Micro RGB vs. OLED: The Physics of Self-Emissive Inorganic Displays

In the language of solid-state physics, the Highest Occupied Molecular Orbital (HOMO) serves as the valence band, and the Lowest Unoccupied Molecular Orbital (LUMO) serves as the conduction band. When a voltage is applied, electrons are injected into the LUMO from the cathode, and holes are injected into the HOMO from the anode.

When an electron and a hole meet in an organic molecule, they form a bound state called an exciton. Unlike free carriers in inorganic crystals, these excitons are strongly localized Frenkel excitons with high binding energy (0.5 – 1.0 eV). The crucial physics limitation here is quantum mechanical spin.

Electrons and holes are fermions with spin $1/2$. When they combine, their spins can align in two ways:

• Singlet State ($S=0$): Spins are anti-parallel ($\uparrow \downarrow$). This state decays rapidly (nanoseconds) via fluorescence, emitting a photon.
• Triplet State ($S=1$): Spins are parallel ($\uparrow \uparrow$). Radiative decay from a triplet to a singlet ground state is "forbidden" by selection rules because it requires a spin flip, which is highly improbable without external perturbation.

According to spin statistics, the combination of uncorrelated electrons and holes results in a ratio of 1:3 for Singlets to Triplets.

• 1st Gen OLED (Fluorescence): Only harvested Singlets. Max Internal Quantum Efficiency (IQE) = 25%. The remaining 75% of energy was lost as heat.
• 2nd Gen OLED (Phosphorescence - PHOLED): Introduced heavy metal atoms (like Iridium or Platinum) into the molecule. The strong spin-orbit coupling of the heavy nucleus mixes the singlet and triplet states, making the "forbidden" transition allowed. This unlocks 100% IQE but suffers from stability issues, particularly in blue emitters.
• 3rd Gen OLED (TADF): Thermally Activated Delayed Fluorescence. These molecules are designed with a very small energy gap ($\Delta E_{ST}$) between the Singlet and Triplet states. Thermal energy at room temperature is sufficient to "upconvert" Triplet excitons back into Singlet states via Reverse Intersystem Crossing (RISC), allowing them to emit light as delayed fluorescence.

The "Blue Problem" in OLEDs is a direct consequence of the high energy of blue photons (~2.8 eV). In PHOLEDs, the long lifetime of triplet excitons leads to a high density of excited states. When two excitons collide (Triplet-Triplet Annihilation), their combined energy can exceed the bond dissociation energy of the organic molecule (typically C-N or C-C bonds), literally breaking the molecule apart. This photochemical degradation is why blue OLED pixels die faster than red or green, necessitating the "Pentile" sub-pixel arrangement where blue pixels are made physically larger to reduce current density.

2. Inorganic Electroluminescence: Band-to-Band Recombination

MicroLEDs abandon organic chemistry in favor of the wurtzite crystal structure of Group III-Nitrides (GaN, InGaN) and Zincblende III-Phosphides (AlGaInP).

Here, light emission is not excitonic but arises from band-to-band recombination of free carriers.

• Band Structure: In a direct bandgap semiconductor like Gallium Nitride (GaN), the minimum of the conduction band and the maximum of the valence band occur at the same crystal momentum ($k=0$).
• Recombination: An electron drops from the conduction band to the valence band, annihilating a hole and releasing energy equal to the bandgap ($E_g$) as a photon ($E = h\nu$).
• Robustness: The binding energy of the atoms in a GaN crystal is vastly higher than in organic molecules. There are no weak C-H bonds to break. A MicroLED can run at current densities $1000\times$ higher than an OLED without immediate degradation.

While robust, III-Nitride LEDs face their own physics demons. GaN is a polar material. When grown on mismatched substrates (like Sapphire or Silicon), strain induces huge internal piezoelectric fields. These fields tilt the energy bands in the quantum wells, spatially separating the electron and hole wavefunctions. This reduces the probability of recombination, a phenomenon known as the Quantum Confined Stark Effect. This is particularly bad for Green and Red InGaN LEDs, where high Indium content creates massive strain—a major hurdle for "True RGB" MicroLEDs.

Part II: The Manufacturing Physics Barrier

If MicroLED is physically superior in brightness and lifespan, why does OLED dominate the market? The answer lies in the physics of mass manufacturing.

1. OLED: The Thermodynamics of Evaporation

The standard manufacturing process for high-end OLEDs (like in iPhones) is Vacuum Thermal Evaporation (VTE) through a Fine Metal Mask (FMM).

• Knudsen Number Regime: VTE operates in a high vacuum where the mean free path of the molecules is longer than the chamber dimensions (Knudsen number $Kn > 1$). Organic molecules travel in straight ballistic lines from the crucible to the substrate.
• The Shadow Mask Limit: To define pixels, a metal mask with tiny holes is placed close to the substrate. However, the mask cannot be infinitely thin (it would sag) or infinitely close (it would scratch). This creates a "shadowing" effect that limits pixel density. Furthermore, the mask heats up from the hot vapor, expanding thermally. This thermal expansion mismatch limits the size of the glass substrate (Gen 6 is the current limit for FMM), making OLED TVs expensive to produce using RGB patterning.

To scale up, manufacturers are trying inkjet printing. Here, the physics shifts from ballistics to fluid dynamics.

• The Coffee-Ring Effect: When a droplet of ink dries, evaporation is faster at the pinned contact line (the edge). Capillary flow rushes liquid from the center to the edge to replenish the loss, carrying the organic solute with it. This results in a pixel with thick edges and a thin center—terrible for uniform emission.
• Marangoni Flow: Engineers must manipulate the Marangoni effect—mass transfer driven by surface tension gradients. By using a mix of solvents with different boiling points and surface tensions, they can induce a recirculating flow inside the drying droplet that counteracts the coffee-ring effect, redistributing the material to form a flat, uniform "U-shape" profile.

2. MicroLED: The Van der Waals Challenge

MicroLEDs are grown on 4-inch or 8-inch wafers, but a TV requires them to be spread out over 65+ inches. This requires Mass Transfer: picking up millions of microscopic chips (smaller than a grain of pollen) and placing them precisely on a backplane.

The physics problem here is that at the micro-scale ( < 10 µm), gravity becomes irrelevant, and surface forces dominate.

• Van der Waals Forces: These are weak intermolecular attractive forces that scale with contact area. For a tiny MicroLED, the van der Waals attraction to its donor wafer is incredibly strong relative to its weight.
• Viscoelastic Adhesion (The Gecko Effect): The leading solution uses elastomer stamps (PDMS). The adhesion of these stamps is rate-dependent.

This kinetic control allows the transfer of millions of LEDs with 99.9999% yield, but the "speed limit" of this physical process fundamentally dictates the cost of the display.

Part III: The "True RGB" vs. Color Conversion Dilemma

One of the fiercest debates in the MicroLED world is how to generate color.

Approach A: True RGB (Inorganic)

Use separate Red, Green, and Blue semiconductor blocks.

• Blue/Green: Made from Indium Gallium Nitride (InGaN).
• Red: Typically made from Aluminum Gallium Indium Phosphide (AlGaInP).

AlGaInP has a fatal flaw at the micro-scale. It has a very high Surface Recombination Velocity ($10^6$ cm/s). When you shrink a red LED to < 10 µm, the ratio of surface area to volume skyrockets. Carriers (electrons/holes) diffuse to the sidewalls and are trapped by surface defects before they can recombine radiatively. This causes the efficiency of red MicroLEDs to plummet below 5% at small sizes, while blue remains near 40-50%.

Approach B: Blue + Quantum Dot Color Conversion (QD-CC)

Use only Blue MicroLEDs (which are efficient and easy to make) and print a layer of Quantum Dots (QDs) on top to convert Blue to Red and Green.

QDs are semiconductor nanocrystals where the exciton wavefunction is confined in all three spatial dimensions. This Quantum Confinement means the bandgap is tunable by simply changing the size of the particle (smaller = bluer, larger = redder).

• Stokes Shift Engineering: A major challenge is Self-Absorption. If a QD emits a photon, a neighboring QD of the same size might re-absorb it. This photon recycling leads to losses. Advanced QDs use a "Core-Shell" structure (e.g., InP core, ZnS shell). By designing a Type-II band alignment, the electron and hole are spatially separated, inducing a large Stokes Shift (a gap between absorption and emission peaks). This ensures that once a photon is emitted, it cannot be re-absorbed by the QD layer, preserving efficiency.
• Blue Leakage: The physics challenge here is optical density. The blue light must be fully absorbed by the thin QD layer. If blue photons leak through the red pixel, color purity is ruined. This requires extremely high absorption coefficients or Distributed Bragg Reflectors (DBR) to recycle the blue light until it is converted.

Part IV: System-Level Physics – Crosstalk and Contrast

1. OLED: Electrical Crosstalk

In high-resolution OLED microdisplays (like those in the Apple Vision Pro), pixels are so close that they share common organic layers (Hole Injection/Transport Layers) to save cost. This creates a path for Lateral Leakage Current. Current intended for one pixel can drift sideways through the conductive organic layers to light up a neighbor. This "Electrical Crosstalk" blurs the image and reduces color gamut.

2. MicroLED: Optical Crosstalk

MicroLEDs are distinct islands, so electrical crosstalk is negligible. However, they are essentially tiny omnidirectional light bulbs. Without proper isolation, photons emitted sideways from a blue pixel can travel through the glass substrate and exit through a neighboring red pixel.

• The Solution: Deep Trench Isolation (DTI). Physical barriers must be etched between pixels and filled with reflective metal or absorbing black matrix material. The physics of Total Internal Reflection (TIR) within the encapsulation layers also must be managed to extract light toward the viewer rather than trapping it in waveguide modes.

Part V: The Future – Porous GaN and Nanostructuring

The most exciting recent development (2024-2025) in MicroLED physics is the use of Nanoporous GaN.

By electrochemically etching the GaN wafer, engineers can create a sponge-like structure full of nanopores.

1. Strain Relief: The porous structure is mechanically compliant. It acts as a stress-relief layer, allowing high-Indium (Red) InGaN layers to be grown on top without the massive strain that usually destroys efficiency. This could finally solve the "Green Gap" and enable efficient InGaN Red MicroLEDs, eliminating the need for the problematic AlGaInP material.
2. Light Extraction: The refractive index of porous GaN can be tuned by changing the porosity. This allows the creation of built-in Distributed Bragg Reflectors (DBRs) underneath the LED, reflecting light upward that would otherwise be lost into the substrate.

Conclusion: The Winner?

As we move toward 2030, the boundaries will blur. Quantum Dots are bridging the gap, and porous semiconductors are solving the strain issues. But for now, MicroLED represents the ultimate "Physics Dream" of a display: a direct connection between electricity and light, unmediated by liquid crystals or fragile organic molecules.