High Efficiency Hybrid White Organic Light-emitting Diodes with Stable Emission Spectrum Based on Two Separately Monochromatic Emission Layers

FANG Da，ZHANG Zhi-qiang，WU Zhen-xuan，ZHANG Chuan，ZHANG Hong-mei∗

(1.Key Laboratory for Organic Electronics and Information Displays＆Institute of Advanced Materials，Institute of The Materials Science and Technology，Nanjing University of Posts＆Telecommunications，Nanjing 210023，China；2.Jiangsu National Synergistic Innovation Center for Advanced Materials(SICAM)，Nanjing University of Posts＆Telecommunications，Nanjing 210023，China；3.State Key Laboratory of Polymer Physics and Chemistry，Changchun Institute of Applied Chemistry，Chinese Academy of Sciences，Changchun 130022，China)∗Corresponding Author，E-mail:iamhmzhang＠njupt.edu.cn

High Efficiency Hybrid White Organic Light-emitting Diodes with Stable Emission Spectrum Based on Two Separately Monochromatic Emission Layers

FANG Da1，2，ZHANG Zhi-qiang3，WU Zhen-xuan1，2，ZHANG Chuan1，2，ZHANG Hong-mei1，2∗

(1.Key Laboratory for Organic Electronics and Information Displays＆Institute of Advanced Materials，Institute of The Materials Science and Technology，Nanjing University of Posts＆Telecommunications，Nanjing 210023，China；2.Jiangsu National Synergistic Innovation Center for Advanced Materials(SICAM)，Nanjing University of Posts＆Telecommunications，Nanjing 210023，China；3.State Key Laboratory of Polymer Physics and Chemistry，Changchun Institute of Applied Chemistry，Chinese Academy of Sciences，Changchun 130022，China)∗Corresponding Author，E-mail:iamhmzhang＠njupt.edu.cn

Highly efficient hybrid white organic light-emitting diode(WOLED)with stable emission spectrum was fabricated by using two separately monochromatic emission layers(EML).The blue emissive layer was based on 14%(mass fraction)trans-1，2-bis(6-(N，N-di-p-tolylamino)-Naphthalene-2-yl)ethene(BNE)doped in BePP2，and the orange emissive layer was based on 1%(mass fraction)Ir(bt)2(acac)doped in a mixed-host of 49.5% (mass fraction)NPB and 49.5%(mass fraction)BePP2.Without the use of any outcoupling techniques，this device can achieve a power efficiency of 39 lm/W at brightness of 100 cd/m2，and slightly rolls off to 27.5 lm/W at brightness of 1 000 cd/m2.There is nearly no color shift with the bias voltages as the Commission Internationale de L’Eclairage(CIE)coordinates are(0.37，0.48)and(0.37，0.47)at 1 000 cd/m2and 10 000 cd/m2，respectively. The stable emission spectrum can be contributed to the well designed ambipolar interlayer which successfully balances the exciton generation evenly on its both sides.

hybrid white organic light-emitting diodes；two emissive layers；stable emission spectrum

1 Introduction

White organic light-emitting diodes(WOLEDs) are of considerable interest in recent years due to their potential applications as full color displays，backlight for liquid crystal displays and large-area solid-state lighting sources[1-4].For the application in solid-state lighting sources，high efficiency，high color rendering index(CRI)，long operation lifetime，low efficiency roll-off and stable emission spectrum are all required.Elegant materials and smart device configurations are the two keys to achieving high device performance.Phosphorescent WOLEDs generally are more efficient due to its demonstrated potentialforachieving 100%internal quantum efficiency by employing both singlet and triplet excitons[5-11].However，deep blue phosphorescent emitter systems still lack high long-term operational stability.On the other hand，pure fluorescent devices are able to show high stability，even with deep blue emission.However，their quantum efficiency is limited due to intrinsical spin statistics. Therefore，the combination of fluorescent and phosphorescent emitters in white OLEDs，which is called hybrid white，can be advantageous over pure fluorescent or pure phosphorescent white OLED.Therefore，the hybrid white OLEDs may result in a good compromise between the high efficiency of phosphorescent emitter systems and the high long-term stability of fluorescent blue emitter systems.However，to achieve high efficiency and low efficiency roll-off，the device engineering must be under consideration，such as charge injection balance，excitons confinement，exciton-formation zone adjustment，etc[12-18]. In general，hybrid WOLEDs can be constructed using two EML architectures，including multi-EML or S-EML with different color emitting dopants[19-23]. Although the S-EML WOLEDs can also realize high efficiency，the spectral variation with luminance is inevitable，which is also the intrinsic property of SEML WOLEDs with complicated operational mechanism[24].Compared to S-EML hybrid WOLEDs，multi-EML counterparts offer a flexible manipulation of each EML as well as precise regulation of the chargeandexcitondistributionindistinct EMLs[25-27].Thus，the multi-EML structures provide a reliable strategy to fabricate better EL performance hybrid WOLEDs.But，the emission spectrum of white OLEDs with separate emission layers for each color may shift with increasing driving current due to changesinthepositionoftherecombination zone[28-29].Among of them，the charge carrier balance is extremely important to achieve stable EL spectrum.

In this paper，we achieved a high performance multi-EML structure hybrid WOLED with stable emission spectrum an ambipolar interlayer.We contribute the stable emission spectrum to the well designed ambipolar interlayer that successfully balances exciton generation evenly on its both sides. The designed device structure is greatly effective to obtain stable white emission spectrum by balance charge carrier transporting.By optimization，the resulting WOLED achieves a power efficiency of 27.5 lm/W at a brightness of 1 000 cd/m2.

2 Experiments

The resulting WOLED was fabricated on a 10 Ω/□indium-tin-oxide(ITO)on glass substrate with a clean.The optimized device in this study was ITO/MoO3(8 nm)/N，N0-di(naph-thalene-1-yl)-N，N0-diphenyl-benzidine(NPB)(80 nm)/Tris(4-carbazoyl-9-ylphenyl)amine(TCTA)(5nm)/ 49.5%(mass fraction)NPB∶49.5%BePP2∶1%Ir-(bt)2(acac)(5 nm)/50%NPB∶50%BePP2(3 nm)/BePP2∶14%BNE(4 nm)/BePP2(40 nm)/ LiF/Al.All layers were prepared by thermal evaporation in a high-vacuum system with pressure of less than 3×10－4Pa without breaking the vacuum.The evaporation rates were monitored by a frequency counter，and calibrated by Dektak 6M profiler.The current-luminance-voltage characteristics were measured by using a Keithley source measurement unit (Keithley 2400 and Keithley 2000)with a calibrated silicon photodiode.

3 Results and Discussion

Fig.1 shows the proposed energy diagram of the hybrid WOLED.The interlayer for this structure is extremely important.On one hand，the fluorescent blue emitter has a low triplet exciton energy，which means that it can efficiently quench the phosphorescence，when phosphorescent excitons energy are transferred to its triplet state.By introducing the interlayer，the device is able to suppress the quenching mechanism，because triplet excitons are transferred via diffusion and Dexter transfer，and Dexter transfer requires spatial overlap of the molecular orbitals of donor and acceptor，it can be suppressed by placing the emitters in separate emission layers and introducing an interlayer between them；on the other hand，the interlayer is a blend layer of an electron transporting material(BePP2)and a hole transporting material(NPB).Therefore，it is able to transport both electrons and holes，and finally make excitons to generate on both sides of the interlayer.

Fig.1 Schematic energy level diagram of the fabricated hybrid white OLEDThe emission layer is composed of orange and

blue emission layers in sequence.The Ir(bt)2-(acac)and BNE are orange and blue dopants，respectively.Using the same blend material(BePP2and NPB)in the orange layer and the interlayer contribute to reduce structural heterogeneities and simplify fabrication processes.

Generally，blue light emission is a cruel factor for the realization of high performance white light OLEDs，because the fluorescent blue can intrinsically have only about 25%internal quantum efficiency (IQE)，whereas the phosphorescent orange can reach 100%of IQE，the EQE naturally is lower for the devices with higher blue content.So，Careful control of the distribution of the excitons generated on both sides of the interlayer is extremely important to obtain efficient white emission[17].Finally，the bipolar orange emission layer is beneficial to broaden exciton formation zone and lower the triplet density，thus，the triplet efficiency roll-off caused by high triplet exciton density could be reduced[20-23].

Fig.2 Current efficiency-power efficiency-current density characteristics of the hybrid WOLED device in forward direction.The inset gives current densityluminance characteristics of the device.

Fig.2 displays the current efficiency-power efficiency-current density characteristics of the hybrid WOLED device in forward direction.The current density-luminance characteristics of the device in the inset of Fig.2.At luminance of 100 cd/m2，the device emits a current efficiency of 37 cd/A and a power efficiency of 39 lm/W.At luminance of 1 000 cd/m2，the device still emits a current efficiency of 34.5 cd/A and a power efficiency of 27.5 lm/W. Obviously，the reduction in efficiency at higher luminance is greatly suppressed.We attribute to thereduced efficiency roll-off due to the balanced charge injection and efficient charge/exciton confinement.

Fig.3 Normalized EL spectra of the hybrid WOLED device at different driving voltages

Fig.3 shows the normalized electroluminescent spectra of the hybrid WOLED device at different driving voltages.Two separate emission bands peaked at 478 nm and 576 nm respectively are clearly observed.The peaks at 478 nm and 576 nm corresponds to the emissions from BNE and Ir(bt)2(acac)，respectively.The EL spectra of the two EMLs based hybrid WOLED device shows stable white light emission spectra.There is nearly no emitting peak-shift with the changing bias voltages and the Commission Internationale de L'Eclairage(CIE)coordinates are (0.37，0.48)at 1 000 cd/m2and(0.37，0.47) at 10 000 cd/m2were respectively achieved.Generally，the emission spectrum of white OLEDs with separate emission layers for each color may shift with increasing driving current due to changes in the position of the recombination zone，or emitter dependent quenching mechanisms[30-34].So，this apparent resistance to the color change under the different voltages indicates the balanced charge carrier injection and transport for exciton recombination.

4 Conclusion

We have demonstrated highly efficient hybrid white organic light-emitting diodes composed of two separate emission layers.It is shown that the ambipolar blend interlayer can balance exciton generation evenly on its both sides，which consequently improve the efficiency and the color stability with brightness.The exciton confinement structure and a broadened exciton formation zone in the orange emitter were employed to improve in the efficiency and reduce the efficiency roll-off.The WOLEDs emit a power efficiency of 39 lm/W and a current efficiency of 37 cd/A at 100 cd/m2，slightly decreasing to 27.5 lm/W and 34.5 cd/A at 1 000 cd/m2.

[1]KIDO J，HONGAWA K，OKUTAMA K，et al..White light-emitting organic electroluminescent devices using the poly(N-vinylcarbazole)emitter layer doped with three fluorescent dyes[J].Appl.Phys.Lett.，1994，64(7):815-817.

[2]D'ANDRADE B W，FORREST S R.White organic light-emitting devices for solid-state lighting[J].Adv.Mater.，2004，16(18):1585-1595.

[3]PARK Y S，LEE S，KIM K H，et al..Exciplex-forming co-host for organic light-emitting diodes with ultimate efficiency [J].Adv.Funct.Mater.，2013，23(29):4914-4920.

[4]SO F，KIDO J，BURROWS P.Organic light-emitting devices for solid-state lighting[J].MRS Bull.，2008，33(7):663-669.

[5]D’ANDRADE B W，HOLMES R J，FORREST S R.Efficient organic electrophosphorescent white-light-emitting device with a triple doped emissive layer[J].Adv.Mater.，2004，16(7):624-628.

[6]D’ANDRADE B W.Lighting:white phosphorescent LEDs offer efficient answer[J].Nat.Photon.，2007，1(1):33-34.

[7]LAAMNSKY S，DJUROVICH P，MURPHY D，et al..Highly phosphorescent bis-cyclometalated iridium complexes:synthesis，photophysical characterization，and use in organic light emitting diodes[J].J.Am.Chem.Soc.，2001，123 (18):4304-4312.

[8]COCCHI M，VIRGILI D，FATTORI V，et al..N∧C∧N-coordinated platinum(Ⅱ)complexes as phosphorescent emitters in high-performance organic light-emitting devices[J].Adv.Funct.Mater.，2007，17(2):285-289.

[9]HO C L，WONG W Y，ZHOU G J，et al..Solution-processible multi-component cyclometalated iridium phosphors forhigh-efficiency orange-emitting OLEDs and their potential use as white light sources[J].Adv.Funct.Mater.，2007，17 (15):2925-2936.

[10]HUANG W S，LIN J T，CHIEN C H，et al..Highly phosphorescent bis-cyclometalated iridium complexes containing benzoimidazole-based ligands[J].Chem.Mater.，2004，16(12):2480-2488.

[11]REINEKE S，BALDO M A.Recent progress in the understanding of exciton dynamics within phosphorescent OLEDs[J]. Phys.Stat.Sol.(a)，2012，209(12):2341-2353.

[12]SU S J，GONMORI E，HISAHIRO S，et al..Highly efficient organic blue-and white-light-emitting devices having a carrier-and exciton-confining structure for reduced efficiency roll-off[J].Adv.Mater.，2008，20(21):4189-4194.

[13]SUN Y R，FORREST S R.Multiple exciton generation regions in phosphorescent white organic light emitting devices[J]. Org.Electron.，2008，9(6):994-1001.

[14]KING S M，AL-ATTAR H A，EVANS R J，et al..The use of substituted iridium complexes in doped polymer electrophosphorescent devices:the influence of triplet transfer and other factors on enhancing device performance[J].Adv. Funct.Mater.，2006，16(8):1043-1050.

[15]HO M H，CHEN T M，YEH P C，et al..Highly efficient p-i-n white organic light emitting devices with tandem structure [J].Appl.Phys.Lett.，2007，91(23):233507.

[16]CHOULIS S A，CHOONG V E，PATWARDHAN A，et al..Interface modification to improve hole-injection properties in organic electronic devices[J].Adv.Funct.Mater.，2006，16(8):1075-1080.

[17]ADACHI C，BALDO M A，FORREST S R，et al..High-efficiency organic electrophosphorescent devices with tris(2-phenylpyridine)iridium doped into electron-transporting materials[J].Appl.Phys.Lett.，2000，77(6):904-906.

[18]KAWAMURA Y，GOUSHI K，BROOKS J，et al..100%phosphorescence quantum efficiency of ir(III)complexes in organic semiconductor films[J].Appl.Phys.Lett.，2005，86(7):071104-1-3.

[19]SASABE H，TAKAMATSU J I，MOTOYAMA T，et al..High-efficiency blue and white organic light-emitting devices incorporating a blue iridium carbene complex[J].Adv.Mater.，2010，22(44):5003-5007.

[20]SU S J，SASABE H，TAKEDA T，et al..Pyridine-containing bipolar host materials for highly efficient blue phosphorescent OLEDs[J].Chem.Mater.，2008，20(5):1691-1693.

[21]KEPLER R G，CARIS J C，AVAKIAN P，et al..Triplet excitons and delayed fluorescence in anthracene crystals[J]. Phys.Rev.Lett.，1963，10(9):400-402.

[22]HE G F，PFEIFFER M，LEO K，et al..High-efficiency and low-voltage p-i-n electrophosphorescent organic light-emitting diodes with double-emission layers[J].Appl.Phys.Lett.，2004，85(17):3911-3913.

[23]WATANABE S，IDE N，KIDO J.High-efficiency green phosphorescent organic light-emitting devices with chemically doped layers[J].Jpn.J.Appl.Phys.，2007，46(3A):1186-1188.

[24]CHEN Y H，ZHAO F C，ZHAO F B，et al..Ultra-simple hybrid white organic light-emitting diodes with high efficiency and CRI trade-off:fabrication and emission-mechanism analysis[J].Org.Electron.，2012，13(12):2807-2815.

[25]WANG Q，HOB C L，ZHAO Y B，et al..Reduced efficiency roll-off in highly efficient and color-stable hybrid WOLEDs: the influence of triplet transfer and charge-transport behavior on enhancing device performance[J].Org.Electron.，2010，11(2):238-246.

[26]ZHAO F C，ZHANG Z Q，LIU Y P，et al..A hybrid white organic light-emitting diode with stable color and reduced efficiency roll-off by using a bipolar charge carrier switch[J].Org.Electron.，2012，13(6):1049-1055.

[27]GUO L Y，ZHANG X L，ZHUO M J，et al..Non-interlayer and color stable WOLEDs with mixed host and incorporating a new orange phosphorescent iridium complex[J].Org.Electron.，2014，15(11):2964-2970.

[28]SUN Y R，GIEBINK N C，KANNO H，et al..Management of singlet and triplet excitons for efficient white organic lightemitting devices[J].Nature，2006，440(7086):908-912.

[29]SUN N，WANG Q，ZHAO Y B，et al..A hybrid white organic light-emitting diode with above 20%external quantum efficiency and extremely low efficiency roll-off[J].J.Mater.Chem.C，2014，2(36):7494-7504.

[30]ZHAO J，YU J S，LIU S Q，et al..Combined host-guest doping and host-free systems for high-efficiency white organic light-emitting devices[J].J.Lumin.，2012，132(8):1994-1998.

[31]LIU B Q，XU M，WANG L，et al..Very-high color rendering index hybrid white organic light-emitting diodes with double emitting nanolayers[J].Nano-Micro Lett.，2014，6(4):335-339.

[32]YE J，CHEN ZH，AN F F，et al..Achieving highly efficient simple-emission layer fluorescence/phosphorescence hybrid white organic light-emitting devices via effective confinement of triplets[J].ACS Appl.Mater.Interf.，2014，6(12): 8964-8970.

[33]ZHU L P，CHEN J S，MA D G.Dopant effects on charge transport to enhance performance of phosphorescent white organic light emitting diodes[J].J.Appl.Phys.，2015，118(17):175503.

[34]XUE K W，HAN G G，DUAN Y，et al..Doping-free orange and white phosphorescent organic light-emitting diodes with ultra-simply structure and excellent color stability[J].Org.Electron.，2015，18:84-88.

方达(1992－)，男，河南洛阳人，硕士研究生，2014年于郑州轻工业学院获得学士学位，主要从事有机发光二极管的研究。

E-mail:1214063309＠njupt.edu.cn

张宏梅(1966－)，女，吉林松原人，教授，博士生导师，2006年于吉林大学获得博士学位，主要从事半导体光电子器件的研究。

E-mail:iamhmzhang＠njupt.edu.cn

光谱稳定的互补色双发光层高效混合白光OLED

方 达1，2，张智强3，吴震轩1，2，张 川1，2，张宏梅1，2∗
(1.南京邮电大学有机电子与信息显示国家重点实验室培育基地，信息材料与纳米技术研究院，江苏南京 210023；2.南京邮电大学江苏国家先进材料协同创新中心，江苏南京 210023；3.中国科学院长春应用化学研究所高分子物理与化学国家重点实验室，吉林长春 130022)

利用两种颜色的发光层制备了光谱稳定的高效混合WOLED。其中蓝光发光层用14%质量分数的BNE掺杂在BePP2中，橙光发光层用1%质量分数的Ir(bt)2(acac)掺杂在49.5%质量分数的NPB和49.5%质量分数的BePP2组成的混合主体中。在不利用任何光耦合技术的条件下，器件在亮度为100 cd/m2时，功率效率可以达到39 lm/W；当亮度提高到1 000 cd/m2时，效率仅发生轻微滚降至27.5 lm/W。器件的光谱稳定，亮度在1 000 cd/m2和10 000 cd/m2时，CIE坐标分别为(0.37，0.48)和(0.37，0.47)。良好的光谱稳定性归结于设计的双极性中间层平衡了其两侧激子的产生。

混合WOLED；双发光层；光谱稳定

2016-07-07；

2016-11-07

中科院百人计划；国家杰出青年科学基金(50325312)；国家重点基础研究发展计划(973)(2009CB623604)资助项目Supported by Hundreds Talents Program of Chinese Academy of Sciences；National Science Fund for Distinguished Young Scholars of China(50325312)；National Basic Research Program of China(973)(2009CB623604)

TN383＋.1

A

10.3788/fgxb20173802.0201

1000-7032(2017)02-0201-06