Lecture 2

OLED Distinctions

OLED’s vs PLED

• Both organi
• OLED made with smaller molecules, PLED made with polymers
• OLED created with vaccum evaporation
• PLED still in research
  • Has potential for very large cheap flexible displays

AMOLED vs PMOLED

• PMOLED’s consume more energy
  • Best for small screens
  • Easy to make
• AMOLED
  • Additional layer of transistors
  • Fsater refresh rates
  • Consumes less power
  • Harder to make

Bottom Emission vs Top Emission Vs Transparent

• Bottom Emission: Cathode is opaque, Anode/substrate transparent
• Top Emission: Anode is opaque, cathode transparent
• Transparent: Both transparent

3 Generations of OLED

1’st Generation: Fluorescent OLED:

• only utilizes singlet pairs
• 25% efficient 2nd Generation: Phosphorescent OLED:
• Utilizes triplet pairs as well

Physics of OLED’s

Traditional LED Operation: In traditional LED, inject charges, fill holes, emit light

Most simple OLED has 3 layers: Annode, cathode, and active layer. This is circa 1965 In OLED: Injection of charges, charge transport, exciton formation, radiative recombination

Charges have to be fast enough to form exciton If exciton is slow/weak, then charges move and don’t recombine radiatively

We want charges to move as fast as possible until the exciton phase

Structure helps with this. See diagram

EBL is electron blocking layer

Learn this diagram

Strong forward bias: When light is emittted Reverse Bias: Hooked up OLED backwards Equilibrium: Not hooked up to power

Qualities of OLED’s

Luminous Efficiency

$${\eta }_{lum}=\frac{{\Phi }_L}{V\cdot I}\left[\frac{lumen}{W}\right]\propto \frac{i}{V\cdot I}\left[\frac{cd}{VA}\right]$$

V: Driving voltage I: Current ${\Phi }_L$: Luminous Flux

Lumens/Watts

proportional to Incandecence (Candellas)/ W

Electroluminescence Quanum Efficiency:

Internal efficiency, disregarding losses in photon transport Assumes Phosphoresence does not exist

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

${\eta }_{PL}$ - Photo-luminescence efficiency. Can be improved by supressing nonradiative channels $r_{st}$ - Ratio of singlets to total excitons Maxes at 25% Increase radiative excitons ${\gamma }_{cap}$ - Excition formation factor Need more minority carrier to balance charge injection and transport

Optimizing OLED’s

Increasing ${\eta }_{PL}$: Suppress non-radiative decay channels

1. Prevent aggregation Quenching 2. Optimize optical design, ensure injected charges don’t recombine too soon 3. Reduce contamination Preventing Aggregation Quenching:

• Prevent close-packing of chromospeheres
• Dilute with matrix substance
• Or design chemicals to not stack

A chemical that does this are polythiophenes Lower Crystalinity leads to higher efficiency

Oligothiophenes also do this:

FET materials desire the opposite properties of this, as seen last lecture

Optical device design

• Many engineering considerations
• Prevent waveguiding
• Prevent reabsorbtion
• Prevent refraction

Increase $R_{st}$: Boost # of radiative excitons

1. Maxes out at 25%

Methods:

Triplet-Triplet Annihilation: (TTA)

• Take two triplets, put it into a singlet
• Seen in Anthracene
• Total possible efficiency of 25% + 37.5% = 62.5%
• In practice, triplets provide only about 15%, raising total efficiency to 40%

Thermaly activated Delayed Fluorescence: (TADF)

• Reverse Intersystem crossing: turning triplet to singlet
• Requires heavy molecules, complex
• Potential 100% efficency

Hybrid Local Charge Transfer: (HLCT)

• Theoretical 100% efficiency as well
• Uses donor-acceptor molecule with triplet and singlet energies that are siminlar

Harvesting Phosphorescence:

• Theoretical 100% PLQY

Three methods:

• Foerster Transfer
• Dexter Transfer
• E-H Direct capture

Issues

• Hard to find materials that don’t have high TTA levels
• Difficult to find blue emitters
• Efficiency drop at large currents
• Dexter transfer is slow because of range

Increase ${\gamma }_{cap}$: Improve e-h balance

Balancing Charge Injection:

Cathode:

• reduce work function by adding alkaline materials
• very useful for blue materials Anode:
• Reduce work function of ITO by:
  • Self-assembled monolayers
  • Spin Coated layers
  • Layer-by-Layer deposition of progressively dedoped layers
  • Oxygen plasma treatment
    • Standard now

Balancing Charge Transport:

Make sure that charges move quickly

• Reduce excess majority carriers
• Introduce separate electrion and hole transporting layers
• Formation of hetrojunctions

Organic Photovoltaics

• Goal is to turn light into electrons.
• Opposite of OLED’s