All-aromatic liquid crystal triphenylamine-based poly(azomethine)s as hole transport materials for opto-electronic applications

Abstract

We have explored the opto-electronic properties of a new series of hole-transport materials based on main-chain triphenylamine-based poly(azomethine)s. 4,4′-Diaminotriphenylamine (TPA) was polymerized under benign conditions with either terephthalaldehyde (TPA-14Ta), 2,5-thiophenedicarboxaldehyde (TPA-25Th) or 1,3-isophthalaldehyde (TPA-13Iso) to yield polymers with an M_n of 5700–16 000 g mol⁻¹. Despite the non-linear, or ‘kinked’, backbone geometry, all polymers form lyotropic solutions in chloroform and this liquid crystal (nematic) ordering could be maintained in the solid film after spin casting. All polymers exhibit high glass-transition temperatures (T_g > 250 °C) and display outstanding thermal stabilities, i.e. 5% wt loss in excess of 400 °C under nitrogen. The HOMO and LUMO energy levels of these polymers were in the range of 5.0–5.3 and 2.4–3.3 eV below the vacuum level, respectively. Introduction of a thiophene heterocycle (TPA-25Th) resulted in a material with a low optical band-gap of approximately 2.0 eV, whereas TPA-14Ta and TPA-13Iso showed optical band gaps of 2.3 and 2.6 eV, respectively. A photovoltaic device based on a TPA-25Th/PCBM blend (1 : 3) showed an EQE of 20% at 500 nm. Under simulated sunlight, the device gives an open-circuit voltage of 0.41 V, a short-circuit current of 1.23 mA cm⁻² and a fill factor of 0.24, leading to a power conversion efficiency of 0.12%.