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-C-71-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.