The Dawn of QLED for the FPD Industry

Rapid improvements in the operating lifetimes of QLEDs, as well as investments into large-scale production lines for inkjet printing, are bringing the electroluminescent application of quantum dots for flat-panel displays closer to commercialization.

by Chaoyu Xiang, Weiran Cao, Yixing Yang, Lei Qian, and Xiaolin Yan

Not long ago, colloidal quantum dots (QDs) emerged as excellent emitters for high-quality flat-panel displays (FPDs). These well-dispersed, nanosize semiconductors offer ideal optical properties for display applications. Due to quantum-confinement effects, QDs’ electrons and holes are trapped in “quantum wells,” bonded as excitons, which are like states in the molecules. The exciton confinement energy determines the QDs’ optoelectronic properties.

This is the beauty of QDs. The “quantum wells” structures, which define the lowest energy state for excitons, reduce the radiative recombination paths, enabling purer emission spectra. Changing the size and shape of QDs makes it possible to manipulate the confinement energy. As a result, the absorption and emission properties of QDs can be tuned to cover the full visible spectrum.

Moreover, the sizes and shapes of QDs can be easily controlled during the colloidal QD synthesis. After years of development, the so-called core-shell structure of QDs was widely adapted to improve photoluminescence quantum yield (PLQY) by enforcing the confinement effect through reducing the nonradiative recombination paths. Nowadays, near-unity PLQY can be achieved. All those properties and factors promise lower energy consumption, higher color purity, and larger color gamut through the use of QDs in FPDs.

Development of QD Applications in FPDs

QDs have been successfully incorporated into LCD products as the downconversion media that generate the white light in backlight units (BLUs). In 2014, several panel makers, including Samsung and TCL, launched their first QD-LCD products. Using QD enhancement films (QDEFs) enabled the panel makers to improve color gamut and enhance luminance at the same time. Thanks to the QDEF technology, LCD was able to rally in the market battle against organic light-emitting diodes (OLEDs) in the high-end, large-area TV market.

More advanced technologies using QDs for downconversion have been proposed in recent years; these include QD color filter (QD-CF) and QD-OLED (incorporating the downconversion of red and green light from QDs on top of blue light from OLEDs). However, a few technical obstacles exist regarding the mass production of these proposed QD photoluminescent technologies. Considering the developing states of red, green, and blue QD materials, the soonest foreseeable technology is the aforementioned hybrid QD-OLED. (Note: though lifetimes for blue OLEDs and blue QLEDs have been a bottleneck for developers, the lifetime for blue OLEDs is much better and is acceptable for commercial use. This is not yet the case for blue QLEDs.) The ultimate solution for QLED is an all-QD emissive display that replaces the blue OLED with QD electroluminescence. This can happen when the blue QD materials are ready (Fig. 1).

Fig. 1:  The evolution of QD technology in flat-panel displays began with QDs for downconversion backlighting and as part of a QDEF film. The next step is a hybrid OLED/QLED display, and the ultimate goal is a fully emissive RGB QLED-based display.

Using QD materials in displays has additional benefits. With regard to the fabrication process, QDs are more suitable for large-area printing. It is easy to produce colloidal QDs dispersed in different solvents through a ligand change, which enables the deposition of QDs on various underlayers. Compared to other solution-processed emitters, colloidal QDs have fewer problems with thin-film casting. For example, phase segregation is unlikely to happen during a QD ink-evaporation process. QD film structures, such as scattered monolayers, multilayers, and close-packed monolayers or multilayers, can be tuned by controlling the processing parameters. And the synthesis of colloidal QDs is simple, which promises a substantial reduction in fabrication costs. Due to the robustness of inorganic materials, QDs are more resistant than organic materials to defects and impurities introduced during the fabrication process. They are also suitable for mass production.

Improving Efficiency

The efficiency of QLED has been improved to the point where it is high enough for display applications. For example, red QLED efficiency was the first color to be reported with more than 20 percent external quantum efficiency (EQE), which is close to the theoretical limit.¹ The highest green OLED now shows an efficiency of 92 current efficiency (Cd/A) from bottom emission, which is also over 20 percent EQE.² Recently, the QD team at TCL has pushed the efficiency of blue QLEDs to more than 23 percent.³

Several efficiency loss factors have been resolved to achieve this high QLED efficiency. First, to incorporate QD into the LED structure, the balance between QD luminescence and electrical charge conduction has to be considered. In a QLED, QDs are close-packed into a 20- to 40-nanometer (nm) film. In this state, the greater the physical space between QDs, the better the operation of the QD luminescence. However, when the physical space is increased, the electrical charge’s ability to move from one QD to another is reduced, leading to a higher resistance in the QD film, which in turn leads to higher power consumption. The surfaces of QDs are covered with surfactants, which protect the QDs during processing. The length of the surfactants and the QD film processing determine the physical space between the QDs, thus balancing QD luminescence and electric charge conduction. ^4–7

Unlike downconverted QDs, which are under light excitation when operating, QLED is under an electric excitation. Thus, electric fields and electric charges play a big role in possible efficiency losses. For example, due to the high resistance of QD film compared to the other layers in QLEDs, a very high electric field, usually an order of 1 MV cm^–1 (megavolts per centimeter), must be applied across the QD layer for suitable operation. Device performance decreases under such high electric fields because of the possibility of positive and negative charges recombining to reduce light emission. Methods to minimize the influence of the electric field include designing new structures of QDs to make them more robust under high electric fields and creating new device architectures to move the QD layers out of the high-electric-field regions in the QLEDs. In the meantime, electric charges also cause a problem. The best scenario is that positive and negative charges are always equal during the QLED operation. However, due to the mismatching of the interface between the QD layer and the other layers in the QLED structure, the unbalanced charges could accumulate in QLEDs to quench QD light emission. This is why a great deal of interface engineering is needed for efficiency improvement.^8,9

Lifetime Issues

Even though an EQE of more than 20 percent has been demonstrated for RGB QLEDs, the lifetime of QLEDs is far behind that of their OLED counterparts. Lifetime is the most critical factor for commercialization. This issue has therefore drawn immense research interest on a global basis. Lifetime progress since 2011 is summarized in Fig. 2. Considerable lifetime value was reported in 2011 by NanoPhotonica, with the introduction of solution-processable zinc oxide (ZnO) as the electron-transport layer (ETL), which provided better electron injection. A green-emitting QLED with a T50 (meaning a lifetime to 50 percent of initial luminance) of more than 5,000 hours lifetime at 100 nits was obtained. ¹⁰ In 2012, a red QLED with a T50 of 10,000 hours at 100 nits was reported.⁸ Two years later, Zhejiang University demonstrated a T50 of 100,000 hours at 100 nits for a red QLED by manipulating charge balancing.¹ In 2015, using accelerated tests, NanoPhotonica demonstrated a red QLED T50 lifetime of 300,000 hours and a green lifetime of 90,000 hours initial at 100 nits. ¹¹

Fig. 2:  QLED lifetime progress, especially for green and red, has advanced steadily over the past eight years.

Although great improvement has been made, a T50 lifetime rating based on 100 nits (T50@100 nits) light output is not suitable for the display industry. Based on panel requirements, a lifetime to 95 percent of initial luminance at 1,000 nits (T95@1,000 nits) or higher is a good criterion to use. Such a criterion is set to make sure the display color quality is good enough during the product’s life span. In 2018, the research team at TCL reported QLEDs with a T95@1,000 nits over 2,100 hours, which is about 10 times higher than previously reported for QLEDs. ¹² Figure 3 shows the lifetime data. This number is the first in which QLED exceeds the industrial requirements of displays, making this technology truly viable for commercial applications.

Fig. 3:  The left plot shows the lifetime measurement data; the right plot contains histograms of T95 lifetime with initial luminance of 1,000 cd/m² measured from 48 ZnSe-QD devices and 48 ZnS-QD devices.¹²

The researchers took several steps to overcome the lifetime block. As mentioned in the previous section, balancing QD luminescence and electric charge conduction is critical in QLEDs. In state-of the-art QLED devices, negative charges are sufficient while the positive charges are much more insufficient. Previous studies^1,10,12 focused on device structures by either matching the QDs’ conduction band for positive charges or impeding negative charges through more complicated designs. TCL researchers took a different approach, targeting the QDs directly.

Thoroughly analyzing the QD structure, the TCL researchers discovered that by replacing the ZnS in QDs with zinc selenide (ZnSe), they could maintain a photoluminescence quantum yield value of more than 80 percent, achieving a high level of QD luminescence while also improving the electric charge equality in QLED and ultimately leading to better device performance. The ZnSe-QD device exhibits a maximum external quantum efficiency (ηEQE) and a current efficiency (ηA) value of 15.1 percent and 15.9 cd/A-1, respectively, which is good enough for commercial use. Moreover, benefiting from the improvement in electric charge equality, the ZnSe-QD-based device showed a moderate voltage increase and a T95 operation lifetime of more than 2,100 hours with an initial luminance of 1,000 cd/m ², and a T50 lifetime at 100 cd/m² of more than 2 million hours.

Solution Processing

QLEDs are “born” to be solution processed. In earlier stages of QLED development, both Samsung and QD Vision demonstrated QLED panels patterned by transfer printing, although they had problems achieving larger sizes and mass production. Inkjet printing technology, due to its precise position control and mask-free capabilities, emerged as a promising method for QLED panel fabrication. In 2016, a research team from South China University of Technology demonstrated a 2-in., 120-ppi full-color active-matrix-QLED panel that contained some inkjet-printed layers and some evaporated layers.¹³ One year later, big panel makers such as TCL and BOE showed their own demo of a full-color active-matrix-QLED (AMQLED) panel with all layers inkjet printed except for the electrodes. In term of mass production, inkjet-printing technology is also making fast progress. JOLED has already produced 21-in. 200-ppi AMOLED panels, using its own Gen 4.5 inkjet-printing line. LGD, BOE, and TCL have also announced the adoption of inkjet-printing technology for higher-generation production lines (Gen 8.5 and above).

A few problems must be addressed in order to commercialize the inkjet printing of displays. Most important is the gap between spincoating and inkjet devices. The efficiency and lifetime of inkjet-printed QLED devices are lower than those of spincoated ones. To date, the QLED performance numbers reported have been based on spincoating. Spincoating is a good method for materials development and screening and is used in primary tests, but it is not suitable for mass production. The ink that spincoating uses is not suitable for inkjet-printing processes, which require additional properties from solvents, including high boiling temperature, high viscosity, more functional groups, etc.

Outlook

With recent rapid developments in both QD materials and panel-fabrication processes, QLED is a promising technology for next-generation displays. We can expect better and larger demos, and even mass production of AMQLED panels in the near future.

In order to make this technology appealing both to panel makers and to the market, however, several issues must be resolved. First, due to their high energy levels, blue emitters are always a problem, and this applies to blue QDs as well. Even though there are ways to compensate for the short lifetime of blue components at the panel level, the common requirement for blue QLED lifetime is hundreds of hours of T95 initial at 1,000 nits. Unfortunately, the T50 lifetime of blue QLEDs is less than 50 hours at 1,000 nits, far less than the lifetimes of red and green QLEDs.³

Another obstacle involves concerns over cadmium (Cd). So far, the QD materials with the strongest QLED performance are based on Cd compounds. Cd-based QDs have advantages in terms of optical properties and synthesis capability. However, Cd is a highly toxic element, and health and environmental concerns arise for its production, use, and recycling. Even though Cd-free QLEDs have reported EQE of more than 20 percent, their lifetime and color purity are still problematic. The frequently cited T50 lifetime of Cd-free QLEDs is less than 100 hours at 500 nits, which equates to around 1,000 hours at 100 nits ( Fig. 2, circle points). The full width at half maximum (FWHM) of the electroluminescent (EL) spectrum is near 40 nm compared to 23 nm from Cd-based counterparts.

Other issues regarding QLED applications relate to making high-resolution panels. The resolution of inkjet printing is around 200 ppi for production lines so far, good for large-area displays with more than 4K resolution. If QLED is to be used on high-end medium- to small-size displays, higher resolution is required. Transfer printing is an easy way to increase resolution but doesn’t work well for mass production in lines over Gen 6.5. For inkjet printing, higher resolution could be achieved through pixel arrangements, but this process is not ready for mass production yet.

In summary, QLED will be the next trend for flat-panel displays, combining electrical driving emission with the excellent emitting properties of QDs – sharp spectra, high luminescence efficiency, and solution-process capability. With recent progress and investment in the technology, we believe the commercialization of QLED is closer than ever.

References

¹X. Dai, Z. Zhang, Y. Jin, Y. Niu, H. Cao, et al., “Solution-processed, high-performance light-emitting diodes based on quantum dots,” Nature 515, 96–99, 2014.

²Reported by NanoPhotonica at 2018 QD Forum, San Diego, CA, US.

³Reported by TCL at 2018 ICDT, Guangzhou, China.

⁴C.R. Kagan, C.B. Murray, and M.G. Bawendi, “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” Phys Rev B 54, 8, 633–8,643, 1996.

⁵B.N. Pal, Y. Ghosh, S. Brovelli, R. Laocharoensuk, V.I. Klimov, et al., “ ‘Giant’ CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance,” Nano Lett. 12, 331–336, 2012.

⁶J. Lim, B.G. Jeong, M. Park, J.K. Kim, J.M. Pietryga, et al., “Influence of Shell Thickness on the Performance of Light-Emitting Devices Based on CdSe/Zn_1–XCdXS Core/Shell Heterostructured Quantum Dots,” Adv. Mater. 26(47), 8,034–8,040, 2014.

⁷H. Shen, W. Cao, N.T. Shewmon, C. Yang, L.S. Li, et al., “High-Efficiency, Low Turn-on Voltage Blue-Violet Quantum-Dot-Based Light-Emitting Diodes,” Nano Lett. 15, 1,211–1,216, 2015.

⁸J. Kwak, W.K. Bae, D. Lee, I. Park, J. Lim, et al., “Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure,” Nano Lett. 12(5), 2,362–2,366, 2012.

⁹D. Bozyigit, O. Yarema, and V. Wood, “Origins of Low Quantum Efficiencies in Quantum Dot LEDs,” Adv. Funct. Mater. 23, 3,024–3,029, 2013.

¹⁰L. Qian, Y. Zheng, J. Xue, and P.H. Holloway, “Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures,” Nat. Photonics 5, 543–548, 2011.

¹¹Y. Yang, Y. Zheng, W. Cao, A. Titov, J. Hyvonen, et al., “High-efficiency light-emitting devices based on quantum dots with tailored nanostructures,” Nat. Photonics 9, 259–266, 2015.

¹²W. Cao, C. Xiang, Y. Yang, Q. Chen, L. Chen, et al., “Highly stable QLEDs with improved hole injection via quantum dot structure tailoring,” Nature Communications, 9(2,608), 2018.

¹³C. Jiang, L. Mu, J. Zou, Z. He, Z. Zhong, et al., “Full-color quantum dots active matrix display fabricated by ink-jet printing,” Sci China Chem 60, 1,349–1,355, 2017.  •

Chaoyu Xiang received his Ph.D. in materials science and engineering from the University of Florida in 2014. In 2016, he joined TCL Corporate Research for QLED development and commercialization. Weiran Cao received his Ph.D. in materials science and engineering from the University of Florida in 2013. After graduation, he joined TCL Corporate Research, continuing his research in QLEDs and their commercialization. Yixing Yang received his Ph.D. in materials science and engineering from the University of Florida in 2011. After graduation, he joined NanoPhotonica and later TCL Corporate Research, engaging in the development of high-performance QLEDs for display applications. Lei Qian received his Ph.D. from Beijing Jiao Tong University. He is the cofounder of NanoPhotonica. In 2015, he joined TCL Industrial Research, leading the QLED research team for the research and development of key technologies. Xiaolin Yan received his Ph.D. from the Plasma Physics Institute of Chinese Academy of Sciences in July of 1999. Yan joined TCL Corporation in 2001. Presently, he is the CTO and senior vice president of TCL Group and the president of TCL Corporate Research. He has served as SID’s regional vice president of Asia since May 2016.