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Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN

Ye YU Wen-yan LIN Xue-kang PENG Yu JIN Zhi-jun WU Yan CHEN

喻叶, 林雯嫣, 彭雪康, 金玉, 吴志军, 陈燕. 利用有效电荷产生层Liq/Al/HAT-CN的叠层有机白光器件中弱微腔效应的研究[J]. 中国光学, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
引用本文: 喻叶, 林雯嫣, 彭雪康, 金玉, 吴志军, 陈燕. 利用有效电荷产生层Liq/Al/HAT-CN的叠层有机白光器件中弱微腔效应的研究[J]. 中国光学, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
YU Ye, LIN Wen-yan, PENG Xue-kang, JIN Yu, WU Zhi-jun, CHEN Yan. Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN[J]. Chinese Optics, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
Citation: YU Ye, LIN Wen-yan, PENG Xue-kang, JIN Yu, WU Zhi-jun, CHEN Yan. Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN[J]. Chinese Optics, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382

利用有效电荷产生层Liq/Al/HAT-CN的叠层有机白光器件中弱微腔效应的研究

doi: 10.3788/CO.20191202.0382
基金项目: 

国家自然科学基金项目 61605049

国家自然科学基金项目 11674111

详细信息
    作者简介:

    喻叶(1995-), 女, 江苏淮安人, 硕士研究生, 主要从事有机电致发光器件的研究。E-mail:

    陈燕(1981-), 女, 福建泉州人, 硕士, 讲师。2003年于南京邮电学院电子信息工程系获得工学学士学位, 2006年于北京大学信息科学技术学院获得工学硕士学位, 主要从事有机电致发光器件、有机光伏器件及有机薄膜晶体管方面研究。E-mail:

  • 中图分类号: TN383+.1;TN312+.8

Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN

Funds: 

the National Natural Science Foundation of China 61605049

the National Natural Science Foundation of China 11674111

More Information
    Author Bio:

    YU Ye(1995—), female, Huai′an, Jiangsu, Master′s Degree Student, primarily engaged in OLED research. E-mail:xiaolvyezi95@163.com

    CHEN Yan(1981—), female, from Quanzhou, Fujian Province, Lecturer. In 2003, she obtained a bachelor′s degree in engineering from the Department of Electronic Information Engineering of Nanjing University of Posts and Telecommunications. In 2006, she obtained a master′s degree in Engineering from the School of Information Science and Technology of Peking University. Currently, she is mainly engaged in research on organic electroluminescent devices, organic photovoltaic devices and organic thin film transistors. E-mail:goldency@hqu.edu.cn

    Corresponding author: CHEN Yan, E-mail:goldency@hqu.edu.cn
图(11) / 表 (1)
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  • 被引次数: 0
出版历程
  • 收稿日期:  2018-06-11
  • 修回日期:  2018-07-13
  • 刊出日期:  2019-04-01

Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN

doi: 10.3788/CO.20191202.0382
    基金项目:

    国家自然科学基金项目 61605049

    国家自然科学基金项目 11674111

    作者简介:

    喻叶(1995-), 女, 江苏淮安人, 硕士研究生, 主要从事有机电致发光器件的研究。E-mail:

    陈燕(1981-), 女, 福建泉州人, 硕士, 讲师。2003年于南京邮电学院电子信息工程系获得工学学士学位, 2006年于北京大学信息科学技术学院获得工学硕士学位, 主要从事有机电致发光器件、有机光伏器件及有机薄膜晶体管方面研究。E-mail:

    通讯作者: CHEN Yan, E-mail:goldency@hqu.edu.cn
  • 中图分类号: TN383+.1;TN312+.8

摘要: 本文使用电荷产生层Liq/Al/HAT-CN制备了蓝黄互补的叠层有机白光器件。通过比较叠层双色器件在相同电流密度下的发光光谱、亮度及电压,阐明了电荷产生层电荷产生及注入过程,并进一步研究了双层结构Liq/Al。在10 mA/cm2电流密度下,叠层白光器件的工作电压为8.3 V,亮度为746 cd/m2,分别为蓝光单节器件(4.2 V,315 cd/m2)与黄光单节器件(4.2 V,426 cd/m2)之和,证明了电荷产生层的有效性。当电流密度为240 mA/cm2时,叠层白光器件获得最高亮度11 420 cd/m2,在1 000 cd/m2的亮度下,电流效率为7.2 cd/A,功率效率为2.6 lm/W。驱动电流密度从10 mA/cm2增加到30 mA/cm2时,蓝光成分比例仅增加5%,证明器件发光性能稳定。针对叠层器件中存在的弱微腔效应,根据微腔理论,通过光学模拟计算进行了深入研究,模拟结果与实际光谱高度吻合,说明了光学模拟计算的准确性。

English Abstract

喻叶, 林雯嫣, 彭雪康, 金玉, 吴志军, 陈燕. 利用有效电荷产生层Liq/Al/HAT-CN的叠层有机白光器件中弱微腔效应的研究[J]. 中国光学, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
引用本文: 喻叶, 林雯嫣, 彭雪康, 金玉, 吴志军, 陈燕. 利用有效电荷产生层Liq/Al/HAT-CN的叠层有机白光器件中弱微腔效应的研究[J]. 中国光学, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
YU Ye, LIN Wen-yan, PENG Xue-kang, JIN Yu, WU Zhi-jun, CHEN Yan. Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN[J]. Chinese Optics, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
Citation: YU Ye, LIN Wen-yan, PENG Xue-kang, JIN Yu, WU Zhi-jun, CHEN Yan. Weak microcavity effect in tandem white organic light-emitting diodes employing an effective charge-generation layer of Liq/Al/HAT-CN[J]. Chinese Optics, 2019, 12(2): 382-394. doi: 10.3788/CO.20191202.0382
    • Organic light-emitting diodes(OLED) have the advantages of self-illuminating, high contrast, wide viewing angle, being thin, and controllable, making them one of the most promising technologies for next-generation solid-state lighting and large-area flat panel displays[1]. In 1993, J. Kido et al. prepared the first high-brightness and high-efficiency white light device[2]. After nearly three decades of development, people′s research on white OLEDs(WOLED) has made remarkable progress. At present, most of the reported WOLEDs use a multi-emissive layer structure, in which three primary colors(red, green and blue)[3] or two complementary colors(blue and orange or yellow) are mixed in the multilayer structure[4]. It is easier to achieve white light emission by adjusting the thickness of each luminescent layer or by doping the concentration of the luminescent dye. From a practical point of view, the spectrum of luminescence of the device remains stable over a large current density range, which is an important factor in measuring the quality of WOLEDs. However, when the driving current changes, the luminescence spectrum changes drastically due to the movement and energy transfer of the exciton recombination region[5]. A white light OLED with a stacked structure can effectively solve the above problems. The upper and lower two light-emitting units in the laminated structure device are not in direct contact with each other, allowing stable luminescent performance to be achieved[6-7]. In addition, the stacked device also has the following advantages:(1)Since the operating current at the same brightness is small, high efficiency can be maintained under a higher brightness, which is advantageous for weakening the quenching phenomenon of the excitons and improving the life of the device, and makes it is easy to meet the requirements of solid-state lighting; (2)The organic layer of the laminated device is thick, which can effectively avoid the short-circuit problem often faced by large-area devices[8], which is beneficial to the production of large-area flat panel displays. The large area WOLED is one of the future developments.

      The charge-generating layer connected to the light-emitting unit is the most critical part of the laminated device. Under the applied electric field, electrons and holes generated in the charge-generating layer are respectively injected into the adjacent electron transport layer(ETL) and hole transport layer(HTL). In the light-emitting layer, photons are generated in combination with holes on the anode side and electrons on the cathode side. In this paper, an electron injection layer(EIL)/electron acceptor structure is used as a charge-generating layer, which has good charge generation, good injection performance and extremely high optical transparency. In this structure, an interface between an electron acceptor and an adjacent HTL generates an electric charge and electrons are injected from the electron acceptor into the adjacent ETL. Typical electron acceptors include transition metal oxides(MoO3, WO3 and V2O5) and organics(HAT-CN)[9-12]. Commonly used EILs include metal compounds and ultra-thin double layers of aluminum(LiF/Al and Liq/Al)[12-13]. Liq/Al/HAT-CN is widely used in laminated white light devices due to its excellent charge generation and injection abilities and its extremely high optical transparency[14]. The working principle is that Liq reacts with Al, and a lithium atom is formed[15-16], which acts as an active alkali metal that dopes and enhances the electrical conductivity in the organic film. The n-type doping can effectively increase the carrier concentration in the organic material[17]. HAT-CN has no absorption in the visible wavelength range and has the lowest molecular unoccupied orbital(LUMO) depth, as well as a strong electron absorption capacity[14]. It is a highly efficient electron acceptor material.

      We used fluorescent dyes DPVBi and Rubrene to make blue and yellow monochromatic units, respectively, and designed complementary color laminated white light devices. The device exhibited good source color stability and white light characteristics over a wide range of operating current densities. When the current density was increased from 10 mA/cm2 to 30 mA/cm2, the International Commission on Illumination(Commission Internationale de L'Eclairage, CIE) color coordinates change only from (0.311, 0.320) to (0.300, 0.313).

      Although the charge-generating layer and the ITO anode have high transmittance in visible light, the multilayer OLED still has a relatively obvious weak microcavity effect due to the large number of layers in the laminated OLED[19]. We conducted an in-depth study of the weak microcavity effect through specific experiments and optical simulation calculations using SimOLED software.

    • An ITO(Indium tin oxide) glass substrate with a scratch-free surface was used and washed with an ultrasonic cleaner. The substrate was immersed in a 5% Decon 90 solution at 60 ℃ for 5 minutes, washed twice with deionized water, and then immersed in a 2% concentration of WIN-188 solution for 10 minutes. It was then finally rinsed with absolute ethanol and dried in an oven. The oxygen plasma-treated substrate was fed to an LN-1103SC multi-source vapor deposition system for thermal evaporation coating. The gas pressure in the evaporation chamber was kept below 5.0×10-5 Pa during the production process to ensure quality in the organic film. An SI-TM 216D quartz crystal film thickness monitor was used to monitor the rate of evaporation and the thickness of the film. The operating voltage, current, brightness and spectrum of the device were measured using a Keithley 2400 programmable power supply, an LS-110 luminance meter, a MAYA2000PRO fiber spectrometer, and a PR655 spectrometer. The test procedure was carried out in a glove box filled with high purity nitrogen at room temperature.

      In this paper, Ir(ppy)3(Tris(2-phenylpyridinato) iridium(Ⅲ)) is used as a green dye, Rubrene (5, 6, 11, 12-Tetraph enylnaphthacene) as a yellow dye, and DPVBi(4, 4′- Bis(2, 2-diphenylethenyl)-1, 1′-biphenyl) as a blue light dye. Alq3(Tris(8-hydroxy-quinolinato)aluminium) is used as a yellow light-doped parent material, BCP(4, 7-diphenyl-1, 10-phenanthroline) is used as a hole blocking layer, HAT-CN(1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile) is used as a hole injection layer and electron blocking layer, Bphen(4, 7-diphenyl-1, 10-Phenanthroline) is used for an electron transport layer, Liq(8-hydroxy-quinolinato lithium) is used as an electron injection layer, and metal Al is used as cathode.

    • We designed a set of green and yellow two-color stacked devices to illustrate the working mechanism of charge-generating layer Liq/Al/HAT-CN. Devices 1, 2, 3, and 4 are no-charge-generating layers. HAT-CN(15 nm), Al(1 nm)/HAT-CN(15 nm) and a complete charge-generating layer are embedded between monochromatic units:

      (1) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/NPB(60 nm)/Alq3:Rubrene (5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al

      (2) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/HAT-CN(15nm)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al

      (3) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/Al(1 nm)/HAT-CN(15 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al

      (4) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/Liq(1 nm)/Al(1 nm)/HAT-CN(15 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al

      Fig. 1 shows the transmittance of the charge-generating layer, which exceeds 85% in the visible range. This is very beneficial for the fabrication of high-performance stacked devices. Fig. 2 shows the spectrum and brightness of four devices at the same current density of 5 mA/cm2. The spectrum was measured using a PR655 spectrometer. The measured value is the absolute value of the brightness and the integrated area of the spectral area indicates the intensity of the luminous intensity. The HOMO level of NPB is -5.4 eV, which is 1 eV different from Bphen(-6.4 eV). It is difficult for cavities to overcome the injection barrier and reach the yellow light unit. Therefore, device 1 emits only green light with a brightness of 292 cd/m2 and a high voltage of 33.1 V. In device 2, a 15 nm thick HAT-CN is inserted between two monochromatic units. The LUMO energy level of HAT-CN can reach -5.7 eV. Electrons jump from the NPB HOMO level to the HAT-CN LUMO and cavities are generated at the same time. The cavities are transmitted to the yellow light unit under the action of the electric field, causing the yellow light component to appear in the spectrum of device 2. Since the LUMO level of Bphen(-3.0 eV) is much higher than that of HAT-CN, the generated electrons are difficult to inject into the green light unit. The brightness and voltage of device 2 do not change much with respect to device 1. Device 3 inserts a thin layer of Al between HAT-CN and Bphen. The Al atoms form self-doping in the organic layer, which improves the mobilitiy of electrons and facilitates electron injection into the green light unit. Therefore, the brightness of the device 3 is higher at 1 165 cd/m2, the operating voltage drops to 15.9 V and the carrier transfer characteristics are significantly improved. After the addition of the thin layer of Liq, the luminescence of device 4 is significantly enhanced(2 855 cd/m2) in comparison to that of device 3, with an operating voltage reduced to 8.3 V. The reason is that Liq reacts with Al to form lithium, which acts as an active alkali metal that dopes and enhances the electrical conductivity of the organic functional layer. The detailed process of charge generation and injection is shown in Fig. 3.

      图  1  电荷产生层在可见光区的透过率

      Figure 1.  Transmittance of the charge generation layer in the visible spectrum

      图  2  绿黄双色器件电致发光光谱

      Figure 2.  Electroluminescence spectra of the green-yellow dual-color device

      图  3  电荷产生层工作原理图

      Figure 3.  Working schematic of the charge-generating layer

      To further illustrate how the two-layer structure Liq/Al improves electron injection efficiency, we have fabricated the set of single-electron devices. Fig. 4 shows the Ⅰ-Ⅴ curve for the set. The HOMO level of BCP is nearly 1.4 eV higher than the work function of ITO(-4.7 eV), effectively blocking the hole injection. Because of this, the current of this group of devices can be considered as electron current. The current density of device 1 increases rapidly with an increase in voltage, while device 2 has almost no current because of the 2.7 eV electron in- jection barrier between HAT-CN and BPhen. Device 3 has a thin layer of Liq inserted between the HAT-CN and Bphen, wherein there is still no current, which shows that the addition of Liq does not reduce the effects of the electron injection barrier. When a two-layer thin layer structure Liq/Al is inserted between the HAT-CN and Bphen, the current increases rapidly. This shows that the electron injection barrier is significantly reduced and that a Liq/Al double layer structure can effectively improve electron injection efficiency.

      图  4  单电子器件的电流-电压特性

      Figure 4.  Ⅰ-Ⅴ characteristics of hole-excluded devices

      Based on the above conclusions, we have prepared blue and yellow monochromatic devices and blue-yellow complementary laminated white light devices:

      (1) ITO/HAT-CN(5 nm)/NPB(60 nm)/DPVBi(15 nm)/Bphen(50 nm)/Liq(1 nm)/Al

      (2) ITO/HAT-CN(5 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 15 nm)/Bphen(50 nm)/Liq(1 nm)/Al

      (3) ITO/HAT-CN(5 nm)/NPB(60 nm)/DPVBi(15 nm)/Bphen(50 nm)/Liq(1 nm)/Al(1 nm)/HAT-CN(15 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 15 nm)/Bphen(50 nm)/Liq(1 nm)/Al。

      Fig. 5 shows the normalized electroluminescence spectrum of a monochromatic device and a laminated white light device at a current density of 10 mA/cm2. The peak wavelength of the blue light is 470 nm and the peak value of the yellow light is 567 nm. It can be seen that the monochromatic units in the laminated device are effectively illuminated. The white light device has a CIE color coordinate of (0.311, 0.320), which is very close to standard white light of coordinate (0.333, 0.333).

      图  5  蓝光单色器件、黄光单色器件和叠层白光器件的光谱

      Figure 5.  Spectra of blue, orange and tandem white devices

      Fig. 6 shows the current efficiency and power efficiency curves of the tandem white device at a luminance of 1 000 cd/m2. Its current efficiency was 7.2 cd/A and its power efficiency was 2.6 lm/W.

      图  6  叠层白光器件电流效率和功率效率曲线

      Figure 6.  Current efficiency and power efficiency curves of the tandem white device

      Fig. 7 is a graph showing the luminance-current-voltage characteristic of the device. When current density was 240 mA/cm2, the maximum brightness was 11 420 cd/m2. In Tab. 1, at current densities of 10 mA/cm2 and 20 mA/cm2, the operating voltages of the stacked devices were 8.3 V and 8.9 V, respectively, and the luminances were 746 cd/m2 and 1 378 cd/m2, respectively. These were approximately the voltage and brightness of the blue(4.2 V and 4.5 V; 315 cd/cm2 and 603 cd/cm2) and yellow(4.2 V and 4.6 V; 426 cd/cm2 and 763 cd/cm2) monochromatic devices. The results confirm the efficiency of the charge-generating layer.

      图  7  单色器件与叠层白光器件的Ⅰ-Ⅴ和L-I特性曲线

      Figure 7.  Ⅰ-Ⅴ and L-I curves for the monochromatic device and the tandem white device

      表 1  电流密度为10 mA/cm2和20 mA/cm2下单色器件与叠层器件发光特性

      Table 1.  Luminous characteristics of the monochromatic device and the tandem device at current densities of 10 mA/cm2 and 20 mA/cm2.

      Voltage/V
      J=10/20
      (mA/cm2)
      Luminance/(cd·m-2)
      J=10/20
      (mA/cm2)
      DPVBi 4.2/4.5 315/603
      Rubrene 4.2/4.6 426/763
      Tandem white 8.3/8.9 746/1 378

      Fig. 8 shows the spectrum, color coordinates and blue light ratio(BLR) of a white light device at different operating current densities(10 mA/cm2, 20 mA/cm2 and 30 mA/cm2). It can be seen that when the current density is increased from 10 mA/cm2 to 30 mA/cm2, the spectral change is small. The color coordinates only changes by (-0.011, -0.007), and the blue component ratio increases from 46.4% to 51.5%. The light source is stable in the range of large driving current density and both are in the white light region. The color rendering index(CRI) is an important parameter for evaluating white light sources. The CRIs at different current densities are 74.4, 74.4 and 74.3, respectively. This indicates that the tandem white device has very stable color rendering performance and is a good illumination device.

      图  8  不同电流密度下叠层白光器件电致发光光谱、色坐标、蓝光比例以及CRI

      Figure 8.  Electroluminescence spectra, color coordinates, blue light ratios and CRI of tandem white device at different current densities

    • The spectra of the tandem white device at angles of 0°, 30° and 60° were measured. As shown in Fig. 9, as the angle increases, the peak from DPVBi undergoes a blue shift and the spectral change is large. In the illustration, the white light device is calculated for every 5° in the range of 0° to 90° and then compared with the Lambert law. It was found that there is a large deviation between the experimental curve and the Lambertian curve. The luminous intensity has a significant angular dependence and there is a weak microcavity effect in the device.

      图  9  叠层器件发光特性随角度的变化

      Figure 9.  Luminous characteristics of tandem devices vary with angles

      The resonant mode of the organic microcavity satisfies the Fabry-Perot equation:

      (1)

      where λ is the cavity resonance wavelength, m is the mode number, θ is the exit angle of light from the microcavity and Le is the effective cavity length. The microcavity effect in the device is closely related to the cavity length Le, as is the thickness of the device. We prepared a set of tandem white devices with different thicknesses (d). As shown in Fig. 10, as the thickness of the device increases, the spec- trum changes and the yellow light emission is suppressed.

      图  10  叠层器件光谱随器件厚度的变化

      Figure 10.  Spectra of tandem devices vary with thickness of device

      Based on the microcavity theory, we conducted in-depth research through optical simulation calculations. Using SimOLED software to simulate the spectrum of the same structure device as in the experiment, the experiment and simulation results were compared at 0° and 30° angles. As shown in Fig. 11, the two are highly consistent, indicating that the simulation calculation is accurate and that the weak microcavity effect does exist. The small difference between the experiment and its simulation is a result of the deviation in film thickness during the procedure.

      图  11  0°和30°观测角度下实验与模拟光谱的比较

      Figure 11.  Comparison of experimental and simulated spectra at observation angles of 0° and 30°

      The electroluminescence spectra of devices with microcavity effects can be described using the Fabry-Perot formula [20]:

      (2)

      Ra and Rc are the internal reflectances of the two electrodes, ϕa and ϕc are the phase changes of the beam after reflection, Ta is the transmittance of the anode, L is the total optical thickness, I0(zi, λ) is the radiation of the luminescent layer, I(λ) is the light radiation after passing through the substrate, zi is the optical length between the light-emitting sub-layer i and the cathode, and θ0 is the angle at which the light-emitting layer emits the light beam. The spectrum can be obtained by calculating I(λ).

    • This paper explains the charge generation and injection process in the charge-generating layer by comparing the luminescence spectrum, brightness and voltage of a laminated two-color device at a consistent current density. Based on this, a complementary tandem white device was prepared. At a current density of 10 mA/cm2, the laminated device has an operating voltage of 8.3 V and a luminance of 746 cd/m2, which is the sum of the tandem blue device(4.2 V, 315 cd/m2) and tandem yellow device(4.2 V, 426 cd/m2). This illustrates the effectiveness of the charge-generating layer. When the current density is 240 mA/cm2, the laminated device obtains a maximum luminance of 1 420 cd/m2. At the luminance of 1 000 cd/m2, the current efficiency is 7.2 cd/A, and the power efficiency is 2.6 lm/W. The CIE color coordinates of the white light are (0.311, 0.320), which is close to the standard white light of (0.333, 0.333). When the current density increases from 10 mA/cm2 to 30 mA/cm2, the color coordinate changes by (-0.011, -0.007) and the blue component increases by only 5%. The chromaticity of the light source is stable, and the color rendering is greater than 74, showing that it has the potential for being a lighting device. With regards to the weak microcavity effect that exists in the laminated device, we have carefully studied it through optical simulations and obtained simulation results that are highly consistent with the actual experimental results, indicating that the optical simulation calculation is accurate.

      ——中文对照版——

    • 有机电致发光器件(OLED)具有自发光, 对比度高, 视角广, 厚度薄, 可制作柔性显示等优点, 已成为用于下一代固态照明和大面积平板显示最有潜力的技术之一[1]。1993年, J.Kido等人制备了第一个高亮度、高效率的白光器件[2], 经过近30年的发展, 白光OLED的研究取得了令人瞩目的进展。目前, 大多数已经报道的白光OLED都采用多发光层结构, 在多层结构中混合三原色(红色, 绿色和蓝色)[3]或两种互补色(蓝色和橙色或黄色)[4], 通过调节各发光层的厚度或者发光染料的掺杂浓度就能较容易实现白光发射。从实用化的角度看, 在较大的电流密度范围内, 器件的发光光谱是否保持稳定是衡量白光OLED品质的重要因素。然而, 多发光层结构在驱动电流变化时, 由于激子复合区域发生移动及能量转移, 使得发光光谱随之剧烈变化[5]。叠层结构器件由于器件内上下两个发光单元没有直接接触, 故能够获得稳定的发光性能[6-7]。因此, 采用叠层结构的白光OLED可以有效解决以上问题。此外, 叠层器件还具有以下优势; (1)其相同亮度下的工作电流较小, 可在较大的亮度下保持较高的效率, 有利于减弱激子的淬灭现象, 提高器件的寿命, 易于满足固态照明的要求; (2)叠层器件的有机层较厚, 可以有效避免大面积器件经常面临的短路问题[8], 有利于制作大面积平板显示, 而大面积平板显示正是有机白光器件未来的发展趋势之一。

      连接发光单元的电荷产生层是叠层器件中最关键的部分, 在外加电场作用下, 电荷产生层内产生的电子和空穴分别注入相邻的电子传输层(ETL)和空穴传输层(HTL), 在发光层中与阳极侧的空穴和阴极侧的电子结合产生光子。本文采用电子注入层(EIL)/电子受体结构为电荷产生层。该结构具有良好的电荷产生、注入性能和极高的光学透明度, 其中电子受体和相邻HTL之间的界面处产生电荷, 电子从电子受体注入到相邻电子传输层。典型的电子受体包括过渡金属氧化物(MoO3, WO3和V2O5)和有机物(HAT-CN)[9-12]。常用的EIL包括金属化合物和铝的超薄双层(LiF/Al和Liq/Al)[12-13]。由于Liq/Al/HAT-CN优良的电荷生成和注入特性以及极高的光学透明性[14]而被广泛用于叠层白光器件。其工作原理是Liq与Al发生了反应, 生成了锂原子[15-16], 从而对有机薄膜进行了n型掺杂, n型掺杂能够有效增加有机材料中载流子浓度[17]。HAT-CN在可见光波长范围内无吸收, 并且分子最低未占据轨道(LUMO)深, 有着强电子吸收能力[14], 是一种高效的电子受体材料。

      本文使用荧光染料DPVBi和Rubrene分别制作蓝、黄单色单元, 设计了互补色叠层白光器件。器件在较大的工作电流密度范围内表现出良好的光源颜色稳定性和白光特性。当电流密度从10 mA/cm2增加到30 mA/cm2时, 国际照明委员会(Commission Internationale de L′Eclairage, CIE)色坐标仅从(0.311, 0.320)变化到(0.300, 0.313)。

      尽管电荷产生层与ITO(Indium Tin Oxide)阳极在可见光内都具有很高的透过率, 但由于叠层OLED的层数较多, 多光束干涉的影响使得叠层OLED仍然存在较为明显的弱微腔效应[18-19]。本文通过具体实验和SimOLED软件进行光学模拟计算, 深入研究了弱微腔效应。

    • 选用表面无划痕的ITO玻璃基片, 使用超声波清洗机清洗。在60 ℃下, 基片浸入浓度为5%的Decon 90溶液中清洗两次, 每次5 min, 用去离子水清洗两次后, 放入2%浓度的WIN-188溶液浸洗10 min, 最后用无水乙醇冲洗干净, 放入烘箱内干燥。将经氧等离子处理后的基片送入LN-1103SC多源气相沉积系统进行热蒸发镀膜。制作过程中蒸镀腔内的气压应始终保持在5.0×10-5 Pa以下, 以保证有机薄膜的质量。使用SI-TM216D石英晶体膜厚监控仪来观察蒸镀的速率和薄膜的厚度。器件的工作电压、电流、亮度、光谱等使用keithley2400程控电源、LS-110亮度仪、MAYA2000PRO光纤光谱仪和PR655光谱仪测量。测试过程均在室温下充满高纯氮的手套箱内进行。

      本文中使用Ir(ppy)3(Tris(2-phenylpyridinato) iridium(Ⅲ))为绿光染料, Rubrene(5, 6, 11, 12-Tetraph enylnaphthacene)为黄光染料, DPVBi(4, 4′-Bis(2, 2-diphenylethenyl)-1, 1′-biphenyl)为蓝光染料, Alq3(Tris(8-hydroxy-quinolinato)aluminium)为黄光掺杂的母体材料, BCP(4, 7-diphenyl-1, 10-phenanthroline)为空穴阻挡层, HAT-CN(1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile)为空穴注入层和电子阻挡层, Bphen(4, 7-diphenyl-1, 10-Phenanthroline)为电子传输层, Liq(8-hydroxy-quinolinato lithium)为电子注入层, 阴极为金属Al。

    • 本文设计了一组绿黄双色叠层器件来说明电荷产生层Liq/Al/HAT-CN的工作机理。器件1、2、3、4分别为无电荷产生层, 单色单元间嵌入HAT-CN(15 nm), Al(1 nm)/HAT-CN(15 nm)和完整的电荷产生层:

      (1) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15nm)/Bphen(65 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al;

      (2) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/HAT-CN(15 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al;

      (3) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/Al(1 nm)/HAT-CN(15 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al;

      (4) ITO/HAT-CN(5 nm)/NPB(60 nm)/TcTa:Ir(ppy)3(15%, 15 nm)/Bphen(65 nm)/Liq(1 nm)/Al(1 nm)/HAT-CN(15 m)/NPB(60 nm)/Alq3:Rubrene(5%, 20 nm)/Bphen(60 nm)/Liq(1 nm)/Al。

      图 1为电荷产生层的透过率曲线, 在可见光范围内透过率都超过85%, 这十分有利于制作高性能的叠层器件。图 2为4个器件在相同电流密度5 mA/cm2下的光谱及亮度。光谱使用PR655光谱仪测量, 测量值为亮度绝对值, 光谱面积积分大小表示发光强度的大小。图 3为电荷产生层电荷产生与注入的详细过程。其中, NPB的HOMO能级为-5.4 eV, 与Bphen(-6.4 eV)之间相差1 eV, 空穴难以克服注入势垒到达黄光单元, 因此器件1仅发出绿光, 亮度为292 cd/m2, 电压高达33.1 V。器件2在两个单色单元间插入一层厚度为15 nm的HAT-CN, HAT-CN的LUMO能级深-5.7 eV, 电子从NPB的HOMO能级跳跃到HAT-CN的LUMO能级, 也将同时生成空穴, 空穴在电场作用下传输至黄光单元, 因此器件2的光谱中出现黄光成分。由于Bphen的LUMO能级(-3.0 eV)远高于HAT-CN, 产生的电子难以注入绿光单元, 器件2的亮度及电压相对于器件1并没有发生太大变化。器件3在HAT-CN和Bphen之间插入了Al薄层, Al原子在有机层中形成自掺杂, 提高了电子迁移率, 有利于电子注入绿光单元, 因此器件3的亮度增加至1 165 cd/m2, 工作电压降至15.9 V, 载流子传输特性改善明显。而在添加Liq薄层后, 器件4发光明显增强(2 855 cd/m2), 且远大于器件3, 工作电压相应地下降到8.3V。原因是Liq与Al发生了化学反应, 生成的锂原子起到活泼碱金属掺杂的作用, 增强了有机功能层的电导率。电荷产生与注入的详细过程见图 3

      为了进一步说明双层结构Liq/Al如何提高电子注入效率, 本文制作了一组单电子器件。图 4是一组单电子器件的Ⅰ-Ⅴ曲线。BCP的HOMO能级比ITO的功函数(-4.7 eV)高出了将近1.4 eV, 有效地阻挡了空穴的注入, 因此该组器件的电流可以认为是电子流。由图 4可见, 器件1的电流密度随着电压增加而迅速增加, 而器件2却几乎没有电流, 这是因为HAT-CN与BPhen之间有高达2.7 eV电子注入势垒; 器件3在HAT-CN与Bphen之间插入Liq薄层, 依旧没有电流, 这说明Liq的加入并没有减小电子的注入势垒, 当在HAT-CN与Bphen之间插入双层薄层结构Liq/Al后, 电流迅速增加, 这说明电子的注入势垒显著降低了, Liq/Al双层结构可有效提高电子注入效率。

      根据上述结论, 本文制备了蓝、黄单色器件和蓝黄互补叠层白光器件:

      (1) ITO/HAT-CN(5 nm)/NPB(60 nm)/DPVBi(15 nm)/Bphen(50 nm)/Liq(1 nm)/Al;

      (2) ITO/HAT-CN(5 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 15 nm)/Bphen(50 nm)/Liq(1 nm)/Al;

      (3) ITO/HAT-CN(5 nm)/NPB(60 nm)/DPVBi(15 nm)/Bphen(50 nm)/Liq(1 nm)/Al(1 nm)/HAT-CN(15 nm)/NPB(60 nm)/Alq3:Rubrene(5%, 15 nm)/Bphen(50 nm)/Liq(1 nm)/Al。

      图 5为单色器件和叠层白光器件在10 mA/cm2电流密度下的归一化电致发光光谱, 蓝光波长峰值在470 nm处, 黄光主峰峰值为567 nm, 可见叠层器件中单色单元均有效发光。白光器件的CIE色坐标为(0.311, 0.320), 非常接近标准白光(0.333, 0.333)。

      图 6为叠层白光器件的电流效率和功率效率曲线。可见, 在1 000 cd/m2的亮度下, 电流效率为7.2 cd/A, 功率效率为2.6 lm/W。图 7为器件的亮度-电流-电压特性曲线, 可见, 当电流密度为240 mA/cm2时获得最高亮度11 420 cd/m2表 1中, 在电流密度为10 mA/cm2和20 mA/cm2时, 叠层器件的工作电压分别为8.3 V和8.9 V, 发光亮度分别为746 cd/m2和1 378 cd/m2, 两个值分别约为蓝光(4.2 V和4.5 V; 315 cd/cm2和603 cd/cm2)与黄光(4.2 V和4.6 V; 426 cd/cm2和763 cd/cm2)单色器件的电压之和与亮度之和, 证实了电荷产生层的高效性。

      图 8为白光器件在不同工作电流密度(10、20和30 mA/cm2)下的光谱, 色坐标以及光谱中的蓝光成分比例(Blue Light Ratio, BLR)。可以看到, 电流密度从10 mA/cm2增加到30 mA/cm2时, 光谱变化不大, 色坐标仅改变(-0.011, -0.007), 蓝光成分比例从46.4%增加到51.5%, 即在较大驱动电流密度范围内光源颜色稳定, 且均在白光区内。显色指数(Color Rendering Index, CRI)是评价白光光源的重要参数, 不同上述电流密度下的CRI分别为74.4、74.4和74.3, 说明该叠层白光器件具有非常稳定良好的显色性能, 是良好的照明器件。

    • 本文测量了叠层白光器件在测量角度分别为0°、30°和60°下的光谱, 结果如图 9所示。可见, 随着角度的增加, 来自DPVBi的峰发生蓝移, 光谱变化较大。插图中, 将白光器件从0°到90°, 发光强度每隔5°进行计算, 并与朗伯(Lambert)规律进行对比, 发现实验曲线与朗伯曲线之间有较大偏离, 说明器件的发光强度具有明显的角度依赖性, 器件中存在着弱微腔效应。

      有机微腔的谐振模式满足Fabry-Perot方程:

      (1)

      其中, λ是腔谐振波长, m为模式数, θ为光从微腔内出射的出射角度, Le是腔体的有效腔长, 器件中微腔效应与腔长Le, 即器件厚度密切相关。本文制备了一组厚度(d)不同的叠层白光器件, 如图 10所示, 随着器件厚度的增加, 光谱随之发生变化, 黄光发光被抑制。

      根据微腔理论, 通过光学模拟计算进行了深入研究。使用SimOLED软件模拟与实验相同结构器件的光谱, 将在0°和30°观测角度下实验与仿真结果进行了比较, 如图 11所示。结果显示, 二者高度吻合, 说明了模拟计算的准确性, 也说明弱微腔效应确实存在。实验与模拟之间的微小区别来源于具体操作过程中的薄膜厚度偏差。

      存在微腔效应的器件的电致发光光谱可以使用Fabry-Perot公式[20]来描述:

      (2)

      其中, RaRc为两个电极的内部反射率, ϕaϕc为光束经反射后相位的变化, Ta为阳极的透过率, L为总光学厚度, I0(zi, λ)是发光层的辐射, I(λ)是经过衬底后的光辐射, zi是发光亚层i与阴极之间的光学长度, θo是发光层出射光束的角度, 通过计算I(λ)可以获得光谱。

    • 本文通过比较叠层双色器件在相同电流密度下的发光光谱、亮度及电压, 解释了电荷产生层中电荷产生及注入过程, 并以此为基础制备了互补叠层白光器件。在10 mA/cm2电流密度下, 叠层器件的工作电压为8.3V, 亮度为746 cd/m2, 分别为蓝光单节器件(4.2 V, 315 cd/m2)与黄光单节器件(4.2 V, 426 cd/m2)之和, 说明了电荷产生层的有效性。叠层器件当电流密度为240 mA/cm2获得最高亮度11 420 cd/m2, 在1 000 cd/m2的亮度下, 电流效率为7.2 cd/A, 功率效率为2.6 lm/W。白光的CIE色坐标为(0.311, 0.320), 接近标准白光光源(0.333, 0.333)。电流密度从10 mA/cm2增加至30 mA/cm2时, 色坐标变化(-0.011, -0.007), 蓝光成分仅增加5%, 光源色度稳定, 且显色性均大于74。上述结果说明所设计器件具有作为照明设备的潜质。针对叠层器件中存在的弱微腔效应, 通过光学模拟计算进行了仔细研究, 模拟结果与实际光谱高度吻合, 说明了光学模拟计算的准确性。

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