Solar Cells and Photovoltaic Effect

Solar Cells

• Solar cells, also known as photovoltaic (PV) cells, convert light energy into electrical energy using the photovoltaic effect.
• Solar cells are p-n junction diodes, and their electrical characteristics (current, voltage, resistance) change when exposed to light.
• Individual solar cells are combined to form modules, commonly known as solar panels.
• A typical single-junction silicon solar cell produces a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.
• Combining multiple cells into a large solar panel generates substantial amounts of renewable energy.

How Solar Cells Work

• Photons from sunlight strike the solar cell.
• The photons' energy creates free electrons that accumulate in the n-type silicon.
• An external circuit allows electrons to flow, generating an electrical current.

Photovoltaic Effect

• This effect converts light energy to electrical energy in certain semiconductor materials.
• The process directly converts light energy to electricity without any intermediate steps.

Solar Cell Construction

• Similar to a junction diode, but construction slightly differs.
• A thin layer of p-type semiconductor is grown on a thicker n-type semiconductor.
• Fine electrodes are applied to the p-type layer, and a p-n junction is created beneath it.
• A current-collecting electrode is placed on the bottom of the n-type layer.
• The entire assembly is encapsulated in glass to protect it from damage.

Working Principle of Solar Cells

• Incident light photons supply enough energy to create electron-hole pairs in the junction.
• This disrupts thermal equilibrium within the junction.
• Free electrons in the depletion region quickly move to the n-type side.
• Similarly, holes move to the p-type side.
• This electron-hole separation creates a photovoltage.
• An external circuit connected across the junction allows current flow.

I-V Characteristics of Solar Cells

• Graph showing the relationship between current and voltage output.
• "Short Circuit Current" (Isc) - maximum current when no load is applied.
• "Open Circuit Voltage" (Voc) - maximum voltage when no current flows.
• "Maximum Power Point" (MPP) - point where maximum power is generated (Vmp, Imp).
• "Fill Factor" (FF) - ratio of maximum power to the product of Voc and Isc. This quantifies how efficiently the solar cell converts light into usable energy.

Solar Cell Efficiency

• Efficiency is the ratio of energy output to energy input from the sun.
• Factors influencing efficiency include incident sunlight spectrum, intensity, and the solar cell's temperature.

Materials Used in Solar Cells

• Materials must have a bandgap near 1.5 eV for optimal light absorption and electrical efficiency.

• Common materials include Silicon, GaAs, CdTe, and CuInSe2.

• Criteria for materials: suitable bandgap, high electrical conductivity, and readily available, low-cost raw materials.

Advantages of Solar Cells

• No air pollution.
• Long lifespan.
• Low maintenance costs.

Disadvantages of Solar Cells

• High installation costs.
• Low efficiency in cloudy conditions.
• No energy generation at night.

Applications of Solar Cells

• Powering small devices (calculators, watches).
• Large-scale applications (spacecraft, renewable energy systems).

Photodetectors

• Devices that detect light and generate an electronic output signal.
• Output signal is proportional to the incident light power.

Types of Photodetectors

• Photodiodes.
• Phototransistors.
• Photoconductive detectors.
• Phototubes/Photoemissive detectors.
• Photomultipliers.

Photodiode Construction

• Uses a p-n junction with an intrinsic layer (i). Sometimes called a p-i-n or PIN photodiode.
• Light absorbed in the depletion region generates electron-hole pairs.
• Output photocurrent is proportional to the light intensity.

Photovoltaic vs Photoconductive Modes of Operation

• Photovoltaic: Similar to a solar cell; creates a voltage from light across a load. Not typically as linear or wide-ranging as the photoconductive mode in terms of the power levels it can handle.
• Photoconductive: Applies a reverse bias to the diode and measures resulting current. Can have high responsiveness over a wide range of light power levels to be much more linear.

Applications of Photodetectors

• Radiometry/photometry applications.
• Optical power measurements.
• Various optical sensors, such as light barriers.
• Fiber optics communications.
• Laser rangefinders and metrology.
• Image sensors (cameras).