Prof. D. Venkataraman (DV)Department of ChemistryUMass Amherst

dv@chem.umass.eduhttp://thedvgroup.com

UMass Amherst

 Making Solar Cells

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Why Care about Photovoltaic Cells?

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US needs ~98 Quads of Energy

68% Waste

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Energy1 Quad = 1015 BTU (Quadrillion British Thermal Units) = 8,007,000,000 Gallons of gasoline = 1018 joules

Units of Energy

Power = Energy transfer rate1 Watt = 1 joule of energy transferred in 1 sec

1 kW h = 3.6 x 106 joules (3.6 megajoules)

US Energy Need/Person/Year= ~1012 Joules = Energy from 1 nuclear bomb

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US needs ~98 Quads of Energy

68% Waste

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Why Care about Photovoltaic Cells?

World Economic Forum

US Cell Phone Need = 327 MWFukushima Reactor = 460 MWWorld Need = 5.6 GW

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~106 J/day/m2

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Solar Cells

Target = 1020 Joules/y

~10% of US area = 1020 J/y Renewable Source of Energy

Energy Magnitude

Cost = ~$0.25/kW h Economic Viability No CO2 emissions during energy

conversion

Production of Si:SiO2 + C Si + CO2

1 ton of Si = 1.6 tons of CO2

Environmental Impact

“Photovoltaic energy production will benefit global society by creating jobs, distributing energy supplies, and preventing pollution that results in health, and environmental degradation.”In Photovoltaics — a path to sustainable futures, Futures, 2002, 7, 663-674

Equity

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“I’d put my money on the sun and solar energy. Whata source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”

Thomas EdisonIn 1931 to Henry Ford and Harvey Firestone

From Current Thinking by Heather Rogers inNew York Times, June 3, 2007

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Efficiency of Photovoltaic Cells Depend on Absorption in the Solar Spectrum Charge SeparationCharge MobilityCharge Collection

Photovoltaic Cells

Exciton

Active Layer

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Single Junction Photovoltaic Cells

Source: NREL

Active Layers: Crystalline Silicon Amorphous Silicon Gallium Arsenide Copper-Indium-Gallium-Selenide (CIGS) Titanium dioxide/dyes Carbon Nanotubes, Carbon-based Polymers

& Molecules

Inorganic

HybridOrganic PVs

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Solar Cells

Energy Magnitude Economic Viability Environmental Impact Equity

Energy Magnitude Economic Viability Environmental Impact Equity

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Organic Solar Cells

Making active layers materials is not energy intensive Carbon-based active layers are inexpensive Carbon-based active layers are light Flexible solar cells

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Exciton Diffusion Distances in OPV and Si Cells

Si or III-V Cells

Exciton diffusion distance >100 nm

Excitons loosely bound

Organic/Hybrid/Dye-sensitized

Exciton diffusion distance <10 nm

Excitons tightly bound (Frenkel Excitons)

Low dielectric constant

10 nm

Gregg, B. A., Excitonic solar cells. Journal of Physical Chemistry B 2003, 107 (20), 4688-4698.

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