Light field displays provide multiple views, allowing a user to receive a separate view in each eye. While current displays in this category provide an interesting viewing experience, a captivating light field display requires a very high pixel density, very low angular separation between views, and a large viewing angle. It is desired that a user experiences smooth transitions between viewing zones, while maintaining an independent and perceivable view from the adjacent views. A fundamental requirement in achieving these viewing parameters is controlling the output characteristics of the emission source. Organic light-emitting diodes (OLEDs) bound in a microcavity allow control of the spectral bandwidth and output angle of the resulting light.
Organic light-emitting diodes consist of thin-film layers of organic material coated upon a substrate, generally made of glass, between two electrodes. OLEDs have a characteristic broad spectral width and Lambertian intensity distribution profile. The thin-film layers disposed between the anode and cathode commonly include one or more of an Organic Hole-Injection Layer (HIL), an Organic Hole-Transporting Layer (HTL), an Emissive Layer (EML), an Organic Electron-Transporting Layer (ETL), and an Organic Electron-Injection Layer. Light is generated in an OLED device when electrons and holes that are injected from the cathode and the anode (electrodes), respectively, flow through the ETL and the HTL and recombine in the EML.
A method for controlling the output characteristics of light is the use of a microcavity. The microcavity is formed between two mirrors. The first mirror can be a metal cathode and the second mirror may be a layered stack of non-absorbing materials. The layered stack of non-absorbing materials is referred to as a distributed Bragg reflector (DBR). A DBR is an optical mirror comprised of multiple pairs of two different dielectric layers with different refractive indices in an alternating order. The highest reflectivity is attained when the layer thicknesses are chosen such that the optical path length of each layer is one quarter of the resonance wavelength, commonly referred to as the Bragg Wavelength, λBragg. Three main design variables affecting the output characteristics of a microcavity are the reflectance of the top and bottom surfaces (i.e. opposing mirrors), and the optical path length. The optical path length between the mirrors is to equal a multiple of the wavelength. The wavelength of the light output by such resonant OLED structure is dependant, in part, upon this optical path length of the microcavity. The optical path length in the cavity can be manipulated in different ways, one of which is changing the thickness of the layers that make up the microcavity. A challenge for OLEDs, which are suitable for light-field displays, is how to determine the optimum optical path length of the microcavity to decrease the spectral bandwidth and output angle.
The current design process for microcavity OLEDs, as known in the prior art, includes creating the initial OLED design, defining the output characteristics, designing the reflective surfaces and determining the material thicknesses. The OLED is then fabricated and tested. This process is repeated until the desired output is attained from the fabricated OLED structure. This is a time consuming and costly process.
U.S. Pat. No. 6,917,159 B2 (microcavity OLED device) describes the use of a microcavity within an OLED to improve efficiency and intensity of output. Metallic mirrors are used instead of DBRs as they are easier to fabricate. This results in greater energy lost and less control of the reflectance. As metallic structures are used for both reflectors, one must be semi-transparent. This is to improve the output efficiency on the OLED and improve the chromaticity, however, for fine-tuning the output parameters, the fabrication of the semi-transparent electrode/reflector would need to be very precise to be able to control the reflectance. Also, the reflectance has a maximum value which is lower then achievable by a DBR since the metal will absorb some of the energy.
Patent publication WO2017051298A1 (Distributed Bragg Reflector on Color Conversion Layer with Microcavity for Blue OLED Lighting Application) describes the fabrication techniques and geometries categorized by device components. This patent publication refers to the fabrication and design for blue OLEDs only. An unknown OLED deposition process is applied for the DBR and the structure includes the combination of inorganic and organic layers to create a bottom emitting device with a flexible substrate and the pixels here are controlled using a passive matrix.
U.S. Pat. No. 7,489,074 (Reducing or Eliminating Color Change for Microcavity OLED Devices) and US 2006/0066220 A1 (Reduction or Elimination of Color Change with Viewing Angle for Microcavity Devices) disclose a multi-layered mirror structure at the bottom of the OLED structure, resulting in a bottom-emitting device. The structure of the OLED includes the use of two electrodes on either side of the emission layer. The disclosed device includes a light modulation thin film on a front surface of the substrate, one of which is a microstructure that redistributes wavelengths so the outputted emission spectrums have the same perceived color.
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