Effect of layered structures on the location of emissive regions in organic electroluminescent devices

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Abstract
Effect of layered structures on the location of emissive regions was studied in four types of organic electroluminescent (EL) devices: a single-layered (SL) device consisting only of an emissive layer (EML), two types of double-layered (DL-H and DL-E) devices in which a hole-transport layer (HTL) or an electron-transport layer (ETL) is attached to an EML, and a triple-layered (TL) device in which an EML is sandwiched between a HTL and an ETL. As EML, HTL and ETL material, 9, 10-bis[4-(diphenylamino)styryl]anthracene, 4,4′-bis[(3-methylphenyl)phenylamino]biphenyl and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene, respectively, were used. Within EML layers, a thin sensing layer doped with a squarilium dye, 2,4-bis[4-diethylamino)-2- hydroxyphenyl]cycrobutenediylium-1,3-dioxide was inserted. The change in emission intensity from the dopant, when the location of the sensing layer was systematically varied, gave information on emissive regions in each type of EL device. The emissive region in the SL device extended through the EML, and that in the DL-H device resided near the HTL/EML boundary. On the contrary, those in the DL-E and TL devices were located within a 10-nm-wide region adjacent to the EML/ETL boundary. Moreover, the emission efficiencies of the DL-E and TL devices were found to be higher than those of the SL and DL-H devices. It was experimentally demonstrated that the carrier recombination within the narrow region adjacent to the EML/carrier transport layer boundary gave high emission efficiency.