Will OLEDs Replace Small TFT-LCDs?

Some OLED proponents believe that their technology is a "disruptive" technology that will challenge TFT-LCDs in the rapidly growing market for small- and medium-sized displays. But, although OLED sales will grow appreciably, domination of TFT-LCDs is not likely.

by Barry Young

IN November 2000, Dr. Alan Heeger – distinguished scholar and, at the time, a recent recipient of the Nobel Prize in Chemistry – announced to the world at Intertech's OLED 2000 conference in San Diego that organic light-emitting-diode (OLED) displays were a "disruptive" technology that would replace LCDs as the dominant display technology in the next 10 years. Uniax, the company Heeger had founded to develop polymer-OLED (PLED) technology, had recently been purchased by DuPont. DuPont announced that it would marshal its mighty corporate resources in support of that technology, but it had little or no expertise in OLEDs and was using Heeger's Uniax as a jumping-off point.

Heeger pointed to the OLED's thin form factor, low power consumption, wide color gamut, high contrast ratio, wide viewing angle, fast response time, and low-temperature manufacturing process compared to those of thin-film-transistor liquid-crystal-displays (TFT-LCDs), and claimed these obvious advantages as evidence for his "disruptive" claim. He put the OLED vs. TFT-LCD comparison in a class with integrated circuits vs. vacuum tubes, automobiles vs. the horse or bicycle, and LCDs vs. CRTs. At the time, this author took the outrageous position of challenging the Nobel Prize winner, not by refuting the prediction that OLEDs would be successful, but by disagreeing with the position that OLEDs would be truly "disruptive."

Today, even as we enter the inevitable downturn portion of the crystal cycle, TFT-LCDs are perhaps the healthiest and fastest-growing high-technology industry in the world. In 2004, year-to-year unit sales were up 43%, revenues were up 45%, display glass was up 48%, and we expect continued growth in the next 5 years as TFT-LCDs completely dominate the huge information-technology (IT) market and target the even-larger TV market. A very short 4 years have gone by and it is really too soon to judge the outcome. But there are a number of signs indicating that OLEDs are not "disruptive," but represent, at best, a step in what has become the evolution of the flat-panel display (FPD).

 


Table 1: Comparison of OLED vs. TFT-LCD Performance Characteristics
 
Status
Forecast
2000
2004
2010
Thin form factor
+++++
++++
+++++
Wide color gamut
+++
++
=
Low power consumption
– – –
+
High contrast ratio
+++++
+++++
+++++
Wide viewing angle
+++++
+++
++
Fast response time
+++++
++++
++
Low-temperature manufacturing process for flexible substrates
P
P
+++
Manufacturing cost
– – – – –
– – – –
+
Imaging sticking
– – – – –
– – –
Differential aging
– – – – –
– – – – –
Short material lifetime
– – – – –
– – –
=
Highly reflective surfaces
– – –
– –
Low efficiency material and relatively low external quantum efficiency
– – – – –
– – –
=

Legend: +, Better; +++++, Significant competitive advantage; P, Promise (not yet practical); 
=, No competitive advantage; –, Worse; – – – – –, Competitive disadvantage.

 

TFT-LCDs seem to be an inviting target for the creative and bold scientist, for if one had to start from scratch to design a display, TFT-LCDs would be the last design selected. The process is extraordinarily difficult, with hundreds of process steps, a large capital investment, and a high dependence on the cost of materials. Then, there are the viewing difficulties caused by shining light through an amorphous material that responds relatively slowly to an electrical charge and has optical characteristics that can change substantially with the angle at which light passes through it.

Nonetheless, history is littered with technologies that would have reduced the capital investment, increased the performance, and eliminated the sensitive material dependence. For example:

• One very smart analyst claimed that LCOS would replace large-area TFT-LCDs because with LCOS "pixels are free."

• The venture-capital community and a number of very large U.S.- and Japanese-based companies bet what is probably more than a billion U.S. dollars on the promise of field-emission-display (FED) technology.

• Polymer-dispersed liquid-crystal (PDLC) technology was at one time the darling of the chemistry community.

We have seen proponents claim that displays could be made from carbon, semiconductors, carbon nanotubes, and a host of other materials, but LCDs have always prevailed. The reasons could be the subject of another article. However, let us take Heeger's claims one at a time.

• Thin form factor. It is certainly true that OLED-display thickness is less than 2 mm, while TFT-LCDs with a backlight have a minimum thickness of 4–6 mm, going up to more than an inch for larger displays. Moreover, within a year or two, OLEDs could be half their current thickness as thin-film encapsulants are commercialized.

• Wide color gamut. This claim was really a projection because, at the time, OLED materials with any lifetime were less than acceptable in terms of percentage of the NTSC color gamut. However, the chemists have since taken OLEDs to more than 80% of NTSC. (But LCDs are now starting to use LED backlights, which will match OLED performance.)

• Low power consumption. At the time, only passive-matrix OLEDs (PMOLEDs) existed, which actually used more power than comparable LCDs. Currently, in the most significant market, mobile-telephone OLEDs are being challenged by transflective LCDs, which use virtually no power in the reflective mode and almost as little as OLEDs in the transmissive mode.

• High contrast ratio. This is a clear strength of OLEDs, which have a natural black, while LCDs struggle to eliminate the light from the backlight (in trans-missive mode).

• Wide viewing angle. This remains a strength of OLEDs, even as LCDs claim 170° viewing angles (but at a minimum contrast ratio of 10:1).

• Fast response time. OLED performance is in the tens of microseconds, compared with 8–10 msec for LCDs. LCDs havemade great strides, but OLEDs will always have an advantage in high-speed video.

• Low-temperature manufacturing process. Although this claim is valid, until the process is applied to flexible substrates it is of no commercial value.

OLED technology also has some disadvantages compared to LCD technology.

• Imaging sticking. The emitting material will eventually fatigue, leaving a residual image, as is the case in other emissive technologies. Engineers are working on solutions to compensate for the problem.

• Differential aging. The red, green, and blue phosphors age at different rates, which is common to all emissive technologies. The changes are readily visible since the human eye can detect differences of luminance down to 1%. Electronic solutions are being developed to mitigate the problem.

• Short material lifetime. Material lifetime in 2000 was at the 2000–3000-hour level and had improved to approximately 10,000 hours by the end of 2004. Lifetime will have to improve substantially in the next few years – and it will probably do so.

• Highly reflective surfaces. The OLED's cathode is a reflective material, which in bright sunlight seriously degrades the image. There has been great progress in mitigating the reflection problem.

• Low-efficiency material and relatively low external quantum efficiency. Initial OLED efficiencies were not promising, but the progress made over the last 4 years is so great that there is some chance that OLED efficiencies could challenge today's standard lighting technologies within 5–10 years.

As can be inferred from Table 1, the promise of OLEDs in 2000 was great, but it is clear that the potential of TFT-LCDs to close the gap was underestimated and that the promise of OLEDs has yet to be realized. Still, there are quite strong signs that OLEDs can realize some of that potential in the next 5 years.

Getting Practical

The theoretical comparisons are easy and fun to do, but from a practical viewpoint, where do OLEDs stand today and how well will they do over the next 5 years? Some points are very clear.

• The PMOLED that could be built at very low cost and would have a wide range of applications does not exist. The passive-matrix approach is limited to relatively small OLEDs, and they are power-hungry because of the pulsed driving scheme and the associated high voltages. As a result, although there is a place for PMOLEDs, it is one with limited growth potential.

• Many bought into the theory that PMOLEDs could provide a lower-cost higher-performance solution than LCDs, which is indicated by the fact that there are no less than 30 companies practicing the art.

• The active-matrix OLED (AMOLED), which uses low-temperature polysilicon (LTPS) for its thin-film pixel-driving transistors, is at the early stages of commercialization and should experience strong growth beginning in 2005 as SK Display, AU Optronics, ST-LCD, RiTdisplay, Pioneer, Casio, Samsung SDI, LG Electronics, Seiko-Epson, and TMDisplay enter the market.

 

Young_Fig_1_tif

Fig. 1: OLED revenues by application (millions of U.S. dollars) are plotted against overall growth (yellow line, in percent). Growth is projected to be substantial, but penetration in most applications is modest through 2008, as indicated in Table 2.

 


Table 2: OLED Share of the Small-to-Medium-Sized-Display Market (%)
 
2003
2004
2005
2006
2007
2008
Automobile monitor
0.0
0.0
0.3
1.5
3.2
5.2
Automotive
0.0
0.0
0.5
1.1
2.4
5.2
Camcorder
0.0
0.0
0.6
7.3
17.7
26.0
Car audio
30.3
21.0
19.5
23.0
26.8
31.0
Digital camera
0.4
0.8
2.2
8.8
13.2
16.9
DVD
0.0
0.0
0.7
7.4
12.8
14.3
Game
0.0
0.0
1.5
6.6
7.4
8.1
Handheld TV
0.0
0.0
0.0
5.2
6.4
8.1
Industrial
0.5
1.2
4.1
6.8
8.8
12.6
Mobile telephone
0.0
0.0
3.1
3.9
10.6
13.5
Mobile telephone sub      
20.3
24.0
31.5
MP3 player      
5.0
5.1
5.1
Notebook PC
0.0
0.0
0.0
0.1
0.3
0.6
PDA
0.0
0.0
11.4
16.0
21.3
24.3
Toy
0.0
0.0
0.2
0.5
1.0
2.0
Watch
0.0
0.0
4.1
8.1
10.9
14.2
Wearable
0.0
22.3
22.4
19.9
17.5
15.2
Others
0.4
0.5
0.6
0.7
0.8
0.9
Total
1.3
1.2
2.6
3.1
5.2
6.3

 

The primary challenges to commercializing AMOLEDs in 2005 are

• Poor LTPS yields resulting from the poor uniformity of the excimer-laser annealing (ELA) process.

• High LTPS costs, which result from a combination of high capital expenditure for the array process (greater than 50% more than for amorphous-silicon arrays of comparable size) and low yields.

• Developing TFTs that will function reliably and with a common threshold voltage in continuous operation.

• Scaling the organic deposition process, which is more of a challenge for the small-molecule evaporation process than for the solution-based process used with polymer materials. However, solution-based full-color commercial processes are not yet functioning, while full-color small-molecule OLEDs are selling in the millions of units.

As a result of all the above, it is expected that AMOLEDs will be limited to panel sizes of less than 10 in. through 2006. Material performance and lifetime are also key to growth. Currently, commercial OLED displays offer lifetimes of 3000–10,000 hours, which is satisfactory for mobile telephones and other small- and medium-sized applications but well below the 30,000–50,000-hour requirements for IT and TV applications. There has been substantial progress in both lifetime and efficiency, but a long journey remains.

AMOLEDs cannot depend on LTPS backplanes to compete in the large-area-applications market because LTPS represents less than 5% of total active-matrix capacity and has proved practical only up to fourth-generation (730 ´ 920 mm) sizes. Amorphous silicon (a-Si), on the other hand, can support seventh-generation substrates at (approximately) 2000 ´ 2000 mm. It is subject to low electron mobility and low TFT reliability in OLED applications, but there are several efforts under way to use a-Si backplanes for AMOLEDs, not the least of which are those of RiTdisplay, Casio, AU Optronics, and Kyocera.

Organic TFTs (OTFTs) carry great promise for combining printable TFTs and OLEDs in the production of roll-to-roll flexible low-cost displays with high performance. However, OTFTs have even lower electron mobility than a-Si and have not reached the stage at which their reliability can be tested. They therefore remain a vision for the future, not a commercial technology for the present.

Unfortunately, we are left at the end of 2004 without an answer to the question of whether OLEDs are "disruptive" or just a continuation in the evolution of FPD technology. In a practical sense, we can obtain our best insights into the situation by looking at the current trends in OLED shipments and revenues (Fig. 1 and Table 2). In 2004, OLED revenues were $334 million, up 42% on a year-to-year basis, but represented only a 1.2% market share, which is actually down from 2003 as the rest of the market grew faster than the OLED segment.

But 2005 and 2006 are expected to be breakthrough years, with revenues forecast to be $832 million and $1.2 billion, up 147% and 47%, respectively, as AMOLEDs take hold. The market share is forecast to grow by more than 250% by 2006, reaching 3.1%.

So, was Alan Heeger's prediction that OLED technology would be "disruptive" presumptuous or insightful? OLED practitioners are working their way through a myriad of problems in order to commercialize the technology. Today, OLEDs offer some of the advantages outlined in 2000, but LCDs are catching up quickly. The OLED-display image is an improvement over that of LCDs, but the low yields, small substrates, and immature process technology make them more expensive. It is likely that these process issues will be overcome to satisfy the evolutionary component. But it will take major scientific breakthroughs in the chemical and physical components of organic devices for OLEDs to become truly "disruptive." •

 


Barry Young is Senior Vice President and CFO of DisplaySearch, 1301 S. Capital of Texas Hwy., Suite B125, Austin, TX 78746; telephone 512/459-3126, fax 512/459-3127, e-mail: barry@displaysearch.com.