An A–D–A small molecule based on the 3,6-dithienylcarbazole electron donor (D) unit and nitrophenyl acrylonitrileelectron acceptor (A) units for solution processed organic solar cells

Abstract

A new A–D–A small molecule (SM) based on a 3,6-dithienylcarbazole donor unit at the center and nitrophenyl acrylonitrile acceptor units as the terminal ends was synthesized by the Pd catalyzed Stille coupling reaction for application as a donor material in the organic bulk heterojunction solar cells. The SM possesses a low lying HOMO energy level at −5.34 eV and optical band gap of 1.74 eV. The photovoltaic performance of the SM was investigated with PC₆₀BM and PC₇₀BM as acceptors cast from THF with 3% CN additive and without additive. The power conversion efficiency (PCE) of the organic solar cells based on SM:PC₆₀BM cast from THF solvent reached 1.94% with a high V_oc of 0.94 V, a J_sc of 6.08 mA cm⁻² and FF of 0.34, where as the PCE of SM:PC₇₀BM cast from THF solvent was 2.65% with a V_oc of 0.96 V, a J_sc of 7.26 mA cm⁻² and FF of 0.38%, under the illumination intensity of 100 mW cm⁻². The higher PCE of the organic solar cells based on PC₇₀BM as an electron acceptor has been attributed to its strong absorption in the visible region than PC₆₀BM. The PCE of the organic solar cells has been further improved to 3.76% and 4.96% for SM:PC₆₀BM and SM:PC₇₀BM, respectively, cast with CN–THF solvent.