Solution-processable star-shaped conjugated molecules with multiple p-n branches: synthesis and photovoltaic properties

Well-defined π conjugated star-shaped molecules have attracted much research interest in the field of organic electronics, due to their good monodispersity, high purity, excellent reproducibility, facile functionalization, interesting two/three-dimensional architecture and potential intriguing isotropic optoelectronic properties. Here two new p-n type star-shaped conjugated molecules, S1 and S2, with phenylcarbazole as core and benzothiodiazole derivatives as arms, were designed and synthesized. They are readily soluble in common organic solvents. The structures of the intermediates and resulting molecules were adequately confirmed by ¹H NMR, ¹³C NMR, MALDI-TOF, and elemental analysis. S1 and S2 exhibited good thermal stability according to thermal gravimetric analysis; cyclic voltammetry revealed low-lying HOMO and LUMO energy levels as well as narrow band gaps. The HOMO/LUMO energy levels and band gaps could be facilely modulated by varying the content and sequence of p and/or n type moieties. With incorporating 3-hexylthiophene π bridge unit, S2 film shows a broad absorption band from 300 nm to 650 nm in films with absorption edge red-shifted by ca. 38 nm in comparison with that of S1. Additionally, S2 possessed amorphous properties, while S1 exhibited crystalline properties in powder. Density functional theory (DFT) calculation was also conducted to further understand the electronic properties. Solution-processed organic solar cells based on a blend of S1 and PC70BM (1:3, w/w) exhibited power conversion efficiency (PCE) of 0.34% with a short circuit current density (J_sc) of 1.5 mA/cm₂ and an open circuit voltage (V_oc) of 0.83 V under the illumination of AM1.5, 100 mW/cm². Improved performance was achieved for S2 with PCE of 0.5%, J_sc of 1.85 mA/cm², and a high V_oc of 1.0 V under identical conditions.