Tuning of the Polymeric\nNanofibril Geometry via Side-Chain\nInteraction toward 20.1% Efficiency of Organic Solar Cells

Constructing fibril morphology has been believed to be\nan effective\nmethod of achieving efficient exciton dissociation and charge transport\nin organic solar cells (OSCs). Despite emerging endeavors on the fibrillization\nof organic semiconductors via chemical structural design or physical\nmanipulation, tuning of the fibril geometry, i.e., width and length,\nfor tailored optoelectronic properties remains to be studied in depth.\nIn this work, a series of alkoxythiophene additives featuring varied\nalkyl side chains connected to thiophene are designed to modulate\nthe growth of fibril aggregates in cutting-edge polymer donors PM6\nand D18. Molecular dynamics simulations and morphological characterizations\nreveal that these additives preferentially locate near and entangle\nwith the side chains of polymer donors, which enhance the conjugated\nbackbone stacking of polymer donors to form nanofibrils with the width\nexpanding from 12.6 to 21.8 nm and the length increasing from 98.3\nto 232.7 nm. This nanofibril structure is feasible to acquire efficient\nexciton dissociation and charge transport simultaneously. By integrating\nthe fibril PM6 and L8-BO as the donor and acceptor layers in pseudo-bulk\nheterojunction (p-BHJ) OSCs via layer-by-layer deposition, an improvement\nof power conversion efficiency (PCE) from 18.7% to 19.8% is observed,\ncontributed by enhanced light absorption, charge transport, and reduced\ncharge recombination. The versatility of these additives is also verified\nin D18:L8-BO OSCs, with enhanced PCE from 19.3% to 20.1%, which is\namong the highest values reported for OSCs.