High-Crystallinity π-Conjugated Small Molecules Based on Thienylene–Vinylene–Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties

Narrow-band-gap small molecules with π-extended backbones are promising donor materials for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). Herein, a series of acceptor-donor-acceptor (A-D-A) photovoltaic small molecules incorporating thienylene-vinylene-thienylene (TVT) as a central D unit and alkyl-substituted rhodanine or 2-(1,1-dicyanomethylene)rhodanine as terminal A units are designed and synthesized. Their physical properties including photoabsorption, electronic energy levels, hole mobility, and morphological characteristics are systematically investigated. Using solvent vapor annealing (SVA), the morphologies of the BHJ photoactive layers composed of these small-molecule donors and a [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) acceptor can be properly modulated. As a result of increased crystallinity of the donors and desired phase segregation between the donors and PC71BM upon rapid SVA treatment, the photovoltaic performances of the resultant OSC devices undergo drastic enhancement. The results reported here indicate that high-efficiency small-molecule OSCs can be achieved through rational design of the TVT-based molecular framework and optimization of the nanoscale phase-segregated morphology via proper SVA treatment.