OLEDS

OLED Distinctions

OLEDs vs. PLEDs

• Commonality: Both are organic light-emitting devices.
• Differences:
  1. OLEDs:
     • Made from smaller organic molecules.
     • Typically manufactured using vacuum evaporation techniques.
  2. PLEDs:
     • Utilize polymers for their structure.
     • Still primarily in the research phase.
     • Potential for producing large, cheap, flexible displays.

AMOLED vs. PMOLED

• PMOLEDs:
  • Consume more energy.
  • Best suited for small screens.
  • Simple and inexpensive to manufacture.
• AMOLEDs:
  • Incorporate an additional layer of transistors.
  • Offer faster refresh rates and lower power consumption.
  • More challenging and costly to produce.

Bottom Emission vs. Top Emission vs. Transparent OLEDs

• Bottom Emission: Cathode is opaque; anode and substrate are transparent.
• Top Emission: Anode is opaque; cathode is transparent.
• Transparent OLEDs: Both the anode and cathode are transparent.

Generations of OLED Technology

1. 1st Generation – Fluorescent OLEDs:
   • Utilize only singlet exciton pairs.
   • Maximum efficiency: 25%.
2. 2nd Generation – Phosphorescent OLEDs:
   • Exploit both singlet and triplet exciton pairs.
   • Significantly higher efficiency compared to the first generation.

---

Physics of OLEDs

Traditional LED Operation

• Inject charges to fill holes and emit light.

Basic OLED Structure

• Most basic design includes three layers:
  1. Anode
  2. Cathode
  3. Active Layer
  • Introduced around 1965.
• Process:
  1. Charge injection.
  2. Charge transport.
  3. Exciton formation.
  4. Radiative recombination.

Key Concepts

• Charge Speed: Charges must move quickly enough to form excitons before they recombine non-radiatively.
• Forward Bias: Light is emitted when the device is powered correctly.
• Reverse Bias: Occurs when the device is connected incorrectly, preventing light emission.
• Equilibrium: The device is not powered, and no light is emitted.

Electron Blocking Layer (EBL)

• Prevents excess electron migration, ensuring efficient recombination in the active layer.

---

Qualities of OLEDs

Luminous Efficiency

$${\eta }_{\text{lum}}=\frac{{\Phi }_L}{V\cdot I}\left[\frac{\text{lumens}}{\text{W}}\right]$$

Where:

• $V$: Driving voltage.
• $I$: Current.
• ${\Phi }_L$ Luminous flux.

Electroluminescence Quantum Efficiency ${\eta }_{EL}$

• Represents internal efficiency, ignoring photon transport losses.

$${\eta }_{EL}={\eta }_{PL}\cdot r_{st}\cdot {\gamma }_{cap}$$

Where:

• ${\eta }_{PL}$: Photoluminescence efficiency. Improved by suppressing non-radiative channels.
• $r_{st}$: Ratio of singlet excitons to total excitons (max: 25%).
• ${\gamma }_{cap}$: Exciton formation factor. Improved by balancing charge injection and transport.

---

Optimizing OLED Performance

Improving ${\eta }_{PL}$: Suppress Non-Radiative Decay Channels

1. Prevent Aggregation Quenching:
   • Avoid close packing of chromophores.
   • Dilute active materials with a matrix substance.
   • Design non-stacking chemical structures (e.g., polythiophenes, oligothiophenes).
     

Oligothiophene efficiency increases

• Lower crystallinity improves efficiency.

2. Optimize Optical Design:
   • Minimize waveguiding, reabsorption, and refraction losses.
   • Ensure charges recombine only in desired regions.

Increasing $r_{st}$: Boost Radiative Excitons

• Max Efficiency: 25%.

Methods:

1. Triplet-Triplet Annihilation (TTA):

   • Converts two triplets into a singlet.
   • Theoretical efficiency: 62.5%.
   • Practical efficiency: ~40%.

2. Thermally Activated Delayed Fluorescence (TADF):

   • Reverse intersystem crossing (triplets to singlets).
   • Requires complex, heavy molecules.
   • Potential 100% efficiency.

3. Hybrid Local Charge Transfer (HLCT):

   • Donor-acceptor molecules with similar singlet and triplet energies.
   • Theoretical efficiency: 100%.

4. Harvesting Phosphorescence:

   • Theoretical photoluminescence quantum yield (PLQY): 100%.
   • Techniques: Foerster transfer, Dexter transfer, and E-H direct capture.

Challenges:

• Materials with low TTA.
• Blue emitters are challenging.
• Efficiency drop at high currents.
• Dexter transfer is slow due to its short range.

Improving ${\gamma }_{cap}$: Enhance Charge Balance

1. Balancing Charge Injection:

   • Cathode: Add alkaline materials to reduce work function.
   • Anode: Modify ITO work function using:
     • Self-assembled monolayers.
     • Spin-coated layers.
     • Layer-by-layer deposition of dedoped layers.
     • Oxygen plasma treatment (standard method).

2. Balancing Charge Transport:

   • Reduce excess majority carriers.
   • Add separate electron and hole transporting layers.
   • Form heterojunctions for efficient recombination.

---