Company USA
Quantum dots have a combination of performance attributes that make them the ideal lumophore for incorporation into light-emitting devices (LEDs) for ultrathin flat-panel displays. They offer high photoluminescence quantum yield, are capable of emitting light from 100% of electrically generated excitons, are solution-processable, and have high stability and differential stability (when compared to different color lumophores of the same class).
Quantum dots (QDs) can have extremely narrow emission and absorption bands (the widths of which are related to the size distribution of the sample). They can be very efficient emitters, with photoluminescence quantum efficiencies as high as any known lumophore, and their emission can be tuned continuously through large portions of the electromagnetic spectrum. As an example, cadmium selenide (CdSe) QDs can be tuned to emit light anywhere from 470 to 640 nm-almost the entire visible spectrum. Advances in the synthesis of QDs by research groups worldwide have demonstrated the incorporation of this material set into a range of useful devices, and have broadened the range of semiconductors that can be made using this wet-chemistry technique.
The wide spectral tunability of QDs allows them to become the only lumophore necessary in a full-color flat-panel display. Their narrowband emission translates into a wide color gamut; thus, QD displays have the potential for color saturation better than liquid-crystal and organic-LED (OLED) displays. This narrowband property also allows for the creation of high-efficiency LEDs across the range of the visible spectrum, including blue and red, where light would otherwise be lost to the ultraviolet and infrared, respectively. As an inorganic emitting body, they are more stable than most organic emissive materials.
Producing quantum-dot displays
Because QDs can be processed in solution, they are compatible with all solution thin-film-deposition techniques. However, these techniques typically place requirements on the surface onto which the QDs are deposited, and do not allow the QD monolayer to be laterally patterned. These two shortcomings make the previously available techniques far from ideal for fabrication of QD-LEDs onto a useful flat-panel-display semiconductor transport material. In contrast, quantum-dot contact printing is a dry deposition technique for QD solids in which no solvent or other impurities ever contact the device substrate during device fabrication, enabling incorporation of QDs into either small-molecule or polymer structures.
Displays based on QD-LEDs are large-area emitters consisting of a single monolayer of QDs in an organic LED structure. This requires nanometer-level deposition precision over extremely large areas-liquid-crystal displays are fabricated on substrates more than two meters on a side. The crucial technological advance that enables QD-LED fabrication relies on two distinct breakthroughs: QD monolayer assembly and QD contact printing.
The company’s development of techniques for the deposition of QDs as a densely packed monolayer on top of a conjugated organic film relies on a detailed understanding of the surface chemistry of the QDs, the solvent, and the surface onto which the QDs are deposited. Once these parameters are understood and controlled, facile and rapid assembly of the high-quality films necessary for achieving high device performance and uniformity can be achieved.
The second step is transferring this QD film into the display device at exactly the appropriate position. Quantum-dot contact printing was invented to solve this problem. Quantum-dot contact printing adds design freedom to the device materials stack that is independent of the QD chemistry. We can now position the QD monolayer with nanometer precision in the light-emitting zone of the device, boosting the device efficiency and ensuring high color saturation. This process can yield a highly uniform QD layer thinner than 10 nm over square inches of display substrate. With scaling up of equipment and additional process control, this fabrication method will be extended to over two meters, which is necessary for low-cost display manufacture.
Technology advantages
Quantum dots have a combination of performance attributes not achievable with other lumophores. The ideal OLED lumophore would have high photoluminescence quantum yield, be capable of emitting light from 100% of electrically generated excitons, be solution processable, and have extremely high stability and differential stability (to eliminate image sticking and white-point drift effects). Polymers, dendrimers, fluorescent, and phosphorescent small molecules have all undergone extensive development, but have yet to yield a single material set that meets the complete set of industry needs. Quantum dots are a new class of lumophores that have the promise to meet all these needs simultaneously.
Quantum-dot LEDs have theoretical performance limits that meet or exceed all other display technologies. Today, phosphorescent OLEDs have the best demonstrated efficiency of any nonreflective display technology, but QD-LEDs have the potential to exceed OLED luminous efficiency by more than 20%, reducing power consumption significantly. The color saturation of 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. The red and green devices exceed the HDTV standard, while the blue QD-LED CIE color coordinates lie just inside the standard. All three color QD-LEDs have reproducible, stable current-voltage characteristics, with turn-on voltages of 2 to 4 V and operating voltages of 5 to 9 V.
A prototype display developed by the company demonstrates the visual appeal of the company’s QD-LED technology. Monochrome passive-matrix displays with 64 x 32 pixels and a 1.4 in. diagonal emitting in vivid red and green colors have already been developed. The form factor is that of a cell-phone display, and its ultrathin 1.5 mm profile lends itself to the slim-format phones popular today.
