Why can't an a-Si:H solar cell rely on diffusion to separate the photogenerated carriers as much as a c-Si solar cell does?
The higher bandgap (around 1.7 eV) prevents the photogenerated carriers from diffusing through the intrinsic layer of the a-Si:H cell.
Because the absorption coefficient of a-Si:H is 70 times higher than that of c-Si in the visible region of the solar spectrum.
Because the diffusion length is around 300 nm, while in c-Si it is around 300 microns.
Because a-Si:H suffers from light induced degradation.
The main reason why an a-Si:H (amorphous silicon) solar cell cannot rely on diffusion as much as a c-Si (crystalline silicon) solar cell to separate the photogenerated carriers is because of its higher bandgap. The bandgap of a-Si:H is around 1.7 eV, which prevents the photogenerated carriers from diffusing effectively through the intrinsic layer of the cell.
Additionally, the absorption coefficient of a-Si:H is approximately 70 times higher than that of c-Si in the visible region of the solar spectrum. This means that a-Si:H absorbs a larger portion of the incident light, leading to a higher number of photogenerated carriers. The higher absorption coefficient also implies a shorter diffusion length for the photogenerated carriers in a-Si:H compared to c-Si.
The diffusion length of photogenerated carriers in a-Si:H is typically around 300 nm, whereas in c-Si, it is around 300 microns. This means that in a-Si:H, the carriers can only travel a relatively short distance before recombining, which limits their separation and collection efficiency.
Lastly, a-Si:H can suffer from light-induced degradation over time. This degradation can further reduce the separation and collection efficiency of photogenerated carriers, making diffusion-based carrier separation less effective.
In summary, the higher bandgap, higher absorption coefficient, shorter diffusion length, and potential light-induced degradation make diffusion less effective in separating the photogenerated carriers in a-Si:H solar cells compared to c-Si solar cells.
To understand why an a-Si:H (amorphous silicon) solar cell cannot rely on diffusion to separate the photogenerated carriers as much as a c-Si (crystalline silicon) solar cell does, we need to consider a few important factors.
1. Bandgap: The bandgap of a material determines the minimum energy required for an electron to be excited from the valence band to the conduction band. In a-Si:H, the bandgap is around 1.7 eV, while in c-Si, it is around 1.1 eV. The higher bandgap of a-Si:H limits the diffusion of photogenerated carriers because they need to overcome a greater energy barrier to move through the material.
2. Absorption coefficient: The absorption coefficient of a material indicates how strongly it absorbs light at different wavelengths. In the visible region of the solar spectrum, the absorption coefficient of a-Si:H is approximately 70 times higher than that of c-Si. This high absorption coefficient means that light is absorbed more quickly and closer to the surface of the a-Si:H solar cell, reducing the diffusion path length for photogenerated carriers.
3. Diffusion length: The diffusion length is a measure of how far photogenerated carriers can move through a material before recombining. In a-Si:H, the diffusion length is typically around 300 nm (nanometers), whereas in c-Si, it can be around 300 microns (micrometers). The shorter diffusion length in a-Si:H further limits carrier diffusion and separation within the material.
4. Light-induced degradation: Another limitation of a-Si:H solar cells is their susceptibility to light-induced degradation, commonly known as the Staebler-Wronski effect. Prolonged exposure to light can cause a decrease in cell performance due to changes in the material's structure and properties, leading to decreased carrier separation efficiency.
In conclusion, the combination of a higher bandgap, higher absorption coefficient, shorter diffusion length, and susceptibility to light-induced degradation makes it less viable for an a-Si:H solar cell to rely on diffusion to separate photogenerated carriers as effectively as a c-Si solar cell does.