TFT-LCD is the dominant technology in the current market but it has limitations. There are stringent specifications for displays which need to operate in extreme temperature ranges; lifetime requirements and a demand for lower costs make the adoption of other technologies difficult. At the SID virtual Vehicle Conference in October 2020, several papers discussed alternatives to current solutions including OLED, full-array local dimming (FALD) LCD, dual cell, Organic LCD dual cell and MicroLED display technology.
TFT-LCD: Current Technology of Choice
In his keynote speech John Schneider from Ford Motor Company said,
“For today and into the near future, LCD remains the technology of choice for automotive designs. Improvement in packaging, color, contrast, luminance and of course relentless design efficiency that continues to deliver highly attractive cost points keep LCD at the top of the list”.
Key automotive display attributes are size, resolution, luminance, contrast, color gamut, color depth, low reflectivity, free format, thin design, touch capable, robustness (environmental), flexible 2D/3D, transparent-capable, power consumption and cost. His presentation also pointed out that there is a major shift in automotive customer expectations.
"Compute is the new horsepower, data service the new currency, and experiences are the new product".
Automotive specifications, environmental concerns, long design cycle and lifetime requirements have caused the auto display market to lag behind the consumer electronics market. Shifts in consumer expectations will impact attributes and performance expectation of automotive displays in the future. The current dominant technology of a-Si TFT LCD lags consumer displays in terms of contrast ratio, black levels, form factors and resolutions. Higher resolution display products based on LTPS or Oxide TFT and miniLED backlight solutions are already entering the market. Designers are considering OLED displays as possible solutions especially due to their higher contrast ratio, black levels and thinner flexible form factors.
OLED: Designers’ Choice
According to Texas Instrument’s presentation, auto display requires more than 1000 cd/m² brightness, a wide temperature range (operating between -40° C and +85° C), long lifetime, production support, mechanical reliability and electromagnetic compatibility. OLED solves many of the LCD design challenges such as no backlight, high contrast ratio (>100,000:1-1,000,000:1), incredibly thin form factors, a fraction of power usage and allows for curved displays. High temperature reliability and lifetime, low luminance, cost and ambient reflections are OLED’s disadvantages (the presentation noted). These issues create challenges for the mass adoption of OLED auto display.
LG Display demonstrated an automotive cockpit design using flexible OLED at DisplayWeek 2020 including a 12.3-inch cinematic sound display (2500x 900, 750 nits) and 27-inch center information display plus co-driver display (5500x 900, 750 nits). According to the Ford Motor keynote presentation at the Vehicle Display conference, recent design innovations have resulted in significant improvements in OLED performance against environmental requirements and durability. OLED cost structures are improving but a substantial gap remains. They can see a future where OLED is much more prevalent in auto interiors.
FALD (Full Array Locally Dimmed): Higher Contrast & Power Savings
Globally dimmed edge-lit LCDs, which are most common in auto display, lack the desired contrast ratio and black levels. The TI paper indicated that FALD (Full Array Local Dimming) backlight architecture shows the potential to improve the contrast ratio of LCDs to near OLED levels while also providing power savings over traditional backlight methods. As mentioned in the paper, a liquid crystal layer blocks or passes through the backlight per RGB subpixel to create each pixel but liquid crystal can’t block 100% of the light when showing a dark pixel. Dark/black pixels are still partially illuminated resulting in the display not having a deep true black.
The paper further conveyed that in FALD backlight, LEDs under the LCD panel are divided into many small zones. The brightness of each zone is adjusted according to different display content. Automotive HMIs (Human Machine Interfaces) have lots of black backgrounds. FALD backlight display has the potential to improve the contrast ratio, display darker blacks and brighter white colors as well as reducing power consumption. The paper noted that local dimming can sometimes cause a halo effect, which is mura-effect where black/dark pixels are non ideally illuminated due to light leakage or zone overlap. A higher numbers of zones helps to avoid mura. TI has developed a 12-inch local dimming backlight display reference design with 384 zones. In the FALD solution there is trade-off between local dimming, price (more zones means higher cost) and image quality improvement.
Dual-Cell: Very High Contrast
A presentation from Visteon noted that dual-cell TFT LCD solution could offer excellent black background performance and very high contrast. Dual-cell TFT technology has been under research for more than 10 years. Recently it has been getting some traction in the TV market.
In a dual-cell system a second monochrome TFT cell is used to increase the number of backlight zones. The paper said
“it is possible to drive more than 300K micro-zones on a monochrome TFT with gray shade control. It doesn’t suffer from image burn in and high luminance is possible”.
Historically in dual-cell technology 2.x to 2.5X more backlight power is needed due to lower system transmission. Visteon has developed micro-Zone dual-cell technology where several modifications were done to increase system efficiency: there was a modification to the inter-cell diffuser and use of a proprietary light control method. (Vehicular Display Makers Explore Dual-Cell LCD) Visteon developed backlight efficiency improvements and improved the driving monochrome TFT gray scale capability to smooth edges. Also a monochrome TFT gray scale is used to provide optimized gamma function and eliminate the color cell leakage component. According to the presentation microZone eliminates black mura issues commonly associated with color TFTs and it looks very similar to OLED but with no image burn-in and higher maximum luminance. Visteon paper noted that most important dual cell development is reducing power consumption as with their microZone product, the power increase is only 1.3 times even with a color improvement to 96% NTSC.
Organic LCD(OLCD) Dual-cell: High Contrast with Design Flexibility
OLCD is like a standard TFT LCD but with film used instead of glass. It is conformable as the display bend radius can be as low as 10mm. It is free-form and lightweight. According to the FlexEnable presentation at the conference, it is 10 times lighter than glass TFT-LCD and OLCD enables conformable, large area, long lifetime and high brightness display at low cost.
The company reported that by using TAC film (commonly used in polarizers) as a substrate material it can bring glass-like optical performance to Flex LCD. The presentation pointed out that dual-cell TAC films allow close coupling of the cells. OLCD dual cell leads to: true pixel level dimming, lower cost and higher transmission and better transmission from a simple stacks. It further added that trade-offs of the dual-cell approach using glass are thickness, weight, brightness at wide angles and mura.
These trade-offs are a direct result of the very large separation between the modulation cell and display cell caused by the glass thickness. Reducing the inter-cell separation is the key to higher performance and lower cost. Direct on polarizer OLCD dual-cell leads to: pixel level dimming (no Halo), lower cost and higher transmission from a simpler stack because of very low inter-cell spacing. Dual cell OLCD can offer very high contrast with conformability format. Novares has already integrated curved, glass-free OLCDs into its Nova concept car in 2019. (Novares are an investor in Flexenable - editor Flexenable Boosts Dual LCD for Automotive (and....)
MicroLED: The Future?
MicroLED provides the best features of LCD and OLED display and has the potential to outperform both technologies. However, it is not in volume production due to manufacturing, production, scaling and yield issues. Technology innovations are critical in enabling cost reductions and volume productions so that microLED can compete effectively with OLED and LCD in future.
VuReal presented the enabling features of their MicroLED for automotive applications. The presentation said that MicroLED is the only solution to address new automotive display demands of enhanced UX (user experience), improve safety and offer better connectivity. It can provide AR for HUD (Heads Up Display), curved displays for center stacks, 3D experience for instrument cluster, and better M2M (machine to machine) and M2H experience with MicroLED external display.
In the VueReal Micro-Solid printing technology process MicroLEDs are printed directly from a cartridge to a display substrate selectively. It has multi cartridge transfer (>100 cartridges at once) and multi color cartridge (high alignment accuracy). Its MicroVue inspection system can be used to identify defects, bin for performance, and protect microLEDs during printing, thus improving yields. This technology has allowed VuReal to demonstrate a printed 10μm LED with 166ppi and >60% transparency.
The company is planning to sell its inspection solution and Micro-Solid printing technology in the future. Some companies are already demonstrating MicroLED for auto display by using different processes. At the DisplayWeek 2020 virtual conference, AUO demonstrated a 12.1" (169 ppi) microLED display with a glass backplane and a 9.4" (228 ppi) flexible display with PI (Polyimide) co-developed with Playnitride. Also Tianma showcased a 7.56" (114 ppi), >600cd/m² flexible display and a 5.04" (114 PPI), 600cd/m² LTPS TFT microLED display for automotive applications.
Vehicle display will transition into a new era in future driven by customers’ expectations, higher connectivity, the growth in electric vehicles and a shift towards autonomous vehicles. With the move towards new functionality, increases in display size, performance and the number of displays, ithe ndustry will start to embrace new solutions such as FLAD, Dual-cell, miniLED and new display technology such as OLED and MicroLEDs. (SD)
Sweta Dash is the founding president of Dash-Insights, a market research and consulting company specializing in the display industry. For more information, contact email@example.com or visit www.dash-insights.com