Mike Logan , Display & Input Technology Manager, andersDX
Choosing the right display technology for your next application requires an eye for detail
As a feedback mechanism, a colour display is hard to beat in any application. In non-consumer applications the use of a large, full colour display may have seemed opulent in the past, but continued price erosion means it is becoming more commercially viable to use graphical colour displays in a wider range of applications.
Of course, if the the product only needs a power indicator then an LED is still a good option, and segment displays still make commercial sense in applications that only need to convey simple information. However, in the industrial, medical and automotive markets the trend is towards employing a graphical user interface that can provide much more than basic feedback.
Choosing to use a full colour display is only the start of the decision process; the question soon turns to what kind of display technology is the ‘right’ one for a given application. The need to constantly innovate in this area may be driven predominantly by consumer demand, but all sectors and application areas can now choose from a wide range of display solutions, creating a design dilemma for engineers.
Rise of the OLED
The latest chapter in the display technology saga is, arguably, the AMOLED, or Active Matrix Organic LED. Although the technology is now relatively mature it has yet to become as prolific as LCD displays that still rely on backlights, but that may be changing.
While most graphic colour displays employ thin-film transistor (TFT) technology to control the colour and light intensity of every pixel displayed, only AMOLEDs are truly emissive displays; they generate their own light instead of modulating and filtering the light provided by a single, common source.
AMOLEDs use TFT technology to implement their active pixel addressing scheme, which sets them apart from their passive matrix cousins (PMOLED). Both types generate light through the all-important emissive electroluminescent layer, formed from a thin layer of organic compound, but PMOLEDs use a simpler ‘row/column’ pixel addressing scheme inherited from earlier LCD designs, which imposes the need for refresh rates high enough to maintain acceptable image persistence.
AMOLEDs, on the other hand, allow each pixel to maintain its state between refresh cycles, through the integration of an active (transistor and capacitor) circuit on each pixel. This removes the need for high refresh rates purely for image persistence, which normally translates to comparably lower operating power consumption.
However, as with most display technologies that have evolved over the years, one type doesn’t necessarily supplant the other; while AMOLEDs have many advantages over PMOLEDs, there are still many applications where a PMOLED display would be the more suitable option. Similarly, the use of other forms of display technology using backlit LCD technology may represent the right solution in specific applications. This blurred line only increases the design complexity.
Today, the single biggest application area for AMOLEDs is probably high-end smart phones, but with new manufacturing facilities and advanced processes coming online, costs and MOQs are coming down. It is becoming easier for the embedded sector to access AMOLED technology. OEMs can now choose an AMOLED display in standard sizes or have bespoke solutions designed for them, bringing the benefits and advantages of OLED technology to a wider range of products.
When choosing the most appropriate display technology for a given application, price, availability and performance are always the key factors to consider. Display technologies differ significantly in terms of viewing angle, brightness, response time, power and size.
As it represents an evolutionary step in display technology, it follows that AMOLED should offer gains in terms of the performance metrics mentioned, and it does. AMOLED displays offer a wider viewing angle than TFT LCD displays, yet are significantly thinner in profile (see below for an overview of the construction of AMOLED technology). Their construction also allows more of the light to reach the top layer (normally glass) and so they are generally perceived as being brighter, too. Manufacturer’s data shows that for a given luminance of, say, 150cd/m2, AMOLEDs can appear almost 50% brighter than TFT-LCDs.
In practice, this means that a TFT-LCD display would need to be brighter (use more power) to be appear as bright as an AMOLED display, particularly in sunlight. Conversely, the AMOLED display could operate at lower power and still be readable in direct sunlight.
For industrial and medical applications, operating temperature can be critical. AMOLEDs compare favourably with even the best TFT-LCDs, making them suitable for environments that would be considered harsh in terms of consumer applications, but standard in industrial, medical and automotive markets.
These benefits ensure that AMOLED displays will successfully transition from the very high volume consumer (smart phone) market, to medium and high volume applications commonly found in all other sectors.
By way of illustration, Truly Semiconductors is one of China’s largest manufacturers of small and medium sized display modules, producing both AMOLED and TFT-LCD displays. Its own data shows that AMOLED offers superior performance in terms of contrast ratio and response times. Truly Semiconductors has recently invested in a new 4.5G production line that will use LTPS to manufacture AMOLED displays with a pixel density of up to 500ppi; the company is also running an amorphous silicon process to manufacture TFT-LCDs with a pixel density of 300ppi. Both technologies are available in the UK and Ireland through andersDX.
This alone would make it more applicable in many applications, but the benefits extend to include lower dynamic power, further widening the potential applications to include portable equipment, such as home health care (another burgeoning sector), and the Industrial IoT.
AMOLED technology offers much higher response rates than TFT-LCD. High response times directly influence the update rate; for a frame rate of 60 frames per second (60Hz), a minimum response time of 16ms would be required. It follows that a higher response rate results in a clearer image. This can be particularly important in industrial or medical applications, where the clarity of the image could be crucial.
Although in terms of performance and size, newer AMOLED technology normally wins, it is certainly not right for every application.
The most immediate is cost. AMOLED technology comes at a premium of 100% compared to standard TN TFT displays, and 50% compared to TFT IPS technology, which in truth offers very similar viewing angle and brightness to AMOLED though is somewhat larger and more power hungry.
MOQs for AMOLED technology are also higher. Although we can offer AMOLED for industrial applications, MOQs are still of the order of 50,000 per year – much less than the millions required even a short time back, but still too high for many industrial applications. Leadtimes also are longer on AMOLED – roughly double that on TFT at present.
Finally, lifetime can rule out AMOLED in professional electronics applications. Consult the datasheet for the operational hours on your preferred AMOLED or TFT display, but expect in the region of 20-30,000 hours for AMOLED and 50,000 hours for TFT.
There are strong technical and commercial reasons for choosing AMOLED technology when developing the next-generation of industrial, medical and automotive applications. Its superior performance, flexibility (often literally) and availability compare favourably with existing display technologies and it is destined to become a popular choice with OEMs and their customers.
The emission principle behind OLEDs uses the injection and recombination of holes and electrons in an emitting layer, which absorbs the energy of the hole-electron pairs to emit light. The layers required to achieve this are sandwiched between an anode and cathode, making the equivalent circuit an LED.
The process used in the creation of the active matrix, which is fundamental to AMOLED technology, is generally based on a poly-silicon process, as opposed to the amorphous silicon process typically used in the manufacture of TFT-LCDs.
The substrate is invariably glass when creating displays, which has a deformation temperature of 650C. For this reason a ‘low temperature’ poly-silicon (LTPS) process is required when manufacturing AMOLEDs. While more complex than an amorphous silicon process, LTPS results in much higher carrier mobility, contributing to AMOLED’s superior performance. For example, higher mobility enables smaller transistor geometries, which in turn results in a larger effective area, a high aperture ratio and higher resolution.
An LTPS process also lends itself better to creating curved and flexible displays, features that are already apparent in some smart phones. The ability to create curved and irregular displays is arguably even more applicable when developing user interfaces for industrial machinery, medical equipment or automotive dashboards.
For further information visit www.andersdx.com .