In the last seven years, perovskite-based solar cell efficiency has gone from less than 4% to over 22% – which is the fastest rise in efficiency ever seen in the solar PV industry.
What is a “perovskite”? It is essentially a salt with a lot of desirable properties for solar cells. Some of these 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 it’s 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 get from the valence band into the conduction band.
Band Gap
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 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 into the conduction band.
Absorption Coefficient / Image Credit: nasa.gov
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.
Printed Solar Cell / Image Credit: onelab.mit.edu
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. We are confident that these challenges will be overcome, and perovskite solar cells will play a large role in the growth of the solar industry within the next five years.
Main Image Credit: www.oxfordpv.com