The light extraction problem in MicroLED
MicroLED technology promises brighter, more efficient, and longer-lived displays than OLED. But there is a fundamental optical problem: gallium nitride (GaN), the semiconductor material at the heart of every MicroLED, has a refractive index of approximately 2.4. Air has an RI of 1.0. At this interface, total internal reflection traps the majority of light inside the semiconductor.
For conventional LEDs, this problem is partially solved by chip shaping, roughened surfaces, and encapsulant domes. But MicroLEDs — with pixel sizes below 50 micrometres — cannot use these bulk approaches. The pixel dimensions are too small for mechanical shaping, and encapsulant domes would destroy the display's pixel-level contrast. A materials-based solution is needed.
Graded refractive index approach
Instead of a single abrupt GaN-to-air transition, a graded RI stack creates a smooth optical gradient from the semiconductor to air. Each layer in the stack has an RI slightly lower than the layer below it, reducing the reflection at each interface and suppressing total internal reflection.
The optimal RI profile transitions from approximately 2.0 (near the GaN surface) to 1.4 (near the air interface) over several thin layers. This gradient allows photons at angles that would otherwise be totally reflected to escape the semiconductor, dramatically increasing the fraction of generated light that reaches the viewer.
Kriya's solution: nanoparticle dispersions from RI 1.40 to 2.00
Kriya's bottom-up synthesised nanoparticle dispersions provide the RI building blocks for graded light-extraction layers. By controlling the nanoparticle loading in each layer, RI values from 1.40 to 2.00 are achieved with precision.
Key advantages over alternative approaches:
- Haze below 0.3% — essential for display-grade optical clarity
- Wet-coat processing — compatible with wafer-level and panel-level manufacturing
- RI tunability in fine increments — enabling optimised gradient profiles
- PFAS-free formulation across the entire RI range
Measured efficiency gains
Kriya's graded RI light-extraction coatings have been evaluated on MicroLED test structures under controlled conditions. The measured results:
- Light extraction efficiency gain: 30 to 40% compared to uncoated GaN
- Broadband enhancement across the visible spectrum
- Angular emission profile compatible with display viewing angles
- No degradation of colour purity or spectral characteristics
These are measured values from validation experiments, not simulation-only predictions. The efficiency gain translates directly to either brighter displays at the same power or equivalent brightness at 30 to 40% lower power consumption — a critical metric for wearable and mobile MicroLED products.
Processing for MicroLED
Applying coatings to MicroLED arrays requires compatibility with pixel-level feature sizes and the temperature sensitivity of the device stack. Kriya's coatings are applied via:
- Spin coating for wafer-level processing
- Ink-jet deposition for pixel-level selectivity (under development)
- Spray coating for large-panel applications
Cure temperatures are compatible with MicroLED backend processing constraints. The coatings do not require high-temperature annealing — sol-gel grades can be cured at moderate temperatures compatible with the device thermal budget.
From R&D samples to volume production
Kriya currently provides MicroLED light-extraction coatings at R&D and pilot scale. The company's manufacturing infrastructure — designed capacity of over 100,000 kg per year for nanoparticle dispersions — is capable of scaling to the volumes anticipated as MicroLED transitions from pilot to mass production. Engagement typically follows a structured path: material evaluation, process integration, device-level validation, then volume supply agreement.