磷光白光有机电致发光器件载流子平衡及性能优化的研究

【作者】 雷霞；

【导师】 蒋亚东；

【作者基本信息】 电子科技大学 ， 光学工程， 2012， 硕士

【摘要】 磷光白光有机电致发光器件（PhWOLED）虽然在发光亮度、效率等方面取得了一定的进展，但仍存在许多问题，如器件亮度、效率衰减（roll-off）严重，色漂移大等。本文围绕PhWOLED载流子平衡及其性能优化展开深入研究，具体做了以下几方面的工作：1．为获得高效的具有双发光层结构的PhWOLED，我们先优化磷光染料iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]picolinate（Firpic）在N,N′-dicarbazolyl-3,5-benzene（mCP）中的掺杂浓度，得到高效的蓝光器件。在此基础上，通过固定Firpic的浓度，调节[2-(4-tertbutylphenyl)benzothiazolato-N,C2′]iridium(acetylacetonate)[(t-bt)2Ir(acac)]在4,7-diphenyl-1,10-phenanthroline（BPhen）中的浓度，确定了最佳染料掺杂浓度，并从理论上分析了染料掺杂浓度的变化对器件发光特性的影响。2．发光层中载流子的平衡性直接影响器件的性能，在上述最佳器件结构基础上，我们固定染料的掺杂浓度，分析了不同黄光主体mCP、4,4′-bis(9-carbazolyl)-1,1′-biphenyl（CBP）、1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene（TPBi）、BPhen对发光层中载流子注入和传输的影响。结果表明当BPhen为黄光主体时，发光层中电子空穴平衡性更好，载流子复合区域得到拓宽，此时白光器件的蓝光成分和效率均显著增加。此外，我们制备了具有不同发光层的白光器件和单载流子器件，进一步研究了基于BPhen为黄光主体的PhWOLED的发光机理、染料分子对载流子传输的影响及其性能优异的原因。3．我们围绕基于BPhen为黄光主体的PhWOLED，通过在两发光层之间引入不同厚度的隔层：mCP和p-bis(triphenylsilyl)benzene（UGH2），调节激子在两发光层之间的分布，进一步优化了白光器件的性能。4．我们引入了3nmBPhen、3nmmCP、3nmBPhen与mCP按质量比1:1混合的薄膜，系统地分析了不同性质的隔层对器件中载流子、激子的影响，进而对器件性能的影响。结果表明在双发光层的界面处引入3nmmCP时有利于平衡发光层中载流子、激子的分布、同时抑制了三线态激子的淬灭。器件的最大亮度、电流效率、功率效率、分别为46714cd/m2、35.4cd/A、14.5lm/W，并且在亮度为40000cd/m2时，电流效率仍高达28.8cd/A，roll-off小；色坐标在较大电压范围内稳定。5．通过分析具有不同bis(1,2-dipheny1-1H-benzoimidazole)iridium(acetylacetonate)[(pbi)2Ir(acac)]厚度的OLED的电致发光特性，我们研究了(pbi)2Ir(acac)薄膜在电压驱动下的淬灭效应以及其对器件中载流子复合的影响。6．通过改变电子阻挡层（EBL）的厚度，进一步调节了载流子在发光层的分布，分析了三线态激基缔合物随着激子分布的变化。当EBL降低至5nm时，器件中没有了三线态激基缔合物的发射，(pbi)2Ir(acac)分子的发光占主导。当没有EBL时，器件中有两个复合区域，(pbi)2Ir(acac)的磷光绿光发射结合NPB的荧光蓝光发射产生了磷光荧光混合WOLED，此时器件的在9V电压下的色坐标为（0.33，0.42），位于白光区域；器件在亮度1042cd/m2下的最大电流效率为10.4cd/A，在亮度为10000cd/m2时，电流效率只下降为8.0cd/A。发光区域的拓宽，使得磷光发光区域激子淬灭减弱，器件体现出较小的roll-off。更多还原

【Abstract】 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.更多还原