Thienoisoindigo (TIIG)-based small molecules for the understanding of structure–property–device performance correlations

Abstract

In this contribution, a series of small molecule semiconductors based on the recently conceived thienoisoindigo (TIIG) and three different end-capping moieties (benzene (Bz), naphthalene (Np), and benzofuran (Bf)) with varied electron-donating strength and conformations has been synthesized by Suzuki coupling and utilized for organic photovoltaics (OPVs). Incorporation of different end-capping blocks onto the TIIG core facilitated the tuning of optical properties and the electronic structure (HOMO/LUMO energy levels), solid-state morphology and performance in OPVs. It is apparent that the bandgaps within this series (TIIG-Bz, TIIG-Np, and TIIG-Bf) were progressively red-shifted and the absorption coefficients were enhanced by increasing the conjugation length and/or the donor ability of the end-capping units. In addition, HOMO and LUMO levels were shown to simultaneously follow changes made to the end-capping moieties. The best performing OPVs using TIIG-Np : PC₇₁BM exhibited a power conversion efficiency (PCE) of 1.81% with J_sc = 7.15 mA cm⁻², FF = 0.39, and V_oc = 0.66 V. With the aim of exploring underlying structure–property relationships for this new class of molecular systems, we have quantitatively investigated various morphological structures in both the pristine small molecule films and small molecule/PC₇₁BM blend films using a combination of grazing incidence wide angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). In this study, a correlation between the molecular structure, thin film morphology, and photovoltaic properties of these conjugated small molecules was established that provides guidance for the molecular design of new photovoltaic semiconductors based on TIIG units.