Aim
To investigate the factors which influence the output from a solar cell. The factors I am choosing to investigate are the light intensity. This will be how much voltage is supplied, and also the distance of the lamp to the solar cell. I have chosen these because I believe these are the most important factors, and well give us the best results. The results will give us definite results and systematic results, such as a linear graph.
Hypothesis
I predict that as the distance between the lamp and the solar cell the less current the solar cell will produce. Secondly I believe that with higher voltage the solar cell will produce more current, because the light intensity of the lamp will increase.
Explanation of Hypothesis
The same amount of energy from the lamp will be given out throughout, however as the light becomes more dilute the further away it moves from the lamp. In a small area around the lamp the light will be concentrated and therefore give more energy to the solar cell. As the energy moves further out becoming more dilute, the same energy is in that area but it is more spread out and therefore the solar cell does not receive all of the energy. Some of the energy is lost to the surroundings. Also our preliminary experiment informed us on how to carry out the experiment and what results we may get.
Apparatus
For the experiment we will require:
- 12 Volt lamp
- Variable low voltage supply
- Solar cell
- Digital voltmeter & digital ammeter
- 0-100 milliamp ammeter
- Meter rules
- Wires
- Crocodile clips
Keeping the Experiment Fair
When testing the different input powers as a variable, I will keep constant the distance, temperature, frequency and orientation of the solar cell.
When testing the distance as a variable, I will keep the input power, temperature, frequency and orientation of the solar cell constant.
Method
- Set up the apparatus as shown in the diagram
- I will check to make sure the constants are fixed to give fair results
- When testing the distance as a variable I will increase the distance by 2 cm each time up to 20cm, keeping the voltage fixed at 12V
- When testing the power input as a variable I will change the input power from 1V – 11 V, keeping the distance fixed at 5cm
- I will record each ammeter, voltmeter and mA reading
- Then repeat I will repeat each experiment to make sure that our results are reliable and not a one off fluke
Results
Varying the input power, distance is fixed
Repeating this experiment
Varying the distance, 12V input power constant
Repeating this experiment
Average of the Output Power
Graphs
Analysis of the graphs
Analysing the graphs for varying the input power, we can clearly see that as the power out increases the output power increases. This shows us that as there is more power (W) produced, the current produced by the solar cell increases (mA). This graph is as we expected because with more the power, the bulb becomes brighter, and this means that more current is produced by the solar cell.
The graph showing the varying distance is more difficult to analyse. The line shows that as the distance increases, the current produced by the solar cell decreases. This is as expected because as the distance increases, the light intensity becomes less concentrated and less energy reaches the solar cell to be converted. We would like to see a linear relationship though, showing that as the decrease in current is proportional to the increase in distance. We can get a straight line graph by inversing the relationship.
Now we can see from the new table that I have divided each original distance by 1. For example 2 cm becomes 1/2 which equals 0.5 cm. This gives us an inverse relationship. The graph for this table looks like this:
This graph shows us a straight line, which proves to us that the relationship cannot be something different such as 1/r3. A straight line gives definitive results, we can now say that the distance from the bulb to the solar cell is inversely proportional to the amount of current the solar cell produces.
Conclusion
We can conclude from the experiment that as the light intensity decreases the current produced by the solar cell decreases. Therefore as the power is increased and the light intensity increases the current produced by the solar cell increases. We found there is an inversely proportional relationship between the distance between the bulb and the solar cell, to the amount of current produced by the solar cell.
We can find the light intensity of the bulb at different voltages by using this equation:
P= IV P= 11V x 1.95 A= 21.45 W
Intensity= P/A 21.45/ 4π r2 21.45 / 4 x π x 52
21.45 / 314.16 = 0.068 W/cm2
As the distance between the solar cell and the lamp increases, the area between them increases. This means that as the light travels in a bigger area it becomes more dilute and more energy is lost to the surroundings. When the area between the two is small then the light is concentrated and the solar cell receives more energy to convert to electricity.
C is the unit candela. Candela is the unit for measuring light intensity. The candela is the luminous light intensity, in a given direction that emits a monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
Monochromatic radiation refers to electromagnetic radiation of a single wavelength.
Solar cells are photovoltaic cells; this means that they convert light into electricity. Photovoltaic cells are made of semiconductors. For example silicon, this is the most commonly used. When a photon of light hits the cells some of it is absorbed and so the energy from the light is absorbed into the semiconductor. This energy knocks an electron free, allowing it to move freely. The photovoltaic cell has electric fields to make the electrons freed to flow in a certain direction. Metal contacts are placed on the top and bottom of the cell which draws the current off for use.
Silicon is has 14 electrons, meaning that its outer shell has only 4 electrons in and is half full. Therefore the silicon shares electrons with 4 other silicon atoms. This makes a crystalline structure. However this is poor for conducting because it has no free electrons. This means that the silicon must be modified before using it in the solar cell.
A silicon semiconductor contains impurities. When impurities are added to silicon, such as phosphorus, it makes the material a much better conductor. This is because the phosphorus has five electrons in its outer shell and not four. This means although the phosphorous still bonds with its neighbouring silicon atoms, there is one free electron which doesn’t form part of the bond. This means when energy is added that it is much easier to knock free a phosphorus electron because it was not bonded. This means that a lot more electrons break free than in pure silicon. These electrons can carry charge and are called free carriers. Making the silicon impure is called doping.
When the silicon is doped with phosphorus, it becomes N-type. This means that it is negative because it has more electrons. When silicon is doped with boron, which has only 3 electrons in its outer shell it forms the P-type junction (positive). There is a hole for each absence of an electron, which makes the silicon positive.
When the two P and N-type silicon are added together they mix and form a barrier at the junction between the two. This makes it difficult for electrons on the N-side to cross to the P-side. This makes equilibrium at the junction, with an electric field separating both sides. The electric field allows electrons to flow from the P-side to the N-side but not in the opposite direction. It allows the current to only flow in one direction, acting as a diode.
(The electrons can only travel from p-type to n-type)
Now when a light photon hits the cell with enough energy it will usually free an electron and a hole. If this occurs near the electric field the field will send the electron to the N-side and the hole to the P-side. This causes the neutrality to be disrupted and now electrons will flow back to the P-side, after being sent to the N-side, to unite with the holes. This will cause a current, and the cells electric field causes a voltage.
A solar cell is only able to absorb around 15% of the suns light energy, because the energy is not monochromatic, but of all different wavelengths. Some wavelengths will not have enough energy to knock free electrons. Therefore the energy just passes through. Some wavelengths have too much energy and only some is used to knock free electrons and the rest is lost. This accounts for 70% of the energy loss. Other reasons for energy loss are the silicon’s high internal resistance.
Some 5% of losses are saved by coating the semiconductor in antireflective coating. This is because silicon is shiny and therefore some energy would be lost due to it being reflected of the surface. This is stopped by the coating.
Evaluation
Our experiment could be improved by repeating the experiment another time to prove that our results were correct. Also our results may have been affected by the way the solar cell was facing. It was not facing the bulb fully at all times.
During the experiment we had to be cautious of working with the electricity, because we may have got an electric shock.