Solar cell principle and applications

Principle of solar cell (photovoltaic) technology

A solar cell is a p-n junction semiconductor diode (usually silicon) that converts sunlight directly into DC electricity by the photovoltaic effect.

1. A silicon wafer has an n-type layer (doped with phosphorus: extra electrons) on top of a p-type layer (doped with boron: extra holes). At the junction, electrons diffuse into the p side and holes into the n side, leaving a depletion region with a built-in electric field (about 0.6–0.7 V for silicon).
2. When photons with energy greater than the band gap of silicon (1.1 eV) strike the cell, they create electron–hole pairs.
3. The built-in field sweeps electrons towards the n side and holes towards the p side, so the charges separate.
4. With metal contacts on the front (grid) and back, and an external load connected, electrons flow through the load, giving DC current. Photons with less energy than the band gap pass through, and the extra energy of higher-energy photons becomes heat.

   Sunlight (photons)
     | | | |
  ===========  front grid contact
  |  n-type  |   (electrons)
  |----------|   junction / depletion layer
  |  p-type  |   (holes)
  ===========  back contact
      \___[ Load ]___/
         current flow

A typical cell gives about 0.5–0.6 V and, under full sun, about 35–40 mA/cm²; cells are connected in series and parallel to make modules.

Types of PV technology

• Monocrystalline silicon (efficiency 18–22%)
• Polycrystalline silicon (15–18%)
• Thin film: amorphous silicon, CdTe, CIGS (10–15%)
• Emerging: perovskite and multi-junction cells (above 25–30% in labs)

Applications

1. Solar home systems (SHS): lighting and phone charging in remote homes in Nepal (AEPC has installed hundreds of thousands).
2. Water pumping: drinking water and irrigation.
3. Grid-connected rooftop and ground-mounted plants.
4. Telecommunication towers, remote health posts and schools.
5. Street lighting, traffic signals and signboards.
6. Hybrid systems with micro-hydro or diesel.
7. Consumer products: calculators, watches, torches, chargers.
8. Space satellites and transport: solar cars, boats.

Advantages and limits

• Free fuel, silent, long life (25 years), little maintenance and modular.
• Low efficiency, high initial cost, intermittent output, needs storage and area, and DC to AC conversion losses.