Enhanced molecular planarity for high-performance photodetectors: Effect of backbone conformation via alkyl substitution position control in dithienoquinoxaline-based conjugated polymers

Abstract

Conjugated polymers (CPs) with donor and acceptor units exhibit a flexible conformation due to the rotation of the single bond of the bridge groups which greatly hinders a precise study of their photodetection properties. Moreover, the modulation mechanism related to complex factors of CPs molecular conformation on photodetection performance is not clear, causing CP-based organic photodiodes (OPDs) to generally suffer from high dark current and low specific detectivity. In this work, we synthesized two novel PBQx-i-4F and PBQx-o-4F CPs by combining fluorinated BDT with DTQx units using i- and o-alkylated side chain-based thiophene linkages, respectively. Quantum simulation and photo-physics study revealed that PBQx-i-4F displayed a dominant trans conformation, a more planar molecule structure, and high utilization efficiency of photoinduced excitons. The OPDs based on PBQx-i-4F exhibited higher on–off ratios by over two orders of magnitude and higher detectivity (Dsh*) by nearly one order of magnitude compared with those based on PBQx-o-4F. PBQx-i-4F with the i-alkyl substitution on the thiophene linkage displayed enhanced planarity, resulting in more ordered packing and larger population of face-on orientation in film, where decreased trap density and trap-assisted recombination led to reduced dark current and improved charge transport. In summary, our work provides an insight into the relationship between the fine-tuning of the molecular conformation, planarity, and packing of CPs and their photodetection properties in OPDs.