Working outside, in a sunny place (or indoors, under a 100-watt incandescent bulb), set the multimeter to the DC voltage scale so it can measure a few volts. Using the red clip lead, connect the positive terminal of the meter to the positive terminal of the solar cell. Then use the black clip lead to connect the common (COM) terminal of the meter to the negative terminal of the solar cell (see photos below).
Measure the open circuit voltage (Voc) across the solar cell. This is the voltage when no current is flowing through the cell. Since no current flows through a perfect voltmeter, a voltmeter measures the open circuit’s voltage.
Tilt the solar cell in sunlight or lamplight and notice how the Voc changes. The solar cell measured for the setup shown below, for example, had a Voc = 1.2 volts in full sunlight.
Flip over the solar cell (see photo below), and watch what happens to the meter reading. In our setup, the reading of 0.16 volts shows what happens when almost no light reaches the collectors.
Flip the solar cell face-up again so the light hits it directly, and set the meter to “DC amperes” on a scale that will measure a few amperes of electrical current. Use a red clip lead to connect the positive terminal of the meter to the positive terminal of the solar cell. Then, use a black clip lead to connect the common (COM) terminal of the meter to the negative terminal of the solar cell. (Note that there may be a separate terminal for measuring amperes. If that’s the case, you’ll need to move the input lead to that terminal.)
The maximum current that a solar cell can produce occurs when a wire is connected across the terminals. This is called the short-circuit current, or Isc. Like a wire, an ammeter has very low resistance, so will register a measurement similar to a short circuit.
Note the Isc through the solar cell. In our setup, the solar cell measured Isc = 0.48 amps in full sunlight (your results may vary).
Try tilting the solar cell. How does the current change?
In the image below, we again show the connections on the back of the solar cell.
To investigate a solar-powered motor, put a piece of masking tape on the shaft of the electric motor so it creates a tiny flag (see photo below). Make sure the motor still spins freely with the masking tape in place.
Connect the two terminals of the solar cell to the two terminals of the electric motor. (The photos below show the front and back of the solar cell so you can see the connections.) Flip the solar cell face-up and notice how the motor shaft spins when it’s in the sun. Tilt the solar cell to maximize motor speed, and then tilt it away from its maximum orientation. (Be careful not to shade the solar cell as you hold it.) Notice that the motor speed is greatest when the solar cell is oriented perpendicular to a line from the sun to the solar cell.
Measure the voltage across the motor as it runs at maximum speed by connecting the meter as you did in Investigation 1 while leaving the motor connected. This array of connections is called a parallel circuit (see photo below).
Then set the multimeter to measure current, and connect it in a single loop with the motor and solar cell (see photo below). This arrangement is referred to as having the meter in series. In our experiment, the solar cell and motor had V = 1.1 volts and I = 0.11 amps.
Calculating the power of a solar cell
The power of a solar cell is the product of the voltage across the solar cell times the current through the solar cell. Here’s how to calculate the power the solar cell delivers to the motor:
The maximum theoretical power from our solar cell, Pmax, is the product of the Voc and Isc.
Pmax = Voc * Isc = 1.2V * 0.48A = 0.58W
The actual power, Pactual, delivered by the solar cell to the motor, in practice, is equal to the voltage across the motor, V, times the current through the motor, I.
P = V * I
For the solar cell and motor we used, the electrical power delivered to the motor was
P = 1.1V * 0.11A = 0.12W
Calculate the solar cell’s efficiency
The efficiency of the solar cell is the electrical power out divided by the solar power in. You can use the estimate for the maximum theoretical power to calculate the maximum theoretical efficiency, E, of the solar cell.
Here’s how to calculate the efficiency of the solar cell using the sun:
First, calculate the solar power arriving at the solar cell by multiplying the intensity of the sun by the area of the solar cell. The solar intensity from the sun, Si, over a given area at the surface of the earth is approximately 1,000 watts/m2.
Use a ruler to measure the active area, A, of your solar cell (see photo below).
The cell in this experiment measured 5 cm by 5 cm.
A = 5cm * 5cm = 25cm2 = 0.0025m2
The solar power, Ps, intercepted by a cell this size is
Ps = Si * A = 1,000W/m2 * 0.0025m2 = 2.5W
The maximum theoretical efficiency, E, of the solar cell is estimated to be
E = Pmax / Ps = 0.58W / 2.5W = 23%
The actual efficiency of the solar cell when providing power to the motor was
E = Pactual / Ps = 0.12W / 2.5W = 4.8%