Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-2-one functionality for efficient bulk-heterojunction devices

Abstract

Two simple semiconducting donor–acceptor–acceptor (D–A1–A) modular, small molecule, non-fullerene electron acceptors, 2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (P3), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of P2 and P3 were directly compared with those of a structural analogue, 3-((4-((4-(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one (P1), which was designed based on a D–A format. All of these new materials comprised an electron rich triphenylamine (TPA) donor core (D) and electron deficient chromen-2-one terminal core (A). In the simple D–A system, TPA and chromenone were the terminal functionalities, whereas in the D–A1–A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping the D/A units constant. The inclusion of A1 was primarily done to induce cross-conjugation within the molecular backbone and hence to generate low band gap targets. The physical and optoelectronic properties were characterized by ultraviolet–visible (UV–Vis), thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited broadened absorption spectra, for instance panchromatic absorbance in case of P3, excellent solubility and thermal stability, and energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with P1, P2 and P3 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: P3-based device showed an impressive enhanced power conversion efficiency of 4.21%, an increase of around two-fold with respect to the efficiency of the best P3HT: P1-based device (2.28%). Our results clearly demonstrate that the D–A1–A type small molecules are promising non-fullerene electron acceptors in the research field of organic solar cells.