Solution-processed indacenodithiophene-based small molecule for bulk heterojunction solar cells
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 (Voc) of 0.93 V, a short-circuit current (Jsc) 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.