Eastman Kodak Company (Rochester, NY) inventor Keith B. Kahen developed a method of forming quantum dot based LEDs using low cost manufacturing techniques to deposit the LED layers on inexpensive substrates such as glass, plastic, metal foil, or silicon. According to U.S. Patent 7,615,800, Kodak’s manufacturing method for “all inorganic” light emitting diodes (LEDs) based on quantum dot emitters combines many of the desired attributes of crystalline LEDs with those of organic LEDs while overcoming disadvantages of inorganic LEDs.
Kodak’s inorganic quantum dot LEDs emitters are formed by low cost deposition techniques and the individual layers show good conductivity performance. Using quantum dots as the emitters in light emitting diodes confers the advantage that the emission wavelength can be simply tuned by varying the size of the quantum dot particle. As such, spectrally narrow (resulting in a larger color gamut), multi-color emission can occur from the same substrate. If the quantum dots are prepared by colloidal methods and not grown by high vacuum deposition techniques then the substrate no longer needs to be expensive or lattice matched to the LED semiconductor system.
Semiconductor light emitting diode (LED) devices have been made since the early 1960's and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers comprising the LEDs are based on crystalline semiconductor materials which require ultra-high vacuum techniques for their growth, such as, molecular organic chemical vapor deposition. In addition, the layers typically need to be grown on nearly lattice-matched substrates in order to form defect-free layers. These crystalline-based inorganic LEDs have the advantages of high brightness (due to layers with high conductivities), long lifetimes, good environmental stability, and good external quantum efficiencies. The usage of crystalline semiconductor layers that results in all of these advantages, also leads to a number of disadvantages. The dominant ones are high manufacturing costs, difficulty in combining multi-color output from the same chip, the need for high cost and rigid substrates.
It is an advantage of Kahen's invention to provide a way of forming a light emitting layer, whose emitting species are quantum dots, that is simultaneously luminescent and conductive. The light emitting layer includes a composite of conductive wide band gap nanoparticles and shelled quantum dot emitters. A thermal anneal is used to sinter the conductive nanoparticles amongst themselves and onto the surface of the quantum dots. As a result, the conductivity of the light emitting layer is enhanced, as is electron-hole injection into the quantum dots. To enable the quantum dots to survive the anneal step without a loss in their fluorescent efficiency (since the organic ligands passivating the quantum dots boil away during the anneal process), the quantum dot shells are engineered to confine the electrons and holes, such that, their wavefunctions do not sample the surface states of the outer shell.
It is also an advantage to incorporate the conductive and luminescent light emitting layer in an all inorganic light emitting diode device. The electron and hole transport layers are composed of conductive nanoparticles; in addition, separate thermal anneal steps are used to enhance the conductivities of these layers. All of the nanoparticles and quantum dots are synthesized chemically and made into colloidal dispersions. Consequently, all of the device layers are deposited by low cost processes, such as, drop casting or inkjetting. The resulting all inorganic light emitting diode device is low cost, can be formed on a range of substrates, and can be tuned to emit over a wide range of visible and infrared wavelengths. In comparison to organic-based light emitting diode devices, its brightness should be enhanced and its encapsulation requirements should be reduced.