The main technical parameters of solar cells
The main technical parameters of solar cells include: open circuit voltage, short circuit current, maximum power, voltage and current at maximum power, fill factor, conversion efficiency, equivalent series resistance, etc. The values of the above parameters can be measured by a solar cell sorter.
1. Open circuit voltage (Uoc)
The open circuit voltage of a solar cell refers to the terminal voltage when the temperature is 25°C, the solar cell is placed in AM1.5 (according to Baidu Encyclopedia: AM's full English name is air-mass, and its Chinese translation is atmospheric mass, and its value is the actual distance of light passing through the atmosphere divided by the vertical thickness of the atmosphere.
AM1.5 means that the actual distance of light passing through the atmosphere is 1.5 times the vertical thickness of the atmosphere. Because most countries such as China, the United States, and Europe are at latitudes with spectral conditions of AM1.5, the spectral conditions of AM1.5 are used as the standard) and the solar cell is exposed to a light source with an irradiance of 100mW/cm2, and the solar cell is not connected to a load (i.e., open circuit). The open circuit voltage of commonly used solar cells (silicon solar cells) is about 0.6V.
The open circuit voltage is related to the temperature. As the temperature rises, the voltage drops slightly. The open circuit voltage has nothing to do with the cell area.
2. Short-circuit current (Isc)
The short-circuit current of a solar cell refers to the output current when the cell is short-circuited when the temperature is 25°C and the solar cell is placed under the spectrum conditions of AM1.5 and the irradiance is 100mW/cm2.
The short-circuit current is related to the cell area, light intensity, and temperature. The larger the cell area and the stronger the light intensity, the larger the short-circuit current; as the temperature rises, the short-circuit current increases slightly.
3. Maximum output power (Pm)
The working voltage (personal understanding: the working voltage is the voltage when the load is connected) and the working current (personal understanding: the working current is the current when the load is connected) of the solar cell change with the change of the load resistance. The working voltage and working current values corresponding to different resistance values are plotted into a curve to obtain the volt-ampere characteristic curve of the solar cell.
If the product of the working voltage and working current corresponding to a certain resistance value is greater than the product of the working voltage and working current corresponding to all other load resistance values, that is, the product of the working voltage and working current corresponding to the resistance value is the largest, then the product is the maximum output power (Pm). The working voltage at this time is the optimal working voltage (Um), and the working current at this time is the optimal working current (Im).
The relationship between the maximum output power, the optimal working voltage, and the optimal working current is: Pm=UmIm.
IV. Filling factor (FF)
The filling factor of a solar cell refers to the ratio of the maximum output power to the product of the open circuit voltage and the short circuit current. The formula for the filling factor is: FF=Pm/(Uoc·Isc)=Um·Im/(Uoc·Isc). The value of the filling factor is less than 1, and the solar cell with a larger filling factor value is better.
V. Conversion efficiency (η)
The conversion efficiency of a solar cell refers to the ratio of the maximum output power to the incident light energy. The conversion efficiency is the maximum energy conversion efficiency of a solar cell. The formula for conversion efficiency is: η=Pm/(A·Pin)=FF·Uoc·Isc/(A·Pin). Among them, A is the area of the cell, Pin is the incident light power per unit area, Pin=1000W/m2=100mW/cm2. Conversion efficiency is related to the cell structure, cell material properties, cell operating temperature, radioactive particle radiation damage, and environmental changes.
VI. Equivalent series resistance
The equivalent series resistance inside the solar cell can affect its forward volt-ampere characteristics (personal understanding: because the PN junction is in the cut-off state when reversely energized, the equivalent series resistance does not affect the reverse volt-ampere characteristic curve of the solar cell, but only affects the forward volt-ampere characteristic curve of the solar cell) and short-circuit current. The increase in series resistance can reduce the fill factor and conversion efficiency of the solar cell.