A julolidine functionalized benzimidazoline n-dopant: optimizing molecular doping in fullerene derivatives by modulating miscibility

Abstract

Poor miscibility with the most popular n-dopants of polar 1H-benzimidazolyl molecules and limited doping efficiency have severely retarded the development of highly efficient n-doped organic semiconductors for organic thermoelectrics (OTE) applications. One effective strategy to enhance miscibility involves introducing hydrophilic triethylene-glycol-type (TEG-type) side chains into n-type organic semiconductors. In this contribution, an alternative strategy is introduced for miscibility modulation by replacing the dimethylaniline group of the representative n-dopant of 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI-H) with julolidine; 9-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-julolidine (JLBI-H) was attained with more lipophilicity. JLBI-H was found to modulate its miscibility with three representative n-type fullerene derivatives: PC₆₁BM, PTEG-1, and PTEG-2 with increasing hydrophilicity in an opposite trend to N-DMBI-H. The optimal PC₆₁BM film doped with 15 mol% JLBI-H exhibited a remarkably high conductivity (σ) of 1.25 S cm⁻¹ at 100 °C, one of the highest values reported so far for solution-processed PC₆₁BM films. The optimized PTEG-1 film doped with 15 mol% JLBI-H achieved a room-temperature σ of approximately 2.0 S cm⁻¹, comparable to that doped with 30 mol% N-DMBI-H. In contrast, the σ of the PTEG-2 film doped with 15 mol% JLBI-H is only one-tenth of the one doped with 22 mol% N-DMBI-H. The doping efficacy of JLBI-H on the three fullerene derivatives is significantly influenced by the initial microstructure of the host films, the miscibility level between the dopant and the hosts and the unique thermal annealing treatment to trigger doping, as evidenced by a collaborative analysis on electrical behaviors, GIWAXS and AFM. This study demonstrates the effectiveness of tailoring the structure of molecular n-dopants to modulate the miscibility between host–dopant pairs and achieve high electrical performance.