by Olivier Nowak, Wicht Technologie Consulting
After years spent playing catch up with LCDs, organic light emitting diode (OLED) displays seem to be finally closing the distance. The first mobile phone sporting an OLED main display - the BenQSiemens S88 - was introduced earlier this year. It includes a 2.2-inch, 176 × 220 full-colour active matrix OLED (AMOLED) screen from AU Optronics and several major display manufacturers (Samsung SDI, CMEL, and TPO Displays) have now signalled their intentions to offer AMOLED displays by the end of the year, targeting the mobile phone market, and marking a turning point for OLEDS.
Mobile phones have always been envisioned as a key application although the market has not developed as initially forecasted. In fact, mobile phones oems have used OLED for sub-displays since 2003 but due to price, reliability and availability issues, have not trusted the main display to OLEDs. In the meantime, falling prices have made thin-film transistor (TFT) and colour super twisted nematic (CSTN) LCDs the choice for displays in high-end phones. and lower to mid-range phones respectively.
Since their introduction seven years ago, passive OLED displays have been successful in applications that require text and/or simple graphics. Many music players, auto-radios, sub-displays for mobile phones and video players employ OLEDs. But large scale manufacturing is difficult. Production yields hover around 70%. And the largest facilities can deliver only a few tens of thousand of higher resolution displays per month. This is enough for audio/video players but not quite up to the throughput requirements of the mobile phone market, or even for the high-end segment.
Active Matrix OLEDs reap the benefits of LCD advances
Passive matrix OLED (PMOLED) displays work by addressing each pixel along a row sequentially, line by line. This concept is no longer viable beyond about 128×128 pixels. Therefore passive displays are well suited to low resolution static (not video) displays.
Active displays (LCD or OLED) have a backplane consisting of thin-film silicon below the active layer. Each red/green/blue subpixel is driven by one or more transistors. Whereas an LCD pixel is driven in voltage, an OLED pixel must be driven in current (10s–100s µA per column for PMOLED and 10–100 µA for AMOLED). The backplanes developed for LCDs cannot be re-used as is for OLEDs, and the production of backplanes for OLEDs has proven elusive. However, OLEDs could massively benefit from the low temperature polysilicon (LTPS) backplanes conceived for LCD. Ltps backplanes are well-suited to small displays, are thinner and consumes less energy than the standard amorphous silicon (a-Si) backplanes. More importantly for OLEDs, they can handle the higher currents needed to switch the pixels more efficiently. However the manufacturing of LTPS still faces issues of uniformity and yields.
Two of the early manufacturers of OLED displays have recently exited the market. Pioneer, the first company to commercialise an OLED display in 1999 for car radio applications, has given up on its Eldis joint-venture with Sharp for the production of AMOLEDs. It will also cease the production of PMOLEDs. The joint-venture SanyoKodak Displays, for a long time the sole producer of AMOLEDs, has also been dissolved.
Sony remains the only Japanese company with a demonstrated capacity for volume production of AMOLEDs. Toshiba Matsushita Displays (tmd) has showcased AMOLEDs up to 3.5” but has not announced a date for commercialisation. Hitachi and Epson are also developing OLEDs.
Things are markedly different in Taiwan. AU Optronics already offers AMOLEDs. CMEL, RiTDisplays and TPO Displays also plan to introduce AMOLED technology by the end of 2006. RiTDisplays is credited with being one of the largest OLED manufacturers with Samsung. Univision and Opto Tech also have plans for AMOLED beyond their current PMOLED offerings.
In South Korea, Samsung (the largest manufacturer of LCDs for mobile phones and the third largest producer of mobile phones) has invested $465 million in a fourth-generation AMOLED facility (glass panels of 730×920 mm). Further investment may bring the initial capacity of 20 million displays per year in 2007 to 54 million displays per year later on. Lg Philips LCD Co. (lpl) seems to be taking a more conservative approach, having only a second generation facility (370×470mm) for AMOLED. Hyundai LCD has a pilot line. Elsewhere in Asia, new manufacturers are also emerging for PMOLEDs. These include Beijing Visionox Technology (Kunshan, China) and Ness Display (Singapore).
Market reality and forecast
There are many numbers circulating for market volumes and value of OLED displays. Some 2003 forecasts, problably made as a result of euphoria surrounding the first commercial AMOLED, were quickly revised downwards as AMOLEDs failed to emerge en masse. Even so, many subsequent market forecasts have continued to be overly optimistic. With hindsight, the market for OLEDs was about $500m in 2005 ($520m according to iSupply, $491m according to DisplaySearch), or about 10% growth over 2004. Forecasts predict a market worth over $1bn in the next few years.
In terms of price point, passive OLED displays still cost about 20% more than an LCD of corresponding size, resolution and colour depth. Active OLED displays can cost up to twice as much as an equivalent TFT LCD.
Lifetime suffices for mobile
Lifetime—defined as the time after which a pixel loses half its brightness—is a major issue for OLEDs. The organic active materials present in the sub-pixels do not age at the same rate, resulting in distortion of colour rendering over time. More importantly, the blue sub-pixels age quite quickly. Commercial OLED displays are typically rated at 10,000–15,000 h (depending on brightness). This suffices for a mobile phone, whose assumed useful life is two years, but not for a television or monitor. An LCD, on the other hand, easily achieves 50,000 h lifetime.
Next target: TV displays
A prototype 13-inch AMOLED display was presented by Sony as early as five years ago. Most display manufacturers have already showcased one or more prototype large OLED display. One of the latest is from Samsung - a 40-inch single sheet AMOLED display with an a-Si backplane (April 2006).
However, OLED TV sets are still many years away. From a technical point of view, the main hurdles are the backplane and the operating lifetime. Televisions require 50,000 h lifetime and brightnesses in the range of 500 cd/m, compared to 120–150 cd/m for mobile applications. Although an OLED can be made to emit at the required brightness, this has a dramatic impact on its lifetime. And its lifetime (about 10,000 h) is already five times too short at lower brightness levels.
Announcements from several manufacturers planning to introduce TV products as early as 2008 therefore seem optimistic. Previous announcements that touted products in time for the 2008 Olympic Games have already fallen by the wayside. In the meantime, LCD technology keeps improving and getting cheaper, all the time raising the market entry barrier for OLEDs.
Thin is in
Contrary to LCD, OLEDs do not require a backlighting unit (BLU) and are therefore only around 1.5 mm thick (compared to 34 mm for a small TFT LCD with the BLU). This property is especially interesting in view of the trend toward ultra-thin mobile phones.
In an interview given to its corporate newsmagazine in December 2005, Ryoji Chubachi, Sony’s President and Electronics CEO said: “[Sony]does not consider OLED devices a replacement for TFT LCD. Rather, we would like to capitalize on the paper-thin characteristics of OLED device technology to develop interesting new applications.”
Smart cards and clothing could be some of the beneficiaries. Siemens has presented a prototype OLED display card as far back as 1999.
Many conceivable applications rely on the potential of the OLED to be printed, e.g. by inkjet. Polymer OLEDs (P-OLED). although commanding only a few percentage points of the overall OLED market now, show great potential in this area.
Printing offers a major advantage: the substrate need not be glass but could be an inexpensive plastic film, opening the door to flexible displays. Incidentally, the costs (capital and operation) for printing are expected to run at less than the current processes. Printed displays would come in three broad segments:
• Ultra-cheap low-resolution, likely monochrome displays, e.g. to be placed on packages. The question remains of how to power these displays.
• Very large colour displays (several meters in size) for advertisements, both indoors and outdoors
• Flexible displays, e.g. for e-paper. A flexible film that can encapsulate and protect the organic materials (from oxygen, humidity, light) long term remains to be developed. The first steps towards flexible displays could be curved and then bendable displays.
Time will tell if it is wise to bypass the competition with LCD by essentially entering into competition with paper.
Lighting with white OLEDs
Whether or not OLEDs succeed as a display technology, GE, Philips, Osram and others are writing a new chapter in the OLED saga—developing white OLEDs that could be pasted in sheets to transform any surface into a light source.
In an strange twist, the us company UDC is developing a white OLED panel with the LCD backlighting unit market in mind.