OLED display technology overcomes some limits of semiconductors

Diodes, like transistors and computer chips, rely on semiconductivity. Silicon is a semiconducting chemical element that is neither a metallic conductor nor a nonmetallic insulator. The electrons in its atoms are more constrained than the roaming electrons in metals, but less so than the tightly bonded electrons in nonmetals.

Doping adds small amounts of impurities to the crystal structure of a semiconductor that slightly increase its conductivity by creating either a deficiency (P-type) or an excess (N-type) of electrons in the crystal depending on the doping material.

A diode consists simply of P and N type semiconductors stuck together.

Bonding a P and an N causes electrons to migrate away from the bonded junction in both the P and N materials, creating a depletion zone on both sides of the junction.

Connecting the N side to the positive terminal of a DC circuit attracts electrons to the N side and widens the depletion zone. Reversing the terminals repels electrons toward the junction on the N side and repels holes toward the junction on the P side, which narrows the gap of the depletion zone.

At some specific energy (voltage), the circuit reaches a critical level, and the depletion zone vanishes allowing a current to flow.

Because it requires energy to tear electrons free from their atoms to create holes on the P side, the energy of electrons on the N side is greater than the energy of holes on the P side. Therefore, electrons in the N side lose energy as they fall into holes on the P side.

The holes are all the same ‘depth,’ and when electrons fall in, they all release the same amount of energy in the form of photons. Because the energy loss is the same for every electron, they each emit light of the same frequency, so the light is monochromatic.

LED (light-emitting diode) technology has advanced to emit higher frequencies with the discovery of new semiconductors and doping techniques. Infrared LEDs in remote controls led to red, yellow, green and blue LEDs, which finally allowed for producing full color images.

Older LED display screens have notable disadvantages, many of which OLED displays solve.

Today’s LED displays are a misnomer. To have a true LED screen requires each pixel to be an LED. This is not a problem for huge Jumbotron displays in stadiums and arenas. For TVs, and even more so for handheld displays, LEDs are too big and it is too expensive to produce and control tiny multiple millions of microscopic LEDs on the screen.

TVs marketed as LED are actually LCD with LED backlights, which replaced older LCD displays with CFC backlights.

OLEDs (organic light-emitting diodes) use organic polymers rather than inorganic semiconductors to produce light. OLEDs produce light similarly to LEDs, but the differences are many and significant for small displays.

OLEDs are thin, comparable to the width of a hair. They can bend, and OLEDs produce their own light like LEDs but are small enough to pack the three primary colors required for each pixel into a small space. They can be printed using 3-D printers. They produce extremely black blacks because they are printed on a black substrate, so that when the pixel is not illuminated there is no backlight leakage as with LCDs.

On the downside, OLEDs are less rugged because of their vulnerability to oxidation and water damage, so they must be encased to prevent contact with the elements.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.