1. Solar Cells 3 generations of solar cells:
Starting with 1st, photovoltaics
Bulk technologies – wafer-based, typically 8cm in diameter, 3mm in thickness usually cut from a crystalline silicon
Monocrystalline- absolutely pure

2. Photoelectric Effect
This effect is in direct contradiction to the laws of classical physics and without which solar cells would not exist.
Here light travels in the form of photons with energy described by:
Photons pass through- energy of photon is less then bandgap energy of silicon semiconductor
Photons are reflected- photon energy is greater then the band gap energy of silicon
Photon is absorbed- photon energy is within the bandgap energy of silicon

3. Not all photons are reflected some are absorbed or transmitted
When a photon is absorbed, the energy of the photon is transferred to an electron in the crystal lattice
This is known as the photovoltaic effect
Solar cells are made of semiconducting material, traditionally Silicon
Over 95% of all solar cells produced worldwide are silicon semiconductors
Silicon semiconductors can be doped on one side to produce p-material (boron doped silicon) or the other side can be doped to produce more n-material (phosphorous doped silicon)

4. These valence electrons escape their orbits leaving holes
Energy of the photon is transferred to the valence electrons in the n-type layer
These valence electrons escape their orbits leaving holes
(electrons are majority carriers, holes are minority carriers)
This creates mobile electron-hole pairs

5. In the p-type layer electrons are the minority carriers and holes are the majority carriers

6. When p-type and n-type layers are placed together a p-n junction is formed and current begins to flow
When electrons from the n layer move into the p layer a depletion zone is formed
This zone separates the positive and negative charges and prevents a flow between them – an electric field is created
Np-junctions are really made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa)
A diffusion of electrons occurs from the n-type side of the junction in to the p-type side of the junction. Electrons diffuse across the p-n junction and recombine with holes on the p-type side. Forming depletion zone. This separation of positive and negative charge is an electric field, allowing current to flow only one way (like a diode).

7. The flow of electrons is a DC current (I)
The electric field of the cell is a voltage (V)
Here power (P) is given by:
The diode created by the electric field allows current to flow in only one direction across the junction
Connecting the sides of the cell externally will cause electrons to flow to their original p side to meet with holes

8. Many of these cells are connected to in order to create a solar panel
By connecting the cells in series a higher voltage is obtained
By connecting the cells in parallel a higher current is obtained
Any number of modules can be connected together to give the desired electrical output

9. Efficiency ( ) Maximum power (Pm) in W
Irradiance of input light (E) measured in W/m2
Surface area or solar cell (Ac) in m2
Fill factor (FF)
Open circuit voltage (Voc)
Short circuit current (Isc)
Higher cell temps => lower output => lower efficiency
Efficiency- % of power converted (from absorbed light to electrical energy) and collected
The 1st generation of solar cells, (photovoltaic) are comprised of one layer of p-n junction
Starting at 6% efficiency – 12%
Cannot convert more than 25% of the solar energy into electricity because IR radiation does not have enough energy to separate positive and negative charges in the material
Efficiency is improved in 2nd generation photovoltaics by layering p-n junctions. Each layer designed to absorb a successively longer wavelength of light (lower energy) – absorbing more ot the solar spectrum – increasing amount of electrical energy produced
At least 30% efficiency
3rd generation- semiconductor that does not rely of p-n junctions (dye sensitive cells, organic polymer cells, quantum dot solar cells)

10. 2nd gen. – each layer is designed to absorb longer wavelengths
2nd gen. – each layer is designed to absorb longer wavelengths. Absorbing more of the solar spectrum. More power output.

11. Thin-Films
Less material needed to create solar cell but less energy conversion efficiency
However, multi-layer thin films may have higher efficiencies then silicon wafers
Use flexible resin film substrates instead of glass sheet substrates

12. Solar Shingles Thin film PVs can be used as shingles
Cells shown here are triple junction
Each shingle has a pair of wires coming off its back so the system can be wired inside the attic
People love the technology but they don’t want it on their house because it is “ugly”
Shingles are highly absorptive can actually add to the buildings thermal load
Thus, shingles need to be thermally isolated from the building so they don’t heat it

13. Conductive Polymers Built from thin films of organic semiconductors
The high-efficiency cells made from GaAs where used in Deep Space 1

14. Dye-sensitive solar cells
Absorption occurs in dye molecules
Electrons are passed on to the n-type TiO2, holes are passed to an electrolyte on the other side of the dye
Heat and UV light cause cells to degrade
Low-cost production
Moderate efficiency (less then 10%)

15. Quantum Dots
Semiconductor quantum dots can slow the cooling of hot electrons
This could enhance efficiencies up to 66%
Combination of quantum dots with polymers creates cells that can absorb IR radiation
Spray on solar power cells
Solar cell window coating- power home appliances with the absorbed IR radiation

16. Energy Storage
Energy produced by solar cells can be stored in batteries
For domestic systems it is most effective to run the meter backward- in effect selling the electricity to the grid
When electricity is needed and the solar cells are not producing the electricity can be bought back from the network
Not all energy is used immediatly
Batteries good for small amounts of energy, but batteries don’t have long life span and large batteries are very expensive

17. References
Dye-Sensitized Solar Cells. European Institute for Energy Research April 2006 <
Green, Martin A. Solar Cells. New Jersey:Prentice-Hall, 1982.
Highlights of the 2003 NCPV and Solar Program Review Meeting. NREL April 2006 <
Lovgre, Stefan. Spray-On Solar-Power Cells Are True Breakthrough. National Geographic News. 14 Jan April 2006 <
Photodetectors. HyperPhysics. 20 April 2006 <
PV Cells. Specmat.com. 23 April 2006 <
Solar Cells. 23 April 2006 <
Solar Cell. Wikipedia. 25 April 2006 <
Solar Electricity. The Electricity Forum. 23 April 2006 <
U.S. Department of Energy. Roofing. 10 Dec April 2006 <
U.S. Department of Energy. Solar Shingles. 5 Jan April 2006 <
Wave-Particle Duality. HyperPhysics. 25 April 2006 <