OLED

Think about the difference between a mirror and a firefly. A mirror only shows you things when light shines on it — it can't make its own light. But a firefly makes its own glow from inside its body!

Old TV screens are like mirrors — they need a big light behind them (called a backlight) to show you pictures. But OLED screens are like millions of tiny fireflies, and each one can light up on its own!

This is why when an OLED screen shows something black, it's REALLY black — those tiny "fireflies" just turn completely off. On an old TV, black areas are actually just the backlight trying to hide, so they look more gray.

OLED screens can also bend because they're super thin — like a piece of paper instead of a thick glass sandwich. That's why some phones can fold in half!

Historical context: Scientists first noticed that organic materials could emit light when electrified in the 1950s at Nancy University in France. But it took decades before this became practical.

The breakthrough came in 1987 when Ching Wan Tang and Steven Van Slyke at Kodak created the first efficient OLED device using a two-layer structure. Then in 1990, Cambridge researchers showed that polymers (long-chain molecules) could work too, making manufacturing easier.

Sony released the first OLED TV in 2007 — an 11-inch screen that cost $2,500. Today, OLED dominates premium smartphones and is rapidly taking over the TV market.

How it works:

• Organic layer: Special carbon-based compounds sandwiched between electrodes
• Electroluminescence: When voltage is applied, electrons and "holes" meet in the organic layer and release energy as light
• Color: Different organic compounds emit different colors (red, green, blue)

AMOLED vs PMOLED:

• PMOLED (Passive Matrix): Rows and columns controlled sequentially. Simple, cheap, but limited to small displays (~3 inches). Used in fitness bands, car dashboards.
• AMOLED (Active Matrix): Each pixel has its own thin-film transistor (TFT). Enables large, high-resolution displays. What you see in phones and TVs.

Advantages over LCD:

• Infinite contrast ratio: True blacks (pixel off = no light)
• Faster response time: ~0.1ms vs ~5ms for LCD (no motion blur)
• Wider viewing angles: No backlight bleed, consistent color at angles
• Thinner and lighter: No backlight assembly needed
• Flexible possible: Enables foldable/rollable displays
• Per-pixel dimming: HDR content looks dramatically better

Disadvantages:

• Burn-in: Static images can cause permanent image retention
• Lifespan: Blue OLEDs degrade faster than red/green
• Cost: More expensive to manufacture than LCD
• Brightness: Peak brightness historically lower than high-end LCDs (though gap is closing)

Device physics:

OLEDs operate through charge injection and recombination. The anode (typically ITO — Indium Tin Oxide) injects holes, while the cathode (low work function metal) injects electrons. These carriers travel through transport layers and recombine in the emissive layer.

HOMO/LUMO energy levels:

Organic semiconductors have discrete molecular orbitals. The HOMO (Highest Occupied Molecular Orbital) acts like a valence band; the LUMO (Lowest Unoccupied Molecular Orbital) like a conduction band. The energy gap determines emission wavelength.

Fluorescence vs Phosphorescence:

• Fluorescence: Only singlet excitons emit light (25% max internal quantum efficiency)
• Phosphorescence: Heavy metal atoms (Ir, Pt) enable triplet harvesting (100% theoretical IQE)
• TADF: Thermally Activated Delayed Fluorescence — purely organic materials that harvest triplets without heavy metals

Manufacturing methods:

• Vacuum thermal evaporation: Traditional method. Precise but wasteful (~70% material loss). Used by Samsung.
• Inkjet printing: Solution-processed. Less waste, potentially cheaper. JOLED commercialized this in 2017.

The blue emitter challenge:

Efficient, stable blue phosphorescent emitters remain elusive. The high triplet energy required (~2.8 eV) causes rapid degradation of both emitter and host materials. Current solutions:

• Fluorescent blue: Stable but inefficient (~7% EQE)
• Phosphorescent blue: Efficient but short-lived (T50 < 10,000 hours)
• TADF blue: Promising but color purity and roll-off issues persist
• Hyperfluorescence: TADF sensitizer + fluorescent emitter — combines benefits

QD-OLED (Samsung Display):

Hybrid architecture using blue OLED to excite quantum dot color converters. Achieves wider color gamut (~90% BT.2020) than either technology alone. First commercialized in 2022 for premium monitors and TVs.

Tandem architectures:

Stacking multiple emission units with charge generation layers (CGLs) between them. Doubles or triples efficiency/lifetime at the cost of higher driving voltage. Essential for high-brightness applications.

MicroOLED for AR/VR:

OLEDs fabricated directly on silicon backplanes (OLEDoS). Enables ultra-high pixel densities (>3000 PPI) required for near-eye displays. Sony leads with chips for Apple Vision Pro. Key challenges: thermal management and achieving sufficient brightness for see-through AR.

Emerging research directions:

• Perovskite LEDs: Potential for lower-cost, high-efficiency emitters
• Stretchable OLEDs: Beyond foldable — truly elastic displays
• Machine learning for materials discovery: Accelerating identification of new emitter candidates