Chapter – 12 Organic Light Emitting Diodes Molecular Orbitals HOMO and LUMO

Molecular Orbital Theory Molecular orbital (MO) theory describes covalent bond formation as a combination of atomic orbitals to form molecular orbitals. In the H2 molecule 2 singly occupied 1s atomic orbitals combine to produce 2 molecular orbitals. Each molecular orbital can occupy maximum 2 electrons. The number of MO's must equal the number of atomic orbitals which combined to produce them. The bonding MO (denoted 1) is lower in energy. - ground state - both 1s electrons reside The antibonding MO (denoted 2* ) is higher in energy. - 2* is empty. If EM radiation of the energy equal to the energy gap between 1 and 2* strikes the molecule, one or both of the bonding electrons may be excited (promoted) to the 2* orbital.

Molecular Orbital Description of Ethylene (C2H4) Atomic No. of C = 6. Electronic configuration: 1s2, 2s2 2p2 The carbon atom doesn't have enough unpaired electrons to form the required number of bonds, so it needs to promote one of the 2s2 pair into the empty 2pz orbital. Hybridization There is only a small energy gap between the 2s and 2p orbitals, and an electron is promoted from the 2s to the empty 2p to give 4 unpaired electrons. When the carbon atoms hybridize their outer orbitals before forming bonds (with hydrogen atom), they only hybridize three of the orbitals rather than all four. They use the 2s electron and two of the 2p electrons, but leave the other 2p electron unchanged. The C-C sigma () bond in ethylene results from the overlap of two 2sp2 atomic orbitals producing two MO’s (one 1 bonding and one 2* antibonding). The C-C pi () bond in ethylene results from the side-to-side overlap of two 2p atomic orbitals producing two MO’s (one 1 bonding and one 2* antibonding).

1 is called a HOMO (Highest occupied Molecular Orbital) and 2 1 is called a HOMO (Highest occupied Molecular Orbital) and 2* is called a LUMO (Lowest Unoccupied Molecular Orbital). In unsaturated compounds, the HOMO  LUMO excitation is generally a   * transition (or ‘n’  * transition). The  and * MO’s are not involved.

HOMO and LUMO Levels HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The energy difference between the HOMO and LUMO is termed the HOMO–LUMO gap. Homo  is analogues to valance band while LUMO is analogues to Conduction band. The difference of the CB-VB is band gap and on similar terms LUMO-HOMO is the band gap and is energy levels required for the conduction to take place.   HOMO and LUMO of a molecule. Each circle represents an electron in an orbital; when light of a high enough energy is absorbed by an electron in the HOMO, it jumps to the LUMO.

Importance of HOMO - LUMO The HOMO-LUMO gap of organic molecules are significant as they relate to specific movements of electrons. Conducting organic material a lower gap between the two states. There are ideal systems of organic polymers that come close to being conductors, close to metals. Organic semiconductors have applications in high tech transistors for expensive circuits, LED's, lasers (mouse pad, barcode scanners, etc), and many other materials packed into your phone.

Measurements of HOMO, LUMO energies Experimental techniques Cyclic voltammetry XPS spectroscopy, UV-Vis spectroscopy. Theoretical technique Density Functional Theory (DFT)

Distribution of electron density in frontier molecular orbitals using DFT calculations  Four N,N′bis(dinitrophenyl) derivatives Diketopyrrolopyrroles (DPPs) were found as efficient materials both in organic dye-sensitized (DS) and and bulk heterojunction (BHJ) and solar cells (SC).

Fluorescence Emission Fluorescence emission mechanism in organic materials

Phosphorescence Emission Phosphorescence emission mechanism in organic materials

Evolution of OLEDs

Organic Light Emitting Diodes Glass ITO (anode) HIL HTL EML HBL ETL Cathode (HIL: hole injection layer, HTL: hole transport layer, EML: emissive layer, ETL: electron transport layer, EIL: electron injection layer, HBL: hole blocking layer)

EML: Doped emissive layer Block diagram of OLED device  where ITO (Indiun tin oxide) is anode, HTL is hole transport layer, EML is emitting layer, HBL is hole blocking layer, ETL is electron transport layer and metal cathode is calcium or magnesium, or lithium fluoride-aluminum  

Efficiency & Stability Evolution of Organic Light Emitting Diodes Anode EML Cathode Pope (1963), Helflich (1965) Monolayer (Thick crystals) 1965 ETL HIL HTL Doped EML HBL Heterostructures Multilayers HBL : ▶ Hole Blocking layer ▶ Exciton confinement P-doped PIN OLED N-doped EIL Doped transport layers K. Leo, U. Dresden 2002 Efficiency & Stability Doped 1985 2-layers EK US patent # 4539507 # 4769292 1988

Block diagram of conventional OLED manufacturing process

Device structures of bottom and top emission OLEDs

Device structure of light emitting polymer

Transparent OLED device structure

White OLEDs White light emission configurations: (a) two color mixing, and (b) three color mixing

White OLEDs Single Layer White OLED (a) Fluorescent and (b) Phosphorescent

Current status of OLED devices First Generation OLED devices - Based on Fluorescent materials Second Generation OLED devices - Based on phosphorescent materials Third Generation OLED devices - Based on Thermally activated delayed fluorescence (TADF) materials. TADF is considered a very promising path towards metal-free efficient OLED emitters. Several companies are developing TADF based materials - most notably Cynora and Kyulux which was spun-off from the Kyushu University in Japan.

Evolution in OLED devices and technology

HyperFluorescence (HF) technology all manage to convert almost 100% of the energy into light. HF can be thought of as a combination of the high efficiency of TADF and the narrow spectrum of Fluorescence.

OLED Lighting Market by End-Users Architectural Sector Hospitality Sector Commercial Sector Automotive Sector Residential Sector Industrial Sector

Fig. 1 Market drivers