An Iminostilbene Functionalized Benzimidazoline for Enhanced n-Type Solution Doping of Semiconducting Polymers for Organic Thermoelectrics

Doped organic semiconductors play a central role in the development of several innovative optoelectronic and energy harvesting applications. Currently, the realization of thermoelectric generators, which require both hole- and electron-transporting materials with high electrical conductivity, is strongly hindered by the scarce availability of stable solution-processable n-dopants and their limited efficiency. Herein, the synthesis of 4-(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)-dibenzazepine (IStBI), a novel derivative belonging to the well-known family of the benzimidazoline compounds, is presented. The functionalization with the planarized and rigid iminostilbene substituent allows, without significantly affecting the compound electronic structure, an efficient intercalation of the dopant molecules inside the ordered regions of thin films of the benchmark n-type polymer poly(N,N′-bis-2-octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5′-2,2′-bithiophene) P(NDI2OD-T2). Consequently, a maximum electrical conductivity of (1.14 ± 0.13) × 10−2 S cm−1 is recorded, exceeding by one order of magnitude what previously achieved upon solution doping of the reference P(NDI2OD-T2) with benzimidazoline derivatives. The thermoelectric power factor is also simultaneously increased. The findings confirm that tailoring of the dopant chemical structure to improve structural interactions with the host semiconductors can be employed as a successful strategy to achieve more effective n-doping, helping to close the performance gap with p-type materials.