Solution-processed indacenodithiophene-based small molecule for bulk heterojunction solar cells

Abstract

A novel small molecule with a acceptor–donor–acceptor (A–D–A) structure, IDT(BTTh₂)₂, containing an electron-rich indacenodithiophene (IDT) unit as core, flanked by electron-deficient 2,1,3-benzothiadiazole (BT) units and end-capped with hexyl-substituted bithiophene units, has been synthesized for photovoltaic application. IDT(BTTh₂)₂ shows a broad absorption in the visible range with an optical band gap of ca. 1.80 eV and possesses a relatively deep HOMO level at −5.21 eV. The solution-processed bulk heterojunction solar cells based on the blend of IDT(BTTh₂)₂/PC₇₁BM (1 : 3, w/w) without using any treatment such as a solvent additive or thermal annealing, showed a power conversion efficiency (PCE) of 4.25% with a high open-circuit voltage (V_oc) of 0.93 V, a short-circuit current (J_sc) of 9.42 mA cm⁻² and a fill factor (FF) of 48.5%, under the illumination of AM 1.5G at 100 mW cm⁻². These results indicate that indacenodithiophene-based small molecules are promising for bulk heterojunction solar cells.