The simulation illustrates how diffusion and drift affect the spatial distribution of charge carriers – two processes fundamental to understanding solar cells.
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Description
In an illuminated semiconductor material, free charge carriers are generated: electrons and holes. Both initially move randomly through the material (diffusion). Holes form when electrons leave their lattice sites – they migrate as neighbouring electrons fill the vacancies. Electron current and hole current flow in opposite directions.
At the p-n junction inside a solar cell, an electric field is present – the space charge region (SCR). This field arises from the different doping on each side and deflects charge carriers in a directed way: electrons towards the positive side, holes towards the negative side (drift).
Charge carriers have a limited lifetime τ between generation and recombination. The longer the lifetime, the greater the diffusion length L – the average distance a carrier travels:
\[ L = \sqrt{D \cdot \tau} \]
With:
- \( L \) – diffusion length
- \( D \) – diffusion coefficient
- \( \tau \) – carrier lifetime
The animation makes this visible: at short lifetimes, carriers barely leave their point of origin. At longer lifetimes, the probability increases that carriers reach the space charge region and are separated by drift – which manifests as a voltage at the contacts.
Note: This is an analogy model. Diffusion is represented as motion with a random but constant direction; in reality, the direction changes continuously. Further simplifications concern recombination and the exact field distribution within the SCR.
Physical Background
When a semiconductor is illuminated, electrons are excited from the valence band into the conduction band, generating electron-hole pairs. Carriers that diffuse to the space charge region are separated by the built-in electric field and directed towards their respective contacts – producing a usable electric voltage. The lower the recombination rate and the greater the diffusion length, the more efficiently the solar cell operates.
Overview
| Title | Solar Cell – Diffusion and Drift |
| Target audience | Teachers and lecturers |
| Features | Full-screen mode lossless scaling large displays and projectors supported |
| Licence | MIT |