Freezing the p-i-n Interlayer with a Symmetric Phenolic Compound to Achieve Favorable Vertical Morphology and Efficient Pseudo-Planar Heterojunction Organic Solar Cells with Excellent Stability

Abstract

Although achieving a suitable vertical phase separation morphology is essential for improving charge transport efficiency, reducing charge recombination, and ultimately boosting the efficiency of organic solar cells (OSCs), simple and effective methods to construct the ideal morphology with long-term stability remain lacking. Herein, 4,4'-Dihydroxybiphenyl (BPO) featuring bisphenol functional groups is introduced into the PM6/BTP-eC9 system as a donor modification layer to fabricate efficient and stable pseudo-planar heterojunction (PPHJ) OSCs. BPO with orthogonal solvent is spin-coated as an interlayer between donor and acceptor layers, effectively preventing the upper layer solution from washing away or damaging the underlying film. The surface energy of PM6 is altered by BPO, leading to a larger Flory-Huggins interaction parameter for PM6/BPO/BTP-eC9, favoring the formation of vertical phase separation morphology. Consequently, an outstanding efficiency of 20.03% was achieved for the PM6/BPO/L8-BO:BTP-eC9-based ternary device. Moreover, BPO can be utilized as morphological stabilizer to mitigate the rapid diffusional motion of donor and acceptor molecules, thereby stabilizing the metastable morphology of the p-i-n layer. Therefore, the PPHJ devices with BPO protective layer demonstrated excellent stability. This work presents a simple and effective strategy for optimizing vertical phase separation morphology and improving stability in PPHJ OSCs by freezing the p-i-n interlayer.