Do you know these three factors that affect the efficiency of solar cells?

Jun 16, 2022 Solar Cell Foundation and Development

Do you know these three factors that affect the efficiency of solar cells?

(1) Theoretical calculation of solar cell efficiency
Solar cell efficiency is one of the most important parameters of solar cells. When the solar cell is working, sunlight illuminates the solar cell to generate a photocurrent, and its current density is J1, which generates a forward bias voltage on the P-N junction of the solar cell, resulting in a dark current current density Jd. After measuring the area of ​​the solar cell, the current density can be converted into the amount of current, and then the conversion efficiency of the solar cell can be calculated by formula 1-3.

(2) Influence of series-parallel resistance
The working circuit diagram of the solar cell can be equivalent to that shown in Figure 1. In this figure, RS is the series resistance, which is mainly composed of the bulk resistance, surface resistance, front and back grid line resistance, grid line and silicon surface contact resistance of the solar cell, etc. Part of the composition; Rsh is a parallel resistance, mainly due to the internal defect of the silicon wafer or the side leakage resistance caused by the unclean edge. In general, the larger Rsh is, the better, and the smaller RS ​​is, the better. In an ideal situation, Rsh can be considered as infinite, Ish can be ignored, and RS can be equal to zero.

Figure 1 - Solar Cell Equivalent Circuit
Figure 1 – Solar Cell Equivalent Circuit

The series resistance and the parallel resistance will directly affect the fill factor of the solar cell efficiency. The larger the series resistance RS and the smaller the parallel resistance, the more the fill current drops and the more the fill factor decreases. Under a certain amount of sunlight, the solar cell generates a certain photo-generated current. If there are regional high-conductivity impurities in the solar cell or the edge etching is not complete, it will have a shunting effect on the generated photo-generated current, so that the P-N junction barrier is exceeded. Effective current decreases. This is equivalent to connecting a resistor in parallel with the P-N junction, which is called a parallel resistor. A parallel resistor is actually an equivalent resistor that doesn’t exist. The smaller the parallel resistance is, the more obvious the shunt effect is. In order to increase the utilization rate of the photo-generated current, it is better to have a larger parallel resistance. The parallel resistance has been accepted as a basic electrical parameter to describe the battery characteristics.

Although the parallel resistance is not a physical resistance, in the solar cell equivalent circuit diagram, the parallel resistance is represented by the physical equivalent resistance. For single-junction (P-N junction) solar cells, a circuit model combining series resistance, parallel resistance, etc. can be given to describe the equivalent working process of the solar cell. There are many factors that affect the actual parallel resistance of solar cell efficiency components, such as short-circuit channels at the edge of the silicon wafer (which can be caused by dirt), poor film deposition quality, and short-circuit channels formed by pinholes in thin-film cells, which will reduce the parallel resistance value. .

In the actual measurement system, we mathematically fit the part of the photovoltaic I-V characteristic curve close to V=0, or directly calculate the reciprocal of dI/dV, which is actually to calculate the differential of the photovoltaic I-V characteristic curve close to V=0. Reciprocal, as a measure of the parallel resistance of the solar cells. In this way, for solar cells or modules with poor fill factor, the series resistance can be qualitatively used for process optimization and analysis; for solar cells or modules with better fill factor, due to the fluctuation of electrical signals measured by the test equipment, The limitation of the mathematical fitting method is that for the same sample, the measured values ​​will be different for many times, so many measurement systems generally do not give the parallel resistance value.

The series resistance can be expressed as follows:
Series resistance = silicon wafer substrate resistance + diffusion sheet resistance + grid line resistance + contact resistance after sintering. In the formula, the substrate resistance is determined by the silicon wafer; the diffused sheet resistance can be adjusted with the change of the junction depth; the grid resistance is mainly determined by the screen printing parameters. If the series resistance is simply reduced, the grid line should be made of materials with low resistivity, and the cross-sectional area should be as large as possible. Large cross-sectional area will increase the shading of the solar cell; the contact resistance mainly depends on sintering.

The series resistance RS affects the short-circuit current. The increase of RS will reduce the short-circuit current, but has no effect on the open-circuit voltage. The parallel resistance reflects the leakage level of the solar cell. The leakage current can theoretically be attributed to the parallel resistance. The parallel resistance affects the open-circuit voltage of the solar cell. The reduction of Rsh will reduce the open-circuit voltage, but it has no effect on the short-circuit current because the fill factor FF is defined as the ratio of the maximum output power Pm to ISCVOC, which is the maximum power rectangular area on the I-V characteristic curve. ImVm and ISCVOC. The ratio of the area of ​​the rectangle. The smaller the series resistance and the larger the parallel resistance, the closer the fill factor of the solar cell is to 1. On the I-V characteristic curve of the solar cell, the curve is closer to a rectangle, indicating that the higher the conversion efficiency of the solar cell, the better the performance of the solar cell. it is good. (If you know more about the factors that affect battery efficiency, welcome to visit Tycorun Battery to discuss and learn with us).

(3) Influence of temperature
The open circuit voltage of a solar cell is determined by the forbidden band width and Fermi level of the semiconductor material of the solar cell. Since the higher the temperature, the closer the Fermi level of the semiconductor is to the valence band, so when the temperature increases, the open circuit voltage of the solar cell decreases. It can be understood that the curve between the temperature and the open circuit voltage is probably a negative slope. The straight line of values, during the solar module certification process, is called to detect the voltage temperature coefficient of the solar module.

The effect of temperature on the I-V characteristic curve is shown in Figure 2. When the temperature rises, the short-circuit current increases. From the change of the area enclosed by the I-V characteristic curve and the coordinate axis, it can be seen that the increase of the specific temperature will reduce the open-circuit voltage. The effect is small, indicating that the temperature increase has a negative impact on the photoelectric conversion efficiency of solar cells. The temperature-short-circuit current curve is a straight line with a slightly positive slope, which is called the current temperature coefficient of the solar cell in the testing of solar module certification.

Figure 2 - The effect of temperature on the I-V characteristic curve, Figure 3 - The effect of temperature on the P-V characteristic curve
Figure 2 – The effect of temperature on the I-V characteristic curve, Figure 3 – The effect of temperature on the P-V characteristic curve

The effect of temperature on the P-V characteristic curve is shown in Figure 3. When the temperature increases, the total output power of the solar cell shows a downward trend (V decreases greatly, while I rises very small, so P=VI decreases)

(4) Influence of power loss
There are many factors that affect the power loss of solar cells, among which band gap mismatch, series and parallel resistance, grid line shading, and anti-reflection coating are the main factors affecting the power reduction of solar cells.

①The band gap does not match. That is, the loss caused by the mismatch between the energy of the photon and the forbidden band width E of the solar cell. For a solar cell of a given material, photons smaller than the forbidden band width cannot excite the positron-negative electron pair to form photogenerated carriers, which do not contribute to the efficiency of the solar cell, but also cause the vibration of the lattice to generate heat, Negative effects on solar cells. Photons larger than the band gap can excite photogenerated carriers, but energy above the band gap will be converted into heat, which can also negatively affect the solar cell. The more the photon energy is higher than the Eg, the greater the proportion of sunlight energy that is lost. In theory, photons with energy just equal to or slightly larger than the forbidden band width Eg are the most suitable for solar cell power generation.

②Losses caused by series and parallel resistance. When the solar cell works for the load, a part of the electric power will be lost due to the certain resistance of the solar cell itself and the grid lines and other components.

③The shielding of the grid line and the encapsulation glass affects the absorption of light energy. At present, the collection of carriers on the light-receiving surface of silicon solar cells relies on very fine silver grids. These grid lines for collecting carriers will cover 5% of the surface area of ​​the solar cell, and then a corresponding proportion of light energy will be lost. Since solar cells are generally very thin, tempered glass is generally used when packaging components. When sunlight passes through the tempered glass, a certain percentage of light energy will be lost.

④ Light energy loss caused by anti-reflection film and passivation layer. An anti-reflection layer is generally applied on the surface of a solar cell to reduce the reflection of light on the surface of the cell. When the thickness of the anti-reflection layer is controlled to make the incident light waves interfere and cancel, the proportion of photons entering the solar cell body can be increased. However, the anti-reflection layers we make are generally aimed at certain wavelengths of light, and photons of other wavelengths will have some adverse effects on the absorption of light energy by solar cells. At the same time, the anti-reflection layer itself will also lose a part of the light energy.

There are many other factors that cause energy loss in solar cells, such as the recombination of carriers by crystal defects in solar cells. If we can fully understand and control these factors, it is beneficial to improve the efficiency of solar cells.

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