Chapter – 12 Organic Light Emitting Diodes

1. Chapter – 12 Organic Light Emitting Diodes MolecularOrbitals HOMO and LUMO

2. 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 1satomic orbitals combine to produce 2 molecular orbitals. Eachmolecular 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.

3. 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).

4. 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.

5. 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.

6. 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.

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

8. 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).

9. Fluorescence Emission Fluorescence emission mechanism in organic materials

10. Phosphorescence Emission Phosphorescence emission mechanism in organic materials

12. Evolution of OLEDs

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

14. 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

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

22. Block diagram of conventional OLED manufacturing process

23. Device structures of bottom and top emission OLEDs

24. Device structure of light emitting polymer

25. Transparent OLED device structure

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

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

28. 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.

29. Evolution in OLED devices and technology

30. 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.

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

32. Fig. 1 Market drivers