Study On White Organic Light-emitting Devices

Organic light-emitting devices (OLEDs) have been attracting muchattention due to their potential applications in flat-panel displays. Since Tangdemonstrated the high brightness OLEDs at low operating voltage for the firsttime in 1987, materials and devices for OLEDs have developed rapidly, andsmall size OLED displays have been commercially available. Compared withhomochromous devices, the structure of white OLEDs (WOLEDs) are complexdue to the fact that the spectra of WOLEDs should fully span the the entirevisible spectrum. WOLEDs can be used in not only white displays but also fullcolor displays combined with color filters, backlights for liquid crystal displaysand even illumination light sources. In this thesis, we introduce three kinds ofWOLEDs with different materials and structures, and study the characteristicsof these devices and the principles for adjusting the chromaticity of whiteemission. Multilayer structures are commonly used in WOLEDs due to its easyfabrication processing, convenient chromaticity-tuning and high luminance andefficency. WOLEDs are demonstrated by inserting a thin layer of tris(8-hydroxyquinoline) aluminum (Alq) doped with4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl) (DCJTB)into N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) 117吉林大学博士学位论文layer. Alq without doping is used as an electron-transporting layer and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine or BCP) as anexciton-blocking layer. NPB layers are separated by the doped Alq layer, thelayer that sandwiched between BCP and doped Alq layers acts as ablue-emitting layer, and the other as a hole-transporting layer. The doped Alqlayer acts as a red and green-emitting as well as chromaticity-tuning layer,whose thickness and position as well as the concentration of DCJTB in Alqpermit the tuning of the device spectrum to achieve a balanced white emissionwith Commission Internationale De L′Eclairage coordinates of (0.33, 0.33). Thedevice has a maximum luminance of 6623 cd/m2 and a maximum externalefficiency of 1.36 %, corresponding to 2.56 cd/A. More highly efficientWOLEDs can be achieved if more efficient dyes are used in this kind ofdevices. Interest in poly-p-phenylenevinylene (PPV) has increased dramaticallysince the discovery that polymers on the basis of PPV can be used as alight-emitting material in electroluminescent devices. Because of definitestructure, oligomers as model compounds for the corresponding polymers and asactive materials themselves have been paid more and more attention.Characteristics of two novel derivatives of oligo(phenylenvinylene) (OPV),2,5,2',5'-tetrakis(4'-biphenylenevinyl)-biphenyl (TBVB) and 2,5-diphenyl -1,4-distyrylbenzene with two trans- double bonds (trans-DPDSB), synthesized byProf. Yuguang Ma's group in the Key Laboratory for Supramolecular Structureand Materials of Ministry of Education, Jilin University, are investigated. BlueOLEDs are fabricated by using TBVB and trans-DPDSB as emitting layer,respectively. The device with 30-nm-thick TBVB attains the maximumluminance of 2154 cd/cm2, efficiency of 1.62 cd/A and CIE coordinates of(0.20, 0.26)at 14, 5 and 10 V, respectively. CIE coordinates of (0.16, 0.09) fordevice with 10-nm-thick trans-DPDSB and 10-nm-thick BCP, which are almostidentical to the coordinates of National Television System Committee (NTSC) 118吉林大学博士学位论文standards (0.14, 0.08) are recorded at 16 V. The device attains the the maximumluminance of 1337 cd/cm2 and efficiency of 1.17 cd/A at 16 and 5 V,respectively. WOLEDs based TBVB or trans-DPDSB and an ultrathin layer of5,6,11,12-tetraphenylnaphthacene (rubrene) are fabricated. The structure ofdevices is simple and may favor the lifetime of WOLEDs. A fairly pureWOLED with CIE c...