An A′–A–D–A–A′ type small molecule based on 2,7-carbazole for solution-processed organic solar cells with high open-circuit voltage

Abstract

A new A′–A–D–A–A′ type low band gap small molecule Cz-TBT-CAC8 with 2,7-carbazole as the center donor (D), benzothiadiazole as an electron acceptor (A) and alkyl cyanoacetate as another electron acceptor (A′) has been designed and synthesized as the donor material for the solution-processed bulk hetero-junction solar cells. This small molecule possesses a low lying HOMO level at −5.30 eV and an optical band gap of 1.73 eV. The solar cell based on Cz-TBT-CAC8/PC₆₁BM blend film spin-coated from chlorobenzene solution exhibits a PCE of 2.87% with a high V_oc of 1.05 V. After adding 0.2% DIO into the chlorobenzene solution as an additive, the PCE was further improved to 3.95% with a V_oc of 1.03 V, a J_sc of 7.32 mA cm⁻² and a FF of 0.52.