Ether-soluble hole-transporting polymers based on triphenylamine/phenothiazine moieties with shallow HOMO levels†
Hole-transporting polymers, PTPA-F, PTPA-Cz, PTPA-P, PTPA-T and PPT-T, were synthesized using triphenylamine or phenothiazine as the backbone unit, and oligo(ethylene glycol) (OEG) as the side chain, respectively. Thereinto, PTPA-F, PTPA-Cz and PTPA-P emit blue light and own wide bandgaps larger than 2.8 eV, while PTPA-T and PPT-T have narrower bandgaps of 2.56 eV and 2.35 eV, respectively. All of the polymers are soluble in ether solvents, such as dioxane and dioxolane, but insoluble in alcoholic solvents. Moreover, the resulting polymers have shallow highest occupied molecular orbital (HOMO) energy levels in the range of −5.12 to −4.87 eV, which can be adopted as electron donors to construct charge carrier generation layers (CGLs) (with zinc oxide (ZnO) as the electron acceptor) by the orthogonal solvent process. The PTPA-P/ZnO based CGL device exhibits the best charge carrier generation and transferability with the highest current density of over 600 mA cm−2 at 8 V, which is on a par with the PEDOT:PSS/ZnO based device. The results reveal that these hole-transporting polymers have great potential to fabricate highly efficient CGLs for tandem organic light-emitting diodes (OLEDs).