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.

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方達(dá)(1992－)，男，河南洛陽人，碩士研究生，2014年于鄭州輕工業(yè)學(xué)院獲得學(xué)士學(xué)位，主要從事有機(jī)發(fā)光二極管的研究。

E-mail:1214063309＠njupt.edu.cn

張宏梅(1966－)，女，吉林松原人，教授，博士生導(dǎo)師，2006年于吉林大學(xué)獲得博士學(xué)位，主要從事半導(dǎo)體光電子器件的研究。

E-mail:iamhmzhang＠njupt.edu.cn

光譜穩(wěn)定的互補(bǔ)色雙發(fā)光層高效混合白光OLED

方 達(dá)1，2，張智強(qiáng)3，吳震軒1，2，張 川1，2，張宏梅1，2?
(1.南京郵電大學(xué)有機(jī)電子與信息顯示國家重點(diǎn)實(shí)驗(yàn)室培育基地，信息材料與納米技術(shù)研究院，江蘇南京 210023；2.南京郵電大學(xué)江蘇國家先進(jìn)材料協(xié)同創(chuàng)新中心，江蘇南京 210023；3.中國科學(xué)院長(zhǎng)春應(yīng)用化學(xué)研究所高分子物理與化學(xué)國家重點(diǎn)實(shí)驗(yàn)室，吉林長(zhǎng)春 130022)

利用兩種顏色的發(fā)光層制備了光譜穩(wěn)定的高效混合WOLED。其中藍(lán)光發(fā)光層用14%質(zhì)量分?jǐn)?shù)的BNE摻雜在BePP2中，橙光發(fā)光層用1%質(zhì)量分?jǐn)?shù)的Ir(bt)2(acac)摻雜在49.5%質(zhì)量分?jǐn)?shù)的NPB和49.5%質(zhì)量分?jǐn)?shù)的BePP2組成的混合主體中。在不利用任何光耦合技術(shù)的條件下，器件在亮度為100 cd/m2時(shí)，功率效率可以達(dá)到39 lm/W；當(dāng)亮度提高到1 000 cd/m2時(shí)，效率僅發(fā)生輕微滾降至27.5 lm/W。器件的光譜穩(wěn)定，亮度在1 000 cd/m2和10 000 cd/m2時(shí)，CIE坐標(biāo)分別為(0.37，0.48)和(0.37，0.47)。良好的光譜穩(wěn)定性歸結(jié)于設(shè)計(jì)的雙極性中間層平衡了其兩側(cè)激子的產(chǎn)生。

混合WOLED；雙發(fā)光層；光譜穩(wěn)定

2016-07-07；

2016-11-07

中科院百人計(jì)劃；國家杰出青年科學(xué)基金(50325312)；國家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃(973)(2009CB623604)資助項(xiàng)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