White light‐emitting organic electroluminescent devices using the poly(N‐vinylcarbazole) emitter layer doped with three fluorescent dyes (original) (raw)

1994, Applied Physics Letters

in flat panel displays and solid lighting sources. Particularly, white OLEDs (WOLEDs) are promising devices for important applications in full-color flat panel displays with color filters. Generally, the emission spectrum of WOLEDs should be broad and continuous covering the entire visiblelight spectral region [1]. The emitting light color of ideal WOLEDs can be characterized by Commission International de L'Eclairage (CIE) chromaticity coordinate of (0.33, 0.33). To obtain WOLEDs, many methods have been reported. On the one hand, the small molecule based-WOLEDs will be commonly obtained through using multilayer devices, each of luminous layer emits a primary color (i.e. red, blue or green) to generate white-light emission [2, 3]. The charge blockers is necessary to confine charges/carries and excitons within desired regions to improve color purity, but at the same time, the high driving voltages and low power efficiencies are the primary drawbacks due to superimposed structure in multilayer white devices [4]. On the other hand, the polymer based WOLEDs will be obtained through using single-layer structure in which two or three fluorophore or phosphor dyes are blended into one emissive host [5-8]; but there are some inherent problems in multiple dopants WOLEDs, such as color stability under different driving voltage, accurate control of individual dopant concentration, and insufficient Förster energy transfer between chromospheres [4]. What's more, another particularly promising approach is to use aggregates, such as excimers or exciplexes, to get WOLEDs [9-12], which will reduce the number of emissive components and simplify fabrication process, but the power efficiency is still low and need to be improved. Exciplex is considered as one kind of excited state complexes that is formed by an electronically excited state donor molecule, D* (or acceptor A*), with a complementary acceptor molecule, A (or donor D), in their ground Abstract We report a method to achieve white organic light-emitting diodes (WOLEDs) with poly {N,N′-bis (4-butylphenyl)-N,N′-bis(phenyl)-benzidine} (poly-TPD) as single emitting layer. Monomer emission, excimer and electromer emission are all observed from poly-TPD, and combining them the white light emission with color coordinate of Commission International de L'Eclairage (CIE) (0.37, 0.31) was achieved. Double electron transport layer with apposite thickness 1,3,5-tris(1-phenyl-1H-benzimida-zol-2-yl) benzene (TPBi) (30 nm) and tris(8-hydroxyquinolato) aluminum (Alq3) (20 nm) were introduced to improve the performance of WOLEDs. The brightness reached 1000 cd/ m 2 at 22 V with an efficiency of 1.0 cd/A.