Bridging the Gap Between Chemistry and Physics - PowerPoint PPT Presentation

Bridging the Gap Between Chemistry and Physics ������������������������������ " �������������������� ��������� : � ������������ " Prof. Monzir Abdel-Latif Chemistry Department Dec. 2015

Electronic Transitions

Photochemical Absorption and Luminescence 6

Internal Conversion Internal conversion (IC) is a radiationless deactivation process whereby excited molecules return to the ground state without emission of a photon. This process lacks rigid understanding but seems to be the most efficient deactivation process in luminescence spectroscopy, since most molecules do not show electroluminescence. However, molecules with close electronic energy levels, to the extent that their vibrational energy levels of ground and excited states are overlapped, are believed to cause efficient internal conversion. 7

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Intersystem Crossing Electrons present in the first excited electronic level can follow one of three choices including emission of a photon to give fluorescence, radiationless deactivation to ground state, or intersystem crossing (ISC). The process of intersystem crossing involves transfer of the electron from an excited singlet to a triplet state. This process can actually take place since the vibrational levels in the singlet and triplet states overlap. However, crossing of the singlet state to the triplet state involves a flip in electron spin in order to satisfy the triplet state. 9

Factors Affecting Luminescence (1) Paramagnetic metal ions : Essentially non- fluorescent (promote ISC) (2) Increasing atomic number : Fluorescence reduced (promote ISC) InQ 3 < GaQ 3 <AlQ 3 (3) Temperature and solvent effects (4) Structural rigidity 10

Effect of Dissolved Oxygen Dissolved oxygen largely limits electroluminescence although it promotes intersystem crossing. This may be explained on the basis that the ground state of oxygen is the triplet state and it is easier for an electron in the triplet state to transfer its energy to triplet oxygen rather than performing a flip in spin and relax to singlet state. Therefore, oxygen will be excited and what we really observe is oxygen emission rather than electroluminescence. It is for this reason that oxygen should be totally excluded. 11

Effect of Oxygen 12

Electroluminescence 13

�������������������� � ������������������������������������������������ ���������������������������������������������� ��������������������������������������������������� ������������������ Ionization energy; E I The first ionization energy, E I , of an atom/ion is the minimum energy which is required to remove an electron from an atom.

Semiconductors 21

Doping – Add Impurities N-type P-type 22

Light Emitting Diodes (LEDs)

Basic Idea Behind Emission Basic Idea Behind Emission Molecular Light Energy Systems Excitation 26

Excitons • An electron and a hole form a hydrogen-like bound state! Called exciton! – Annihilation • Electron falls into hole • Energy emitted • Energy released as electron falls into hole – May turn into vibrations of lattice – heat – May turn into photons (only in some materials) • Infrared light (if gap ~ 1 eV) • Visible light (if gap ~ 2-3 eV) – May excite other molecules in the material 28

Organic Semiconductors (OLEDs) • Similar physics to LEDs but – Non-crystalline – No doping; use cathode/anode to provide needed charges – Fluorescence/phosphorescence enhance exciton � light probability Manufacturing advantages • – Soft materials – very malleable – Easily grown – Very thin layers sufficient Many materials to choose from • Relatively easy to play tricks • – To increase efficiency – To generate desired colors – To lower cost • Versatile materials for future technology

The basic OLED Anode Cathode Conductive Layer Emissive Layer

The basic OLED The holes move more efficiently in organics • Anode Cathode Conductive Layer Emissive Layer

The basic OLED • The holes move more efficiently in organics • Excitons begin to form in emissive layer Anode Cathode Conductive Layer Emissive Layer

The Exciton Exits in a Flash Excitons eventually annihilate into • – Molecular vibrations � heat (typical) – Photons (special materials) Light can be generated indirectly: • – Exciton can transfer its energy to a suitable molecule – Molecule is thus excited – Returns to ground state via fluorescence or phosphorescence • Greatly increases likelihood (per exciton) of light emission • Also allows for different colors and flexibility – determined by the light-emitting molecule(s), not the exciton

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Multiple Layer Hererostructure OLED 42

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Polymer OLEDs Cathode Cathode Emissive polymer Emissive polymer Anode Conducting polymer Substrate Anode Substrate 44

Electrodes Electrodes Anode: Indium-tin-oxide (ITO): 4.5-4.8 eV Au: 5.1 eV Pt: 5.7 eV Cathode: Ca: 2.9 eV Mg: 3.7 eV Al: 4.3 eV Ag: 4.3 eV Mg : Al alloys Ca : Al Alloys 45

Typical JV Curves 46

OLED (Polymer) PPV MEH-PPV OxdEH-PPV O n O n O C H 3 CzEH-PPV n N O N n O n C 6 H 13 N C 6 H 13 DHF-PPV 47

Small molecular OLEDs — Structure Structure Single layer device Cathode P-n junction device Organic Layer Multiple layers device Anode Cathode Cathode Substrate Electron transport layer Electron Injection layer Hole transport layer Electron transport layer Anode Emissive layer Substrate Hole transport layer Hole Injection layer Anode Substrate 48

Solar Cells

2010 Production by Cell Type Source: PV News, May 2011

Primary Photovoltaic (PV) Markets ��

In September 2011 there were protests at a Chinese PV factory over pollution in the river Chemical and Engineering News, September 26, 2011

Thin Film Solar Cells Cadmium Telluride Solar Cells • Direct bandgap, E g =1.45eV • Good efficiency (Record:17.3%) • High module production speed • Long term stability (20 years)

CdTe CdTe Solar Cell with Solar Cell with CdS CdS window layer window layer Back Contact: Cathode Metal Absorber layer P-type CdTe 3~ 8 um Window Layer N-type CdS 0.1 um Transparent Conducting Oxide 0.05 um ~ 1000 um Glass Superstrate Front Contact: Anode Incident Light CdS: tends to be n-type, large bandgap(2.42eV) 55

Cu(In x Ga 1-x )Se 2 Solar Cells • World record efficiency = 20.4 %. • Many companies are evaporating, printing, sputtering and electrodepositing it. • Some are manufacturing ~30-50 MW/yr. • Handling a 4-element compound is tough. Shell Solar, CA Global Solar Energy, AZ Energy Photovoltaics, NJ ISET, CA Wurth Solar, Germany ITN/ES, CO SULFURCELL, Germany NanoSolar Inc., CA CIS Solartechnik, Germany DayStar Technologies, NY/CA Solarion, Germany MiaSole, CA Solibro, Sweden HelioVolt, Tx CISEL, France Solyndra, CA Showa Shell, Japan SoloPower, CA Honda, Japan

Conversion efficiency I V � m m Fill factor FF I V sc oc Conversion efficiency � I V FF I V � � � m m sc oc P P in in • FF; I sc ; V oc should be maximized for efficient solar cells! | 58

Polymer Solar Cells

Organic Solar cells bulk heterojunction Polymer PCBM (donor) (acceptor) Power conversion efficiency ~ 5 - 6% | 61

Photoinduced Charge Generation An ultra-fast e - transfer occurs between Conjugated Polymer / Fullerene composites upon illumination. The transition time is less than 40 fs. ACCEPTOR DONOR exciton O n O MDMO PPV PCBM 3,7 - dimethyloctyloxy methyloxy 1-(3-methoxycarbonyl) propyl-1- PPV phenyl [6,6]C 61 | 64

The driving force! • Donor and acceptor LUMO energy offset ! -3.5 eV OMe -4.2 eV O -5.2 eV -6.0 eV PCBM Polymer Ultrafast phenomena ! | 65

Polymer-Solar Cells - FI LM PREPARATI ON Spin Casting is a easy coating technique for small areas. Material loss is very high. Doctor Blade Technique was developed for large area coating Doctor Blade has no material loss | 66

Optimization eff = I sc * V oc * FF / I inc I sc Tuning of the Transport Properties - Mobility V oc Tuning of the Electronic Levels of the Donor Acceptor Systems FF Tuning of the Contacts and Morphology I inc Tuning of the Spectral Absorbance/Absorbing more light (low band gap and high absorptivity) | 67