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

We have explored the opto-electronic properties of a new series of hole-transport materials based oil 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 degrees C) and display Outstanding thermal stabilities, i.e. 5% wt loss in excess of 400 degrees 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 hand gaps of 2.3 and 2.6 eV, respectively. A photovoltaic device based oil a TPA-25Th/PCBM blend (1 : 3) showed an EQE of 20% at 500 nm. Under simulated Sunlight, the device gives all 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%.