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Making a Greener TFT-LCD

Recently, green technology has become a key driver for TFT-LCD development. Green technologies in TFT-LCDs include product designs that are environmentally friendly, alterative manufacturing processes, and more. Achieving high transmittance of an LCD cell as well as reducing overall system power requirements are key elements, along with clean process alternatives for manufacturing. These are the most promising approaches for green TFT-LCDs. The authors from AU Optronics will describe the process involved in making a greener LCD.

by Po-Lun Chen and Ming-Kwan Niu

THE FIRST WAVE OF TFT-LCD development involved performance aspects such as contrast ratio, viewing angle, luminance, response time, and motion image quality. Cost reductions to allow more consumers to afford TFT-LCDs or to stimulate customers to purchase more TFT-LCDs were the second wave. Recently, a third wave of development has begun – green products and processes.

Green Considerations at AUO

AUO launched its Green Solutions initiative in 2008, a company-wide mission designed to promote environmentally friendly innovations, procurements, production, logistics, service, and recycling. AUO's environmental goals for the coming 3 years include the achievement of an 80% waste-recovery rate, reducing water consumption per substrate size by 70% from 2004 levels, and reducing greenhouse gas emissions per substrate size by 70% from 2004 levels. In addition, AUO has begun actively pursuing business opportunities such as entering the green energy market, making its move from achieving energy efficiency to creating energy, with plans including the development of a new factory for solar-panel production.

After much improvement and optimization, Auo's 46-in. "Eco Plus" TV panel promises a 30% weight reduction, 30% thickness reduction, and 30% energy-consumption reduction over previous similarly sized models. This panel also promises 66% product packaging efficiency with reductions in packaging space and shipment weight. Green logistics have been introduced into product design, so as to effectively enhance transportation efficiency and mitigate impacts to the environment

Cell Transmittance Improvement

For a TFT-LCD, power consumption can be reduced through better transmittance and smarter backlight loading. The following improvements have been implemented to achieve this.

VA-LC modes: To achieve a significant transmittance improvement, the following aspects need to be improved: LC transmission, aperture ratio, and pixel-rendering methods. For better performance from a TFT-LCD panel, AUO developed a series of vertical-alignment (VA) technologies over the past few years (Fig. 1). Recently, the company developed its AMVA5 technology not only to improve the contrast ratio to 16,000:1, but also to enable an LC transmission improvement of 30% compared to AMVA1 in 2005. This was accomplished by effectively improving the LC disclination line using newly developed polymer-stabilized vertical-alignment (PSA) technology.1

 

Fig_1

Fig. 1: AUO's recently developed AMVA5 technology significantly improves contrast ratio. (There is no AMVA4).

 

The key control mechanism in PMVA is to impose protrusions on a color-filter (CF) substrate in order to make VA-LC subpixels (single red, green, or blue pixels) having a four-domain orientation. The key concept in AUO's AMVA2 for eight-domain VA is to use a capacitive coupling method with an ART transistor that can provide good color-washout performance. AMVA2 is an improved version of AMVA in terms of contrast ratio. PSA, as shown in Figs. 2(a) and 2(b), was applied to AMVA3 to improve the transmittance for an eight-domain VA, and AMVA5 provides further improvements in contrast ratio and transmittance via storage-capacitor modification and CF material optimization.

 

(a)   Fig_2a
(b)   Fig_2b

Fig. 2: AUO's AMVA5 PSA technology provides good transmittance that in turn reduces power consumption. (a) The technology incorporates the basic PSA process and the LC molecular reorientation principle. (b) Themicroscopic photography of the uni-pixel shows that the PSA operates at different gray scales.

 

Pixel aperture ratio: Another way to improve cell transmittance is to enlarge the aperture ratio of a pixel. Several methods used to achieve a higher aperture ratio can be adopted, including narrow bus-line design, closer electrode arrangement, and black- matrix (BM) shielding area shrinkage. For narrow bus-line design, a lower resistivity metal such as copper (Cu) should be considered. The Cu process3 and design are other important aspects for aperture-ratio improvement, especially for products larger than 65 in. and 2k x 4k at 120 Hz. For closer electrode (bus lines and pixel electrode) arrangement, ultra-high aperture (UHA), color filter on array (COA), shielding metal under the bus line, or placing a bus line under the shielding metal structures are possible ways to improve aperture ratio. These methodologies can provide about 10–20% aperture-ratio improvement, depending on the pixel size and LC modes used. For the shrinkage of the BM shielding area, an acceptable optimization between light leakage and BM shielding can be considered. The assembly accuracy of the upper and lower glass substrate during manufacturing also influences the BM shielding-area design quite a bit. The BM shielding area is always tightened if the assembly process is not accurate. COA also provides a good solution because the BM as well as the R, G, and B color layers are all fabricated in the bottom substrate.

RGBW rendering: Pixel rendering with red, green, blue, and white subpixels is also an effective way to improve transmittance. A color LCD is made with a color filter with red, green, and blue subpixels. However, more than two thirds of the light from the backlight is filtered out. If white subpixels can make up a specific ratio of the total display area, the transmittance of the LCD will be increased. A technique for mapping the color reproduction from a conventional red, green, and blue color system to a red, green, blue, and white color system can be developed without sacrificing too much color performance. By using this method, the transmittance can be improved by about 20%. However, faded color and the existence of some artifacts are two challenges of RGBW rendering.

Backlight and Electronics System

Local dimming is an effective way to reduce the power consumption of a TFT-LCD. With this technology, the backlight does not need to be continuously on at the highest brightness; the power consumption can thus be reduced by a factor of 2 or more. A 32-in. TFT-LCD, for example, with a conventional back-lit driving method, needs 100 W for the module, but only 50 W or less are needed for a modulethat utilizes local dimming, and the CR can also be significantly improved. LED backlighting is necessary for the local-dimming design.

Color-sequential displays without a color filter. Color-sequential displays without CFs4 can reduce power consumption by a factor of about 50%. This methodology also provides a boost in contrast ratio. However, there are no effective LC modes that can provide fast response times for a field-sequential operation. Optically compensated bend (OCB), ferroelectric LC (FLC), and the currently popular blue-phase LC are three LC modes that could possibly provide a response time smaller than 1 msec. However, OCB requires a complicated optical film compensation that is currently not adopted for TV-LCD applications. (For more aboult OCB mode and blue phases, see the November 2008 and November 2009 issues of Information Display, respectively.) FLC has a very fast response time but requires a small cell gap of about 1 μm so that it also can not be used for current TFT-LCD applications. Blue-phase LC will provide a fast response of below 1 msec for each gray-to-gray switching, but several issues with the material itself still need to be resolved.

Green Product Design: Thickness Reduction with LED-Backlit Modules. Another key to delivering a green product is to reduce the module thickness. It not only saves the materials used but also provides advantages when being transported and packaged. The most popular way to reduce the thickness of a TFT-LCD module is to use LED backlighting. Conventional direct-type CCFLs result in a module thickness from 100 to 40 mm. An edge-type LED module design is required to reduce the thickness as well as the weight. Thus, a module as thin as 20 mm or even thinner is possible. However, the trade-off between local dimming (by using direct 2-D array LEDs) and thickness reduction (by using edge-type LEDs) needs to be considered. The main focus of the direct 2-D array LED module is to introduce a good radiation pattern by optimizing the number of LED chips used and the thickness of the gap between the LEDs and optical films. The main focus of the edge-type LED module is to introduce an light-guide plate (LGP) to maintain a uniform light distribution. Figure 3 shows an LED-backlight module for TFT-LCD TVs. It provides thickness reduction and local dimming.

 

Fig_3

Fig. 3: Progress for LED-backlight modules for TFT-LCD TVs from 2009 through 2013 includes thinner form factors and improved dimming functions.

 

Table 1 sums up the key approaches that AUO employs in fabricating more eco-friendly panels. AMVA5, Cu bus line, and COA are all technologies used for pixel aperture-ratio improvement. By using these methods, a product that consumes less power can be delivered. Besides, slim LEDs not only provide better color reproduction, they also make slimmer and lighter-weight products.

 


Table 1: AUO uses several methods, as shown below, in order to achieve a low-power, lightweight product.
Technology AMVA5 Cu COA Slim LED
Green advantage High cell transmittance High aperture ratio High aperture ratio Thin & lightweight
Size 55-in. FHD 55-in. FHD 46-in. FHD 42-in. FHD
Frame rate 60 Hz 120 Hz 60 Hz 60 Hz
Brightness 500 nit 500 nit 500 nit 500 nit
Power consumption 150 W (30% ↓) 150 W 68 W (x 1/2.5)
BLU type CCFL CCFL CCFL Side LED
Driving scheme 3 sides
Thickness 11 mm (x 0.31)
Spec. w/ normal technology 220 W 4 sides 170 W 35 mm
Announced at FPD Intl. 2009 Display Taiwan '09 China FPD 2010 FPD Intl. 2009

 

Green Manufacturing

It is also very important to keep reducing the power consumption and waste generated when products are produced. AUO has set targets of a 90% process water-recovery rate, a 90% construction waste-recovery rate, and 21% in total energy savings.

AUO's LEED-Certified Fab

AUO's L8A fab (Gen 8.5) is the world's first Leadership in Energy and Environmental Design (LEED), a U.S.-based internationally recognized green building-certification-program gold-certified TFT-LCD fab. There are only five LEED gold-certified facilities in the world at this time, and among them the AUO fab is the largest in terms of facility size and is also the world's first TFT-LCD hybrid fab, consisting of both Gen 8.5 and 7.5 lines. It can deliver 21% in total energy savings, compared to a fab with-out green technology incorporated, equivalent to US$9 million per year when fully operational. The site is designed to achieve a 90% water-recovery rate, saving 3 million tons of fresh tap water annually – enough to fill 1430 standard swimming pools. In addition, 90% in construc-tion waste has been reclaimed. Furthermore, with extensive tree plantings, as well as 130M kWh of power savings per year, the AUO Gen 8.5 fab can deliver a significant 87,000 carbon emission reduction annually, an effect equiva-lent to that of 23 New York City Central Parks.

In order to make all this possible, the following innovations have been put into effect:

• (1) Uninterrupted Exhaust-Driven Wind-Power Generator: AUO installed wind turbines on top of some air outlets. These turbines, operating at speeds selected so as not to affect air-flow efficiency, are capable of generating more than 100 kWh of electrical power per day.

• (2) Waste Heat Recycling: The waste heat from the chilling system is used to generate the preheating and reheating energy for the MAUs (Fig. 4).

• (3) Dual-Temperature Chilled-Water System: In the past, AUO used water chilled to a single temperature (8°C) to cool all process equipment. But for the new fab, the company implemented temperatures in dual mode (8 and 14°C) to meet the cooling demands of different equipment. (Energy consumption simulations helped ensure the feasibility of this method beforehand.)

• (4) Water Inter-Use System (WIS): The WIS uses an innovative design that connects manufacturing process points so as to save 335,000 tons of water annually.

 

Fig_4

Fig. 4: Waste-heat recycling: the thermal chamber exhaust is designed to be recycled in a clean room to spare the de-humidifier from using power. The equivalent power savings is as high as 1.2M kWh/year in a fab.

 

Green Manufacturing Alternatives

Ink-Jet Printing (IJP) for the CF Process. This is another key green consideration to reduce power consumption and waste during production. The conventional process for array fabrication is a vacuum-based photo-lithography process. A five-mask array process is used to repeat a cycle of functional layer deposition, photoresist (PR) coating, photoexposure, PR developing, etching, and PR stripping. If we can directly deposit the functional layer with the expected pattern, only a few minutes instead of the usual one- to two-day cycle is required. Printing methods including gravure printing, flexo-printing, and ink-jet printing (IJP), which can dispense the functional layer in targeting positions, are good methods of implementing direct patterning. AUO has tried to implement IJP processes to fabricate photo-spacers, alignment layers, and even R, G, and B layers of a CF. However, IJP cannot only save the material consumption, but also simplify the process steps. Figure 5 shows an illustration of R, G, and B fabrication for both the conventional process and the IJP process.

 

Fig_5

Fig. 5: R, G, and B fabrication can be used for both the conventional and the IJP processes.

 

The use of the IJP CF process is estimated to reduce CO2 consumption by 20,087 tons/year based on a Gen 7.5 fab with 120K capacity. It is equivalent to the CO2 consumption that would be used in driving a car 139,000 times around the island of Taiwan. (Note: 1 tree = 4.5 kg/year; 1 car = 0.2 kg/km). Besides, it also saves on material consumption and the number of photomasks used. Table 2 shows the advantage of IJP for the CF process.

 


Table 2: Ink-jet printing has advantages for the CF process.
  Conventional Ink-Jet Technology
Photomask – RGB mask method – RGB mask free
Process simplicity – Complicated: Exposure & development system – Simple: Printing system
Material consumption – Consuming more material: Two thirds of RGB material lift-off – Material saving: RGB printing to the right position
Equipment & space – More equipment & space 
   Three photo-lines required
– Equipment & space saving: Printing system

 

IJP for Cell Process: IJP can also be used in cell processes for polyimide (PI) layer coating and spacing imposition. The conventional way to coat PI layers for LC alignment consists of roller coating by using an APR plate attached to a cylindrical roller. The PI is continuously supplied by another roller that requires more material (PI material and the consumption part) consumption compared to IJP. Figure 6 shows the operational principles for conventional and IJP PI processes. IJP can also be applied to impose the spacers on the glass substrate for the post-step cell assembly. The conventional way of imposing spacers is by photo-stripping, in which PR coating, photo-exposure, and stripping are all required. A small portion of the coated PR remains as the spacer. The IJP spacer process can eliminate a large amount of chemical use and also reduce equipment complexity. It seems to be a very good method for green production. Figure 7 compares the operational principles for conventional and IJP PI processes.

 

Fig_6

Fig. 6: Operational principles for the conventional polyimide (PI) process and the IJP PI process are shown.

 

Fig_7

Fig. 7: Operational principles for conventional spacer process and the IJP PI process are compared.

 

Four-mask array process: The current TFT process requires five repeating photolithography steps and is therefore referred to as a five-mask process. Each mask includes thin-film deposition, PR coating, photo-exposure, etching, and PR stripping as mentioned above. If one photomask exposure can be removed from the process, it means the company can save not only the cost of one photomask, but also one PR coating and stripping process. Thus, manufacturing space as well as chemicals can be saved.

The Carbon Footprint of AUO's 32-in. Module

By converting the total power consumption during raw-material preparation, product manufacturing, customer usage, and final disposal to carbon emissions tells us the product's contribution from cradle to grave. The carbon estimation and calculation on four steps of the total pipeline are listed below:

• (1) Stage 1 (Raw Material): Issuing a questionnaire to obtain carbon footprint data; secondary data is used when primary data is not available

• (2) Stage 2 (Manufacturing): Inventory based on the experience of ISO14064 & ISO14040, and data obtained from primary data.

• (3) Stage 3 (User Usage): Measuring the power consumption based on Energy Star 3.0 STD and then follow-up with Top Runner, Japan's program to set the efficiency standards for a wide variety of products, to perform the calculation.

• (4) Stage 4 (Disposal): Based on the principle of dismantling WEEE 3R products, we calculated the ratio of recovery and recycling of materials, then referred it to the WEEE directive database for calculation.

Our estimation shows that about 60% of carbon emissions derive from consumer usage, while 28% comes from raw-material preparation and another 12% from manufacturing. These findings imply that very low power consumption is the most important feature in reducing TFT-LCD carbon emissions, while the use of better materials with low carbon emission and green processes for manufacturing are another two key principles.

Power-Consumption Discussion

We have already discussed green technology in terms of both product and manufacturing advances adopted and developed by AUO. Another area of interest is the analysis of the power consumption of a product during its entire lifetime. Two key considerations are the power used during manufacturing and the power used to operate the display. If we take 1 year as the standard calculated time period for power use and translate this power into CO2 emission, we find that the CO2 emission for AUO's eco- designed products are 53.3, 77.6, and 2.5 kg for AMVA5, COA, and slim LEDs, respectively. Table 3 shows the estimated performance of CO2 reduction brought about by different methods.

 


Table 3: Shown is the estimated performance of CO2 reduction by different methods.
Technology CF by IJP Exhaust-driven-wind facility AMVA5 COA Slim LED
Green advantage • CO2 emission ↓
• Materials ↓
• CO2 emission ↓
(generate electricity)
• CO2 emission ↓
(power saving)
• CO2 emissionm ↓
• Materials ↓
• CO2 emission ↓
(Transport saving)
Size 46-in. (G7.5: 6 up) 46-in. (G7.5: 6 up) 55-in. FHD 46-in. FHD 42-in. FHD
Power consumption 150 W (70 W ↓) 68 W (102 W ↓)
Thickness 11 mm ( x0.31)
Specs. w/normal technology 220 W 170 W 35 mm
Power saving
(per pcs/per year use)
• 5.6 kWh ↓
(during production)
• 0.0047 kWh ↓
(during production)
• 127.8 kWh ↓
(5 hours/day in use)
• 186.2 kWh ↓
(5 hours/day in use)
(Transported 200 km by car
& 2000 km by flight)
CO2 reduction
(per pcs/per year use)
• 2.3 kg ↓
(during production)
• 0.002 kg ↓
(during production)
• 53.3 kg ↓
(5 hours/day in use)
• 77.6 kg ↓
(5 hours/day in use)
• 2.5 kg ↓
(per transportation)

Total CO2 consumption saving by IJP-CF: Remark:
• 20,067 tons/year (based on G7.5 120K) • 1 car = 0.2 kg/km (1 cargo: 1000 pcs)
• Equivalent to the electricity consumption: 20,087,000 kg/year = 48,208,800 kWh/year • 1 flight = 32.5 kg/km (8 cargo: 8000 pcs)
  • 1 kW device @ 1 day:
     – 1 day = 24 hours ® 1 kW x 24 hours = 24 kWh (electricity)
     – 24 kWh ~ 21 kg (coal)
     – 24 kWh ~ 10 kg (CO2)

 

Conclusion

Green TFT-LCDs can be achieved through innovative designs to achieve more efficient transmittance and smart backlighting systems. It seems that the high transmittance of an LCD cell as well as low power considerations from a systems point of view, as well as clean process alternatives for manufacturing, are the most promising approaches for green TFT-LCDs. We also drew up a roadmap of product carbon footprint reduction, aiming at a 30% decrease in carbon footprint levels from 2009 by 2012, to help generate an all-new low-carbon product for consumers.

References

1T-S. Chen, et al., "Invited Paper: Advanced MVA III Technology for High-Quality LCD TVs," SID Symposium Digest 40, 776 (2009).

2P-L. Chen, et al. "Advanced MVA for High-Quality LCD-TVs," SID Symposium Digest 37, 1946-1949 (2006).

3C-N. Lin, et al., "A New Structure of Cu Compound / Cu as the Metal Electrode for TFT-LCD Process," Proc. IDW '07, 89-92 (2007).

4H-P. D. Shieh, et al., "Invited Paper: An LCD-TV Powered by a Battery?" SID Symposium Digest 40, 228(2009). •

 


Po-Lun Chen is Director, Interface Component Business Unit, at AU Optronics Corp. He can be reached at pl.chen@auo.com.