Short circuit current is the current passing through a solar cell when voltage is zero across the solar cell, which happens when a solar cell is short circuited. Usually it is denoted Isc
How Isc is generated
The short circuit current results from collection and generation of light generated carriers. For a perfect solar cell with highly efficient loss mechanisms, both short circuit current and the light generated would both be matching. Consequently, the short circuit current is the greatest current that may be drawn out from the solar cell.
Factors affecting Isc
It depends on a multiple factors as stated below:
- The solar cell area: for removing the dependence of the solar cell area, it is typical to use the short circuit current density ,Jsc in units of mA/cm2 instead of the short-circuit current;
- The photons number , which is describes the incident light source power. A solar cell’s Isc is directly proportional to the light intensity;
- The incident light’s spectrum.
- The absorption and reflection and other optical properties of the solar cell
- The probability of collection of the solar cell, that mainly rely on the lifetime of minority carrier lifetime in the base and the surface passivation.
For comparison of solar cells having same type of material, the most important material factors are the surface passivation and diffusion length. In a cell that has a perfectly passivated surface and generation is uniform, the short circuit current equation can be approximately:
ISC = qG (Ln + LP )
G: the generation rate
Ln and Lp: respectively the electron and hole diffusion lengths.
Although several assumptions are made for this equation that are not necessarily true for the conditions in most solar cells, the equation above still show that the short circuit current strongly depend on and the diffusion length and the generation rate.Laboratory devices have measured short circuit currents for commercial solar cell in the range between 28 mA/cm2 and 35 mA/cm2.
Illuminated Current and Short Circuit Current
IL is the current generated from the light inside the solar cell and is the accurate term to use in solar cell equation. The externally measured current for short circuit current is Isc. As Isc is typically equal to IL, the two terms are interchangeably used and for easiness the solar cell equation uses Isc instead of IL. In case of really high series resistance ( greater than 10 Ωcm2) Isc is smaller than IL and the solar cell equation using Isc is incorrect.
A different assumption is that the illumination current ( IL ) only depends on the incoming light and does not depend on the voltage across the cell. Yet, IL differs with voltage in the drift field solar cells case and when lifetime of carrier is a function of the injection level as in the case of defected multicrystalline materials.