Tetracyanobutadiene- and dicyanoquinodimethane-functionalized D′-A-(π)-D-(π)-A-D′ type indacenodithiophene small-molecule chromophores: Synthesis, optoelectronic properties and photovoltaic response

Abstract

Tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ)-functionalized π-conjugated small-molecule chromophores (SMCs) have received continuous attention in organic optoelectronic devices. Herein, four D′-A-(π)-D-(π)-A-D′ type SMCs (IDT-PCN, IDT-TCN, IDT-T-PCN, and IDT-T-QPCN) based on TCBD/DCNQ were designed and synthesized by strategically changing A and D′ groups and inserting π-bridge to investigate of their molecular structures, optical and electrochemical properties and other functional properties. These symmetrical SMCs were easily obtained through shorter reaction steps relative to unsymmetrical molecules. Results demonstrated that the end-capped D′ (benzene and thiophene) has a limited effect on molecular structures and optoelectronic properties, whereas the the insertion of π-bridge (thiophene) and change of A unit (TCBD versus DCNQ) can evidently redshift absorption spectra, reduce optical bandgaps (E_g^opt) and lower HOMOs. As a result, IDT-PCN and IDT-TCN exhibit similar absorption spectra and E_g^opt as well as HOMO/LUMOs; in contrast, for IDT-T-QPCN, the most redshifted absorption band, narrowest E_g^opt (1.16 eV) and highest-lying HOMO/LUMOs are obtained. Theoretical calculations indicate these D′-A-(π)-D-(π)-A-D′ molecules possess a good planar geometry along the main conjugated molecular backbone, facilitating effective π-electron delocalization and strong ICT effect from the D to A units; while the end-capped D′ and far = C(CN)₂ group are almost orthogonal with the main conjugation backbone. Moreover, the photovoltaic response of these new SMCs was evaluated by fabricating binary and ternary organic solar cells (OSCs). Results reveal that both PBDB-T:SMC and SMC:PC₇₁BM binary systems produced rather poor device efficiencies (below 1 %). Inspiringly, compared to the control PM6:Y6 binary device, the PM6:Y6:IDT-T-PCN ternary OSCs with IDT-T-PCN as the third component showed enhanced photostability and an obviously improved efficiency (17.36 %), the highest for TCBD-based active materials. On a larger picture, this finding argues the influence of the A group and π-bridge on manipulating the optoelectronic properties of TCBD/DCNQ-based D′-A-(π)-D-(π)-A-D′ chromophores and the application potential of these SMCs in constructing efficient ternary OSCs, which opens more avenues for the rational development of TCBD/DCNQ-based SMCs for efficient OSCs and other optoelectronic applications.