And God said, let there be light: and there was light.

Milestones of Photochemistry/physics/biology

Primitive Earth Age: Small gas molecules + Sun light Nucleic acids and Proteins 1805: Young’s double slit experiment (Wave nature of light) 1864: Electromagnetic Wave theory of light (Maxwell) 1886: First organic photochemical reaction by Giacomo Luigi Ciamician 1887: Photoelectric effect (Heinrich Hertz) Early 1900’s: Grotthus- Draper law Early 1990s: Beer-Lambert law 1900: Black body radiation: Quantum nature of light (Max Planck) 1905: Quantum theory of radiation (Einstein). 1909: Wave-particle duality (Einstein) 1912: Stark-Einstein law 1916: Concept of spontaneous absorption and emission (Einstein) 1917: Concept of stimulated emission: Concept of MASER and LASER (Einstein) 1918: Nobel prize: Max Planck: Energy quanta 1921: Nobel Prize: Einstein: Photoelectric effect 1926: Nomenclature of the light quantum as photon (Gilbert Lewis) 1927: Nobel Prize: Arthur Compton: Compton effect: Photons carry momentum 1953: Discovery of MASER: Charles H. Townes 1960: Discovery of LASER: Theodore H. Maiman 1967: Nobel prize: Manfred Eigen: Flash Photolysis to study ms-ns processes 1971: Nobel prize: Gerhard Herzberg: Electronic structure of molecules 1992: Nobel prize: Rudolf A. Marcus: Theory of electron transfer reactions 1997: Nobel prize: Steven Chau: Laser cooling and trapping of atoms 1999: Nobel prize: Ahmed H. Zewail: Femtosecond laser spectroscopy 2000: Development of attosecond laser. 2008: Nobel prize: Osamu Shimomura: Green fluorescent protein 2009: Nobel prize: Charles K. Kao: Optical fibre cable

Giacomo Luigi Ciamician: Father of Modern Molecular Photochemistry

BornAugust 25, 1857(1857-08-25)Trieste, Austria

DiedJanuary 2, 1922 (aged 64)Bologna, Italy

Education University of Vienna

Employer University of Bologna

His first photochemistry experiment was published in 1886 and was titled “On the conversion of quinone into quinol by light”.

Fundamental theories of Photochemistry

Grotthus-Draper law: Only that light which is absorbed by a system can cause chemical change.

Stark-Einstein law: One quantum of light is absorbed per molecule of absorbing and reacting substance that disappears.

Beer-Lambert law:

Fundamental theories of Photochemistry

Fundamental theories of Photochemistry Selection rules:

1. Radiation transitions (Absorption, fluorescence) Allowed transitions: g u, e.g.: S0 S1 and T1 S0

Forbidden transitions: g g, u u, e.g.: S1 T1

2. Radiationless transitions (vibration relaxation, Internal conversion, Intersystem crossing) Allowed transitions: g g, u u, e.g.: S1 T1 Forbidden transitions: g u, e.g.: S0 S1 and T1 S0

Instrumentation

Femtosecond Transient Absorption Spectroscopy

Femtosecond Transient Absorption Spectroscopy

Femtosecond Transient Absorption Spectroscopy

Femtosecond fluorescence upconversion setup

Femtosecond Spectroscopy lab

• Electrons can jump between “bands”

• Incident light with energy ≥ than the “band gap” energy can be used to excite the electrons

Absorption of Light by Ionic Compounds

17

So What Does this Mean for Solar Cells?• In dye-sensitized solar

cells…– Talk about highest

occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)

• In single-crystal silicon solar cells…– Talk about

“conduction band” (excited states) and “valence band” (ground states)

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A Closer Look at Solar Cells• How do

traditional, silicon-based solar cells and newer, dye-sensitized solar cells work?

• What are the advantages and disadvantages of each type of cell?

Silicon-based solar cell

Dye-sensitized solar cell

How a Silicon-Based Solar Cell Works

• A positive “hole” is left in the electron’s place

• This separation of electrons and holes creates a voltage and a current

• Light with energy greater than the band gap energy of Si is absorbed

• Energy is given to an electron in the crystal lattice

• The energy excites the electron; it is free to move

Click image to launch animation (requires web access)

How a Dye-Sensitized Cell Works

Click image to launch animation (requires web access)

• Light with high enough energy excites electrons in dye molecules

• Excited electrons infused into semiconducting TiO2, transported out of cell

• Positive “holes” left in dye molecules

• Separation of excited electrons and “holes” creates a voltage

P=1.367kWm-2 - the solar constant – solar radiation power outside the Earth’s atmosphere

Taken from: S. M. SZE; Physics of Semiconductor Devices; Second Edition; John Wiley & Sons;New York; 1981

SOLAR SPECTRAL IRRADIANCE