OLED, AMOLED, POLED, PMOLED, flexible, bendable, edge… Display panel terminology seems to be forever expanding, making it tougher than ever to understand what’s actually going into the latest and greatest smartphone panels. As LG Display ramps up its plastic OLED (POLED) production for smartphones, it is inevitable that comparisons are going to start being drawn with the leading small form factor OLED panel supplier – Samsung.
Samsung is well known for marketing its Super AMOLED display tech, which forms the backbone of its latest curved edge smartphone design. But what’s the difference between POLED vs AMOLED, and what do these acronyms mean for our future phones?
OLED, the basics
To start with, let’s get to grips with the building blocks of OLED technology, ignoring the P and AM prefixed for now. All of these panels are built from millions of organic light emitting diodes, tiny little components that light up when current is applied to them. These are arranged in various red, green, and blue configurations in order to produce a full range of colors. One of the benefits of OLED over the competing LCD design is that these light emitters can be switched completely off, which gives the technology deep blacks and an excellent contrast ratio.
The next question is how do you pass the current to these pixels. There are two ways of doing this, using a passive wiring matrix or an active wiring matrix. Passive matrix displays provide current to an entire row of LED one at a time, which isn’t ideal but it is cheap. Active matrix introduces a capacitor and thin-film transistor network that allows each pixel to be driven individually at the right voltage. This driving matrix is part of the panel that its on top of the substrate, which is usually glass.
Now, all high resolution, smartphone sized displays need to use active matrix technology, as passive matrix requires higher and higher voltages the more pixels you introduce. This degrades the LED lifetime, making it impractical for use in high resolution phone panels.
Yes, you guessed it, that’s simply where the AM in Samsung’s AMOLED comes from. However, all smartphone OLED panels, including those from LG Display will be using active matrix technology too. Anything else simply isn’t viable.
The move to plastic substrates
Now that we know the layered structure of an OLED display, we can move on to the plastic part. While the first wave of OLED panels were built using glass substrates, the desire for more interesting form factors has seen this layer move over to use more flexible plastic components. While glass is fixed and rigid, plastic substrates can be more easily formed into new shapes, even if the end result doesn’t end up being highly flexible. Not to mention that working with plastics is much more cost effective than moulding glass.
The use of a plastic substrate (POLED) allows for more interesting form factors, thinner panels, and flexible properties.
A range of plastics have been used tried and tested for flexible displays, including polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). However, the switch from a glass substrate also has to accommodate the type of TFT technologies used, in order to bring down their manufacturing temperatures or use of plastics that can withstand higher temperatures. As a result, flexible OLED manufacturers are using polyimide plastics (PI) that can better withstand high TFT manufacturing temperatures. The type of substrate and heating process used also defines the flexibility of the display.
On the TFT side, a-Si and more expensive poly-Si (based on LTPS) can both be used to drive a flexible display, but we should be aware that these offer different performance characteristics. LTPS technology is better suited for OLED panels because of its higher electron mobility when compared to a-Si, which is important for driving current hungry LEDS. But even these characteristics can vary depending on the manufacturing technique.
What is meant by electron mobility is the amount of current that can be provided to the LED components. a-Si typically has lower electron mobility than poly-Si, which means slower on/off switching times and larger required transistor sizes, making it tricker to produce very high resolution panels. So if you’re building a flexy QHD panel, poly-Si is imperative. Leakage defines the amount of waste across the panel, with more leakage requiring higher voltages which reduced the lifetime of the LEDs. Therefore, the manufacturing process used here is of paramount importance to determining a panel’s perfromance, and will vary somewhat from manufacturer to manufacturer even if they’re building similar displays.
The somewhat confusing part is that Samsung’s edge AMOLED displays are built on plastic substrates with poly-Si TFT technology, such as the one inside the Galaxy S8. LG Display’s POLED technology is clearly using plastic and poly-Si is assumed for adequate performance too. So it’s absolutely possible to build a plastic substrate, active matrix OLED panel, and that’s what both of the big two panel manufacturers are doing when it comes to curved mobile displays.
Other panel differences
Having said that, this doesn’t mean that LG Display and Samsung are producing identical panels. We’ve already mentioned that the quality of the TFT layer and plastic compound can make a difference to display performance, and so can the type of organic materials used on the LEDs, and the sub-pixel layout of the individual RGB diodes.
Between peak brightness, color gamut, and subpixel layouts, there can be a range of differences even between panels of the same type.
Because different color LEDs offer different brightnesses and shelf life, particularly the blue light which degrades the quickest, panel manufacturers can opt to use different LED materials, such as small molecule, polymer, or phosphorescent, to optimize their design. This then necessitates different subpixel layouts in order to balance the panel white color, gamut, and resolution.
We know that LG’s POLED powered G Flex 2 carried a diamond pentile matrix, which is the same as what Samsung is using in its latest Galaxy S8 Super AMOLED panel. So, it possible that upcoming panels from Samsung and LG Display may boast a number of similarities.
Put simply, POLED is, as the name suggests, an OLED display with a plastic substrate. AMOLED, a term mostly used by Samsung to market its OLED technology, doesn’t strictly have to be made on plastic substrate, but this is certainly how the company is achieving its curved design for its flagship smartphones.
AMOLED displays can be built on plastic too, it's mostly just a case of marketing.
Other than that, there are certain to be some differences in the underlying panel technology, but these are not bound by the names used to describe the panels. There will be subtle differences between POLED and AMOLED displays, in terms of brightness, color gamut, white point, etc, just as there are already differences between Samsung’s own AMOLED panels. But for most consumers, this will be of little consequence.
The mobile display market has mostly built a consensus around the best ways to produce flexible panels, and the underly principle – active matrix OLED on a flexible plastic substrate – applies equally to both of the big two manufacturers. Despite the different names, LG Display and Samsung aren’t worlds apart in their approach to producing high-end panels for smartphones.