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Investigating the effect of intensity on the power of solar cells.

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Introduction

Investigating the effect of intensity on the power of solar cells This experiment involves changing the intensity of light falling on different cells and measuring their power outputs. Higher intensity of light means that there are more photons hitting the surface of the cell per unit area per second. The more hit the cell, the more rapidly the electrons move across the p-n junction, so the larger the emf produced. If the rate of movement of electrons is inhibited, then the greater the rate of supply of photons (intensity), the more will not successfully excite an electron, so the lower the efficiency of the cell. I therefore predict that the higher the intensity, the greater the emf across the cell, the greater the power output of the cell, and the lower the efficiency of the cell. Method The intensity of the light falling on the cell will vary with the separation of the light source and the cell according to the inverse square law: intensity ? 1/d2 so I can vary the intensity by changing the distance between the light source and the cell. I have calculated the amount of light hitting the cell by the ratio of area of the cell to the ratio of area over which light is spread: intensity = power from bulb x surface area of cell 4 ? ...read more.

Middle

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. ...read more.

Conclusion

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. ...read more.

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