Star-shaped benzotriindole-based donor-acceptor molecules: Synthesis, properties and application in bulk heterojunction and single-material organic solar cells

Abstract

The search of the novel building blocks for π-conjugated donor-acceptor (D-π-A) molecules remains an urgent task to design promising materials for organic solar cells (OSCs) and other electronic devices. Here we report on the design and synthesis of two star-shaped D-π-A small molecules based on benzotriidole (BTI) electron-donating core, BTI(2T-DCV-Hex)3 and BTI(2T-CNA-EHex)3, end-capped with either hexyldicyanovinyl or 2-ethylhexylcyanoacetate acceptor groups. Comprehensive investigation and comparison of the optical, thermal and physicochemical properties of these molecules and their analogue with the triphenylamine (TPA) core, N(Ph-2T-DCV-Hex)3, revealed the effect of the electron-withdrawing groups and type of the donor core on their properties. The BTI-based material BTI(2T-DCV-Hex)3 differs from the amorphous TPA-based analogue by high crystallinity and blue-shifted absorption and luminescence spectra. The change of electron-withdrawing group from hexyldicyanovinyl to 2-ethylhexylcyanoacetate leads to a higher energy of the lowest unoccupied molecular orbital, lower solubility and several times higher photoluminescence quantum yield in solutions achieving 67%. Evaluation of the photovoltaic performance of these materials in single-material OSCs and as a donor material in bulk heterojunction OSCs with PC71BM as an acceptor revealed that the devices based on BTI(2T-DCV-Hex)3 are more efficient as compared to those based on BTI(2T-CNA-EHex)3. In comparison to N(Ph-2T-DCV-Hex)3, the photovoltaic devices based on BTI(2T-DCV-Hex)3 showed the comparable performance in bulk heterojunction OSCs and two times higher performance (about 1%) in single-material OSCs. As a result, we conclude that the BTI core is a promising block for the design of semiconducting materials for organic photovoltaics and other related applications.