Organic Electronics: Test 1

All Material for T1

Electrode

An electrical conductor used to make contact with a non-metallic part of a circuit

Semiconductor

partially conducts electricity, allowing electrons to flow throughout the circuit when a certain voltage is applied

HOMO

(Highest Occupied Molecular Orbital) is the highest-energy molecular orbital containing electrons

LUMO

(Lowest Unoccupied Molecular Orbital) is the lowest-energy molecular orbital that is empty

How is the energy gap between HOMO and LUMO calculated

E_gap = hc/λ (where h is planks, c is speed of light, and λ is wavelength of absorption)

What does increasing conjugation do to the absorption frequency

increases the absorption frequency

Electron Withdrawing Groups (+σ)

 Raise the LUMO energy

Lower the HOMO energy

Increase the energy gap between HOMO and LUMO ( ∴ ↓ λ absorbed)

Electron Donating Groups (-σ)

Lower the LUMO energy

Raise the HOMO energy

Decrease the energy gap between HOMO and LUMO( ∴ ↑ λ absorbed)

Fluorescence

the process of an electron in the First Singlet Excited State (S_1) radiatively

decaying, dropping from the LUMO to the HOMO releasing light fast

Phosphorescence

the process of an electron (usually in a large molecule with a large

magnetic moment) in the First Singlet Excited State (S_1) moving to the First Triplet Excited State (T_1) via Intersystem crossing (electron reverses its spin) then radiatively decaying,

slowly dropping to the HOMO releasing light

Stoke’s Shift

The shift in peaks/wavelength which shows a loss of energy between absorption and emission spectra’s mirror images

Increased conjugation shifts the light emitted from…..

blue to red

A photon can excite an electron…..

from the HOMO (𝑆_0) to the LUMO (𝑆_1) and

beyond to the 𝑆_2. If it is excited to 𝑆_2, it will lose its energy though Internal Conversion Molecular Vibration and fall to 𝑆_1.

Fluorescence only occurs…

when the electron radiatively decays from 𝑆_1 to 𝑆_0.

How do OLEDs work?

by injecting charges: an electron in LUMO meets a hole (electron vacancy in HOMO) to form an exciton that releases photons.

The coupling of two electron spins in different MOs can come together as…..

a Singlet state or one of three Triplet states

Hole Transport

Occurs at the HOMO when an electron moves from one molecule to another, effectively “moving” the hole from one molecule to another

Electron Transport

an extra electron in the LUMO moving from a molecule to an adjacent molecule

Hole mobility increases when

an electron has a higher IP (ionization potential), aka a shallower HOMO

Electron mobility increases when

An atom has a larger EA (electron affinity), which has a deeper LUMO

Excitons

Excited states bound with their hole on the ground state and can hop from molecules as entities until they are separated into free charge carriers

Work Function

Amount of Energy (Voltage) required to make electrons escape the surface metal.

Basic OLED composed of what layers

High Work Function Metal

Organic Semi-Conductor

Low Work Function Metal

Four Steps of OLED Function (in order)

1. Charge Injection

2. Charge Transport

3. Charge Combination

4. Radiative Decay

Charge Injection

• Holes enter the organic semiconductor from the Anode, overcoming the hole injection barrier dropping to the semiconductor’s HOMO.

• Electrons enter the organic semiconductor from the Cathode, overcoming the electron injection barrier jumping to the semiconductor’s LUMO.

Charge Transport

Both charges are transported through semiconductor towards one

another.

Charge Combination

Within the Emissive Layer (EL) the hole and electron meet at the combination zone, forming an exciton in the process

Radiative Decay

The relaxation of the exciton through light emission

Anode

Where oxidation occurs (loss of electrons)

Cathode

Where reduction occurs (gain of electrons)

Conjugation

Overlap of pi orbitals