Highly efficient homojunction organic light-emitting diodes

Abstract

Highly efficient blue p–i–n homojunction phosphorescent top-emitting organic light emitting diodes (TOLEDs) and white homojunction bottom-emitting phosphorescent OLEDs based on two different bipolar host materials, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) and (4-(diphenyl(4-(1-phenyl-1H-benzimidazol-2-yl)phenyl)silyl)phenyl)-diphenylamine (p-BISiTPA), are reported. The pyridine-containing bipolar transport 26DCzPPy is used as the host material for blue TOLEDs in this study. By electrical doping, the carrier injection and transport characteristics are greatly improved. The optical structure is optimized for of blue light emission to enhance the color purity and improve the view characteristics. As a result, the optimized blue p–i–n homojunction phosphorescent TOLED exhibits a maximum current efficiency of 36.5 cd A⁻¹, a maximum power efficiency of 28.0 lm W⁻¹, and an external quantum efficiency (EQE) of up to 16.3%. The device emits a saturated blue color with the full width at half maximum of 50 nm and the color shift with viewing angles is almost eliminated. In order to realize high performance homojunction white OLEDs (WOLEDs) and further simplify the device structure, the tetraarylsilane-based p-BISiTPA with a triplet energy value at 2.69 eV and superior bipolar charge transport properties is employed without p- and n-doping in the charge transport layers. The WOLED, based on three primary-color emitters, shows a peak power efficiency of 32.9 lm W⁻¹, EQE of up to 16.4%, and rather stable spectra with a color-rendering index at 76. The results from both TOLEDs and WOLEDs demonstrate that the homojunction structure works well and homojunction OLEDs with performances comparable to those of heterojunction devices can be achieved by structure optimization.