Near-white and tunable electrophosphorescence from bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II)-based organic light emitting diodes

Abstract

A new phosphorescent platinum(II)-pyridyltriazolate complex, bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II), Pt(ptp) 2, was synthesized and incorporated into organic light emitting diodes for evaluation as electrophosphorescent dopant. Single crystal X-ray diffraction shows that the complex forms columnar stacks stabilized via strong intermolecular Pt(II) ⋯ Pt(II) interactions (3.289 Å), making it amenable to form excimers. Three types of phosphorescent emissions are seen for neat and doped thin films of Pt(ptp) 2 into 4,4′-bis(carbazol-9-yl)triphenylamine (CBP): structured monomer emission in the blue–green region ( λ max ∼ 480 nm), unstructured excimer emission in the yellow region ( λ max ∼ 550 nm), and rather broad unstructured extended excimer emission in the orange–red region ( λ max ∼ 600 nm). By varying the doping level in x% Pt(ptp) 2:CBP thin films, the ratio of monomer to excimer emission could be adjusted leading to tuning both the photoluminescence (PL) and electroluminescence (EL) wavelengths. Doping levels of ∼5–10%v were found to be optimal for both EL efficiency and white color coordinates resulting from simultaneous monomer and excimer emissions. Peak power and luminous efficiencies obtained were 9.8 lm/W and 14 cd/A, respectively, while the peak external quantum efficiency was 6.6% in a standard OLED device structure. The emissions for the 7.5–15%v Pt(ptp) 2:CBP devices are characterized by CIE coordinates of ∼(0.3, 0.5), rendering near-white EL with green hue. The CRI increased with dopant concentration, reaching 56 at the 45% doping level. More optimal white EL color could not be attained by fine-tuning the doping level. The devices exhibit good stability at higher current density and brightness levels, which is atypical of phosphorescent organic light emitting diodes. Strategies to attain higher efficiencies and CRI are proposed based on the results of this work.