OLED has become a major display technology trend and it continues to gain traction in high-end display products. The active use of OLEDs kickstarted around 2008 when Nokia released the world’s first OLED phone – the N85. OLED was soon adopted by other players. The following… (Featured image is intended for representational purpose alone and has been sourced from https://pixabay.com/en/tablet-smartphone-laptop-hard-drive-626093/)
OLED has become a major display technology trend and it continues to gain traction in high-end display products. The active use of OLEDs kickstarted around 2008 when Nokia released the world’s first OLED phone – the N85. OLED was soon adopted by other players.The following year, Samsung applied the OLED screen in its i7110. AMOLED screens were also featured in Google’s Nexus One phone ─ the first phone in the Nexus series released in early 2010. Apple, though lately, has introduced its first OLED phone ─ iPhone X, in 2017. OLEDs are becoming the screen of choice for many display manufacturers.
OLED scores over other screen technologies such as LCD owing to the PenTile technology. Back in the late 2000s, the phones were based on LCD technology and incorporated the conventional RGB stripe pattern, the manufacturing technology for which was fairly established. However, a growing demand for better viewing angles and contrast ratio led companies to adopt OLEDs. The OLED layer replaced the liquid crystal layer in LCD screens (the TFT backplane, polarizers, and color filters remaining intact), providing better control capability over individual subpixels and negating the need of a backlight. (For a detailed understanding of these layers, read LCDs to OLEDs – Evolution of Display Technology)
The earlier manufacturing process for OLED screens, such as the Fine Metal Mask, posed challenges in achieving pixel density comparable to that of LCDs. Further, the blue subpixel has the luminous efficiency lower than the red and green sub-pixels; so the blue sub-pixels need to be driven at high currents, resulting in faster degradation compared to the red and green sub-pixels.
PenTile Technology to the rescue – RGBG and RGBW pixel layouts
Clairvoyante is among the early companies that worked to overcome these limitations and developed the PenTile technology – a subpixel-sharing layout that is widely used today in most smartphone displays. Samsung acquired IP assets of Clairvoyante in 2008 and formed Nouvoyance to further develop the PenTile technology.
Some of the techniques to improve the overall performance of screens based on OLED technologies include shifting the white balance towards the cooler blue and designing pixel layouts different from conventional RGB stripes. One the earliest pixel layouts in PenTile technology was the RGBG layout adopted in phones such as Samsung Galaxy S and S III. This layout leverages human vision, which is more sensitive to green color, by increasing the number of green subpixels compared to red and blue. The layout included twice as many green subpixels as blue subpixels and red subpixels. While RGBG reduced the number of subpixels per inch, it still achieved a resolution comparable to that of the then LCD screens without affecting the image quality.
Another pixel layout in the PenTile matrix family is the RGBW layout which is used in the Samsung Galaxy Note 10.1 (2014 edition). The white pixel in the RGBW layout enhances the brightness of the displayed image, helping in reducing the overall power required for displaying an image of given brightness.
Migrating from PenTile technology to conventional RGB pattern and back
Samsung diverged from its PenTile technology towards the conventional RGB stripe, which it called Real Stripe, in the Samsung Infuse 4G. It was the first Samsung phone to adopt the Super AMOLED Plus display – a marketing name used by Samsung for its Real Stripe pattern – and in Samsung Galaxy S II. Although this layout was believed to be better than PenTile technology in terms of brightness, power saving and reduced pixelation, Samsung and other manufacturers leveraged the Real Stripe only in a few phones. Samsung reverted to PenTile technology in its Galaxy S3 and defended the use of PenTile, stating that OLED screens with PenTile technology – especially the ones with 230+ PPI – lasted longer compared to OLEDs with the RGB stripe pattern. Today, OLED phones with PenTile technology are considered superior to LCD phones.
Diamond PenTile pixel layout ─ the most widely used pixel arrangement
In 2013, Samsung adopted the Diamond PenTile pixel arrangement; another addition to the PenTile matrix family. It was introduced in the Samsung Galaxy S4 and since then has been adopted in all successive Galaxy S series phones. Samsung Galaxy S9 and Galaxy S9+ are likely to feature the Diamond PenTile pixel layout. Apple’s iPhone X includes the Samsung Diamond PenTile pixel arrangement, which Apple has branded as the Super Retina Display.
The Diamond PenTile pixel arrangement has an oval-shaped green sub-pixel and the red and blue sub-pixels in diamond. The blue subpixel in Galaxy S4 was larger in size than the red and green sub-pixel probably to compensate for its low luminosity. However, the Diamond PenTile pixel arrangement adopted in Galaxy S5 appears to include a slightly different arrangement – all the sub-pixels were diamond shaped and the red and blue pixels appear to be of the same size. The same size of the red and blue sub-pixels may be due to improvements in various layers, which may have possibly resulted in less difference between the luminosity of the red and blue sub-pixels in the Galaxy S5.
A preliminary study of the iPhone X screen conducted by iRunway indicates that the sizes of the green, blue and red sub-pixels are different from one another.
iRunway conducted a comprehensive teardown analysis of the iPhone X and provides an in-depth research of its OLED display. For deeper insights, click here.
Technologies adopted by Samsung and Apple in their phones
Apple adopted a dual domain pixel arrangement in its phones, which was introduced in the iPhone 6. This is an RGB stripe with the pixel edges skewed with respect to the edges of the screen. The arrangement seems to improve the overall display performance – reduced brightness and color shifts with viewing angles compared to those of conventional RGB stripe pattern in LCD screens.
Other display technologies and OLED’s future
Displays that can be read in direct sunlight, unlike LCD and OLED screens, without much trouble are available in commercial products. However, such technologies have not been adopted in smartphones due to certain limitations. These displays include another layer in place of the OLED stack in OLED screens or liquid crystal layer in LCDs depending on the capability of the material to produce its own light. For instance, the electrophoretic displays used in Amazon Kindle come in black and white and have long refresh time which is not suitable for smartphones or televisions. The 3Qi Multimode technology by Pixel Qi combined electronic ink used in electrophoretic displays with LCD technology to produce a display that can work in both closed room and bright sunlight. This technology, however, did not fare well and the company went out of business in 2015. Others include electrofluidic displays by Gamma Dynamics and Photonic Ink by Opalux.
QLED and microLED are technologies that show the potential of entering large-scale production in the coming years. However, these technologies right now are being talked with respect to televisions and not small screen smartphones. Full Active LCD technology used in smartphones such as the LG G6 has the potential to compete with OLED and conventional LCD technologies.
In 2017, Apple reportedly invested around $2.7 billion in LG’s OLED technology. It was believed that this step would help Apple fulfill its demand for OLEDs and lower Samsung’s dominance. If the reports of Apple investing in LG’s OLED technology is true, then OLED is there to stay with us at least for a few years. Even some of the iPhones to be released in 2018 are rumored to include OLED screens. Companies such as Japan Display, Sharp, and BOE are reportedly gearing up to acquire a share in the OLED market. It is interesting to see how companies compete with one another and lure customers to remain on top of the game. It is the end user who is going to be benefited from more competitors in the OLED space.
Featured image is intended for representational purpose alone and has been sourced from https://pixabay.com/en/tablet-smartphone-laptop-hard-drive-626093/