# What are the performance parameters of solar cells?

The main performance parameters of solar cell modules are: short circuit current, open circuit voltage, peak current, peak voltage, peak power, fill factor and conversion efficiency. The concept of these performance parameters is the same as the main performance parameters of silicon solar cells, but differs in specific values.

(1) Short circuit current (Isc)
When the solar cell is placed under the illumination of a standard light source and the output terminal of the solar cell module is short-circuited, the current flowing through both ends of the solar cell is the short-circuit current of the cell module. The method of measuring the short-circuit current is to connect the two ends of the solar cell with an ammeter whose internal resistance is less than 12. The short-circuit current varies with the light intensity.

(2) Open circuit voltage (Uoc)
Under the illumination of the light source, when both ends are open (the positive and negative poles of the solar cell module are not connected to the load), the output voltage of the solar cell is the open-circuit voltage. The unit of open circuit voltage is V. The open circuit voltage of a solar cell module varies with the number of cells connected in series. The open circuit voltage of a module with 36 cells connected in series is about 21V. The open circuit voltage of the battery can be measured with a high internal resistance DC millivoltmeter.

(3) Peak current (Im)
The peak current is also called the maximum working current or the optimum working current. The peak current refers to the working current when the solar cell module outputs the maximum power, and the unit of the peak current is A.

(4) Peak voltage (Um)
The peak voltage is also called the maximum working voltage or the optimum working voltage. The peak voltage refers to the working voltage when the solar cell outputs the maximum power, and the unit of the peak voltage is V. The peak voltage of the module varies with the increase or decrease of the number of cells in series. The peak voltage of the module with 36 cells in series is 17-17.5V.

(5) Maximum output power
If the selected load resistance value can maximize the product of output voltage and current, the maximum output power can be obtained, which is represented by the symbol Pm. The working voltage and working current at this time are called the optimum working voltage and optimum working current, which are represented by symbols Um and Im respectively, Pm=UmIm.

The product of the rated voltage and the rated current is the rated power. The rated output power is the maximum output power under normal conditions (can work for a long time). The rated output power of a solar cell is related to the conversion efficiency. Generally speaking, the higher the conversion efficiency of a battery module per unit area, the greater the output power. The current conversion efficiency of solar cells is generally between 14% and 17%. The output power of each square centimeter of cells is 14 to 16mW, and the output power of solar cell modules per square meter is about 120W.

(6) Peak power (Pm)
Peak power is also called maximum output power or optimum output power. Peak power refers to the maximum output power of the solar cell module under normal working or test conditions, that is, the product of the peak current and the peak voltage: Pm=ImUm. The unit of peak power is watts. The peak power of a solar cell module depends on the solar irradiance, the solar spectral distribution and the operating temperature of the module, so the measurement of the solar cell module should be carried out under standard conditions. The test standard for solar cells: the spectral distribution when the air quality is AM1.5 (for specific regulations, please refer to the relevant national and international standards in my country), the incident solar irradiance is 1000W/m², and the temperature is 25°C. The output power of the solar cell under this condition is defined as the peak power of the solar cell.

(7) Fill factor (ff)
Fill factor refers to the ratio of the maximum power of the solar cell module to the product of the open-circuit voltage and the short-circuit current: ff=Pm/(Ise/Uoc). It reflects the variation characteristics of the output power of the battery with the load, and the fill factor is one of the important parameters to characterize the pros and cons of solar cells. The larger the fill factor, the better the solar cell performance.

The fill factor mainly depends on the following factors.

①It increases with the increase of the forbidden band width of the battery material. For example, the fill factor of high-quality arsenide solar cells can often reach 0.87-0.89, while silicon cells can only reach 0.75-0.82. Generally, the fill factor of solar cell modules is between 0.5 and 0.8.

②When the series resistance increases, the bypass resistance decreases, and there are defects such as defects and impurities in the PN junction, the ff will become smaller. For the same solar cell, within a certain range of light intensity, the fill factor increases with the decrease of light intensity.

③ When the temperature rises, although the working current of the solar cell increases, the working voltage will drop, and the latter will drop more, so the total output power will drop, so try to keep the solar cell at a lower temperature. Work.

(8) Conversion efficiency (η)
The conversion efficiency of a solar cell refers to the maximum energy conversion efficiency when the optimal load resistance is connected to the external loop, which is equal to the ratio of the output power of the solar cell to the energy incident on the surface of the solar cell, that is, n=Pm (the peak power of the battery module). )/[A (effective area of ​​battery module) Pin (incident light power per unit area)], where Pin=1000W/m²=100mW/cm².

The photoelectric conversion efficiency of solar cells is an important parameter to measure the quality and technical level of the battery. It is related to the structure, junction characteristics, material properties, operating temperature, radiation damage of radioactive particles and environmental changes of the battery. Width is the most direct relationship. First, the forbidden band width directly affects the maximum photogenerated current, that is, the size of the short-circuit current. Since the photon energy in sunlight varies, only those photons whose energy is larger than the forbidden band width can generate photo-generated electron-hole pairs in the semiconductor, thereby forming a photo-generated current. Therefore, if the forbidden band width of the material is small, the number of photons smaller than it is larger, and the obtained short-circuit current is larger; on the contrary, if the forbidden band width is larger, the number of photons larger than it is smaller, and the obtained short-circuit current is smaller. However, it is not suitable for the forbidden band width to be too small, because the photons with energy greater than the forbidden band width convert the remaining energy into heat energy after exciting the electron-hole pair, thus reducing the utilization rate of the photon energy. Secondly, the forbidden band width directly affects the size of the open circuit voltage. The magnitude of the open circuit voltage is inversely proportional to the magnitude of the reverse saturation current of the PN junction. The larger the band gap, the smaller the reverse saturation current and the higher the open circuit voltage.