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Photoelectric Effect
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- Photoelectric effect, 1st demonstrated by Einstein, relies on the interaction of photons and electrons and their behaviour during interaction.
- Flash a beam of light on to some material like metal it will knock-off electrons, collect these electrons at an electrode. Connect the electrode to a wire and there you have electricity.
Working of PV Cells
- A photovoltaic panel creates electricity when a photon hits the silicon surface and pushes an electron out of the top layer of the silicon and across an electrical junction inside the panel.
- The movement of this electron creates a useful voltage. When wires are connected to produce a circuit, this voltage means that current will flow, eventually taking the displaced electron back to the top layer.
- Solar cells work best in strong sunlight but will also generate some power on an overcast day from the diffused light that gets through the clouds.
- Nature of electrical energy from PV cells
- DC current
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Types of PV Cells
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- 1st gen: Single crystal PV cells
- 2nd Gen: Thin-film PV cells
- 3rd Gen: Perovskites
1st Generation PV cells
- These are basically silicon-based.
- Single crystal silicon: have the highest efficiency, around 20-25%, high-cost manufacturing
- Amorphous silicon (powdered): efficiency is 5%, used in calculators
2nd Generation PV cells: Polycrystalline PV cells
Cadmium Telluride:
- CdTe can be deposited on thin sheets. So you can make flexible PV panels.
- CdTe absorbs sunlight readily to release electrons. So thin sheets can be made.
- Efficiency ~ 15%
- Disadvantage is cadmium is highly toxic.
- Tellurium is not available as it is a rare earth metal. It is found in under-water ridges.
CIGS: Copper Indium Gallium Selenide
Advantage:
- Similar to CdTe, it can absorb sunlight readily. Besides it is not toxic like CdTe.
- Disadvantage: Indium is in short supply
Multi-Junction PVs: Gallium-Arsenic, Indium, Germanium
- Layers of semiconductor and metal
- Main advantage is high efficiency because different materials absorb different wavelengths from the sun.
3rd Generation PV cells
Perovskites
- The main advantage of perovskites is they can used to make PV paints which can be painted on conformal surfaces like cars.
- Very recently organic-perovskite solar cells have reached efficiency of a record-breaking 24%
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