Solar cells are photovoltaic cells that use sunlight as a light source, and they are becoming more and more important in the national economy and people's livelihood. Because of its light weight, high reliability, long life, and ability to withstand various environmental changes, it has become an important energy source in space technology. Therefore, solar cells are a very promising energy source.
(1) Photovoltaic effect
In the dark state, the p-n junction is in thermal equilibrium, and the diffusion current is equal to the drift current. When light is irradiated, its balance is destroyed and unbalanced carriers are generated.
Photovoltaic ceramic tile
(2) Photoelectric conversion efficiency
Sunlight is a continuous spectrum, and light of different wavelengths has different energy. When the photon energy is equal to the forbidden band width, the photoelectric effect can be directly generated, and the light energy is converted into electric energy. When the photon energy is greater than the forbidden band width, that part of the energy equivalent to the forbidden band width is converted into electrical energy. The excess energy is transferred to the crystal lattice, strengthens the lattice vibration, and becomes heat energy loss. When the energy of the photon is less than the band gap, it becomes heat energy lost or transmitted in the same way, thus reducing the efficiency of converting sunlight into electrical energy.
When the energy of photons is greater than the band gap, the excess energy will also cause heat loss. If the band gap of the photocell is wider, the loss of low-energy photons will be greater and the photoelectric conversion efficiency will be lower. Therefore, use a wide band gap The material is disadvantageous for solar cells. However, the photon energy is greater than the band gap, and the excess energy also causes heat loss. If the band gap of the photocell is too narrow, the loss caused by high-energy photons will also cause the photoelectric conversion efficiency to decrease, and it is not suitable for photovoltaic cell materials.
In fact, the conversion efficiency of solar cells is not only limited by the photon excitation utilization rate, but also by the reflection loss of the surface of the material, the recombination loss of electrons and holes and other factors. Theoretical research shows that the theoretical value of conversion efficiency can reach about 25%, but the maximum value of the experiment is less than 23%, and the conversion rate of general products is below 10%. After comprehensively considering various factors affecting the photoelectric conversion efficiency, it is known that the band gap of the photosensitive material is preferably 1.0~1.6 eV.
Band gap of some semiconductor materials
From the point of view of the band gap, Si, Cu2S, GaAs, CdTe, etc. are all suitable for manufacturing solar cells. Among them, Si and GaAs are often used as monocrystalline or polycrystalline thin film solar cell materials, while Cu2S and CdTe are often used as ceramic solar materials.