Solar Cells: How the Photovoltaic Effect Makes Electricity

Silicon, the Band Gap and Doping

By Akanksha Kumari · Science faculty, The Cavalier · B.Tech, 15 years teaching Science

In a solid, the energy levels of billions of atoms merge into two broad bands. The valence band holds the electrons that bind the atoms together. The conduction band above it is where electrons can roam and carry current. Between them lies the band gap (E_g), a range of forbidden energies. Its size decides what kind of material you have.

Silicon is in group 14 of the periodic table (see our periodic table page), so each atom has four valence electrons and shares one with each of four neighbours in a diamond-like crystal. At room temperature, heat frees only a few electrons. Each one leaves behind a vacancy called a hole, which acts like a positive charge as neighbouring electrons hop into it. Pure silicon is a poor conductor. The remedy is doping: replacing a tiny fraction of the silicon atoms, a few parts per million, with an impurity atom.

A semiconductor's resistance falls as it heats, opposite to a metal, because heat creates more carriers. The same doped silicon makes chips too, as in our article on the Sanand semiconductor units.

The p-n Junction: Where Light Becomes Voltage

A solar cell is a large, flat p-n junction: a very thin n-type layer on a thicker p-type wafer. At the boundary three things happen, in order.

1. A built-in field forms. Electrons from the n side drift across and fill holes on the p side. This leaves positive ions on the n side and negative ions on the p side, a strip of the order of a tenth of a micrometre wide with almost no free carriers: the depletion region. Its electric field points from n to p, and the potential difference across it, the barrier potential, is about 0.7 V for silicon (0.3 V for germanium).
2. Light creates pairs. A photon with energy above the band gap breaks a bond and makes one electron-hole pair.
3. The field separates them. It sweeps the electron to the n side and the hole to the p side. The n side becomes the negative terminal, the p side the positive terminal. Connect a bulb across them and electrons flow round the outside circuit to the back contact, where they meet holes again.

The three steps, generation, separation and collection, are the whole story. No chemical reaction occurs and nothing is used up, which is why a cell differs from the batteries on our electrochemical cells page. A battery stores chemical energy; a solar cell converts incoming light and stops the moment the light stops. The same junction is used four ways, and UPSC likes to ask which is which.

A solar cell's I-V curve lies in the fourth quadrant because the cell supplies current. An ordinary diode is non-ohmic: its current is not proportional to voltage.

One Photon, One Electron: Photovoltaic versus Photoelectric

In 1905 Albert Einstein proposed that light arrives in packets, or photons, each with energy E = hν. One photon frees at most one electron, and only if its energy is large enough. He received the 1921 Nobel Prize in Physics for the law of the photoelectric effect, not for relativity. The same idea explains both effects below.

Edmond Becquerel, the father of Henri Becquerel who found radioactivity (see our radioactivity page), saw a voltage when light fell on electrodes in a liquid in 1839. The first practical silicon solar cell came from Bell Laboratories in 1954, at about 6% efficiency.

A green photon (500 nm) carries 1240 ÷ 500 = 2.48 eV. Only 1.1 eV is used to lift one electron across; the other 1.38 eV, more than half, becomes heat.

How Much Sunlight Becomes Power?

The Sun delivers about 1.36 kW per square metre above the atmosphere (the solar constant). Panels are rated at 1,000 W per square metre, roughly a clear day at ground level. A cell's efficiency is electrical power out divided by light power in.

Why not 100%? Photons below the band gap are not absorbed, photons above it waste the excess as heat, some carriers recombine before reaching the contacts, and some light is reflected. For a single-junction cell, William Shockley and Hans Queisser showed in 1961 that no design can beat about 33%, at a band gap near 1.3 to 1.4 eV. Silicon, at 1.1 eV, tops out nearer 29%. Silicon panels on sale today deliver roughly 20-24%, which is why engineers stack two materials, each tuned to a different part of the spectrum.

The cell works on the visible and near-infrared part of the spectrum; our electromagnetic spectrum page shows how photon energy rises from infrared to ultraviolet.

Cell, Module, Array and the Types of Panel

One silicon cell gives only about 0.5 to 0.6 volt under load. So cells are wired up in stages. A module has 60, 72 or more cells in series (voltages add) under glass, with bypass diodes that let current skirt a shaded cell, and it is rated in watt-peak (Wp). An array joins modules in series for higher voltage and in parallel for higher current. Shade on one cell can cut the output of a whole string, which is why the bypass diodes matter.

Inverter, Solar Thermal and the Grid

A cell produces direct current (DC). Homes and the grid run on alternating current (AC) at 50 Hz. The inverter converts one to the other and keeps the array at its maximum-power point. A grid-connected inverter shuts itself off in a blackout so that no power flows back to line workers. An off-grid system adds a charge controller and a battery, because panels give nothing at night.

Solar thermal is a different technology: it turns sunlight into heat, not electricity. Rooftop water heaters and solar cookers are the familiar forms, and concentrating plants use mirrors to make steam for a turbine. PV also works in diffuse light; concentrating plants need strong direct sunlight.

The PIB backgrounder puts India's total installed capacity at around 560 GW (October 2026), of which 308 GW (55%) was non-fossil as of 31 July 2026. For coal, hydro and wind beside solar, see our energy resources page.

India's Solar Push and the International Solar Alliance

Our article on PM-Surya Ghar explains how a household gets connected, and the agrivoltaics article covers PM-KUSUM and farm-top panels. Manufacturing is the gap: India installs more panels than it makes cells for, which explains PLI and ALMM, in the same spirit as the PLI schemes in other industries. Two landmarks come up in quizzes: Modhera in Gujarat, India's first 24x7 solar-powered village, and the Gandhi Solar Park at the United Nations headquarters in New York.

How UPSC Has Asked It: Past Papers Solved

Real CDS and NDA questions. UPSC asks which device converts light into electricity, who explained the photoelectric effect, and a steady stream of solar current affairs.

Practice MCQs (with answers)

1. A solar cell converts
   1. electrical energy into light
   2. light into electrical energy
   3. heat into sound
   4. chemical energy into electrical energy
2. An n-type semiconductor is made by doping silicon with
   1. boron
   2. aluminium
   3. phosphorus
   4. gallium
3. A p-type silicon crystal is
   1. positively charged overall
   2. negatively charged overall
   3. rich in free neutrons
   4. electrically neutral
4. In a working solar cell, the junction field sweeps electrons towards the
   1. n side, which becomes the negative terminal
   2. p side, which becomes the negative terminal
   3. p side, which becomes the positive terminal
   4. n side, which becomes the positive terminal
5. Silicon has a band gap of about 1.1 eV. Which photon will NOT create an electron-hole pair in it?
   1. Infrared of 0.8 eV
   2. Red light of 1.8 eV
   3. Green light of 2.5 eV
   4. Violet light of 3.1 eV
6. A 2 m² panel of 18% efficiency in sunlight of 1,000 W/m² gives
   1. 180 W
   2. 1,000 W
   3. 2,000 W
   4. 360 W
7. The device that changes the DC output of a solar array into AC is the
   1. transformer
   2. rectifier
   3. inverter
   4. charge controller
8. The International Solar Alliance was launched at the climate conference held in
   1. Kyoto
   2. Paris
   3. Copenhagen
   4. Glasgow

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