OLED Displays

Organic Light-Emitting Diodes (OLEDs)

OLEDs contain organic compounds made of carbon and other substances to light up (Illuminate) and display characters, icons and graphics in a wide variety of colors that are readable in daylight and nighttime. Since they generate their own illumination, they are called an emissive display.

LCDs (Liquid Crystal Display) are readable in strong ambient light (sun, office light) but require a backlight for readable in dark environments.

Key advantages of OLEDs include:

OLEDs draw less power than LCDs containing a LED backlight
A faster refresh rate which produces a sharper contrast
Higher brightness without increased power consumption
An advantage for battery powered products
Thinner than TFT and character displays
Wider operating temperature allowing the display to operate to -40C
Wider viewing angle
Produce darker blacks than LCD technology
An OLED with an off pixel produces a dark black that is not possible for LCDs
LCDs can produce brighter white with their LED backlight

AMOLED vs. PMOLED

OLEDs are broken down into two categories: PMOLED (Passive Matrix Organic Light-Emitting Diode) and AMOLED (Active Matrix Organic Light-Emitting Diode).

The word ‘matrix’ just means how the display is driven. Below is a rough drawing of a matrix of pixels. Each pixel is either on or off to generate an image.

There are five rows with each row containing several pixels.

PMOLED

Passive displays control/refresh one row at a time. Once that row is refreshed, the control moves to the next row and so on. That means that the rows are on a small percentage of time which limits their size.

Another word for this is duty cycle which is the ratio of time ON vs. time OFF. This can be seen in refrigerators where the compressor is on for ten minutes for every thirty minutes off.

Duty Cycle = 10 minutes (on) / 40 minutes (total time on and off) = 25% duty cycle

The lower the duty cycle, the more time before the display is refreshed, the more power required to illuminate the pixel, and the duller the contrast.

Once control moves to the next line of pixels, the brightness of the previous line of pixels fades. To fix this and provide an even light for the human eye, more power is required to increase the brightness when the pixels are refreshed.

If your display has 5 rows, then you need to make the pixel 5 times brighter which equates to 5 times more power. This increased brightness reduces the display’s half-life.

Half-life is the amount of time in hours for the display to be half as bright as when first turned on. This is not the amount of time for it to burn out, but just to grow dim.

Even with this set back, PMOLEDs are popular because they are less expensive and easier to manufacture.

AMOLED

AMOLEDs produces a higher quality image by adding one capacitor for each pixel. A capacitor is a mini battery that stores power and illuminates the pixel after it has been refreshed. This allows the pixel to stay active and not fade away when refreshed.

The capacitors increase complexity and therefore, AMOLED’s are more expensive, but requires much less power and can produce larger displays than PMOLED modules.

Advantages of AMOLEDs:

Wider viewing angle
Thinner displays since there is no need for a backlight
More colors
Brightness of each pixel is independently controlled.
LCDs can independently control each pixel, but not the brightness of that pixel. The brightness is control by the LED backlight that sits behind all the pixels.
A sharper contrast
Contrast is the difference between the brightest and the darkest color

Disadvantages of OLEDs:

They may have a possible burn in due to half-life. If some pixels are on more often than others, the high usage pixels could dim over time.
The brighter each pixel, the more power required.
OLEDs contain an organic material that may lead to a shorter life-time than LCDs
Pixels

Pixels (aka picture element) are very tiny dots to create the image or character or icon. The more pixels in an area, the sharper the resolution.

A 320 x 240 graphic display contains 320 pixels (or dots) along the x-axis and 240 pixels along the y-axis for a total of 76,800 pixels.

Each pixel is its own light source and consists of three sub-pixels that are Red, Blue and Green (RGB). Each pixel can be driven at different intensities to generate a wide possibility of colors.

Many displays can generate between 64,000 to 64,000,000 unique colors by mixing a combination of Red/Green/Blue.

Viewing Angles

OLEDs offer a much wider viewing angle than TFTS and LCDs (Liquid Crystal Displays). LCDs (FSTN) can provide up to a 120-degree viewing angle, but there will be a viewing bias. That is, one of the four views (top, bottom, left or right) will have the sharpest viewing angle.

Lifetime

At one time, the lifetime of an OLED ranged from 10K to 15K hours. Now, it is much closer to 100K hours.

Lifetime for an OLED is when the image becomes burned in.

Touch Screens

Many graphic OLEDs contain a built-in touch screen. Touch screens are not standard on character displays but can be added.

There are two main types of touch screens

Resistive Touch Screens (RTS)

RTS consists of combining two layers of material with a small air gap between each layer. When the user applies pressure to the top layer, it makes contact with the lower layer and creates a short circuit. The touch screen is then able to calculate the exact location of the touch.

Resistive touch screen’s advantage is the freedom for the user to choose any type of stylus; A stylus is a device used to make contact and apply pressure to the top layer of the touch panel such as a bare finger, fingernail, gloved finger, edge of a credit card or the soft end of a pen.

They are cheaper than Capacitive touch screens and easier to integrate.

Capacitive Touch Screens (CTS)

Capacitive touch screens are found in many consumer products such as cell phones, tablets and medical equipment. The touchscreen panel contains an insulating material, in many cases it is glass, coated with a transparent conducting material such as indium tin oxide (ITO). The human skin is also an electrical conductor, so that when the surface of a finger contacts the surface of the glass, the electrostatic field is interrupted. This disturbance locates the position of contact.

Capacitive is growing in popularity since it can pinch and zoom. Two features that are not available in most RTS.
CTS is more expensive, more integrated and not as rugged as RTS.

Character OLEDs

Character OLEDs can display letters, numbers and punctuation marks. They contain a controller driver with a built-in character table that reduces programing steps when developing the display’s firmware.

Character LCD Table

Displaying a letter is as easy as sending the ASCII number to the controller. The controller than draws the letter at the specified location and automatically refreshes each character to maintain a sharp contrast.

OLED vs. LCD Power Consumption

OLEDs are an emissive technology, meaning that the characters/segments glow when the display is on. This is also known as a negative mode display meaning the background is dark and the letters/segments are light colored.

Negative mode modules stand out to catch the user’s attention and allow the display to be readable at night but draw additional current than a Liquid Crystal Display.

LCDs do not glow and without a backlight they can operate on as little as 1mA. The addition of a LED backlight is necessary for nighttime operation but will increase current by 15mA to 90mA depending on the number of LEDs and how brightly they are driven.

Graphic OLEDs

Graphic OLEDs can display letter, numbers, images, video etc. They use dots (pixels) in a matrix pattern. The dots are turned on and off by the customer’s firm wear.

Graphic displays are identified by the size of the glass that is measured, in inches, along the diagonal. Common sizes include .91”, 1.54”, 2.42” and others.

The resolution of graphic displays is measured by the number of dots along the horizon (X-axis) by the number of dots along the vertical (Y-axis). The more dots, the sharper the contrast. Examples are 320×240, 180×180 etc.

Monochrome vs. Multicolor Display

Why aren’t all OLEDs multicolor?

Most people think all OLEDs as a multi-color displays that can generate between 64K to 64M unique colors from the combination of Red, Green and Blue pixels.

But there is a demand for monochrome displays because they produce one color background (normally black for an OLED) and a different color character, normally white for OLEDs but can be other colors.

Monochrome displays produce a sharp contrast that makes it easy them easy to read. Many non-consumer industries such as industrial, construction, test and measurement and even medical applications use monochrome to provide the sharpest image possible.