Environmentally Friendly Quantum Dots for Display Applications

E. Jang SAIT, Electronics, Suwon, Korea, email: [email protected]

Abstract— Ever since the physics of (QD) was Color IQ technology, which was the glass tube optic using Cd- discovered, much research effort has been carried out for based QD. After that, TCL, Philips, BOE, Hisense and Konka more than 30 years, and lots of applications adopting QDs applied the same technology for their TV and monitor have been proposed. Especially, wide color gamut displays products. Nanosys has been very active in commercializing using QDs as active light emitting materials have drawn QD-film through the joint development with film-fabricating much attention. And, the QD-based consumer displays such companies such as 3M, LMS, Hitachi, and Nitto Denko. as LED TVs, tablets, and special monitors are now on the Starting from Amazon’s Kindle fire tablet, QD color market. They provide best color gamut, reasonable power converting film, which is called as QDEF, was adopted in the ASUS’s laptop and monitor, and TVs of Hisense, TCL, Vizio, efficiency, and affordable price showing superior competitive AUO. In China, Najing technology started to provide QDs to edge to OLED technology. However, still there are issues film makers such as Sangbo and Poly-OE, and work with TCL and argues using containing materials in practical and Hisense to produce TVs. Nanoco is one of the pioneer consumer devices. In spite of the European RoHS companies to have developed the Cd-free QDs from the Exemptions, we need to be aware the environmental risk of beginning, and they have collaborated with Dow chemical and producing large quantity of Cd-containing materials and Merck for the mass-production. using them in the consumer electronics. And, this growing apprehension for environmental issues formed great We reported the fabrication of 46” prototype TV with limitation for QD’s applications. Therefore, we have white LED BLU using Cd-based QDs in 2010 [1], and dedicated to develop more environmentally friendly InP completed the QD film technology using Cd-based QDs in based QDs that showed considerably high efficiency and 2012. However, we decided not to commercialize the QD TV products because we concluded not to use any saturated color spectrum compared to the Cd-containing environmentally harmful materials inside our products even materials. The structure of Cd-free QD was specially tailored though the minimal usage of Cd was still legitimate according for display applications and the synthetic process was to the RoHS exemptions. After the decision, we started the optimized to produce reliable materials in commercial scales. development of the QDs with no environmental problem to In order to improve the efficiency and stability of the QDs in solve the health and safety issues over entire manufacturing the devices operating under severe atmosphere, specific processes. Aside from the environmental policy, it was the composite materials were designed and the fabrication right time to start another challenge on the new materials process was optimized. From 2015, Samsung has released because high levels of performances with Cd-QDs were Cd-free QD adopted UHD TV for major product line-up already achieved. The RoHS exemption had been renewed which show the best color gamut among the current displays. several times to allow the usage of Cd-QDs in electronic Now we are trying to make additional breakthroughs in devices for a while, but in 2018, the European Parliament displays by using established QD material platform and finally decided not to extend the exemption after 2019. broaden the technology to wider optoelectronic applications. II. CD-FREE QD I. INTRODUCTION Many researchers from academia and industries have been interested in the InP QD synthesis. In a quite early stage, InP After the quantum confinement effect was discovered in QDs were prepared in strongly coordinating solvent and the early 1980’s, many chemists tried to prepare ideal ligands, and as a result, the nanoparticle growth took more quantum dot materials and develop synthetic methods for than days. Then, Peng [2] reported very effective reaction uniform size, core/shells, and shape-controlled structures. conditions using octadecene as a solvent, so that the crystal Alongside the fundamental investigations, many potential growth became very fast compared to the previous method. applications were suggested such as laser, bio labeling, His continuous report [3], which was about the wavelength printable TFT, color converting LED, electrically driven LED, control of InP from 450nm~750nm and the possibility of , as well as memory devices, and these are all making stable InP QDs in hydrophilic conditions, patented by several leading technology groups. From 2010, considerably alleviated the concerns about the practical industries started to produce prototypes and commercial applications using InP QDs. After that, subsequent reports products in display applications such as large sized TVs, disclosed the method to improve the quantum efficiency and tablets, laptops, and monitors. Fig.1 shows the recent FWHM as well as the understanding of the surface structures industrial technology trends of QD display development and and growth mechanisms [4~6]. The progress of QDs was changes in exemptions in European RoHS. The first summarized in Fig. 2 in terms of QE according to the emission commercial product came out from Sony with QD Vision’s

978-1-7281-1987-8/18/$31.00 ©2018 IEEE 38.2.1 IEDM18-875 range. After the preparation of the first core/shell structure of composition to make patterns through jetting process. Cd-QDs, which showed 50% QE, it took about 20 years to Compared to the conventional LCDs, the emissive color filter prepare QDs with 100% QEs, and it’s still hard to find showed almost perfect viewing angle from 178 degree. Also, efficient blue light emitting one. For InP QDs, the progress much attention was paid to the electrically driven QD-LEDs was relatively very fast and it took less than 10 years to show (Fig. 9). Unlike the color converting applications, a blue light higher than 85%. Now, our QDs showed higher than 95% of emitting QD is required to be developed. For InP, the bandgap QE, and there are only small rooms for the improvement in of the bulk is about 1.4eV, the diameter should be reduced to 2 each color. To obtain efficient InP QDs, we designed multi nm for the blue emission. We prepared the InP QDs emitting element core and gradient shell structure. ZnSe and ZnS at 460nm with relatively broad FWHM and 60% QE. Another composition gradient was controlled to reduce the lattice promising candidate for blue emission is ZnSe QD, and now mismatch between InP and ZnS (Fig. 3). The oxidative many people focus on this structure [7, 8]. In QD-LED device interface was induced to remove defects on the interfaces of fabrication, it is important to control ligands of QDs for III-V core and II-VI shell. To enhance the crystallinity, we efficient charge transfer. Other charge transport layers should maintained relatively high temperature (about 340C) during be optimized and stabilized with more robust structure. Fig. 10 the synthesis. Also, we tried to design the process to be as shows the progress in QD-LEDs including both of Cd-QDs simple as possible to cut down the cost, and increase the and Cd-free QDs. Currently, Cd based QD-LEDs show very production yield by controlling the uniformity of size, shape, high external QEs which are close to theoretical limit, and the and suppressing by-products. performances of the Cd-free QD LED improves quite fast. However, QD-LED device fabrication process involves We have optimized critical parameters in synthesis to solution printing steps which have many latent issues of purity make the process repeatable and reproducible in large scale. and reliability. As the soluble inkjet process develops, We have collaborated with Hansol chemical since 2013 and fundamental understandings of the interfaces are required in transferred our synthetic recipe after a verification of the the nano/micro structures of layered LEDs. reaction conditions with 20 L bench reactor in the lab. Currently, we prepared green and red QDs with FWHM less V. SUMMARY than 33nm in the lab, and these were reproduced as 35nm in We have developed efficient Cd-free QD materials, and the production scale (Fig. 4). produced them in large scale. In display applications, InP based QDs have been used as color converters first, in the film III. QLED TV and second in the color filter structure. In those applications, The current Samsung QLED TVs comprise InP QD color the formulation technology of QDs with polymers has been converting films and blue LEDs. After we prepared the green developed as well, and this can be expanded to patterning and red QDs, mixed them with photo-curable polymers, then technologies such as photo lithography and inkjet printing. For made films with protective barrier against moisture and the QD-LEDs, charge injection and recombination properties oxygen. Finally, the QD film was put in front of blue light have been investigated to optimize the device performances. guiding panel, then it became a white BLU which showed Beyond the display technology using QDs, the sensor devices wider color gamut than conventional phosphor white LED- using NIR light absorbing QD materials are one of our BLU (Fig. 5 & 6). There are several standards for color space, interests in the future. and we set the color coordination of green and red QDs to REFERENCES maximize the agreement to the DCI standard. [1] E. Jang, S. Jun, H. Jang, J. Lim, B. Kim, and Y. Kim, “White-Light- When we developed the TV set, we organized a cross Emitting Diodes with Quantum Dot Color Converters for Display functional team covering materials synthesis, interface design, ”, Adv. Mater., 22, pp3076–3080, 2010 product evaluation, and mass production. It was also [2] D. Battaglia, and X. Peng, "Formation of High Quality InP and InAs necessary to handle the environmental issue, predict market Nanocrystals in a Noncoordinating Solvent" Nano Letters, 2 (9), pp 1027–1030, 2002 size, estimate, cut down the cos, build the eco system, and [3] R. Xie, D. Battaglia, and X. Peng, “Colloidal InP Nanocrystals as make a strong patent position. Since we released InP based Efficient Emitters Covering Blue to Near-Infrared” J. Am. Chem. Soc., QD TVs in 2015, there have been progresses in brightness 129 (50), pp 15432–15433, 2007 and color gamut of TVs. The most recent product showed [4] L. Li and P. Reiss, “One-pot Synthesis of Highly Luminescent InP/ZnS Nanocrystals without Precursor Injection”, J. Am. Chem. Soc., 130 (35), average brightness of 800nit with 100% color volume. And, pp 11588–11589, 2008 we are still working to reduce the FWHM of green QD to less [5] J. Lim, W. Bae, D. Lee, M. Nam, J. Jung, C. Lee, K. Char, and S. Lee, than 30nm for targeting BT2020 as a next step. “InP@ZnSeS, Core@Composition Gradient Shell Quantum Dots with Enhanced Stability”, Chem. Mater., 23 (20), pp 4459–4463, 2011 IV. FUTURE DISPLAY APPICATIONS [6] P. Ramasamy, N. Kim, Y. Kang, O. Ramirez, and J. Lee, “Tunable, There are other chances to use QD optical modules instead Bright, and Narrow-Band Luminescence from Colloidal Indium Phosphide Quantum Dots”, Chem. Mater., 29 (16), pp 6893–6899, of the film such as emissive color filter using blue BLU or 2017 pixelated micro LEDs (Fig. 7). The advantage of using [7] H. Shen, W. Cao, N. Shewmon, C. Yang, L. Li, and J. Xue, “High- emissive color filter with backlights is the prefect viewing Efficiency, Low Turn-on Voltage Blue-Violet Quantum-Dot-Based angle (Fig. 8), and, QD is the only light emitter for this Light-Emitting Diodes” Nano Lett., 15 (2), pp 1211–1216, 2015 application in terms of the size and stability. To make the QD [8] H. Shen, W. Cao, N. Shewmon, C. Yang, L. Li, and J. Xue, “High- Efficiency, Low Turn-on Voltage Blue-Violet Quantum-Dot-Based color filter, we prepared QDs and mixed them with photoresist Light-Emitting Diodes” Nano Lett., 15 (2), pp 1211–1216, 2015 to make patterns through photo-lithography, or formulate ink

IEDM18-876 38.2.2 Form ‘13 ‘14 ‘15 ‘16 ‘17

QD Vision Cd-QD Samsung M&A (Boston, 200kg) TCL QD Rail (TV) SONY Philips, BOE (Cd) (TV, 13 line-up) (Monitor) Hisense, Konka (TV)

Nanosys: Cd-QD Cd-QD Cd-free QD (Milpitas, 2000kg) 3M LMS/Hitachi QMC/Nitto Denko Dongwoo Finechem (QDEF) (QDEF) (Film) DIC (Color filter) Amazon ASUS Hisense TCL Vizio, AUO ASUS (Kindle) (Laptop) (TV) (TV) (TV) (Monitor) QD Film Najing Tech Sangbo Poly-OE TCL Hisense (Cd QD) (Cd/Cd-free) (Film) (Film) (TV) (TV) Cd-free QD Nanoco/Dow Chem. Wah Hong Nanoco/Merck (Mass Production) (Film) (Mass Production) Samsung Cd-free QD Samsung Samsung ‘10 QD-LED TV (SUHD TV) (QLED TV) (Monitor) Samsung proto

Exemption#39  expire Exemption#39 (lighting)  Exemption#39  EP Veto Exemption#39 replace  CdSe QDs color converting RoHS July ‘14 extend to July ’17 (May ‘15) display (2 years)  Expire for all categories on Oct. ‘19

Fig. 1. Recent industrial developments of QD displays and exemptions in RoHS

SAIT SAIT ('13/100%) Peng ('10/100%) 100 ('16/100%) SAIT* SAIT* SAIT ('17/95%) ('15/85%) Ajou Univ ('10/95%) Henan Univ. ('16/85%) SAIT SNU Abdullah Gul Univ. 80 SNU ('15/83%) ('03/85%) ('13/81%) ('16/78%) ('11/80%) DGIST SAIT Hongik Univ ('17/67%) CEA-CNRS ('15/63%) ('05/60%) ('10/70%) 60

(%) Univ. East Anglia Ajou Univ. Ghent Univ. Ajou Univ. NUS (‘08/60%) ('11/60%) ('15/60%) ('16/60%) QE U. Chicago ('03/50%) Ocean Univ. ('99/50%) ('10/45%) Cd-free Red QY (%) 40 Peng Peng CEA-CNRS Ajou Univ. Cd-free Green QY (%) ('03/40%) ('10/40%) ('16/40%) ('07/40%) Cd-free Blue QY (%) Cd Red QY (%) 20 Cd Green QY (%) KIST Cd Blue QY (%) ('12/5%) 0 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017 2019 Year Fig. 2. Progress of QDs’ quantum efficiency in the literatures Fig. 3. Optimized structure of green (left) and red (right) light emitting InP/ZnSeS QD

Fig. 4 Photo luminescent spectra of green and red QDs prepared in lab scale reactor and mass production (left), 20 L bench scale reactor (middle), 100L pilot scale reactor (right)

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Fig. 5. Structure of QLED TV 400 500 600 700 Wavelength (nm)

Fig. 6. The spectra of conventional phosphor BLU (up) and QLED BLU

90º 90º 135º 45º 135º 45º

Cathode (Al) 180º 0º 180º 0º Electron Injection Electron Transport 225º 315º 225º 315º QD Emission 270º 270º Hole Transport Anode (ITO) Conventional CF : 90º Emissive CF : 178º Substrate (Glass) Hole Injection

Fig. 8. Brightness at different viewing angle Fig. 9. Schematic drawing of current-driven Fig. 7. Schematic drawing of QD color filter with from the LCD display with conventional color QD-LED structure conventional LCD backlight filter (left) and QD color filter (right)

Peng NPI Changchun

Phosphorescent OLED (~20%) QD Vision NPI 10 SNU Yang Henan (430nm, 7.8%) Yang Fluorescent OLED (~6%) Ginger Bawendi Bawendi Yang (607nm, 2.5%) SAIT SAIT SNU (518, 3.5%) SAIT SNU

1 Changchun

(%) Giali Thermal SNU annealing EQE Alivisatos C/S QD

0.1 Bawendi Red QD (Cd) Green QD (Cd) Blue QD (Cd) Red QD (Cd-free) Green QD (Cd-free) Blue QD (Cd-free) Alivisatos SAIT 0.01 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017 2019 2021

Fig. 10. The progress of QD-LEDs’ external QE in the literature

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