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

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.

Efficient fluorescent red, green, and blue organic light-emitting devices with a blue host of spirobifluorene derivative

“…Numerous red fluorescence dyes have been synthesized and studied for fabricating an excellent red OLED, including pyran, chromene, and isophorene containing 4-(dicyanomethylene)-2-methyl-6-[4-(dimethylaminostyryl)-4H-pyran] (DCM) type red dyes [2][3][4][5][6][7][8][9][10][11][12], porphyrin-type macrocyclic red dyes [13][14][15], and squarylium dyes [16][17][18]. Among these red fluorescence dyes, DCM type dyes remain the most efficient material when applied for the red OLEDs, particularly for the dye namely (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) (DCJTB) [4,5].…”

Charge carrier trapping effect by luminescent dopant molecules in single-layer organic light emitting diodes

“…In particular, in the electroluminescent process, we are extremely interested in the energy transfer and carrier trapping phenomena between host and guest molecules, which definitely influences ⌽ EL . In the previous studies, although two emission mechanisms for energy transfer and carrier trapping were proposed for each device, 4,5,[16][17][18] it was still unclear which processes in each host-guest combination were dominant. Thus, in the present study we experimentally elucidated carrier trapping and energy transfer processes by comparing two typical dopant molecules: rubrene and coumarin 6.…”

Organic Light-Emitting Diodes (OLEDs): Materials, Photophysics, and Device Physics

“…To maximize η ext , all of these factors should be nearly 100 %. A high γ can be attained by the construction of appropriate multilayer structures [8,13]. High Φ PL can be achieved by using emissive materials that suppress nonradiative recombination, and these materials are often doped into a host layer with a wide energy gap to minimize the concentration quenching of excitons that usually occurs at high concentrations.…”