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

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Photochemical Absorption and Luminescence

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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.

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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

• rder to satisfy the triplet state.

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(1) Paramagnetic metal ions : Essentially non- fluorescent (promote ISC) (2) Increasing atomic number : Fluorescence reduced (promote ISC) InQ3 < GaQ3 <AlQ3 (3) Temperature and solvent effects (4) Structural rigidity

Factors Affecting Luminescence

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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

• bserve is oxygen emission rather than
• electroluminescence. It is for this reason

that oxygen should be totally excluded.

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Effect of Oxygen

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Electroluminescence

• Ionization energy; EI

The first ionization energy, EI, of an atom/ion is the minimum energy which is required to remove an electron from an atom.

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Semiconductors

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Doping – Add Impurities

N-type P-type

Light Emitting Diodes (LEDs)

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Energy Molecular Systems Excitation

Basic Idea Behind Emission Basic Idea Behind Emission

Light

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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

Organic Semiconductors (OLEDs)

• Similar physics to LEDs but

– Non-crystalline – No doping; use cathode/anode to provide needed charges – Fluorescence/phosphorescence enhance excitonlight 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

Anode Cathode

• The holes move more efficiently in organics

Conductive Layer Emissive Layer

The basic OLED

Anode Cathode Conductive Layer Emissive Layer

• The holes move more efficiently in organics
• Excitons begin to form in 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

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Cathode Emissive polymer Anode Substrate Cathode Conducting polymer Anode Substrate Emissive polymer

Polymer OLEDs

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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

Electrodes Electrodes

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Typical JV Curves

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OLED (Polymer)

n

C 6H 13 C 6H 13

DHF-PPV

O O C H 3

n

MEH-PPV

n

PPV

N O

n

CzEH-PPV

O N N O

n

OxdEH-PPV

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Cathode Organic Layer Anode Substrate

Single layer device

Small molecular OLEDs — Structure Structure

Cathode Hole transport layer Anode Substrate Electron transport layer

P-n junction device

Electron transport layer Hole transport layer Anode Substrate Emissive layer Electron Injection layer Cathode Hole Injection layer

Multiple layers device

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

Cadmium Telluride Solar Cells

• Direct bandgap, Eg=1.45eV
• Good efficiency (Record:17.3%)
• High module production speed
• Long term stability (20 years)

Thin Film Solar Cells

CdTe CdTe Solar Cell with Solar Cell with CdS CdS window layer window layer

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Glass Superstrate Transparent Conducting Oxide N-type CdS P-type CdTe Metal Back Contact: Cathode Front Contact: Anode Window Layer Absorber layer Incident Light 3~ 8 um 0.1 um 0.05 um ~ 1000 um

CdS: tends to be n-type, large bandgap(2.42eV)

Cu(InxGa1-x)Se2 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 ITN/ES, CO NanoSolar Inc., CA DayStar Technologies, NY/CA MiaSole, CA HelioVolt, Tx Solyndra, CA SoloPower, CA Wurth Solar, Germany SULFURCELL, Germany CIS Solartechnik, Germany Solarion, Germany Solibro, Sweden CISEL, France Showa Shell, Japan Honda, Japan

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Conversion efficiency

• FF; I sc; Voc should be maximized for efficient solar

cells!

• c

sc m m

V I V I FF

• Fill factor

Conversion efficiency

in

• c

sc in m m

P V I FF P V I

Polymer Solar Cells

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Organic Solar cells

Polymer (donor) PCBM (acceptor)

Power conversion efficiency ~ 5 - 6%

bulk heterojunction

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Photoinduced Charge Generation

MDMO PPV

3,7 - dimethyloctyloxy methyloxy

PPV PCBM 1-(3-methoxycarbonyl) propyl-1- phenyl [6,6]C61

O O n

DONOR ACCEPTOR

An ultra-fast e- transfer occurs between Conjugated Polymer / Fullerene composites upon illumination. The transition time is less than 40 fs. exciton

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The driving force!

• Donor and acceptor LUMO energy
• ffset!

OMe O

Polymer

PCBM

• 6.0 eV
• 5.2 eV
• 4.2 eV
• 3.5 eV

Ultrafast phenomena!

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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

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Optimization

eff = Isc * Voc * FF / Iinc

Isc Tuning of the Transport Properties - Mobility Voc Tuning

• f

the Electronic Levels

• f

the Donor Acceptor Systems FF Tuning of the Contacts and Morphology Iinc Tuning of the Spectral Absorbance/Absorbing more light (low band gap and high absorptivity)

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Enhancing devices efficiency

• Optimize the materials properties

– Matching solar spectrum! NIR materials – Relative position of the energy levels of the donor and acceptor

• ptimal offset between LUMO (D) – LUMO(A)

for electron transfer at least 0.3 – 0.5 eV

• Optimize the morpholog

– microscopic phase separation ( exciton diffusion length ~ 5 – 7 nm )

Dye-Sensitized Solar Cells

Quantum Dots Solar Cells

Solar Cells Using Non-Toxic Abundant Materials

Solar Cells Using Non-Toxic Abundant Materials

• CuInGaSe2 – 20.4 % efficient – thin film architecture
• Cu2ZnSnS4 (CZTS) is similar to CuInGaSe2 in many ways

Raw Material Costs Cu - $3.35/lb Zn - $1.59/lb Sn - $6.61/lb S – $0.02/lb Ga - $209/lb In - $361/lb Se - 2002 $4, 2007 $33/lb Absorptivity ~104 Band gap ~ 1.45-1.5 ev

Cu Sn Zn S

CZTS History

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CZT(S,Se) Efficiency vs. Time

• # of CZTS Papers vs. Time
• IBM World Record – 9.6%
• AQT-Clemens Record – 9.3%
• CZTS research base growing fast

TEG : triethylene glycol; ODE : octadecyldecene; ODA : octadecylamine

CZTS crystal structure. Orange: Cu, grey: Zn, blue: Sn, yellow: S.

Thank You