In this blog post, I will provide an easy-to-understand overview of the basic principles and structures of LCD and OLED, as well as the pros and cons of each technology.

Basic Concepts of Displays

Displays are core components that serve as the “face” of most electronic devices, such as TVs, computers, and smartphones. As electronic devices have become more sophisticated, the demand for higher resolution and more vivid images has grown, driving continuous advancements in display technology. In this field, where Korean companies have a strong presence, LCD and OLED have established themselves as the leading technologies.
A pixel refers to the smallest square unit on a screen that displays a single color, while resolution refers to the total number of pixels that make up the screen. For example, a resolution of 1024 × 768 means the screen consists of 1024 pixels horizontally and 768 pixels vertically. In this context, ppi (pixels per inch) represents the number of pixels per square inch and indicates pixel density. The higher the ppi, the more natural and sharp the image appears, similar to how dots are placed more densely in pointillism. For instance, smartphones with higher ppi typically offer higher-quality screens.
Most color displays use the three primary colors of light—RGB (Red-Green-Blue)—to produce color. Therefore, a single pixel is structured to produce these three colors, and by combining brightness and color, it creates the images we see.

Principles and Structure of LCDs

LCD stands for Liquid Crystal Display, and its basic operating principle utilizes the polarization of light and the molecular alignment of liquid crystals. First, when a backlight emits white light from behind the display, this light passes through a polarizing filter, leaving only light that vibrates in a specific direction (polarized light). If light polarized in one direction encounters a polarizing filter at a right angle, the light cannot pass through, resulting in a dark state.
Liquid crystals appear similar to liquids but have the property of changing their molecular alignment in response to electrical signals. When no voltage is applied, the liquid crystal molecules act to rotate the direction of light vibration in a specific manner, allowing light that has passed through the rear polarizer to also pass through the front polarizer. This results in a visible image. Conversely, when voltage is applied to change the arrangement of the liquid crystal molecules, the rotation of the light changes, preventing it from passing through the front polarizer and creating a dark pixel. This process is used to control the brightness of each pixel.
An actual LCD panel consists of multiple layers, including a backlight, polarizing filters, a liquid crystal layer, and a color filter. Each pixel contains a transistor that controls how much voltage is applied to specific areas of the liquid crystal, thereby determining brightness and color. The white light from the backlight passes through the color filter, where it is separated into RGB colors to produce the desired image.
Disadvantages of LCDs include relatively slow response times due to the time required for the liquid crystal to align under voltage, and the difficulty of making the panel very thin due to the numerous components. Additionally, because a backlight is always required, there are limitations in displaying true black, and constraints on implementing flexible (bendable) displays make them less suitable for certain applications.

Principles and Advantages/Disadvantages of OLED

OLED stands for Organic Light Emitting Diode, a display technology that uses fluorescent organic materials to emit light on its own. Unlike LCDs, it does not require a backlight or liquid crystals, resulting in a relatively simple structure.
The basic structure consists of an anode and a cathode on a substrate, with layers through which current flows stacked in between. When a voltage is applied, electrons are supplied from the cathode, and holes (where electrons have been removed) are created at the anode. When electrons and holes meet and recombine in the emissive layer, the energy is released as light, causing the pixel to emit light. These pixels come together to form the screen.
The advantages of OLEDs include the ability to produce deeper blacks because they do not require a backlight due to their self-emissive nature, as well as high energy efficiency and fast response times. Additionally, they offer wide viewing angles, and when the substrate is made of plastic, it enables the creation of flexible displays that can be bent or folded. For these reasons, OLEDs are highly sought after in smartphones, high-end TVs, and wearables.
On the other hand, challenges include the limited lifespan of organic materials, burn-in issues, and manufacturing costs and yield problems associated with large-area or high-density production. Research and process improvements aimed at reducing production costs and extending lifespan are currently underway.
In summary, LCDs use polarization and electrical control of liquid crystals to modulate light; while their structure is complex, they are widely adopted. OLEDs, with their self-emissive structure, offer advantages such as better contrast ratios, response speeds, and flexibility, but challenges remain in terms of manufacturing and lifespan. The two technologies coexist in different applications and markets, each leveraging its own strengths.