Solar cells are renewable energy machines that turn sunlight directly into electric energy. A single-junction solar cell uses one semiconductor band gap, so it can only use photon energy in certain ways. This creates a basic efficiency ceiling called the Shockley-Queisser limit.
Understanding this limit helps explain why even excellent solar panels cannot convert all incoming sunlight into electricity.
When a photon has less energy than the band gap, it passes through or is absorbed without freeing an electron. When a photon has more energy than the band gap, only about the band gap energy can become useful electrical energy and the extra energy becomes heat. Other losses come from recombination, reflection, resistance, and unavoidable voltage limits.
For an ideal single-junction cell under normal sunlight, the maximum efficiency is about 33 percent.
Key Facts
- Photon energy is E = hf = hc/λ, so shorter wavelength light has higher energy.
- A semiconductor absorbs a useful photon when E_photon ≥ E_g, where E_g is the band gap energy.
- Photons with E_photon < E_g usually do not create electron-hole pairs and their energy is mostly not converted.
- For E_photon > E_g, excess energy is lost as heat through thermalization.
- Electrical power output is P = IV, where I is current and V is voltage.
- The Shockley-Queisser limit for an ideal single-junction solar cell under standard sunlight is about 33 percent efficiency.
Vocabulary
- Band gap
- The band gap is the minimum energy needed to move an electron in a semiconductor into a state where it can help carry current.
- Photon
- A photon is a packet of light energy whose energy depends on its frequency or wavelength.
- Single-junction solar cell
- A single-junction solar cell is a photovoltaic device made with one main semiconductor junction and one band gap.
- Thermalization
- Thermalization is the loss of extra photon energy as heat after an electron is excited above the band gap.
- Recombination
- Recombination is the process in which an excited electron falls back into a hole, losing energy before it can do useful electrical work.
Common Mistakes to Avoid
- Assuming all absorbed light becomes electricity is wrong because absorbed photons can lose energy as heat or through recombination before producing useful current.
- Ignoring the band gap is wrong because photons below the band gap cannot create useful electron-hole pairs in a single-junction cell.
- Thinking higher energy photons are fully converted is wrong because energy above the band gap is mostly lost through thermalization.
- Using 100 percent as the target efficiency for one solar cell is wrong because the Shockley-Queisser limit sets an ideal maximum near 33 percent for a single-junction device.
Practice Questions
- 1 A solar cell receives 1000 W/m^2 of sunlight. If it operates at the Shockley-Queisser limit of 33 percent, what electrical power per square meter could it ideally produce?
- 2 A photon has energy 2.0 eV and enters a solar cell with band gap 1.1 eV. How much energy is available beyond the band gap to be lost mainly as heat?
- 3 Explain why a single-junction solar cell cannot be optimized for both low energy infrared photons and high energy blue photons at the same time.