Efficient tandem polymer light-emitting diodes with PTPA-P/ZnO as the charge generation layer

Abstract

Tandem polymer light emitting diodes (tandem PLEDs) consisting of two EL units, connected by a charge generation layer (CGL), exhibited the advantages of improved luminous efficiency and lifetime. Here, the ether-soluble PTPA-P and alcohol-soluble ZnO were used as the CGL for a tandem PLED with a configuration of ITO/ZnO/PEIE/P-PPV/PEDOT:PSS/PTPA-P/ZnO/PFN-OX/P-PPV/MoO₃/Al. The water wetting ability of the P-PPV layer was obviously improved by oxygen plasma treatment, which enabled PEDOT:PSS to be spin-coated on the surface directly. Appropriately choosing an orthogonal solvent and a cross-linked material ensured the successful fabrication of an eight-layered device by solution processing. The whole connecting layer of PEDOT:PSS/PTPA-P/ZnO/PFN-OX has a high optical transmittance, which ensures the extraction of the light generated in the tandem PLED. The PTPA-P/ZnO CGL has more effective charge generation ability than the PEDOT:PSS/ZnO CGL, due to the shallower HOMO level of PTPA-P and the larger contact area between PTPA-P and ZnO, compared with those based on PEDOT:PSS. Due to the higher charge generation ability of the PTPA-P/ZnO CGL the tandem device exhibited a high luminous efficiency of 26.9 cd A⁻¹ and an external quantum efficiency (EQE) of 10.4%, which were over 2 times those in the relevant single-emitting-layer device.