Electrical and Computer Engineering Undergraduate Laboratory
ECE 494 - Electrical Engineering Laboratory III

# Part B - Semiconductor Devices Experiment Series 3 – Solar Cell

## Solar Cell - Pre-laboratory assignment

1. Draw the equivalent circuit representing a solar cell. Use a current source representing the photoelectric current and resistors representing the shunt and series resistances.
2. Simulate I-V characteristic of the device. Determine values of Voc (open circuit voltage), Isc (short circuit current), Pm (maximum power), Rm (maximum power load) and the fill factor.

## Solar Cell Experiments

1. Measurements in dark. Measure the solar cell I-V characteristic (in dark) using Agilent U2722A source measure unit. Position the cell face down so that it is not exposed to light. In these measurements you treat the solar cell as a diode. Use the same methods for obtaining forward and reverse bias characteristics as in the diode experiment series (1). In the reverse bias use voltage up to 0.6 V. The purpose of these measurements is to determine the diode parameters: saturation current, ideality factor and the two resistances.
2. Measurements with the incandescent light source. Position the halogen lamp approximately 8 cm (3 inches) directly above the cell. Make sure that the lamp and the cell positions are stable and do not change during these measurements. Do not keep the solar cell under the light for long time since the lamp heats-up the cell, which affects its characteristics. Plan your measurements, program the source measure unit, and turn on the lamp only when needed.

Use Agilent U2722A source measure unit to obtain the I-V curve in the fourth quadrant of the I-V plot. Now the cell generates current (the dark I-V curve is shifted down below the V axis) and the source measure unit acts as a current sink with the voltage that can be programmed (stepped). Plot the I-V curve, using the solar cell convention showing the generated current positive. Plot also the power (P-V curve), find the maximum power Pmax and the corresponding current Ipm and voltage Vpm.

To obtain the open circuit voltage (VOC), connect a voltmeter (multimeter) across the cell terminals. The high impedance of the voltmeter will give you a true value of the open circuit voltage. Compare this measured VOC with the corresponding value form the I-V curve. Find also the short circuit current Isc from this curve. Now calculate the fill factor FF.

Calculate also the specific resistance Rpm corresponding to the maximum power point. The solar cell connected to a load resistance Rl = Rpm will deliver maximum power. This will be used to measure the solar cell efficiency in part (3).
3. Measurements with the solar radiation simulator. Select a resistance as close as possible to Rpm from the resistance substitution box or individual resistors and connect it as a load resistor across the solar cell terminals. Place your solar cell under the calibrated solar illuminator and measure the voltage across the load resistor corresponding to the maximum power. Note the reading of the sun illuminator output in units of Sun. From this measurement and the illuminated cell area you will be able to determine the efficiency of the solar cell. Measure also the open circuit voltage (VOC) with the solar radiation simulator and compare it with the one measured with the incandescent light source.

## Report and Analysis

Present clearly all schematics of the experimental circuits, showing the diode, resistors and the instruments used for measuring voltage and current. Extract parameters of the diodes: saturation current, ideality factor, equivalent series and shunt resistances from measurements (1).

Present the I-V plot of the solar call. Report the values of VOC, ISC, Pmax, Rpm and FF (fill factor) from the measurements (2).

Estimate the solar cell efficiency from the measurements (3). The solar cell used in this lab has the area of 4 cm2 (20mm x 20 mm). The solar irradiation unit of 1 Sun = 1.0 kW/m2.

## Points to discuss:

• How the values of the solar cell parameters compare with the parameters of the diodes tested in experiments series 1? What may explain these differences?
• Is the measured value of the solar cell efficiency reasonable?