Solar Cell Lab

Solar energy was the focus of our lab experiment today. Our goal was to find the relationship between light intensity and voltage and also between light’s wavelength and voltage.
We wanted to prove that greater distance results in a decrease of both light intensity and voltage.

Let me just give you some background information about solar energy and photovoltaics [read solar cells] so you have a basic knowledge of the subject before moving on to the experiment. I’ll start off with voltage and current.

CURRENT is a moving charge
VOLTAGE is the amount of energy per charge required to move charge around a circuit

The relationship between these two is seen in an electrical circuit. In a circuit, like the one pictured below, it is voltage that drives current. To better explain how this works I’ll use an analogy and hopefully this will make a bit more sense. Think of an electric circuit like a pipe system. In a pipe system, a pump pushes water through a closed pipe. This pipe is like the wire of an electric circuit and the pump is the battery. In a pipe, pressure drives water through the pipe much like an electric circuit in which voltage, generated by the battery, drives the current (electrons).

With the relationship between voltage and current explained we can move on to the big stuff. First, there’s photovoltaics which is a fancier way of saying solar cells. These cells provide a direct current of constant electricity. The amount of voltage/current of them is dependent on wavelength of light which is the length of a single cycle of the wave. Light intensity is a measure of the energy of light. Higher intensity means greater current and voltage because of the increase in the amount of generated photons.

Just these simple definitions and explanations should be enough to help develop a greater sense of what’s happening in the experiment. That’s finally out of the way so in the words of Marvin Gaye, “Let’s Get It On.”
Here’s the equipment what we used for the experiment:

• One solar cell (pictured on right)
• One voltage probe
• One NXT adaptor
• NXT with light sensor
• One light source
• Labview VI
• Ruler
• Colored film filters (red, orange, purple, blue)
• Excel sheet

Outline of what we were to do:
Part I: Steps (measurements of voltage)
1. With no light
2. With light 0 cm away
3. Distance 1 (varied by student groups): 5 cm
4. Distance 2 (varied by student groups): 10 cm
5. Distance 3 (varied by student groups): 15 cm
Part II: Re-do with 4 colored filters – red, orange, purple, and blue (used in this order)
Part III: Graph Results – voltage vs. intensity (varies by distance); voltage for 4 different filters

Here are our results with no filters

No Light                        0 cm                     5 cm                       10 cm                       15 cm
-0.01469                           0.47285                  0.46002                  0.42153                      0.30606
-0.02752                           0.54983                  0.42153                    0.39587                     0.30606
-0.04035                           0.47285                  0.4087                     0.47285                      0.25474
-0.04035                          0.51134                    0.4087                     0.48568                      0.30606
0.06229                            0.51134                   0.46002                   0.42153                       0.24191
-0.01469                           0.47285                  0.4087                      0.38304                      0.37021
-0.02752                           0.537                       0.44719                    0.39587                      0.42153
-0.02752                           0.51134                   0.42153                    0.48568                      0.34455
-0.02752                           0.46002                 0.4087                      0.44719                       0.35738
0.01097                             0.49851                  0.39587                   0.39587                       0.44719
avg:-0.01469                    avg:0.499793 avg:0.424096                avg:0.430511             avg:0.335569

The first column shows voltage with no light. You can see that results are mostly negative in number. This means that when no light is present, voltage is at its lowest because light is not as readily detected. The second column shoes results when light is 0 cm away, we held the flashlight directly against the solar cell. Here, voltage, and in turn light intensity, is greatest. This is evidence that the closer/more direct light is to the cell, the greater voltage/light intensity will be. With the following three sequences, voltage/light intensity decreases with increased distance away from the solar cell. This supports our theory that greater distance results in a decrease of both light intensity and voltage.

We used the colored filters in this order: red, orange, purple, blue. We weren’t sure what kind of results to expect. Here are our results using the colored filters:

Red                                   Orange                                   Purple                                        Blue
0.4087                                0.49851                                      0.34455                                         0.39587
0.39587                              0.47285                                      0.39587                                         0.37021
0.49851                              0.47285                                      0.31889                                         0.37021
0.51134                               0.48568                                     0.34455                                         0.35738
0.47285                              0.46002                                     0.2804                                           0.26757
0.48568                              0.42153                                      0.31889                                         0.25474
0.4087                                0.46002                                     0.35738                                         0.26757
0.39587                              0.47285                                      0.29323                                         0.2804
0.46002                             0.47285                                      0.30606                                         0.29323
0.39587                             0.48568                                      0.29323                                          0.29323
avg:0.443341                   avg:0.470284                            avg:0.325305                    avg:0.315041

Filters yielded voltage/light intensity from greatest to least: orange, red, purple, blue. From these results we found that the darker the color value of the filter, the darker light it let through. The solar cell detected most light from the orange and the least from the blue. This means that bright/lighter light is more easily detected when passing through lighter/brighter color values than through darker/deeper color values and that is why the voltage/light intensity was greater for orange and red than it was for purple and blue. Furthermore, filters only transmits one wavelength of color to pass through whereas no filter allows all wavelengths to pass and that is why voltage/light intensity is greater with no filter versus with a filter regardless of color.

This lab gave me a lot of insight into some things I see in my everyday life such as the differences in light that I’ve noticed in head lights versus tail lights. Now I’ll know the background behind those differences in light. Pretty cool.