Short-Circuit Current

The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). Usually written as ISC, the short-circuit current is shown on the IV curve below.

iv curve
IV curve of a solar cell showing the short-circuit current.

The short-circuit current is due to the generation and collection of light-generated carriers. For an ideal solar cell at most moderate resistive loss mechanisms, the short-circuit current and the light-generated current are identical. Therefore, the short-circuit current is the largest current which may be drawn from the solar cell.

The short-circuit current depends on a number of factors which are described below:

  • the area of the solar cell. To remove the dependence of the solar cell area, it is more common to list the short-circuit current density (Jsc in mA/cm2) rather than the short-circuit current;
  • the number of photons (i.e., the power of the incident light source). Isc from a solar cell is directly dependant on the light intensity as discussed in Effect of Light Intensity;
  • the spectrum of the incident light. For most solar cell measurement, the spectrum is standardised to the AM1.5 spectrum;
  • the optical properties (absorption and reflection) of the solar cell (discussed in Optical Losses); and
  • the collection probability of the solar cell, which depends chiefly on the surface passivation and the minority carrier lifetime in the base.

When comparing solar cells of the same material type, the most critical material parameter is the diffusion length and surface passivation. In a cell with perfectly passivated surface and uniform generation, the equation for the short-circuit current density can be approximated as:

$$J_{SC}=q G\left(L_{n}+L_{p}\right)$$

where G is the generation rate, and Ln and Lp are the electron and hole diffusion lengths respectively. Although this equation makes several assumptions which are not true for the conditions encountered in most solar cells, the above equation nevertheless indicates that the short-circuit current depends strongly on the generation rate and the diffusion length.

The short circuit current, ISC, is the short circuit current density, JSC, times the cell area:

$$I_{SC}=J_{SC} A$$

Silicon solar cells under an AM1.5 spectrum have a maximum possible current of 46 mA/cm2. Laboratory devices have measured short-circuit currents of over 42 mA/cm2, and commercial solar cell have short-circuit currents between about 28 mA/cm2 and 35 mA/cm2.

Solar cell short circuit current as a function of bandgap
In an ideal device every photon above the bandgap gives one charge carrier in the external circuit so the highest current is for the lowest bandgap.

Illuminated Current and Short Circuit Current (IL or Isc ?)

IL is the light generated current inside the solar cell and is the correct term to use in the solar cell equation. At short circuit conditions the externally measured current is Isc. Since Isc is usually equal to IL, the two are used interchangeably and for simplicity and the solar cell equation is written with Isc in place of IL. In the case of very high series resistance (> 10 Ωcm2) Isc is less than IL and writing the solar cell equation with Isc is incorrect.

Another assumption is that the illumination current IL is solely dependent on the incoming light and is independent of voltage across the cell. However, IL varies with voltage in the case of drift-field solar cells and where carrier lifetime is a function of injection level such as defected multicrystalline materials.