LCDs to OLEDs – Evolution of Display Technology

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Display technology has been an ever-intriguing space, evolving dramatically since the days of the first television sets. Liquid crystals were first observed in the 19th century; however, it was only in the 1960s that the foundation of modern-day liquid crystal displays (LCDs) was laid by James... Featured image is intended for representational purpose alone and has been sourced from https://pxhere.com/en/photo/597333

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Display technology has been an ever-intriguing space, evolving dramatically since the days of the first television sets. Liquid crystals were first observed in the 19th century; however, it was only in the 1960s that the foundation of modern-day liquid crystal displays (LCDs) was laid by James Fergason, who was then an associate director at the Liquid Crystal Institute at Kent State University. He discovered the twisted nematic effect of liquid crystals and patented its application in the 1970s. This technology made the displays superior to earlier LCDs that worked on a dynamic scattering method and were low in performance and also power hungry.

Today, the world has developed various versions of LCDs that are way better than the Twisted Nematic (TN) LCDs, the major one being In-Plane Switching (IPS) LCD. The market for display technology has progressed and has advanced from LEDs to OLEDs, with continued progression.

iPhone X & its OLED – Going Behind the Screen 

iRunway conducted a comprehensive teardown analysis of the iPhone X and provides an in-depth research of its OLED display. For deeper insights, click here.

 

TN LCD – How it works

FIG. 1 – Main components of a TN LCD

In a typical TN LCD, liquid crystals are in the nematic phase, i.e., in the form of rod-shaped molecules that self-align along their long axis and achieve long-range directionality. These liquid crystals are placed between two glass substrates – one including electrodes (typically made up of the transparent Indium Tin Oxide (ITO)) in rows, and the other including electrodes in columns. The intersection of each horizontal and vertical electrode defines a sub-pixel that represents the smallest area on a screen through which the intensity of light can be controlled. In a typical pixel arrangement for a color display, three sub-pixels – one red, one green and one blue – form a pixel.

Alignment layers, typically made up of polyimide, are arranged at right angles to each other. They are placed over electrodes and include parallel grooves to align liquid crystals close to the layers. This aids the liquid crystal layer (LC layer) to take a twisted shape.

A rear polarizing filter polarizes the unpolarized light generated from the backlight. The LC layer helps modulate the amount of light required for generating the image. In the absence of the LC layer, no light will pass through the combination of two filters.

FIG. 2 – Working of a TN LCD

Thin-film transistors (TFTs) on the rear glass substrate are connected to electrodes via capacitors and are driven by a driver IC to provide voltages to these electrodes. When no voltage is applied, light polarized by the rear polarizing filter orients itself with the liquid crystals and aligns with the polarization axis of the front polarizing filter. This allows light to pass through the front polarizing filter with minimal obstruction. A voltage applied across the LC layer using the electrodes untwists the LC layer between the electrodes. This misaligns the light with respect to the polarization axis of the front polarizing filter and results in lesser light passing through the front polarizing filter.

The white light modulated by the LC layer is typically passed through a color filter placed between the front electrode and the front polarizing layer. This produces different colors on the screen.

LEDs powering LCDs

What differentiates an LCD screen from an LED screen is the backlight. Although earlier LCDs included CCFLs for generating white light, current LCDs use LEDs to provide light and are hence called LED screens. These LEDs are distributed either behind the display panel (back-lit LED screens) or along the edge of the display panel (edge-lit screens). Typically, diffusion layers in between the backlight and the rear polarizing filter are used to distribute light.

IPS LCD – the next step

TN LCD was a breakthrough technology at the time of its introduction. However, the presence of many layers reduced the overall efficiency of the LCD. Additionally, the color vs. power tradeoff in TN LCDs resulted in a poor color gamut (different colors that a device can reproduce). Improving the color gamut meant increasing power consumption of the display.

Fergason’s technology was adopted in watches, calculators and early mobile phones. Even mobile phones and devices with color displays manufactured in the early 2000s such as the Apple iPhone, Apple iPhone 3G and 3GS included TN LCDs. However, technological advancements and the need for displays with better viewing angles and color gamut led to the development of IPS LCDs.

FIG. 3 – Working of an IPS LCD

Unlike in TN LCDs, IPS LCDs have both electrodes – the anode and the cathode – in the same plane. This allows for easy manipulation of the intensity of light, bettering the viewing angle and color reproduction. Further, as the liquid crystals in an IPS LCD are loosely bonded to other layers, the liquid crystals easily rotate in a plane parallel to the TFT plane and color filters, resulting in better response time.

Since its first practical implementation by Hitachi Displays in 1996, IPS LCDs have been widely adopted in devices such as displays and smartphones. Apple included an IPS display for the first time in iPhone 4, and since then, has used it in every iPhone till iPhone 8 Plus.

However, IPS LCDs also require backlight, leaving scope for improvement in contrast ratio. There was also a growing demand for a display with even better color gamut.

Entering the OLED era

The OLED stack replaces the LC layer in LCDs and is capable of producing its own light, negating the need for a backlight. A typical OLED display includes a TFT backplane, the OLED stack and a polarizing filter on top. Fewer layers and no backlight give OLED displays two major advantages – flexibility and thinness. Fewer layers also mean better viewing angles. OLEDs also have better color gamut.

The OLED stack includes an anode and a cathode. A voltage difference between the two electrodes causes movement of holes and electrons and their recombination in the emission layer, thus producing light.

FIG. 4 – Main components of an OLED screen and the OLED stack

The advantage that OLEDs have over other display screens is good contrast ratio that is brought about by better control in blocking the light passing through them. Despite all the improvements, none of the technologies so far have been able to achieve an ideal display – a screen that can display full colors stays, is easy on the eye in all lighting conditions, and uses less power. With more players in the smartphone space than before and with more alternative technologies, it will be interesting to see how the smartphone world unfolds itself in the coming future.

Watch out for our upcoming blogs on technology that betters OLED performance and future of smartphone technology.

iPhone X & its OLED – Going Behind the Screen 

iRunway conducted a comprehensive teardown analysis of the iPhone X and provides an in-depth research of its OLED display. For deeper insights, click here.

 

Featured image is intended for representational purpose alone and has been sourced from https://pxhere.com/en/photo/597333

Kushal Jain
Kushal Jain

An instrumentation and control engineer by profession, Kushal wishes to be a strategy consultant someday, focussing on advanced electronics and semiconductors. When not analyzing complex fabrication patents, you will find him following current affairs and technology.


Girajala Ramcharan
Girajala Ramcharan

An electrical and electronics engineer by profession and inventor by heart, Ramcharan wants to contrive a technology completely novel.


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