Abstract

Objective

The goal of this experiment is to determine how changes in incoming light intensity affect the output of solar cells.

Introduction

Solar cells are electronic devices that can transform light energy into an electric current. Solar cells are semiconductor devices, meaning that they have properties that are intermediate between a conductor and an insulator. When light of the right wavelength shines on the semiconductor material of a solar cell, the light creates a flow of electrons. Small solar cells, like the one used in this project, can be used in circuits to charge batteries, power a calculator, or light an LED.

In this project, you'll use the solar cell to power an LED. You'll use a digital multimeter to measure the current flowing through the LED when the solar cell is illuminated by light bulbs with different levels of light output (and different wattages). How will the current produced vary as the intensity of the light falling on the solar cell increases?

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• semiconductor,
• solar cell (also called photovoltaic cell),
• light emitting diode (LED),
• lumens (unit of light intensity),
• voltage (V),
• current (I),
• resistance (R),
• Ohm's law (V = IR, or I = V/R, or R = V/I),
• power.

Questions

• How do solar cells turn light into electricity?

Bibliography

• These two webpages are a good place to start for information on how solar cells work:
  • Aldous, S., 2006. "How Solar Cells Work," HowStuffWorks.com [accessed December 13, 2006] http://science.howstuffworks.com/solar-cell.htm
  • Barron, J. and students, 2004. "Solar Cells." Miramar High School, Miramar, FL. [accessed December 13, 2006] http://library.thinkquest.org/04apr/00215/energy/solar/solar_cells.htm.
• On this page you can build virtual circuits with batteries and resistors, then test your circuit by throwing a switch to light up a bulb. If there's too much current, the virtual light bulb blows up, too little current, and the bulb won't light. When you get the current right, the bulb glows brightly.
  Unknown, 1999. "Ohm's Law." Physics Department, University of Oregon. [accessed December 13, 2006] http://zebu.uoregon.edu/nsf/circuit.html#Ohm
• This page has lots of information about incandescent light bulbs:
  Klipstein, Jr., D.L, 1996–2006. "The Great Internet Light Bulb Book, Part 1," Donald L. Klipstein, Jr. [accessed December 13, 2006] http://members.misty.com/don/bulb1.html.
• This webpage has useful information on LEDs:
  Hewes, J., 2006. "Light Emitting Diodes (LEDs)," The Electronics Club, Kelsey Park Sports College [accessed December 13, 2006] http://www.kpsec.freeuk.com/components/led.htm.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• a small solar cell, such as:
  • part number 215319, rated at 3 V, 100 mA (full sun) from Jameco Electronics, or
• digital multimeter, available at Amazon.com
• an LED (light emitting diode), such as:
  • part number 333851 from Jameco Electronics;
• alligator clip leads, such as:
  • part number 10444 from Jameco Electronics;
• clamp-on lamp with reflector, rated for at least 100 W bulbs (available at your local hardware store),
• 5 light bulbs with different light output, such as:
  • 15 watts (about 110 lumens),
  • 40 watts (about 440–505) lumens),
  • 60 watts (about 840–890 lumens),
  • 75 watts, (about 1150–1210 lumens), and
  • 100 watts (about 1670–1750 lumens).
  • Note that all bulbs should fit the reflector light socket, and
  • all bulbs should have the same type of glass, either all clear or all frosted.

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Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
2. Use the alligator clip leads to connect the LED and the digital multimeter (DMM) in series with the solar cell.
   • Connect the red wire of the solar cell to the longer lead (anode) of the LED.
   • Connect the shorter lead (cathode) of the LED to the red probe of the DMM. Note that some DMMs have separate sockets for the red probe for reading current and voltage. Make sure that the red probe is in the correct socket for reading current.
   • Connect the black probe of the DMM to the black wire of the solar cell to complete the circuit.
   • Set the DMM to read DC current in the 200 mA range.
   • Safety note: Never use this circuit arrangement with a battery in place of the solar cell. Too much current would flow, and the LED would pop like a flash bulb. With a battery, you need a current-limiting resistor in series with the LED. Recommended current for LEDs is typically 20 mA. Since the solar cell produces a fairly small maximum current when illuminated with light bulbs, connecting the LED without a current-limiting resistor is OK.
3. Install the 100 W light bulb in the clamp-on lamp (with the lamp unplugged). Plug it in, turn it on, and place it directly above the solar cell, at a known distance (say, 30 cm).
4. The LED should light up brightly. If it doesn't, re-check all of your connections.
   • Make sure the alligator clips are making good contact on metal.
   • Make sure that the red wire from the solar cell is connected to the longer lead of the LED.
   • Make sure that the red probe of the DMM is in the correct socket for measuring current, and that the DMM is set to read DC current (200 mA range).
5. Record the current reading from the DMM. If the value is too low, switch the meter to the next-lowest current range (usually 20 mA).
6. Turn off the lamp, unplug it, and allow the bulb to cool. Replace the 100 W bulb with the 75 W bulb and repeat steps 3–5. Repeat this for the 60, 40, and 15 W bulbs.
7. Make a graph of LED current (y-axis) vs. light bulb intensity in lumens (x-axis). (You should be able to find the output in lumens printed on the light bulb package.)
8. What relationship do you find between incident light intensity (i.e., light falling on the solar cell) and the output current of the solar cell?

Variations

• Since you have a package of ten LEDs, repeat the experiment with different LEDs. How much does the current vary for different LEDs with the same light input to the solar cell? Calculate the average and standard deviation of the LED current for each input light intensity.
• Power in DC circuits is proportional to the square of the current. Make a graph of current squared vs. light intensity. What relationship do you find?
• For more advanced students, can you devise a way to figure out the internal resistance of the solar cell? Is the internal resistance constant, or does it vary with incident light intensity?
• Test solar cell power output as a function of the angle of the incoming light. Keep the distance and brightness of the light source constant, but vary the angle of the incoming light.
• Another variation would be to measure the power output of the solar cell as a function of the ambient temperature (see the Science Buddies project: A Cool Way to Make Electricity: Solar Cell Power Output vs. Temperature.
• For another experiment that involves measuring current through LEDs, see the Science Buddies project: How Does LED Brightness Vary with Current?
• A more advanced project idea would be to measure the power output of the solar cell as a function of the color of the incoming light. You should do background research to learn how light energy varies with wavelength (color). You will also need to know the spectral (color) characteristics of your light source. For this reason, it would be a good idea to use sunlight for this project. You can get a booklet with 100 color filters for about $10 plus shipping. A search on "color filter booklet" should turn up multiple sources.

• For more science project ideas in this area of science, see Energy & Power Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on:

• Corlett, N., 2003. "How Does the Intensity of Light Affect Output of Solar Cells?," California State Science Fair Abstract [accessed December 13, 2006] http://www.usc.edu/CSSF/History/2003/Projects/J0706.pdf.

Last edit date: 2011-11-04 11:00:00