Rough trials showed the following:
∙ Using the thin layer polycrystalline cell, I found that pressing contacts onto the surface of the cell scratched it, so I taped wire pickups onto the top and bottom with transparent tape, as shown on the pictures. I noticed that the readings obtained for the current produced by the cell were inconsistent between ammeters (see Experiment Five). I therefore changed my method for the amorphous and monocrystalline cells and used the voltage readings and the resistance of the voltmeter to work out the power output of the cell by P = V2/R.
∙ Having the cell and bulb standing on the table and changing the horizontal distance between them meant that the top and bottom of the cell got a different amount of light, and the distribution changed with distance. To ensure that the same amount of light was reaching each region of the cell (as near as possible), I held the bulb upside-down on a clamp stand, with the cell under it, as shown in Figure 2.
∙ The easiest way to change the height of the bulb, was to draw a line around it with a felt pen at the same height as the filament, as shown in Figure 1. I then used the shadow projected onto the ruler taped to the retort stand to measure the height.
∙ To ensure that the bulb was centred above the cell, I used a fabric tape measure to measure the distance from the line on the bulb to each corner of the cell.
The experiment set-up that I used is shown in Figure 2.
Table 1 is a summary of the results (averages) used to plot the graphs of the efficiencies of the three types of cell. The graph for the amorphous silicon cell is shown overleaf, the other two in Appendix 1. The result ringed with red on the graph is obviously an anomaly, so I repeated the experiment, and the result is ringed with purple on the graph. It is possible that the readings causing the anomaly were for 50 mm rather than 150 mm separation, but were entered in the table under 150 mm.
Error analysis
All measurements of distance taken during the experiment are with rulers accurate to 1mm. However, for the height of the cell, the shadow from the line drawn around the bulb was about 2mm thick, so the accuracy is estimated as +/- 0.004 m in column B of the results tables shown in Appendix 1. The voltmeter and ammeter used to measure the power output of the cell were accurate to 0.5 units, so the combined error of the voltage and current reading is estimated to be +/-1 for the power into the bulb. The digital multimeter was theoretically accurate to 0.01 units, but fluctuated during use as an ohmmeter so I estimated its accuracy to about one tenth of the value it was reading and rounded the value to the nearest integer, hence the value of 1E+07 for the resistance of the voltmeter and inaccuracy of +/- 1E+06. For the voltage (and current) readings, and the calculated efficiency, I used the deviation of my five readings to estimate the error. The error for the intensity of light falling on the cell (power into cell) was calculated by putting the values of error estimated for the distance, area etc. into the equation used to calculate the intensity of light. The calculated and estimated inaccuracies are shown on the results table in Appendix 1 and represented on the graphs in the form of error bars. Inclusion of constant errors like the bulb efficiency and the fact that the light is not spread out over a sphere due to the fact that part of the bulb was covered by the wooden base. I estimated these errors as about 20% and included them as error bars in the graphs.
Conclusion
The results show that, for monocrystalline and amorphous silicon cells the efficiency does indeed decrease with an increase in intensity. The effect that increasing the intensity had on the efficiency of the cell decreased as the intensity increased, resulting in a curved graph with asymptotes. The case for the polycrystalline is exactly the opposite- the efficiency increases with the light intensity and then plateaus out.
The reason that the amorphous and monocrystalline cell acted in this way could be due to the physical factors limiting the rate of conduction and the way in which boron-phosphorus p-n junctions work. If electrons can only move slowly, the number of electrons in the lowest filled band of the n-type semiconductor will depend on the rate at which electrons move to it and on the rate at which they are removed. If photons are supplied more quickly, this results in more electrons being excited and moving to the lowest unfilled band of a phosphorus atom, therefore less electrons in the lowest filled band as they are removed from here more quickly. Since there are fewer electrons available for photoexcitation, more of the photons hitting the cell will not successfully excite an electron, so the efficiency will be lower.
The reason that the thin-layer polycrystalline cell acted differently to the monocrystalline and amorphous cells could be due to the increase in conductivity of semiconductors with an increase in current flowing through them. The efficiency of the polycrystalline cell is lower than that of the other types of cells, even more so when the efficiency calculated with the same calculation as the other two cells (Efficiency 2 on the table) is considered. The low activity of these cells means that the number of electrons in the lowest unfilled band ready to be excited is never in short supply, so the drop in efficiency at higher intensities does not occur. Instead, the competing factor has an effect: p-n junctions become better conductors, the greater the current flowing through them, so the greater the intensity the better the cells are at causing a current to flow. The levelling-off of the graph could be due to the photon rate effect competing with this.
It should be noted that the V2/R method of calculating power and efficiency for the polycrystalline cell gave much smaller calculated power (see "Power out of cell" column on polycrystalline table in Appendix 1). This could be suggestive that the value measured for the resistance of the voltmeter is too high, therefore the real efficiency values being much larger. I therefore used V*I, as shown in "power out of cell 2"This does not, however, explain the fact that the amorphous cell came out as the most efficient cell. This high efficiency could be explained by the fact that the cell is smaller and its surface area harder to measure than the other cells, so the error may be the factor that causes this.