基于双极性发射的蓝光和白光OLED

Efficient blue and white organic light emitting devices based on a single bipolar emitter 基于单层双极发射的高效蓝光和白光基于单层双极发射的高效蓝光和白光OLED Silu Tao Chun Sing Lee and Shuit Tong Lee Center of Super Diamond and Advanced Films COSDAF and Department of Physics and Materials Science City University of Hong Kong Hong Kong SAR China Xiaohong Zhang Nano organic Photoelectronic Laboratory and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100080 China Received 30 March 2007 accepted 30 April 2007 published online 5 July 2007 Excellent bipolar carrier transport properties of 2 7 dipyrenyl 9 9 dimethyl fluorene DPF have been elucidated by using different device structures A nondoped device using DPF as host emitter showed highly efficient blue emission with a maximum efficiency of 6 0 cd A and CIE coordinates of x 0 15 and y 0 19 Another device based on rubrene doped DPF as emission layer gave pure high efficiency white emission with good color stability a maximum efficiency of 10 5 cd A and CIE coordinates of x 0 28 and y 0 35 The excellent bipolar transport capability and high performance as both emitter and host suggest that DPF is an efficient and versatile material for various applications in organic light emitting devices 用不同器件结构阐述了DPF的优良双极载流子传输特性 用DPF为主体的不掺杂器件 显示出高效的蓝光发射 并伴有最大电流效率6 0 cd A CIE坐标 0 15 0 19 另一个基 于rubrene掺杂到DPF作为发光层的器件 获得了纯的并且高效的白光发射 同时具有好的 色稳定性 该器件最大的电流效率为10 5 cd A CIE坐标 0 28 0 35 优良的双极传输能 力和作为发射体和主体都具有的高性能 说明DPF对于OLED不同应用是一个高效并且通 用的材料 Organic light emitting devices OLEDs have attracted much attention due to their wide applications in full color flat panel displays and solid state lighting Considerable progresses have been made in the past decade on both OLED materials and device structures Compared to greenemitting materials the performance of blue emitting materials still needs further improvement Good blue emitting materials are important not only as blue emitters but also as hosts for dopant emitters to facilitate efficient green and white emission Further white organic light emitting devices are becoming important for practical applications as solid state lighting and backlights for liquid crystal displays 由于OLDEs在全色平板显示和固态照明领域广泛的应用已经引起了极大的关 注 过去十年中关于OLED的材料和器件结构方面都取得了相当大的进展 与 绿光材料相比较 蓝光发射材料的性能仍需要进一步改进 好的蓝光发射材料 不仅对于作为蓝光发射体是重要的 对于掺杂体中作为主体去促进高效绿光和 白光发射也是重要的 另外 白色olde在固态照明和液晶显示的背光源的实际 应用中变得重要 One possible way to improve device performance is to use good emitters with bipolar charge transport property There are reports on bipolar emitting materials in OLEDs however due to donor acceptor D A architectures most of them emit in longer wavelength such as in green and yellow region and very few were on bipolar blue emitters In addition the reported bipolar materials usually have a D A structure which usually leads to fluorescence quenching in solid film 一种可能改进器件性能的方式是用带有双极电荷传输特性的优良发射体 在 OLEDs领域有关于双极发射材料的报道 然而 由于受体结构 它们中的大部 分发射较长波长 例如绿光和黄光区域 极少在双极蓝光发射 另外 已经报 道的双极材料通常有D A结构 该结构在固态薄膜中通常导致荧光淬灭 It has been reported that fluorene derivatives with suitable substituents in the fluorene backbone are efficient blue emitters In particular the fluorene derivative 2 7 dipyrenyl 9 9 dimethyl fluorene DPF with pyrene groups at 2 and 7 positions exhibited a high thermal stability and good blue emitting property In this letter we show that DPF not only has highly efficient blue emission but also excellent bipolar charge transport properties for use as a high performance host for dopant emitters 已经报道了在芴的主干上带有合适取代基的芴的衍生物是高效的蓝光发射体 特别是这种芴的衍生物DPF 它在2和7的位置上带有芘团 展示了高的热稳定 性和良好的蓝光发射特性 在这篇文章中 我们不仅展示了DPF具有高效蓝光 发射 也展示了DPF对于用作掺杂体的高性能主体的优良双极电荷传输特性 Figure 1 a shows the chemical structures of DPF 4 4 bis N 1 naphthyl N phenylamino biphenyl NPB was used as a hole transport layer HTL 2 2 2 1 3 5 benzenetriyl tris 1 phenyl 1H benzimidazole TPBI as an electron transport layer ETL tris 8 hydroxyquinoline aluminum Alq3 as an emitting layer and an electron transport layer and 5 6 11 12 tetraphenyl naphthacene Rubrene as a yellow dopant Experimental procedures were described in detail elsewhere To investigate the electroluminescence and charge transporting properties of DPF six device structures were used I ITO DPF 100 nm MgAg II ITO DPF 60 nm Alq3 60 nm MgAg III ITO DPF 60 nm TPBI 60 nm MgAg IV ITO NPB 80 nm DPF 40 nm MgAg V ITO NPB 70 nm DPF 30 nm TPBI 50 nm MgAg and VI ITO NPB 70 nm DPF dopant 20 nm TPBI 50 nm MgAg 图1 a 显示DPF的化学结构 NPB用于空穴传输层 TPBi用于电子传输层 Alq3 用于发光层和电子传输层 Rubrene作为黄光掺杂 实验过程以被在其他地 方详细介绍 为了研究DPF的电致发光和电荷传输特性 制备了六组器件 I ITO DPF 100 nm MgAg II ITO DPF 60 nm Alq3 60 nm MgAg III ITO DPF 60 nm TPBI 60 nm MgAg IV ITO NPB 80 nm DPF 40 nm MgAg V ITO NPB 70 nm DPF 30 nm TPBI 50 nm MgAg VI ITO NPB 70 nm DPF dopant 20 nm TPBI 50 nm MgAg FIG 1 Color online a Molecular structure of DPF To ascertain the bipolar carrier transport properties of DPF we examined device I comprising of a single DPF layer sandwiched between the anode and cathode device I The single layer device gives a bright blue emission with a peak at 480 nm and CIE coordinates of x 0 17 and y 0 28 The similarity of the electroluminescence EL and photoluminescence PL spectra of DPF film indicates that EL of the device arises from the emission of the singlet excited state of DPF Table I lists the key performance parameters of all six devices Device I shows a maximum current efficiency of 0 8 cd A or an equivalent power efficiency of 0 6 lm W Figure 1 b shows the turn on voltage defined as the driving voltage needed to produce a brightness of 1 cd m2 of device I which is relatively low for a single layer device giving 3 cd m2 at 4 5 V The high efficiency and low turn on voltage of the single layer device I indicate that DPF has decent bipolar transport properties for both holes and electrons 为了确定DPF的双极载流子传输特性 我们检查了器件I 它是由在阳极和 阴极间夹一单层DPF构成的 该单层器件发出了很亮的蓝光发射 伴有480 nm 发光峰 CIE坐标为 0 17 0 28 相似的PDF的EL和PL发光光谱表明 器件的 EL发射源于DPF的单限激发态 表I列出了六个器件的关键性能参数 器件I显 示出0 8 cd A最大的电流效率对应功率效率0 6 lm W 图I b 显示开启电压由 驱使电压使器件I的亮度达1 cd m2决定的 相对单层器件该电压相对较低 在 4 5V时发光 3 cd m2 单层器件I的高效和低的开启电压表明 DPF对于空穴和电子都具有相当好的双 极传输特性 To further investigate the bipolar carrier transport properties of DPF we fabricated devices II III and IV In the typical two layer device II DPF was used as HTL and Alq3 as ETL 进一步研究DPF的双极载流子传输特性 我们制作了器件II III 和 IV 在 典型的双层器件II中 DPF被用于HTL同时 Alq3 作为 ETL The DPF Alq3 device gives green emission peaking at 540 nm due to Alq3 emission Table I shows that the twolayer device has a maximum efficiency of 4 1 cd A 1 7 lm W which is comparable to or even better than the performance of the prototypical NPB Alq3 two layer device In device III Alq3 is replaced by TPBI which has a lowlying HOMO level and is often used as both ETL and hole blocker 该DPF Alq3的器件由于Alq3发光所以在540 nm处发出绿色发光峰 表I显示双层 器件最大电流效率为4 1 cd A 功率效率为1 7 lm W 可以与典型的NPB Alq3双 层结构器件的性能相比较 甚至比它还要好 器件III中 Alq3被TPBI取代 由此具有低的HOMO能级同时经常被用作ETL和空穴阻挡 Device III gives blue emission peaking at 473 nm due to DPF with a maximum power efficiency of 2 8 lm W Due to the large hole injection barrier at the DPF TPBI interface holes and thus excitons are primarily confined in DPF leading to electroluminescence purely from DPF 器件III由于DPF所以在473 nm处有蓝色发光峰 伴有最大功率效率为2 8 lm W 由于在DPF TPBI界面存在大的空穴注入势垒 空穴和激子被主要限制在 DPF中 导致电致发光纯来自于DPF Data of devices II and III confirm that DPF has good hole transport and emission properties Device IV with a NPB DPF two layer structure gives efficient and bright blue emission peaking at 480 nm due to DPF emission 器件II 和 III的数据证实了DPF具有好的空穴传输和发光特性 带有NPB DPF双 层结构的器件IV 由于DPF发光 导致高效同时高亮度蓝光发射 发射峰在480 nm处 This device has a low turn on voltage of 2 8 V and a high current efficiency of 6 9 cd A 4 1 lm W In device IV DPF acts both as the emitting layer and electron transport layer The low turn on voltage and the high efficiency indicate that DPF has high electron mobility and can act as a good electron transport layer 该器件具有低的开启电压2 8V 和高的电流效率6 9 cd A 4 1 lm W 在器件 IV中 DPF同时作为发光层和电子传输层 低的开启电压和高的效率表明DPF 具有高的电子迁移率和可作为好的电子传输层 Although DPF has good electron and hole transport properties the single layer device device I has a much lower efficiency than the two layer device device II This is partly due to the poor matching of the HOMO and LUMO levels of DPF respectively with the work function of the anode and cathode 尽管DPF具有好的电子和空穴传输特性 单层器件I比双层器件II具有更低的功 率效率 部分由于DPF的HOMO和LUMO能级分别与阳极和阴极的功函数具有 不好的匹配 The imbalance between hole and electron currents is another weakness which contributes to the lower efficiency of single layer device I Although high efficiency blue emission with a maximum efficiency of 4 1 lm W was achieved in device IV with a two layer structure the color is nevertheless blue green with peak emission at 480 nm and CIE coordinates of x 0 16 and y 0 28 空穴和电子间流动的不平衡是导致单层结构的器件I的低效率的另一个缺点 尽 管双层结构的器件IV获得了伴有4 1 lm W最大功率效率的高效的蓝光发射 它 的颜色是蓝绿色 发光峰在480 nm CIE坐标 0 16 028 These values deviate from pure blue CIE coordinates around x 0 14 and y 0 18 To improve device performance and color purity we fabricated a three layer device with a configuration of ITO NPB DPF TPBI MgAg device V 这些值偏离存蓝光CIE坐标 0 14 0 18 为了改进器件的性能和色纯度 我们 制备了三层器件 结构为ITO NPB DPF TPBI MgAg 即器件V The three layer device V gives a bright blue emission with I V L characteristics as shown in Fig 2 a The device has a turn on voltage of only 3 3 V a maximum brightness of 9560 cd m2 at 9 5 V and a current density of 165 mA cm2 The EL spectrum of the device the inset of Fig 2 a shows a peak at 468 nm and CIE coordinates of x 0 15 and y 0 19 三层结构的器件V具有很亮的蓝光发射 它的I V L特性显示在图2 a 中 该 器件的开启电压仅为3 3V 在9 5V时最大亮度为9560 cd m2 电流密度 165mA cm2 它的EL光谱在图2 a 的插图中 发光峰在468 nm 处同时 CIE 坐标 0 15 0 19 FIG 2 Color online a I V L characteristics of device V and b efficiency vs current density of device V Inset is the schematic energy level diagram of device V Figure 2 b depicts the dependence of efficiency on current density of the three layer device V revealing a maximum current efficiency of 6 0 cd A 3 6 lm W The schematic energy level diagram of the device shows that the holeinjection barrier at NPB DPF junction is only 0 3 eV and the electron injection barrier at TPBI DPF junction is negligible these two low barriers facilitate the injection of both holes and electrons into the DPF layer 图2 b 描绘了器件V基于电流密度的效率曲线 最大电流效率为6 0 cd A 3 6 lm W 器件的能级原理图显示出在NPB DPF结处空穴注入势垒仅为0 3 eV 在 TPBI DPF结处的电子注入势垒是可以忽略的 这两个低的势垒促使空穴和电子 注入到DPF层中 As the mobility of holes in hole transporting organic materials is usually much larger than that of electrons in electron transporting materials consequently holes commonly transport faster to the ETL and even further to the cathode rather than recombining with electrons in the emission layer EML 由于在空穴传输有机材料中的空穴迁移率通常大于在电子传输材料中电子的迁 移率 结果是空穴更快到达电子传输层 甚至进一步到达阴极而不是与电子在 EML层复合 Since TPBI has a lower HOMO of 6 2 eV than DPF at 5 7 eV leading to a 0 5 eV barrier which slows down hole injection from DPF to TPBI Both factors favor efficient recombination of holes and electrons in the emission layer 由于TPBI 6 2eV 具有低于DPF 5 7eV 的HOMO能级 导致0 5eV的势垒 由此降低了DPF到TPBI的空穴注入 这两个因素都有利于空穴和电子在发光层 有效的复合 In addition the good bipolar transport properties and the high fluorescence quantum efficiency of DPF also contribute to the high performance of the blue OLED using DPF emitter 另外 DPF良好的双极传输特性和高荧光量子效率也有助于用DPF得到高性能 的蓝光OLED Significantly by combining the blue emission of DPF and the orange emission of rubrene we achieved white emission in a device structure of ITO NPB 70 nm rubrene DPF 0 5 20 nm TPBI 50 nm Mg Ag The structure was similar to the blue emitting device V except that the DPF emitting layer was doped with rubrene and reduced in thickness to 20 nm 明显地 通过结合DPF的蓝光发射以及红荧烯的橙光发射 我们能获得白光发 射 器件结构为 ITO NPB 70 nm rubrene DPF 0 5 20 nm TPBI 50 nm Mg Ag 该结构除了DPF发光层掺入了红荧烯同时厚度减小到 20nm外与蓝光发射的器件V相似 Figure 3 a shows the fluorescence spectra of DPF and rubrene in dilute dichloromethane solution and the absorption spectrum of rubrene DPF and rubrene emit in blue and orange respectively but the absorption spectrum of rubrene shows some overlap with the emission spectrum ofDPF It means that the energy transfer from DPF to rubrene is possible when rubrene is doped into DPF Therefore with the right ratio of rubrene doped in DPF we can achieve white emission by balancing blue and orange emissions 图3 a 显示DPF和红荧烯在稀释的二氯烷溶液中的荧光光谱以及红荧烯的吸 收光谱 DPF和红荧烯分别发蓝光和橙光 但是红荧烯的吸收光谱显示与DPF 的发射光谱部分重叠 这说明当红荧烯被掺到DPF中时 能量从DPF转移到红 荧烯是可能的 因此 适当掺杂比率的红荧烯掺到DPF中 我们能够通过平衡 蓝光和程光发射来获得白光发射 Depending on rubrene concentrations three layer device VI can give efficient emission varying from blue to yellow At 2 rubrene concentration device VI gives yellow emission due to rubrene indicating complete energy transfer from DPF to rubrene When doping concentration is reduced to 0 5 device VI gives efficient white emission with excellent color stability The EL spectra of the white emitting device at different applied voltages are shown in Fig 3 b It shows that the EL spectra and CIE coordinates of the device remain almost the same at a driving voltage between 6 and 9 V Excellent color stability of the device against voltage is primarily due to the device structure having only one emission layer 基于不同红荧烯掺杂浓度 三层器件VI可获得从蓝到黄高效的发光 在2 红 荧烯掺杂浓度 器件VI由于红荧烯而发射黄光 表明从DPF到红荧烯发生了全部 能量转移 当掺杂浓度降低到0 5 器件VI得到高效的白光发射并伴有良好的 色稳定性 在不同电压的白色发光器件的发光光谱如图3 b 结果表明 驱动 电压在6V到9V之间 该器件的发光光谱和CIE坐标几乎相同 该器件相对电压具 有良好的色稳定性主要是由于器件的结构只有一个发光层 FIG 3 Color online a Photoluminescence spectra of DPF and rubrene and absorption spectrum of rubrene b EL spectra of device VI at different applied voltages The device VI exhibits white emission with a maximum efficiency of 10 5 cd A 4 2 lm W and CIE coordinates of x 0 28 and y 0 35 which are close to standard white emission Efficiency increases quickly to 10 5 cd A and remains high even when the current density increases to 200 mA cm2 It is worthwhile to point out tha