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CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件

尹茂军 潘腾 刘士浩 谢文法 张乐天

尹茂军, 潘腾, 刘士浩, 谢文法, 张乐天. CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件[J]. 中国光学, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
引用本文: 尹茂军, 潘腾, 刘士浩, 谢文法, 张乐天. CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件[J]. 中国光学, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
YIN Mao-jun, PAN Teng, LIU Shi-hao, XIE Wen-fa, ZHANG Le-tian. Top-emitting red, green and blue organic light-emitting devices with CdSe/ZnS quantum dots down-conversion films[J]. Chinese Optics, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
Citation: YIN Mao-jun, PAN Teng, LIU Shi-hao, XIE Wen-fa, ZHANG Le-tian. Top-emitting red, green and blue organic light-emitting devices with CdSe/ZnS quantum dots down-conversion films[J]. Chinese Optics, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431

CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件

doi: 10.3788/CO.20191206.1431
基金项目: 

国家自然科学基金项目 61774074

吉林省科技发展计划项目 20190101024JH

详细信息
  • 中图分类号: TN383

Top-emitting red, green and blue organic light-emitting devices with CdSe/ZnS quantum dots down-conversion films

Funds: 

National Natural Science Foundation of China 61774074

Science and Technology Development Plan of Jilin Province 20190101024JH

More Information
    Author Bio:

    YIN Mao-jun (1993—), male, Hegang, Heilongjiang, graduated with a master's degree and received a bachelor's degree from Jilin University in 2016, He is mainly engaged in research on organic light-emitting devices.E-mail:zlt@jlu.edu.cn

    ZHANG Le-tian (1977—), female, Changchun Jilin, professor, received her Ph.D.from Jilin University in 2004, She is mainly engaged in research on organic optoelectronic devices.E-mail:zlt@jlu.edu.cn

    Corresponding author: ZHANG Le-tian, E-mail:zlt@jlu.edu.cn
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  • PDF下载量:  6
  • 被引次数: 0
出版历程
  • 收稿日期:  2019-01-18
  • 修回日期:  2019-03-09
  • 刊出日期:  2019-12-01

CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件

doi: 10.3788/CO.20191206.1431
    基金项目:

    国家自然科学基金项目 61774074

    吉林省科技发展计划项目 20190101024JH

  • 中图分类号: TN383

摘要: 顶发射结构可实现100%的开口率,对高品质有机发光器件(OLED)显示具有重要意义。但目前OLED显示的传统制作方法难度较大,且器件的光谱稳定性较差。为了降低OLED显示的制作难度并进一步提高器件的光谱稳定性,本文将CdSe/ZnS量子点与聚甲基丙烯酸甲酯溶液相混合,并通过压印法制备了具有微结构阵列的红光与绿光量子点下转换膜,将其与高效顶发射蓝光OLED和彩色滤光片相结合,实现了红(R)、绿(G)、蓝(B)三色顶发射OLED。RGB器件的最大电流效率分别为3.6、21.9和10.6 cd/A,色坐标分别为(0.70,0.30)、(0.24,0.62)和(0.10,0.20)。此外,器件的光谱与色坐标几乎不随电压变化,且同时具有优良的光谱角度稳定性。这种实现OLED彩色化的方法具有制作简单、成本较低等优点,具有广阔的发展前景。

English Abstract

尹茂军, 潘腾, 刘士浩, 谢文法, 张乐天. CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件[J]. 中国光学, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
引用本文: 尹茂军, 潘腾, 刘士浩, 谢文法, 张乐天. CdSe/ZnS量子点下转换膜的红、绿、蓝顶发射有机发光器件[J]. 中国光学, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
YIN Mao-jun, PAN Teng, LIU Shi-hao, XIE Wen-fa, ZHANG Le-tian. Top-emitting red, green and blue organic light-emitting devices with CdSe/ZnS quantum dots down-conversion films[J]. Chinese Optics, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
Citation: YIN Mao-jun, PAN Teng, LIU Shi-hao, XIE Wen-fa, ZHANG Le-tian. Top-emitting red, green and blue organic light-emitting devices with CdSe/ZnS quantum dots down-conversion films[J]. Chinese Optics, 2019, 12(6): 1431-1441. doi: 10.3788/CO.20191206.1431
    • Organic electroluminescent devices(OLEDs) play an important role in display fields due to their uniquely bright, self-luminescent and flexible characteristics. After a long period of development[1-5], OLED technology has gradually gone into industrialization stage. Nowadays OLED-based products such as smartphones, computers and TVs, are popular in our daily life. Till now, most OLED display panel manufacturers adopt the method based on independent red(R), green(G) and blue(B) pixels. However, the method need fine metal mask technology to achieve high resolution display, and the fine metal mask technology is difficult and costly due to its requirements to extremely high dimensional and positional accuracy. Moreover, since organic luminescent materials used for achieving three primary color have different performances in their lifetimes, color distortion problems are serious as time goes on for OLED display panel fabricated by the method with independent RGB pixels. Besides, another method is also proposed to achieve RGB OLEDs display by combining white OLEDs with color filter(CF)[6-7]. This method does not require mask alignment, which reduces manufacturing difficulty and production costs. However, conventional white OLEDs are usually a multi-layered structure. Their exciton recombination region always moves with bias voltage changes, thereby changes emission spectra[8-9]. These issues still prevent further developments of OLED commercialization.

      In addition to the above two methods, there is also a method using multicolor OLED with down-conversion layers and color filter. A blue OLED is generally used as a pump source, and its produced blue light is used to pump down-converting materials to obtain light of other colors[10-13]. This method needs simple fabrication process without requirement for fine metal mask and has free choice in luminescent materials, making it suitable for achieving large-scale OLED displays. Moreover, since the intensity of red and green light from down-converting materials is controlled by blue light, RGB emission from such device has high color stability. However, current down-conversion materials are mostly organic materials and few organic materials have strong absorption in the blue region, resulting in low conversion efficiency. Quantum dots (QDs) have emerged as a new type of luminescent materiel for luminescent devices due to their advantages in tunable color, narrow spectra and high photochemical stability. More importantly, QDs have high photoluminescent efficiency and wide absorption, thereby they are ideal down-conversion materials[14-15].

      Additionally, compared to conventional bottom-emitting devices, top-emitting devices have advantage in the choice of substrate materials. In this way, driving circuits for active driving OLEDs(AMOLEDs) can be fabricated under the top-emitting devices, thereby eliminating the competition between the pixel′s light-emitting area and its driving circuit, which increases the aperture ratio. However, the top-emission devices have a serious spectral angle dependence problem that color coordinates changes obviously under different viewing angles, which is unsuitable for high-quality display. To improve the angular characteristics of the top-emitting devices, a light scattering layer with microstructure can be introduced on surface of semitransparent top electrode[16-17]. Such light scattering layer can scatter light without affecting the internal structure of top-emitting devices, thereby improving its angular characteristics. Microstructure array can be integrated into the down-conversion layer to achieve both down-conversion and scattering functionalities. In addition, due to a mismatch in the refractive indices, there is total reflection at the interface between the down-conversion layer and the air when light emits out of the down-converting layer. It causes a portion of light to be confined in the down-conversion layer, reducing the luminous efficiency of device. Integrating an array microstructure on the surface of the down-conversion layer can attenuate the total reflection[18] and extract light that is confined in the down-conversion layer, which is beneficial to improve device efficiency.

      Considering above, red and green quantum dots down-conversion films with microstructure are prepared by integrating CdSe/ZnS quantum dots into polymethyl methacrylate and using transfer imprint process, and then they are combined with efficient top-emitting blue OLED and color filters to achieve RGB emission using the same organic functional layers. These RGB OLEDs exhibit a maximum current efficiency of 3.6, 21.9 and 10.6 cd/A, respectively, and their chromaticity coordinates are (0.70, 0.30), (0.24, 0.62) and (0.10, 0.20), respectively. Spectra and chromaticity coordinates of RGB OLEDs are almost invariable under different voltages, and emission of such RGB OLEDs also shows excellent angle stability.

    • Before preparing top-emitting OLEDs, glass substrate was cleaned with Decon 90 detergent and de-ionized water ultrasonically in sequence, and then dried at 120 ℃ for 10 min and treate by UV ozone treatment for 5 min. After that, the clean substrate was placed in a multi-source organic molecular vapor deposition system, for depositing top-emitting blue OLEDs. A quartz crystal film thickness monitor was used to simultaneously observe deposition rate and thickness of functional layers during evaporation process. The rate of evaporation was controlled at about 0.1 nm/s, and the chamber was always maintained at a vacuum of about 6×10-4 Pa during the evaporation process.

      Red and green CdSe/ZnS quantum dots were purchased from Suzhou Xingshuo Nano Technology Co., Ltd. PL peak of the red quantum dot was located at 630 nm, its diameter was 25 nm and its quantum yield was ≥85%. The PL peak of the green quantum dot was located at 550 nm, its diameter was 25 nm, and its quantum yield was ≥85%. To fabricate a microstructure array down conversion layer, 100 μL toluene solution of polymethyl methacrylate(PMMA)(concentration of 300 mg/mL) was mixed with 1 μL toluene solution of red CdSe/ZnS quantum dot(concentration of 100 mg/mL) and 10 μL toluene solution of green CdSe/ZnS quantum dot(concentration of 50 mg/mL), respectively. And then these two kinds of solution were stirred for 5 minutes to obtain uniform solution, respectively. The above two kinds of quantum dot-PMMA mixed solution were sprayed on two silicon substrates with same microstructure, respectively, and then heated at 70 ℃ for 40 minutes to evaporate the solution and form solid thin film. Finally, the cured down-conversion films with array microstructure were separated from the silicon substrates.

      The brightness, current, voltage and electroluminescence spectrum of the devices were tested using the Otsuka Distributed Photometer Test System(GP-500). Each device was tested in atmosphere at room temperature.

    • Schematic diagram of devices R, G and B based on blue top-emitting OLED(TOLED) is shown in Fig. 1. The blue TOLED is used as a pump source, and a red quantum dot down-conversion film and a green quantum dot down-conversion layer with a micro-cylindrical lens array are covered on the blue TOLED, respectively. Red and green quantum dots absorb part of blue light to produce red and green photoluminescence(PL) emission, respectively. Then, color purity of red, green and blue emission is further improved through filtering action of color filters. Here, RGB emission can be obtained by combining blue TOLED, quantum dot down-conversion layers and color filters, and the whole devices(including color filter and down-conversion layer) used to achieve R, G and B emission are marked as device R, G and B, respectively. The details of structure of the blue TOLED is glass substrate silver (Ag, 100 nm)/molybdenum trioxide(MoO3, 3 nm)/4, 4′-cyclohexyl bis(N, N-bis(4-methylphenyl)aniline (TAPC, 40 nm)/4, 4′, 4′-tris(carbazol-9-yl)triphenylamine(TCTA, 5 nm)/TCTA:bis(4, 6-difluorophenylpyridine-N, C2) Picolinyl hydrazide(TCTA:Firpic, 25 nm)/1, 3, 5-tris((3-pyridyl)-3-phenyl) benzene(TmPyPB, 50 nm)/8-hydroxyquinoline lithium(Liq, 2 nm)/Samarium(Sm, 20 nm)/TAPC(60 nm), wherein the doping ratio of host TCTA and guest Firpic in emitting layer is 8:1.

      图  1  红、绿、蓝顶发射器件的结构示意图

      Figure 1.  (Color online)Schematic diagram of devices R, G and B based on blue TOLED

      Transmittance of the used color filters is shown in Fig. 2. Each filter only allows light with specific wavelengths to pass and blocks light with other wavelengths, which can greatly improve the color purity of RGB emission. The transmittance of the filters is also very important for the device performances. Higher transmittance leads to that more emission is utilized, and thus the efficiency of the entire device is improved.

      图  2  彩色滤光片的透过率

      Figure 2.  Transmittance of the used color filters

      Fig. 3 is absorption spectra of red and green quantum dots, and normalized electroluminescence (EL) spectra of devices TOLED, R, G and B. EL spectrum of blue TOLED shows a main peak at 475 nm and a side peak at 500 nm. Device B obtained by combining device TOLED with blue color filter shows pure blue emission with a main peak at 475 nm. The main peak of device G is located at 550 nm, which is mainly derived from the PL of the green light quantum dot. The green emission from device G also has a side peak at 508 nm, and it should be attributed to the EL emission of device TOLED that is not absorbed by the green light quantum dots. The emission peak of device R is at 635 nm, corresponding to PL emission peak of red quantum dots.

      图  3  红、绿、蓝器件和器件TOLED的归一化发光光谱,以及红光、绿光量子点的吸收谱

      Figure 3.  Normalized EL spectra of devices R, G, B and TOLED, and absorption(Abs.) spectra of red and green QDs

      As shown in Fig. 4, chromaticity coordinates of devices R, G and B are (0.70, 0.30), (0.24, 0.62), and (0.10, 0.20), respectively, and 75.2% NTSC color gamut is achieved by such RGB emission. Apparently, properties of quantum dot materials, such as narrow emission peaks and high color purity, make it easy to achieve wide color gamut. And color filters, which are widely used in liquid crystal display technology, can effectively improve the color quality. Here, full color emission is achieved by a simple method using the combination of blue top-emitting OLED, quantum dots down-conversion layers and color filters. Furthermore, there is still room for improvement in color gamut, such as selecting quantum dot materials with specific wavelength and matched filter. Emission wavelength of quantum dot materials can be easily tuned to cover entire visible wavelength range by changing particle sizes. Therefore, quantum dot materials with specific emission wavelength can be selected according to requirements for achieving wide color gamut.

      图  4  红、绿、蓝器件的色坐标

      Figure 4.  Chromaticity coordinates of devices R, G and B

      As a pump source, blue TOLED is critical to the resulting devices R, G and B, therefore high performance blue TOLED is important for constructing full color display using the method with down-conversion layer and color filter. As shown in Fig. 5, after a series of optimizations on the device structure, the optimized blue TOLED was obtained with a maximum current efficiency of 37 cd/A. After down-conversion and filter actions, devices R, G and B can still obtain a maximum current efficiency of 3.6, 21.9, and 10.6 cd/A, respectively. The inset is photographs of red and green down-conversion films consisting of quantum dots and micro-cylindrical lens array under ultraviolet light. It can be seen that the down-converting films produce very bright PL emission. The current density-voltage-luminance characteristics of these devices are shown in Fig. 6. Since the down-conversion film is placed outside the device and does not change the internal device structure, the current density of each device is nearly same. The feature allows blue TOLED and down-conversion films to be individually optimized, increasing the flexibility of device design.

      图  5  红、绿、蓝器件和器件TOLED的电流效率-亮度特性曲线,插图为红光与绿光量子点下转换膜在紫外光照射下的照片

      Figure 5.  Current efficiency-luminance characteristics of devices R, G, B and TOLED, the insets are photographs of down-conversion films with red and green QDs under UV light

      图  6  红、绿、蓝器件和器件TOLED的电流密度-电压-亮度特性曲线

      Figure 6.  Current density-voltage-luminance characteristics of devices R, G, B and TOLED

      Imprinting technology is a common method used for fabricating microstructures due to its simplicity, short manufacture time and low cost. Using imprinting technology, microstructure with high resolution and consistency can be obtained. As shown in Fig. 7, the down-conversion film obtained by the imprint method has a uniformly distributed cylindrical array on its surface. These micro-cylinders have a regular shape with a clear outline, and its diameter and height are about 20 μm and 5 μm, respectively. In our previous work, we compared the performance of OLEDs based on a down-conversion film without microstructure[19]. Results showed that the microstructure of the down-conversion film is beneficial to improve light extraction and angular characteristics of devices.

      图  7  微结构量子点下转换膜的扫描电镜图

      Figure 7.  Scanning electron micrograph(SEM) of down-conversion film with a micro-cylindrical lens array

      We also tested the spectral stability of devices R, G and B. Fig. 8 shows normalized spectra of devices R, G and B at different voltages. It can be seen that as voltage increases from 4 V to 8 V, the spectra of devices R, G and B remain stable. And the chromaticity coordinates of devices R, G and B only change (0.000 9, 0.001 6), (0.000 4, 0.000 6) and (0.000 9, 0.001 2), respectively. The stable spectra of devices R, G and B are obtained because their host device TOLED is a monochromatic device whose spectrum does not change with voltage. Under excitation of blue emission, the emission intensity of quantum dots is also stable so that the resulting devices R, G and B have very high spectral stability.

      图  8  红、绿、蓝器件在不同电压下的归一化发光光谱

      Figure 8.  Normalized EL spectra of devices R, G and B at different voltages

      Fig. 9 shows normalized EL spectra of devices R, G and B under different viewing angles. Device R has a very high spectral angular stability since its spectrum hardly changes with viewing angle. The chromaticity coordinates of device R only change from (0.696 2, 0.304 1) at 0° to (0.692 9, 0.305 5) at 60°. When the viewing angle changes from 0° to 60°, the intensity of the side peak(at 508 nm) of the green device is gradually weakened relative to the main peak(at 550 nm), and the chromaticity coordinates also change from (0.244 1, 0.619 8) to (0.265 3, 0.622 1). It is mainly because the emission of the green device consists of two parts, wherein the main peak is derived from PL emission of green light quantum dot and the side peak is derived from EL emission of TOLED. And the PL emission of green quantum dots is not affected by the microcavity effect, while the EL emission of TOLED is. Due to microcavity effect, EL emission of TOLED is mainly concentrated the forward direction. As the angle increases, intensity of the EL emission of TOLED decreases. However, the microstructure of the down-conversion film can scatter emission from device TOLED and down-conversion layer to make emission more dispersed. Eventually, the spectral angular stability of the green device is still very high. Since emission of the blue device is completely derived from the device TOLED so that blue emission is influenced by the microcavity effect. Nevertheless, as the angle increases, the spectrum of device B still keep stable. The color coordinates of device B only change from (0.100 7, 0.195 5) to (0.105 2, 0.174 7) when viewing angle increases from 0° to 60°.

      图  9  红、绿、蓝器件在不同视角下的归一化发光光谱

      Figure 9.  Normalized EL spectra of devices R, G and B under different viewing angles

    • In this paper, red and green down-conversion films with micro-cylindrical lens array were fabricated using CdSe/ZnS quantum dots and polymer PMMA. And RGB OLEDs were successfully constructed by combining highly-efficient top-emitting blue OLEDs with down-conversion films and color filters. These RGB OLEDs show a maximum current efficiency of 3.6, 21.9, and 10.6 cd/A, respectively. Color purities of the red, green and blue devices are high, and their chromaticity coordinates are (0.70, 0.30), (0.24, 0.62) and (0.10, 0.20), respectively. 75.2% NTSC color gamut can be achieved by such RGB emission. Besides, these RGB OLEDs also show high color stability under different bias voltage and viewing angle. This study provides a simple and feasible method for achieving full color OLED display.

      ——中文对照版——

    • 有机电致发光器件(OLED)因其特有的轻薄、自发光和柔性等特点,在显示领域中占有重要的地位。OLED技术经过长期发展[1-5],已经逐渐开始产业化,基于OLED的智能手机、电脑和电视等产品已经开始大规模量产。目前,实现全彩OLED的方式主要有3种。其中之一为OLED显示面板制造企业多数采用的红(R)、绿(G)、蓝(B)像素独立发光法。这是一种基于精细金属掩模板的真空蒸镀技术,但其制作难度大,成本高,且对金属掩模板的尺寸精度和定位精度的要求都非常苛刻。而且由于不同发光颜色的有机材料的寿命不同,从而导致显示器件存在色彩失真的问题。另一种方法是将白光OLED与彩色滤光片(CF)结合[6-7],通过对白光进行滤色来制作RGB OLEDs。这种方法不需要掩模对位,降低了制作难度与生产成本。但传统的白光OLED通常为多层结构,器件内部的激子复合区会随电压的改变而发生移动,从而使光谱发生变化[8-9]。这些问题都直接影响了OLED商业化的进展。

      除了以上两种方法,还有一种采用下转换结构来实现全彩OLED的方法。一般采用蓝光OLED作为泵浦源,并用其产生的蓝光来泵浦下转换材料,从而得到其他颜色的光[10-13]。这种方法的制作过程简单,无需掩模,在材料的选择上也更灵活,适合制作大尺寸器件。而且红光与绿光的强度受蓝光控制,器件的色稳定性较强。但这种技术存在效率较低的问题,其主要原因在于下转换层转换效率不高。目前采用的下转换材料多为有机材料,而有机材料的吸收通常位于紫外波段,从而导致蓝光泵浦的转换效率过低。量子点(QDs)作为一种新型的发光材料,目前在发光器件应用方面取得了很大的进展。它具有颜色可调、光化学稳定性高等优势,而且它的发光光谱非常窄,有利于在显示器件上应用。更重要的是其具有很高的光致发光效率,因而将其作为下转换材料是目前量子点材料的一个重要应用方向[14-15]

      相比于普通的底发射器件,顶发射器件可制备于任意衬底上。这样就可以将有源驱动OLED(AMOLED)的像素驱动电路制作在器件的下方,以消除像素发光面积与驱动电路间相互的竞争,从而提高开口率。但顶发射器件存在较严重的光谱角度稳定性问题,不同视角下的色坐标变化较为明显,不适合制作大面积显示器件。为了改善顶发射器件的角度特性,可以在器件顶电极外部引入具有微结构的光散射层[16-17]。它可以在不影响顶发射器件内部结构的情况下,对光进行散射,从而改善顶发射器件的角度特性。综合考虑,可以将微结构阵列制作在下转换层表面,使其同时具有下转换和散射的作用。此外,光线在下转换层中传输时,由于折射率不匹配,在下转换层与空气界面处存在全反射。这会使一部分光被局限在下转换层内部,使器件的发光效率下降。针对这一问题,在下转换层表面制作微结构阵列可以减弱这种全反射[18],把原本局限在下转换层内部的光提取出来,从而提高器件的效率。

    • 制作顶发射OLED器件时,首先用Decon 90洗涤剂对玻璃衬底进行清洗,然后在去离子水中进行超声清洗,干燥后再进行紫外臭氧处理。经过这一系列步骤后,将干净的衬底放进多源有机分子气相沉积系统,准备进行顶发射蓝光OLED的制备,并使用石英晶体膜厚监测仪来同步观察不同金属层及有机层的沉积速率和厚度。在进行蒸镀的整个过程中,蒸镀速率控制在约为0.1 nm/s,且腔室始终维持在约6×10-4 Pa的真空度。

      采用的红光与绿光CdSe/ZnS量子点购买自苏州星烁纳米科技有限公司,其中红光量子点的PL峰位于630 nm,半峰宽为25 nm,量子产率≥85%。绿光量子点的PL峰位于550 nm,半峰宽为25 nm,量子产率≥85%。制作微结构量子点下转换膜时,首先将100 μL的聚甲基丙烯酸甲酯(PMMA)甲苯溶液(浓度为300 mg/mL)分别与1 μL的红光CdSe/ZnS量子点甲苯溶液和10 μL的绿光CdSe/ZnS量子点甲苯溶液(量子点溶液的浓度分别为100 mg/mL和50 mg/mL)混合,并搅拌5 min,得到均匀混合的量子点-PMMA甲苯溶液。将上述两种量子点-PMMA混合溶液分别滴涂在硅基微结构模板上,并在70 ℃下加热40 min,通过加热来促进溶液挥发,最后将加热固化后的微结构量子点下转换膜与硅基微结构模板分离。

      器件的亮度、电流、电压以及电致发光光谱等性能均是采用日本大塚分布式光度计测试系统(GP-500)进行测试。各器件的测试皆在室温条件下的大气环境中进行。

    • 器件结构如图 1所示,顶发射蓝光OLED(器件TOLED)被用作泵浦源,分别将表面具有微型圆柱阵列的红光量子点下转换膜和绿光量子点下转换膜覆盖在器件TOLED上,量子点吸收部分蓝光的能量,产生光致发光(PL),从而得到红光与绿光。然后通过彩色滤光片的过滤作用,进一步提高色纯度,并最终得到RGB三组器件。器件TOLED的结构为玻璃衬底/银(Ag,100 nm)/三氧化钼(MoO3,3 nm)/4, 4′-环己基二(N, N-二(4-甲基苯基)苯胺)(TAPC,40 nm)/4, 4′, 4′-三(咔唑-9-基)三苯胺(TCTA,5 nm)/TCTA:双(4, 6-二氟苯基吡啶-N, C2)吡啶甲酰合铱(TCTA:Firpic,25 nm)/1, 3, 5-三((3-吡啶基)-3-苯基)苯(TmPyPB,50 nm)/8-羟基喹啉锂(Liq,2 nm)/钐(Sm,20 nm)/TAPC(60 nm),其中发光层TCTA:Firpic的掺杂比例为8:1。

      实验所采用的滤光片的透过率如图 2所示,每种滤光片仅允许特定波长的光通过,而将其他波长的光过滤掉,能够大幅提高器件的色纯度和色域。同时,滤光片的透过率也对器件的性能非常重要,更高的透过率意味着有更多的能量被利用,进而提高整个器件的效率。

      图 3是器件的归一化发光光谱以及两种量子点的吸收谱。器件TOLED的发光峰位于475 nm,同时在500 nm左右有一个侧峰。将器件TOLED与滤光片结合得到蓝光器件,其发光峰位于475 nm。过滤后得到的绿光器件的主峰位于550 nm,这主要来源于绿光量子点的PL。同时绿光器件在508 nm处存在一个侧峰,其主要来源于器件TOLED的电致发光,这部分光在下转换过程中未被绿光量子点吸收,在穿过下转换膜后与量子点的PL发光混合在一起。红光器件的发光峰位于635 nm,这来源于被蓝光激发的红光量子点。

      图 4所示,RGB三组器件的色坐标分别为(0.70,0.30)、(0.24,0.62)和(0.10,0.20),器件达到了75.2%的NTSC色域。量子点材料的发光峰非常窄,色纯度高,非常适合应用于显示领域。同时,彩色滤光片已经在液晶显示中被广泛采用,且技术较为成熟,能够有效地提升显示器件的色彩品质。本文通过将顶发射OLED与量子点和彩色滤光片结合,通过一种较为简单的方式,实现了顶发射OLED的全彩化。这种器件的色域具有较大的提升空间,可通过选择特定发光波长的量子点材料与合适的滤光片来进一步提升色域。而且,可通过改变其自身尺寸来调节量子点材料的发光颜色,发光波长基本覆盖了整个可见光范围,种类丰富。可根据不同需求来选择具有特定发光波长的量子点材料,从而改变器件的色坐标与色域。

      器件TOLED作为泵浦源,它的性能对于最终得到的RGB-OLEDs至关重要,而这也是采用下转换结构制作显示器件的关键问题。如图 5所示,通过对器件结构的一系列优化,本文最终制备的顶发射蓝光器件TOLED的最大电流效率达到了37 cd/A。过滤后得到的RGB三组器件的最大电流效率分别为3.6、21.9和10.6 cd/A。插图为两种颜色的微结构量子点下转换膜在紫外光照射下的照片,可以看到下转换膜产生了明亮的发光。器件的电流密度-电压-亮度特性曲线如图 6所示,由于下转换膜覆盖在器件外部,不会改变器件的内部结构,所以各器件的电流密度相同。同时,在实验过程中,这种特点也允许对器件TOLED和下转换膜各自单独进行优化,增加了器件设计的灵活性。

      压印技术是一种制作微结构的常用方法,其具有制作工艺简单、加工时间短和成本低等优势,而且制得的微结构能达到较高的分辨率,一致性好。本文通过压印法制作的微结构下转换膜,如图 7所示,其表面具有均匀分布的圆柱阵列。可见,这些微型圆柱的形状规则,轮廓清晰,直径约为20 μm,高度约为5 μm。在本课题组之前的工作中,比较了采用表面平坦和表面具有微结构的下转换膜的器件性能[19]。结果显示,下转换膜表面的微结构对器件的光取出和角度特性具有较为明显的改善作用。

      接下来,对器件的光谱稳定性进行了测试。图 8是RGB三组器件在不同电压下的归一化光谱,可以看到,随着电压由4 V增加到8 V,RGB器件的光谱保持稳定,色坐标仅分别改变了(0.000 9, 0.001 6)、(0.000 4,0.000 6)和(0.000 9,0.001 2)。这是因为器件TOLED是单色器件,其光谱不随电压发生变化。而在稳定的蓝光激发下,量子点的发光光谱也保持稳定,所以最终得到的RGB OLEDs具有非常高的光谱稳定性。

      图 9是器件在不同视角下的光谱。首先,红光器件具有非常强的光谱角度稳定性,它的光谱几乎不随角度发生改变,色坐标仅由0°时的(0.696 2,0.304 1)变化到60°时的(0.692 9,0.305 5)。而在角度由0°变化到60°时,可以观察到绿光器件的侧峰(位于508 nm)的强度相对于主峰(位于550 nm)逐渐减弱,色坐标也由(0.244 1, 0.619 8)变化到了(0.265 3, 0.622 1)。这主要是因为绿光器件的发光由两部分组成,其中主峰来源于绿光量子点,而侧峰来源于器件TOLED。绿光量子点不受微腔效应影响,发光更为分散,强度会随着角度的增加而缓慢减弱。但由于顶发射器件TOLED存在微腔效应,它的发光主要向正前方集中,随着角度增加,发光强度会快速减弱。这两部分光在角度增大时产生衰减的速度不同,两者间的相对比例将发生改变,从而导致整个光谱发生变化。而下转换膜表面的微结构阵列,可以对器件TOLED发射的较为集中的光进行散射,改变光的传输路径,使其变得更加分散。最终,这两部分光随角度增加而产生衰减的速度变得更加接近,绿光器件的光谱角度稳定性得到改善。由于蓝光器件的发光全都来源于器件TOLED,因此受到微腔效应的影响,随着角度增加,光谱逐渐蓝移,色坐标由(0.100 7, 0.195 5)变化为(0.105 2, 0.174 7)。

    • 本文采用CdSe/ZnS量子点与聚合物PMMA制备了表面具有微型圆柱阵列的红光与绿光量子点下转换膜,并通过与高效顶发射蓝光OLED和彩色滤光片结合,成功制备了RGB OLEDs,实现了顶发射OLED的全彩化。红、绿、蓝三组器件的色坐标分别为(0.70,0.30)、(0.24,0.62)和(0.10,0.20),采用以上红、绿、蓝单色器件为三基色的彩色显示的色域达到了75.2%NTSC,且同时具有较强的光谱电压稳定性和光谱角度稳定性。本项研究为OLED显示器件的产业化提供了一种简单可行的方法。

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