The demand for larger, brighter, curved, higher resolution, and higher contrast ratio displays in automotive applications is stronger than ever. New types of in-vehicle displays are becoming increasingly popular. Currently, TFT LCDs dominate the flat panel display technology in the automotive industry. However, OLED and micro-LED displays are gaining more attention from car manufacturers due to their excellent display effects, low energy consumption, high flexibility, and ultra-thinness. This article compares these different display technologies and discusses the 2T1C pixel driver for LCD displays and the 7T1C/2C pixel driver for OLED and micro-LED displays.
Introduction
This article discusses TFT LCD, OLED, and micro-LED displays in automotive applications. The article comprehensively compares characteristics of TFT LCD, OLED, and micro-LED displays and the techniques of different pixel drivers. Also, TFT technology and the power techniques of automotive displays are discussed.
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This article focuses on the basics of automotive displays, including the trends, challenges, and display architecture. Pixel drivers are introduced to help understand the power technology in automotive displays..
AutomotiveDigitalCabinTrends
In recent years, the automotive industry has rapidly evolved in areas such as connectivity, electrification, autonomous driving, and shared mobility. This evolution has reshaped cockpit design to enhance the user experience. The demand for larger, curved, higher resolution, and higher contrast ratio displays, as well as new types of in-vehicles displays, is growing stronger. According to IHS Markit Automotive Display Market Tracker, 161.5 million automotive display panels were shipped in 2018, and shipments are expected to exceed 200 million units by 2025.
To enhance the cabin experience, modern vehicles feature multiple types of displays: the instrument cluster, the center information display (CID), heads-up display (HUD), passenger display, smart e-mirror display, side mirror display, and rear entertainment display. The instrument cluster provides the driver with key information such as speed and fuel gauge status. The HUD projects crucial information onto the windshield. The rear seat entertainment displays and passenger displays are part of the infotainment system, allowing passengers to watch movies or engage in other entertainment activities. The digital camera monitor system (CMS) is replacing exterior rearview mirrors with two to three cameras, and the side mirror displays and e-mirror displays enhance the driver’s visual perception of the surroundings.
According to IHS Markit Automotive Display Market Tracker, 10% of automotive display panel shipments were 10-inch panels in 2018, and this is expected to grow to 18.4% by 2025.1 In recent years, 12.3-inch and larger display panels have become the mainstream size for instrument cluster displays.
Since the Tesla Model 3 introduced a 15-inch touchscreen in 2017, automotive display panels have trended toward being larger, with higher resolution, higher contrast ratios, and free-form designs. In 2019, the Li Xiang ONE featured a pillar-to-pillar display that includes two 12.3-inch and 16.2-inch screens. In 2023, the BMW 3 Series introduced a 14.9-inch curved touchscreen with local dimming technology, stretching from the driver’s side to the center console. The concept car VISION EQXX is equipped with a 47.5-inch pillar-to-pillar screen that uses local dimming technology. The trend of automotive displays can be summarized as shown in Figure 1.
Figure 1.Trends in cabin displays.
The full-windshield HUD on BMW’s concept car, Neue Klasse, will enter production in 2025. This innovative HUD technology allows visible displays across the entire width of the windshield for all passengers. The lower edge of the windshield, with higher light intensity and contrast, shows relevant information to the driver and passengers. Additionally, there is a free-form central display.
Display Panel Architecture
The TFT LCDs in Figure 2, OLEDs in Figure 3, and micro-LEDs in Figure 4 represent three distinct technologies that revolutionize visual display capabilities.
TFT LCDs utilize liquid crystals sandwiched between two glass substrates. The bottom substrate is embedded with TFTs, while the upper substrate serves as a color filter. These liquid crystals align to modulate the rotation of light passing through them by controlling the current flow through the transistors, which causes changes in the electric field. Each pixel with a different color is generated by illuminating the color filter in varying proportions.
Figure 2. TFT LCD display structure.
In contrast, OLED displays do not require a backlight due to their self-emissive capability. The basic structure of OLEDs consists of an organic light-emitting layer on indium tin oxide (ITO) glass. This organic light-emitting layer is sandwiched between two low work function metal electrodes: the upper cathode and the bottom anode.
When an external voltage is applied to the cathode and anode, the electron transport layer (ETL) and hole transport layer (HTL) inject electrons and holes into the organic light-emitting layer with controlled volume and speed. This process causes the OLEDs to emit light. Red, green, and blue light can be produced by using different chemical materials in the OLEDs. Consequently, OLED displays are thinner, more energy efficient, and offer superior color reproduction and contrast.
Figure 3. OLED display structure.
Micro-LED displays are a recent advancement that use arrays of microscopic LEDs as individual pixels. Typically, the chip size of micro-LEDs is within 50 µm, making them hardly visible to the human eye. Due to their tiny size and advanced assembly technology, the illumination sources for red, green, and blue light can be integrated into a single pixel point, eliminating the need for color filters and liquid crystals in micro-LED displays.
Each micro-LED in the pixel emits its own light, offering high brightness, deep blacks, and excellent energy efficiency. These technologies represent significant strides in display innovation, each offering unique advantages in terms of structure and performance. Micro-LED displays are suitable for applications ranging from smartphones and televisions to augmented reality, wearables, and automotive displays.
Figure 4. Micro-LED display structure.
As TFT LCDs are a relatively mature technology with outstanding cost advantages, LCDs are currently the dominant flat panel display technology in the automotive industry. However, OLED displays and micro-LED displays are drawing more attention from car manufacturers.
OLED displays offer excellent display effects, low energy consumption, high flexibility, and ultra-thinness. Micro-LED displays are emerging as the next-generation display technology, enabling curved display designs with enhanced brightness and contrast, thereby adding flexibility to in-cabin screen designs.
However, OLED displays suffer from image retention, causing pixel degradation after displaying static images for a long time, and their lifespan is shorter than that of LCDs. Micro-LED displays are expensive due to the challenges in commercializing mass production.
The detailed differences between TFT LCD, OLED, and micro-LED displays are summarized in Table 1.
Table 1. Comparison of LCD, OLED, and Micro-LED Displays
| TFT LCD | OLED | Micro-LEDs | |
| Method | Backlight | Self-emissive | Self-emissive |
| Luminance Efficiency | Low | Medium | High |
| Contrast | Medium | High | High |
| Response Time | ms | µs | ns |
| Power Consumption | High | Medium | Low |
| Compactness | Low | Medium | High |
| Lifetime | Long | Medium | Long |
| Viewing Angles | Low | High | High |
| Cost | Low | Medium | High |
The existing TFT LCD displays can be upgraded with a mini-LED (submillimeter light-emitting diode) backlight source and local dimming technology. Mini-LEDs are scaled-down conventional LEDs and serve as a bridge to micro-LEDs. LEDs with dimensions smaller than 200 micrometers are categorized as mini-LEDs, while LEDs under 100 micrometers are categorized as micro-LEDs.5
Although mini-LEDs can primarily serve as a backlight source in LCD displays, they improve the thickness and contrast performance of LCD displays, while offering cost-effective solutions.
Pixel Drivers
Various colors are synthesized by mixing the three primary colors (red, green, and blue). The mixture of these three primary colors forms a pixel, as shown in Figure 5. Each pixel consists of three sub-pixels, which are managed and combined in one pixel.
TFT LCD, micro-LED, and OLED displays use different methods to drive these sub-pixels due to their distinct display technologies and manufacturing processes. For example, the Tesla Model 3 features a 15.4-inch TFT LCD display with a resolution of 1920 by 1200 pixels, resulting in a total of 6.91 million sub-pixels.
Figure 5. Pixel.
In a TFT LCD display, the equivalent circuit of a sub-pixel, which controls the electric field across the liquid crystal, is shown in Figure 6. It comprises 1T2C (one transistor, one liquid crystal capacitor, and one storage capacitor). The gate driver provides a positive voltage, called voltage gate high (VGH), to turn on the TFT, and a negative voltage, called voltage gate low (VGL), to turn off the TFT. The picture information is transmitted to the source driver, which charges the liquid crystal capacitor (CLC). The storage capacitor (CST) acts as a buffer to prevent leakage current from the CLC. More pixel drivers in TFT LCD displays are discussed in the article “New Driving Structure to Increase Pixel Charging Ratio for UHD TFT-LCDs With High Frame Rate”.4
Image retention or flicker in TFT LCDs is caused by parasitic capacitance (CGD) existing between the gate node and drain node of the TFT. When the picture content changes and the TFT turns off from the on state, a voltage drop on the CLC is caused by a capacitive voltage divider between CGD and CLC||CST. To improve panel performance consistency, the common backplane voltage (VCOM) is introduced and tuned to the center of the pixel voltage during the pixel transition time.
Figure 6. A conventional pixel driver.
The topology of popular pixel drivers in micro-LED and OLED displays is similar but more complex than in TFT LCD displays due to the fabrication process and integration of TFT circuits with LEDs on a glass or polyimide substrate. Consequently, LEDs in each pixel are driven individually with their own brightness.
As shown in Figure 7, a simple pixel driver called 2T1C (two transistors and one storage capacitor) is described in the article “Driving Technologies for OLED Display”.5 In this pixel driver, the analog signal of LED emission is sent to TFT M1. The threshold voltage (VGS) is then stored in (CST), which is used to drive TFT M2 in the saturation region, as shown in Figure 8. The driving TFT M2 maintains the LEDs at a constant current with the positive voltage (VDD) and cathode voltage (VSS). The saturation operation driving method of this 2T1C pixel driver has the advantage of extending the LED’s lifetime compared to the linear region operation of the driving TFT.
Figure 7. A 2T1C pixel driver of OLED or micro-LED.
Figure 8. MOS transistor output characteristics.
There are, however, disadvantages to the 2T1C pixel driver, which include the Mura problem and threshold voltage shift under electric bias. The Mura problem is the uneven brightness in a display’s uniformity, which is mainly caused by variations in the manufacturing process, such as the density of the TFT layer, uniformity of LED forward voltage and threshold voltage, etc. These effects cause image quality issues. Although the best fabrication process cannot overcome the threshold voltage shift, pixel circuits with voltage feedback methods and threshold voltage shift overcompensation methods have been proposed to improve image quality.
The 7T1C driving method proposed in the article “Image Quality Enhancement in Variable-Refresh-Rate AMOLED Displays Using a Variable Initial Voltage Compensation Scheme” is shown in Figure 9. This 7T1C pixel circuit has three operation stages, as shown in Figure 10: initialization, compensation, and emission. The TFT M4 is used for the diode connection of driving TFT M3. During compensation, the voltage stored in CST from the source driver maintains the LED emission. The TFTs M1, M6, and M7 are used to prevent the LED from turning on. Additionally, a 7T2C pixel circuit was proposed in the article “A Highly Uniform Luminance and Low-Flicker Pixel Circuit and Its Driving Methods for Variable Frame Rate AMOLED Displays.”
Figure 9. Schematic of a 7T1C pixel driver.
Figure10. Driving sequence of 7T1C compensation pixels:(a) initialization,(b) compensation, and (c)emission.
Currently, the display backplane technology has been developed from hydrogenated amorphous silicon (a-Si:H) TFT to low temperature polycrystalline silicon (LTPS) TFF and low temperature polycrystalline silicon and oxide (LTPO) TFT constitute the next generation backplane technology for consumer electronics. The a-Si:H TFT has a low carrier mobility (1 cm2/Vs), which results in the large size of the backplane and leads to more power consumption. The LTPS TFT has superior carrier mobility (>50 cm2/Vs) so that it is applied in the OLED display. The LTPS TFT usually has a high off-current. However, the LTPO TFT has a low off-current. Thus, the hybrid pixel scheme combining the LTPS and LTPO TFTs is considered for use in OLED/micro-LED display backplanes.
Conclusion
Displays in vehicles play an increasingly crucial role in the cabin experience, with growing expectations on enhancing visibility, safety, user experience, etc. from the consumers. Automotive displays face significant challenges in delivering outstanding image quality with high resolution and contrast ratio, free form, large size, and cost-effective solutions.
In this article, the characteristics of TFT-LCD, OLED, and micro-LED display are discussed. To achieve better display performance, the pixel driver of OLED and micro-LED becomes more complicated than the TFT-LCD display. Thus, OLED and micro-LED displays are expensive due to the challenges in commercializing mass production. Mini-LED LCD displays with local dimming technology serve as a bridge to micro-LED and OLED displays.
The article has been written by YujieBai, Senior Applications Engineer, Analog Devices
Yujie Bai is a senior applications engineer at Analog Devices and is responsible for support and application of automotive power products. Yujie joined Maxim Integrated (now part of ADI) in 2020 and holds a master’s degree in electrical engineering from Miami University (Ohio).
