Passive Matrix OLED displays – Still alive and looking great

Passive Matrix OLED (PMOLED) displays have been around for many years. At their highest volume point, color PMOLED displays were used as the front window on many mobile flip phones. They peaked in commercial popularity when Osram was selling their Pictiva brand of PMOLED displays in the mid-2000s. When Osram exited the OLED display market in 2007, the noise settled way down because theirs were the only PMOLED displays being sold nationally, by the Osram reps and distributors like Avnet. But PMOLED displays have continued to be made in volume for a wide variety of applications, and I am glad to say that it is a product DTS is upporting, through Inteltronic.

Plus and Minus of PMOLED Displays

PMOLED displays are monochrome displays with several features that make them stand above any monochrome LCD. An OLED display is essentially micro-LEDs

• Ultra High Contrast – Typical contrast of 2000:1
• Wide Viewing Angle – Since this is an emissive display, the OLED material is on the top surface, so viewing angles are to 89°. Here are three Inteltronic displays for comparison. From left to right are:

1. 4.3” TFT, model LMIX0430NTN247-4RTP (no, I did not create that image)
2. 3.12″ 256 x 64 blue graphic OLED, model UG-5664ALBEF01
3. 2 x 20 yellow character OLED, model UC-2002ASYAG01

                Picture #1 is from directly above the displays:

                Picture #2 is from at least a 55° angle to the 6:00 direction:

 As you can see, the TFT image is just about gone but the OLED is very visible.

• Very Thin – All of Inteltronic’s PMOLED displays are 2.0mm or thinner
• Wide Operating Temp – Most of the Inteltronic PMOLED displays operate from -30 to +85°C; many operate down to -40°C
• Response time – Since this is a solid state display, response is in nSec rather than mSec
• Simple interface – all of these display have either 3 or 4 SPI, or 68XX / 80XX parallel interfaces; most of them offer all of these as standards
• Evaluation Kits – all of the Inteltronic PMOLED displays have Evaluation Kits available for under $100, which can dramatically speed up you development process. Here is a picture of a typical Eval Kit:

The potential negatives depend on your requirements:

• The brightest of these displays is around 150 nits; even with the high contrast, it is not considered sunlight readable
• The PMOLED displays available from Inteltronic are generally small; due to the motherglass size they are cost effective up to 3.12”
• Product Life – there have been great improvements made in PMOLEDs in the past 5 years. But the longest product life is still dependent on the display’s color. Here are typical MTBF in hours to 50% brightness, by color:
  • Yellow – 40K
  • Green –   40K
  • Blue –       10K
  • White –    10K

Power Usage is an interesting topic, because it is entirely dependent on the image shown. Since each pixel is a small LED, if that specific pixel is not on, then no power is applied to it. Here is a chart showing the different amounts of power that is used in a yellow 2.7” 128 x 64 monochrome PMOLED display, as the image activates more and more pixels:

One important point – a comparable 2.7” monochrome STN display requires .55W sustained with the LED backlight on.

Inteltronic also offers color PMOLED displays, which function similar to a CSTN color display – a monochrome display with a color filter. The power usage chart is even more dramatic with this 1.5” 128 x 128 color display:

A complete list of all of the Inteltronic PMOLED displays can be found on the DTS LCD website – http://dtslcd.com/id68.html

Demonstration units are available on several models. Please contact DTS at bwaddell@dtslcd.com if you would like to see a demo, get a quote on a PMOLED display or an Evaluation Kit, or if you have any questions about the products.

One of the questions that I often hear is “Where is the TFT industry going?” That can mean a lot of different things depending on who is asking the question, but for the sake of this entry I will focus on what it means to a typical commercial/industrial user. I think this is a question that is really looking for two answers: 1) What products have at least one alternate source, so I won’t be stuck on an island if it goes away; and 2) what products will be around for at least 5 years, once my production starts.

With that in mind I took an unofficial poll of several key suppliers to the U.S. commercial markets to see what sizes are now available as standard products. The list that I created below is all products that have at least two suppliers, so a second source is an option.

A few comments about this list:

• The latest trend I am seeing is for higher resolutions at the smaller sizes. There are several examples on this list, which were not part of the ‘standard’ TFT offering even one year ago.  The first is the 3.5” HVGA or 320 x 480 TFT. It changes the 3.5”form factor slightly from the traditional 3:4 to a 2:3 ratio, but the resolution goes to 164 dots per inch. Another model receiving increasing interest is the 4.3” WVGA or 800 x 480 TFT. This raises the resolution to a whopping 213 dpi.As the volumes increase the prices will come down, which will make these more attractive. The only question to be raised is whether your application truly needs that many pixels in a screen that small.
• The 3.0” 240 x 400 TFT has become a popular model, and there are many suppliers now available.  It is a good size for a handheld device, and many of these have excellent optical image quality.
• There is still concern over the length of time that manufacturers will support any TFTs at the 2.2” and 2.4” sizes. Many of these products are still being driven from the cell phone markets, and are prone to changes or EOL notices with little recourse.
• For the best price/performance, in my opinion the top 3 would be:
  • 4.3” WQVGA
  • 3.5” QVGA Landscape
  • 3.0” 240 x 400
• One final caveat – even though all of the listed displays have at least one alternate source, there are still few (if any) direct drop-in replacements. Every supplier does something slightly different – the number of pins in the connector, flex location, outer dimensions, integrated driver ICs. So even though there may be an alternate source, if your product goes away there will still likely be some engineering redesign work in your immediate future.

This year’s SID show was much larger than the 2009 version in San Antonio. That was due to three primary reasons:  1) A much easier location for Asian suppliers to get to than San Antonio; 2) An upswing in the economy; and 3) The H1N1 outbreak was in full swing last spring, which kept many from traveling. All of the key players were in attendance, and everyone had new products on exhibit. I just wanted to post a few observations about the show.

1)      3-D - Like most of the other attendees that I spoke to, I was not very impressed with the large 3D screens that were on display.   Samsung and LG both had large 3D screens with active shutter glasses on display. The image quality did not appear to be consistent from the center of the screen to the sides; one demo was showing a soccer game, and while the person with the ball in the center of the screen was clear, the players on the outer portions of the screen did not appear to move with the same clarity. The effect eventually became somewhat disorienting. And most of us agreed that we did not have any burning desire to watch TV in the evening when you have to wear glasses, especially if like me you don’t wear glasses all day. It seemed to be a year or two away from being perfected.

2)      RGBW – This is a topic that we will hear more about in the future. There is growing discussion and development in using a White sub-pixel in addition to Red, Green and Blue. The arguments are very compelling – a white color filter transmits almost twice the light of RGB, so moving to an RGBW filter can increase brightness, and/or reduce power. It also can provide a perceived increase in resolution, so this is a technology that could help in all applications. Sharp has announced that they are developing a technology which adds a Yellow sub-pixel, so more will be heard about RGBY also.

3)      Projected Capacitive everywhere – It seemed like every company that has a fab is now in the P-Cap touch screen business. There had to be at least 15 to 20 companies that were touting their P-Cap touch screens at the show. There is still a variety of differing technologies being used, particularly to the manner in the area of P-Cap controller ICs and the number of touch points, which shows that the technology is still evolving. But there is no question that this is becoming a dominant technology, and I would expect that as volumes increase in the commercial/industrial sector there will be more simplified controller options that will become standardized.

The largest and most important LCD display trade show in the U.S. will be in Seattle this year. The Society for Information Displays – known by all as SID – has an annual “Display Week” of conferences and exhibits in different cities around the U.S. This year’s show will be in Seattle the week of May 24^th. The conferences run all week; the exhibits are open Tuesday through Thursday, 5/25 – 5/27. Here are the times for the exhibits at this year’s show:

• Tues., May 25    10:30 am – 6:30 pm
• Wed., May 26    9:00 am – 5:00 pm
• Thurs., May 27  9:00 am – 2:00 pm

There are several great reasons why you may want to go to the SID show this year:

•  It is a terrific opportunity to see all of the major display makers at one time. Virtually all of the industrial TFT manufacturers will be exhibiting, and most of them use this opportunity to show off their new displays, and the best of their current lineup
• It is by no means limited to flat panel displays. There will be every type of product remotely related to displays at the show – touch screens, lighting, controller boards, films and coatings, you name it
• It is an excellent chance to see new and emerging technologies, and to talk to the exhibiting companies to see when (or if) they plan to sell into your marketplace
• The show does get very busy at times – especially on Tuesday – but it’s not like CES, so if you have patience you should have no problem having an in-depth discussion with all the companies you are interested in
• There is an extensive series of conferences on a broad series of topics, where you can find out the latest technology or future directions from the biggest names in the industry. I have sat through many of these presentations and they are generally excellent and extremely informative
• It is not a huge show, so it doesn’t take days to get through it. You should have no problem talking to every company you wish in a day or two.

A personal note – I will be working the booth for one of the primary companies I represent, Inteltronic. Inteltronic’s booth number is 1012, which is just inside the rear hall of the Washington State Convention and Trade Center. Inteltronic has a pretty compelling story to tell, and a very good price to performance ratio. General Digital, another company I work closely with who is an expert in high reliability displays and value-add services for more harsh environments (and a big contributor to the Sunlight Readability Part II blog entry), will be in booth 401. If you are planning to be at the show and would like to meet with either of these companies, please let me know in advance so we can be sure to set a time for a booth tour, and talk about what they have that best suits your needs.

I will also be wandering the show all week, trying to find out who and what is new, and finding more topics for the commercial/industrial blog. If you are not planning to attend the show and want to know about any specific topics or companies, please let me know and I’ll check it out!

Here is a link to the SID show website:  http://www.sid.org/conf/sid2010/sid2010.html

If your application has an absolute sunlight readable requirement and you are not entirely budget constrained, there are other options available to you than just trying to knock down the outside reflection.  Here is the same diagram as in part 1 which highlights the reflection points, where the base configuration is a TFT display with a resistive touch screen:

There are four different reflective surfaces that can all be addressed either individually or collectively:

1. Front of touch screen or protective glass
2. Rear of touch screen
3. Front of TFT display
4. TFT display brightness

1. Front of touch/glass – In addition to using a circular polarized touch screen, you can also add index-matched films and anti-reflective coatings, which can greatly reduce the amount of reflection and increase light transmissivity. There are a wide variety of options and several suppliers in this market niche. I currently work with General Digital of South Windsor, CT, who in my experience is among the best in this field. General Digital started their OptiLabs division in 2001 and has a wide variety of products and solutions available, depending on your need. One of their products is what they call ‘GenFlective Technology’, which increases light output and contrast by up to 100% without adding any power or heat. The difference is impressive as you can see below – for more information Click Here.

2. Rear of touch/glass – The best way to eliminate the reflection here is to fill the space between the touch screen and the display by Optically Bonding the two together. Here is the definition of Optical Bonding from General Digital: “Optical Bonding is the affixing of two optical elements to one another using a liquid adhesive. The qualifier optical implies that the adhesive is transparent, has a suitable refractive index and is made under adequate control such that there are no significant variations in the optical properties within a single bond. In this way, we differentiate bonding from lamination.” By filling the space between TFT and touch you eliminate the air gap, and therefore you eliminate all front and rear surface reflections. It also greatly increases the product’s durability.

Here is a GREAT series of images from General Digital that graphically shows the impact that AR Coating and Bonding can have:

If optical bonding is not desired another option is to add Anti-Reflective film or coating to the rear of the touch screen.

3. Front of TFT Display – One important note. If you plan to optically bond a touch screen or protective glass to your display, there is no reason to get the display with an Anti-Reflective coating. There are actually two good reasons not to:

• Since the optical bonding fills the air gap, there is no reflective surface to worry about
• Optical bonding material does not adhere will to anti-reflective films or coatings

4. TFT Brightness – Modifying a backlight to make a product truly sunlight readable is an easier proposition than it used to be with the onset of LED backlights, but it is still a much more complex product than just adding a string of LEDs on a rail. Here are just a few of the design considerations:

• The LEDs need to be sequenced in multiple strings, to ensure that if one LED fails the backlight doesn’t look dim
• The LED strings shouldn’t be sequential. For example, if the LED rail has 40 LEDs and they are designed in 8 strings of 5 LEDs, if 6-10 went dark it would be visible to the user. However, if LEDs 6, 11, 16, 21 and 26 went dark they would be spaced far enough apart that the typical user would not be able to tell the difference
• Depending on the display size, there can be a series of different heat sinks, to be sure the whole LED rail doesn’t overheat
• It is important to think in terms of “What do I need” rather than “How much can I get”.  A 2,000 nit TFT display will be visible from anywhere, but how much heat and power can you handle in your cabinet? 

Like most good designs, the key is to approach the design as a system question. A display with an optically bonded touch screen with AR or AR/AG coating on the front will not need to have nearly the same amount of light output coming from the display, because all of the actions taken to reduce the reflection will give a big boost to the contrast ratio. Again, this may not be cheap, but if you need to be able to see your display outdoors it will be money well spent.

Sunlight readability is probably the one design question I run into the most, and it is also probably the hardest question to give a straight answer to. My initial answer is the same every time – “Well, it depends”. There are loads of variables that go into a display that can be seen in the sunlight. What I want to do is to try and break it down into smaller parts, and then take a deeper dive over several entries.

The first question, as in most things, has to do with money. If your application is in a harsh environment where money is not a major issue –such as in a cockpit, or a military project – then the solution can include ultra-high bright custom backlights, enhanced films and optical bonding. That is NOT what this entry is about. For most of us we need to try and find a sunlight readable solution within a budget, and that is the focus of this Blog entry. I’ll address the higher end options in a future entry.

The key to sunlight readability is not the brightness, it’s the contrast ratio. While most industrial TFTs are rated with contrast ratios of 500:1 or greater, the reality is that if you can get an effective contrast ratio of around 3:1 or greater the human eye can see it; and at a 6:1 or greater contrast ratio you can see it very clearly. It is a simple equation:    Brightness / Reflection

 So in order to improve the contrast ratio you can work on both sides of the equation – increase the brightness, and reduce the reflection. 

Here are the key areas that need to be considered.

• Brightness - There is not a ‘magic number’ of nits that makes a TFT readable outdoors. I have heard people say anywhere from 500 to 1,000 nits; I have seen 800 nit displays look terrible, and 350 nit TFTs look great outside. In general the brighter the better; if you can get a display that is 600 nits or greater as a standard product, that will give you a much better image outside – depending on the reflection. From a price/performance standpoint, you should always look for a standard off-the-shelf high-bright display if at all possible. There are several companies that can offer custom high-bright backlights (I represent a few, if you have a need let me know), but be aware that it is a much more complex and costly process than just putting a bank of bright LEDs on a rail. There are other considerations to balance – specifically power and heat – that can have a big impact on what products are feasible for you to use.
• Reflection – To begin with, it’s important to understand where all of the reflection points are. Most companies either use a touch screen, or protective glass to keep the display secure. And if you’re not planning to cover the display with something then you probably should. To get a better picture of the reflection issue, here is a drawing of an LCD with a Resistive touch screen, courtesy of Gunze USA.

As you can see,  there are 5 layers that can all cause reflection:

1. R1 – Touch screen outer surface
2. R2 – First ITO layer
3. R3 – Second ITO layer
4. R4 – Touch screen rear surface
5. R5 – LCD Front surface

Of the three reflection points that can be adjusted – front and back of the touch screen, and the front of the display – the simplest and most cost-effective is to focus on the outer surface. By reducing the outer reflection, you can effectively reduce the reflection of each additional layer.

 A note: What is the difference between Anti-Glare and Anti-Reflection? Most TFTs come with an anti-Glare (AG) surface coating. An AG coating has a rough or etched surface so there is no mirror effect. It does not reduce reflection, it just diffuses it; so the reflection is still there, just not as centralized. Anti-Reflection (AR) coatings actually reduce the amount of reflection by absorbing some of the light. The downside to AR is that it does have a slight mirror effect, and can be more conducive to smudging, which of course is a problem with a touch screen, where touching is the whole point. There are also touch screens with an AR/AG film or coating option now available from select suppliers. These have both anti-reflective properties, but the AG reduces the smudging problem.

One of the best touch screen options for reducing glare uses a Circular Polarizer. Here is another image from Gunze on how the Circular Polarizer (‘CP’) works:

A CP-based touch screen does an excellent job of reducing the reflection. The only downside is that it is not an inexpensive technology, and the touch screens are definitely higher in cost than a standard model. But if you have an outdoor product, you can’t afford to use a cheap alternative.

There are other touch products that target the sunlight readable market now available, such as the EclipsTouch family from Panjit.

In the next entry we will discuss more robust sunlight readable options.

If any of you have seen a demo of a full color, Active Matrix OLED display (AMOLED), you would have to agree that it is one of the best looking displays you have ever seen. But it appears that this product family may be going away from the commercial market – or at least on hiatus for a while.

All of the standard AMOLED displays that are currently available to commercial customers all come from the same source, Chi Mei EL Corporation (CMEL). CMEL is 100% owned by CMO, and their sole focus has been AMOLED. There are several companies that currently sell these AMOLED displays in the U.S., such as Powertip, OSD and AZ Displays; they are all essentially the same CMEL displays. Standard products are available in 2.4”, 2.8”, 3.4”, 4.3” and 7.6” diagonal sizes.

CMEL’s parent company CMO recently merged with two other large TFT manufacturers, Innolux and Toppoly. This was a major consolidation in the TFT world; the joint company is now the 3^rd largest TFT company in the world and the largest in Taiwan, surpassing AUO.

As a result of this merger, CMEL recently sent a notice to their partners to immediately stop recommending and selling their AMOLED displays for all new designs. A quote from the official release: “CMEL definitely will be impacted by this merge; we don’t know what it will be at this moment. But it is highly possible to stop all the development and production of CMEL products and transfer to new AMOLED technology and new product like top emission or others. To minimize the impact from this high potential risk, we request all agents should stop promotion of CMEL products to new customers and discussion of new projects with all customers from now.”

AMOLED has always been a terrific technology, but there were inherent drawbacks that kept it from mass acceptance in the commercial space. The positives – contrast of 10,000:1 and viewing angles of 89° in all directions – make an AMOLED an impressive display indeed. And the expected life improved to 20,000 hours. But there were drawbacks too:

• Power – Since every pixel is an LED, this is either a blessing or a curse. With a black background and light icons, the power usage can be much less than a TFT. But if all pixels are on, the power draw is about 3X that of a TFT
• Brightness – maximum of about 200 nits
• Reflection – since OLEDs are mounted directly to a substrate, there is no way to put a reflective backing behind the image and use the sun. And 200 nits outside just doesn’t cut it, even with ultra-high contrast
• Cost – AMOLEDs are difficult to make, so the yield was low relative to TFTs. Pricing is not on par with TFT even in small sizes, and there is a pretty steep curve in price as the glass size increases

This does not mean that AMOLED displays are going away by any means. There is much new development in the AMOLED world, but it is focused on high-volume consumer applications such as cell phones, digital cameras and even TVs (expensive ones). For example, Samsung just announced a new phone using their ‘SuperAMOLED’ technology that is getting rave reviews, with a 3.3” WVGA AMOLED that is sunlight readable – for more info click here.  But none of these products are targeted for the lower volume commercial markets, where long life support is a must.

Here is a recent roadmap from DisplaySearch of recent and planned AMOLED product development.

One of the readers sent me an email that addresses an issue I heard countless times over the years. They use monochrome LCDs (both standard and custom, and in their industry they truly need parts to be around for 10 to 20 years. This particular customer is in the medical field, but it could easily be a number of other industries too – military, industrial or agricultural. But medical may be the industry where making a change to an existing product is most painful, because for every change a whole new cycle of FDA approvals start over – which pulls engineers off new designs. So the question was if it’s possible to design a mono LCD that can be guaranteed 10+ years of life.

The short answer is probably not, but I think there are a couple ways to improve your odds.  The most important thing is to understand what the problem actually is. Here is a picture of a typical chip on glass monochrome graphic LCD – this particular model is a 1.5” 96 x 64 display from one of my suppliers, RPMTRONICS:

The LCD driver IC is in the top center of the display, mounted on the glass and coated in black epoxy, and that IC is the real problem. LCD driver ICs are made by a relatively small number of companies such as Samsung, Epson, Toshiba, Renesas and Sitronix to name a few. As the volume of TFT displays soared, more and more LCD driver suppliers converted their mono IC lines to TFT drivers.

If you look at the big picture it’s hard to fault them. When I was at Optrex from 2004-06 one of our primary mono IC suppliers was Renesas. When the iPod Nano hit peak volumes, Apple sourced TFTs from multiple suppliers at a rate of over 10 million TFTs per month, and Renesas had a big piece of the LCD IC business. So Renesas obsoleted dozens of mono LCD driver ICs to convert the factories. It is hard to blame Renesas, but it was a nightmare for the thousands of mono LCD designs that were suddenly abandoned.

I don’t believe any mono IC supplier will make a 10 year promise, but it is possible they might be able to guarantee at least 3 years – still not enough. I believe there are three possible ways to get the maximum life out of your design:

1. Buy the driver ICs – Chips like the kind used in COG displays have a shelf life in ESD sealed bags of 1 to 3 years. You could make a last time buy to the LCD manufacturer for them to hold the ICs, so they can continue to build the displays after the IC is EOL, if that option is available. This is not a long term fix because the closer you get to the max shelf life the greater the chance the ICs will fail, but it does extend the time for you to complete your redesign
2. Use an LCD with an industry standard base IC – The idea is that if one supplier obsoletes their IC, the LCD manufacturer has another option that is compatible. In the case of a 2 x 16 character LCD there are numerous ICs that are compatible to the old HD44780 architecture (KS0066U, ST7066U, HD66712, etc). But generally the higher the resolution the more difficult finding a cross is.
3. Use an MCU with built-in LCD drivers – there is a wealth of MCUs with on-board LCD driver control available now. The idea here is to buy an LCD that doesn’t have an LCD IC on board, and keep all of the controls on your PCB. Here is another LCD, again from RPMTRONICS, that is just glass with pins:

By off-loading the LCD drive function, a change in the LCD driver IC world doesn’t affect you. And you have a much better chance of getting long-term commitments from the MCU supplier, because they focus on the commercial market. To get a custom glass + pins LCD designed and built is very inexpensive – maybe $1-3K in tooling, with another $2-3K for a custom LED backlight. This is a trivial investment for a potential long term solution. Again, as the resolution goes up so does the complexity, so you will need to work with your MCU supplier’s FAE.

If anyone has any other suggestions PLEASE post it here, I think everyone would appreciate it.

I recently was involved in a design where the customer needed a 7” wide touch screen that could handle a 1 pound steel ball drop test from 52”. The user also needed to be able to make contact with a glove. Due to the display size and the glove, the only technologies that seemed viable were 4-wire resistive (the 5 and 8-wire resistive displays were too physically large around the perimeter) and projected capacitive.  I worked with several touch screen companies on this, and here is what I found:

• A Glass-Film-Glass resistive touch screen cracked at about 18-20”. The outer glass surface is sufficient for handling a sharp object or a less strenuous drop test, but it couldn’t hold up in this test
• A Projected Capacitive glass touch screen cracked at a height of 16”

The only touch screen to pass the test was a 4-wire resistive polycarbonate touch screen, because of its inherent flexibility. Then the question became one of thickness. The beauty of a polycarbonate T/S is that it can be built up layer by layer to whatever thickness is required, and is still inexpensive. In this case the touch screen functioned at a 6mm thickness; further testing may prove that it will work at a 5mm or even 4mm thickness.

The other question to be addressed is how much flex the touch screen has, and how far from the TFT glass surface it will need to be without actually flexing into the glass. This can also cause issues with the images when seen through such a thick touch screen off-angle, but that’s one of the tradeoffs this app requires. Like every other LCD design, it became a priority issue, and in this case durability was the key.

Bob Waddell has been involved in commercial LCD sales and technical support for 20 years. Prior to launching DTS Bob was the Business Development Manager for Avnet supporting all LCD and related projects for the Central U.S. Previous positions were at Optrex; the manufacturers rep for Sharp and Fujitsu in Michigan; and Marshall Industries. His focus has always been to help provide the right solution for each customer’s application, and it is this experience that he hopes to share with the LCD engineers and users through this forum