DISPLAYS & UIS
LED display technology has long been regarded as the essence of flat panel display technology - its optical performance, in terms of accurate colour reproduction, and image quality and clarity, is quite exceptional.
Although successful in some markets, what has prevented organic light-emitting diode (OLED) displays from becoming a more established and widely used display technology, and one that can offer a viable, long-term replacement for more traditional flat panel displays (FPD) such as thin-film transistor (TFT) LCD technology?
Evolving technology
Achieving efficient and effective OLED manufacturing processes have until recently presented a barrier to cost-effective OLED production.
One of the issues affecting OLED displays has been the limited lifetime of OLED materials - particularly the organic compounds used as light emitters. Recently, there have been many advances, which have led to significant improvement in materials and a resultant improvement in product lifetimes.
OLED success
OLED display technology has seen considerable success in high volume OEM applications such as smart watches, top-of- the-range mobile phones, and large-size
OLEDcomes of age O
(>50-inch) high-end smart TVs. OLED adoption in other markets and industrial applications has been limited.
How does an OLED work? An OLED uses an organic compound as an emissive electroluminescent (EL) film layer that emits light when an electrical current is applied. An OLED device consists of two electrodes and several function layers: an electron transport layer (ETL), an emitting layer (EML) and a hole transport layer (HTL). When a bias voltage is applied between the anode and cathode electrodes, electron and hole recombine in the emitting layer and light is emitted in response to the current flow.
OLED manufacturing
Existing manufacturing techniques for OLED display technology are complex and expensive, resulting in high production costs. This is one of the main reasons for the relative lack of success of OLEDs in the medium-size display sector.
Conventional OLEDs are produced using a technique known as the vapour deposition method. This production technique requires large vacuum chambers to accommodate a vapour deposition source and the panel substrate. The EL layer is formed by vaporising the EL material, with a fine metal mask employed to form individual pixels on the substrate.
Justin Coleman, Display Division Manager at Review Display Systems explores the latest innovations in printed OLED technology
For colour (RGB) OLEDs, three individual
mask layers are required and as pixel resolutions increase aligning the pixel mask precisely becomes increasing challenging. In addition, the vapour deposition method is relatively inefficient in its utilisation of the EL materials. Japan-based OLED manufacturer, JOLED
has developed a pioneering ink-jet printing technique which forms emissive RGB subpixels by printing directly onto the panel substrate. The innovative printing technology realises a more efficient and cost-effective production process - typically around 30-50% cheaper compared to vapour deposition methods. The printed OLED substrate is processed
under normal atmospheric conditions without the need for a vacuum chamber or any metal masking layers. Waste material is completely avoided as only the precise amount EL material is deposited on the substrate during the printing process. This new manufacturing process for OLED
displays is also scalable which enables a range of panel sizes to be mass-produced using the same print head technology. While the printing method still requires advanced technical capabilities, it represents a major innovation in OLED manufacturing, as the production process is simple, efficient, and adapted for medium to large-size OLED production.
36 JULY/AUGUST 2021 | ELECTRONICS TODAY
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