotes the more extended backbone order of Y6 molecules, consequently reducing the fractal dimensions and passivating deep traps. By applying this strategy to another high-performance system, D18:L8BO, a benchmark efficiency of 19.6% is achieved, among the highest efficiencies reported for LBL OPVs.