Backbone Fluorination of Benzodithiophene-Based Hole-Transporting Polymers for Enhanced Organic Transistors and Nanocrystal Photovoltaics

Abstract

Chemical substitution is a propitious strategy for optimizing the charge transport properties of π-conjugated donor–acceptor (D–A) semiconducting materials in organic electronic devices. To explore the effects of fluorine substitution on the electronic and structural properties of organic field-effect transistors (OFETs) and photovoltaics (PVs), two new benzo[1,2-b:4,5-b′]dithiophene (BDT)-based hole transport polymers (HTPs) were synthesized and characterized. The BDT monomers consisting of 2,6-bis(trimethytin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene monomer (BDT monomer), and (4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) (FBDT monomer) were combined with 2,5-dibromofuran to produce BDT-Fu and FBDT-Fu HTPs. Fluorine integration significantly improved the molecular structure, optical, electrochemical, and morphological properties of these polymers, and the optoelectronic properties of the resulting devices. In FBDT-Fu, the fluorination enhanced crystallinity, optical absorption, and morphology, leading improvement in hole mobility of 3.49 × 10^–3 cm² V^–1 s^–1 in optimized poly(methyl methacrylate) (PMMA)-gated OFETs, with an on/off current ratio exceeding 10³. Consequently, FBDT-Fu-based silver bismuth sulfide (AgBiS₂) nanocrystal PVs achieved a power conversion efficiency of 5.5%, a high fill factor of 55.46%, and an open-circuit voltage of 0.504 V under 1-sun illumination. This molecular design strategy offers an effective approach for optimizing the electrical properties of organic conjugated semiconductors for next-generation optoelectronic devices.