Revealing the Side-Chain Dependent Ordering Transition of Highly-Crystalline Double-Cable Conjugated Polymers

Highly-crystalline conjugated polymers are important for microstructure analysis and charge transport in organic electronics. In this work, we have developed a series of highly-crystalline double-cable conjugated polymers for application in single-component organic solar cells (SCOSCs). These polymers contain conjugated backbones as electron donor and pendant perylene bisimide units (PBIs) as electron acceptor. PBIs are connected to the backbone via alkyl units varying from hexyl (C6H12) to eicosyl (C20H40) as flexible linkers. The highly-crystalline nature of these materials allows us to systematically study the effect of the length of the alkyl linkers on the nanostructure and photovoltaic performance. In particular, we find that for double-cable polymers with short linkers, the PBIs tend to stack in a head-to-head fashion, resulting in large d-spacing (e.g. 64 A for the polymer P12 with C12H24 linker) along the lamellar stacking direction. When the length of the linker groups is longer than a certain length, the PBIs instead adopt a more ordered packing likely via H-aggregation, resulting in short d-spacings (e.g. 50 A for the polymer P16 with C16H32 linker). Evidence for this transition is provided by X-ray diffraction measurements along with cryo-transmission electron microscopy measurements, where different packing motifs of the PBI units are clearly imaged. The different packing facilitated by longer linker groups is associated with improved exciton separation and charge transport, resulting in enhanced efficiencies of SCOSCs based on the polymer P16. The findings in this work demonstrate that double-cable conjugated polymers can be an important family of highly-crystalline conjugated polymers. Furthermore, this work demonstrates how the precise molecular packing of the acceptor units influences the photovoltaic performance of SCOSCs.