Tungsten Oxide Buffer Layers Fabricated in an Inert Sol-Gel Process at Room-Temperature for Blue Organic Light-Emitting Diodes
Tungsten Oxide Buffer Layers Fabricated in an Inert Sol-Gel Process at Room-Temperature for Blue Organic Light-Emitting Diodes
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DOI：
10.1002/adma.201301627
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发表时间：
2013-08-14
影响因子：
29.4
通讯作者：
Colsmann, Alexander
中科院分区：
文献类型：
--
作者：
Hoefle, Stefan;Bruns, Michael;Colsmann, Alexander
有机半导体器件的卷对卷和片对片溶液加工被广泛认为是实现低成本光电器件如太阳能电池、晶体管和有机发光二极管（OLED）的关键技术。如今，用于OLED的最有效的发射层是根据主体-客体概念制造的，其中电荷在被混合的发光染料收集之前从电极通过缓冲层注入主体。[1]为了得到更高的转换效率，使用磷光发光体，例如三（苯基吡啶）铱（Ir（ppy）3，绿色发光）。[2]对于未来的应用，发蓝光的OLED是最重要的，因为它们能够制造白色发光器件，从而打开通向普通照明的途径。研究得最好且经常使用的蓝色发光磷光染料之一是双（4，6-二氟苯基吡啶-N，C2）吡啶并铱（FIrpic）。FIrpic的合适主体是4，4′，4 ″-三（咔唑-9-基）-三苯胺（TCTA）[3，4]，因为它表现出比FIrpic更高的三重态能量。[5，6]为了将空穴注入TCTA或其他宽带隙有机半导体，需要具有相等或更深的传输能级（TCTA：Ehomo = 5.7 eV）的空穴注入层[7，8]。在溶液处理的器件中，用于空穴注入到发光层中的常用导电聚合物共混物聚（3，4-亚乙基二氧噻吩）：聚（苯乙烯磺酸盐）（PEDOT：PSS）表现出功函数Φa ≤ 5.2eV [9，10]，因此不是蓝色OLED的最佳空穴注入层，因为它经常在与发光层的界面处形成空穴注入势垒。此外，由于PEDOT：PSS的强吸湿性，在器件制造期间或之后的水吸收可能在操作期间稍后损坏OLED。蒸发的过渡金属氧化物，如三氧化钨（WO 3），[11，12]三氧化钼（MoO 3）[13，14]或五氧化二钒（V2 O 5）[15，16]，具有较高的功函数，Φa（WO 3）≤ 6.65 eV，Φa（MoO 3）≤ 6.9 eV和Φa，（V2 O 5）≤ 6.85 eV，[17]分别显得更适合于空穴注入OLED。[18]虽然环境大气的污染会降低金属氧化物的功函数（例如WO 3：Φa，空气为5.7 eV，[19] MoO 3：Φa，空气为5.4 eV，[20]），但它们仍然能够实现良好的电荷载流子注入。因此，它们已被用于空穴注入到小分子有机发光二极管（SMOLED）中的宽带隙发射极中，所述小分子有机发光二极管通过真空中的热蒸发制造。[11]另一方面，在有机太阳能电池中对稳定的和溶液可加工的电荷载流子传输层的需求引起了过渡金属氧化物层的前体工艺的全面发展。最近，溶液处理的电荷载流子传输层从MoO 3，WO 3或V2 O 5已被检查在有机太阳能电池。[21-23]大多数氧化物到金属氧化物的转化在高温和环境条件下进行，因为该过程需要氧气或水或两者的存在。不幸的是，这与OLED制造的特定要求非常冲突，其中，到目前为止，惰性处理是强制性的，以确保合适的器件寿命。在这项工作中，我们研究了溶液处理的蓝色SMOLED包括TCTA：FIrpic发射层和WO 3空穴注入层（HIL）。后者已经通过在室温下水解转化两种不同的乙醇钨前体来应用，一种在氮气气氛下处理，因此能够实现惰性OLED制造工艺链。乙醇钨（W（OEt）5.
Roll-to-roll and sheet-to-sheet solution processing of organic semi-conductor devices are widely considered as key technologies towards low-cost optoelectronic devices such as solar cells, transistors and organic light-emitting diodes (OLEDs). Today, the most efficient emission layers for OLEDs are fabricated according to the host-guest concept where charges are injected from the electrodes through buffer layers into the host before being collected by intermixed luminescent dyes.[1] In order to yield higher conversion efficiencies, phosphorescent emitters such as tris (phenylpyridine) iridium (Ir (ppy) 3, green emission) are used.[2] For future applications, blue emitting OLEDs are of utmost importance as they enable the fabrication of white emitting devices and hence open pathway to general lighting. One of the best investigated and often used blue emitting phosphorescent dyes is bis (4, 6-difluorophenylpyridinato-N, C2) picolinatoiridium (FIrpic). A suitable host for FIrpic is 4, 4′, 4 ″-tris (carbazol-9-yl)-triphenylamine (TCTA),[3, 4] as it exhibits a higher triplet energy than FIrpic.[5, 6] In order to inject holes into TCTA or other wide band-gap organic semiconductors, a hole injection layer with an equal or deeper transport energy level (TCTA: Ehomo≈ 5.7 eV)[7, 8] is needed. The commonly used conductive polymer blend for hole injection into the emission layer in solution processed devices, poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate)(PEDOT: PSS), exhibits a work function of Φa≈ 5.2 eV [9, 10] and hence is not an optimum hole injection layer for blue OLEDs since it often forms hole injection barriers at the interface to the emission layer. In addition, due to the strong hydroscopic nature of PEDOT: PSS, water uptake during or after device fabrication can damage the OLED later during operation. Evaporated transition metal oxides such as tungsten trioxide (WO 3),[11, 12] molybdenum trioxide (MoO 3)[13, 14] or vanadium pentoxide (V2o 5)[15, 16] with higher work functions of Φa (WO 3)≈ 6.65 eV, Φa (MoO 3)≈ 6.9 eV and Φa,(V2o 5)≈ 6.85 eV,[17] respectively appear much more suitable for hole injection into OLEDs.[18] Though contamination with ambient atmosphere can reduce the work function of metal oxides (eg WO 3: Φa, Air≈ 5.7 eV,[19] MoO 3: Φa, Air≈ 5.4 eV,[20]), they nevertheless enable good charge carrier injection. Consequently, they have been used for hole injection into wide band-gap emitters in small molecule organic light emitting diodes (SMOLEDs) that were fabricated by thermal evaporation in vacuum.[11] On the other hand, the need for stable and solution processable charge carrier transport layers in organic solar cells gave rise to a comprehensive development of precursor processes for transition metal oxide layers. Recently, solution processed charge carrier transport layers from MoO 3, WO 3 or V2o 5 have been examined in organic solar cells.[21–23] Most precursor-to-metal oxide conversions take place at high temperatures and under ambient conditions since this process requires the presence of either oxygen or water or both. Unfortunately, this is very much in conflict with the particular requirements for OLED fabrication where, as of today, inert processing is mandatory in order to ensure suitable device life-times. In this work we investigate solution processed blue SMOLEDs comprising TCTA: FIrpic emission layers and WO 3 hole injection layers (HIL). The latter have been applied by hydrolytic conversion of two different tungsten ethoxide precursors at room temperature, one being processed under nitrogen atmosphere, hence enabling an inert OLED fabrication process chain.Tungsten (V) ethoxide (W (OEt) 5 …