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Quantum-Dot and Quantum-Rod Displays – the Next Big Wave

Photoluminescent and electroluminescent quantum dots and quantum rods are the subject of rapidly expanding research efforts.  These materials possess performance benefits – particularly in the areas of luminescence and color – that the authors believe will reshape the display industry as we know it.

by Kai Wang and Xiao Wei Sun

ALTHOUGH there are different opinions as to what the hottest topics in display technology currently are, no one can deny that quantum dots (QDs) are among the hottest.  At Display Week 2016, we witnessed a dramatic increase in quantum-dot activity.  The first quantum-dot presentation at SID’s annual Display Week technical symposium – an invited paper from Samsung – was made in 2010.  It was, therefore, amazing to find that just 6 years later there were five full sessions dedicated to QDs and quantum rods (QRs) and eight sessions containing QD/QR presentations.  There were a total 51 papers related to QDs/QRs at Display Week 2016, including both oral and poster presentations.

Defining Quantum Dots

QDs, QRs, the larger perovskite QDs, and other luminescent nanocrystals (LNCs) have rapidly been developed in both academia and industry due to their outstanding luminescence.  Their performance is especially good for color,   including highly saturated colors (narrow-bandwidth emission), precisely tunable emission wavelengths based on quantum-size effects, and high quantum yields, which are beneficial for wide-color-gamut display and high-quality (meaning a high color-rendering index) lighting.  In 2013, Sony released the first commercialized QD-enabled display – its Triluminous TV, in which a Color IQ optical subsystem from QD Vision was adopted to enhance LCD panels.  According to a report from Custom Market Insights earlier this year, the overall QD flat-panel-display market will reach US$110 billion by 2017.1

There are two types of QD/QR displays: photoluminescent (PL), in which QDs/QRs are used as backlights for LCDs, and electroluminescent (EL), in which QDs/QRs are self-emissive through electrical excitation.  For PL, the next challenges concerning LNCs for the display and lighting industries will probably concern new materials that are more environmentally friendly, with higher quantum yields and even better color saturation (narrower emission) as well as new LNC composites with better long-term operational stabilities.  Moreover, some rod-shaped LNCs with strong polarized emission also have huge potential in decreasing the power consumption of LCD panels.  For EL, there are many issues that need to be resolved, such as QD surface modification for EL applications, QD ink for printed displays, balance in carrier injection, appropriate hole-transport materials, etc.  In this article, we will provide a brief review of the emerging QD and QR display technologies, including those designed to meet the aforementioned challenges that were presented at Display Week 2016.

Quantum Dots for PL Applications

Due to environmental regulations surrounding cadmium, performance benchmarking between cadmium-containing and cadmium-free QD displays has been highlighted of late.  At Display Week 2016, QD Vision demonstrated quantitatively that cadmium selenide (CdSe) based systems outperform indium phosphide (InP) based systems in terms of luminance, color gamut, and power consumption.2,3  The TVs used in the study included four different configurations of backlit LCDs: (1) CdSe QDs in edge-optics configuration, (2) QD enhancement film (QDEF), (3) InP-based QDEF, and (4) conventional white LEDs with red and green phosphors.  The demonstration showed that the CdSe QD solutions achieved the widest color gamut and the highest energy efficiency.  Though improvements have been made in InP QDs, their deficiencies in full width at half maximum (FWHM), efficiency, and operational stability indicate a performance gap compared to that of CdSe QDs.  InP QDs show a wider FWHM, causing a drop in color gamut.  Moreover, the lower external quantum efficiency of InP QDs decreases the luminance and increases the energy usage of these displays.  In addition, non-QD solutions showed even poorer performance compared to their QD counterparts.  At the exhibition at Display Week, QD Vision showed a demo of three QD-backlit LCDs and an OLED display.  Figure 1 shows performance benchmarking of TVs based on OLEDs (203 W), CdSe QDs (148 W), InP QDs (202 W), and white LEDs (160 W).  The TV based on CdSe QDs shows the most vivid display performance and the lowest power consumption.  (Note: specs including the model and year of the OLED TV used for comparison are not known.)

Fig. 1:  QD Vision showed side-by-side performance benchmarking of TVs with different technologies demonstrated at the show.  Top left: OLED; top right: CdSe QDs (Color IQ); bottom left: white LED; bottom right: InP QDs.

It should be noted that quantum-dot-maker Nanosys also had a similar side-by-side TV demonstration at Display Week 2015 (for which the company won a Best-in-Show award), and also in 2016.

QD Vision also proposed a new standard called “Color Nits” for measuring brightness and luminance in the latest high-performance displays that support high dynamic range and wide color gamut.4  The “Color Nits” metric is derived from a formula that takes into account varying spectral profiles and the subtle differences between perceived brightness and actual luminance.  The new concept integrates aspects of the so-called Helmholtz– Kohlrausch effect, which states that more-saturated colors can appear brighter than less-saturated ones of equivalent luminance, meaning that wider color gamut results in higher perceived brightness levels.  QD Vision called the measuring standard the first comprehensive metric that will be able to compare the brightness of all Rec.2020 display implementations.

Although LCD TVs that use cadmium-containing QDs have the potential to provide the best display performance, according to the European Union’s Restriction of Hazardous Substances (RoHS) regulations, the cadmium concentration in consumer-electronics products must be less than 100 ppm in order to be compliant.  Currently, there is an exemption in the European Union for cadmium in display applications, but it is due to expire in June 2018; therefore, a “greener” QD is a high priority for the industry.

Nanosys has developed a new type of “greener” quantum dots, its Hyperion QDs, by combining green CdSe and red InP QDs.5  The FWHM color saturation of Hyperion QDs is less than 25 nm for green and 42 nm for red. [25 nm is very narrow for green QDs, but 42 nm is rather large for red QDs.  The FWHM of red QDs is always less than 35 nm (even 30 nm) for cadmium-based QDs.]  Benefiting from this, LCD panels using quantum-dot-enhanced film (QDEF) with Hyperion QDs are able to reach a high level of >90% BT.2020 gamut as shown in Fig. 2(a).  Most importantly, the cadmium concentration of the Hyperion QDs is less than 95 ppm, which fully meets the requirements of RoHS.  This is an important advancement in color performance for consumer electronics.

Nanoco demonstrated a new heavy-metal-free QD film, CFQD, at Display Week, as shown in Fig. 2(b), by using a roll-to-roll manufacturing process.6,7  This red QD film is flexible, color tunable, highly efficient, easy to handle, and can be cut to any shape.  The film can be integrated into an existing light fixture with paired phosphor/LED backlights to produce a light with an appropriately correlated color temperature (CCT) of 6500K (to replicate daylight) and a high color-rendering index (CRI) with Ra > 95 and R9 = 92, at a luminous efficiency of 109 lm/W.

Fig. 2:  (a) Nanosys modeled gamut coverage of different films with wavelengths optimized for maximum BT.2020 coverage.5  (b) Nanoco has developed cadmium-free quantum-dot film rolls and strip lights.

Chinese quantum companies also showed eye-catching performance.  Najing Technology Company, with support from Zhejiang University, showed a new method to synthesize QDs with narrow FWHM (<20 nm) and high PL quantum efficiencies at a low temperature of 160°C.8  In products, the PL quantum yield of the QDs for light-converting devices remained high.  In addition, QD light-converting films based on the QDs were demonstrated, as shown in Fig. 9(a).

Zhonghuan Quantum Tech, another rising QD company with the support of the authors’ institution, the Southern University of Science and Technology (SUSTech), released a new technology based on quantum-dot luminescent microspheres (QLMS).9,10  QLMS, as shown in Fig. 3, is a new type of a highly robust QD composite featuring high efficiency, narrow FWHM, and excellent long-term operational stability.  It is actually a sphere full of QDs coated with inorganic and organic protection layers.

Fig. 3:  Zhonghuan Quantum Tech has developed quantum-dot luminescent microsphere (QLMS) technology, which incorporates on-chip-type LEDs based on green and red QLMS and their display applications.9,10

QLMS is fully compatible with current LED packaging processes and can be used as phosphors for direct on-chip applications, meaning tubes or films are not required.  QLMS will make it more convenient and cost effective for manufacturers to adopt QDs in flat-panel displays with wide color gamut and LED lighting with better color rendering.  For example, QLMS-based white LEDs with a CCT of 5500K, high luminous efficiency of 142.5 lm/W, and a high color-rendering index of Ra = 90 and R9 = 95 provide more vivid rendering colors of objects.

Yonsei University proposed an advanced LCD with patterned QD film and a short-pass reflector (SPR) that uses the principle of distributed Bragg reflection.11  As shown in Fig. 4(a), the patterned QD film and SPR are applied to an LCD to enhance the optical efficiency dramatically by separating the green and red QDs matched with their respective color filters (CFs) to reduce light-energy attenuation by using color-filter selection and absorption and reflecting the backward emission of QDs to a forward direction, preventing the absorption loss of backward emission by the backlight unit.

Compared to an LCD with a mixed QD film, the optical power of blue, green, and red light in an LCD with a patterned QD film and SPR is increased by 869%, 256%, and 85%, respectively.  Moreover, the LCD with the patterned QD film and SPR results in an increased color gamut with a broad-band color filter while maintaining the maximum optical intensity.

National Chiao Tung University proposed a new method to deposit low-heavy-metal (ZnCdSeS) and heavy-metal-free (ZnCuInS/ZnS) (Cd<100 ppm, Pb-free) based RGB quantum dots on top of the conventional twisted-nematic (TN) liquid-crystal device to improve the viewing angle of the LCD, as shown in Fig. 4(b).12  The non-vacuum low-materials-consumption solution process by aerosol-jet deposition has the advantage of direct printing on black-matrix-patterned glass.  The eco-QD-array TN-LCD demonstrates wide viewing angle, high transmittance, and fast response time (<3 msec) under low driving voltage (4 V).

Fig. 4:  (a) The schematic mechanism of an LCD with patterned QD film and SPR structure11 and (b) an eco-QD array on a twisted-nematic LC cell architecture.12

The University of Central Florida optimized optical efficiency and color gamut simultaneously to realize Rec.2020 for PL QD LCDs.13  Results indicate that 97% of the Rec.2020 color gamut can be achieved while maintaining a reasonably high optical efficiency for both fringe-field-switching and multi-domain vertical-alignment LCDs by properly selecting QD wavelengths and slightly modifying the transmission spectra of the color filter, which means the Rec.2020 color gamut is no longer exclusive to laser displays.

Since the expected lifetime of a television product is at least 20,000–30,000 hours, accelerated test methods and platforms should be developed for QD backlights specifically.  3M developed measurement techniques that allow researchers to directly measure and then estimate how long the QD-based film can survive in many typical display applications.14 3M created four accelerated aging platforms – High Intensity Light Testers (HILTs), Screening High Intensity Light Testers (SHILTs), Testboxes, and Mini Testboxes – to help predict in-device lifetime.  These systems enable various levels of control over the sample temperature and the blue flux to which the sample is exposed.  Table 1 shows the specifications of the accelerated aging platform proposed by 3M for QD-based film.


Table 1:  3M’s specifications for its accelerated aging platform for QD-based film include High Intensity Light Testers (HILT), Screening High Intensity Light Testers, (SHILT), Testboxes (Testbox), and Mini Testboxes (Mini).14
   
Accelerated Aging Platform
Parameter Application SHILT HILT Testbox Mini Testbox
Temperature (°C) ~50 50–85 85 50–85 50
Blue Incidence (mW/cm2) <10 ~1000–7500 300 ~20 400
Configuration Recycling System Single pass Single pass Recycling Recycling
Time to L85*/∆x, ∆y < 0.01 (hours) 20,000–30,000 ~10–50 1200 ~1500–3000 150
*L85 = 85% of luminance emission at beginning of life.

Quantum Rods for PL Applications

Besides QDs, semiconductor quantum rods are attractive emerging nanomaterials that have been applied to displays and lighting.  A quantum rod is a type of core-shell nano-crystal with an aspect ratio of more than 1:1 (e.g., 5:1).  There are two main types of quantum rods: dot-in-rod and rod-in-rod.  Display and lighting applications utilize the characteristic properties of QRs, e.g., tunable emission spectra, large absorption cross section, large Stokes shift, and especially polarized emission along their long axes derived from their anisotropic crystal structures.  A backlight that emits linearly polarized light would make it possible to increase the transmission through the polarizers.  Such properties are universally applicable for colloidal core-shell QRs, e.g., heterogeneous CdSe/CdS dot-in-rods.  These features make QRs promising for low-power and wide-color-gamut LCDs.

Merck demonstrated the use of cadmium selenide/cadmium sulfide (CdSe/CdS) in two configurations for display applications, including the backlight and emissive color pixels for LCD and OLED panels, respectively.15,16  For the backlight, sheets consisting of nanofibers with embedded QRs were fabricated by collecting electrospun nanofibers on a drum rotating at high speed.  The sheets, as shown in Fig. 5(a), exhibited polarized fluorescence emission with a polarization ratio of 0.6 as well as a very high out-coupling efficiency.  Moreover, Merck fabricated QR emissive color pixels with a resolution of 81 ppi using an ink-jet-printing process, as shown in Fig. 5(b)).  The color gamut of the pixels covers 70% of the BT.2020 standard.  This demonstration indicated that it is possible for QRs to be used in color pixels in LCDs.

Fig. 5:  (a) A polarized emission of the quantum-rod nanofiber sheet image and (b) a micrograph of color pixels filled with green and red QRs. (Merck)15,16

The Centrum for Angewandte Nanotechnologie successfully developed a continuous flow reactor for producing high-quality CdSe/CdS quantum rods in red and green for optoelectronic devices.17  The polarization of a single QR reaches 0.9.  This development is important for the systematic investigation of yield and polarization of these QRs in relation to properties such as length, width, and core size.

Ghent University showed that QRs can be aligned with their long axis parallel to an applied electric field with sufficient amplitude by using the dip-coating technique.18  A substrate with ITO electrodes in a finger pattern was slowly pulled out of a suspension with CdSe/CdS quantum rods while an ac electric field was applied.  The Ghent researchers demonstrated that this procedure makes it possible to align and fix the QRs in the desired orientation.  The resulting layer emits linearly polarized light with a high degree of polarization, as shown in Fig. 6(a).

The Hong Kong University of Science and Technology disclosed thin liquid-crystal polymer (LCP) films with uniformly dispersed QRs.19.  Photoalignment has been used to align the QR in the LCP thin film as shown in Fig. 6(b).  An order parameter of 0.87 was achieved.  The QR films illuminated by the blue light emit polarized light with an extinction ratio of 5:1.  The proposed QREF film shows an extended color gamut like QDEF film, but also an increase in the polarization efficiency by 20% compared to the existing system.

Fig. 6:  Images from Ghent University include (a) top left: the fluorescence of oriented QRs under a polarizing microscope; top right: the dependency of the PL intensity on the azimuth angle of the polarizer; and bottom: the fluorescence microscopy of a particular region on the substrate with oriented QRs with the polarizer parallel (left) and perpendicular (right) to the applied electric field.18  (b) Hong Kong University of Science and Technology’s image shows the process flow of QR-dispersed aligned LCP films.19

The Southern University of Science and Technology (SUSTech) proposed a new tributyl-phosphine-assisted method to synthesize uniform CdSe/CdS quantum rods having a peak emission at 630 nm, high absolute quantum yield, a narrow FWHM of 27 nm, and a large Stokes shift of 165 nm.20  The alignment of the quantum-rod nanofibers was achieved by electrospinning.  A large-scale active luminance enhancement film (ALEF) based on well-aligned quantum-rod nanofibers was fabricated with a polarization of 0.45 over a 5-cm2 area, as shown in Figs. 7(a)–7(c).9,21  When the ALEF is adopted in a backlight, the luminance of the LCD can be improved by 18.4%.  This research indicates that ALEF is a very promising backlight solution for highly efficient and wide-color-gamut LCDs.

Some perovskite QDs also possess the feature of polarized emission.  Nanograde showed cadmium-free CsPbX3 (X=Cl, Br, and I) perovskite QDs, with an FWHM of 20 nm for a 530-nm peak wavelength and 30 nm for a 630-nm peak wavelength, for wide-color-gamut displays.22  This RoHS-conforming material depicts a performance better than that of CdSe.  Moreover, SUSTech demonstrated full inorganic CsPbX3 (X=Br, I, and mixed halide systems Br/I) perovskite QDs with peak wavelengths from 517 to 693 nm and narrow FWHM from 22 to around 30 nm, which can meet 103% of the Rec.2020 standard,23  More importantly, as shown in Fig. 7(d), some perovskite QDs showed highly polarized photoluminescence.  The polarization of CsPbI3 is as high as 0.36 in hexane and 0.40 in film without any alignment.  The CsPbX3 perovskite QDs with a narrow FWHM and high polarization properties seem to possess great potential for display applications.

Fig. 7:  These images from SUSTech include (a) a transmission-electron-microscopy image of quantum rods in QRs/PMMA nanofiber; (b) an ALEF under daylight (top) and 365-nm UV light (bottom); (c) an LCD without ALEF (top) and with ALEF (bottom)21 and (d) polarizations of different perovskite QDs.23

Quantum-Dot LEDs (QLEDs)

Thus far, we have discussed the latest progress of PL QD/QR displays presented at Display Week 2016.  Although PL QD/QR displays can achieve very impressive performance in terms of wider color gamut and lower power consumption, EL QD displays possess more advantages, not only in wide color gamut but also in flexibility, fast response, and printable displays.  Many companies, such as Samsung, BOE, TCL, etc., have put a lot of effort into EL QD displays.  The QD LED (QLED) is the core device for EL QD displays.  In the following, we will introduce QLED technologies showcased at Display Week 2016.

NanoPhotonica demonstrated multilayered QLED structures that exhibited high current and power efficiencies of 6.1 cd/A and 5.0 lm/W, 70 cd/A and 58 lm/W, and 12.3 cd/A and 17.2 lm/W for blue-, green-, and red-emitting QLEDs, respectively, as shown in Fig. 8.24  The QLEDs exhibit highly tunable emission spectra, constituting the first report of QLEDs covering a wide color gamut of ~90% of the Rec.2020 standard.  The QLEDs also show a nearly Lambertian angular emission pattern, desirable for wide viewing angles in displays.  Furthermore, they demonstrated a champion green QLED exhibiting 21% external quantum efficiency (EQE), 82 cd/A, and 79.8 lm/W, the highest reported green QLED efficiency in the literature and also comparable to phosphorescent OLEDs used in commercial AMOLED displays.  In addition, encapsulated green and red QLEDs show outstanding lifetimes of >100,000 and >280,000 hours (as predicted from early life-testing data).

Fig. 8:  Images from NanoPhotonica include (a) a schematic of the device structure and energy-level diagram of existing QLEDs with ZnO nanoparticles as the electron-transport layer; (b) the normalized EL spectra of red, green, and blue QLEDs with a multilayered structure and an inset of images of red, green, and blue QLEDs; and (c) the current efficiency and EQE vs. luminance of a champion green QLED from NanoPhotonica.24

Najing Technology Company also showed RGB-colored QLEDs with EQEs of 20%, 18%, and 14% for red, green, and blue colors, respectively, as shown in Fig. 9(b).  With EL peaks at 640 nm (red), 467 nm (blue), and 520 nm (green), one could obtain QLEDs in a wide color gamut, namely, 106.7% of Rec.2020.8

Fig. 9:  A QLCD with film types (QLCF) in manufacturing lines and (b) QLED devices in operation (Najing Tech).8

Brunel University in the UK reported red QLEDs that exceed the color coordinates requirement set by Rec.2020.25  By using appropriate device architecture and QD size, researchers achieved a high current efficiency of 4 cd/A for A CIE(x, y) of (0.708, 0.292), meeting the Rec.2020 specification.  A high efficiency of 10 cd/A at 10,000 cd/m2 and 5.8 lm/W for A CIE(x, y) of (0.695, 0.305) was achieved by employing a novel hole transporter.  The researchers also demonstrated devices that exceeded the Rec.2020 color coordinates (namely, 0.712, 0.288). Besides using these red QLEDs alone in HDTV with a wide color gamut, this development promises the possibility of hybrid pixels (red QD, green PHOLED, and blue TADF) in the near future.

The University of Central Florida proposed a hybrid white OLED (WOLED) technology that combines QD narrow red emitters with existing state-of-the-art blue and green organic emitters for high-efficacy high-color-quality solid-state lighting, as shown in Fig. 10(a).26  Spectra analysis indicates this approach will lead to white OLEDs (WOLEDs) that could achieve high color quality (CRI ≥ 91, R9 ≥ 32) at a high luminous efficacy of radiation (≥359 lm/W).  In addition, the University of Central Florida researchers demonstrated an integrated sensing platform based on QLEDs.27  The use of QLEDs as an excitation source can improve the power efficiency by 57% and above.

SUSTech has developed transparent and all-solution-processed QLEDs.28  Sputtered ITO was adopted as transparent electrodes.  To reduce the plasma damage caused by the sputtering, ZnO nanocrystals with a thickness of 82 nm were employed as buffer layers and electron-transport layers.  As a result, damage- free QLEDs were demonstrated with a high averaged transparency of 70% as shown in Figs. 10(b) and 10(c).  The transparent QLEDs exhibited an EQE of 5% and a current efficiency of 7 cd/A, which is comparable to that of devices using conventional Al electrodes.

Fig. 10:  Image (a) shows a tandem structure for a red QLED integrated hybrid WOLED.26  Image (b) demonstrates the transmission spectrum of the ITO electrodes and devices, along with an inset of the QLEDs.  Image (c) is a photo of the transparent QLEDs in front of a mirror (SUSTech).28

Prepare for a Quantum Leap

With the advantages of wide color gamut (narrow FWHM), size-dependent emission wavelength, high quantum efficiency, low power consumption, flexibility, printability, and low cost, QD/QR technology is emerging in displays.  This represents a revolution in color: PL QD/QR display technology will rejuvenate LCD technologies in terms of color, and EL QD technology based on QLEDs will challenge the dominant position of OLEDs in flexible displays.  The next big wave of display technology is coming.  By using quantum dots and quantum rods, we may anticipate some change in the game of displays.

References

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2S. Coe-Sullivan, “The Quantum Dot Revolution: Marching Towards the Mainstream,” SID Digest of Technical Papers (2016).

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4QD Vision, http://coloriq.com/

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13R. Zhu, Z. Luo, H. Chen, Y. Dong, and S-T. Wu, “Quantum Dot LCDs for Rec.2020,” SID Digest of Technical Papers (2016).

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20J. Qin, J. Hao, W. Chen, J. Deng, D. Wang, B. Xu, D. Wu, and K. Wang, “A Rapid, Highly Emissive Procedure Synthesize of Giant Pure Red Core-Shell Quantum Rods by Using Modified Tributylphosphine Assisted Method, SID Digest of Technical Papers (2016).

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24J. R. Manders, J. Hyvonen, A. Titov, K. P. Acharya, J. Tokarz-Scott, Y. Yang, W. Cao, Y. Zheng, L. Qian, J. Xue, and P. H. Holloway, “High Efficiency and Ultra-Wide Color Gamut Quantum Dot LEDs for Next Generation Displays, SID Digest of Technical Papers (2016).

25P. Kathirgamanathan, M. Kumaraverl, L. M. Bushby, S. Ravichandran, N. Bramananthan, and S. Surendrakumar, “Quantum Dot Electro-luminescence: towards Achieving the REC 2020 Colour Co-Ordinates,” SID Digest of Technical Papers (2016).

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28W. Wang and S. Chen, Transparent Quantum Dot Light Emitting Diodes with Sputtered ITO Electrodes,” SID Digest of Technical Papers (2016).  •


Kai Wang and Xiao Wei Sun are with the Department of Electrical & Electronic Engineering in the College of Engineering at Southern University of Science and Technology (SUSTech), in Shenzhen, Guangdong, China.  They can be reached at wangk@sustc.edu.cn and sunxw@sustc.edu.cn, respectively.