Apple's Fourth Invention Covering Next-Gen Quantum Dot Displays surfaced today at the U.S. Patent Office
To date Patently Apple has posted three patent reports regarding Apple's possible use of quantum dot displays. The first report was posted in February 2014, the second in July 2016 and the third last month titled "Apple Invents Quantum Dot Hybrid Pixels for Future Power Efficient Displays that Could Deliver 'Pure Color'." Quantum dot QD-LED displays will offer better power efficiency than OLED alone. Today the US Patent & Trademark Office published yet another patent application on this subject matter titled "Quantum Dot Spacing for High Efficiency Quantum Dot LED Displays."
State of the art displays for phones, tablets, computers and televisions utilize glass substrates with thin-film transistors (TFTs) to control transmission of backlight through pixels based on liquid crystals. More recently emissive displays such as those based on organic light emitting diodes (OLED) have been introduced because they can have a faster response time, and be more power efficient, allowing each pixel to be turned off completely when displaying black or dark colors. Even more recently, quantum dot light emitting diodes (QD-LEDs) have been introduced as an alternative display technology, potentially being more power efficient than OLEDs.
Quantum dots are semiconductor materials where the size of the structure is small enough (e.g. less than tens of nanometers) that the electrical and optical characteristics differ from the bulk properties due to quantum confinement effects. For example, the emission properties of quantum dots are related to their size and shape in addition to their composition.
Apple notes that when an electric field is applied to a QD-LED electrons and holes move into the quantum dot layer where the electrons and holes are captured in the quantum dots and recombine, emitting photos. The emission wavelength can be tuned by changing the size of the quantum dots. Typically, smaller quantum dots emit bluer light (higher energy) and larger quantum dots emit redder light (lower energy).
Apple's invention relates to Quantum dot (QD) layers and QD-LED displays structures. In one embodiment, a QD layer includes a matrix of quantum dots in which each quantum dot includes a core, a shell around the core, and a metal oxide coating around the shell. In an embodiment, the shells of adjacent quantum dots are spaced apart by an average distance of 5-10nm, for example, to mitigate the impact of Forster resonance energy transfer (FRET) within the QD layer. In an embodiment, the metal oxide coatings of the quantum dots have an average thickness of 2.5-5nm.
Apple's patent FIG. 5 below is a schematic cross-sectional view illustration of a quantum dot including ligands bound to the shell. FIG. 6 is a schematic cross-sectional view illustration of quantum dot film including a mixture of quantum dots with metal oxide coatings and quantum dots with ligands
More specifically, Apple notes that in patent FIGS. 5-6 we're able to see a QD layer that may include a second matrix of QDs 500 dispersed in a first matrix of QDs 300. As shown, the QDs 500 of the second matrix may be smaller than the QDs 300. Additionally, the QDs 500 may include organic or inorganic ligands 506 bound to the shells 104 of the QDs 500. In an embodiment, a QD layer including a mixture of semiconductor core-shell QDs 100, 500 with metal oxide coated QDs 300 may create spacing between QDs to mitigate or eliminate FRET. Such a mixture may additionally create carrier trap sites (e.g. charge trapping sites on the emitter for electrons and holes) on the QDs 100, 500, which may enhance QD excitation in the device.
In another aspect, the metal oxide coatings may be a material system that is QD-LED device compatible. In accordance with embodiments, the metal oxide coatings may create a charge transporting matrix that may have the effect of increasing a recombination zone. As a result, device efficiency and lifetime may additionally be extended. A variety of metal oxide materials may be used based on band gap and band position relative to surrounding layers within a QD-LED device.
In some embodiments, the metal oxide coating material may be selected to be within a specified energy level of an adjacent HTL or ETL, both of which may be formed of organic or inorganic materials. For example, the metal oxide coating material may have a conduction band within 1.0 eV, or more specifically within 0.5 eV of a conduction band or lowest unoccupied molecular orbital (LUMO) of an adjacent ETL, or a valence band within 1.0 eV, or more specifically within 0.5 eV of a valence band or highest occupied molecular orbital (HOMO) of an adjacent HTL. Conventionally, HOMO and LUMO are used to characterize organic semiconductors, while valence band and conduction band are used to characterize inorganic materials such as semiconductors and metal oxides.
Obviously this is a complex filing that engineers in this field will understand far better than us mere mortals. For them and anyone brave enough to review then entire filing can here.
Apple's patent application 20170271605 was filed back in Q2 2016. Graphic Credit: S. Kelly/JQI – Artists rendition of a spherical quantum dot (fluorenscing ensemble of spheres).
Considering that this is a patent application, the timing of such a product to market is unknown at this time.
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