Enhancing color purity and efficiency of white organic light-emitting diodes using a double-emitting layer

This paper presents white organic light-emitting diodes (WOLEDs) based on a novel double-emitting layer consisting of blue and white emitters. A blue fluorescent host of 4,40-bis(2,2-diphenylvinyl)-1,10-biphenyl (DPVBi) doped with 1,4-bis[2-(3-N-ethylcarbazoryl) vinyl] benzene (BCzVB) is used as the blue emitter, and this blue matrix doped with 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9enyl)- 4 H-pyran (DCJTB) is used as the white emitter. In this double-emitting system, the blue emitter not only emits but also assists energy transfer from DPVBi to DCJTB. More significantly, this blue emitter is expected to act as an effective trapping site for holes. It leads to the efficient recombination of electron-hole pairs in the emission region. The white emitter is used to fabricate WOLEDs, in which DCJTB, tris-(8-hydroxyquinoline) aluminum (Alq3) and DPVBi contribute to the red, green and blue emissive components, respectively. We have observed that this strategy can greatly improve the device performances such as better chromaticity, improved color stability and enhanced efficiency. Through the optimization of the device structure, a balanced white emission with Commission Internationale de L'Eclairage (CIE) color coordinates of (0.33, 0.37) was obtained. This device showed stable color coordinates, i.e. the maximum color shift is less than 0.02 units on CIE color coordinates at the current density range of 4-200 mA cm(-2). The maximum luminance achieved was 21 044 cd m(-2) at a driving voltage of 17 V, and the maximum luminance efficiency reached was 9.12 cd A(-1) at the luminance of 292 cd m(-2). Such excellent performance shows that this double-emitting layer structure has great potential as a white light source for eventual applications.