Solution-processed benzotrithiophene-based donor molecules for efficient bulk heterojunction solar cells

Abstract

In this study we used convergent syntheses to prepare two novel acceptor–donor–acceptor (A–D–A) small molecules (BT4OT, BT6OT), each containing an electron-rich benzotrithiophene (BT) unit as the core, flanked by octylthiophene units, and end-capped with electron-deficient cyanoacetate units. The number of octylthiophene units affected the optical, electrochemical, morphological, and photovoltaic properties of BT4OT and BT6OT. Moreover, BT4OT and BT6OT possess low-energy highest occupied molecular orbitals (HOMOs), providing them with good air stability and their bulk heterojunction (BHJ) photovoltaic devices with high open-circuit voltages (V_oc). A solar cell device containing BT6OT and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) in a 1 : 0.75 ratio (w/w) exhibited a power conversion efficiency (PCE) of 3.61% with a short-circuit current density (J_sc) of 7.39 mA cm⁻², a value of V_oc of 0.88 V, and a fill factor (FF) of 56.9%. After adding 0.25 vol% of 1-chloronaphthalene (CN) as a processing additive during the formation of the blend film of BT6OT:PC₇₁BM (1 : 0.75, w/w), the PCE increased significantly to 5.05% with values of J_sc of 9.94 mA cm⁻², V_oc of 0.86 V, and FF of 59.1% as a result of suppressed nanophase molecular aggregation.