Quantum-Dot Light-Emitting Devices for Displays
A new technology is being developed that combines the familiar construction of a small- molecule organic LED with an inorganic emissive core that uses quantum dots. QD-LEDs combine the soluble nature of polymers for ease of manufacture with the potential for high- efficiency emission similar to phosphorescent materials.
by Greg Moeller and Seth Coe-Sullivan
QUANTUM DOTS (QDs), used as the emissive material in an organic light-emitting device (OLED), combine the performance and manufacturing advantages of both small-molecule and polymer materials into a single material set. The market penetration of OLED technologies, both small molecule (SM-OLED) and polymer (PLED), is being slowed by a combination of performance limitations and manufacturing-process challenges. Although consumers are attracted to the visible advantages inherent to emissive-display technologies and panel manufacturers continue to see the advantages of solution-based manufacture, competing technologies continue to win new designs because the combined advantages of high performance and solution processability have not previously been offered in one OLED material set. This article proposes the use of quantum dots incorporated into a standard OLED structure as a single platform that offers both high performance and solution processability.
Performance
In the past decade, much effort has been devoted to the continuous improvement of the basic OLED materials set. Although the number of possible molecules and polymers is theoretically infinite, the multi-parameter optimization of organic materials for lifetime, efficiency, color, and manufacturing process to date has prevented the creation of a clear leader for the red, green, and blue emissive materials used in OLEDs.
Small-Molecule and Polymer OLED Materials. The unique advantages of SM-OLED and PLED emissive materials have not previously been offered in a unified package. Fluorescent small molecules have demonstrated the best combination of lifetime and manufacturability to date, as exemplified by the multitude of products on the market today. Fluorescent SM-OLEDs are found in MP3 players, cellular phones, digital cameras, and the microdisplay accessory to Apple's iPod. Phosphorescent small molecules are the clear leader in efficiency, with internal quantum efficiencies that approach perfection. However, of these choices only polymeric materials currently have the key advantage of being solution-processable, which seems to be a necessary attribute to reach substrate sizes up to and beyond Gen 4. QDs blend all of the key advantages of these disparate material sets, leveraging OLED advancements in materials and device stack architecture, by only replacing the emissive layer.
Quantum Dots. Using inorganic QDs as the emissive layer combines the attractive characteristics of small-molecule and polymer materials, while potentially relaxing the stringent OLED packaging requirements. The understanding of QDs (see Fig. 1), also known as nanocrystals, requires only a modest knowledge of the principles of quantum mechanics and solid-state semiconductor physics.
In a conventional bulk semiconductor, the electrons and holes may be best described by particle wave functions of some finite size (typically on the order of 10 nm). In a confined system such as a QD, these wave functions are not able to extend fully, and hence exist at some raised energy state relative to the bulk case. Quantum mechanics allows us to calculate the relative energies of a particle within such a confined system [for an extensive summary, see Efros and Rosen, Ann. Rev. Mater. Sci. 30, 475 (2000)], but intuition is sufficient to tell us that this energy increases as the level of confinement is raised. For QDs, the degree of confinement is determined by the particle size, and, hence, as the QD diameter is reduced, the energy corresponding to the electron and hole wave functions are similarly increased. Thus, an emissive electron–hole pair recombination event will release more energy (bluer light) when the QD diameter is decreased.
The function of the overcoating shell shown in Fig. 1 is explained by considering the QD surface. The QD crystal structure necessarily breaks down at any crystal surface because the periodic structure is not continued. This gives rise to surface states of different energy levels, which may lead to non-radiative relaxation pathways, and thus a reduction in the efficiency of emission. Overcoating mitigates all of these effects. By adding another shell of semiconductor to the crystal, the core's dangling bonds are satisfied. They are instead replaced with dangling bonds at the new surface, further from the QD core. Using a semiconductor with a wider bandgap than the core material, allows the electronic wave function to remain well confined within the core. Thus, the wave function has only minimal overlap with the dangling bonds and the surface states and gives rise to a QD with higher photoluminescence quantum yield.
The extremely narrow emission band that yields saturated-color-emitting quantum-dot light-emitting devices (QD-LEDs) is also due to quantum confinement effects. In a bulk three-dimensional semiconductor, the density of available states above the conduction band rise very quickly [g(E) µ E1/2]. However, for an idealized zero-dimensional QD, g(E) becomes a series of discrete states with extremely narrow bandwidths.
The capping ligands allow the QDs to remain stable in colloidal suspension. In practice, this means that the QDs do not precipitate out of solution while they are being grown to a specific size, and that they can be deposited using liquid-processing techniques. In an electrical device, the caps play an important role in charge transport to the QD emissive states, as well as in stabilizing the device morphology.
Thus, quantum dots combine the soluble nature of polymers with the potential for the high-efficiency emission of phosphorescent materials. Because of the inorganic nature of QDs, they are inherently more stable in the presence of water vapor or oxygen than their organic-semiconductor counterparts. Due to their quantum-confined emission properties, their luminescence is extremely narrow-band and yields highly saturated color emission. Finally, because the nanocrystal diameter controls the QD optical band gap, the process for identifying and optimizing luminescent properties is greatly simplified. In practice, colloidal suspensions of QDs (i.e., solutions) have been made that (1) emit anywhere across the visible and infrared spectrum (see Fig. 2), (2) are orders of magnitude more stable than organic lumophores, (3) have full-width at half-maximum (FWHM) of emission below 20 nm, (4) have quantum yields as high as 90%, and (5) can be incorporated into working QD-LEDs when married with commercially available organic transport layers.
QD-LED Structure. To make efficient QD-LEDs, a device structure quite similar to an SM-OLED is employed (see Figs. 3 and 4). Electron-transport, hole-transport, hole-injection, and hole-blocking layers all have similar roles to play in such a device.
Instead of using a host:dopant-based emissive layer as used in phosphorescent or fluorescent OLEDs, a single monolayer of QDs is used as the emissive layer (as depicted in Fig. 3). The importance of the monolayer lies in the fact that the QD to QD-charge transport is not an efficient process and leads to higher-voltage devices, and hence an increase in power consumption. This monolayer need not be perfect over the entire display pixel, but rather is quite tolerant to defects of both omission and addition, which makes the task of manufacture much simpler.
QD-LED Properties. QD-LEDs have theoretical performance limits equivalent to those of phosphorescent materials, and the stable nature of inorganic QDs suggest the potential for significant lifetime improvements. The color saturation of manufactured red, green, and blue QD-LEDs is represented by their position on the CIE diagram relative to the high-definition-television (HDTV) standard color triangle as seen in Fig. 5. The red and green devices are in excess of the HDTV standard, while the blue QD-LED CIE color coordinates lie just inside the standard as a result of the red tail seen in the blue QD-LED EL spectrum (Fig. 6). All three color QD-LEDs have reproducible, stable, current–voltage (I–V) characteristics, with turn-on voltages of 3–5 V and operating voltages of 8–12 V.
MIT/QD Vision, Inc.
Fig. 1: Artist's rendition of a core-shell-cap semiconductor nanocrystal quantum dot.
QD-LED luminous efficiency has a theoretical limit equivalent to phosphorescent OLEDs (100 lm/W) as opposed to fluorescent OLEDs (25 lm/W). Hence, QD-LEDs have the potential to be an extremely low-power-consumption display alternative. The combination of solution processibility and 100-lm/W peak efficiency is unique amongst OLED emissive material options. Additionally, because the luminescent decay time of QDs (tens of nanoseconds) is an order of magnitude faster than that of typical phosphorescent small molecules (hundreds of nanoseconds), their efficiency can be conserved even for high brightness (1000 cd/m2) displays, a trait that is necessary for performance competition with plasma displays. QD-LEDs will enable the combination of high brightness with long lifetime (greater than 20,000 hours); hence, differentiating itself from LCDs, OLEDs, and plasma displays alike.
A second effect of the increased stability of QDs as compared to that of organic lumophores is the relaxation of the requirements on OLED packaging. Glass/glass or glass/metal packages are the norm for OLED displays today, but many companies (such as Vitex and DuPont) are pursuing thin-film barrier coatings that will allow a near 50% reduction in OLED thickness and weight by eliminating the cover glass or metal can. The requirements for water vapor and oxygen permeation through these barriers are remarkably stringent, and hence this feat has remained elusive for many years. By using inorganic QDs that are inherently more stable to water vapor and oxygen, it may be possible to relax this requirement, easing the introduction of such thin-film barrier packaging schemes.
MIT/QD Vision, Inc.
Fig. 2: CIE chromaticity diagram depicting the potential QD emission colors relative to the NTSC color triangle (dotted lines).
MIT/QD Vision, Inc.
Fig. 3: Atomic-force-microscope (AFM) image of a QD monolayer used as the emissive layer of an OLED.
Manufacturing
Manufacturing challenges restrict the rate and diversity of market penetration of OLEDS by restricting access to motherglass sizes larger than Gen 4. The technological drivers of this restriction are numerous, but can be broken down into two categories: (1) challenges with the deposition of the emissive material and (2) challenges with existing backplane technologies. QD-LEDs are a frontplane technology and are consequently focused on solving the former, not the latter. However, it seems reasonable to assume that progress will be made on backplane technologies given the resources being applied.
Solution Phase Deposition. Emissive-material deposition on Gen 4 and above motherglass can be achieved by using solution-based manufacturing techniques. The most often cited example of a solution-based manufacturing technique which can be applied to display manufacturing is ink-jet printing, as pioneered by companies such as Cambridge Display Technology, Litrex, and Epson.
Great strides have been made in PLED production to increase the accessible substrate sizes. Progress has also been made in the effort to increase substrate size for SM-OLED displays, but problems in creating a Gen 4 shadow mask still remain. As evidence of this progress, Samsung has announced its intent to start a Gen 4 PLED display line using ink-jet technology. This tool is expected to reach production by late 2006 or 2007.
Although polymer-based OLEDs have made significant progress toward larger substrates by using ink-jet printing, the devices manufactured with existing polymer materials have suffered, in general, from lower color saturation and less-efficient emission than their small-molecular counterparts, which comprise most of today's OLED market.
Vapor-Phase Deposition. The lack of a solution-based small- molecule emissive material has been a significant challenge to SM-OLED manufacturers. In the manufacture of SM-OLED displays, patterning of the red, green, and blue layers is an area with opportunity for significant improvement. Current production methods involve the thermal evaporation of materials through a shadow mask. The low material-utilization rate during this step results in the need for frequent chamber cleaning. The wasted material also creates dust that reduces yield.
Improvements have been made to increase material utilization for OLED manufacture. The switch from point-source to linear-source evaporation has increased material utilization more than six-fold, from less than 5% to more then 30%. Organic vapor-phase deposition (OVPD) is another promising technology aimed at increasing the material utilization rate.
MIT/QD Vision, Inc.
Fig. 4: Diagram of the proposed device design.
:
MIT/QD Vision, Inc.
Fig. 5: CIE diagram showing actual QD-LED color coordinates relative to the NTSC color triangle (dotted lines).
To maximize OLED performance, it is clear that direct deposition of higher performance solution-based materials has the most promise. An emissive material with the performance of SM-OLEDs and the manufacturing process advantages of PLEDs would be a decidedly positive event for both, allowing the industry to leverage its efforts to become more competitive with LCDs.
QD-LED Manufacturing. Because QDs are solution-processable, the manufacturing advantages associated with polymeric materials are available to QDs as well. Some techniques that have been successfully demonstrated include phase separation, ink-jetting, drop-casting, and Langmuir-type techniques.
Perhaps the best-known QD-LED fabrication technique is phase separation during spin-casting. Phase separation is an established method for forming large-area ordered monolayers of colloidal nanocrystal QDs. The QD thin films are formed in a single step by spin-casting a mixed solution of aromatic organic materials and aliphatic-capped QDs. The two different materials phase-separate during solvent drying, forming the desired QD monolayer assembled on top of the organic-semiconductor contact. The robustness and flexibility of this phase-separation process has been shown, as well as how the properties of the resulting films can be controlled in a precise and repeatable manner. Parameters such as solution concentration, solvent ratio, QD-size distribution, and QD aspect ratio can all affect the resulting thin-film structure. Controlling these factors allows the creation of QD-LEDs with high efficiency and high color saturation. However, since this process relies upon spin-casting, it results in only a monochrome-display platform.
For full-color displays, a fabrication process that would allow for in-plane patterning of QD monolayers is desirable, provided that it not place additional constraints on the materials or device designs with which it can be used. Ink-jetting is one such process. QD Vision has developed other novel manufacturing techniques that will enable patterned QD monolayer deposition. These new processes will allow QD-LED displays to be fabricated over large areas in a high-throughput high-yield process, allowing the technology to transfer from lab scale, where it is today, into pilot production and eventually manufacturing.
Conclusion
Quantum dots have the potential to simultaneously improve the material stability, lifetime, efficiency, and color purity of OLEDs, while allowing for solution-processing techniques to be employed in large-area manufacture. OLEDs continue to show promise as a display technology, with the potential for both high performance and access to large motherglass. Although performance improvements rely upon the development of new materials, the substrate size is constrained primarily by manufacturing techniques. QD-LEDs leverage large segments of the already emplaced OLED infrastructure, such as current-driven backplanes and hermetic packaging techniques, both of which have undergone tremendous improvement in recent years. Thus, QD-LEDs have an important role to play within the OLED technology roadmap. •
MIT/QD Vision, Inc.
Fig. 6: Emission spectra of red, green, and blue QD-LEDs.
Seth Coe-Sullivan is a founder and Chief Technology Officer of QD Vision, Inc., 313 Pleasant St., Watertown, MA 02472; telephone 617/926- 0001, e-mail: scoe-sullivan@qdvision.com. Greg Moeller is a founder and Director of Business Development at QD Vision; e-mail: gmoeller@ qdvision.com.