Could someone explain to me in details (or point me to appropriate sources) why the short-circuit current is less than the photogenerated current, resulting in an internal quantum efficiency less than unity?
Dear Omar
The short-circuit current ISC 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).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 IL are identical. In the case of high series resistance (> 10 Ωcm2) Isc is less than IL .
The ISC is also reduced due to recombination which results in loss of photogenerated charge carriers.
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.
Dear Omar, to undestand the difference between the IL and Isc you can use also the electrical equivalent circuit of PV cells or module. You can refer to single or double diode model, in any case you can see that when the positive and negative electrodes of a PV cell are short cirtcuited, IL flows through the Rsh and Rs that in this case are in parallel. The diode(s) is (are) open as the voltage applied to it (them) is lower than its threshold.
Thank you for the clear explanation and different viewpoints Dr. Vasilopoulou and Dr. Tina.
The short-circuit current of a solar cell is less than the light-generated current because of the internal resistance of the cell, i.e. because of the internal leakage current.
Consider the equivalent circuit of a solar cell. The internal resistance is represented by a series resistance and a shunt resistance. A part of the light-generated current flows through the shunt resistance; this current is not available at the external terminals of the cell.
The thickness of the solar cell should be equal (or less) to the width of depletion layer. Larger than this makes unnecessary resistance, which is a cause of low short-circuit current than photo current. Another one is matching problems of the heterojunction which hampered to increase the short-circuit current. You can anneal your sample after making solar cell to solve matching problems.
In Solar Cell Short Circuit Current is equals to the Light generated current - Reverse saturation current(exp (qv/kt) - 1). If Solar cell is ideal or no reverse saturation current/ leakage current/recombination current (opposite current) only that time short circuit current will be equal to the photgenerated current. But practically its not possible.
This opposite current is due to thermal losses, resistance losses and recombination losses. So always try to minimize these losses in your solar cell.
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