What is a “perovskite”? It is essentially a salt with a lot of desirable properties for making a solar cell. Some of these impressive properties include:
- A tunable energy band gap
- A high absorption coefficient
- Ability to be printed
A Tunable Energy Band Gap
A solar cell’s energy band gap is one of its most important properties, and is defined as the energy difference between the valence and conduction bands. In order for a solar cell to generate electricity, the electrons need to jump from the valence band into the conduction band.
A large band gap means that most of the light which hits the solar cell doesn’t have enough energy to move electrons from the valence to the conduction band.
A small band gap means that light energy will excite the electrons high into the conduction band. However, the electrons then fall to the bottom of the conduction band and a lot of the energy gets lost in thermalization.
With perovskite solar cells, the band gap can be changed and optimized by altering the halide content in the material. It turns out that the ideal band gap is around 1.34 eV.
A High Absorption Coefficient
The absorption coefficient indicates how well light is absorbed by a solar cell. A cell with a a low absorption coefficient will allow light to pass further into it before the photon gets absorbed. In cases where the solar cell is very thin, certain wavelengths will pass straight through it. Once the light is absorbed by the cell, its energy goes towards exciting electrons from the valence band into the conduction band.
Perovskite solar cells have such a high absorption coefficient that thin films of around 500 nm can absorb the entire visible solar spectrum.
Ability to be Printed
The main advantage of printing solar cells is the low cost of production. The low cost is due to the fact that there is very little wasted material, and no vacuum processing is required.
Downsides of Perovskite Solar Cells
As with all technologies, there are certain downsides. With perovskite solar cells, these downsides include:
- Sensitivity to water (since salts take up water, and perovskite is essentially a salt)
- Corrosion of some metal electrodes
- Methylammonium can leave the film fairly easily
Current perovskite research is primarily aimed at overcoming the downsides listed above.