3,4-Difluoropyridine based polymer additives for optimizing aggregation and phase separation enable 20.07 % efficiency layer-by-layer organic solar cells

Layer-by-layer (LbL) solution-processed organic solar cells (OSCs) have emerged as a promising approach for achieving controlled vertical phase separation and optimized donor-acceptor interfaces. However, precise morphology control still remains challenging for further elevating OSC performance. In this work, we report a molecular design strategy by utilizing conjugated polymer additives to optimize the active layer morphology for fabricating efficient LbL-OSCs. Three 3,4-difluoropyridine based polymers with systematically modified side chains, namely PDFN-H, PDFN-S and PDFN-F, have been prepared to regulate donor polymer aggregation and phase separation. Among them, PDFN-H additive demonstrated superior performance because of its lower miscibility with D18 donor, thus providing enhanced driving force for molecular packing and phase separation. Incorporation of 5 wt% PDFN-H led to a well-defined bicontinuous network morphology with improved charge transport pathways, finally resulting in an impressive power conversion efficiency (PCE) of 20.07 %. Detailed morphological and photophysical studies revealed that the enhanced performance stems from faster exciton dissociation, more balanced charge transport and suppressed recombination. This work demonstrates the effectiveness of rationally designed polymer additives for optimizing LbL-OSC performance and provides valuable insights for the development of more higher efficiency organic photovoltaic devices.