Solution-processable two-dimensional conjugated organic small molecules containing triphenylamine cores for photovoltaic application

Abstract

Two solution-processable two-dimensional conjugated organic small molecules based on triphenylamine (TPA) cores, TPA-BT-C8 and TPA-3Th, were designed and synthesized. As to TPA-BT-C8, two arms of the TPA core are symmetrically connected with a thiophene donor group and a benzothiadiazole acceptor group, while the third arm consists of a strong acceptor group of 2-(5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (DCM) connected through a trans double bond with the TPA core. For TPA-3Th, two arms of its TPA core are composed of only donor group, terthiophene, whereas the third arm consists of an acceptor group of cyano-n-octyl acetate connected through a trans double bond with the TPA core. The investigation indicated that TPA-BT-C8 has a lower energy band gap and wider absorption than TPA-3Th due to the strong intramolecular charge transfer effect in TPA-BT-C8. The two molecules have a deep highest occupied molecular orbital (HOMO) energy level. Bulk heterojunction photovoltaic devices were fabricated using TPA-BT-C8 or TPA-3Th as the donor and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) as the acceptor. All the devices have a high open-circuit voltage (V_oc) of about 0.9 eV. Devices based on TPA-BT-C8 have a much higher short circuit current (J_sc) (8.47 mA cm⁻²) and power conversion efficiency (PCE) (2.26%) than devices based on TPA-3Th (4.32 mA cm⁻², 1.21%), resulting from wider solar light absorption of TPA-BT-C8 and better compatibility and film-formation ability of TPA-BT-C8 with PCBM than TPA-3Th. Incident photon-to-electron conversion efficiency (IPCE) spectra also confirmed that TPA-BT-C8 based devices have a wider and red-shifted response range than TPA-3Th based devices, which leads to a higher performance of the former devices.