1. Energy diagram of a silicon solar cell Electrons and holes are pulled in opposite directions by the opposite charges of the ionized dopants at the p n-junction. It is important to make sure that the carriers travel to the contacts without getting trapped. Single crystal Si does best. Electrons and holes build up an open-circuit voltage that keeps reducing the band bending until the bands are flat. VBM CBM

2. Slow progress in silicon photovoltaics, compared to Moore’s law (no miniaturization, mainly volume) 2005 1976 Module price decreases by 20% for every doubling of cumulative production 2011 2012 Silicon Wafer Technologies NREL + recent data points Price war Si shortage

3. Lose photons below the band gap. Lose the kinetic energy of hot electrons This part is converted to electrical power 1/3 Efficiency limit: 33% for a single junction (Shockley-Queisser)

4. Efficiency versus cost Physics Today, March 2007, p. 37 1 $ /W Goal High end Low end Low end designs are more cost-effective (less $ /W).

5. More flexibility by combining nanoparticles, molecules, and electrolytes Dye-sensitized solar cell (Grätzel cell) Porphyrin dye: A metal atom in a cage of 4 nitrogens

6. General design of a solar cell: 4 energy levels, 3 materials to play with. EFEF EFEF e V OC + Acceptor Absorber Donor Small energy drop: Large voltage Large energy drop: Large current Want both

7. Grätzel, Nature 414, 338 (2001) and J. Phys. Chem. C 112 (2008) Lose half the voltage on the donor side. Energy levels in a Grätzel cell

8. Area of solar cells required for the US electricity production 1 kW/m 2 (Incident solar power)  ¼(Useful daylight)  0.16(Efficiency of a solar cell)  10 10 m 2 (100  100 km 2 ) = 0.4 TWElectric power consumed in the US 0.7 TW could be generated by all the rooftops in the US (NREL study).

9. How much would it cost ? 1 $ /W (Price of solar cells per Watt)  0.4 TW (Electric power generated in the US) = 0.4 T $ = 0.4 Trillion Dollars Mechanical support structure adds significant costs. But one can design buildings to provide the support.

10. Inexpensive thin film solar cells Use nanoparticles, molecules, polymers Less material, low temperature processing Print solar cells like newspaper, roll-to-roll Nanoparticle ink on metal foil (Nanosolar)

11. Price comparison between solar and fossil energy Solar energy is free, while fossil fuels need to be paid for. One-time cost for solar, continuous costs for fossil energy. Energy payback time matters for solar energy (1-4 years).Energy payback time Typical warranty is 25 years. The price of solar cells is only about 1⁄3 of the total cost. The rest is for the support structure, the converter, labor. $ /W $ /Ws