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Factors Affecting Conversion Efficiency of Solar Cells

Author: Source: Datetime: 2016-09-28 16:04:43
The open-circuit voltage U increases with the increase of the band width E, but on the other hand, the short-circuit current density J decreases as the band width E increases. As a result, it is expected that the peak of the efficiency of the solar cell will occur anywhere in a certain E.


With the increase of temperature, the efficiency of N decreased. I is very sensitive to temperature T, the temperature also play a major role in U. For Si, for every 1℃increase in temperature, U decreases by about 0.4% of the room temperature, and N also decreases by about the same percentage.
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Photo - carrier lifetime

For a semiconductor solar cell, the longer the composite lifetime of the photo-generated carriers, the greater the short-circuit current I will be. In indirect bandgap semiconductor materials such as Si, a considerable number of carriers can also be generated at a distance of 100 μm from the PN junction. If the lifetime of the photogenerated carriers in these positions is greater than 1 μs, they can be collected by the PN and transported to the external circuit. In a direct bandgap material, such as GaAs or Gu2S, a compound lifetime of 10 ns is sufficient. The long lifetime of the carriers also reduces the dark current and increases U. The key to achieving long life is in the preparation of materials and lifePo4 battery pack production process, to avoid the formation of composite center. In the process, appropriate and often related to the process, you can make the composite center removed, thus extending the life.

Light intensity

Focusing sunlight on solar cells allows a small solar cell to generate a large amount of electrical energy. Assuming that the intensity of light is concentrated X times, the input power per cell area and I will increase by a factor of X, and U will increase with time. The output power is increased by more than X times, and the conversion efficiency is improved by the converging result.

Doping concentration

Another factor that has a significant effect on U is the semiconductor dopant concentration. The higher the doping concentration, the higher the U. At present, the doping concentration is about 10 ^ 6cm-3 in Si solar cells, about 10 17cm-3 in direct-gap materials solar cells, in order to reduce the series resistance, the former diffusion region doping concentration is often Higher than 10 ^ 19cm-3, so the effect of heavy doping in the diffusion zone is more important.
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Surface recombination rate

The low surface recombination rate contributes to an increase in I, and the recombination rate of the front surface is difficult to measure and is often assumed to be infinite. One type of cell, called a back surface field, was designed to produce a solar cell using a P-type material, which diffused a P + additional layer on the back of the cell before the metal contacts.

Series resistance

In any actual solar cell, there are series resistance, the source can be lead, metal contact grid or lifePo4 batteries body resistance. However, in general, the series resistance mainly comes from the thin diffusion layer. The current collected by the PN junction must pass through the surface layer and then into the nearest metal wire. This is the route where the resistor is present. Obviously, the series resistance can be reduced by the metal wire. The effect of a certain series resistance R is to change the position of the I-U curve.

Metal gate lines and light reflection

The metal grids on the front surface can not transmit sunlight. In order to maximize I, the area occupied by the metal gate lines should be minimized. In order to make the R small, the metal grid lines are generally made of dense and thin shapes. Because of the presence of sunlight reflection, not all of the light can enter the Si. The reflectivity of bare Si surface is about 40%. The use of an antireflective film is effective in reducing the reflectivity.

The conversion efficiency of a lifePo4 battery is the ratio of its output power to the input power. In order to obtain high efficiency, it is desired to have a large short-circuit current, a high open-circuit voltage, and a large fill factor, and a short-circuit current is large if the solar cell is made of a material having a small band gap E. Good manufacturing process and a good battery design, even if the smallest carrier complex, but also make short-circuit current increases. Solar cells with E large material, then have a high open-circuit voltage. The fill I-U factor is a measure of the steepness at the inflection point of the curve, which can be made smaller by series resistance. Normally, when the open-circuit voltage is high, FF is also large. The conversion efficiency increases with the increase of light intensity and the decrease of temperature. With E values between 1.2-1.6eV made of  lifePo4 batteries, is expected to achieve maximum efficiency. Direct-bandgap semiconductors for thin-film cells are preferred because they absorb photons near the surface.

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