| Recently, the efficiency of phosphorescent white organic light-emitting device(PhWOLED) has been dramatically improved, which is competitive with that offluorescent lamps. In addition to efficiency enhancement, several aspects of PhWOLEDare under intense discussions, especially lower efficiency roll-off and higher colorstability. In this paper, some basic and systematic works have been carried out toimprove the charge carrier balance and performance of PhWOLED.1. In order to obtain high efficiency PhWOLED with a double emissive layerstructure, firstly we fabricated a blue OLED by adjusting the doping concentration ofiridium(III) bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′] picolinate (Firpic) inN,N′-dicarbazolyl-3,5-benzene (mCP) host. After that, we altered the dopingconcentration of [2-(4-tertbutylphenyl) benzothiazolato-N,C2′] iridium (acetylacetonate)[(t-bt)2Ir(acac)] in4,7-diphenyl-1,10-phenanthroline (BPhen) host at a fixedconcentration of Firpic to obtain the PhWOLED.2. Charge carrier balance of the emissive layer has a great influence on the deviceperformance. By analyzing the characteristics of mCP,4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP),1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), BPhen based PhWOLEDs, it was found that that the utilization ofBPhen as a yellow host greatly improved blue emission and efficiency of thePhWOLED due to the improved charge carrier balance and broadened recombinationzone. Moreover, the devices with various EMLs structure and single-carrier deviceswere further investigated to reveal the emission mechanism of BPhen based PhWOLED,the influence of dopants on the charge carriers and the reason of improved performance.3. We inserted various interlayers with different thickness between the two EMLsto adjust the distribution of excitons in the device. The interlayers are the mCP andp-bis(triphenylsilyl)benzene (UGH2) respectively.4. The influence of interlayer on the charge carriers and performance ofPhWOLED was studied with the assistance of three different interlayers consisting of3 nm BPhen, mCP, BPhen mixing mCP inserted between the blue and yellow emittinglayers. The results showed that3nm mCP interlayer was beneficial for the balance ofcharge carriers and excitons, and suppressing the exciton quenching processes. ThePhWOLED with mCP interlayer had the highest performance, exhibiting stable whiteemission with slight CIE coordinates variation at a wide luminance range, with amaximum luminance of46,714cd/m2, a current efficiency of35.4cd/A, a powerefficiency of14.5lm/W, especially the current efficiency was as high as28.8cd/A at40,000cd/m2, which means that the efficiency roll-off was rather low.5. By studying the electroluminescence characteristics of phosphorescent OLEDswith an undoped bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate)[(pbi)2Ir(acac)] EML of various film thicknesses, the concentration quenching of(pbi)2Ir(acac) monomer emission and the influence of quenching effect on excitonrecombination were investigated.6. To further explain the relationship between the triplet excimer emission and thechange of exciton distribution, we fabricated several devices with different electronblocking layer (EBL). When the EBL was decreased from25nm to5nm, there was notriplet excimer emission and the (pbi)2Ir(acac) was the dominant. A WOLED with lowefficiency roll-off was realized by combining the blue fluorescent emission from NPBand green phosphorescent emission from (pbi)2Ir(acac). The CIE coordinates of theWOLED at a bias of9V are (0.33,0.42), located in the white region. The maximumcurrent efficiency is10.4cd/A at1042cd/m2, which slightly shifted to8.0cd/A at10,000cd/m2, exhibiting a low efficiency roll-off. The low roll-off was due to thebroadened recombination zone and reduced quenching effects in the EML when no EBLwas employed. |
PDF Full Text Request