The photoelectric conversion efficiency of solar panel cells is an important parameter to measure the quality and technical level of the solar panel. It is related to the structure, junction characteristics, material properties, operating temperature, radiation damage of radioactive particles, and environmental changes of the solar panel. How to choose the best solar panel for cloudy conditions?

First, the forbidden bandwidth directly affects the maximum photogenerated current, that is, the size of the short-circuit current. Due to the different energy of photons in sunlight, only those photons whose energy is larger than the forbidden bandwidth can generate photo-generated electron-hole pairs in the semiconductor, thereby forming a photo-generated current. Therefore, if the forbidden bandwidth of the material is small, the number of photons smaller than it will be larger, and the obtained short-circuit current will be larger; on the contrary, if the forbidden bandwidth is larger, the number of photons larger than it will be smaller, and the obtained short-circuit current will be smaller. But the bandgap metric is converted into thermal energy, which reduces the utilization of photon energy.

Secondly, the forbidden bandwidth directly affects the open circuit the magnitude of the 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 bandgap, the smaller the reverse saturation current and the higher the open-circuit voltage.

The main performance parameters affecting the outdoor solar panel components 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 panel is placed under the illumination of a standard light source, and the output end of the solar panel module is short-circuited, the current flowing through both ends of the solar panel 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 1Ω. The short-circuit current varies with the light intensity.

(2) Peak current (Im)

The peak current is also called the maximum working current or the best working current. The peak current refers to the working current when the solar panel cell module outputs the maximum power, and the unit of the peak current is A.

(3) Peak voltage (Um)

The peak voltage is also called the maximum working voltage or the best working voltage. The peak voltage refers to the working voltage when the solar panel cell outputs the maximum power, and the unit of the peak voltage is V. The peak voltage of the module changes with the increase or decrease of the number of cells in series. The peak voltage of the module with 36 solar panel cells in series is 17-17.5V

(4) Open-circuit voltage (Uoc)

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

(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=Um X Im

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 panel cell is related to the conversion efficiency. Generally speaking, the higher the conversion efficiency of a solar panel module per unit area, the greater the output power. The current conversion efficiency of solar panel cells is generally between 14% and 17%. The output power of each square centimeter of solar panel cell 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 optimal output power. The peak power refers to the maximum output power of the solar panel cell module under normal working or test conditions, that is, the product of the peak current and the peak voltage: Pn=Im X Um. The peak power of a solar panel 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 of solar panel cells: the spectral distribution when the air quality is AM1.5 (for specific regulations, please refer to the relevant national and international standards), the solar radiation emitted by people is 1000W/m², and the temperature is 25°C. The output power of the solar panel cell under this condition is defined as the peak power of the solar panel cell.

(7) Fill factor

Fill factor (ff) refers to the ratio of the maximum power of the solar panel cell module to the product of the open-circuit voltage and the short-circuit current: f=Pm/(IsU.) It reflects the variation characteristics of the output power of the solar panel with the load, and the fill factor is one of the important parameters to characterize the pros and cons of solar panel 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 bandwidth of the solar panel material. For example, the fill factor of high-quality gallium arsenide solar cells can often reach 0.87 to 0.89, while that of silicon cells can only reach 0.75 to 0.85. Generally, the fill factor of solar panel cell modules is between 0.5 and 0.8.

②The series resistance increases, the shunt resistance decreases, and when there are defects such as defects and impurities in the PN junction, ff will become smaller. For the same solar panel 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 panel 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 panel cell working at a lower temperature.

(8) Conversion efficiency (η)

The conversion efficiency of a solar panel 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 panel cell to the energy incident on the surface of the solar cell, that is, η =Pm (peak power of battery module) / [A (effective area of ​​battery module) Pin (incident light power per unit area)], where Pin = 1000W/m² = 100mW/cm²