An Investigation into the Factors, which affect the Voltage Output of a Solar Cell

Hayley Gardner

My aim is to try and find out how much the voltage is affected when exposing different sized areas of a solar cell to a light source. From this I will also establish the energy of each photon and approximately, the number of freed electrons, which can make an electric current flow.

I know that light consists of packets or quanta of energy called photons. When electromagnetic radiation such as light shines on materials (usually metals), which emit electrons the light photons containing energy are captured by the electrons. This means that the electron absorbs the energy from a photon thus allowing it to escape from the surface of its material. For each light photon landing on the surface of a material which emits electrons, an electron can be ‘free’.

I know that solar cells contain thin wafers of silicon protected by glass. When light photons strike the surface of the solar cell, energy from the photon is absorbed by an electron. The electron needs a certain minimum energy to escape the material but excess energy or surplus energy is transferred to the electron as kinetic energy. Thus creating an electric force, this pushes the electrons around a circuit, known as an electric current, when the solar cell is connected up. The size of the voltage depends on the number of flowing or ‘freed’ electrons.

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From looking at the methodology of my preliminary experiment I know that there are many other variables. In my preliminary experiment my main aim was to investigate as many other variables as I could think of, which might affect the output or voltage. I did this because by considering all the other variables I am able to regulate them, keeping my experiment as fair as possible. The other variables or independent variables that I have noted from my preliminary investigation include:

Keeping the light source (lamp or ray box) the same distance from the solar cell through out all experiments that I carry out. I will place the solar cell 15cm away from the light filament. I will establish this distance by ensuring that my light source contains a transparent bulb.
Keeping the colour of my light source the same. From my knowledge of the photoelectric effect I know that the shorter the wavelength of light, the higher the energy of photon. Therefore a photon of ultraviolet for example which has an extremely short wavelength will have about twice as much energy of a infrared photon of light which has a very long wavelength. I will use white light in my investigation.
Keeping the solar cell at the same angle to the light source through out the experiment.
Keeping the light emission by the light source the same. I will not alter the electrical input to the light source. As some of the output energy is emitted as light photons.

I know that the formula for calculating electrical power is P = IV or

power = voltage x current.

From my preliminary experiment I have also taken in to account other ways in which to make my results more reliable and easier to record. They are placing a white piece of paper under the solar cell and the light box and drawing around them. This enables me to see exactly where my apparatus are situated therefore allowing me to keep the distance between them the same. Another way in which I will make my results more reliable is by placing black paper to act as a ...

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power = voltage x current.

The Results of my Preliminary Experiment

As my Preliminary Experiment table shows I was able to record ten different voltage readings.

My method for collecting my results as shown above are as follows:

Collecting all equipment and arranging it as shown on the diagram. I then drew around my apparatus on to white paper and surrounded my apparatus with black paper.
I measured accurately 10 points on to black card, which were situated 1cm apart. I then began to cover up the solar cell, which was facing the ray box. I did this 1cm at a time until the whole solar cell had been covered, careful as I held the card not to obstruct the light shinning on the rest of the solar cell that was revealed.
I recorded the voltage every time I covered up the solar cell.

Equipment List Safety Precautions

1 Solar Cell Do not set the voltage to high on the power pack

Voltmeter Do not handle apparatus with wet hands

Power Pack Don’t unplug apparatus while socket is switched on

Ray Box

White and Black Paper

Ruler

Connecting Wires

From my preliminary experiment I am able to make some modifications to my final experiment. These are to repeat my experiment 3 times. By doing this I will ensure a more accurate results and reduce the amount of anomalous results and I am also able to record averages of my results.

I will also record one extra result which is having all of the solar cell revealed. From this I will be able to work out the percentage decrease as more and more of the solar cell is covered.

My preliminary results prove to me that as the amount of solar cell surface, which is revealed, is decreased the voltage will also decrease. They are directly proportionate to one another. Therefore I predict the average results will show that as the surface area of the solar cell is decreased by half, the voltage will also decrease by half. My prediction is based on my scientific knowledge because if the solar cell’s area is decreased by half then the light which contains packets of energy known as photons will only be able to strike half the solar cell. Hence producing only half the out put voltage a whole solar cell area would produce.

Results Table 1

Results Table 2

Results Table 3

The Same or Very Similar results (only 0.01 differences)

Results Table to show the Average of my Experiments

In this experiment I have collected enough results to support my predictions. I have repeated the experiment three times, and I have calculated the average voltage for each experiment. I also have an overall voltage for each of the experiments, as I have taken an average reading, by adding results from experiment 1, 2 and 3 and dividing the total by 3.

My results appear to be as I predicted when looking at both the individual experiments and the average voltage output when I progressively revealed 1cm of Solar cell at a time.

I predicted that the average results would show that as the surface area of the solar cell is decreased by half, the voltage will also decrease by half.

My prediction has not just proved correct for just the average result table. It appears that all three individual experiments follow the general trend of the average results, therefore supporting my prediction.

When observing my individual results tables I have noticed that I have collected some anomalous results. The difference in voltage between 4cm and 5cm is significantly bigger than that of all the other differences that I have collected. The same anomalous results occur through all of my individual experiments and therefore are also reflected in my average results table and my graph.

This continual occurrence suggests to me that I have not created this anomalous result through inaccurate testing. I think the quality of the equipment I have used has affected my results.

I have examined the equipment that I have used and I have observed that the solar cell is made up of individual strips of solar panel. Therefore I predict the panel, which lies at the 4cm, and 5cm line is faulty. If I were to investigate this experiment further I would test if my prediction, to see if my justification of my anomalous results was correct.

I have also noticed another significant difference in result table 2 at 8cm and 9cm. However when looking at result tables 1 and 3 the same significant difference does not occur. This supports the need for calculating averages. I think that I have done some inaccurate testing and if I were to develop this investigation I would ensure accurate testing throughout my experiments.

I have observed all three results tables and have noticed that there are many results which are exactly the same or extremely close with a difference of only 0.01 of a voltage. An example of these results is at 0cm on tables 2 and 3 they both read 1.72 volts. This could suggest I have collected the majority of my results through very accurate testing and paying a lot of attention to the factors, which I have listed under fair testing. I have highlighted the same or similar results.

My graph clearly shows the anomalous result between 4cm and 5cm. I would have expected it to follow the trend and produce approximately 1.25 volts. I have drawn a line of best fit and I have noticed from this that only 2 points sit on the line of best fit. This would conclude that my results are fairly scattered but still show the general trend that I predicted, of the volts increasing when the area of the solar cell also increases.

From looking at my graph I feel that I still do not have enough evidence to support why my results do not follow my prediction in detail but only in a general trend.

Therefore I have decided I am going to further my investigation and test the voltage output of every single panel of cell which make up the whole solar cell. From my scientific knowledge I am going to predict that I will find that each of the solar panels will differ in terms of sensitivity. This scientifically means that the material, which emits the electrons, is blocking out the light containing photons coincidentally not freeing the electrons and therefore not making a powerful circuit. This has probably occurred through cheap and poor manufacturing of the solar cell.

I have decided that I am going to investigate the voltage output of each panel of the solar cell through following the basic plan of my original experiment.

The method that I will under in this further investigation will be as follows:

I will collect all the same equipment, which I have used in my original investigation. I will do this so my investigations remain fair and accurate. I will also arrange the equipment according to the diagram, which I also used for my original experiment.
I will use white paper and draw around my apparatus to mark their positions so I can check the distance between the light source and the solar panels always remain the same. I will also surround my apparatus with black paper to block out any unwanted light.
I will accurately measure and draw the sizes of the different panels on the solar cell on black card. I will then begin to individually reveal each of the solar panels, which will face the ray box.
I will not work across the solar cell revealing the solar panels until the whole solar cell is exposed to the light. I will reveal no more than one panel at a time. I will be careful as I hold the card not to obstruct the light, which is shinning on the reveal solar panel.
I will record the voltage every time I reveal a solar panel.
When I have recorded the voltage for the individual solar panels I will record the voltage when the whole solar panel is revealed. I will do this so I check that the each solar panel result is accurate.
I will repeat this process three times and find the average for each result.

I predict that the solar panel which lies at 4cm will be considerably higher than the panel which lies at 5cm. This prediction is based on my results that I have collected in my original experiment. I also predict that the average volts for the first and the last panel of the solar cell will also be significantly lower than that of the other panels. I think this because these are the smallest panels on the whole solar cell.

The Average Results of my Further Investigation

I think I have collected enough results to support my predictions. The first and the last solar panel are clearly lower than every other panel. The first is significantly lower, but I expected this as the first panel measures in at just under a 1cm and all the other panels though different in size measure in at over 1.2cm. This proves to me that the smallest size can make a huge difference in the voltage output.

Considering this I think the voltage output difference for each of the solar panels has proved why my results followed the general trend that I predicted but don’t follow it in detail. I think for the original test to be completely fair I would of had to make sure that the solar panels were all the same size and all the same sensitivity. Therefore generating a more reliable line, which probably would have, sat closer to the line of best fit on my graph.

The other prediction, which I had made, was also proved correct. The panel, which lies at 4cm, has a voltage output of 0.47 while the panel, which lies at 5cm, has a voltage output of 0.58. The sensitivity of these two panels is very significant therefore having a big effect on my original experiment and henceforth producing an anomalous result.

From my results I have recognized a general trend however I have not established clear pattern because my results are very scattered and they also contain anomalies. I think this is due to the poor quality equipment that I used because I feel that I have followed a clear and precise method.

I think that if I extended my investigation further I could eventually find a clear pattern of results and have my previous predictions fully supported. If I were to extend this investigation again, I would use a solar cell containing panels of all the same size and all the same sensitivity, which would produce extremely similar voltages. I would also use equipment of a higher standard therefore improving the reliability of my results. I would use the method that I followed in my original experiment and would repeat it three times and find an average set of results.

I would also plot my further set of results on a graph and compare it with that of my original experiment and I would notice the difference and see to what extent the size and sensitivity of the individual panels have on the voltage output of the whole solar cell.