有机电致发光的能量传递和载流传输研究
|
摘要
有机半导体具有许多有用的特性,是一种非常吸引人的材料。基于有机半导体材料的电子器件,如有机发光器件(OLED)、场效应晶体管等由于在成本和柔性方面比无机器件有优势,引起了人们的极大兴趣。近年来,由于其特性和潜在应用价值,基于有机小分子材料的有机发光器件的研究较热。主要原因是由于大量的有机材料在可见光谱范围内荧光量子效率高,因此,适合于做显示器。
光发射是载流子复合形成的单重态激子辐射衰减的结果。然而,不是所有的载流子复合会对发光有贡献,：一部分激子在体内或界面以非辐射或淬灭方式衰减。因为形成激子时,单重态与三重态激子的比例是大约1：3,在系统间串阅过程充分的情况下,荧光辐射的内量子效率约为25%,而磷光辐射的内量子效率约为100%。在这点上,对于基于有机分子的光电子器件,掺杂是一项重要技术。掺杂经常被用于调节发光颜色和提高发光效率。因此,对于基于能量传输从主体到客体的有机电致发光器件,发光层中适当的荧光掺杂可以使器件效率更高。这样,可以调节发光波长,并通过优化主体和客体材料提高器件效率。为降低电压和提高效率必须使载流子的注入变得容易,改善注入和传输的一种有效方法是对有机层进行掺杂。传输层掺杂的控制对器件起着至关重要的作用。本论文将在各种有机电致发光器件的制备、特性、载流子传输性能的改善方面进行研究。P(1)
有机电致发光二极管(Organic light-emitting diodes, OLEDs)由于其具有自发光、宽视角和在平板显示中的应用而备受瞩目。高效低驱动电压的红光OLED器件对于有机全彩显示的发展起到了至关重要的作用。通常用一个掺杂体系来获得红光,而此体系由主体材料和红色掺杂物所组成。当主体材料掺杂少量红色荧光染料,则该掺杂会发出高亮度的红光。
红色OLEDs通常采用将红色掺杂剂掺入主体有机材料,但大多数研究广泛使用基于具有优异稳定性和电荷传输能力的8-羟基喹啉铝(tris-(8-hydroxyquinoline) aluminum, Alq3)作为主体,并且从主体到红色掺杂的能量传递在获得红色放光中起到了重要作用。尽管如此,对于由于在主体和客体之间存在不完全的能量传输以及在高浓度掺杂情况下会发生浓度淬灭,这种掺杂体系若想获得纯度高和高效率的红光还存在一定难度。
最近,一种用于激发完整的能量连续传递过程的共掺杂方法得到了发展,正好解决了上述问题。在共掺杂体系中,发光层由主体材料A和两个客体材料B、C组成,能量传递过程从A到B,再由B到C。通过使用这种共掺杂方法,基于主体Alq3红光OLED器件发光得到了巨大提高。但是,因为在当今OLED显示应用方面,大量使用DCJTB这种荧光红色染料掺杂到Alq3主体这种方式,得到较低的电子迁移率(5.2×10-6cm2/V·s),被证实不能同时满足红光所有的需求。此外,由于Alq3本身能级的限制,其仅能作为红光和绿光OLED器件的主体材料,而在光敏处理蓝光掺杂中无能为力。因此,在有机电致发光二极管领域,获得一个高效、低驱动电压、发饱和红光的二极管将成为一项挑战。
在本文中,我们设想一个宽禁带的9,10-di (2-naphthyl) anthracene(ADN),该聚合物以其高的热稳定性而被广泛用来作蓝光OLED器件的主体材料。因其有较宽的禁带宽度,被看做在全彩色OLED器件中非常有前途的主体材料。此外,由于ADN拥有较高的电子迁移率,能达到3.1×10-4cm2/(V·s),它也被看做未来能降低驱动电压的理想材料。但是当ADN被用于红光时,由于其禁带宽度(3.2eV)而导致从ADN到DCJTB的直接能量传递效率不高,并且降低了ADN的重叠发光和DCJTB的吸收。为了克服这个问题,我们使用辅助掺杂剂C545T,该材料禁带宽度介于ADN和DCJTB之间,以提高能量传递效率。一个广泛接受的事实是,在不同客体或主体之间传递能量,Forster能量传递形式在提高发光亮度上占主要作用。选择C545T作为辅助掺杂剂提高了电致发光效率和器件驱动电压。在共掺杂发光层,ADN作为能量给体,C545T作为敏化剂,DCJTB作为红色荧光染料。
实验中,我们制备了两种染料掺杂器件,并且对其特性加以比较。对于红光器件而言比较优化的器件结构如下：glass substrate/ITO (80 nm)/m-MTDATA (20 nm)/NPB (10 nm)/EML (x nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)。对于共掺杂ADN基的器件,35nm厚的发光层包含了ADN基(ADN：red dopant:assistant dopant)和共主体器件([ADN:Alq3]:red dopant:assistant dopant). m-MTDATA用于空穴注入层,NPB为空穴传输层,Alq3用于电子传输材料,LiF andAl分别作为电子诸如层和阴极。有机层和阴极通过在1.0×10-5 Torr气压下真空蒸镀而成。电致发光谱(electroluminescent spectra)和色坐标(Commission International d'Eclairage, CIE)通过PR-650色度计。电流-电压特性和亮度特性分别用Keithley 2400 dc Source Meter和Minolta LS-110 luminance meter进行测量。
结果与讨论：发现掺杂型有机电致发光器件对得到发光亮度高、稳定和色彩选择性是一个有效方法。在主客体之间中存在两种能量传输机制,(1) Forster能量传递和(2) Dexter能量传递。对于在发光层的荧光受体,能量传递机制被证明为Forster形式且较宽(40-100 A)。Forster能量传递(FET)是给体和受体共振偶极-偶极耦合,同时Dexter传递是通过遵循自旋保护定则从给体到受体的激子扩散。FET是非辐射传递,分子A(受体)的偶极子跃迁产生一个场来诱导刺激分子D(给体)的发射。给体和受体从基态到激发态的迁移必须遵循Forster传递形式。单谱线激子传递能量到掺杂分子的单态常基于如下Forster过程：1D*+1A→1D+1A* (1)
其中,左上角的1表示单线态,星号表示激发态。因此,一个有效的传递需要给体发射和受体吸收光谱重叠。根据Forster理论,给体发射光谱和受体吸收光谱的重叠部分导致给体和受体间的高的能量传递速率,也提高了受体的发光效率。因此,高度重叠满足了能量传递更高速率和对给体受体间能量传递基本要求。因为激发的给体弛豫,其能量由染料分子间的强库伦作用力来传递,并且能量传递的速率由主体和客体分子间的距离R0决定,该至关重要的距离称之为Forster临界半径。该能量传递速率由下式表达：其中τ。给体激发态的平均寿命,对应的和Forster临街半径以下式表达：其中心为阿伏伽德罗常数,nd是受体或给体的折射率,β2是取向因子, J(λ)为重叠区域。有效重叠J(λ)能够在下式辅助下进行计算：其中Fd(v)是给体荧光光谱,在公式2.4中已给出定义；εa(v)是受体分子脉冲消失相干光谱；υ是能量波数。这表明了Forster临界半径(R0)独立于荧光给体和受体吸收之间的重叠区域,同时也反映了在迁移过程中共振的必要。因此,我们在本文中研究了在发光层中的能量传递过程机制。我们就这样首先分别研究了给体(ADN)和受体(DCJTB)分子的光致发光(photoluminescence, PL)和吸收光谱。从分析ADN和DCJTB光谱显示较差的重叠,我们利用C545T作为敏化剂来辅助ADN到DCJTB的能量传递。图1.1显示了ADN、C545T和DCJTB的PL谱和光学吸收谱。从图1.1可以看出,ADN的PL谱和DCJTB的吸收谱之间较差重叠。这说明从ADN到DCJTB的能量传递低效。为了克服该问题,我们用C545T作为敏化剂来辅助使ADN到DCJTB的能量得以有效传递。观察ADN和C545T的光谱可以清楚地看到ADN的PL谱线与C545T吸收谱之间存在良好的重叠。因此,与[ADN:DCJTB]体系相关的区域比较之后得到,[ADN：C545T]混合体系的光谱显示了在直线一下的区域(重叠区)相当大。通过光谱位置(PL谱和吸收谱)的判断,我们决定使用C545T作为辅助掺杂剂来帮助提高从ADN到所谓的红色染料DCJTB之间的能量传递。我们同时也能观察到C545T的PL谱和DCJTB吸收谱之间具有较好的光谱重叠,这也说明能量能够有效从ADN传递到C545T,然后接着从C545T传递到DCJTB.这种级联机制能够有效提高器件性能。
如图1.2所示,不同浓度下的C545T的ADN基器件的EL光谱以及最优浓度1wt%DCJTB的EL光谱。从中可以看出,当发光层只掺杂有1wt%浓度的DCJTB时,只有获得最大吸收波长为608 nm的橙光(CIE=0.568,0.414),这是由于ADN的自发光,并且DCJTB红色的峰值,从ADN到DCJTB能量传递的效率低。我们可以推断这些低效能量转移现象与ADN发射光谱和吸收光谱的DCJTB之间的重叠有关。当DCJTB在更高浓度下(2wt%),获得红光,但主要由于浓度淬灭而使cd/A效率下降。为了提高能量传递效率和抑制浓度猝灭现象,我们将C545T作为DCJTB/ADN体系的辅助掺杂剂。我们将少量的C545T作为共掺杂剂掺入发光层中(1wt%DCJTB:0.2wt%C545T),得到色坐标为(0.590,0.401)。当把C545T的站咋浓度提高到0.5wt%或者1wt%,在光谱中出现了C545T的发光峰。这一现象可能是由于C545T具有更高的荧光效率,从ADN到C545T的能量传递比从ADN到DCJTB更容易。实验结果表明,掺杂浓度为DCJTB:C545T=[1wt%:0.2wt%]的器件性能最好。
表格1.1是共掺杂器件的性能数据。通过比较掺杂了C545T和未掺杂C545T器件的性能,我们发现即使掺杂微量的C545T也会导致器件驱动电压的大幅度下降。而浓度猝灭现象则由于采用了ADN(主体材料)而明显减弱。根据"solvent state polarization "理论,DCJTB的发光强烈依赖于偶极子。而在处于非极性状态时,它只能发黄光。因此,只有通过将极性很强的材料与它掺杂才能发红光。因此我们可以用一种共主体系统,通过很强极性的Alq3来使DCJTB发纯红光。在上一实验中我们看到,基于ADN的共掺杂器件的色度没有明显的改善。为了提高器件的发光特性,我们制备了共主体结构的有基点至发光器件。图1.2是优化的共主体器件1:0.2(wt% DCJTB:wt% C545T).的的光谱图。从图上我们可以看到,当ADN:Alq3=50:50时,器件发红光,色坐标为(0.629,0.359)。通过进一步调节ADN:Alq3的比率,我们可以得到很好侧坐标的红色发光。当ADN:Alq3=80：20时,我们得到红色发光的色坐标为(0.618,0.373)。C545T,DCJTB和ADN共掺杂器件效率和色度的提高可以通过以下来解释。在基于ADN的器件中,可能由三个过程：(1)注入的电子和空穴在主体(ADN)中复合。此时,ADN处于激发态,他将能量通过Forster能量传递形式传递给DCJTB, DCJTB发红光。(2)注入的电子和空穴在主体(ADN)中复合。此时,ADN处于激发态,他将传递给C545T,然后C545T传递给DCJTB, DCJTB发红光。(3) DCJTB作为陷阱中心。注入的电子和空穴在DCJTB中行车那个激子从而发红光。
尽管ADN对浓度猝灭有一定的一致作用,但是在DCJTB浓度比较高的时候,发生了浓度猝灭现象,降低了器件的效率。对于共掺杂器件,当DCJTB的浓度比较低时,过程(1),(2)和(3)都会起作用。但是,过程(2)是主要的。注入的空穴和电子再ADN中复合并形成激子。在Forster能量传递理论中,能量传递的概率正比于掺杂剂的发射光谱和受主材料吸收光谱的重叠面积。不同于于ADN和DCJTB光谱的较小重叠面积,ADN和C545T光谱的重叠面积相当大。
为了研究共主体体系的能量传递特性,我们制备了不同ADN和Alq3比例的器件(0：100,50：50,70：30,and 80：20).研究结果表明,随着ADN浓度的增加器件的驱动电压逐渐降低。在共掺杂体系中,当ADN：Alq3=0：100时,驱动电压为11.65V。当改变比例至50：50时,驱动电压迅速下降至7.76 V。表格2是共主体器件的性能数据。
众所周知,载流子的迁移率是影响载流子注入和传输平衡的最重要因素。ADN良好的双极性是器件驱动电压降低的一个重要原因。而且,在ADN中电子和空穴的传输速度几乎相等。我们认为,正式它的这种特性导致了器件效率的提高。
总之,我们用宽禁带材料ADN作为主体材料,DCJTB和C545T作为共掺杂剂,实现了良好的红色有机电致发光器件。通过使用C545T作为辅助掺杂剂,有效地提高了发光效率。能量传递特性可以通过主体到掺杂剂的阶梯能量传递来解释。发光色度的改善可能是由于Alq3的极化作用影响了共主体体系的发光光谱。通过共主体体系我们得到了良好的器件性能：电流效率为3.5 cd/A,色坐标为(0.618,0.373),发光峰为620 nm。通过共主体体系,器件的驱动电压明显降低。这一结果可能是由ADN高的电子迁移率所引起的。与常规器件相比,共主体器件的电流效率提高了三倍而驱动电压则降低了38%。P(2)
自从Tang和Van Slyke实现了有机电致发光,众多荧光材料被用来作为主体或掺杂剂。降低驱动电压是实现高的功率效率和寿命的重要条件。目前,主要有两种方法来降低驱动电压。一是加强电荷的注入,可以通过使用低功函数阴极或修饰来实现。
增强空穴的注入通常通过对ITO表面处理或加入修饰层来实现。对于后一种方法来说,空穴注入曾的质量对于器件的驱动电压,效率和稳定性都有重要的影响。空穴注入曾有利于降低空穴注入势垒从而降低驱动电压和提高效率。另一方面,我们可以在空穴注入曾和发光层中掺杂受主和掺杂剂。在空穴注入曾和发光层中掺杂受主和掺杂剂会引起主体和掺杂剂之间的电荷转移,导致更高的自由电荷浓度从而在有机层和电极之间形成欧姆接触。另一方面,由于通常在有机材料中空穴的迁移率要比电子的迁移率高的多,这就使得保持发光层中的载流子平衡更加困难。为了解决这一问题,我们研究了不同的空穴传输层,电子传输层,空穴阻挡层和电子阻挡层材料。
在这一部分工作中,我们制备了单一发光层的有机电致发光器件,在这一结构中TBADN和DSA-Ph分别作为主体材料和蓝色掺杂剂。为了降低驱动电压,提高效率我们用MoO3和BPhen分别作为空穴注入层和电子传输层。
实验所用的器件结构为：glass substrate/ITO/(buffer layer) molybdenum oxide (MoO3) or 4,4',4"-tris (N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA)/N, N'-bis-(naphthalene-1-yl)-N,N'-diphenylbenzidine(NPB)/[TBADN:DSA-Ph(3%)]/ETLs/ LiF/Al. TBADN和DSA-Ph分别作为主体材料和蓝色掺杂剂。Alq3和BPhen作为电子传输层,LiF作为电子注入层,Al是阴极。
在OLED器件中,空穴从阳极注入到空穴注入层的HOMO能级,电子从阴极注入到电子注入层的LUMO能级。由于他们能级的不同,为了得到最大可能的电荷载流子注入,使得OLED器件的电流漂移被限制在有机层则需要欧姆接触。但是,假如在电极/有机层的接触面能垒足够高,那么将会发生注入限制。通常而言,为了得到可能的最低电压,那么必须在有机层和电荷注入层之间的界面拥有欧姆接触,并且使得两种载流子的漂移迁移率最大化。这样的话,OLED中的缓冲层(空穴注入层)的质量就显得尤为重要,它对驱动电压,效率以及器件稳定性至关重要。阳极缓冲层能够降低ITO和空穴传输层(HTL)之间的能垒,因此它对提高表面电荷注入、最终降低驱动电压和提高器件效率不无裨益。
我们使用MoO3作为缓冲层。MoO3是一个宽禁带材料,禁带宽度约为3.1 eV,当使用MoO3在ITO之上作为修饰层,空穴注入得以提高。在另一方面,通过调节电子传输层,我们获得一个有显著低驱动电压和最大效率的理想器件。为了对MoO3作为空穴注入层的器件进行深入研究,我们制备了一系列器件。m-MTDATA也作为器件的一部分,以下是所有器件的详细结构：
Cell-MD/A:ITO/m-MTDATA (30 nm)/NPB (10 nm)/EML (35 nm)/Alq3 (12 nm)/LiF (0.8)/Al(100nm)
Cell-MD/B:ITO/m-MTDATA (30 nm)/NPB (10 nm)/EML (35 nm)/BPhen (12 nm)/LiF (0.8nm)/Al(100nm)
Cell-Mo/B:ITO/MoO3 (5 nm)/NPB (30 nm)/EML (35 nm)/BPhen (12 nm)/LiF(0.8 nm)/Al(100nm)
Cell-Mo/A:ITO/MoO3 (5 nm)/NPB (30 nm)/EML (35 nm)/Alq3 (12 nm)/LiF (0.8 nm)/Al (100nm)
在这部分中,我们研究了基于m-MTDATA器件的EL谱线,该器件同时以Alq3和BPhen作为电子传输层。图2.1a显示了器件Cell-MD/A和器件Cell-MD/B(在20 mA cm-2下驱动)的EL谱线。我们可以从器件Cell-MD/A和器件Cell-MD/B的EL谱中看到在456 nm处出现一个峰(FWHM=56 nm),并且其肩峰在495 nm处。在465 nm处的峰缘于主体TBADN,495 nm处的肩峰归因于DSA-Ph。如图2.1a所示,除了器件Cell-MD/A在495 nm的肩峰密度有所不同以外,器件Cell-MD/A和器件Cell-MD/B的EL谱线没有明显的区别。这个区别导致了器件Cell-MD/B具有更好的色纯度。在电流密度为20 mA cm-2下的器件Cell-MD/A和器件Cell-MD/B的色坐标分别为(0.156,0.229)和(0.157,90.187)。我们能从图中得出用BPhen取代Alq3的器件Cell-MD/B在色坐标中得到了更好的提高。在色坐标中的
漂移很有可能缘于BPhen。对于这种在色坐标中的漂移现象的机制我将在下文中予以阐明。
在这部分,我们通过与m-MTDATA基器件的比较,研究了MoO3对EL谱的影响。EL谱线显示由于电荷过量地空穴注入和较差的色彩质量。由图2.1b可知,其发光光谱的主波峰均在465 nm,并且在495 nm处其光谱有一肩峰。在电流密度为20 mA cm-2时,器件Cell-Mo/A和器件Cell-Mo/B的色坐标分别为(0.176,0.286)和(0.209,0.302)。与不加MoOx的器件相比,其发射光谱的色坐标有一定的往天蓝偏移,以及半峰宽(FWHM)值也趋于不等,分别为66 nm和90 nm。这些主要是因为光谱在长波出的拓宽造成的。对于上述现象,我们可以解释为：一方面,在电场的作用下,电子和空穴各自朝着相反方向往不同极性的电极运动,于是在发光层形成了一定浓度的不同极性载流子。又因为在OLED中最常用的空穴传输材料的空穴的迁移率(10-4 cm2/V·s)要比Alq3的电子迁移率高出来两个数量级。当大量的空穴通过发光层时,其与电子的复合在EML与ETL的界面处,所以,与器件没有MoOx的器件相比,器件Cell-Mo/A和Cell-Mo/B在较长波长处(大约520)得到扩展。而对于这种扩展主要是因为电子空穴在Alq3层的复合发光所致。而且,从图中可以看出,器件Cell-Mo/B的半峰宽(66 nm)要小于器件的Cell-Mo/A。这也是导致器件Cell-Mo/A的色坐标不稳定的原因所在。对于上面的情况,我们认为在OLED器件中对空穴阻挡不利所致,因此也导致了额外的的复合发光。另外,还由于电子振动也是引起这些器件的发光光谱的不同形状。
另一方面,对于用BPhen作为ETL的器件Cell-Mo/B的色坐标较为稳定。从(0.209,0.302)到(0.176,0.283)。对于这一性能上的改善主要是由于BPhen对空穴的阻挡作用,从其能级可以清楚的看到,从TBADN的HOMO能级到Alq3的HOMO能级差仅为0.1eV,而从TBADN到BPhen的势差到达0.9 eV,从而使得在OLED器件中的载流子得到平衡,并且将载流子局限于发光层。
值得注意的是：我们发现用器件Cell-Mo/A和Cell-Mo/B的驱动电压要明显低于器件Cell-MD/A和Cell-MD/B,具体的各个器件的性能见表2.1。由表可知,在电流密度为20 mA cm-2时,器件Cell-Mo/A, Cell-Mo/B, Cell-MD/A, Cell-MD/B,的驱动电压分别为9.8 V,7.9 V,5.4 V,和7.3 V。很明显器件Cell-Mo/B的具有最低的驱动电压。这主要是因为由于MoO3极大的降低了空穴的注入势垒,使得其驱动电压降低。比如在大约8 V时,器件Cell-Mo/B的发光亮度为16305 cdcm-2,而其他器件要远远低于这一数值,分别仅为261,1735,3662 cd cm-2.从表中可以看出,器件Cell-MD/A和Cell-Mo/A其发光效率迅速升高,然后在较高电流密度(40-400 mA cm-2)时,逐渐下降；而对于器件爱你Cell-MD/B和Cell-Mo/B的发光效率在较大电流密度(50-400 mA cm-2)时,基本保持稳定,其电流猝火现象并不明显。而对于器件Cell-Mo/B具有最高功率效率(4.7 lm/W),明显要高出其他器件,这主要是因为MoO3极大的提高了空穴的注入,从而降低了器件的驱动电压,并且与有高电子迁移率的BPhen做电子传输层,很好的促进了载流子的平衡。根据我们的测量,M003的HOMO能级为5.43 eV,与NPB的HOMO能级(5.4eV)非常接近；而m-MTDATA的HOMO能级为5.1 eV,与NPB之间存在较大的势差(0.3 eV)。另一方面,在电子迁移方面,我们所使用的BPhen是一种高电子迁移率(3.9×10-4-5.2×104 cm2/V·s)的ETL,同时其有作为空穴阻挡层。
为了进一步研究载流子的注入和平衡,我们制备了一系列的'only'器件(空穴-only-器件和电子-only-器件),其结构组成为：Cell-EA:ITO/Alq3(12 nm)/LiF (0.8 nm)/Al(150 nm) Cell-EB:ITO/BPhen(12 nm)/LiF (0.8 nm)/Al(150 nm) Cell-HMT:ITO/m-MTDATA (20 nm)/NPB (10 nm)/Al(150 nm) Cell-HM3:ITO/MoO3 (30 nm)/NPB (10 nm)/Al(150 nm)
正如我们所知,功率效率取决于载流子的注入和传输,而电流效率比较取决于载流子的注入,同时也取取决于载流子的平衡。由3.6图可知,在低电压下器件Cell-HMT和Cell-EA的电流密度是比较接近的,这就是说器件Cell-MD/A的电子和空穴能够更好的平衡。这就是器件Cell-MD/A在低电压下有更高的电流效率的原因。但是,由于发光层中电子和空穴不平衡,导致在高电流密度下电流效率逐渐降低。对于基于Cell-Mo/A的器件,器件Cell-HM3的电流密度大于器件Cell-EA,因此发光层中电子和空穴不平衡,导致了效率降低。
表3.2基于Alq3和BPhen的电子only器件和基于m-MTDATA和MoO3的空穴only器件的Ⅰ-Ⅴ特效另一方面,在较低的电流密度下,尽管器件Cell-MD/A有最高的功率效率,但是器件Cell-Mo/B比器件Cell-MD/A电流效率稍低。因此,在高电流密度情况下,器件Cell-Mo/B较器件Cell-MD/A有更好电子空穴平衡。这是因为Mo03有很高的空穴注入能力,降低驱动电压。同时,空穴的注入能力强于电子传输层的电子注入能力。过量的空穴注入导致空穴电子的不平衡,因而,电流效率也降低了。
总之,利用MoO3作为空穴注入层和BPhen作为电子传输层,器件的功率效率较使用其他电子传输层和空穴注入层的器件提高74%。在电流密度为20 mA cm-2,驱动电压为5.4 V,。这比器件Cell-MD/A的驱动电压降低了46%。器件Cell-Mo/B的优越性能归因于MoO3具有很高的空穴注入能力,以及BPhen的高电了传输能力。这使得器件的功率效率提高,同时降低了器件的驱动电压。同时,由于电子和空穴平衡,器件的电流效率提高。功率效率的提高降低器件的能耗,而降低器件的能耗是下一代OLED面板进入实用化的关键因素之一。P(3)
我们知道,电子和空穴复合区的平衡时提高有机电致发光器件效率和稳定性的关键因素。因此,测量非晶态有机半导体载流子的迁移率对于有机电致发光有着重要的意义。TOF (time-of-flight)是测量有机半导体薄膜载流子迁移率的一个常用方法。空间电荷限制法也是测量半导体薄膜载流子迁移率的一个常用方法。尽管空间电荷限制法已经有了比较完善的理论体系,但是这种方法缺很少用于有机材料载流子迁移率的测量。其原因是这种方法通常要求电极与半导体薄膜要有良好的欧姆接触。如果在邮寄薄膜和电极之间插入一层缓冲层来实现欧姆接触,则空间电荷限制法就有望被用于有机材料载流子迁移率的测量。研究表明,通过引入LiF来形成欧姆接触,而后用空间电荷限制法所测得的Alq3的电子迁移率为5.2×10-6cm2/(V·s)(在0.8 MV/cm)这一结果与用TOF法测得的结果吻合的很好。但是,用空间电荷限制法来研究掺杂有机半导体(Liq-doped BPhen)的电子迁移率还没有报道过。在这部分的工作中,我们用空间电荷限制法研究了电子传输材料BPhen与Alq3掺杂体系的电子迁移率。对于掺杂体系(BPhen doped with 33 wt% Liq)电子迁移率的测量,我们用LiF来作为缓冲层材料形成欧姆接触。因为它可以通过降低Al和掺杂薄膜的注入势垒来有效的增强载流子的注入。不同厚度掺杂体系薄膜的电子迁移率通过空间电荷限制法来测量。
为了研究n型掺杂体系薄膜(BPhen:x wt% Liq)的电子注入和传输能力,我们制备了一系列的only-器件：
Cell-E1:ITO/BPhen:0 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)
Cell-E2:ITO/BPhen:10 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)
Cell-E3:ITO/BPhen:33 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)
Cell-E4:ITO/BPhen:50 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)图3.1是不同Liq掺杂浓度的BPhen薄膜的电流密度随着电压变化的特性曲线。我们可以看到,掺杂了Liq之后,电流密度明显增加而电压也有显著地下降。电流密度随着Liq浓度的上升而增加,当Liq的浓度达到50%时,电流密度开始下降。因此,我们得出Liq的最佳浓度为33%。表格1是当only器件的电流密度在40和100 mA/cm2时的电压数据。从表格我们可以看到,电压随Liq掺杂浓度的增加而降低,当掺杂浓度为33%时,电压达到极小值。结果也表明,在更高的掺杂浓度下电压有轻微的上升。导致这一结果的原因可能是由于在高掺杂浓度下的浓度猝灭。因此,我们选择33%掺杂浓度的器件来测量迁移率。选取了最佳厚度之后,我们以ITO/ [BPhen:33 wt% Liq (x nm)]/LiF (1 nm)/Al (150 nm)的结构制备了only-器件,其中x的值为50,100,150,200,300 nm。J-V曲线在高压和低压区情况下显示出两个截然不同的区域：肖特基势垒和空间电荷限制区。随着电压的增加,J-V曲线显示出空间电荷限制的特性。空间电荷限制可以表达为：E是电场强度,ε和ε0分别是相对介电常数和真空介电常数,L是有机薄膜的厚度。而且,以log-log的坐标形式来看,在低压区斜率小于2,而在高压区电流密度与电压的平方成比例。这一现象表明了电荷传输过程中的空间电荷限制特性。电荷的迁移率依赖于电场强度,可以由Poole-Frenkel方程来描述：μ0是0电场强度下的迁移率,β是Poole-Frenkel因了。合并公式1和2,我们可以得到依赖于电场强度的空间电荷限制表达式：图3.2是电流密度随电场强度平方根的变化曲线。拟合曲线和实验数据的良好吻表明电流在这一区间遵循空间电荷限制。有机层的相对介电常数为3,真空介电常数为8.85×10-12F/m.实验结果表明,随着掺杂层厚度的增加,电荷的迁移率也相应加,当膜层的厚度达到150 nm时,迁移率达到饱和。这是因为,在厚的膜层里边,界面对于电荷迁移率的影响大大降低导致了比较高的电荷迁移率。随着膜层的继续加厚,迁移率受厚度的影响更小,逐渐接近于掺杂体系的本征迁移率。在电场强度为0.3 MV/cm时,测得300 nm膜层的迁移率为5.2×10-3 cm2/(V·s)。这一结果比先前报道的3.4×10-4 cm2/(V·s)高了一个数量级。然而我们所要测量的是在有机电致发光器件中有机层的迁移率,因为在OLED器件中BPhen的厚度只有大概50nm。当我们把掺杂有机层的厚度从50 nm增加至300 nm,电了的迁移率从1.7×10-4cm2/(V·s)增加至5.2×10-3cm2/(V·s)。根据报道,
Alq3的迁移率表现出与我们的实验结果相同的趋势。而TOF法不能用来测量实际OLED器件中有机层的迁移率(因为这种方法要求有机膜的厚度要达到几个微米以上)。在最初的阶段,β随着膜层的增厚而单调上升,这一结果也与PF曲线相吻合。
Pool Frenkel因子β是电场激活能,它与非晶薄膜的无序度有关。然而,随着膜厚的增加,β趋于饱和(当厚度大于150 nm时),它的饱和值为8.1×10-3(cm/V)1/2.另外,当膜厚大于150 nm时,迁移率和β趋于饱和也说明了测量所得的迁移率反映了掺杂体系的真实迁移率。然而,当电场强度为0时,随着膜厚从50 nm增加至300nm,电子迁移率也从5.53×10-6增加至7.63×10-4 cm2/(V·s)。
总之,我们的实验结果表明空间荷限制法可以应用于测量BPhen和Liq掺杂膜电子迁移率的测量。研究发现,Liq掺可以有效的提高BPhen的电子迁移率。我们发现,当掺杂魔的厚度大于150nm时,膜厚的增加对于迁移率的影响很小,此时的迁移率接近于膜的本征迁移率。在膜厚为300 nm时,测得的掺杂膜迁移率为5.2×10-3cm2/(V·s),这一值高于本征的BPhen。应用于OLED器件的掺杂BPhen薄膜的迁移率也可以用空间电荷限制的方法测得。当掺杂薄膜为(Liq 33wt%) 50 nm时,测得的迁移率为1.7×10-4cm2/(V·s).这些结果对于设计和提高OLED器件的性能有重要的意义。
Organic semiconductors are a fascinating class of materials, with a wide range of properties. Electronic devices based on organic semiconductors, such as Organic Light-Emitting Diodes (OLEDs) and field-effect transistors (FETs) have attracted much interest as possible inexpensive and flexible alternatives to inorganic devices. In recent years, the investigation of organic light emitting diode based on small organic molecules has attracted growing interest, due to their attractive characteristics and potential applications to flat panel displays. The primary reason is that large numbers of organic materials are known to have high fluorescence quantum efficiencies in the visible spectrum. Hence, they are ideally suited for multicolor display applications.
The light emission is the result of radiative decay of singlet excitons formed by recombination of charge carriers. However, not all charge carriers which recombine contribute to the generation of light:a part of the excitons may decay non-radiatively or may be quenched in the bulk or at interfaces. Because the ratio of singlet exciton formation to triplet exciton formation under electrical excitation is approximately 1:3 due to spin statistics, organic fluorescent emitters are limited to 25% internal quantum efficiency whereas phosphorescent emitters can in principle reach a quantum efficiency of 100% if the intersystem crossing process is efficient. In this regards, doping is an important technology for electronic imaging and optoelectronics devices based on molecular materials. Emissive doping is usually used for tuning emission colors and enhancing luminescence efficiency. Therefore, efficient organic devices could be demonstrated by using highly fluorescent dye molecules as the emissive dopant in the emitting layer of OLED that based on energy transfer from host to guest molecules. Hereby, the emission wavelength can be tuned in the desired way and the efficiency has been improved by optimization of the properties of the guest and host molecules. In order to achieve low driving voltage and high efficiency in OLED devices, it is necessary to facilitate the injection of charges. One effective approach to enhance carrier injection and transport is to conductively dope the organic layer. This conductive and control doping in the transport layers plays a crucial role for the OLED. The work performed in this dissertation is mainly based on the fabrication and characterization of various types of organic light emitting diodes, focusing on the improvement and charge transport phenomena.
Here, red organic light-emitting devices were constructed that based on a wide band gap host emitting system of 9,10-di(2-naphthyl)anthracene (ADN) co-doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,1 1H-10(2-benzothiazolyl)-quinolizine-[9,9a,lgh] coumarin (C545T) as an assistant dopant. The typical device structure was glass substrate/ITO/4,4',4"-tris(N-3-methylphenyl-N-phenylamino) triphenylamine(m-MTDATA)/N-bis-(naphthalene-1-yl)-N,N diphenylbenzidine(NPB)/[ADN:DCJTB:C545T/Alq3/LiF/Al]. It was found that C545T dopant did not emit by itself but did assist the energy transfer from the host (ADN) to the red emitting dopant. The red OLEDs realized by this approach not only enhanced the emission color, but also significantly improved the EL efficiency. The EL efficiency reached 3.5 cd/A at a current density of 20 mA cm-2, which is enhanced by three times compared with devices where the emissive layer is composed of the DCJTB, doped ADN. The saturated red emission was obtained with Commission International De L'Eclairage (CIE) coordinates of (x=0.618, y=0.373) at 620 nm, and the device driving voltage is decreased as much as 38%. We attribute these improvements to the assistant dopant (C545T), which leads to the more efficient energy transfer from ADN to DCJTB. These results indicate that the co-doped system is a promising method for obtaining high-efficiency red OLEDs.
In this work, we demonstrated blue organic light-emitting devices based on wide band gap host material,2-(t-butyl)-9,10-di-(2-naphthyl)anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO3) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline(BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO3(5 nm)/NPB(30 nm)/[TBADN:DSA-Ph(3wt%)](35 nm)/BPhen(12 nm)/LiF(0.8 nm)/Al(100 nm). It was found that the Cell-Mo//B-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA cm-2, its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA//Alq3 is based on, respectively. The J-V curves of'hole-only'devices reveal that a small hole injection barrier between MoO3//NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.
Charge carriers in organics transport via hopping mechanism, which is quite different from that in inorganic crystalline semiconductors. This makes the charge carrier mobility in organics rather low and strongly dependent on electrical field and temperature, leading to the measurement of the mobility in organics much difficult. The conventional methods to determine the mobility in organics include surface charge decay, transient photocurrent or electroluminescence measurement, and time-of-flight measurement. However, all these methods require determination of transit time of charge carriers, which is complicated, and need intricate instruments. The current through organic devices is space charge limited and governed by the dc mobility. In principle, the information of charge carrier mobility should be reflected in their current-voltage characteristics, i.e., the mobility could be derived from processing or transforming their current-voltage characteristics. One method relating to this idea is space charge limited current (SCLC) technique.
In this work, the electron mobility of 4,7-diphenyl-1,10-phenanthroline (BPhen) doped 8-hydroxyquinolinato-lithium (Liq) at various thicknesses (50-300 nm) has been estimated by using space-charge-limited currents measurements. It has been observed that the electron mobility of doped BPhen (33 wt.% Liq:BPhen) approaches its true value when the thickness is more than 200 nm. The electron mobility of 33 wt.% Liq: BPhen at 300 nm is found to be-5.2×10-3 cm2/(V·s) (at 0.3 MV/cm) with weak dependence on electric field, which is about one order of magnitude higher than that of pristine BPhen (3.4×10-4 cm2/(V·s)) measured by space-charge-limited currents. For thickness typical of organic light-emitting devices, the electron mobility of doped BPhen is also investigated. Keywords
Organic Light Emitting Diodes, Co-doped, BPhen. Electron mobility, Space Charge Limited Current (SCLC),8-hydroxyquinolinonate-lithium (Liq)
光发射是载流子复合形成的单重态激子辐射衰减的结果。然而,不是所有的载流子复合会对发光有贡献,：一部分激子在体内或界面以非辐射或淬灭方式衰减。因为形成激子时,单重态与三重态激子的比例是大约1：3,在系统间串阅过程充分的情况下,荧光辐射的内量子效率约为25%,而磷光辐射的内量子效率约为100%。在这点上,对于基于有机分子的光电子器件,掺杂是一项重要技术。掺杂经常被用于调节发光颜色和提高发光效率。因此,对于基于能量传输从主体到客体的有机电致发光器件,发光层中适当的荧光掺杂可以使器件效率更高。这样,可以调节发光波长,并通过优化主体和客体材料提高器件效率。为降低电压和提高效率必须使载流子的注入变得容易,改善注入和传输的一种有效方法是对有机层进行掺杂。传输层掺杂的控制对器件起着至关重要的作用。本论文将在各种有机电致发光器件的制备、特性、载流子传输性能的改善方面进行研究。P(1)
有机电致发光二极管(Organic light-emitting diodes, OLEDs)由于其具有自发光、宽视角和在平板显示中的应用而备受瞩目。高效低驱动电压的红光OLED器件对于有机全彩显示的发展起到了至关重要的作用。通常用一个掺杂体系来获得红光,而此体系由主体材料和红色掺杂物所组成。当主体材料掺杂少量红色荧光染料,则该掺杂会发出高亮度的红光。
红色OLEDs通常采用将红色掺杂剂掺入主体有机材料,但大多数研究广泛使用基于具有优异稳定性和电荷传输能力的8-羟基喹啉铝(tris-(8-hydroxyquinoline) aluminum, Alq3)作为主体,并且从主体到红色掺杂的能量传递在获得红色放光中起到了重要作用。尽管如此,对于由于在主体和客体之间存在不完全的能量传输以及在高浓度掺杂情况下会发生浓度淬灭,这种掺杂体系若想获得纯度高和高效率的红光还存在一定难度。
最近,一种用于激发完整的能量连续传递过程的共掺杂方法得到了发展,正好解决了上述问题。在共掺杂体系中,发光层由主体材料A和两个客体材料B、C组成,能量传递过程从A到B,再由B到C。通过使用这种共掺杂方法,基于主体Alq3红光OLED器件发光得到了巨大提高。但是,因为在当今OLED显示应用方面,大量使用DCJTB这种荧光红色染料掺杂到Alq3主体这种方式,得到较低的电子迁移率(5.2×10-6cm2/V·s),被证实不能同时满足红光所有的需求。此外,由于Alq3本身能级的限制,其仅能作为红光和绿光OLED器件的主体材料,而在光敏处理蓝光掺杂中无能为力。因此,在有机电致发光二极管领域,获得一个高效、低驱动电压、发饱和红光的二极管将成为一项挑战。
在本文中,我们设想一个宽禁带的9,10-di (2-naphthyl) anthracene(ADN),该聚合物以其高的热稳定性而被广泛用来作蓝光OLED器件的主体材料。因其有较宽的禁带宽度,被看做在全彩色OLED器件中非常有前途的主体材料。此外,由于ADN拥有较高的电子迁移率,能达到3.1×10-4cm2/(V·s),它也被看做未来能降低驱动电压的理想材料。但是当ADN被用于红光时,由于其禁带宽度(3.2eV)而导致从ADN到DCJTB的直接能量传递效率不高,并且降低了ADN的重叠发光和DCJTB的吸收。为了克服这个问题,我们使用辅助掺杂剂C545T,该材料禁带宽度介于ADN和DCJTB之间,以提高能量传递效率。一个广泛接受的事实是,在不同客体或主体之间传递能量,Forster能量传递形式在提高发光亮度上占主要作用。选择C545T作为辅助掺杂剂提高了电致发光效率和器件驱动电压。在共掺杂发光层,ADN作为能量给体,C545T作为敏化剂,DCJTB作为红色荧光染料。
实验中,我们制备了两种染料掺杂器件,并且对其特性加以比较。对于红光器件而言比较优化的器件结构如下：glass substrate/ITO (80 nm)/m-MTDATA (20 nm)/NPB (10 nm)/EML (x nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)。对于共掺杂ADN基的器件,35nm厚的发光层包含了ADN基(ADN：red dopant:assistant dopant)和共主体器件([ADN:Alq3]:red dopant:assistant dopant). m-MTDATA用于空穴注入层,NPB为空穴传输层,Alq3用于电子传输材料,LiF andAl分别作为电子诸如层和阴极。有机层和阴极通过在1.0×10-5 Torr气压下真空蒸镀而成。电致发光谱(electroluminescent spectra)和色坐标(Commission International d'Eclairage, CIE)通过PR-650色度计。电流-电压特性和亮度特性分别用Keithley 2400 dc Source Meter和Minolta LS-110 luminance meter进行测量。
结果与讨论：发现掺杂型有机电致发光器件对得到发光亮度高、稳定和色彩选择性是一个有效方法。在主客体之间中存在两种能量传输机制,(1) Forster能量传递和(2) Dexter能量传递。对于在发光层的荧光受体,能量传递机制被证明为Forster形式且较宽(40-100 A)。Forster能量传递(FET)是给体和受体共振偶极-偶极耦合,同时Dexter传递是通过遵循自旋保护定则从给体到受体的激子扩散。FET是非辐射传递,分子A(受体)的偶极子跃迁产生一个场来诱导刺激分子D(给体)的发射。给体和受体从基态到激发态的迁移必须遵循Forster传递形式。单谱线激子传递能量到掺杂分子的单态常基于如下Forster过程：1D*+1A→1D+1A* (1)
其中,左上角的1表示单线态,星号表示激发态。因此,一个有效的传递需要给体发射和受体吸收光谱重叠。根据Forster理论,给体发射光谱和受体吸收光谱的重叠部分导致给体和受体间的高的能量传递速率,也提高了受体的发光效率。因此,高度重叠满足了能量传递更高速率和对给体受体间能量传递基本要求。因为激发的给体弛豫,其能量由染料分子间的强库伦作用力来传递,并且能量传递的速率由主体和客体分子间的距离R0决定,该至关重要的距离称之为Forster临界半径。该能量传递速率由下式表达：其中τ。给体激发态的平均寿命,对应的和Forster临街半径以下式表达：其中心为阿伏伽德罗常数,nd是受体或给体的折射率,β2是取向因子, J(λ)为重叠区域。有效重叠J(λ)能够在下式辅助下进行计算：其中Fd(v)是给体荧光光谱,在公式2.4中已给出定义；εa(v)是受体分子脉冲消失相干光谱；υ是能量波数。这表明了Forster临界半径(R0)独立于荧光给体和受体吸收之间的重叠区域,同时也反映了在迁移过程中共振的必要。因此,我们在本文中研究了在发光层中的能量传递过程机制。我们就这样首先分别研究了给体(ADN)和受体(DCJTB)分子的光致发光(photoluminescence, PL)和吸收光谱。从分析ADN和DCJTB光谱显示较差的重叠,我们利用C545T作为敏化剂来辅助ADN到DCJTB的能量传递。图1.1显示了ADN、C545T和DCJTB的PL谱和光学吸收谱。从图1.1可以看出,ADN的PL谱和DCJTB的吸收谱之间较差重叠。这说明从ADN到DCJTB的能量传递低效。为了克服该问题,我们用C545T作为敏化剂来辅助使ADN到DCJTB的能量得以有效传递。观察ADN和C545T的光谱可以清楚地看到ADN的PL谱线与C545T吸收谱之间存在良好的重叠。因此,与[ADN:DCJTB]体系相关的区域比较之后得到,[ADN：C545T]混合体系的光谱显示了在直线一下的区域(重叠区)相当大。通过光谱位置(PL谱和吸收谱)的判断,我们决定使用C545T作为辅助掺杂剂来帮助提高从ADN到所谓的红色染料DCJTB之间的能量传递。我们同时也能观察到C545T的PL谱和DCJTB吸收谱之间具有较好的光谱重叠,这也说明能量能够有效从ADN传递到C545T,然后接着从C545T传递到DCJTB.这种级联机制能够有效提高器件性能。
如图1.2所示,不同浓度下的C545T的ADN基器件的EL光谱以及最优浓度1wt%DCJTB的EL光谱。从中可以看出,当发光层只掺杂有1wt%浓度的DCJTB时,只有获得最大吸收波长为608 nm的橙光(CIE=0.568,0.414),这是由于ADN的自发光,并且DCJTB红色的峰值,从ADN到DCJTB能量传递的效率低。我们可以推断这些低效能量转移现象与ADN发射光谱和吸收光谱的DCJTB之间的重叠有关。当DCJTB在更高浓度下(2wt%),获得红光,但主要由于浓度淬灭而使cd/A效率下降。为了提高能量传递效率和抑制浓度猝灭现象,我们将C545T作为DCJTB/ADN体系的辅助掺杂剂。我们将少量的C545T作为共掺杂剂掺入发光层中(1wt%DCJTB:0.2wt%C545T),得到色坐标为(0.590,0.401)。当把C545T的站咋浓度提高到0.5wt%或者1wt%,在光谱中出现了C545T的发光峰。这一现象可能是由于C545T具有更高的荧光效率,从ADN到C545T的能量传递比从ADN到DCJTB更容易。实验结果表明,掺杂浓度为DCJTB:C545T=[1wt%:0.2wt%]的器件性能最好。
表格1.1是共掺杂器件的性能数据。通过比较掺杂了C545T和未掺杂C545T器件的性能,我们发现即使掺杂微量的C545T也会导致器件驱动电压的大幅度下降。而浓度猝灭现象则由于采用了ADN(主体材料)而明显减弱。根据"solvent state polarization "理论,DCJTB的发光强烈依赖于偶极子。而在处于非极性状态时,它只能发黄光。因此,只有通过将极性很强的材料与它掺杂才能发红光。因此我们可以用一种共主体系统,通过很强极性的Alq3来使DCJTB发纯红光。在上一实验中我们看到,基于ADN的共掺杂器件的色度没有明显的改善。为了提高器件的发光特性,我们制备了共主体结构的有基点至发光器件。图1.2是优化的共主体器件1:0.2(wt% DCJTB:wt% C545T).的的光谱图。从图上我们可以看到,当ADN:Alq3=50:50时,器件发红光,色坐标为(0.629,0.359)。通过进一步调节ADN:Alq3的比率,我们可以得到很好侧坐标的红色发光。当ADN:Alq3=80：20时,我们得到红色发光的色坐标为(0.618,0.373)。C545T,DCJTB和ADN共掺杂器件效率和色度的提高可以通过以下来解释。在基于ADN的器件中,可能由三个过程：(1)注入的电子和空穴在主体(ADN)中复合。此时,ADN处于激发态,他将能量通过Forster能量传递形式传递给DCJTB, DCJTB发红光。(2)注入的电子和空穴在主体(ADN)中复合。此时,ADN处于激发态,他将传递给C545T,然后C545T传递给DCJTB, DCJTB发红光。(3) DCJTB作为陷阱中心。注入的电子和空穴在DCJTB中行车那个激子从而发红光。
尽管ADN对浓度猝灭有一定的一致作用,但是在DCJTB浓度比较高的时候,发生了浓度猝灭现象,降低了器件的效率。对于共掺杂器件,当DCJTB的浓度比较低时,过程(1),(2)和(3)都会起作用。但是,过程(2)是主要的。注入的空穴和电子再ADN中复合并形成激子。在Forster能量传递理论中,能量传递的概率正比于掺杂剂的发射光谱和受主材料吸收光谱的重叠面积。不同于于ADN和DCJTB光谱的较小重叠面积,ADN和C545T光谱的重叠面积相当大。
为了研究共主体体系的能量传递特性,我们制备了不同ADN和Alq3比例的器件(0：100,50：50,70：30,and 80：20).研究结果表明,随着ADN浓度的增加器件的驱动电压逐渐降低。在共掺杂体系中,当ADN：Alq3=0：100时,驱动电压为11.65V。当改变比例至50：50时,驱动电压迅速下降至7.76 V。表格2是共主体器件的性能数据。
众所周知,载流子的迁移率是影响载流子注入和传输平衡的最重要因素。ADN良好的双极性是器件驱动电压降低的一个重要原因。而且,在ADN中电子和空穴的传输速度几乎相等。我们认为,正式它的这种特性导致了器件效率的提高。
总之,我们用宽禁带材料ADN作为主体材料,DCJTB和C545T作为共掺杂剂,实现了良好的红色有机电致发光器件。通过使用C545T作为辅助掺杂剂,有效地提高了发光效率。能量传递特性可以通过主体到掺杂剂的阶梯能量传递来解释。发光色度的改善可能是由于Alq3的极化作用影响了共主体体系的发光光谱。通过共主体体系我们得到了良好的器件性能：电流效率为3.5 cd/A,色坐标为(0.618,0.373),发光峰为620 nm。通过共主体体系,器件的驱动电压明显降低。这一结果可能是由ADN高的电子迁移率所引起的。与常规器件相比,共主体器件的电流效率提高了三倍而驱动电压则降低了38%。P(2)
自从Tang和Van Slyke实现了有机电致发光,众多荧光材料被用来作为主体或掺杂剂。降低驱动电压是实现高的功率效率和寿命的重要条件。目前,主要有两种方法来降低驱动电压。一是加强电荷的注入,可以通过使用低功函数阴极或修饰来实现。
增强空穴的注入通常通过对ITO表面处理或加入修饰层来实现。对于后一种方法来说,空穴注入曾的质量对于器件的驱动电压,效率和稳定性都有重要的影响。空穴注入曾有利于降低空穴注入势垒从而降低驱动电压和提高效率。另一方面,我们可以在空穴注入曾和发光层中掺杂受主和掺杂剂。在空穴注入曾和发光层中掺杂受主和掺杂剂会引起主体和掺杂剂之间的电荷转移,导致更高的自由电荷浓度从而在有机层和电极之间形成欧姆接触。另一方面,由于通常在有机材料中空穴的迁移率要比电子的迁移率高的多,这就使得保持发光层中的载流子平衡更加困难。为了解决这一问题,我们研究了不同的空穴传输层,电子传输层,空穴阻挡层和电子阻挡层材料。
在这一部分工作中,我们制备了单一发光层的有机电致发光器件,在这一结构中TBADN和DSA-Ph分别作为主体材料和蓝色掺杂剂。为了降低驱动电压,提高效率我们用MoO3和BPhen分别作为空穴注入层和电子传输层。
实验所用的器件结构为：glass substrate/ITO/(buffer layer) molybdenum oxide (MoO3) or 4,4',4"-tris (N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA)/N, N'-bis-(naphthalene-1-yl)-N,N'-diphenylbenzidine(NPB)/[TBADN:DSA-Ph(3%)]/ETLs/ LiF/Al. TBADN和DSA-Ph分别作为主体材料和蓝色掺杂剂。Alq3和BPhen作为电子传输层,LiF作为电子注入层,Al是阴极。
在OLED器件中,空穴从阳极注入到空穴注入层的HOMO能级,电子从阴极注入到电子注入层的LUMO能级。由于他们能级的不同,为了得到最大可能的电荷载流子注入,使得OLED器件的电流漂移被限制在有机层则需要欧姆接触。但是,假如在电极/有机层的接触面能垒足够高,那么将会发生注入限制。通常而言,为了得到可能的最低电压,那么必须在有机层和电荷注入层之间的界面拥有欧姆接触,并且使得两种载流子的漂移迁移率最大化。这样的话,OLED中的缓冲层(空穴注入层)的质量就显得尤为重要,它对驱动电压,效率以及器件稳定性至关重要。阳极缓冲层能够降低ITO和空穴传输层(HTL)之间的能垒,因此它对提高表面电荷注入、最终降低驱动电压和提高器件效率不无裨益。
我们使用MoO3作为缓冲层。MoO3是一个宽禁带材料,禁带宽度约为3.1 eV,当使用MoO3在ITO之上作为修饰层,空穴注入得以提高。在另一方面,通过调节电子传输层,我们获得一个有显著低驱动电压和最大效率的理想器件。为了对MoO3作为空穴注入层的器件进行深入研究,我们制备了一系列器件。m-MTDATA也作为器件的一部分,以下是所有器件的详细结构：
Cell-MD/A:ITO/m-MTDATA (30 nm)/NPB (10 nm)/EML (35 nm)/Alq3 (12 nm)/LiF (0.8)/Al(100nm)
Cell-MD/B:ITO/m-MTDATA (30 nm)/NPB (10 nm)/EML (35 nm)/BPhen (12 nm)/LiF (0.8nm)/Al(100nm)
Cell-Mo/B:ITO/MoO3 (5 nm)/NPB (30 nm)/EML (35 nm)/BPhen (12 nm)/LiF(0.8 nm)/Al(100nm)
Cell-Mo/A:ITO/MoO3 (5 nm)/NPB (30 nm)/EML (35 nm)/Alq3 (12 nm)/LiF (0.8 nm)/Al (100nm)
在这部分中,我们研究了基于m-MTDATA器件的EL谱线,该器件同时以Alq3和BPhen作为电子传输层。图2.1a显示了器件Cell-MD/A和器件Cell-MD/B(在20 mA cm-2下驱动)的EL谱线。我们可以从器件Cell-MD/A和器件Cell-MD/B的EL谱中看到在456 nm处出现一个峰(FWHM=56 nm),并且其肩峰在495 nm处。在465 nm处的峰缘于主体TBADN,495 nm处的肩峰归因于DSA-Ph。如图2.1a所示,除了器件Cell-MD/A在495 nm的肩峰密度有所不同以外,器件Cell-MD/A和器件Cell-MD/B的EL谱线没有明显的区别。这个区别导致了器件Cell-MD/B具有更好的色纯度。在电流密度为20 mA cm-2下的器件Cell-MD/A和器件Cell-MD/B的色坐标分别为(0.156,0.229)和(0.157,90.187)。我们能从图中得出用BPhen取代Alq3的器件Cell-MD/B在色坐标中得到了更好的提高。在色坐标中的
漂移很有可能缘于BPhen。对于这种在色坐标中的漂移现象的机制我将在下文中予以阐明。
在这部分,我们通过与m-MTDATA基器件的比较,研究了MoO3对EL谱的影响。EL谱线显示由于电荷过量地空穴注入和较差的色彩质量。由图2.1b可知,其发光光谱的主波峰均在465 nm,并且在495 nm处其光谱有一肩峰。在电流密度为20 mA cm-2时,器件Cell-Mo/A和器件Cell-Mo/B的色坐标分别为(0.176,0.286)和(0.209,0.302)。与不加MoOx的器件相比,其发射光谱的色坐标有一定的往天蓝偏移,以及半峰宽(FWHM)值也趋于不等,分别为66 nm和90 nm。这些主要是因为光谱在长波出的拓宽造成的。对于上述现象,我们可以解释为：一方面,在电场的作用下,电子和空穴各自朝着相反方向往不同极性的电极运动,于是在发光层形成了一定浓度的不同极性载流子。又因为在OLED中最常用的空穴传输材料的空穴的迁移率(10-4 cm2/V·s)要比Alq3的电子迁移率高出来两个数量级。当大量的空穴通过发光层时,其与电子的复合在EML与ETL的界面处,所以,与器件没有MoOx的器件相比,器件Cell-Mo/A和Cell-Mo/B在较长波长处(大约520)得到扩展。而对于这种扩展主要是因为电子空穴在Alq3层的复合发光所致。而且,从图中可以看出,器件Cell-Mo/B的半峰宽(66 nm)要小于器件的Cell-Mo/A。这也是导致器件Cell-Mo/A的色坐标不稳定的原因所在。对于上面的情况,我们认为在OLED器件中对空穴阻挡不利所致,因此也导致了额外的的复合发光。另外,还由于电子振动也是引起这些器件的发光光谱的不同形状。
另一方面,对于用BPhen作为ETL的器件Cell-Mo/B的色坐标较为稳定。从(0.209,0.302)到(0.176,0.283)。对于这一性能上的改善主要是由于BPhen对空穴的阻挡作用,从其能级可以清楚的看到,从TBADN的HOMO能级到Alq3的HOMO能级差仅为0.1eV,而从TBADN到BPhen的势差到达0.9 eV,从而使得在OLED器件中的载流子得到平衡,并且将载流子局限于发光层。
值得注意的是：我们发现用器件Cell-Mo/A和Cell-Mo/B的驱动电压要明显低于器件Cell-MD/A和Cell-MD/B,具体的各个器件的性能见表2.1。由表可知,在电流密度为20 mA cm-2时,器件Cell-Mo/A, Cell-Mo/B, Cell-MD/A, Cell-MD/B,的驱动电压分别为9.8 V,7.9 V,5.4 V,和7.3 V。很明显器件Cell-Mo/B的具有最低的驱动电压。这主要是因为由于MoO3极大的降低了空穴的注入势垒,使得其驱动电压降低。比如在大约8 V时,器件Cell-Mo/B的发光亮度为16305 cdcm-2,而其他器件要远远低于这一数值,分别仅为261,1735,3662 cd cm-2.从表中可以看出,器件Cell-MD/A和Cell-Mo/A其发光效率迅速升高,然后在较高电流密度(40-400 mA cm-2)时,逐渐下降；而对于器件爱你Cell-MD/B和Cell-Mo/B的发光效率在较大电流密度(50-400 mA cm-2)时,基本保持稳定,其电流猝火现象并不明显。而对于器件Cell-Mo/B具有最高功率效率(4.7 lm/W),明显要高出其他器件,这主要是因为MoO3极大的提高了空穴的注入,从而降低了器件的驱动电压,并且与有高电子迁移率的BPhen做电子传输层,很好的促进了载流子的平衡。根据我们的测量,M003的HOMO能级为5.43 eV,与NPB的HOMO能级(5.4eV)非常接近；而m-MTDATA的HOMO能级为5.1 eV,与NPB之间存在较大的势差(0.3 eV)。另一方面,在电子迁移方面,我们所使用的BPhen是一种高电子迁移率(3.9×10-4-5.2×104 cm2/V·s)的ETL,同时其有作为空穴阻挡层。
为了进一步研究载流子的注入和平衡,我们制备了一系列的'only'器件(空穴-only-器件和电子-only-器件),其结构组成为：Cell-EA:ITO/Alq3(12 nm)/LiF (0.8 nm)/Al(150 nm) Cell-EB:ITO/BPhen(12 nm)/LiF (0.8 nm)/Al(150 nm) Cell-HMT:ITO/m-MTDATA (20 nm)/NPB (10 nm)/Al(150 nm) Cell-HM3:ITO/MoO3 (30 nm)/NPB (10 nm)/Al(150 nm)
正如我们所知,功率效率取决于载流子的注入和传输,而电流效率比较取决于载流子的注入,同时也取取决于载流子的平衡。由3.6图可知,在低电压下器件Cell-HMT和Cell-EA的电流密度是比较接近的,这就是说器件Cell-MD/A的电子和空穴能够更好的平衡。这就是器件Cell-MD/A在低电压下有更高的电流效率的原因。但是,由于发光层中电子和空穴不平衡,导致在高电流密度下电流效率逐渐降低。对于基于Cell-Mo/A的器件,器件Cell-HM3的电流密度大于器件Cell-EA,因此发光层中电子和空穴不平衡,导致了效率降低。
表3.2基于Alq3和BPhen的电子only器件和基于m-MTDATA和MoO3的空穴only器件的Ⅰ-Ⅴ特效另一方面,在较低的电流密度下,尽管器件Cell-MD/A有最高的功率效率,但是器件Cell-Mo/B比器件Cell-MD/A电流效率稍低。因此,在高电流密度情况下,器件Cell-Mo/B较器件Cell-MD/A有更好电子空穴平衡。这是因为Mo03有很高的空穴注入能力,降低驱动电压。同时,空穴的注入能力强于电子传输层的电子注入能力。过量的空穴注入导致空穴电子的不平衡,因而,电流效率也降低了。
总之,利用MoO3作为空穴注入层和BPhen作为电子传输层,器件的功率效率较使用其他电子传输层和空穴注入层的器件提高74%。在电流密度为20 mA cm-2,驱动电压为5.4 V,。这比器件Cell-MD/A的驱动电压降低了46%。器件Cell-Mo/B的优越性能归因于MoO3具有很高的空穴注入能力,以及BPhen的高电了传输能力。这使得器件的功率效率提高,同时降低了器件的驱动电压。同时,由于电子和空穴平衡,器件的电流效率提高。功率效率的提高降低器件的能耗,而降低器件的能耗是下一代OLED面板进入实用化的关键因素之一。P(3)
我们知道,电子和空穴复合区的平衡时提高有机电致发光器件效率和稳定性的关键因素。因此,测量非晶态有机半导体载流子的迁移率对于有机电致发光有着重要的意义。TOF (time-of-flight)是测量有机半导体薄膜载流子迁移率的一个常用方法。空间电荷限制法也是测量半导体薄膜载流子迁移率的一个常用方法。尽管空间电荷限制法已经有了比较完善的理论体系,但是这种方法缺很少用于有机材料载流子迁移率的测量。其原因是这种方法通常要求电极与半导体薄膜要有良好的欧姆接触。如果在邮寄薄膜和电极之间插入一层缓冲层来实现欧姆接触,则空间电荷限制法就有望被用于有机材料载流子迁移率的测量。研究表明,通过引入LiF来形成欧姆接触,而后用空间电荷限制法所测得的Alq3的电子迁移率为5.2×10-6cm2/(V·s)(在0.8 MV/cm)这一结果与用TOF法测得的结果吻合的很好。但是,用空间电荷限制法来研究掺杂有机半导体(Liq-doped BPhen)的电子迁移率还没有报道过。在这部分的工作中,我们用空间电荷限制法研究了电子传输材料BPhen与Alq3掺杂体系的电子迁移率。对于掺杂体系(BPhen doped with 33 wt% Liq)电子迁移率的测量,我们用LiF来作为缓冲层材料形成欧姆接触。因为它可以通过降低Al和掺杂薄膜的注入势垒来有效的增强载流子的注入。不同厚度掺杂体系薄膜的电子迁移率通过空间电荷限制法来测量。
为了研究n型掺杂体系薄膜(BPhen:x wt% Liq)的电子注入和传输能力,我们制备了一系列的only-器件：
Cell-E1:ITO/BPhen:0 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)
Cell-E2:ITO/BPhen:10 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)
Cell-E3:ITO/BPhen:33 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)
Cell-E4:ITO/BPhen:50 wt%Liq (80 nm)/LiF (1nm)/Al (100 nm)图3.1是不同Liq掺杂浓度的BPhen薄膜的电流密度随着电压变化的特性曲线。我们可以看到,掺杂了Liq之后,电流密度明显增加而电压也有显著地下降。电流密度随着Liq浓度的上升而增加,当Liq的浓度达到50%时,电流密度开始下降。因此,我们得出Liq的最佳浓度为33%。表格1是当only器件的电流密度在40和100 mA/cm2时的电压数据。从表格我们可以看到,电压随Liq掺杂浓度的增加而降低,当掺杂浓度为33%时,电压达到极小值。结果也表明,在更高的掺杂浓度下电压有轻微的上升。导致这一结果的原因可能是由于在高掺杂浓度下的浓度猝灭。因此,我们选择33%掺杂浓度的器件来测量迁移率。选取了最佳厚度之后,我们以ITO/ [BPhen:33 wt% Liq (x nm)]/LiF (1 nm)/Al (150 nm)的结构制备了only-器件,其中x的值为50,100,150,200,300 nm。J-V曲线在高压和低压区情况下显示出两个截然不同的区域：肖特基势垒和空间电荷限制区。随着电压的增加,J-V曲线显示出空间电荷限制的特性。空间电荷限制可以表达为：E是电场强度,ε和ε0分别是相对介电常数和真空介电常数,L是有机薄膜的厚度。而且,以log-log的坐标形式来看,在低压区斜率小于2,而在高压区电流密度与电压的平方成比例。这一现象表明了电荷传输过程中的空间电荷限制特性。电荷的迁移率依赖于电场强度,可以由Poole-Frenkel方程来描述：μ0是0电场强度下的迁移率,β是Poole-Frenkel因了。合并公式1和2,我们可以得到依赖于电场强度的空间电荷限制表达式：图3.2是电流密度随电场强度平方根的变化曲线。拟合曲线和实验数据的良好吻表明电流在这一区间遵循空间电荷限制。有机层的相对介电常数为3,真空介电常数为8.85×10-12F/m.实验结果表明,随着掺杂层厚度的增加,电荷的迁移率也相应加,当膜层的厚度达到150 nm时,迁移率达到饱和。这是因为,在厚的膜层里边,界面对于电荷迁移率的影响大大降低导致了比较高的电荷迁移率。随着膜层的继续加厚,迁移率受厚度的影响更小,逐渐接近于掺杂体系的本征迁移率。在电场强度为0.3 MV/cm时,测得300 nm膜层的迁移率为5.2×10-3 cm2/(V·s)。这一结果比先前报道的3.4×10-4 cm2/(V·s)高了一个数量级。然而我们所要测量的是在有机电致发光器件中有机层的迁移率,因为在OLED器件中BPhen的厚度只有大概50nm。当我们把掺杂有机层的厚度从50 nm增加至300 nm,电了的迁移率从1.7×10-4cm2/(V·s)增加至5.2×10-3cm2/(V·s)。根据报道,
Alq3的迁移率表现出与我们的实验结果相同的趋势。而TOF法不能用来测量实际OLED器件中有机层的迁移率(因为这种方法要求有机膜的厚度要达到几个微米以上)。在最初的阶段,β随着膜层的增厚而单调上升,这一结果也与PF曲线相吻合。
Pool Frenkel因子β是电场激活能,它与非晶薄膜的无序度有关。然而,随着膜厚的增加,β趋于饱和(当厚度大于150 nm时),它的饱和值为8.1×10-3(cm/V)1/2.另外,当膜厚大于150 nm时,迁移率和β趋于饱和也说明了测量所得的迁移率反映了掺杂体系的真实迁移率。然而,当电场强度为0时,随着膜厚从50 nm增加至300nm,电子迁移率也从5.53×10-6增加至7.63×10-4 cm2/(V·s)。
总之,我们的实验结果表明空间荷限制法可以应用于测量BPhen和Liq掺杂膜电子迁移率的测量。研究发现,Liq掺可以有效的提高BPhen的电子迁移率。我们发现,当掺杂魔的厚度大于150nm时,膜厚的增加对于迁移率的影响很小,此时的迁移率接近于膜的本征迁移率。在膜厚为300 nm时,测得的掺杂膜迁移率为5.2×10-3cm2/(V·s),这一值高于本征的BPhen。应用于OLED器件的掺杂BPhen薄膜的迁移率也可以用空间电荷限制的方法测得。当掺杂薄膜为(Liq 33wt%) 50 nm时,测得的迁移率为1.7×10-4cm2/(V·s).这些结果对于设计和提高OLED器件的性能有重要的意义。
Organic semiconductors are a fascinating class of materials, with a wide range of properties. Electronic devices based on organic semiconductors, such as Organic Light-Emitting Diodes (OLEDs) and field-effect transistors (FETs) have attracted much interest as possible inexpensive and flexible alternatives to inorganic devices. In recent years, the investigation of organic light emitting diode based on small organic molecules has attracted growing interest, due to their attractive characteristics and potential applications to flat panel displays. The primary reason is that large numbers of organic materials are known to have high fluorescence quantum efficiencies in the visible spectrum. Hence, they are ideally suited for multicolor display applications.
The light emission is the result of radiative decay of singlet excitons formed by recombination of charge carriers. However, not all charge carriers which recombine contribute to the generation of light:a part of the excitons may decay non-radiatively or may be quenched in the bulk or at interfaces. Because the ratio of singlet exciton formation to triplet exciton formation under electrical excitation is approximately 1:3 due to spin statistics, organic fluorescent emitters are limited to 25% internal quantum efficiency whereas phosphorescent emitters can in principle reach a quantum efficiency of 100% if the intersystem crossing process is efficient. In this regards, doping is an important technology for electronic imaging and optoelectronics devices based on molecular materials. Emissive doping is usually used for tuning emission colors and enhancing luminescence efficiency. Therefore, efficient organic devices could be demonstrated by using highly fluorescent dye molecules as the emissive dopant in the emitting layer of OLED that based on energy transfer from host to guest molecules. Hereby, the emission wavelength can be tuned in the desired way and the efficiency has been improved by optimization of the properties of the guest and host molecules. In order to achieve low driving voltage and high efficiency in OLED devices, it is necessary to facilitate the injection of charges. One effective approach to enhance carrier injection and transport is to conductively dope the organic layer. This conductive and control doping in the transport layers plays a crucial role for the OLED. The work performed in this dissertation is mainly based on the fabrication and characterization of various types of organic light emitting diodes, focusing on the improvement and charge transport phenomena.
Here, red organic light-emitting devices were constructed that based on a wide band gap host emitting system of 9,10-di(2-naphthyl)anthracene (ADN) co-doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,1 1H-10(2-benzothiazolyl)-quinolizine-[9,9a,lgh] coumarin (C545T) as an assistant dopant. The typical device structure was glass substrate/ITO/4,4',4"-tris(N-3-methylphenyl-N-phenylamino) triphenylamine(m-MTDATA)/N-bis-(naphthalene-1-yl)-N,N diphenylbenzidine(NPB)/[ADN:DCJTB:C545T/Alq3/LiF/Al]. It was found that C545T dopant did not emit by itself but did assist the energy transfer from the host (ADN) to the red emitting dopant. The red OLEDs realized by this approach not only enhanced the emission color, but also significantly improved the EL efficiency. The EL efficiency reached 3.5 cd/A at a current density of 20 mA cm-2, which is enhanced by three times compared with devices where the emissive layer is composed of the DCJTB, doped ADN. The saturated red emission was obtained with Commission International De L'Eclairage (CIE) coordinates of (x=0.618, y=0.373) at 620 nm, and the device driving voltage is decreased as much as 38%. We attribute these improvements to the assistant dopant (C545T), which leads to the more efficient energy transfer from ADN to DCJTB. These results indicate that the co-doped system is a promising method for obtaining high-efficiency red OLEDs.
In this work, we demonstrated blue organic light-emitting devices based on wide band gap host material,2-(t-butyl)-9,10-di-(2-naphthyl)anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO3) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline(BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO3(5 nm)/NPB(30 nm)/[TBADN:DSA-Ph(3wt%)](35 nm)/BPhen(12 nm)/LiF(0.8 nm)/Al(100 nm). It was found that the Cell-Mo//B-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA cm-2, its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA//Alq3 is based on, respectively. The J-V curves of'hole-only'devices reveal that a small hole injection barrier between MoO3//NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.
Charge carriers in organics transport via hopping mechanism, which is quite different from that in inorganic crystalline semiconductors. This makes the charge carrier mobility in organics rather low and strongly dependent on electrical field and temperature, leading to the measurement of the mobility in organics much difficult. The conventional methods to determine the mobility in organics include surface charge decay, transient photocurrent or electroluminescence measurement, and time-of-flight measurement. However, all these methods require determination of transit time of charge carriers, which is complicated, and need intricate instruments. The current through organic devices is space charge limited and governed by the dc mobility. In principle, the information of charge carrier mobility should be reflected in their current-voltage characteristics, i.e., the mobility could be derived from processing or transforming their current-voltage characteristics. One method relating to this idea is space charge limited current (SCLC) technique.
In this work, the electron mobility of 4,7-diphenyl-1,10-phenanthroline (BPhen) doped 8-hydroxyquinolinato-lithium (Liq) at various thicknesses (50-300 nm) has been estimated by using space-charge-limited currents measurements. It has been observed that the electron mobility of doped BPhen (33 wt.% Liq:BPhen) approaches its true value when the thickness is more than 200 nm. The electron mobility of 33 wt.% Liq: BPhen at 300 nm is found to be-5.2×10-3 cm2/(V·s) (at 0.3 MV/cm) with weak dependence on electric field, which is about one order of magnitude higher than that of pristine BPhen (3.4×10-4 cm2/(V·s)) measured by space-charge-limited currents. For thickness typical of organic light-emitting devices, the electron mobility of doped BPhen is also investigated. Keywords
Organic Light Emitting Diodes, Co-doped, BPhen. Electron mobility, Space Charge Limited Current (SCLC),8-hydroxyquinolinonate-lithium (Liq)
引文
[1]L. S. Hung and C. H. Chen, Mater. Sci. Eng.:R:Reps., vol.2002,39:pp.143
[2]A. Bernanose, Br. J. Appl. Phys.,1955,6:S54
[3]A. Bernanose, J. Chim. Phys.,1954,51:255
[4]A. Bernanose, J. Chim. Phys.,1955,52:396
[5]G. D. Short and D. M. Herucles, J. Am. Chem. Soc.,1965,87:1439
[6]M. Pope, H. Kallmann, and P. Magnate, J. Chem. Phys.,1963,38:2042
[7]W. Helfrich and W. G. Schneider, Phys. Rev. Lett.,1965,14:229
[8]M. Sano, M. Pope, and H. Kallmann, J. Chem. Phys.,1965,43:2920
[9]W. G. Schneider and W. Helfrich, J. Chem. Phys.,1966,44:2902
[10]T. C. Werner, J. Chang, and D. M. Hercules, J. Am. Chem. Soc.,1970,92:763
[11]D. F. Williams and M. Schadt, J. Chem. Phys.,1970,53:3480
[12]H. P. Schwob, D. Weitz, and D. F. Williams, Mol. Cryst. Liq. Cryst.,1973,24:271
[13]L.L.T. Bradrey, H. P. Schwob, D. Weitz, and D.F.Williams, Mol. Cryst. Liq. Cryst, 1973,23:271
[14]W. Hwang and K. C. Kao, J. Chem. Phys.,1974,60:3845
[15]M. Witter and I. Zschokke-Granacher, J. Chem. Phys.,1975,63:4187
[16]F. Lohmann and W. Mehl, J. Chem. Phys.,1969,50:500
[17]C. E. Swenberg and M. Pope, Phys. Rev.,1975,12:4113
[18]J. Kalinowski and J. Godlewski, Chem. Phys. Lett.,1975,36:345
[19]J. Kalinowski, J. Godlewski, and R. Signerski, Mol. Cryst. Liq. Cryst.,1976, 33:247
[20]J. Glinski, J. Godlewski, and J. Kalinowski, Mol. Cryst. Liq. Cryst.,1977,48:1
[21]G. G. Roberts, B. S. Keating, P. S. Vincett, andW. A. Barlow, J. Phys. C:Solid State Phys.,1978,11:3847
[22]P. S. Vincett, W. A. Barlow, R. A. Hann, and G. G. Roberts, Thin Solid Films, 1982,94:171
[23]N. P. Sinha, Y. Misra, L.N. Tripathi, and M.Misra, Solid State Commun,1981,39: 89
[24]R. M. Burgess, M. Gouterman, and G. E. Khalil, J. Lumin.,1983,28:377
[25]G. G. Roberts, M. McGinnity, P. S. Vincett, and W. A. Barlow, Solid State Commun.,1979,32:683
[26]P. H. Partridge, Polymer,1983,24:733
[27]P. H. Partridge, Polymer,1983,24:739
[28]P. H. Partridge, Polymer,1983,24:748
[29]P. H. Partridge, Polymer,1983,24:755
[30]S.Hayashi, H. Etoh, and S. Saito, Jpn. J. Appl. Phys.,1986,25:L773
[31]C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett.,1987,51:913
[32]R. Friends, J. Burroughes, D. Bradely, WO Patent (1990),90/13148
[33]R. Friends, J. Burroughes, D. Bradely, US Patent (1993),5247190
[34]L. S. Hung, C. H. Chen, Materials Science and Engineering,2002, R 39:143
[35]M. A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S. R. Forrest, Appl. Phys. Lett.,1994,75:4
[36]L. J. Rothberg and Andrew J. Lovinger, J. Mater. Res.,1996,11:3174
[37]L. S. Hung, C. H. Chen, Materials Science and Engineering,2002, R 39:143
[38]Ta-Ya Chu and Ok-Keun Song, Appl. Phys. Lett.,2007,90:203512
[39]I.-M. Chan, W.-C. Cheng, F. C. Hong, Asia Display/IDW'01,2001, p.1483.
[40]S. A. VanSlyke, C.H. Chen, C. W. Tang, Appl. Phys. Lett.,1996,69:2160
[41]Y. Shirota, Y. Kuwabara, H. Inada, Appl. Phys. Lett.,1994,65:807
[42]Y. Yang, A. J. Heeger, Appl. Phys. Lett.,1994,64:1245
[43]Y. Cao, G. Yu, C. Zhang, R. Menon, A.J. Heeger, Synth. Met.1997,87:171
[44]Z. B. Deng, X. M. Ding, S. T. Lee, W.A. Gambling, Appl. Phys. Lett.1999, 74:2227
[45]D. B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, B. Francois, Appl. Phys. Lett., 1995,67:1659
[46]F. Huang, A.G. MacDiamid, B. R. Hsieh, Appl. Phys. Lett.,1997,71:2415
[47]J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett.1998,73:729
[48]A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, Appl. Phys. Lett.,1998, 72:2147
[49]A. Elschner, F. Bruder, H.-W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S. Thurm, R. Wehrmann, Synth. Met.,2000,111:139
[50]F. Nuesch, F. Rotzinger, L. Si-Ahmed, L. Zuppioli, Chem. Phys. Lett.,1998, 288:861
[51]F. Nuesch, E.W. Forsythe, Q. T. Le, Y. Gao, L.J. Rothberg, J. Appl. Phys,2000, 87:7973
[52]S. F. J. Appleyard, M.R. Willis, Opt. Mater.1998,9:120
[53]S. F. J. Appleyard, S. R. Day, R.D. Pickford, M. R. Willis, J. Mater. Chem.2000, 10:169
[54]C. Ganzorig, M. Fujihira, Appl. Phys. Lett.,2000,77:4211
[55]X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, K. Leo, Appl. Phys. Lett.,2001,78:410
[56]D.-M. Shin, S.-T. Lim, J.S. Choi, J.S. Kim, Thin Solid Films,2000,363:268
[57]S.A. VanSlyke, C.W. Tang, US Patent,1991,569:5,061
[58]Y. Sato, S. Ichinosawa, H. Kanai, IEEE J. Selected Top. Quantum Electron.1998, 4:40
[59]D. F. O'Brien, P.E. Purrows, S. R. Forrest, B. E. Koene, D.E. Loy, M. E. Thompson, Adv. Mater.,1998,10:1108
[60]K. Yamashita, T. Mori, T. Mizutani, H. Miyazaki, T. Takeda, Thin Solid Films 2000,363:33
[61]Y. Shirota, K. Okumoto, H. Inada, Synth. Met.2000,111:387
[62]I.-Y. Wu, J.T. Lin, Y.-T. Tao, E. Balasubramaniam, Adv. Mater.2000,12:668
[63]U. Bach, K.D. Cloedt, H. Spreitzer, M. Gratzel, Adv. Mater.2000,12:1060
[64]U. Mitschke, P. Bauerle, J. Mater. Chem.2000,10:1471
[65]M. Kimura, S.-I. Inoue, K. Shimada, S. Tokito, K. Noda, Y. Taga, Y. Sawaki, Chem. Lett.2000,192
[66]S. Tokito, K. Noda, K. Shimada, S.-I. Inoue, M. Kimura, Y. Sawaki, Y. Taga, Thin Solid Films 2000,363:290
[67]J. F. Hartwig, M. Kawatsura, S.I. Hauck, K.H. Shaughnessy, L.M. Alcazar-Roman, J. Org. Chem.1999,64:5575
[68]M. C. Harris, S.L. Buchwald, J. Org. Chem.2000,65:5327
[69]Y.T. Ao, E. Balasubramaniam, A. Danel, P. Tomasik, Appl. Phys.Lett.2000, 11:933
[70]K.R. J. Thomas, J.T. Lin, Y.-T. Tao, C.-W. Ko, Adv. Mater.2000,12:1949
[71]S. Tokito, K. Noda, H. Fujikawa, Y. Taga, M. Kimura, K. Shimada, Y. Sawaki, Appl. Phys. Lett.2000,77:160
[72]H. Aziz, Z. D. Popovic, N.-X. Hu, A.-M. Hor, G. Xu, Science,1999,283:1900
[73]Z. D. Popovic, S. Xie, N.-X. Hu, A.-M. Hor, D. Fork, G. Anderson, C. Tripp, Thin Solid Films,2000,363:6
[74]H. Aziz, Z.D. Popovic, Appl. Phys. Lett.2002,80:2180
[75]P. Kulkarni, C.J. Tonzola, A. Babel, and S.A. Jenekhe, Chem. Mater.,2004,16: 4556-4573
[76]G. Hughes and M.R. Bryce, J. Mater. Chem.,2005,15:94-107
[77]Y. Hamada, N. Matsusue, H. Kanno, H. Fujii, Jpn. J. Appl. Phys.2001,40:L753
[78]J. Kido, T. Mizukami, US Patent,2000,6,013,384
[79]Z. Liu, J. Pinto, J. Soares, E. Pereira, Synth. Met.2001,122:177
[80]M. Brinkmann, G. Gadret, C. Taliani, N. Masciocchi, A. Sironi, M. Muccini, Synth. Met.2001,121:1499
[81]G. Yu, D. Shen, Y. Liu, D. Zhu, Synth. Met.2001,121:1433
[82]B.J. Chen, W.Y. Lai, Z.Q. Gao, C.S. Lee, S.T. Lee, W.A. Gambling, Appl. Phys. Lett.1999,75:4010
[83]M. StoBel, J. Staudigel, F. Steuber, J. Blassing, J. Simmerer, A. Winnacker, H. Neuner, D. Metzdorf, H.-H.Johannes, W. Kowalsky, Synth. Met.2000,111:19
[84]M. D. Hall, H. B. Schlegel, Chem. Mater.2001,13:2632
[85]M. Sugimoto, M. Anzai, K. Sakanoue, S. Sakaki, Appl. Phys. Lett.2001,79:2348
[86]J. Szmytkowski, W. Stampor, J. Kalinowski, Z.H. Kafafi, Appl. Phys. Lett.2002, 80:1465
[87]S. Yin, Y. Hua, S. Chen, X. Yang, Y. Hou, X. Xu, Synth. Met.2000,111:109
[88]H. Jang, L.-M. Do, Y. Kim, J.G. Kim, T. Zyung, Y. Do, Synth. Met.2001, 121:1669
[89]Y. Shao, Y. Qiu, N.-X. Hu, X. Hong, Chem. Lett.2000:1068
[90]B. J, Chen, X.W. Sun and Y. K. Li, Appl. Phys. Lett.2003,82:3017-3019
[91]D.Z.Garbuzov, V. Bulovic,P.E. Burrows, and S.R. Forrest, Chem. Phys.Lett,1996,249:433
[92]T. Sano, Y. Nishio, Y. Hamada, H. Takahashi, T. Usuki, K. Shibata, J. Mater. Chem.2000,10:157
[93]Y. Liu, J. Guo, J. Feng, H. Zhang, Y. Li, Y. Wang, Appl. Phys. Lett.2001, 78:2300
[94]S. Anderson, M.S. Weaver, A.J. Hudson, Synth. Met.2000,111:459
[95]C. Schmitz, H.-W. Schmidt, M. Thelakkat, Chem. Mater.2000,12:3012
[96]S. Tokito, K. Noda, H. Tanaka, Y. Taga, T. Tsutsui, Synth. Met.2000,111:393
[97]N. Johansson, J. Salbeck, J. Bauer, F. Weissortel, P. Broms, A. Andersson, W.R. Salaneck, Adv. Mater.1998,10:1136
[98]M. A. Khan,Wei Xe, Khizar-ul-Haq,Yu Bai, X. Y. Jiang, Z. L. Zhang, W. Q. Zhu, J. Appl. Phys.,2008,103:014509
[99]S. Naka, H. Okada, H. Onnagawa, and T. Tsutui, Appl. Phys. Lett.2000,76:197
[100]Y. Kijima, N. Asai, and S. Tamura, Jpn. J. Appl. Phys.,1999,38:5274-5277
[101]Y. Sakon, T. Ohnuma, M. Hashimoto, S. Saito, T. Tsutsui, and C. Adachi, U.S. Patent US 5077142 (1991)
[102]S. Naka, H. Okada, H. Onnagawa, and T. Tsutsui, Appl. Phys. Lett.,2000,76:97
[103]Y. Li, M. K. Fung, Z. Xie, S. Lee, L. Hung, and J. Shi, Adv. Mater,2000,14: 1317
[104]D. F. O'Brien, M.A. Baldo, M.E. Thompson, and S.R. Forrest, Appl. Phys. Lett., 1999,74:442-444
[105]Y. Su, H. Huang, C. Li, C. Chien, Y. Tao, P. Chou, S. Datta, and R. Liu, Adv. Mater.,2003,15:884-888
[106]M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Appl. Phys. Lett.,2001, 79:156-158
[107]R.C. Kwong, M.R. Nugent, L. Michalski, T. Ngo, K. Rajan, Y. Tung, M.S. Weaver, T.X. Zhou, M. Hack, M.E. Thompson, S.R. Forrest, and J.J. Brown, Appl. Phys. Lett.,2002,81:162-164
[108]T. Thorns, S. Okada, J. Chen, and M. Furugori, Thin Solid Films,2003,436:264
[109]C. W. Tang, S.A. VanSlyke, C. H. Chen, J. Appl. Phys.1989,65:3610
[110]J. Shi, C. W. Tang, Appl. Phys. Lett.,1997,70:1665
[111]M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature,2000,403:750
[112]J. L. Fox, C. H. Chen, US Patent 4,736,032 (1988)
[113]T. Inoe, K. Nakatani, Japanese Patent 6,009,952 (1994)
[114]T.G. Pavlopoulos, P.R. Hammond, J. Am. Chem. Soc.1974,96:6568
[115]C. H. Chen, C. W. Tang, J. Shi, K.P. Klubek, US Patent 6,020,078 (2000)
[116]C. H. Chen, C.-H. Chien, T.-H. Liu, in:Proceedings of the International Conference on Mater. Adv. Tech. (ICMAT 2001), Singapore,2001, p.221 (Abstracts)
[117]M. Uchida, T. Izumizawa, T. Nakano, S. Yamaguchi, K. Tamao, K. Furukawa, Chem. Mater.2001,13:2680
[118]Y. Sato, Semicond. Semimetals,2000,64:209.
[119]H. Tokailin, M. Matsuura, H. Higashi, C. Hosokawa, T. Kusumoto, SPIE 1910 (1995)38
[120]C. Hosokawa, S. Sakamoto, T. Kusumoto, US Patent 5,389,444 (1995)
[121]T. Kofuji, in:Proceedings of the Electro. J.6th FPD Seminar, vol.81, Tokyo, 1999.
[122]C. Hosokawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett.1995, 67:3853
[123]TDK, in:Proceedings of the Electro. J.6th FPD Seminar, vol.31, Tokyo,1999
[124]J. Shi, C.W. Tang, C.H. Chen, US Patent 5,935,721 (1999)
[125]Y. Liu, J. Guo, H. Zhang, Y. Wang, Angew. Chem. Int. Ed.2002,41:182
[126]G. Rajeswaran, M. Itoh, M. Boroson, S. Barry, T.K. Hatwar, K.B. Kahen, K. Yoneda, R. Yokoyama, T. Yamada, N. Komiya, H. Kanno, H. Takahashi, SID'00 Digest 2000,40:1
[127]Y. Fukuda, T. Watanabe, T. Wakimoto, S. Miyaguchi, M. Tsuchida, Synth. Met. 2000,111:1
[128]C. H. Chen, J. Shi, K.P. Klubek, US Patent 5,908,581 (1999)
[129]N. Komiya, R. Nishikawa, M. Okuyama, T. Yamada, Y. Saito, S. Oima, K. Yoneda, H. Kanno, H. Takahashi, G. Rageswaran, M. Itoh, M. Boroson, T.K. Hatwar, in:Proceedings of the 10th International Workshop on Inorg. And Org. EL (EL'00), Hamamatsu, Japan,4 December 2000, p.347.
[130]Y. Hamada, H. Kanno, H. Fujii, T. Tsujioka, H. Takahashi, ACS Poly Millennial 2000 Abs.,2000, p.167.
[131]T. K. Hatwar, G. Rajeswaran, J. Shi, Y. Hamada, H. Kanno, H. Takahashi, in: Proceedings of the 10th International Workshop on Inorg. And Org. EL (EL'00), Hamamatsu, Japan,4 December 2000, p.31.
[132]R. H. Young, C.W. Tang, A.P. Marchetti, Appl. Phys. Lett.,2002,80:874
[133]Z. Y. Xie, L. S. Hung, S.T. Lee, Appl. Phys. Lett.2001,79:1048
[134]C. H. Chen, C. W. Tang, J. Shi, K.P. Klubek, Thin Solid Films,2000,363:327
[135]B. Chen, X. Lin, L.F. Cheng, C.-S. Lee, W.A. Gambling, S.-T. Lee, J. Phys. D: Appl. Phys.2001,34:30
[136]M. Mitsuya, T. Suzuki, T. Koyama, H. Shirai, Y. Taniguchi, Appl. Phys. Lett. 2000,77:3272
[137]S. Toguchi, Y. Morioka, H. Ishikawa, A. Oda, E. Hasegawa, Synth. Met.2000, 111:57
[138]M. Ichimura, T. Ishibashi, N. Ueda, S. Tamura, in:Proceedings of the 3rd International Conference on EL Mol. Mater. Relat. Phenom. (ICEL-3),O-29, Los Angelos, CA, USA,5-8 September 2001 (Abstracts)
[139]J. Zhang, R.-G. Xiong, Z.-F. Chen, X.-Z. You, G.-H. Lee, S.M. Peng, Chem. Lett. 2001,676
[140]W. Hu, M. Matsumura, M. Wang, L. Jin, Appl. Phys. Lett.2000,77:4271
[141]W. Hu, M. Matsumura, M. Wang, L. Jin, Jpn. J. Appl. Phys.2000,39:6445
[142]Robert J. Silbey, Robert A. Alberty, and Moungi G. Bawendi, Physical chemistry, Hoboken, NJ, Wiley,2005.
[143]Andrew Gilbert, Essentials of Molecular Photochemistry, Oxford, U.K.,1991
[144]Jingsong Huang, Martin Pfeiffer, Ansgar Werner, Jan Blochwitz, and Karl Leo, Shiyong Liu, Appl. Phys. Lett.,2002,80:139
[145]Chang-Ching Chang, Ming-Ta Hsieh, Jenn-Fang Chen, Shiao-Wen Hwang, Jia-Wei Ma, and Chin H. Chen, Society for information display DIGEST,1106(2006)
[146]M. G. Mason, L. H. Hung, C.W Tang, S. T. Lee, K. W. Wong and M. Wang, J. Appl. Phys.1999,86:1688
[147]S. A. VanSlyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett.,1996,69:2160
[148]Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett.,1994,65:807
[149]Z. B. Deng, X. M. Ding, S. T. Lee, and W. A. Gambling Appl. Phys. Lett.,1999, 74:2227
[150]H. J. Jiang, Y. Zhou, B.S.Ooi, Y. Chen, T. Wee,Y. L. Lam, J. S. Huang, and S.Y. Liu, Thin Solid Film 2000,363:25
[151]A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend, N. C. Greenham, P. L. Burn, A. B. Holmes, and A. Kraft, Appl. Phys. Lett.1992,61:2793
[152]C. Ganzoring and M. Fujihira, Appl. Phys. Lett.,2000,77:4211
[153]D. B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, and B. Francois, Appl. Phys. Lett.,1995,67:1659
[154]F. Huang, A. G. MacDiamid and B. R. Hsieh, Appl. Phys. Lett.,2000,71:4211
[155]M. Pfeiffer, A. Beyer, T. Fritz, and K. Leo, Appl. Phys. Lett.,1998,73:3202
[156]I. D Parker, J. Appl. Phys.1994,75:1656
[157]J. Kido, M. Kimura, and Nagai, Science 1995,267:1332
[158]L. S. Hung, C.W. Tang, and M. G. Mason, Appl. Phys.Lett.1997,70:152
[159]J. Kido, and Y. Lizumi, Appl. Phys. Lett.1998,73:2721
[160]Y. Kim, H. Park, and J. Kim, Appl. Phys. Lett.1996,69:599
[161]H. H. Fong, Wallace C. H. Choy, K. N. Hui, and Y. J. Liang, Appl. Phys. Lett. 2006,88:113510
[162]Junji Kido, Toshio Matsumoto, Appl. Phys. Lett.1998,73:2866
[163]Gufeng He, Martin Pfeiffer, Karl Leo, Michael Hofmann, Jan Birnstock, Robert Pudzich and Josef Salbeck, Appl. Phys. Lett.2004,85:3911
[164]Gufeng He, Oliver Schneider, Dashan Qin, Xiang Zhou, Martin Pfeiffer, Karl Leo, Appl. Phys. Lett.2004,95:5773
[165]Hamaguchi, C. Basic Semi. Physics. pp.263, Springer, New Delhi (2001)
[166]Campbell, A. J., Bradley, D.D.C., Antoniadis, H:Synth. Met.2001,22(1):161
[167]Lampert, M. A., Mark, P:Current Injection in Solids. Academic Press, New York (1970)
[168]Blom, P. W. M., de Jong, M.J.M., Vleggaar, J. J.M.:Appl. Phys. Lett.1996 68(23):3308
[169]Fichou, D.:Handbook of Oligo-and Polythiophenes. Wiley-VCH, Weinheim, New York (1999)
[170]Bassler, H.:Phys. Stat. Sol. B 1993,175:15
[171]Horowitz, G., Hajlaoui, M.E., Hajlaoui, R.:J. Appl. Phys.2000,87:4456
[172]Pope, M. and Solowiejczkyk, Y., Mol. Cryst. Liq. Cryst.,1975,30:175
[173]W. Brutting, S. Berleb and A. G. Muckel, Synth. Met.2001,122:99
[174]L. Bozano, S. A. Carter, J. C. Scott, G. G. Malliaras and P. J. Brock, Appl. Phys. Lett.199974:1132
[175]S. Forero, P. H. Nguyen, W. Brutting and M. Schwoerer, Phys. Chem. Chem. Phys.1999,1:1769
[176]Helfrich, W., In Physical and Chemistry of the Organic Solid State, Vol.3 D. Fox, M. M. Labes, and A. Weissberger, Inter-science, New York,1967, chap.1.
[177]P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A. Cronin, and M. E. Thompson, J. Appl. Phys.1996,79:7991
[178]H. Bassler, Phys. Stat. Sol. (b) 1993,175:15
[179]M. A. Baldo, and S. R. Forrest, Phys. Rev. B 2001,64:85201
[1]Tang C W and VanSlyke S A. Appl. Phys. Lett.1987,51:913
[2]Tang C W, Van Slyke S A and Chen C H, J. Appl. Phys.1989,65:3610
[3]Sakamoto G, Adachi C, Koyama T, Taniguchi Y, Merritt C D, Murata H and Kafafi Z H, Appl. Phys. Lett.1999,75:766
[4]Xie Z Y, Hung L S and Lee S T, Appl. Phys. Lett.2001,79:1048
[5]Hamada Y, Kanno H, Tsujioka T and Takahashi H, Appl. Phys. Lett.1999, 75:1682
[6]Jin Y D, Yang J P, Heremans P L, Auweraer M V, Rousseau E, Geise H J and Borghs G, Chem. Phys. Lett.2000,320:387
[7]Ohmori Y, Kajii H, Sawatani T, Ueta H and Yoshino K, Thin Solid Films.2001, 393:407
[8]D'Andrade B W, Blade M A, Adachi C and Brooks, J. Appl. Phys. Lett.2001, 79:1045
[9]Feng J, Li F, Gao W B, Cheng G, Xie W F and Liu S Y, Appl. Phys. Lett.2002, 81:2935
[10]Chen C H, Tang C W, Shi J and Klubek K P, Thin Solid Films.2000,363:327
[11]Shi J and Tang C W Appl. Phys. Lett.2002,80:201
[12]S. C. Tse, S.K. So. M. Y. Yeung, C.F. Lo, S.W. Wen, C.H. Chen, Chem. Phys. Lett.2006,422:354-357
[13]S. W. Cullingan. Andrew C.-A. Chen, Jason U. Wallace, Kevin P. Klubek, Ching W. Tang, Shan H. Chen, Adv. Funct. Mater.2006,16:1481-1487
[14]T. Forster. Discuss. Faraday Soc.1959,7:27
[15]M. Pope, C.E. Swenberg, Electronics Processes in Organic Crystals, Oxford University Pr (Txt), New York,1982
[16]Wang L, Lie G and Qui Y,J. Appl. Phys.2005,97:114503
[17]Luo Y, Aziz H, Popovic Z D and Xu G, Appl. Phys. Lett.2006,89:103505
[18]Chen C H, Tang C W, Shi J and Klubek K P, Macromol. Symp.1997,125:49
[19]Popovic Z D, Aziz H, Ioannidis A, Hu N X and dos Anjos P N M, Synth. Met. 2001,123:179
[20]Bulovic V, Shoustikov A, Baldo M A, Bose E, Kozlov V G, Thompson M E and Forrest S R, Chem. Phys. Lett.1998,287:455
[21]Iou C-Y, Liu T-H, Chen H-H, Shen W-J and Chen C H Proc. SID'03 (Baltimore, MD,21 May), p 971(2003)
[22]Young R H, Tang C W and Marchetti A P, Appl. Phys. Lett.2002,80:874
[23]M. Redecker, D.D.C. Bradley, M. Inbasekaran, W.W. Wu, Ed. P. Woo, Adv. Mater.1999,11(3):41
[24]Musubu Ichikawa, Yasuhiro Horiba, H. Nakatani, Masashi Yamada, Toshiki Koyama, Yoshio Taniguchi, Jpn. J. Appl. Phys.2002,41:2252
[25]M.T. Lee, Y.S. Wu, H.H. Chen, C.H. Tsai, C.H. Liao, C. H. Chen, in: Proceedings of the SID'04 Digest, vol.710,2004
[26]S.W. Wen, M. T. Lee, C.H. Chen, IEEE/OSA J. Display Technol.2005,1:90
[1]C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett.1987,51:913
[2]H.K. Lee, J.H. Seo, J.H. Kim, J.R. Koo, K.H. Lee, S.S. Yoon, Y.K. Kim, J. Korean Phys. Soc.2006,49:1052
[3]G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett.2004,85:3911
[4]R. Meerheim, K. Walzer, M. Pfeiffer, K. Leo, Appl. Phys. Lett.2006,89:061111
[5]Huang J, Pfeiffer M, Werner A, Blochwitz, J, Leo K, and Liu S, Appl. Phys. Lett. 2002,80:139
[6]Parker I D, J. Appl. Phys.1994,75:1656
[7]Kido J, Kimura M and Nagai K, Science 1995,267:1332
[8]Wakimoto T, Fukuda Y, Nagayama K, Yokoi A, Nakada H and Tsuchida M IEEE Trans. Electron Devices 1997,44:1245
[9]Hung L S, Tang C W and Mason M G Appl. Phys. Lett.1997,70:152
[10]Kido J and Lizumi Y Appl. Phys. Lett.1998,73:2721
[11]Ganzorig C and Fujihara M Appl. Phys. Lett.2000,77:4211
[12]Piromreun P, Oh H, Shen Y, Malliaras G, Scott J C and Brock P J Appl. Phys. Lett.2000,77:2403
[13]Mason M G, Hung L H, Tang C W, Lee S T, Wong K W and Wang M J. Appl. Phys.1999,86:688
[14]Kim Y, Park H and Kim J Appl. Phys. Lett.1996,69:599
[15]S. A. Van Slyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett.1996,69:2160
[16]Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y.Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett.1994,65:807
[17]Zheng Z. B, Ding X. M, Lee S. T, and Gambling W.A, Appl. Phys. Lett.1999 74: 82227
[18]Romero D. B, Schaer M, Zupproli L, Cesar B and Rancois B. F, Appl. Phys. Lett. 1995,67:1659
[19]Blochwitz J, Pfeiffer M, Fritz T and Leo K, Appl. Phys. Lett.1998,73:729
[20]Ganzorig C and Fijihira M, Appl. Phys. Lett.2000,77:4211
[21]M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett.1998,73:3202
[22]J. Blochwitz, T. Fritz, M. Pfeiffer, K. Leo, D.M. Alloway, P.A. Lee, N.R. Armstrong, Org. Electron.2001,2:97
[23]F. Huang, A.G. MacDiarmid, B.R. Hsieh, Appl. Phys. Lett.1997,71:2415
[24]J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett.1998,73:729
[25]Ganzorig C and Fijihira M, Appl. Phys. Lett.2000,77:4211
[26]E. Gondek, J. Niziol, A. Danel, I.V. Kityk, M. Pokladko, J. Sanetra, E. Kulig, J. Luminesc,2008128:1831.
[27]W. C. H. Choy, K. N. Hui, H. H. Fong, Y. J. Liang, P. C. Chui, Thin Solid Films, 2006,509:193
[28]J. H. Jou, Y.S. Chui, R.Y. Wang, H.C. Hu, C.P. Wang, H.W. Lin, Org. Electron, 2006,7:8
[29]Meng-Ting Lee, Hsian-Hung Chen, Chi-Hung Liao, and Chih-Hung Tsai, Chin H. Chen, Appl. Phys. Lett.2004,85:3301
[30]S. L. Lai, M. Y. Chan, M. K. Fung, C. S. Lee, and S. T. Lee, Appl. Phys. Lett. 2007,90:203510
[31]J. Shi, and C. W.Tang, Appl. Phys. Lett.2002,80:3201
[32]W.-J.Shen, B. Banumathy, H.-H. Chen, and C. H.Chen, Proceedings of International Display Manufacturing Conference and FPD Expo, Taipei, ROC, 18-21 February 2003, p 741.
[33]M.-T.Lee, Y.-S.Wu, H.-H. Chen,C.-H. Tsai, L.-C, Liao, and C. H. Chen, Proceedings of the Society For Information Display, Seattle, Washington,23-28 May 2004
[34]Zhang Z L 1997 Organic Electrolumies Materials and Devices ed S Miyata and Nalwa H S (New York:Gordon and Breach) chap 5, pp 209
[35]M. A. Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, X. Y. Jiang, Z. L. Zhang, J. Appl. Phys.2008,103:014509
[1]Tang C W, Slyke V., Appl. Phys. Lett,1987,51:913
[2]Tang C W, Slyke V, Chen C. H. J. Appl. Phys,1989,65:3610
[3]Baldo M A, O'Brien D F, You Y, A. Shoustikov, S. Sibley, M. E.Thompson, and S. R. Forrest, Nature (London),1998,395:151
[4]Choong V E, Shi S, Curless J, S. Shi, J. Curless, C. L. Shieh, H. C. Lee, J. Shen, and J.Yang, Appl. Phys. Lett.,1999,75:172
[5]Slyke S A V, Chen C H, Tang C W, Appl. Phys. Lett,1996,69:2160
[6]Di C A, Yu G, Liu Y, X. Xu, Y. Song, and D. Zhu, Appl. Phys. Lett.,2007, 90:133508
[7]Chen Z, Yu J, Sakuratani Y, M. Li, M. Sone, S. Miyata, T. Watanabe, X.Wang, and H. Sato, J Appl Phys,2001,89:7895
[8]Gill W D, J. Appl. Phys,1972,43:5033
[9]Tse S C, Fong H H, So S K. J. Appl. Phys,2003,94:2033
[10]Blom PWM, de JongM J M, Vleggaar J J M., Appl. Phys. Lett,1996,68:3308
[11]Bozano L, Carter S A, Scott J C, G. G. Malliaras, and P. J. Brock, Appl. Phys Lett.,1999,74:1132
[12]Yasuda T, Yamaguchi Y, Zou D C, Jpn J Appl Phys, Part 1,2002,41:5626
[13]Kim S H, Jang J, Lee J Y. Appl. Phys. Lett,2000,89:253501
[14]Chu T Y, Song O K, Appl. Phys. Lett,2007,90:203512
[15]Malliaras G G, Shen Y, Dunlap D H, H. Murata, and Z. H. Kafafi, Appl. Phys. Lett.,2001,79:2582
[16]Khan M A, Xu W, Khizar-ul-Haq, Yu Bai, X. Y. Jiang, Z. L. Zhang, J. Appl. Phys,2008,103:014509
[17]Naka S, Okada H, Onnagawa H, and T. Tsutsui, Appl. Phys. Lett.,2000,76:197
[18]Wang X J, Zhao J M, Zhou Y C, X. Z. Wang, S. T. Zhang, Y. Q. Zhan, Z. Xu, H. J. Ding, G. Y. Zhong, H. Z. Shi, Z. H. Xiong, Y. Liu, Z. J. Wang, E. G. Obbard, X. M. Ding, W. Huang, and X. Y. Hou, J. Appl. Phys,2004,95:3828
[19]Stoβel M, Staudigel J, Steuber F, J. Blassing, J. Simmerer, and A. Winnacker, Appl. Phys. Lett,2000,76:115
[20]Huang J, Pfeiffer M, Werner A, Blochwitz J, Leo K and Liu S, Appl. Phys. Lett. 2002,80:139
[21]Kido J and Matsumoto T, Appl. Phys. Lett.1998,73:2866
[22]He G, Pfeiffer M, Leo K, Hofmann M, Birnstock J, Pudzich R and Salbeck J, Appl. Phys. Lett.2004,85:3911
[23]He G, Schneider O, Qin D, Zhou X, Pfeiffer M and Leo K, Appl. Phys. Lett.2004, 95:5773
[24]Fong H H, Choy W C H, Hui K N and Liang Y J, Appl. Phys. Lett.2006, 88:113510
[25]Khan M A, Xu W, Khizar-ul-Haq, Yu Bai, Fuxiang Wei, X Y Jiang, Z L Zhang, and W Q Zhu, J. Phys D:Appl. Phys,2007,40:6535
[26]P. W. M. Bloom, M. J. M. deJong, and J. J. M. Vleggaar, Appl. Phys. Lett.1996, 68:3308
[27]G. Horowitz, Adv. Mater.1998,10:365
[28]R. G. Kepler, Phys. Rev.1960,119:1226
[29]Malliaras G G, Salem J R, Brock P J, and C. Scott, Phys Rev B,1998,58:R13411
[30]Pal A J, Osterbacka R, Kallman K M, and H. Stubb, Appl Phys Lett,1997,71: 228
[31]Kumar P, Jain S C, Kumar V, et al., Synth Met,2007,154:905
[32]M A Khan, Wei Xu, Khizar-ul-Haq, Xiao Wen Zhang, Yu Bai, X Y Jiang, Z L Zhang and W Q Zhu, J Phys D:Appl Phys,2008,41:225105
[33]Broutting W, Berleb S, Muckl A G. Organic Electron,2001,2:1
[34]Campbell A J, Weaver M S, Lidzey D. G, et al., J. Appl. Phys,1998,84:6737
[35]A. Ioannidis, E. Forsythe, Gao Yongli, M.W. Wu, E.M. Conwell, Appl. Phys. Lett. 1998,72:3038
[36]W. Brutting, S. Berleb, A.G. Muckl, Organ. Electron,2001,2:1
[1]Khizar-ul-Haq, Liu Shan-peng, M A Khan, X Y Jiang, Z L Zhang and W Q Zhu, "Red organic light-emitting diodes based on wide band gap emitting material as the host utilizing two-step energy transfer" Semicond. Sci. Technol.23, 035024 (6pp)(2008)
[2]Khizar-ul-Haq, M. A. Khan, X.Y. Jiang, Z. L. Zhang, X.W. Zhang, Bin Wei, J. M. Wei, "Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO3" Journal of Luminescence 129,1158-1162 (2009)
[3]Khizar-ul-Haq, Khan M A, Jiang X Y, Zhang Z L, Zhang X W, Zhang L, and Li Jun "Estimation of electron mobility of n-doped 4,7-diphenyl-1,10-phenanthroline using space-charge-limited currents" Journal of Semiconductors Vol.30, No.11:114009-1(2009)
[4]M A Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, Fuxiang Wei, X Y Jiang, Z L Zhang, and W Q Zhu, "Highly power efficient organic light-emitting diodes based on p-doped and novel n-doped carrier transport layers" J. Phys. D:Appl. Phys. 40,6535(2007)
[5]M A Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, X Y Jiang, Z L Zhang and W Q Zhu, "Influence of p-doping hole transport layer on the performance of organic light-emitting devices" Semicond. Sci. Technol.23,055014 (5pp) (2008)
[6]M A Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, X. Y. Jiang, Z. L. Zhang, and W. Q. Zhu Z. L. Zhang and W. Q. Zhu, "Electron mobility of 4,7-diphenyl-1,10 phenanthroline estimated by using space-charge-limited currents" Journal of Applied Physics 103,014509(2008)
[7]M A Khan, Wei Xu, Khizar-ul-Haq, X W Zhang, Yu Bai, X Y Jiang, Z L Zhang and W Q Zhu, "Electron injection and transport mechanism in organic devices based on electron transport materials" J. Phys. D:Appl. Phys.41, 225105 (6pp)(2008)
[8]Bai, X. Liu, L. Chen, Khizar-ul-Haq, M.A. Khan, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, "Organic thin-film field-effect transistors with MoO3/Al electrode and OTS/SiO2 bilayer gate insulator" Microelectronics Journal 38,1185-1190(2007)
[9]X Y Jiang, Z L Zhang, Jin Cao, M A Khan, Khizar-ul-Haq, and Wen-Qing Zhu, "White OLED with high stability and low driving voltage based on a novel buffer layer MoOx" J. Phys. D:Appl. Phys.40,5553-5557(2007)
[10]X W Zhang, X Y Jiang, M.A. Khan, Jin Cao, Jun-Wei Ma, L Zhang, Jun Li, Khizar-ul-Haq, W Q Zhu, Z L Zhang, "Enhanced electron injection in organic light-emitting devices by using a composite electron injection layer composed of 8-hydroquinolatolithium and cesium oxide" Solid State Communications 149,652-656(2009)
[11]Bai Yui, Khizar-ul-Haq, M A Khan, Jiang X Y, and Zhang Z L, "OTFT with Bilayer Gate Insulator and Modified Electrode" Journal of semiconductor,Vol.29, No.4 April(2008)
[12]X W Zhang, Jun Li, M A Khan, L Zhang, X Y Jiang, Khizar-ul-Haq, Wen-Qing Zhu and Z L Zhang, "Improved chromaticity and electron injection in a blue organic light-emitting device by using a dual electron-transport layer with hole-blocking function" Semicond. Sci. Technol.24,075021 (5pp) (2009)
[13]Jun Li, X W Zhang, L Zhang, Khizar-ul-Haq, Xue-Yin Jiang, Wen-Qing Zhu, Z L Zhang, "Improving organic transistor performance through contact-area-limited doping" Solid State Communications 149,1826-1830(2009)
[14]Jun Li, X W Zhang, L Zhang, Khizar-ul-Haq, X Y Jiang, Wen-Qing Zhu and Z L Zhang, "Performance enhancement of organic thin-film transistors using WO3-modified drain/source electrodes" Semicond. Sci. Technol.24,115012 (5pp) (2009)
[15]X W Zhang, Jun Li, L Zhang, X Y Jiang, Khizar-ul-Haq, Wen-Qing Zhu Zhi-Lin Zhang, "Top-emitting organic light-emitting device with high efficiency and low voltage using a silver-silver microcavity" Thin Solid Films 518,1756-1759(2010)
[16]Jun Li, X W Zhang, L Zhang, Hao Zhang, X Y Jiang, Khizar-ul-Haq, W Q Zhua, Z L Zhang. "MoOx interlayer to enhance performance of pentacene-TFTs with low-cost copper electrodes" Synthetic Metals volume 160,376-379(2010)
[17]L Zhang, J Li, X. W Zhang, D B Yu, H P Lin, Khizar-ul-Haq, X Y Jiang, Z L Zhang "Low-voltage-drive and high output current ZnO thin-film transistors with sputtering SiO2 as gate insulator" Current Applied Physics (In press-2010)
[2]A. Bernanose, Br. J. Appl. Phys.,1955,6:S54
[3]A. Bernanose, J. Chim. Phys.,1954,51:255
[4]A. Bernanose, J. Chim. Phys.,1955,52:396
[5]G. D. Short and D. M. Herucles, J. Am. Chem. Soc.,1965,87:1439
[6]M. Pope, H. Kallmann, and P. Magnate, J. Chem. Phys.,1963,38:2042
[7]W. Helfrich and W. G. Schneider, Phys. Rev. Lett.,1965,14:229
[8]M. Sano, M. Pope, and H. Kallmann, J. Chem. Phys.,1965,43:2920
[9]W. G. Schneider and W. Helfrich, J. Chem. Phys.,1966,44:2902
[10]T. C. Werner, J. Chang, and D. M. Hercules, J. Am. Chem. Soc.,1970,92:763
[11]D. F. Williams and M. Schadt, J. Chem. Phys.,1970,53:3480
[12]H. P. Schwob, D. Weitz, and D. F. Williams, Mol. Cryst. Liq. Cryst.,1973,24:271
[13]L.L.T. Bradrey, H. P. Schwob, D. Weitz, and D.F.Williams, Mol. Cryst. Liq. Cryst, 1973,23:271
[14]W. Hwang and K. C. Kao, J. Chem. Phys.,1974,60:3845
[15]M. Witter and I. Zschokke-Granacher, J. Chem. Phys.,1975,63:4187
[16]F. Lohmann and W. Mehl, J. Chem. Phys.,1969,50:500
[17]C. E. Swenberg and M. Pope, Phys. Rev.,1975,12:4113
[18]J. Kalinowski and J. Godlewski, Chem. Phys. Lett.,1975,36:345
[19]J. Kalinowski, J. Godlewski, and R. Signerski, Mol. Cryst. Liq. Cryst.,1976, 33:247
[20]J. Glinski, J. Godlewski, and J. Kalinowski, Mol. Cryst. Liq. Cryst.,1977,48:1
[21]G. G. Roberts, B. S. Keating, P. S. Vincett, andW. A. Barlow, J. Phys. C:Solid State Phys.,1978,11:3847
[22]P. S. Vincett, W. A. Barlow, R. A. Hann, and G. G. Roberts, Thin Solid Films, 1982,94:171
[23]N. P. Sinha, Y. Misra, L.N. Tripathi, and M.Misra, Solid State Commun,1981,39: 89
[24]R. M. Burgess, M. Gouterman, and G. E. Khalil, J. Lumin.,1983,28:377
[25]G. G. Roberts, M. McGinnity, P. S. Vincett, and W. A. Barlow, Solid State Commun.,1979,32:683
[26]P. H. Partridge, Polymer,1983,24:733
[27]P. H. Partridge, Polymer,1983,24:739
[28]P. H. Partridge, Polymer,1983,24:748
[29]P. H. Partridge, Polymer,1983,24:755
[30]S.Hayashi, H. Etoh, and S. Saito, Jpn. J. Appl. Phys.,1986,25:L773
[31]C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett.,1987,51:913
[32]R. Friends, J. Burroughes, D. Bradely, WO Patent (1990),90/13148
[33]R. Friends, J. Burroughes, D. Bradely, US Patent (1993),5247190
[34]L. S. Hung, C. H. Chen, Materials Science and Engineering,2002, R 39:143
[35]M. A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S. R. Forrest, Appl. Phys. Lett.,1994,75:4
[36]L. J. Rothberg and Andrew J. Lovinger, J. Mater. Res.,1996,11:3174
[37]L. S. Hung, C. H. Chen, Materials Science and Engineering,2002, R 39:143
[38]Ta-Ya Chu and Ok-Keun Song, Appl. Phys. Lett.,2007,90:203512
[39]I.-M. Chan, W.-C. Cheng, F. C. Hong, Asia Display/IDW'01,2001, p.1483.
[40]S. A. VanSlyke, C.H. Chen, C. W. Tang, Appl. Phys. Lett.,1996,69:2160
[41]Y. Shirota, Y. Kuwabara, H. Inada, Appl. Phys. Lett.,1994,65:807
[42]Y. Yang, A. J. Heeger, Appl. Phys. Lett.,1994,64:1245
[43]Y. Cao, G. Yu, C. Zhang, R. Menon, A.J. Heeger, Synth. Met.1997,87:171
[44]Z. B. Deng, X. M. Ding, S. T. Lee, W.A. Gambling, Appl. Phys. Lett.1999, 74:2227
[45]D. B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, B. Francois, Appl. Phys. Lett., 1995,67:1659
[46]F. Huang, A.G. MacDiamid, B. R. Hsieh, Appl. Phys. Lett.,1997,71:2415
[47]J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett.1998,73:729
[48]A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, Appl. Phys. Lett.,1998, 72:2147
[49]A. Elschner, F. Bruder, H.-W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S. Thurm, R. Wehrmann, Synth. Met.,2000,111:139
[50]F. Nuesch, F. Rotzinger, L. Si-Ahmed, L. Zuppioli, Chem. Phys. Lett.,1998, 288:861
[51]F. Nuesch, E.W. Forsythe, Q. T. Le, Y. Gao, L.J. Rothberg, J. Appl. Phys,2000, 87:7973
[52]S. F. J. Appleyard, M.R. Willis, Opt. Mater.1998,9:120
[53]S. F. J. Appleyard, S. R. Day, R.D. Pickford, M. R. Willis, J. Mater. Chem.2000, 10:169
[54]C. Ganzorig, M. Fujihira, Appl. Phys. Lett.,2000,77:4211
[55]X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, K. Leo, Appl. Phys. Lett.,2001,78:410
[56]D.-M. Shin, S.-T. Lim, J.S. Choi, J.S. Kim, Thin Solid Films,2000,363:268
[57]S.A. VanSlyke, C.W. Tang, US Patent,1991,569:5,061
[58]Y. Sato, S. Ichinosawa, H. Kanai, IEEE J. Selected Top. Quantum Electron.1998, 4:40
[59]D. F. O'Brien, P.E. Purrows, S. R. Forrest, B. E. Koene, D.E. Loy, M. E. Thompson, Adv. Mater.,1998,10:1108
[60]K. Yamashita, T. Mori, T. Mizutani, H. Miyazaki, T. Takeda, Thin Solid Films 2000,363:33
[61]Y. Shirota, K. Okumoto, H. Inada, Synth. Met.2000,111:387
[62]I.-Y. Wu, J.T. Lin, Y.-T. Tao, E. Balasubramaniam, Adv. Mater.2000,12:668
[63]U. Bach, K.D. Cloedt, H. Spreitzer, M. Gratzel, Adv. Mater.2000,12:1060
[64]U. Mitschke, P. Bauerle, J. Mater. Chem.2000,10:1471
[65]M. Kimura, S.-I. Inoue, K. Shimada, S. Tokito, K. Noda, Y. Taga, Y. Sawaki, Chem. Lett.2000,192
[66]S. Tokito, K. Noda, K. Shimada, S.-I. Inoue, M. Kimura, Y. Sawaki, Y. Taga, Thin Solid Films 2000,363:290
[67]J. F. Hartwig, M. Kawatsura, S.I. Hauck, K.H. Shaughnessy, L.M. Alcazar-Roman, J. Org. Chem.1999,64:5575
[68]M. C. Harris, S.L. Buchwald, J. Org. Chem.2000,65:5327
[69]Y.T. Ao, E. Balasubramaniam, A. Danel, P. Tomasik, Appl. Phys.Lett.2000, 11:933
[70]K.R. J. Thomas, J.T. Lin, Y.-T. Tao, C.-W. Ko, Adv. Mater.2000,12:1949
[71]S. Tokito, K. Noda, H. Fujikawa, Y. Taga, M. Kimura, K. Shimada, Y. Sawaki, Appl. Phys. Lett.2000,77:160
[72]H. Aziz, Z. D. Popovic, N.-X. Hu, A.-M. Hor, G. Xu, Science,1999,283:1900
[73]Z. D. Popovic, S. Xie, N.-X. Hu, A.-M. Hor, D. Fork, G. Anderson, C. Tripp, Thin Solid Films,2000,363:6
[74]H. Aziz, Z.D. Popovic, Appl. Phys. Lett.2002,80:2180
[75]P. Kulkarni, C.J. Tonzola, A. Babel, and S.A. Jenekhe, Chem. Mater.,2004,16: 4556-4573
[76]G. Hughes and M.R. Bryce, J. Mater. Chem.,2005,15:94-107
[77]Y. Hamada, N. Matsusue, H. Kanno, H. Fujii, Jpn. J. Appl. Phys.2001,40:L753
[78]J. Kido, T. Mizukami, US Patent,2000,6,013,384
[79]Z. Liu, J. Pinto, J. Soares, E. Pereira, Synth. Met.2001,122:177
[80]M. Brinkmann, G. Gadret, C. Taliani, N. Masciocchi, A. Sironi, M. Muccini, Synth. Met.2001,121:1499
[81]G. Yu, D. Shen, Y. Liu, D. Zhu, Synth. Met.2001,121:1433
[82]B.J. Chen, W.Y. Lai, Z.Q. Gao, C.S. Lee, S.T. Lee, W.A. Gambling, Appl. Phys. Lett.1999,75:4010
[83]M. StoBel, J. Staudigel, F. Steuber, J. Blassing, J. Simmerer, A. Winnacker, H. Neuner, D. Metzdorf, H.-H.Johannes, W. Kowalsky, Synth. Met.2000,111:19
[84]M. D. Hall, H. B. Schlegel, Chem. Mater.2001,13:2632
[85]M. Sugimoto, M. Anzai, K. Sakanoue, S. Sakaki, Appl. Phys. Lett.2001,79:2348
[86]J. Szmytkowski, W. Stampor, J. Kalinowski, Z.H. Kafafi, Appl. Phys. Lett.2002, 80:1465
[87]S. Yin, Y. Hua, S. Chen, X. Yang, Y. Hou, X. Xu, Synth. Met.2000,111:109
[88]H. Jang, L.-M. Do, Y. Kim, J.G. Kim, T. Zyung, Y. Do, Synth. Met.2001, 121:1669
[89]Y. Shao, Y. Qiu, N.-X. Hu, X. Hong, Chem. Lett.2000:1068
[90]B. J, Chen, X.W. Sun and Y. K. Li, Appl. Phys. Lett.2003,82:3017-3019
[91]D.Z.Garbuzov, V. Bulovic,P.E. Burrows, and S.R. Forrest, Chem. Phys.Lett,1996,249:433
[92]T. Sano, Y. Nishio, Y. Hamada, H. Takahashi, T. Usuki, K. Shibata, J. Mater. Chem.2000,10:157
[93]Y. Liu, J. Guo, J. Feng, H. Zhang, Y. Li, Y. Wang, Appl. Phys. Lett.2001, 78:2300
[94]S. Anderson, M.S. Weaver, A.J. Hudson, Synth. Met.2000,111:459
[95]C. Schmitz, H.-W. Schmidt, M. Thelakkat, Chem. Mater.2000,12:3012
[96]S. Tokito, K. Noda, H. Tanaka, Y. Taga, T. Tsutsui, Synth. Met.2000,111:393
[97]N. Johansson, J. Salbeck, J. Bauer, F. Weissortel, P. Broms, A. Andersson, W.R. Salaneck, Adv. Mater.1998,10:1136
[98]M. A. Khan,Wei Xe, Khizar-ul-Haq,Yu Bai, X. Y. Jiang, Z. L. Zhang, W. Q. Zhu, J. Appl. Phys.,2008,103:014509
[99]S. Naka, H. Okada, H. Onnagawa, and T. Tsutui, Appl. Phys. Lett.2000,76:197
[100]Y. Kijima, N. Asai, and S. Tamura, Jpn. J. Appl. Phys.,1999,38:5274-5277
[101]Y. Sakon, T. Ohnuma, M. Hashimoto, S. Saito, T. Tsutsui, and C. Adachi, U.S. Patent US 5077142 (1991)
[102]S. Naka, H. Okada, H. Onnagawa, and T. Tsutsui, Appl. Phys. Lett.,2000,76:97
[103]Y. Li, M. K. Fung, Z. Xie, S. Lee, L. Hung, and J. Shi, Adv. Mater,2000,14: 1317
[104]D. F. O'Brien, M.A. Baldo, M.E. Thompson, and S.R. Forrest, Appl. Phys. Lett., 1999,74:442-444
[105]Y. Su, H. Huang, C. Li, C. Chien, Y. Tao, P. Chou, S. Datta, and R. Liu, Adv. Mater.,2003,15:884-888
[106]M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Appl. Phys. Lett.,2001, 79:156-158
[107]R.C. Kwong, M.R. Nugent, L. Michalski, T. Ngo, K. Rajan, Y. Tung, M.S. Weaver, T.X. Zhou, M. Hack, M.E. Thompson, S.R. Forrest, and J.J. Brown, Appl. Phys. Lett.,2002,81:162-164
[108]T. Thorns, S. Okada, J. Chen, and M. Furugori, Thin Solid Films,2003,436:264
[109]C. W. Tang, S.A. VanSlyke, C. H. Chen, J. Appl. Phys.1989,65:3610
[110]J. Shi, C. W. Tang, Appl. Phys. Lett.,1997,70:1665
[111]M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature,2000,403:750
[112]J. L. Fox, C. H. Chen, US Patent 4,736,032 (1988)
[113]T. Inoe, K. Nakatani, Japanese Patent 6,009,952 (1994)
[114]T.G. Pavlopoulos, P.R. Hammond, J. Am. Chem. Soc.1974,96:6568
[115]C. H. Chen, C. W. Tang, J. Shi, K.P. Klubek, US Patent 6,020,078 (2000)
[116]C. H. Chen, C.-H. Chien, T.-H. Liu, in:Proceedings of the International Conference on Mater. Adv. Tech. (ICMAT 2001), Singapore,2001, p.221 (Abstracts)
[117]M. Uchida, T. Izumizawa, T. Nakano, S. Yamaguchi, K. Tamao, K. Furukawa, Chem. Mater.2001,13:2680
[118]Y. Sato, Semicond. Semimetals,2000,64:209.
[119]H. Tokailin, M. Matsuura, H. Higashi, C. Hosokawa, T. Kusumoto, SPIE 1910 (1995)38
[120]C. Hosokawa, S. Sakamoto, T. Kusumoto, US Patent 5,389,444 (1995)
[121]T. Kofuji, in:Proceedings of the Electro. J.6th FPD Seminar, vol.81, Tokyo, 1999.
[122]C. Hosokawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett.1995, 67:3853
[123]TDK, in:Proceedings of the Electro. J.6th FPD Seminar, vol.31, Tokyo,1999
[124]J. Shi, C.W. Tang, C.H. Chen, US Patent 5,935,721 (1999)
[125]Y. Liu, J. Guo, H. Zhang, Y. Wang, Angew. Chem. Int. Ed.2002,41:182
[126]G. Rajeswaran, M. Itoh, M. Boroson, S. Barry, T.K. Hatwar, K.B. Kahen, K. Yoneda, R. Yokoyama, T. Yamada, N. Komiya, H. Kanno, H. Takahashi, SID'00 Digest 2000,40:1
[127]Y. Fukuda, T. Watanabe, T. Wakimoto, S. Miyaguchi, M. Tsuchida, Synth. Met. 2000,111:1
[128]C. H. Chen, J. Shi, K.P. Klubek, US Patent 5,908,581 (1999)
[129]N. Komiya, R. Nishikawa, M. Okuyama, T. Yamada, Y. Saito, S. Oima, K. Yoneda, H. Kanno, H. Takahashi, G. Rageswaran, M. Itoh, M. Boroson, T.K. Hatwar, in:Proceedings of the 10th International Workshop on Inorg. And Org. EL (EL'00), Hamamatsu, Japan,4 December 2000, p.347.
[130]Y. Hamada, H. Kanno, H. Fujii, T. Tsujioka, H. Takahashi, ACS Poly Millennial 2000 Abs.,2000, p.167.
[131]T. K. Hatwar, G. Rajeswaran, J. Shi, Y. Hamada, H. Kanno, H. Takahashi, in: Proceedings of the 10th International Workshop on Inorg. And Org. EL (EL'00), Hamamatsu, Japan,4 December 2000, p.31.
[132]R. H. Young, C.W. Tang, A.P. Marchetti, Appl. Phys. Lett.,2002,80:874
[133]Z. Y. Xie, L. S. Hung, S.T. Lee, Appl. Phys. Lett.2001,79:1048
[134]C. H. Chen, C. W. Tang, J. Shi, K.P. Klubek, Thin Solid Films,2000,363:327
[135]B. Chen, X. Lin, L.F. Cheng, C.-S. Lee, W.A. Gambling, S.-T. Lee, J. Phys. D: Appl. Phys.2001,34:30
[136]M. Mitsuya, T. Suzuki, T. Koyama, H. Shirai, Y. Taniguchi, Appl. Phys. Lett. 2000,77:3272
[137]S. Toguchi, Y. Morioka, H. Ishikawa, A. Oda, E. Hasegawa, Synth. Met.2000, 111:57
[138]M. Ichimura, T. Ishibashi, N. Ueda, S. Tamura, in:Proceedings of the 3rd International Conference on EL Mol. Mater. Relat. Phenom. (ICEL-3),O-29, Los Angelos, CA, USA,5-8 September 2001 (Abstracts)
[139]J. Zhang, R.-G. Xiong, Z.-F. Chen, X.-Z. You, G.-H. Lee, S.M. Peng, Chem. Lett. 2001,676
[140]W. Hu, M. Matsumura, M. Wang, L. Jin, Appl. Phys. Lett.2000,77:4271
[141]W. Hu, M. Matsumura, M. Wang, L. Jin, Jpn. J. Appl. Phys.2000,39:6445
[142]Robert J. Silbey, Robert A. Alberty, and Moungi G. Bawendi, Physical chemistry, Hoboken, NJ, Wiley,2005.
[143]Andrew Gilbert, Essentials of Molecular Photochemistry, Oxford, U.K.,1991
[144]Jingsong Huang, Martin Pfeiffer, Ansgar Werner, Jan Blochwitz, and Karl Leo, Shiyong Liu, Appl. Phys. Lett.,2002,80:139
[145]Chang-Ching Chang, Ming-Ta Hsieh, Jenn-Fang Chen, Shiao-Wen Hwang, Jia-Wei Ma, and Chin H. Chen, Society for information display DIGEST,1106(2006)
[146]M. G. Mason, L. H. Hung, C.W Tang, S. T. Lee, K. W. Wong and M. Wang, J. Appl. Phys.1999,86:1688
[147]S. A. VanSlyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett.,1996,69:2160
[148]Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett.,1994,65:807
[149]Z. B. Deng, X. M. Ding, S. T. Lee, and W. A. Gambling Appl. Phys. Lett.,1999, 74:2227
[150]H. J. Jiang, Y. Zhou, B.S.Ooi, Y. Chen, T. Wee,Y. L. Lam, J. S. Huang, and S.Y. Liu, Thin Solid Film 2000,363:25
[151]A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend, N. C. Greenham, P. L. Burn, A. B. Holmes, and A. Kraft, Appl. Phys. Lett.1992,61:2793
[152]C. Ganzoring and M. Fujihira, Appl. Phys. Lett.,2000,77:4211
[153]D. B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, and B. Francois, Appl. Phys. Lett.,1995,67:1659
[154]F. Huang, A. G. MacDiamid and B. R. Hsieh, Appl. Phys. Lett.,2000,71:4211
[155]M. Pfeiffer, A. Beyer, T. Fritz, and K. Leo, Appl. Phys. Lett.,1998,73:3202
[156]I. D Parker, J. Appl. Phys.1994,75:1656
[157]J. Kido, M. Kimura, and Nagai, Science 1995,267:1332
[158]L. S. Hung, C.W. Tang, and M. G. Mason, Appl. Phys.Lett.1997,70:152
[159]J. Kido, and Y. Lizumi, Appl. Phys. Lett.1998,73:2721
[160]Y. Kim, H. Park, and J. Kim, Appl. Phys. Lett.1996,69:599
[161]H. H. Fong, Wallace C. H. Choy, K. N. Hui, and Y. J. Liang, Appl. Phys. Lett. 2006,88:113510
[162]Junji Kido, Toshio Matsumoto, Appl. Phys. Lett.1998,73:2866
[163]Gufeng He, Martin Pfeiffer, Karl Leo, Michael Hofmann, Jan Birnstock, Robert Pudzich and Josef Salbeck, Appl. Phys. Lett.2004,85:3911
[164]Gufeng He, Oliver Schneider, Dashan Qin, Xiang Zhou, Martin Pfeiffer, Karl Leo, Appl. Phys. Lett.2004,95:5773
[165]Hamaguchi, C. Basic Semi. Physics. pp.263, Springer, New Delhi (2001)
[166]Campbell, A. J., Bradley, D.D.C., Antoniadis, H:Synth. Met.2001,22(1):161
[167]Lampert, M. A., Mark, P:Current Injection in Solids. Academic Press, New York (1970)
[168]Blom, P. W. M., de Jong, M.J.M., Vleggaar, J. J.M.:Appl. Phys. Lett.1996 68(23):3308
[169]Fichou, D.:Handbook of Oligo-and Polythiophenes. Wiley-VCH, Weinheim, New York (1999)
[170]Bassler, H.:Phys. Stat. Sol. B 1993,175:15
[171]Horowitz, G., Hajlaoui, M.E., Hajlaoui, R.:J. Appl. Phys.2000,87:4456
[172]Pope, M. and Solowiejczkyk, Y., Mol. Cryst. Liq. Cryst.,1975,30:175
[173]W. Brutting, S. Berleb and A. G. Muckel, Synth. Met.2001,122:99
[174]L. Bozano, S. A. Carter, J. C. Scott, G. G. Malliaras and P. J. Brock, Appl. Phys. Lett.199974:1132
[175]S. Forero, P. H. Nguyen, W. Brutting and M. Schwoerer, Phys. Chem. Chem. Phys.1999,1:1769
[176]Helfrich, W., In Physical and Chemistry of the Organic Solid State, Vol.3 D. Fox, M. M. Labes, and A. Weissberger, Inter-science, New York,1967, chap.1.
[177]P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A. Cronin, and M. E. Thompson, J. Appl. Phys.1996,79:7991
[178]H. Bassler, Phys. Stat. Sol. (b) 1993,175:15
[179]M. A. Baldo, and S. R. Forrest, Phys. Rev. B 2001,64:85201
[1]Tang C W and VanSlyke S A. Appl. Phys. Lett.1987,51:913
[2]Tang C W, Van Slyke S A and Chen C H, J. Appl. Phys.1989,65:3610
[3]Sakamoto G, Adachi C, Koyama T, Taniguchi Y, Merritt C D, Murata H and Kafafi Z H, Appl. Phys. Lett.1999,75:766
[4]Xie Z Y, Hung L S and Lee S T, Appl. Phys. Lett.2001,79:1048
[5]Hamada Y, Kanno H, Tsujioka T and Takahashi H, Appl. Phys. Lett.1999, 75:1682
[6]Jin Y D, Yang J P, Heremans P L, Auweraer M V, Rousseau E, Geise H J and Borghs G, Chem. Phys. Lett.2000,320:387
[7]Ohmori Y, Kajii H, Sawatani T, Ueta H and Yoshino K, Thin Solid Films.2001, 393:407
[8]D'Andrade B W, Blade M A, Adachi C and Brooks, J. Appl. Phys. Lett.2001, 79:1045
[9]Feng J, Li F, Gao W B, Cheng G, Xie W F and Liu S Y, Appl. Phys. Lett.2002, 81:2935
[10]Chen C H, Tang C W, Shi J and Klubek K P, Thin Solid Films.2000,363:327
[11]Shi J and Tang C W Appl. Phys. Lett.2002,80:201
[12]S. C. Tse, S.K. So. M. Y. Yeung, C.F. Lo, S.W. Wen, C.H. Chen, Chem. Phys. Lett.2006,422:354-357
[13]S. W. Cullingan. Andrew C.-A. Chen, Jason U. Wallace, Kevin P. Klubek, Ching W. Tang, Shan H. Chen, Adv. Funct. Mater.2006,16:1481-1487
[14]T. Forster. Discuss. Faraday Soc.1959,7:27
[15]M. Pope, C.E. Swenberg, Electronics Processes in Organic Crystals, Oxford University Pr (Txt), New York,1982
[16]Wang L, Lie G and Qui Y,J. Appl. Phys.2005,97:114503
[17]Luo Y, Aziz H, Popovic Z D and Xu G, Appl. Phys. Lett.2006,89:103505
[18]Chen C H, Tang C W, Shi J and Klubek K P, Macromol. Symp.1997,125:49
[19]Popovic Z D, Aziz H, Ioannidis A, Hu N X and dos Anjos P N M, Synth. Met. 2001,123:179
[20]Bulovic V, Shoustikov A, Baldo M A, Bose E, Kozlov V G, Thompson M E and Forrest S R, Chem. Phys. Lett.1998,287:455
[21]Iou C-Y, Liu T-H, Chen H-H, Shen W-J and Chen C H Proc. SID'03 (Baltimore, MD,21 May), p 971(2003)
[22]Young R H, Tang C W and Marchetti A P, Appl. Phys. Lett.2002,80:874
[23]M. Redecker, D.D.C. Bradley, M. Inbasekaran, W.W. Wu, Ed. P. Woo, Adv. Mater.1999,11(3):41
[24]Musubu Ichikawa, Yasuhiro Horiba, H. Nakatani, Masashi Yamada, Toshiki Koyama, Yoshio Taniguchi, Jpn. J. Appl. Phys.2002,41:2252
[25]M.T. Lee, Y.S. Wu, H.H. Chen, C.H. Tsai, C.H. Liao, C. H. Chen, in: Proceedings of the SID'04 Digest, vol.710,2004
[26]S.W. Wen, M. T. Lee, C.H. Chen, IEEE/OSA J. Display Technol.2005,1:90
[1]C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett.1987,51:913
[2]H.K. Lee, J.H. Seo, J.H. Kim, J.R. Koo, K.H. Lee, S.S. Yoon, Y.K. Kim, J. Korean Phys. Soc.2006,49:1052
[3]G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett.2004,85:3911
[4]R. Meerheim, K. Walzer, M. Pfeiffer, K. Leo, Appl. Phys. Lett.2006,89:061111
[5]Huang J, Pfeiffer M, Werner A, Blochwitz, J, Leo K, and Liu S, Appl. Phys. Lett. 2002,80:139
[6]Parker I D, J. Appl. Phys.1994,75:1656
[7]Kido J, Kimura M and Nagai K, Science 1995,267:1332
[8]Wakimoto T, Fukuda Y, Nagayama K, Yokoi A, Nakada H and Tsuchida M IEEE Trans. Electron Devices 1997,44:1245
[9]Hung L S, Tang C W and Mason M G Appl. Phys. Lett.1997,70:152
[10]Kido J and Lizumi Y Appl. Phys. Lett.1998,73:2721
[11]Ganzorig C and Fujihara M Appl. Phys. Lett.2000,77:4211
[12]Piromreun P, Oh H, Shen Y, Malliaras G, Scott J C and Brock P J Appl. Phys. Lett.2000,77:2403
[13]Mason M G, Hung L H, Tang C W, Lee S T, Wong K W and Wang M J. Appl. Phys.1999,86:688
[14]Kim Y, Park H and Kim J Appl. Phys. Lett.1996,69:599
[15]S. A. Van Slyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett.1996,69:2160
[16]Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y.Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett.1994,65:807
[17]Zheng Z. B, Ding X. M, Lee S. T, and Gambling W.A, Appl. Phys. Lett.1999 74: 82227
[18]Romero D. B, Schaer M, Zupproli L, Cesar B and Rancois B. F, Appl. Phys. Lett. 1995,67:1659
[19]Blochwitz J, Pfeiffer M, Fritz T and Leo K, Appl. Phys. Lett.1998,73:729
[20]Ganzorig C and Fijihira M, Appl. Phys. Lett.2000,77:4211
[21]M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett.1998,73:3202
[22]J. Blochwitz, T. Fritz, M. Pfeiffer, K. Leo, D.M. Alloway, P.A. Lee, N.R. Armstrong, Org. Electron.2001,2:97
[23]F. Huang, A.G. MacDiarmid, B.R. Hsieh, Appl. Phys. Lett.1997,71:2415
[24]J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett.1998,73:729
[25]Ganzorig C and Fijihira M, Appl. Phys. Lett.2000,77:4211
[26]E. Gondek, J. Niziol, A. Danel, I.V. Kityk, M. Pokladko, J. Sanetra, E. Kulig, J. Luminesc,2008128:1831.
[27]W. C. H. Choy, K. N. Hui, H. H. Fong, Y. J. Liang, P. C. Chui, Thin Solid Films, 2006,509:193
[28]J. H. Jou, Y.S. Chui, R.Y. Wang, H.C. Hu, C.P. Wang, H.W. Lin, Org. Electron, 2006,7:8
[29]Meng-Ting Lee, Hsian-Hung Chen, Chi-Hung Liao, and Chih-Hung Tsai, Chin H. Chen, Appl. Phys. Lett.2004,85:3301
[30]S. L. Lai, M. Y. Chan, M. K. Fung, C. S. Lee, and S. T. Lee, Appl. Phys. Lett. 2007,90:203510
[31]J. Shi, and C. W.Tang, Appl. Phys. Lett.2002,80:3201
[32]W.-J.Shen, B. Banumathy, H.-H. Chen, and C. H.Chen, Proceedings of International Display Manufacturing Conference and FPD Expo, Taipei, ROC, 18-21 February 2003, p 741.
[33]M.-T.Lee, Y.-S.Wu, H.-H. Chen,C.-H. Tsai, L.-C, Liao, and C. H. Chen, Proceedings of the Society For Information Display, Seattle, Washington,23-28 May 2004
[34]Zhang Z L 1997 Organic Electrolumies Materials and Devices ed S Miyata and Nalwa H S (New York:Gordon and Breach) chap 5, pp 209
[35]M. A. Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, X. Y. Jiang, Z. L. Zhang, J. Appl. Phys.2008,103:014509
[1]Tang C W, Slyke V., Appl. Phys. Lett,1987,51:913
[2]Tang C W, Slyke V, Chen C. H. J. Appl. Phys,1989,65:3610
[3]Baldo M A, O'Brien D F, You Y, A. Shoustikov, S. Sibley, M. E.Thompson, and S. R. Forrest, Nature (London),1998,395:151
[4]Choong V E, Shi S, Curless J, S. Shi, J. Curless, C. L. Shieh, H. C. Lee, J. Shen, and J.Yang, Appl. Phys. Lett.,1999,75:172
[5]Slyke S A V, Chen C H, Tang C W, Appl. Phys. Lett,1996,69:2160
[6]Di C A, Yu G, Liu Y, X. Xu, Y. Song, and D. Zhu, Appl. Phys. Lett.,2007, 90:133508
[7]Chen Z, Yu J, Sakuratani Y, M. Li, M. Sone, S. Miyata, T. Watanabe, X.Wang, and H. Sato, J Appl Phys,2001,89:7895
[8]Gill W D, J. Appl. Phys,1972,43:5033
[9]Tse S C, Fong H H, So S K. J. Appl. Phys,2003,94:2033
[10]Blom PWM, de JongM J M, Vleggaar J J M., Appl. Phys. Lett,1996,68:3308
[11]Bozano L, Carter S A, Scott J C, G. G. Malliaras, and P. J. Brock, Appl. Phys Lett.,1999,74:1132
[12]Yasuda T, Yamaguchi Y, Zou D C, Jpn J Appl Phys, Part 1,2002,41:5626
[13]Kim S H, Jang J, Lee J Y. Appl. Phys. Lett,2000,89:253501
[14]Chu T Y, Song O K, Appl. Phys. Lett,2007,90:203512
[15]Malliaras G G, Shen Y, Dunlap D H, H. Murata, and Z. H. Kafafi, Appl. Phys. Lett.,2001,79:2582
[16]Khan M A, Xu W, Khizar-ul-Haq, Yu Bai, X. Y. Jiang, Z. L. Zhang, J. Appl. Phys,2008,103:014509
[17]Naka S, Okada H, Onnagawa H, and T. Tsutsui, Appl. Phys. Lett.,2000,76:197
[18]Wang X J, Zhao J M, Zhou Y C, X. Z. Wang, S. T. Zhang, Y. Q. Zhan, Z. Xu, H. J. Ding, G. Y. Zhong, H. Z. Shi, Z. H. Xiong, Y. Liu, Z. J. Wang, E. G. Obbard, X. M. Ding, W. Huang, and X. Y. Hou, J. Appl. Phys,2004,95:3828
[19]Stoβel M, Staudigel J, Steuber F, J. Blassing, J. Simmerer, and A. Winnacker, Appl. Phys. Lett,2000,76:115
[20]Huang J, Pfeiffer M, Werner A, Blochwitz J, Leo K and Liu S, Appl. Phys. Lett. 2002,80:139
[21]Kido J and Matsumoto T, Appl. Phys. Lett.1998,73:2866
[22]He G, Pfeiffer M, Leo K, Hofmann M, Birnstock J, Pudzich R and Salbeck J, Appl. Phys. Lett.2004,85:3911
[23]He G, Schneider O, Qin D, Zhou X, Pfeiffer M and Leo K, Appl. Phys. Lett.2004, 95:5773
[24]Fong H H, Choy W C H, Hui K N and Liang Y J, Appl. Phys. Lett.2006, 88:113510
[25]Khan M A, Xu W, Khizar-ul-Haq, Yu Bai, Fuxiang Wei, X Y Jiang, Z L Zhang, and W Q Zhu, J. Phys D:Appl. Phys,2007,40:6535
[26]P. W. M. Bloom, M. J. M. deJong, and J. J. M. Vleggaar, Appl. Phys. Lett.1996, 68:3308
[27]G. Horowitz, Adv. Mater.1998,10:365
[28]R. G. Kepler, Phys. Rev.1960,119:1226
[29]Malliaras G G, Salem J R, Brock P J, and C. Scott, Phys Rev B,1998,58:R13411
[30]Pal A J, Osterbacka R, Kallman K M, and H. Stubb, Appl Phys Lett,1997,71: 228
[31]Kumar P, Jain S C, Kumar V, et al., Synth Met,2007,154:905
[32]M A Khan, Wei Xu, Khizar-ul-Haq, Xiao Wen Zhang, Yu Bai, X Y Jiang, Z L Zhang and W Q Zhu, J Phys D:Appl Phys,2008,41:225105
[33]Broutting W, Berleb S, Muckl A G. Organic Electron,2001,2:1
[34]Campbell A J, Weaver M S, Lidzey D. G, et al., J. Appl. Phys,1998,84:6737
[35]A. Ioannidis, E. Forsythe, Gao Yongli, M.W. Wu, E.M. Conwell, Appl. Phys. Lett. 1998,72:3038
[36]W. Brutting, S. Berleb, A.G. Muckl, Organ. Electron,2001,2:1
[1]Khizar-ul-Haq, Liu Shan-peng, M A Khan, X Y Jiang, Z L Zhang and W Q Zhu, "Red organic light-emitting diodes based on wide band gap emitting material as the host utilizing two-step energy transfer" Semicond. Sci. Technol.23, 035024 (6pp)(2008)
[2]Khizar-ul-Haq, M. A. Khan, X.Y. Jiang, Z. L. Zhang, X.W. Zhang, Bin Wei, J. M. Wei, "Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO3" Journal of Luminescence 129,1158-1162 (2009)
[3]Khizar-ul-Haq, Khan M A, Jiang X Y, Zhang Z L, Zhang X W, Zhang L, and Li Jun "Estimation of electron mobility of n-doped 4,7-diphenyl-1,10-phenanthroline using space-charge-limited currents" Journal of Semiconductors Vol.30, No.11:114009-1(2009)
[4]M A Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, Fuxiang Wei, X Y Jiang, Z L Zhang, and W Q Zhu, "Highly power efficient organic light-emitting diodes based on p-doped and novel n-doped carrier transport layers" J. Phys. D:Appl. Phys. 40,6535(2007)
[5]M A Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, X Y Jiang, Z L Zhang and W Q Zhu, "Influence of p-doping hole transport layer on the performance of organic light-emitting devices" Semicond. Sci. Technol.23,055014 (5pp) (2008)
[6]M A Khan, Wei Xu, Khizar-ul-Haq, Yu Bai, X. Y. Jiang, Z. L. Zhang, and W. Q. Zhu Z. L. Zhang and W. Q. Zhu, "Electron mobility of 4,7-diphenyl-1,10 phenanthroline estimated by using space-charge-limited currents" Journal of Applied Physics 103,014509(2008)
[7]M A Khan, Wei Xu, Khizar-ul-Haq, X W Zhang, Yu Bai, X Y Jiang, Z L Zhang and W Q Zhu, "Electron injection and transport mechanism in organic devices based on electron transport materials" J. Phys. D:Appl. Phys.41, 225105 (6pp)(2008)
[8]Bai, X. Liu, L. Chen, Khizar-ul-Haq, M.A. Khan, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, "Organic thin-film field-effect transistors with MoO3/Al electrode and OTS/SiO2 bilayer gate insulator" Microelectronics Journal 38,1185-1190(2007)
[9]X Y Jiang, Z L Zhang, Jin Cao, M A Khan, Khizar-ul-Haq, and Wen-Qing Zhu, "White OLED with high stability and low driving voltage based on a novel buffer layer MoOx" J. Phys. D:Appl. Phys.40,5553-5557(2007)
[10]X W Zhang, X Y Jiang, M.A. Khan, Jin Cao, Jun-Wei Ma, L Zhang, Jun Li, Khizar-ul-Haq, W Q Zhu, Z L Zhang, "Enhanced electron injection in organic light-emitting devices by using a composite electron injection layer composed of 8-hydroquinolatolithium and cesium oxide" Solid State Communications 149,652-656(2009)
[11]Bai Yui, Khizar-ul-Haq, M A Khan, Jiang X Y, and Zhang Z L, "OTFT with Bilayer Gate Insulator and Modified Electrode" Journal of semiconductor,Vol.29, No.4 April(2008)
[12]X W Zhang, Jun Li, M A Khan, L Zhang, X Y Jiang, Khizar-ul-Haq, Wen-Qing Zhu and Z L Zhang, "Improved chromaticity and electron injection in a blue organic light-emitting device by using a dual electron-transport layer with hole-blocking function" Semicond. Sci. Technol.24,075021 (5pp) (2009)
[13]Jun Li, X W Zhang, L Zhang, Khizar-ul-Haq, Xue-Yin Jiang, Wen-Qing Zhu, Z L Zhang, "Improving organic transistor performance through contact-area-limited doping" Solid State Communications 149,1826-1830(2009)
[14]Jun Li, X W Zhang, L Zhang, Khizar-ul-Haq, X Y Jiang, Wen-Qing Zhu and Z L Zhang, "Performance enhancement of organic thin-film transistors using WO3-modified drain/source electrodes" Semicond. Sci. Technol.24,115012 (5pp) (2009)
[15]X W Zhang, Jun Li, L Zhang, X Y Jiang, Khizar-ul-Haq, Wen-Qing Zhu Zhi-Lin Zhang, "Top-emitting organic light-emitting device with high efficiency and low voltage using a silver-silver microcavity" Thin Solid Films 518,1756-1759(2010)
[16]Jun Li, X W Zhang, L Zhang, Hao Zhang, X Y Jiang, Khizar-ul-Haq, W Q Zhua, Z L Zhang. "MoOx interlayer to enhance performance of pentacene-TFTs with low-cost copper electrodes" Synthetic Metals volume 160,376-379(2010)
[17]L Zhang, J Li, X. W Zhang, D B Yu, H P Lin, Khizar-ul-Haq, X Y Jiang, Z L Zhang "Low-voltage-drive and high output current ZnO thin-film transistors with sputtering SiO2 as gate insulator" Current Applied Physics (In press-2010)