Thin and Light Dual-Cell OLCDs: Bringing Ultrahigh Contrast Performance to TVs and Automotive Displays

IN THE LAST FEW YEARS, HIGH-DYNAMIC-RANGE (HDR) displays have become a hot topic—they are in high demand for TVs and monitors because of their ability to deliver crisp and finely detailed images, enhancing the viewer's experience. More specifically, the term HDR is used to describe TVs that offer ultrahigh contrast and brighter images. High contrast displays also are increasingly required for automotive applications, where light leakage reduction, particularly for night driving, allows displays to be hidden until lit.

The Importance of Contrast

Contrast is one of the key properties to consider when choosing a display, as it directly affects the picture quality. Contrast ratio is a common metric and refers to the luminous intensity (in nits) of the pixel when fully “on” divided by its luminance when fully “off.” A high-contrast ratio is a desired feature for any display, as it more accurately represents the dynamic range we experience in the real world. The challenge of increasing contrast, therefore, can be tackled from both ends—improving the black level and/or luminance. Although it has long been demonstrated that the brightest TVs on the showroom floor sell the best, achieving high contrast in practice often means improving black levels—something that is less noticeable in a brightly lit shop but is an enhancement that is unmistakably better, even to a nonexpert, when watching a movie in a dark room.

A more useful measure of perceived contrast takes into account both dark pixel luminance and surface reflections off ambient light from the screen.1 Increasing the ambient contrast ratio (ACR) requires either a reduction in surface reflection from the screen or higher luminance pixels in the “on” state to overcome the background illumination. This means that in low lighting conditions, ACR is usually high for displays that have low dark pixel luminance, favoring OLEDs, whereas in typical bright daylight conditions, where surface reflections will be high, ACR will be better for LCDs. Today, typical OLED and LCD TVs will have the same perceived contrast (ACR) at around 100 lux background illuminance, corresponding to a dimly lit room (typical office lighting is around 500 lux). Naturally, there are many other parameters to consider to comprehensively compare the contrast performance of two displays, such as the angular dependence of contrast and number of dimming zones.

Static and Dynamic Contrast

Putting ambient illumination aside, many displays are able to cover a higher dynamic range when measured over a sequence of frames than what is possible within any given frame, resulting in a difference between the display's static and dynamic contrast ratio. Static contrast ratio is a measure of the native contrast of the display—a measure under certain test conditions of the maximum ratio of brightest to darkest pixel on the display for a still image.

The dynamic contrast ratio will always be equal to or higher than the static contrast ratio and measures the maximum and minimum luminance a pixel can achieve at different times. The distinction between the two arises because LCDs have dimmable backlights, whose luminance can be adjusted frame by frame. For example, this can be characterized by measuring the pixel luminance in a first frame with the backlight and pixel set to maximum luminance and transmission, respectively, and measured again in the subsequent frame with the backlight and pixel set to minimum luminance and transmission. For OLEDs, both the static and dynamic contrast ratio are the same (and very high), because every pixel has an independently controllable light source.

In practice, a static contrast ratio of 100,000:1 is generally considered to be as good as needed in nearly all environments—most people will not notice the difference of increasing the contrast further than that in most environments. OLED TVs meet this because of their excellent black level, whereas a basic LCD TV will have a static contrast ratio of a few thousand at best, largely defined by the ability of the liquid crystal (LC) optical switch to extinguish the backlight.

Competitive Landscape

With a multi-billion-dollar consumer market to tap into, existing and emerging display technologies are competing to deliver optimal picture quality. At present, there are three main approaches to achieving high contrast TVs, included in products today at a relatively accessible price point for “premium TVs,” broadly starting in the US$1,000–$2,000 range for a 65-inch TV. These are WOLED, miniLED (LCD), and dual-cell LCD. Other variants (such as QD-OLED) are being developed, which have other attributes and advantages but rely on the same basic principles to achieve HDR.

A fourth approach, microLED, also can achieve extremely high contrast, but for TVs, the manufacturing cost (100× higher) is expected to remain out of reach for all but the most spendthrift of consumers for the next decade.

Here we take a brief look at the three technologies already on the market and explain why dual-cell OLCD could be the answer to bringing ultrahigh contrast to displays at a lower cost.

DUAL-CELL LCD

Dual-cell LCD technology is a structure that allows LCDs to compete on contrast with OLED TVs by reaching similar black levels. Dual-cell LCDs achieve exceptionally high contrast by putting two LC cells on top of each other (i.e., adding a second LC cell, which we will refer to as the ‘modulation cell’ between the backlight and the usual ‘display cell'). The modulation cell is similar to the display cell, but does not require subpixellation or color filter, meaning that each pixel on the modulation cell maps to at least three (colored) subpixels on the display. It may be at a lower resolution than the display cell—a common combination being an HD modulation cell with a 4K display cell (one modulation pixel maps to four display pixels, meaning 12 display subpixels). With each cell having a static contrast ratio of more than 1000:1, the combined stack squares the contrast levels to reach more than 1,000,000:1, while carrying over existing performance strengths of LCD, such as high luminance, long lifetime, and no burn-in. Dual-cell TVs are available today (e.g., Hisense's range such as 65SX Dual-Cell TV). These displays not only have high contrast, but also benefit from the other ongoing innovations in LCD, such as quantum-dot enhancement film for ultrahigh color gamut (see Fig. 1).

As with all new technologies, there are trade-offs. Each cell comprises two sheets of display glass, increasing the overall thickness and weight. More importantly, optical issues arise as a result of the stacking of two display cells, and must be compensated for. These include moiré/mura and reduced luminance at wide angles due to parallax. Also, unlike OLED TVs, glass-based dual cells are not capable of true pixel-level dimming, as explained. There is also a significant reduction in transmission or power efficiency.

These performance trade-offs in dual cells are a direct result of the relatively large separation between the modulation cell and display cell caused by the thickness of the display glass. This “inter-cell separation” is a few millimeters and crucially is much larger than the pixel pitch for TV products (e.g., a 55-inch 4K TV has a pixel pitch of 0.317 mm). Having an inter-cell separation of several pixels inevitably causes major parallax issues that must be treated with a suitable compensation film (e.g., light diffusion film) between the two cells.

The addition of a compensation film spreads the light from a pixel in the modulation cell over several pixels in the display cell. This explains why glass dual-cell TVs cannot offer true pixel-level dimming, because even if the two cells are of the same resolution, one pixel illuminated on the modulation cell will illuminate several pixels on the display cell—still excellent performance, but not quite the same as OLED in terms of true pixel-level HDR. Second, the additional diffusion layer between the cells affects the degree of polarization of the light, meaning that four polarizers are needed in total, two for each cell (each cell behaves as an independent optical switch). This further reduces the transmission (power efficiency) and increases the bill of materials of the backlight to maintain the luminance of the TV image.

The Advantages of OLCD for Dual-Cell Design

FlexEnable's development of organic thin-film transistors (TFTs), compatible with low-temperature (sub-100°C) processing, enables OLCDs on ultrathin flexible substrates, such as tri-acetate cellulose (TAC) in place of glass (Fig. 2). TAC is a commonly used plastic in the displays industry, often in polarizers, and as such has ideal optical properties in all regards for use in place of glass as the display substrate. The resulting OLCDs are much thinner, lighter, and conformable around surfaces and benefit from all the strengths and parallel innovations in LCD. OLCD also reuses much of the highly cost-optimized LCD supply chain (e.g., polarizers, backlights, driver integrated circuit, LC), and as a result, its cost structure is close to that of a glass LCD, making OLCD the most affordable, flexible display technology.

However, it is the extreme thinness of OLCD cells, rather than its flexibility, that opens up a new realm of performance for dual-cell LCDs, because it allows the two cells to be stacked extremely close together, bringing huge advantages in optical performance. TAC film is commonly available in 40-μm thickness—10× thinner than typical flat-panel display (FPD) glass for TVs.

Fig. 3a shows a simplified comparison of a conventional glass dual-cell LCD drawn approximately to scale for a 55-inch 4K TV, which has a pixel pitch of 317 μm. The vertical separation of the dimming cell from the display cell is several times larger than the pixel pitch, and this is the cause of moiré, a parallax that requires a compensating optical film between the two cells and then an additional polarizer, thereby adding further separation.

Fig. 3b compares the same scale of an OLCD example dual-cell stack using 40-μm TAC film. The inter-cell spacing is less than a pixel pitch, meaning a simpler stack is possible without the need for compensation film.

The reduction in inter-cell spacing when using a TAC substrate instead of glass brings the cells separation down to less than the pixel pitch.

We have made similar test structures in our labs (with an inter-cell separation of 300 μm) and have demonstrated contrast ratios in excess of 250,000:1, as shown in Fig. 4.2 The entire dual-cell OLCD stack is thinner than a single sheet of FPD glass.

But there is much more that can be done to bring the two cells even closer together by foregoing the TAC substrate altogether. Recently, we have shown that the low-temperature organic TFT process allows for the polarizer itself to be used as the substrate directly, because the highest temperature steps in the entire OLCD manufacturing process are sub-100°C, and there are polarizers that can withstand these process temperatures.

In another recent development, an ultrathin state-of-the-art polarizer from Nitto was used as the display substrate, and a full active-matrix OLCD was shown to be operational.3 The ultrathin polarizer used as a substrate was just 25-μm thick, meaning such a stack employed in a dual-cell OLCD would bring the inter-cell separation down to even less than one-tenth of the pixel pitch, improving optical performance, simplifying the stack, and allowing for the first true pixel-level dimming HDR from an LCD (Fig. 5).

Dual-cell OLCD is also the only dual-cell approach that brings high contrast while retaining flexibility and conformability surfaces, allowing greater design freedom for large displays. For example, large-area, conformable and HDR is a killer combination for automotive surface-integrated displays.

As HDR displays move from premium TVs to become mainstream over the next few years, we can expect even more competition between these new technologies within the TV market and beyond. Each technology has its own technical and manufacturing challenges, and cost will play an important role. Of course, TV performance is about more than contrast alone. Color gamut, refresh rate, resolution, power consumption, cost, and viewing angle all need to be considered.

From：SID-Wiley Online Library