Output of a Solar Cell

None needed.

Investigation 1

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 (V_oc) 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 V_oc changes. The solar cell measured for the setup shown below, for example, had a V_oc = 1.2 volts in full sunlight.

Investigation 2

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.

Investigation 3

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 I_sc. Like a wire, an ammeter has very low resistance, so will register a measurement similar to a short circuit.

Note the I_sc through the solar cell. In our setup, the solar cell measured I_sc = 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.

Investigation 4

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, P_max, is the product of the V_oc and I_sc.

P_max = V_oc * I_sc = 1.2V * 0.48A = 0.58W

The actual power, P_actual, 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, S_i, 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, P_s, intercepted by a cell this size is

P_s = S_i * A = 1,000W/m² * 0.0025m² = 2.5W

The maximum theoretical efficiency, E, of the solar cell is estimated to be

E = P_max / P_s = 0.58W / 2.5W = 23%

The actual efficiency of the solar cell when providing power to the motor was

E = P_actual / P_s = 0.12W / 2.5W = 4.8%

Solar cells transfer energy from the photons in sunlight to the electrons in the solar cell. The more photons of sunlight absorbed by the solar cell, the greater the electric current. That’s why the short-circuit current depends so strongly on the orientation of the solar cell. The maximum voltage, on the other hand, is fixed by the material the solar cell is made of. Solar cells also have an internal resistance, which reduces the voltage available at the terminals when current flows.

Electric power is the product of the voltage across a device and the current through that device. Engineers use the theoretical power to characterize a solar cell. The power provided by the sun per unit area, known as solar intensity, is approximately 1,000 Watts per meter squared. This value is reduced by clouds, haze, and when the radiation from the sun has to travel a longer path through the atmosphere (such as at sunset or sunrise). However, it is a good approximation around midday with a clear sky.

The solar cell has energy losses, so does not covert 100% of the solar power to electricity. Some of the light is reflected from the surface of the solar cell, and some of the light is blocked by the metal lines on top of the solar cell that conduct electricity through the cell. To make a solar cell more efficient, the manufacturers reduce reflected light and minimize cell shading by keeping the area of metal conductors small. Energy is also lost if the energy of the photon is higher than what the solar cell can accept.

To determine how well a solar cell really works, it is important to measure the efficiency with which a solar cell converts the power of sunlight into electric power. There are additional losses when you attach a load to the solar cell. In this Snack, you measured the actual power delivered to a motor, and calculated how the efficiency changed when a load was attached.

An important engineering challenge is to try to maximize the power delivered to the motor using solar power. One way to do this is to combine two solar cells in series or in parallel to see if one combination provides more efficient power conversion than the other (see photos below for ideas). If you have two multimeters, you can set up one to measure current and one to measure voltage.