Creating Impressive Images
with the Small-Sized,
High-Resolution
“OLED Microdisplay”
An OLED Microdisplay with
pixel sizes on
the order of several micrometers
OLED displays are self-emitting displays which boast excellent video response times and vibrant color expression. They have seen wide usage including in televisions, monitors, and smartphones.
Sony has managed to scale down the pixel size, which is generally tens to hundreds of μm (micrometers), to the order of several μm using proprietary OLED technology and silicon semiconductor drive technology. Thus, creating high-resolution panels as small as one inch square while preserving the advantages of OLED has been made possible. This is the so-called OLED Microdisplay technology (Sony defines OLED Microdisplays as those that use CMOS circuits on silicon substrates to drive pixels with a size of roughly 10 μm or less).
OLED Microdisplays have carved out a unique position among various display technologies, including liquid crystals and LEDs, with their high image quality and resolution, small size, and fast response time. In particular, they have been rated highly when used in devices such as EVFs for DSLR cameras and head-mounted displays. We are currently performing development with the expectation that demand will increase along with usage in wearable displays such as AR glasses and VR-HMD.
Comparison between a smartphone OLED and an OLED Microdisplay
It began with EVFs for cameras
Sony began development of the technology that would lead to OLED Microdisplays in 2009. At the time, mirrorless interchangeable-lens cameras were gaining steam, and the fact that the performances of liquid crystal microdisplays were insufficient compared to optical viewfinders was becoming a problem.
The major difference compared to liquid crystal microdisplays is OLED Microdisplays are self-emitting displays. Due to this, their contrast is vibrant, and they are capable of properly reproducing black hues, which liquid crystal has trouble with. Also, color breakup sometimes occurs with liquid crystal microdisplays whenever the camera is shaken. Due to fast response time, this is not seen in OLED displays, and motion blur is difficult to occur.
The first OLED Microdisplay, implemented in 2011, was subsequently used in many camera models’ EVFs. This is currently their main use case.
The engineering principles to
realize miniaturization and
high resolution
To create high image quality, we are utilizing top-emitting white OLEDs with color filters (CF) for devices with OLED Microdisplays. The structure of light emissions and the technology used for the top-emitting white OLED method are shown in the chart.
The silicon substrates used in OLED Microdisplays do not transmit visible light, so a top-emitting method to extract light from the CF glass substrate side is used.
There are two methods of forming color pixels for OLED displays, the white OLED method and the shadow-mask patterning OLED method. Super Top Emission™, announced by Sony in 2007, uses shadow-mask patterning OLED method to form a light-emitting material film for each individual pixel. OLED Microdisplays have pixels on scales less than one tenth of those on regular OLED displays, or below 3μm, for example. These are difficult to process using existing fine metal masks. Thus, we are utilizing the white OLED method, capable of forming a film of the same organic material on the entire surface.
By applying voltage to the electrodes on both ends of the OLED layer on the silicon substrate, the luminescent material emits white light, which is then dispersed by a different CF for each pixel, transmitting the glass substrate. Usually, the smaller the sub-pixel size, the more light and current will be mixed into adjacent pixels, and the characteristics and image quality will be more likely to deteriorate. We have suppressed the deterioration by optimizing the CF structure, controlling the alignment between the silicon substrate and the CF substrate, and optimizing the material and layer composition of the electrodes and OLED layer.
To establish this method, many of Sony’s technological assets were utilized. For example, OLED device design technology developed by Sony over many years, as well as high-efficiency and long-lifetime devices developed jointly with material manufacturers. Additionally, the 3μm CF RGB processing technology uses techniques developed for use in CCD image sensors.
The challenge of precisely
controlling microcurrents
One problem we faced when trying to develop more efficient high-resolution OLED Microdisplays was controlling the current with transistors on silicon substrate.
To make fine-pitch pixels emit light for OLED Microdisplays, it is necessary to precisely control the current, which is less than 1/1,000 of that used in OLEDs for smartphones. The current for emission per control voltage is shown in the following chart.
The vertical axis shows the current and the horizontal axis shows the control voltage. In TFTs used in the drive substrates of smartphone OLEDs, the current is controlled in the area defined by the square of the control voltage and the transistor threshold voltage, but for OLED Microdisplays, control in microcurrent areas defined by exponential functions that would be treated as mere leakage currents using regular CMOS logic is required. In such areas, even a small change in voltage causes an exponential increase or decrease in the amount of current. As a result, the brightness of each pixel varies significantly.
Under these circumstances, how do we control the absolute value of the current? We could not optimize the behavior of the transistor used to control microcurrents by simply changing conditions in a process of trial and error. Instead, we went back to basics and created formulas based on physical properties and then ran simulations to back up the theory. After being assured of the theoretical validity, we performed the trial production which gave us the expected results. Sony’s pixel circuit drive technological assets, developed over many years, were utilized as well. Although requirements for TFT and CMOS circuits differ, we sought the optimal pixel circuit technology for silicon substrates with an emphasis on characteristic stability, successfully establishing the technology in 2017.
The aim is high resolution, high
brightness, fast response time,
and low power consumption
Sony’s OLED Microdisplays make use of advanced technological know-how and expertise and boast pioneering high-resolution technology, making use of years of manufacturing experience. To create images which feel real in EVFs and wearable device displays in the future, even higher resolution, higher brightness, faster response time, and lower power consumption will be required. Especially with regard to wearable devices, the higher power consumption makes the device hotter, and it would become an obstacle to the user experience.
Also, in the entertainment industry, higher refresh rates will be important for games. With increasing framerates due to the improvement of graphics processing power, displays that can fully utilize the capability of graphics are required.
Researching new light sources
using open innovation
The challenge to expand the possibilities of OLED Microdisplays is underway. It involves combining existing visible light OLED Microdisplay technology and near-infrared light emitting materials invisible to humans to research new light sources. Sony is the first company in the world to integrate near-infrared OLEDs in microdisplays.
For the research, the microcavity structure design techniques (using the resonance effect of light between the upper and lower electrodes to make a steep and strong spectrum for the light emitted outside) used in Super Top Emission™ and the current control technology were among the technologies expected to be utilized for near-infrared light. However, we had scant experience with OLED devices that emit near-infrared light and were facing problems related to low luminous efficiencies.
The amount of emitted near-infrared light cannot be expressed in candelas, and instead the term external quantum efficiency (EQE) is used. This is the ratio of the number of outcoupled photons to the number of electrons injected into the light emitting element. Usually, the EQE gained by visible light is around 30% at most. But when the wavelength goes over 900nm to the near-infrared spectrum, it decreases to below 1%.
To tackle this problem, we decided to explore ongoing external research efforts. We collaborated with Kyushu University’s Center for Organic Photonics and Electronics Research, which is researching next-generation high-performance luminescent materials. Thus, near-infrared OLED Microdisplay research continued as a result of open innovation. At the beginning, EQE was stuck at below 0.1%. However, by fusing Sony’s top-emission technology with new materials created by utilizing the material technology of the Center, it has become possible to achieve practical EQE in 900nm-band near-infrared OLED.
Anticipating uses of sensing field
In addition to AR and VR uses in the metaverse (services within a virtual three-dimensional space), near-infrared sensing is expected to see applications in the new fields of IoT and robotics. Three-dimensional measurements which combine Sony’s sensors and small, high-resolution light sources will allow more precise and speedy observations of an object’s shape, possibly leading to rapid progress in sensing technology. Additionally, it may be possible to develop devices with both display and sensing capabilities by adding near-infrared light to visible light displays.
Many Sony engineers are involved in developing OLED Microdisplay technology, including the design and fabrication process of semiconductor devices such as transistors and wiring, pixel circuit and light element design, film formation, CF patterning, and glass encapsulation. This wide-ranging technology allows various experts to contribute at their full potential. Our efforts to provide unique value to Sony’s customers through impressive images will continue in the future as well.
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Researchers
Akitsuna Takagi
Display development is a steady process of finding ways to control the light and electricity within pixels. But once a display is complete, it can stir emotion in a great number of people through the use of wonderful images. The ability to witness our contributions directly is the best part about this job. OLED Microdisplays are a technology with plenty of room for further developments. As an engineer, I feel very satisfied being able to tackle new challenges at the cutting edge of the world’s technology.
Kei Kimura
Being able to see the results of your efforts immediately upon turning on the power feels very intuitive and interesting. Compared to image sensors, OLED Microdisplays are still only a small field, but because of that, each and every person’s ideas can contribute to manufacturing new products, which is a great opportunity for young engineers.
