I read some literature on solar cell, all of which state that the voltage output of schottky-type solar cell will be reduced with rising temperature. But what i have learned from literature in terms of the physical originals is quite unclear. Any one can suggest or comment on this physical mechanism? Thanks.
The Schotty solar cells has a dark current and a photocurrent as the pn solar cells except their reverse situation current may larger than the pn solar cells.
The cell current under illumination can be expressed by the relation
I = Iph - Id,
Id = IS ( exp V/Vt -1)
Id the dark current, IS is the reverse saturation current, V is the cell voltage and Vt the temperature equivalent voltage.
In Schottky diodes the the reverse saturation current is determined by the thermionic emission of electrons fro metal to semiconductor.It can be expressed by:
Is= A* T^2 EXP - PHI/ Vt, where phi is the potential barrier from the metal side.
According to this mode the dark current increases with the temperature for a given voltage. Or for a given current the voltage decreases by increasing the temperature.
The open circuit voltage Voc = Vt ln ( Iph/Is). As the temperature increases Is increases and Voc decreases as observed from the measurements.
So, their behavior is similar to the pn solar cells..
Prof. Zekry,
Thanks for your detailed answer. Following your answer, the dark current describes the emission of electron from metal into conduction band of n-type semiconductor. So the number of electron appearing in the conduction band will be increased by the emission of metal electron. Larger current will be collected from back electrodes if Ohmic contact is provided. But here no physics about cellvoltage is presented based on your interpretation, using the band diagram of metal/semiconductor contact. Based on your theory presented above, i suppose that the increased temperature will lift up the fermi level of metal, leading the potential difference between fermi level in semiconductor and that in metal smaller compared to low temperature case. Do you think my understanding is correct?
Not at all Shi-Jun,
Your understanding is okay except the fer mi level in metals does not change with temperature. Only the distribution of electrons around Fermi level will change by temperature such the by increasing the temperature more electrons will be excited to higher energy level in the metal and so a greater fraction of electrons will be able to overcome the barrier crossing it to the semiconductor. It is merely a heating effect like the thermionic emission into vacuum.
wish you success.
This makes sense. Thanks Prof. Zekry.
But what confuses me is about the transfer of hole left in the valence band of semiconductor after light excitation. How is transported into metal side, so that the photoexcited electron in semiconductor can back through external circuit to recombine it in the metal side?
The equation that relates the current through a schottky junction to the voltage across it has saturation current component. This component relate exponentially with temperature and the barrier height. The voltage V=Vt lnIsc/Isat; when the temperature increases saturation current increases thus reducing the voltage output! Barrier height varies for Schottky diodes but is constant for pn junction type.
- Badges
- Science topic