Optically detected magnetic resonance (ODMR) measurements on conjugated polymers and studies of poly-(n-Vinyl Carbazole) (PVK) based LEDs

Combinatorial screening of poly (9-vinyl carbazole) (PVK) OLEDs is described. Thebrightness of the devices improved 10 fold by treating the poly (3,4-ethylenedioxy-thiophene)/poly (styrene sulfonate) (PEDO/PSS) hole injecting layer with ethylene glycol(EG) and 5 fold with optimizing the thickness of the hole blocking layer (HBL) and electrontransport layer (ETL) which results in increased electron injection. The optimized deviceexhib i ted a maximal rad iance R 200 mW/cm 2 and ex te rna l quan tum ef f ic iency r j e x l -1 .0 %with an electroluminescence (EL) spectrum edge at 350 nm and peak at 410 nm.. IntroductionPVK has been studied since 1946 and since the 70s it is used as the thin photoconductivelayer in xerography [1], Only recently, with developing interests in OLEDs, it is beingconsidered as a hole transport layer (HTL) or as a host material for several other dopingmaterials [2], due to its large gap and high triplet energy [3]. Excimer emission from PVKwas reported as earlier as 1969 [4]. However, it has not been used as an actual emittingmaterial in OLEDs, due to its large highest occupied molecular orbital (HOMO) lowestunoccupied molecular orbital (LUMO) gap of 4.9 eV [5],The HOMO of PVK, measured from the vacuum level, is 6.1 eV. Since the work functionof indium-tin oxide (ITO) is 5 eV, the barrier to hole injection is > 1 eV, and hole injection