Making Solar Cells

1. Making Solar Cells Prof. D. Venkataraman (DV) Department of Chemistry UMass Amherst dv@chem.umass.edu http://thedvgroup.com UMass Amherst

2. Why Care about Photovoltaic Cells?

3. US needs ~98 Quads of Energy 68% Waste

4. Units of Energy Energy 1 Quad = 1015 BTU (Quadrillion British Thermal Units) = 8,007,000,000 Gallons of gasoline = 1018 joules Power = Energy transfer rate 1 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

5. US needs ~98 Quads of Energy 68% Waste

6. Why Care about Photovoltaic Cells? US Cell Phone Need = 327 MW Fukushima Reactor = 460 MW World Need = 5.6 GW World Economic Forum

7. ~106 J/day/m2

8. Solar Cells Target= 1020 Joules/y • Energy Magnitude • ~10% of US area = 1020 J/y • Renewable Source of Energy • Economic Viability • Cost = ~$0.25/kW h • No CO2 emissions during energy conversion • Production of Si: • SiO2 + C  Si + CO2 • 1 ton of Si = 1.6 tons of CO2 • Environmental Impact • Equity “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

9. “I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.” Thomas Edison In 1931 to Henry Ford and Harvey Firestone From Current Thinking by Heather Rogers in New York Times, June 3, 2007

10. Photovoltaic Cells Exciton Active Layer • Efficiency of Photovoltaic Cells Depend on • Absorption in the Solar Spectrum • Charge Separation • Charge Mobility • Charge Collection

11. 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 Hybrid Organic PVs

12. Solar Cells • Energy Magnitude • Economic Viability • Environmental Impact • Equity • Energy Magnitude • Economic Viability • Environmental Impact • Equity

13. 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

16. 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.