Liquid crystal displays (LCDs) are currently the most mainstream human-computer interaction interface. Despite the emergence of new technologies, technologies like the latest Mini-LED still fall under the LCD category. How do we distinguish them in practice, and how do they differ from older technologies?
Liquid crystal (LC) is a physical phase. Due to its unique physicochemical and photoelectric properties, it is widely used in display technology, greatly improving the thinness and lightness of devices, and has become the most prevalent display technology today.
Therefore, basically all the various LCD screens widely discussed today fall under the category of LCD (Liquid-crystal Display). However, the term LCD currently used in the market refers specifically to active-matrix TFT-LCD technology; other technologies such as passive-matrix STN LCD have been phased out.
TFT-LCD stands for Thin-film transistor-liquid crystal display. Each liquid crystal pixel on the LCD is driven and independently controlled by a thin-film transistor integrated on the back, which not only improves response speed but also allows for precise control of color gradation. This is the foundation of current consumer products; its technology is not only quite mature but also inexpensive.
Types of Liquid Crystals
LCD Panel Structure
TFT-LCDs primarily work by sandwiching a layer of liquid crystal between two glass substrates. The upper glass substrate is a color filter, while the lower glass substrate houses transistors. When current flows through the transistors, the resulting electric field deflects the liquid crystal molecules, altering the light. Voltage is then used to determine pixel brightness, and each pixel contains the three primary colors (red, green, and blue) to create the image output.
Although its circuit layout is similar to DRAM, except it's built on glass, its manufacturing process mainly involves creating amorphous or polycrystalline silicon layers, rather than requiring advanced epitaxial transistors.
Based on this technology, products with differentiated quality and cost have been developed. Currently, there are three main types: TN, VA, and IPS panels, differing primarily in the liquid crystal layer.
Twisted nematic liquid crystals (TN liquid crystals) are the lowest-cost type of LCD panel; however, their pixel response is already quite fast, sufficient for most needs. Furthermore, Samsung has further developed B-TN technology, offering faster response and richer colors.
The viewing angle of TN LCDs is a significant problem. VA LCD panels represent a further solution, achieving a viewing angle of nearly 170° even without special compensation films. This is due to replacing TN LCDs with vertically aligned liquid crystals (VALs), resulting in higher contrast. However, VALs are slower and more expensive than TN, placing them in the mid-range market.
Currently, the highest-end TFT-LCD is IPS, employing in-plane switching technology to effectively improve viewing angles and address various problems of traditional TN panels. It boasts excellent viewing angles, consumes less power than VA panels, and is ideal for touchscreens. Early Apple iPhones and iPads used IPS LCDs, but were naturally more expensive.
Due to the rapid development of TFT-LCD manufacturing processes, before the emergence of new LED technologies, discussions about panels primarily revolved around these three main categories. Of course, there were other new varieties such as Samsung's PLS and PVA panels, and Fujitsu's MVA panels, but these were essentially results of maintaining performance while striving to keep costs down, all while offering wide viewing angles. As a result, traditional narrow-viewing-angle TN panels were gradually phased out.
The Success or Failure Lies in Open Cells
Although currently a highly competitive market, panels were once a booming industry in Taiwan. TFT-LCD panel components mainly include glass substrates, backlight modules, polarizers, color filters, and related materials such as optical films. Taiwanese companies are involved in most aspects of the overall supply chain, but well-known giants like AU Optronics and Innolux primarily manufacture open cells and assemble modules.
The open-cell manufacturing process for TFT-LCDs is generally divided into three stages: front-end, mid-end, and back-end. The front-end primarily refers to the fabrication of the TFT glass, a process similar to semiconductor manufacturing, involving coating and etching to embed thin-film transistors onto the substrate glass. The mid-end involves bonding the TFT glass with color filters and adding a polarizing plate. The back-end involves laminating the driver IC and printed circuit board to complete the so-called open cell, but this is actually only a semi-finished product.
The front-end, similar to semiconductors, involves the array and TFT glass manufacturing process.
It's worth noting that although a backlight panel is still required for use, many end-product manufacturers choose to directly purchase open cells to achieve integrated panel packaging and product packaging, allowing for greater design flexibility. This is especially prevalent in mainland China's TV assembly plants, which account for over half of the global production capacity, where the BMS (Backlight Module System) model is quite common.
While directly shipping open cells does indeed yield higher profits than modules, saving panel manufacturers significant material management costs, it's important to note that this is a double-edged sword.
If panel manufacturers focus solely on producing semi-finished products, the high standardization of these products makes it difficult to develop product portfolios to meet diverse functional needs, easily leading to oversupply and ultimately a loss of market dominance. Therefore, some research suggests that the shift to Open Cell technology by panel manufacturers has actually caused prices to plummet, which is detrimental to the capital-intensive panel industry in the long run. This is also one of the reasons why mainland Chinese manufacturers have easily surpassed established giants through price subsidies.