What is Emissive Display Technology?

What is Emissive Display Technology? An end user’s guide to understanding emissive display technology options

What is Emissive Display Technology? Emissive display technology create and emit direct light of their own without any sort of filter. LED displays are an example of such a display technology, hence the “E” in the acronym, which stands for Emitting. LED technology is not the only emissive display technology, however, so we’d like to use this space to expand upon the other styles of emissive displays out there so customers can gain a stronger understanding of which options will best suit their needs. The easiest way to frame this discussion with regard to one particular need is size. Due to their brightness and cost-efficiency, LED displays are considered by many in the industry to be the strongest solution for large-format commercial applications. Smaller emissive display use cases like in-home televisions are the apt fit for OLED displays, an emissive solution with excellent picture quality but pricy scaling costs and less consistent durability. Near-eye displays like wearables are the next level down in size, and that is where the tiny emissive diodes of MicroLED come in. Let’s size up these emissive products, shall we?

LED is for Large-Format Applications

Unlike an incandescent bulb, which produces light as a byproduct of heat, an LED diode produces light through electroluminescence, the production of light in response to the running of an electrical current. Electroluminescence is what happens when negatively charged free electrons, excited by an electrical current, combine with positively charged “electron holes” within a semiconductor. Because the electron holes have a lower energy level than the electrons themselves, the electrons must release energy in order to combine.

This energy is released in the form of photons, or light waves. The length of these waves (and thus the color of their light) is determined by the size of the “energy band gap”’ of the semiconductor. This gap is the energy disparity the electrons must make up to transition from their starting place on the conduction band of the semiconductor to the electron holes on the valence band of the semiconductor. Basically, it is the bridge they must cross. As this bridge gets larger, the color of the resulting light will change from infrared, to red, to violet, and then to ultraviolet. Now, just because these photons are released by the electrons does not mean they necessarily escape the semiconductor. Depending on the coating and shape of the semiconductor, these waves can either escape as light or refract inwards and be wasted as heat. This means that while LEDs are far more efficient than incandescent bulbs, not all LEDs are created equal. You can read more about that topic here.

OLED is Appropriate for In-Home

Like all emissions, OLED diodes also emit light through electroluminescence, the production of light in response to the running of an electrical current. However, while the traditional LEDs used by NanoLumens create light when a current is run through an inorganic semiconductor, an OLED creates light when this current is run through a film of organic compound functioning as a semiconductor. The organic compound within an OLED device varies but the very first OLED devices employed small organic molecules deposited onto substrates using a costly and inefficient process called vacuum deposition. Many OLED products are still made using this process which explains their high price point relative to other technologies. Some recent versions of OLED devices now use large polymer molecules like polyaniline for their conducting layer and polyfluorene for their emissive layer. These organic polymers are more easily produced and deposited onto substrates.

Using an organic compound as a semiconductor, as OLEDs do, is not inherently better or worse than using an inorganic semiconductor. The greatest practical difference derived from using organic compounds is that you’re able to mount the diodes onto a wider variety of substrates. While traditional LEDs perform best when using direct band gap materials, OLEDs can function with indirect band gap materials and thus can be mounted onto substances like silicon, which means their display surfaces can flex, roll, or bend in ways traditional LED display substrates cannot. This presents exciting opportunities for future OLED applications but for now you’ll find OLED largely contained within the in-home television space, an environment where their picture quality excels. Limiting OLED to this space allows for optimal usage of the technology, as pressing it to the brightness levels of traditional LED would cause its performance to erode rather quickly. It’d also be prohibitively expensive in out-of-home sizes.

MicroLED Fits for Near-eye Use

Now that we’ve touched on out-of-home and in-home, let’s address the next size down: in-hand. Or, if referring to AR and VR wearables, on-head. The emissive technology best in these applications is MicroLED, which defines all diodes with a die size below 100 micrometers, most frequently in the 10-20 micrometer range. The reasons this tiny technology has generated such fanfare at industry conferences are easy to see (assuming the displays in question are actually MicroLED). The minuscule pixels of a MicroLED display can hit pixel pitches of 0.15 mm, roughly 80% smaller than the tightest pitches of today’s sharpest traditional LED displays. This extremely narrow pitch allows for far greater pixel density within a board while maintaining brightness levels due to the sheer number of diodes. A trio of challenges inhibiting the proliferation of MicroLED is the difficulties of compressing internal display circuitry, the operations costs of producing so many more diodes with equipment altered from its usual settings, and the potential reordering of supply chain logistics. There is general indecision amongst the display manufacturing community about where and how exactly MicroLED will be most successful moving forward, but the technology is currently deployed in a few personal electronics products. The most common application for MicroLED right now is in smartwatches but it will eventually be useful for virtual reality headsets. Each of these products have extremely small average viewing distances that necessitate the minuscule pixel pitch only MicroLED can reproduce. When it comes to large-format applications however there’s no need for a 0.15 mm pixel pitch because audiences will typically be at least 10 feet away. A general rule in the LED industry is that if you don’t need a tighter pitch it’s not worth paying for. Outside of wearables, no other application needs the most microscopic pitches.

So to briefly summarize emissive display usage, LED is for large-format, OLED is for in-home, and MicroLED is for near-eye uses. There are a few other emissive styles that have phased in and out of the market like plasma but the impact and applicability of those technologies should no longer be much of a factor in your purchasing decisions. Hopefully this explanation of the three primary emissive styles helps narrow your search and furthers your decision-making process!

(2023, January 9). AVIXA. Retrieved from https://xchange.avixa.org/posts/what-is-emissive-display-technology