Donor–acceptor–π–acceptor based charge transfer chromophore as electron donors for solution processed small molecule organic bulk heterojunction solar cells

Abstract Two benzothiazole (BT) based donor–acceptor–π–acceptor (D–A–π–A) molecular system denoted as BT3 and BT4 have been designed, synthesized and their optical and electrochemical properties were investigated. The BT4 show wider absorption profile and lower bandgap as compared to BT3 due to the strong electron withdrawing ability of dicyanoquinodimethane (DCNQ) as compared to tetracyanobutadiene (TCBD). The solution processed bulk heterojunction solar cells were fabricated using BT3 and BT4 as electron donor and PC 71 BM as electron acceptor. The organic solar cells optimized dichloromethane (DCM) processed BT3 :PC 71 BM (1:1) and BT4 :PC 71 BM (1:1) showed PCE of 2.56% and 3.68%, respectively. The higher PCE of BT4 :PC 71 BM is related to the wider absorption of the blend and better ordered domain sizes in the blend as compared to BT3 :PC 71 BM. The devices processed with 1,8-diiodoctane (DIO) additives showed PCE of 3.77% and 5.27%, for BT3 :PC 71 BM and BT4 :PC 71 BM blends, respectively.