The animation illustrates how random and directed movements of particles lead to different spatial distributions.
A random movement is called diffusion, a directed movement is called drift.
Drift and diffusion are crucial for understanding solar cells.
Free charge carriers are generated in a semiconductor material when it is exposed to light. These charge carriers move randomly through the material, a process known as diffusion.
There are two types of charge carriers: free electrons and holes. Holes are formed when electrons are displaced from their original positions in the atomic lattice by light. Holes can also move through the material, as they are filled by electrons from neighboring atoms. Hole current and electron current flow in opposite directions.
Within a solar cell, an electric field forms at the PN junction, creating a region known as the space charge zone (SCZ). This field arises from the differing doping levels on either side of the junction. In the SCZ, charge carriers experience drift, meaning their motion is guided by the electric field rather than random diffusion. Electrons drift toward the positively charged side, while holes move toward the negatively charged side.
In solar cells, free electrons and holes have a statistically limited lifetime—the time between their generation and recombination. The longer this lifetime, the farther they can travel. The diffusion length refers to the average distance a charge carrier moves during its lifetime.
L: Diffusion length
D: diffusion coefficient
τ: the lifetime of the electron or hole
The diffusion coefficient indicates the speed at which the electrons or holes move in a material (referred to as diffusion speed in the animation). The diffusion coefficient depends on the temperature, the material and the properties of the diffusing particles. As a rule, the diffusion coefficient is given in square centimeters per second (cm²/s).
The animation shows that the diffusion length also decreases with a very short service life. The particles hardly leave the place where they were generated by light irradiation. A similar effect occurs when the diffusion speed is low. The longer the lifetime, the greater the probability that the particle will reach the space charge zone.
Important note: This is a pure analogy model. In reality, a diffusion movement is not a linear movement.
Requirements
| Plattform | PC/Mac or Tablet |
| Resolution (min) | 1280 x 720 |
Further information
The GUI elements used in this animation are published under the MIT license: