White organic light emitting diodes based on a novel starburst fluorene derivative

Abstract

White organic light-emitting diodes (WOLEDs) are fabricated by thermal vacuum deposition method based on a novel starburst fluorene derivative of 1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2-yl) benzene (HKEthFLYPh). The photoluminescent (PL) spectrum of the HKEthFLYPh peaks at 374 nm, which overlapped with the absorption spectra of N,N'-di(naphthalen-2-yl)-N,N'-diphenyl-benzine (NPB), tris(8-hydroxyquinolinato)aluminum (Alq) and 5,6,11,12-tetraphenylnaphthacene (Rubrene) materials. And PL spectra of HKEthFLYPh: NPB, HKEthFLYPh: Alq and HKEthFLYPh: Rubrene blend in chloroform solution showed characteristics of NPB, Alq and Rubrene. Using HKEthFLYPh as energy transfer layer, NPB as a hole transporting and blue light-emitting layer, Rubrene as a yellow emissive layer, and Alq as an electron transporter, WOLEDs with a undoped structure are fabricated. The structure of WOLEDs are indium-tin-oxide (ITO)/NPB (40-x nm)/HKEthFLYPh (4 nm)/NPB (x nm)/Rubrene (1 nm)/Alq (40 nm)/Mg: Ag (200 nm). The results demonstrated that when x=0 nm the device showed a yellowish white light with the Commission Internationale de l'Eclairage chromaticity (CIE) coordinates changing from (x-0.37, y=0.40) to (x=0.34, y=0.37) under bias voltage from 4 to 10 V; when x=5 nm the device exhibited a pure white light emission with CIE coordinates changing from (x=0.32, y-0.30) to (x=0.34, y=0.34) under bias voltage from 4 to 10 V. Also, the pure white device had a turn-on voltage (defined as the bias required to produce a measurable luminance of 1 cd/m 2 ) of 3.8 V, a luminance of 1137 cd/cm 2 at a bias voltage of 15 V, and a maximum luminance efficiency of 0.51 lm/W at 4.25 V.