Deciphering the Morphology Evolution of Layer-by-Layer Processing Via In Situ Spectroscopy Measurement for High-Performance Organic Photovoltaics

Although the layer-by-layer (LBL) processing can usually achieve an optimal bulk-heterojunction morphology for high-performance Organic photovoltaics (OPVs), the unambiguous working principles governing the morphology evolution are still lacking. To address this issue, here the phase-separation kinetics of LBL processing are comprehensively studied using in situ spectroscopies, which are very sensitive to the intermolecular interactions, e.g. the molecular packing and D:A phase-separation. Upon casting the nonfullerene acceptor (i.e., the BTP-eC9) solution on top of the polymer donor, i.e., PM6, it is found that 1) the solvent will first swell the polymer and induce a polymer gel, 2) following the polymer gelation, the NFAs will immediately permeate into the cavities of polymer gel and form a molecular-level D:A mixing, and 3) with the evaporation of the solvent, phase-separation between the donor and acceptor occurs and results in a bulk-heterojunction morphology as those achieved with blend-cast processing. With these understandings, more diverse processing conditions have been purposely utilized to dictate the phase evolution process precisely. As a result, a high efficiency of 19.7% is reached, representing one of the best among binary OPVs. Therefore, this work deciphers phase-separation kinetics of LBL processing and should pave the way toward advanced morphology control for high-performance OPVs.