Synthesis, characterization, and photovoltaic properties of a solution-processable two-dimensional-conjugated organic small molecule containing a triphenylamine core

Abstract

A novel solution-processable two-dimensional-conjugated organic small molecule based on triphenylamine (TPA) core, (E)-2-(3-(4-(bis(4-(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c] [1,2,5] thiadiazol-4-yl)thiophen-2-yl)phenyl)amino)styryl)-5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (TPA-TBT), has been designed and synthesized by Knoevenagel reaction. Different from the linear or star-shaped TPA derivatives, in TPA-TBT, two arms of the TPA core are symmetrically connected with the acceptor segment 4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DTBT), but the third arm is connected with a strong acceptor group, 2-(5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (DCM), connected through a trans double bond with the TPA core. TPA-TBT has a relatively low optical band gap (1.91 eV) and a deep highest occupied molecular orbital (HOMO) energy level (−5.53 eV). Bulk heterojunction photovoltaic devices were fabricated using TPA-TBT as the donor and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor with the weight blend ratio of 2:1, 1:1, and 1:2. All the devices have a high open-circuit voltage (Voc) of about 0.9 eV. Device with 1:1 blend ratio and LiF modification to the cathode exhibited a better performance with a short circuit current (Jsc) of 4.86 mA cm−2 and a power conversion efficiency (PCE) of 1.13 %.