4.8 Photodiodes and Solar Cells

Table of Contents - 1 2 3 4 5 6 7 8 9 R S ¬ ­ ®

4.8 Photodiodes and Solar cells

Photodiodes and crystalline solar cells are essentially the same as the p-n diodes which have been described in this chapter. However the diode is exposed to light which yields a photocurrent in addition to the diode current so that the total diode current is given by: where the additional photocurrent, Iph, is due to photogeneration of electrons and holes which each are pulled into the region where they are majority carriers. These carriers cause a photocurrent which opposes the diode current under forward bias. As a result the diode can be used as a photodetector - using a reverse or zero bias voltage - as the measured photocurrent is proportional to the incident light intensity or the diode can be used as a solar cell - using a forward bias - as electrical power is generated under such conditions.

The p-n diode solar cell

Solar cells are typically illuminated with sunlight and are intended to convert the solar energy into electrical energy. The solar energy is in the form of electromagnetic radiation, more specifically "black-body" radiation, due to the fact that the sun has a temperature of 5800 K. The radiation spectrum has a peak at 0.8 eV, while a significant part of the spectrum is in the visible part of the spectrum (400 - 700 nm). The power density is approximately 100 mW/cm2.

Only part of the solar spectrum actually makes it to the earth's surface because of scattering and absorption in the earth's atmosphere, while the angle with respect to a normal to the surface - and therefore also the power density - depends on the time of the day, the time of the year and the latitude of a specific location.

Of the solar light which does reach a solar cell only photons with an energy larger than the energy bandgap of the semiconductor generate electron-hole pairs. In addition one finds that the voltage across the solar cell at the point where it delivers its maximum power is less than the bandgap energy in electron volt. The overall power conversion efficiency of single crystalline solar cells ranges from 10 to 30% yielding 10 to 30 mW/cm2.

The calculation of the maximum power of a solar cell can be illustrated using the above expression for the diode current and by using the solar cell convention which consideres a current coming out of the cell to be positive as it leads to electrical power generation. The power generated depends on the solar cell itself and the load connected to it. As an example a resistive load is shown in the diagram below.


solar.gif
The current and the power as function of the forward bias voltage across the diode are shown in the figure below for a photocurrent of 1 mA:


solar.xls - solar1.gif
On the current-voltage we identify the open-circuit voltage, Voc, as the voltage across the illuminated cell if the current is zero and the short-circuit current, Isc, as the current through the illuminated cell if the voltage across the cell is zero. The short-circuit current is close to the photocurrent while the open-circuit voltage is close to the turn-on voltage of the diode as measured on a current scale similar to that of the photocurrent.

The power equals the product of the diode voltage and current and at first increases linearly with the diode voltage but then rapidly goes to zero around the turn-on voltage of the diode. The maximum power is obtained at a voltage labeled as Vm with Im being the current at that voltage.

The fill factor of the solar cell is defined as the ratio of the maximum power of the cell to the product of the open-circuit voltage, Voc, and the short-circuit current, Isc, or:

4.7 ¬ ­ ® 4.9
© Bart J. Van Zeghbroeck, 1997