Solar PV Cells

• Solar PV is the mainstay of solar energy particularly with decreasing cost of PV cells in the recent times.
• Photo voltaic cell is an example of photodetector devices which work on the basis of photoelectric effect.
• It is a way of directly using sunlight to produce electricity.

Photoelectric Effect

• Photoelectric effect, 1^st 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

Types of PV Cells

1. 1st gen: Single crystal PV cells
2. 2nd Gen: Thin-film PV cells
3. 3rd Gen: Perovskites

1^st  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

2^nd 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. 

3^rd 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%