Highly efficient pure white polymer light-emitting devices based on poly(N-vinylcarbazole) doped with blue and red phosphorescent dyes

Abstract

Efficient white-polymer-light-emitting devices (WPLEDs) have been fabricated with a single emitting layer containing a hole-transporting host polymer, poly(N-vinylcarbzole), and an electron-transporting auxiliary, 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenylene, codoped with two phosphorescent dyes: Iridium(III)bis (2-(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) and home-made Ir-G2 for blue and red emission, respectively. With the structure of ITO/PEDOT:PSS 4083(40 nm)/emission layer(80 nm)/Ba(4 nm)/Al(120 nm), the device showed a maximal luminous efficiency (LE) of 13.5 cd A−1(corresponding to an external quantum efficiency (EQE) of 6.8%), and a peak power efficiency (PE) of 6.5 lm W−1 at 6.0 V. Meanwhile, the device exhibited pure white emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.34, 0.35) at a current density of 12 mA cm−2, which is very close to the equi-energy white point with CIE coordinates of (0.33, 0.33). The device performance can be further optimized when more balanced hole/electron injection is achieved by incorporating a lower conducting type anode buffer layer (PEDOT:PSS) and incorporating poly[(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorenene)-alt-2,7-(9,9-dioctyfluorene)] (PFN) as an electron injection layer at the cathode. The optimized device showed an LE of 24.6 cd A−1 (with an EQE of 14.1%), while the peak power efficiency reached 12.66 lm W−1. Moreover, the WPLEDs showed good electroluminescence (EL) stability over a wide range of operating current density and luminance.