Color Conversion Using Quantum Dots for LCD, OLED, and MicroLED Displays†

Ravi Tangirala*, Ernest Lee*, Charlie Hotz*, Yuichiro Kunai**, Yoshifumi Komatsu**, Yoshihiro Harada**, Megumi Komada**, Masayoshi Tokuda**, Tomohiro Fukuura**

*Nanosys, Inc., Milpitas CA **Sumitomo Chemical Co., Ltd., Osaka, Japan

Abstract Quantum dot color conversion technology will lead to displays with higher brightness, color tunability and viewing angles. We have developed Cd-free QD-photoresist (QDPR) formulations containing specially designed QDs with enhanced blue absorption and green/red emission to achieve >90% BT.2020 coverage at feature resolution of <10 µm. QDPR devices can be fabricated on existing lithographic equipment, making this technology attractive for all kinds of displays including LCD, OLED and microLED.

Author Keywords Quantum dots; color conversion; QD-OLED; microLED; LCD; QD-photoresist;

1. Introduction

Quantum dot enhancement film (QDEF) has enabled LCDs with greater color tunability and brightness, and is now part of the mainstream display market. Quantum dot color conversion (QDCC) technology is the next step in the evolution of quantum dot-based displays, featuring patterned subpixels of red or green QDs in place of traditional pigment-based color filters. The color ‘conversion’ approach, in which white light is generated by absorption of blue light and reemission of red or green, leads to higher efficiencies than the traditional color ‘filtering’ approach. Any blue light source can potentially be used as the backlight in these displays, as shown in Figure 1. This allows QDCC to be applied to LCD, OLED or microLED displays, and harness the benefits of each kind.1

When applied to LCDs, QDCC moves the color generation from the back of the display panel to the front, leading to wider viewing angles than conventional LCDs.1 Applying QDCC to OLEDs can combine the greater contrast achieved by OLEDs with higher brightness and wider color gamut (>90 % BT.2020) achievable through QDs, all at a potentially lower manufacturing cost than current OLED displays.2 For microLEDs, QDCC presents a scalable alternative to the pick-and-place manufacturing method where blue, green and red microLEDs are fabricated separately, diced and then moved into position on the display panel. Since only blue LEDs are needed for operation with the QD layer, the manufacturing process is much simplified.3

QDCC films can be fabricated using additive manufacturing methods such as inkjet printing4 as well as large-scale patterning methods such as photolithography5 or transfer printing.6 Both approaches rely on very high QD loading to convert blue light to green or red within a relatively thin layer. QDs are specifically tailored for high blue absorption and efficient conversion to green or red at the target wavelengths.

For lower resolution (<100 ppi) displays, inkjet printing is attractive due to more efficient materials utilization, gentler processing conditions and lower waste generation. However, the capital expenditure in installing inkjet printing equipment can be a barrier for many applications. The photolithographic approach using a QD-photoresist (QDPR) mixture is more versatile and can be used in applications with a wide range of feature sizes and form factors. An added benefit is that the QDPR route allows users to leverage the existing installed capacity of photolithographic equipment in fabs around the world.

In this paper, we will present current advances in QD development for QDCC technology, and the application of QDPR for color conversion in LCDs and microLEDs.

2. Materials and Methods

Material requirements: Several material requirements must be met for making QDCC a viable product. To achieve RoHS compliance, Cd-free QDs must be used. To achieve high color conversion efficiency, the QDs have to be dispersed well at high concentrations in layers that are 6-10 m thick. The matrix polymer has to be a negative photoresist that can be patterned to the desired feature sizes, down to <10 µm. To ensure retention of the QD optical properties, both the QD surface properties and the photoresist composition must be optimized. The QDs need to be air-processible and stable under various steps of the QDPR deposition, patterning and encapsulation processes, including high temperature baking in air, exposure to aqueous developers, and high vacuum processing. Furthermore, to avoid deactivation from the excited state by surface-mediated energy loss upon aggregation, the QDs must be kept well dispersed in the photoresist during all the above processing steps. Finally, the QDPR films must have operational lifetimes sufficient for a commercial product.

QDPR preparation:  Green and red Indium Phosphide (InP) core-shell QDs were synthesized by traditional methods.7 The

Figure 1. Schematics of QDCC layers applied to LCD, OLED and microLED displays.

LCD-QDCC OLED-QDCC MicroLED-QDCC

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native ligands attached to the QD surface were replaced by hydrophilic ligands through a ligand exchange process. This step makes the QDs compatible with the solvents or monomers used in photoresists. The QD  dispersions were then mixed at high concentrations (25-50% loading) with other components of the optimized photoresist, using special procedures to avoid aggregation of QDs. Scattering particles are added to the formulation to increase the optical path length and absorption of blue light, and also enhance the light extraction efficiency of converted light by reducing total reflection at the interface of the QDPR layer. However, diameter distribution, refractive index and loading of the scattering particle need to be optimized to ensure a low level of ambient light reflection from the QDPR film surface.

QDPR evaluation and sample fabrication: The QDPR formulations were characterized by spin-coating single color films, pre-baking (100 C, 3 min), curing (365 nm, 80 mJ), and post-baking (180 C, 60 min). Emission spectra, λpeak, and FWHM of the films were then measured by placing the films between a blue LED backlight and a detector with an integrating sphere (Ocean Optics: QE65 Pro). Optical density was estimated using a UV-Vis-NIR spectrophotometer (Shimadzu: UV-3600), and quantum efficiency was determined by absolute PL quantum yield spectrometer (Hamamatsu photonics: C9920-02G).

Figure 2 shows the emission spectra (top) and optical properties (bottom) or representative green and red QDPR films. The incident blue light at 450 nm is absorbed by the QDPR layer and converted to green and red respectively with high efficiency.

The optical density, emission wavelength and peak width (FWHM) can all be tuned by varying the film thickness and the ratio of QD to other components in the film. The small amount of blue light leaking through can be eliminated by applying an additional color filter on top of the QDPR layer. In the final

display, this color filter layer is also required for avoiding ambient light reflection from the QDPR film surface.

Figure 3 shows cross-sectional TEM images of patterned films containing QDs. Figure 3a shows the example of a well optimized QDPR formulation, in which QDs are dispersed uniformly even after post-baking. This is achieved through careful design of the polymer structure for promoting good initial QD dispersion, as well as the use of a robust backbone structure that lends durability in high temperature processing conditions. On the other hand, Figure 3b and Figure 3c show partial and full aggregation during post-baking process due to the lack of enough structural optimization. Figure 3d schematically shows how QDs can be deactivated upon aggregation.

3. Evaluation of QDPR patterning For evaluating the deposition and patterning of QDPR films, we spin-coated films on glass or Si wafers, then pre-baked them at 100 C for 3 min. Patterning was performed by exposing the films to 365 nm UV light at a dose of 80 mJ through a mask, followed by development for 1 to 1.5 min using a commonly used KOH developer. Finally, we obtained patterned QDPR films after post-baking them at 180 C for 60 min.

Figure 4a) and b) show a top-view of patterned QDPR features on glass, illustrating precisely controlled red and green stripe patterns properly formed by conventional photolithography technology. Figure 4c) and d) show SEM cross-sectional images of these patterned stripes. By varying the curing conditions, we were able to make the sidewalls more vertical and avoid undercutting. Furthermore, it would be expected in actual devices that black matrix around each pixel could support the vertical shape of the sidewalls.

Green Red

Thickness 10 µm 10 µm

Optical density 1.23 1.80

QY (%) 60% 72%

λpeak 542 642

FWHM 35 37

Figure 2. Emission spectra (top) and optical properties (bottom) of red and green QDPR films with 10 m

thickness.

Figure 3. Cross-sectional TEM images of patterned films after post-baking. a) Well-dispersed QDs with fully-

optimized chemical structure of both QD surface and photoresist, considering high temperature durability. b)

Partially aggregated QDs with optimized only considering initial dispersion. c) Fully aggregated QDs with no

optimization. d) Schematic of energy deactivation of QDs due to aggregation.

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4. Display properties and lifetime

We tested the capability of QDPR films to achieve a wide color gamut by measuring the color coordinates of single-color films. While current films can achieve >75% BT.2020 coverage, this can be improved further by reducing blue light leakage and optimizing the emission wavelengths. Table 1 shows the gamut coverage achieved by the films from Figure 2, as well as the maximum gamut that can be achieved by optimizing the wavelengths and combining the QDPR layer with an additional color filter layer to reduce blue light leakage.

The final device containing QDCC film has to meet operational lifetimes relevant for a commercial product. We tested the stability of a QDPR film sandwiched between a glass substrate and a commercially available barrier film (inorganic barrier layer on PET film; 10-1 to 10-2 g/m2/day at 40 C 90% RH) under a variety of durability conditions. Figure 5 shows the lifetime for representative single color films under high temperature (80 C dry) and high temperature and high humidity (60 C 90% RH) conditions which are commonly used as typical durability conditions in the display field. The QDPR films show enough stability for a long time even under these severe conditions, and are applicable for all kinds of displays, including LCD, OLED and microLED.

5. Impact of Your Research

QD color conversion is applicable for all types of displays, including LCD, OLED and microLEDs. This technology will result in displays with higher efficiency, wider color gamut and viewing angles, while reducing costs by utilizing existing photolithography infrastructure. QDPR can be used for applications across a variety of feature sizes, but will particularly benefit devices with high resolution, such as microdisplays for AR/VR, wearables and mobile applications.

6. References

1. Luo, Z., Yurek, J. "Quantum dots: The technology platform for all future displays", Proc. SPIE 10940, Light-Emitting Devices, Materials, and Applications, 1094010 (1 March 2019)

2. DSCC/OLED-A Quarterly OLED Material Report

3. B. Boerger, N Van Wyck. 2017. “Rapid Assembly of Quantum Dots for MicroLED Micro-Displays.” https://www.ledinside.com/knowledge/2017/11/rapid_assembly_of_quantum_dots_for_microled_micro_displays.

4. Aya Ishizuka, Eiji Otsuki, Takeo Kizaki, Takayuki Miki, Sunao Yoshihara, Ikuro Kiyoto, Ernest Lee, Ravi Tangirala, Charlie Hotz, Heejae Kim, Meeting 5-1: Invited Paper, Cadmium-free Quantum Dot Inkjet Inks for Color Conversion Devices, IDW'18, Dec 2018, Japan.

5. E. Lee, S. Kan, C. Hotz, J. Yurek, Z. Luo, J. Yamanaga, A. Carpenter. 2017. “Ambient Processing of Quantum Dot Photoresist for Emissive Displays.” SID Symposium Digest of Technical Papers. Volume 44, Issue 1, pages 984-987.

6. T.-H. Kim, K. -S. Cho, E. K. Lee, S. J. Lee, J. Chae, J. W Kim, D. H. Kim, et al., “Full-colour quantum dot

Figure 4. Optical images of a) green QDPR and b) red QDPR patterned features on glass. Numbers on the top indicate line width (m). SEM images of c) top-view and

d) cross-sectional-view on these patterned subpixels.

Table 1. Gamut coverage of current QDPR films, and with optimized wavelengths and additional color filter

Wavelengths BT.2020

coverage

Current QDPR films from Figure 2

450/542/642 77%

With optimized wavelengths and additional color filter

462/524/646 96%

Figure 5. Stability test results for red and green encapsulated QDPR films at a) 80 C dry and

b) 60 C 90%RH for up to 1000 hrs.

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displays fabricated by transfer printing”, Nat. Photon. 5, p. 176-182, 2011

7. For example, C.B. Murray, C.R. Kagan, M. G. Bawendi (2000). "Synthesis and Characterization of Monodisperse

Nanocrystals and Close-Packed Nanocrystal Assemblies". Annual Review of Materials Research 30 (1): 545–610.

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