微腔量子点发光二极管彩色像素化技术的研究

  基于II-VI族CdSe的胶体量子点发光二极管（QLED）具有发光效率高、色纯度高、可溶液加工等优点，是下一代新型显示的有力竞争者。为实现全彩QLED显示，量子点发光层必须进行精细的图形化，以形成红、绿、蓝色量子点阵列。目前，图形化量子点的方法主要有喷墨打印、转移印刷、光刻和电泳沉积量子点等，但这些方法都是直接对量子点进行图形化，在3次图形化的过程中，不可避免的会破坏量子点，使QLED的性能低于未进行图形化的QLED。除了直接图形化量子点，还可以通过白光QLED结合彩色滤光片实现红、绿、蓝光发射，但由于彩色滤光片的引入，大大降低了70%的亮度。因此，为满足高分辨率、高亮度、高性能显示的需求，本文提出一种微腔QLED彩色像素化技术。

  首先，采用红、绿、蓝混合量子点作为白光QLED的发光层，通过优化其比例，制备出了发光均匀的白光QLED器件，并研究了白光QLED中混合量子点的能量转移机制。

  其次，引入光学谐振腔，采用100 nm Ag和20 nm Ag作为高反射电极和半反射电极，通过IZO透明相位调节层改变谐振波长，把未图形化的量子点发光层的白光转换为红、绿、蓝单色光射出。所制备的微腔QLED器件在IZO透明相位调节层为50 nm、90 nm和130 nm 时分别发射红、绿、蓝光，在5.5 V电压下，亮度分别为22170、51930和3064 cd/m²。

  再次，通过激子寿命分析微腔对激子复合速率的影响，证明微腔能够加速激子辐射复合；同时，研究微腔中白光QLED的激子能量转移机制，得出微腔可以促进或者抑制量子点间的能量转移。这与微腔白光OLED的调制有着本质区别，且效率高于使用彩色滤光片的白光QLED器件。

  最后，通过光刻技术图形化微腔，制备出无需图形化量子点和无需彩色滤光片的全彩QLED显示器件，其分辨率为每英寸1700 像素（PPI），色域为111% NTSC。该量子点无损的图形化方法为QLED显示器件的高精度像素化提供了一条全新的技术路径，将促进QLED在高分辨率全彩显示如移动显示、微显示、AR/VR等新型显示上的应用。

  Colloidal quantum-dot light-emitting diodes (QLEDs) have the advantages of self-emission, high efficiency, narrow spectrum and low-cost solution processability. Thus, they have been widely investigated as one of the substitutes for organic light-emitting diodes (OLEDs) for next-generation displays. To realize full-color QLED displays, the quantum dots (QDs) must be finely patterned to form an array of red, green, and blue QDs in QLEDs. At present, the primary methods for patterning QDs are inkjet printing, transfer printing, photolithography and electrophoretic deposition. However, all of these methods will damage the QDs during the patterning process.  To avoid pattern the QDs directly, the red, green and blue emission can also be obtained by filtering the white emission of QLEDs with a pixelated color filter array. But the introduction of color filter dramatically reduces the brightness by 70%. Therefore, in order to achieve high-resolution, high-brightness, and high-performance displays, this thesis proposes a QLED color pixelation technology based on the microcavity structure.

  Firstly, we fabricated excellent white emission QLEDs by optimizing the mixing ratio of red, green and blue QDs. In addition, the energy transfer mechanism of mixed quantum dots in white QLEDs was investigated.

  Secondly, we designed a microcavity QLED by using an 100 nm Ag as highly reflective electrodes, a 20 nm Ag as semi-reflective electrodes, and an IZO as phase tuning layers. By varying the thicknesses of the phase tuning layers, the optical interference of the cavities can be manipulated, and consequently, the cavities can selectively convert the white emission of unpatterned QDs as saturated red, green and blue emission with the brightness of 22170, 51930 and 3064 cd/m² at 5.5 V, respectively.

  Thirdly, by analyzing the exciton lifetime, it is proved that the microcavity can accelerate the exciton recombination rate. Meanwhile, the exciton energy transfer mechanism of red, green and blue QDs in the microcavity was studied. It is concluded that the microcavity can promote or inhibit the exciton energy transfer between QDs, which is fundamentally different from the microcavity modulation of white OLEDs. And benefit from the modulation effect of microcavity, the device efficiency is higher than that of white QLEDs with color filters.

  Finally, we realized ultra-high resolution full-color QLED display with a high resolution (~1700 PPI) and a wide color gamut (111% NTSC, National Television Standards Committee) by the photolithographic technology. This QD-patterning-free, CF-free method provides a novel technical path for realizing high resolution QLEDs display, which will promote the application of QLEDs in various displays such as cell phone, virtual reality (VR) and augmented reality (AR) displays.

[1] DAI X, ZHANG Z, JIN Y, et al. Solution-processed, high-performance light-emitting diodes based on quantum dots[J]. Nature, 2014, 515(7525): 96-99.
[2] WON Y H, CHO O, KIM T, et al. Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes[J]. Nature, 2019, 575(7784): 634-638.
[3] PAL B N, ROBEL I, MOHITE A, et al. High-sensitivity p-n junction photodiodes based on PbS nanocrystal quantum dots[J]. Advanced Functional Materials, 2012, 22(8): 1741-1748.
[4] TAN Z, ZHANG F, ZHU T, et al. Bright and color-saturated emission from blue light-emitting diodes based on solution-processed colloidal nanocrystal quantum dots[J]. Nano Letters, 2007, 7(12): 3803-3807.
[5] SHEN H, CAO W, SHEWMON N T, et al. High-efficiency, low turn-on voltage blue-violet quantum-dot-based light-emitting diodes[J]. Nano Letters, 2015, 15(2): 1211-1216.
[6] MEDINTZ I L, UYEDA H T, GOLDMAN E R, et al. Quantum dot bioconjugates for imaging, labelling and sensing[J]. Nature Materials, 2005, 4(6): 435-446.
[7] KAGAN C R, LIFSHITZ E, SARGENT E H, et al. Building devices from colloidal quantum dots[J]. Science, 2016, 353(6302): aac5523.
[8] KIM J H, JO D Y, LEE K H, et al. White electroluminescent lighting device based on a single quantum dot emitter[J]. Advanced Materials, 2016, 28(25): 5093-5098.
[9] WANG R, SHANG Y, KANJANABOOS P, et al. Colloidal quantum dot ligand engineering for high performance solar cells[J]. Energy & Environmental Science, 2016, 9(4): 1130-1143.
[10] LI X, ZHAO Y B, FAN F, et al. Bright colloidal quantum dot light-emitting diodes enabled by efficient chlorination[J]. Nature Photonics, 2018, 12(3): 159-164.
[11] LEE K H, LEE J H, KANG H D, et al. Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large-sized quantum dots[J]. ACS Nano, 2014, 8(5): 4893-4901.
[12] RHEE S, CHANG J H, HAHM D, et al. Tailoring the electronic landscape of quantum dot light-emitting diodes for high brightness and stable operation[J]. ACS Nano, 2020, 14(12): 17496-17504.
[13] ROSSETTI R, NAKAHARA S, BRUS L E. Quantum size effects in the redox potentials, resonance Raman spectra, and electronic spectra of CdS crystallites in aqueous solution[J]. The Journal of Chemical Physics, 1983, 79(2): 1086-1088.
[14] JI W, LIU S, ZHANG H, et al. Ultrasonic spray processed, highly efficient all-inorganic quantum-dot light-emitting diodes[J]. ACS Photonics, 2017, 4(5): 1271-1278.
[15] PENG Z A, PENG X. Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor[J]. Journal of the American Chemical Society, 2001, 123(1): 183-184.
[16] PENG Z A, PENG X. Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative routes: nucleation and growth[J]. Journal of the American Chemical Society, 2002, 124(13): 3343-3353.
[17] DABBOUSI B O, RODRIGUEZ-VIEJO J, MIKULEC F V, et al. (CdSe) ZnS core-shell quantum dots: synthesis and characterization of a size series of highly luminescent nanocrystallites[J]. The Journal of Physical Chemistry B, 1997, 101(46): 9463-9475.
[18] CHEN O, ZHAO J, CHAUHAN V P, et al. Compact high-quality CdSe-CdS core-shell nanocrystals with narrow emission linewidths and suppressed blinking[J]. Nature Materials, 2013, 12(5): 445-451.
[19] REISS P, CARRIERE M, LINCHENEAU C, et al. Synthesis of semiconductor nanocrystals, focusing on nontoxic and earth-abundant materials[J]. Chemical reviews, 2016, 116(18): 10731-10819.
[20] BAE W K, KWAK J, PARK J W, et al. Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient[J]. Advanced Materials, 2009, 21(17): 1690-1694.
[21] ZOU Y, BAN M, CUI W, et al. A general solvent selection strategy for solution processed quantum dots targeting high performance light-emitting diode[J]. Advanced Functional Materials, 2017, 27(1): 1603325.
[22] EFROS A L, EFROS A L. Interband absorption of light in a semiconductor sphere[J]. Soviet physics Semiconductors, 1982, 16(7): 772-775.
[23] WANG Y, HERRON N. Nanometer-sized semiconductor clusters: materials synthesis, quantum size effects, and photophysical properties[J]. The Journal of Physical Chemistry, 1991, 95(2): 525-532.
[24] WANG X, SUN G, LI N, et al. Quantum dots derived from two-dimensional materials and their applications for catalysis and energy[J]. Chemical Society Reviews, 2016, 45(8): 2239-2262.
[25] HINES M A, SCHOLES G D. Colloidal PbS nanocrystals with size-tunable near-infrared emission: observation of post-synthesis self-narrowing of the particle size distribution[J]. Advanced Materials, 2003, 15(21): 1844-1849.
[26] LEE J, SUNDAR V C, HEINE J R, et al. Full color emission from II-VI semiconductor quantum dot-polymer composites[J]. Advanced Materials, 2000, 12(15): 1102-1105.
[27] MOREELS I, JUSTO Y, GEYTER B D, et al. Size-tunable, bright, and stable PbS quantum dots: a surface chemistry study[J]. ACS Nano, 2011, 5(3): 2004-2012.
[28] SHIRASAKI Y, SUPRAN G J, BAWENDI M G, et al. Emergence of colloidal quantum-dot light-emitting technologies[J]. Nature Photonics, 2013, 7(1): 13-23.
[29] WANG H C, LIN S Y, TANG A C, et al. Mesoporous silica particles integrated with all-inorganic CsPbBr3 perovskite quantum-dot nanocomposites (MP-PQDs) with high stability and wide color gamut used for backlight display[J]. Angewandte Chemie International Edition, 2016, 55(28): 7924-7929.
[30] SUN Y, SU Q, ZHANG H, et al. Investigation on thermally induced efficiency roll-off: toward efficient and ultrabright quantum-dot light-emitting diodes[J]. ACS Nano, 2019, 13(10): 11433-11442.
[31] BAE W K, PARK Y S, LIM J, et al. Controlling the influence of auger recombination on the performance of quantum-dot light-emitting diodes[J]. Nature Communications, 2013, 4(1): 1038.
[32] OTTO T, MVLLER M, MUNDRA P, et al. Colloidal nanocrystals embedded in macrocrystals: Robustness, photostability, and color purity[J]. Nano Letters, 2012, 12(10): 5348-5354.
[33] ZHANG F, WANG S, WANG L, et al. Super color purity green quantum dot light-emitting diodes fabricated by using CdSe/CdS nanoplatelets[J]. Nanoscale, 2016, 8(24): 12182-12188.
[34] DENG Y, LIN X, FANG W, et al. Deciphering exciton-generation processes in quantum-dot electroluminescence[J]. Nature Communications, 2020, 11(1): 2309.
[35] BURROWS P E, BULOVIC V, FORREST S R, et al. Reliability and degradation of organic light emitting devices[J]. Applied Physics Letters, 1994, 65(23): 2922-2924.
[36] COE S, WOO W K, BAWENDI M, et al. Electroluminescence from single monolayers of nanocrystals in molecular organic devices[J]. Nature, 2002, 420(6917): 800-803.
[37] CARUGE J M, HALPERT J E, BULOVIĆ V, et al. NiO as an inorganic hole-transporting layer in quantum-dot light-emitting devices[J]. Nano Letters, 2006, 6(12): 2991-2994.
[38] QIAN L, ZHENG Y, XUE J, et al. Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures[J]. Nature Photonics, 2011, 5(9): 543-548.
[39] ZHANG H, SUN X, CHEN S. Over 100 cd A-1 efficient quantum dot light-emitting diodes with inverted tandem structure[J]. Advanced Functional Materials, 2017, 27(21): 1700610.
[40] ZHANG H, CHEN S, SUN X W. Efficient red/green/blue tandem quantum-dot light-emitting diodes with external quantum efficiency exceeding 21%[J]. ACS Nano, 2018, 12(1): 697-704.
[41] WANG O, WANG L, LI Z, et al. High-efficiency, deep blue ZnCdS/Cd x Zn 1-x S/ZnS quantum-dot-light-emitting devices with an EQE exceeding 18%[J]. Nanoscale, 2018, 10(12): 5650-5657.
[42] SONG J, WANG O, SHEN H, et al. Over 30% external quantum efficiency light-emitting diodes by engineering quantum dot-assisted energy level match for hole transport layer[J]. Advanced Functional Materials, 2019, 29(33): 1808377.
[43] YANG Z, WU Q, LIN G, et al. All-solution processed inverted green quantum dot light-emitting diodes with concurrent high efficiency and long lifetime[J]. Materials Horizons, 2019, 6(10): 2009-2015.
[44] LI X, LIN Q, SONG J, et al. Quantum-dot light-emitting diodes for outdoor displays with high stability at high brightness[J]. Advanced Optical Materials, 2020, 8(2): 1901145.
[45] PU C, DAI X, SHU Y, et al. Electrochemically-stable ligands bridge the photoluminescence-electroluminescence gap of quantum dots[J]. Nature Communications, 2020, 11(1): 937.
[46] QIAN L, ZHENG Y, XUE J, et al. Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures[J]. Nature Photonics, 2011, 5(9): 543-548.
[47] SHEN H, GAO Q, ZHANG Y, et al. Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency[J]. Nature Photonics, 2019, 13(3): 192-197.
[48] CHEN S, CAO W, LIU T, et al. On the degradation mechanisms of quantum-dot light-emitting diodes[J]. Nature Communications, 2019, 10(1): 765.
[49] ANANDAN M. Progress of LED backlights for LCDs[J]. Journal of the society for information display, 2008, 16(2): 287-310.
[50] COE-SULLIVAN S, LIU W, ALLEN P, et al. Quantum dots for LED downconversion in display applications[J]. ECS Journal of Solid State Science and Technology, 2012, 2(2): R3026.
[51] SEKITANI T, NAKAJIMA H, MAEDA H, et al. Stretchable active-matrix organic light-emitting diode display using printable elastic conductors[J]. Nature Materials, 2009, 8(6): 494-499.
[52] DAI X, DENG Y, PENG X, et al. Quantum-dot light-emitting diodes for large-area displays: towards the dawn of commercialization[J]. Advanced Materials, 2017, 29(14): 1607022.
[53] WOOD V, PANZER M J, CHEN J, et al. Inkjet-printed quantum dot-polymer composites for full-color ac-driven displays[J]. Advanced Materials, 2009, 21(21): 2151-2155.
[54] ZHENG H, ZHENG Y, LIU N, et al. All-solution processed polymer light-emitting diode displays[J]. Nature Communications, 2013, 4(1): 1971.
[55] KIM B H, ONSES M S, LIM J B, et al. High-resolution patterns of quantum dots formed by electrohydrodynamic jet printing for light-emitting diodes[J]. Nano Letters, 2015, 15(2): 969-973.
[56] JIANG C, ZHONG Z, LIU B, et al. Coffee-ring-free quantum dot thin film using inkjet printing from a mixed-solvent system on modified ZnO transport layer for light-emitting devices[J]. ACS Applied Materials & Interfaces, 2016, 8(39): 26162-26168.
[57] LIU Y, LI F, XU Z, et al. Efficient all-solution processed quantum dot light emitting diodes based on inkjet printing technique[J]. ACS Applied Materials & Interfaces, 2017, 9(30): 25506-25512.
[58] YANG P, ZHANG L, KANG D J, et al. High-resolution inkjet printing of quantum dot light-emitting microdiode arrays[J]. Advanced Optical Materials, 2020, 8(1): 1901429.
[59] ROH H, KO D, SHIN D Y, et al. Enhanced performance of pixelated quantum dot light-emitting diodes by inkjet printing of quantum dot-polymer composites[J]. Advanced Optical Materials, 2021, 9(11): 2002129.
[60] KIM L A, ANIKEEVA P O, COE-SULLIVAN S A, et al. Contact printing of quantum dot light-emitting devices[J]. Nano Letters, 2008, 8(12): 4513-4517.
[61] KIM T H, CHO K S, LEE E K, et al. Full-colour quantum dot displays fabricated by transfer printing[J]. Nature Photonics, 2011, 5(3): 176-182.
[62] SUNG S H, YOON H, LIM J, et al. Reusable stamps for printing sub-100 nm patterns of functional nanoparticles[J]. Small, 2012, 8(6): 826-831.
[63] KIM T H, CHUNG D Y, KU J Y, et al. Heterogeneous stacking of nanodot monolayers by dry pick-and-place transfer and its applications in quantum dot light-emitting diodes[J]. Nature Communications, 2013, 4(1): 2637.
[64] CHOI M K, YANG J, KANG K, et al. Wearable red-green-blue quantum dot light-emitting diode array using high-resolution intaglio transfer printing[J]. Nature Communications, 2015, 6(1): 7149.
[65] NAM T W, KIM M, WANG Y, et al. Thermodynamic-driven polychromatic quantum dot patterning for light-emitting diodes beyond eye-limiting resolution[J]. Nature Communications, 2020, 11(1): 3040.
[66] PARK J S, KYHM J, KIM H H, et al. Alternative patterning process for realization of large-area, full-color, active quantum dot display[J]. Nano Letters, 2016, 16(11): 6946-6953.
[67] WANG Y, FEDIN I, ZHANG H, et al. Direct optical lithography of functional inorganic nanomaterials[J]. Science, 2017, 357(6349): 385-388.
[68] CHO H, PAN J A, WU H, et al. Direct optical patterning of quantum dot light-emitting diodes via in situ ligand exchange[J]. Advanced Materials, 2020, 32(46): 2003805.
[69] YANG J, HAHM D, KIM K, et al. High-resolution patterning of colloidal quantum dots via non-destructive, light-driven ligand crosslinking[J]. Nature Communications, 2020, 11(1): 2874.
[70] ZHAO J, CHEN L, LI D, et al. Large-area patterning of full-color quantum dot arrays beyond 1000 pixels per inch by selective electrophoretic deposition[J]. Nature Communications, 2021, 12(1): 4603.
[71] SONG K W, COSTI R, BULOVIĆ V. Electrophoretic deposition of CdSe/ZnS quantum dots for light-emitting devices[J]. Advanced Materials, 2013, 25(10): 1420-1423.
[72] HAN C W, TAK Y H, AHN B C. 15-in. RGBW panel using two-stacked white OLED and color filters for large-sized display applications[J]. Journal of the Society for Information Display, 2011, 19(2): 190-195.
[73] ISHIBASHI T, YAMADA J, HIRANO T, et al. Active matrix organic light emitting diode display based on “Super Top Emission” technology[J]. Japanese Journal of Applied Physics, 2006, 45(5S): 4392.
[74] LEE B, JU Y, HWANG Y I, et al. Micro-cavity design of bottom-emitting AMOLED with white OLED and RGBW color filters for 100% color gamut[J]. Journal of the Society for Information Display, 2009, 17(2): 151-157.
[75] KASHIWABARA M, YAMADA J, YOKOYAMA S, et al. Display unit, method of manufacturing same, organic light emitting unit, and method of manufacturing same: U.S. Patent 7,973,319[P]. 2011-7-5.
[76] WINTERS D L. Oled microcavity subpixels and color filter elements: U.S. Patent 7,180,238[P]. 2007-2-20.
[77] ZHANG W, LIU H, SUN R. Full color organic light-emitting devices with microcavity structure and color filter[J]. Optics Express, 2009, 17(10): 8005-8011.
[78] JOO W J, KYOUNG J, ESFANDYARPOUR M, et al. Metasurface-driven OLED displays beyond 10,000 pixels per inch[J]. Science, 2020, 370(6515): 459-463.
[79] ANIKEEVA P O, HALPERT J E, BAWENDI M G, et al. Electroluminescence from a mixed red-green-blue colloidal quantum dot monolayer[J]. Nano Letters, 2007, 7(8): 2196-2200.
[80] BAE W K, LIM J, LEE D, et al. R/G/B/natural white light thin colloidal quantum dot-based light-emitting devices[J]. Advanced Materials, 2014, 26(37): 6387-6393.
[81] LEE K H, HAN C Y, JANG E P, et al. Full-color capable light-emitting diodes based on solution-processed quantum dot layer stacking[J]. Nanoscale, 2018, 10(14): 6300-6305.
[82] ZHENG K, ZIDEK K, ABDELLAH M, et al. Directed energy transfer in films of CdSe quantum dots: Beyond the point dipole approximation[J]. Journal of the American Chemical Society, 2014, 136(17): 6259-6268.
[83] LEE K H, HAN C Y, KANG H D, et al. Highly efficient, color-reproducible full-color electroluminescent devices based on red/green/blue quantum dot-mixed multilayer[J]. ACS Nano, 2015, 9(11): 10941-10949.
[84] ZHANG H, SU Q, SUN Y, et al. Efficient and color stable white quantum-dot light-emitting diodes with external quantum efficiency over 23%[J]. Advanced Optical Materials, 2018, 6(16): 1800354.
[85] LIN C L, LIN H W, WU C C. Examining microcavity organic light-emitting devices having two metal mirrors[J]. Applied Physics Letters, 2005, 87(2): 021101.
[86] JORDAN R H, DODABALAPUR A, SLUSHER R E. Efficiency enhancement of microcavity organic light emitting diodes[J]. Applied Physics Letters, 1996, 69(14): 1997-1999.
[87] JUNG B Y, KIM N Y, LEE C, et al. Control of resonant wavelength from organic light-emitting materials by use of a Fabry-Perot microcavity structure[J]. Applied Optics, 2002, 41(16): 3312-3318.
[88] LIU G, ZHOU X, CHEN S. Very bright and efficient microcavity top-emitting quantum dot light-emitting diodes with Ag electrodes[J]. ACS Applied Materials & Interfaces, 2016, 8(26): 16768-16775.
[89] ITO N, SATO Y, SONG P K, et al. Electrical and optical properties of amorphous indium zinc oxide films[J]. Thin Solid Films, 2006, 496(1): 99-103.
[90] ZHANG H, SU Q, CHEN S. Quantum-dot and organic hybrid tandem light-emitting diodes with multi-functionality of full-color-tunability and white-light-emission[J]. Nature Communications, 2020, 11(1): 2826.
[91] KIM S K, PARK M J, LAMPANDE R, et al. Primary color generation from white organic light-emitting diodes using a cavity control layer for AR/VR applications[J]. Organic Electronics, 2020, 87: 105938.
[92] BENISTY H, DE NEVE H, WEISBUCH C. Impact of planar microcavity effects on light extraction-Part I: Basic concepts and analytical trends[J]. IEEE Journal of Quantum Electronics, 1998, 34(9): 1612-1631.
[93] BULOVIĆ V, KHALFIN V B, GU G, et al. Weak microcavity effects in organic light-emitting devices[J]. Physical Review B, 1998, 58(7): 3730.
[94] NEYTS K A. Simulation of light emission from thin-film microcavities[J]. Journal of the Optical Society of America, 1998, 15(4): 962-971.
[95] CHO H, CHUNG J, SONG J, et al. Importance of purcell factor for optimizing structure of organic light-emitting diodes[J]. Optics Express, 2019, 27(8): 11057-11068.