Alkoxythiophene‐Directed Fibrillization of Polymer Donor for Efficient Organic Solar Cells

Abstract Realizing fibrillar molecular framework is highly encouraged in organic solar cells (OSCs) due to the merit of efficient charge carrier transport. This is however mainly achieved via the chemical structural design of photovoltaic semiconductors. In this work, through the utilization of three alkoxythiophene additives, T‐2OMe, T‐OEH, and T‐2OEH, the intermolecular interactions among a series of BDT‐type polymer donors, i.e., PM6, D18, PBDB‐T, and PTB7‐Th, are tuned to self‐assemble into nanofibrils during solution casting. X‐ray technique and molecular dynamics simulation reveal that the alkoxythiophene with (2‐ethylhexyl)oxy (─OEH) chains can attach on the 2‐ethylhexyl (EH) chains of these polymer donors and promote their self‐assembly into 1D nanofibrils, in their neat films as well as photovoltaic blends with L8‐BO. By adapting these fibrillar polymer donors to construct pseudo‐bulk heterojunction (P‐BHJ) OSCs via layer‐by‐layer deposition, generally improved device performance is seen, with power conversion efficiencies enhanced from 18.2% to 19.2% (certified 18.96%) and from 17.9% to 18.7% for the PM6/L8‐BO and D18/L8‐BO devices, respectively. This work provides a physical approach to promote the fibrillar charge transport channels for efficient photovoltaics.