Interplay of Molecular Orientation, Film Formation, and Optoelectronic Properties on Isoindigo- and Thienoisoindigo-Based Copolymers for Organic Field Effect Transistor and Organic Photovoltaic Applications

Abstract

A systematic study on the effects of heteroarenes on the solid state structure and optoelectronic properties of isoindigo analogues, namely, PBDT-IIG and PBDT-TIIG, used in solution-processed organic field effect transistors (OFETs) and organic photovoltaics (OPVs) is reported. We discover that the optical absorption, frontier orbitals, backbone coplanarity, molecular orientation, solubility, film morphology, charge carrier mobility, and solar cell performance are critically influenced by the heteroarenes in the acceptor subunits. PBDT-IIG exhibits good p-type OFET performance with mobility up to 1.03 × 10^–1 cm² V^–1 s^–1, whereas PBDT-TIIG displays ambipolar mobilities of μ_h = 7.06 × 10^–2 cm² V^–1 s^–1 and μ_e = 2.81 × 10^–4 cm² V^–1 s^–1. PBDT-IIG and PBDT-TIIG blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) yield promising power conversion efficiencies (PCEs) of 5.86% and 2.55%, respectively. The excellent mobility of PBDT-IIG can be attributable to the growing fraction of edge-on packing by the interfacial surface treatment. Although PBDT-TIIG could construct a long-range face-on packing alignment to meliorate its photocurrent in OPV applications, the low open-circuit voltage caused by its high-lying HOMO energy level and greater recombination demonstrates the trade-off between light absorption and solar cell performance. Nevertheless, PBDT-TIIG with a PCE of 2.55% is the highest reported PCE to date for the TIIG-based systems