# General characteristics of solar cells

(1) Polarity of solar cells

Solar cells are generally made of P+/N type structure or N+/P type structure, as shown in Figure 1(a) and Figure 1(b). Among them, the first symbols, namely P+ and N+, represent the conductivity type of the semiconductor material on the backside of the solar cell.

The electrical properties of a solar cell are related to the properties of the semiconductor material used to make the cell. When the sunlight is irradiated, the polarity of the output voltage of the solar cell, the P-type side electrode is positive, and the N-type side electrode is negative.

When the solar cell is connected to an external circuit as a power source, the solar cell works in a forward state. When the solar cell is used in combination with other power sources, if the positive electrode of the external power source is connected to the P electrode of the solar cell and the negative electrode is connected to the N electrode of the solar cell, the external power source provides forward bias to the solar cell; if the positive electrode of the external power source is connected to the N electrode of the solar cell The N electrode of the solar cell is connected, and the negative electrode is connected to the P electrode of the solar cell, and the external power supply provides a reverse bias voltage to the solar cell.

(2) Current-voltage characteristics of solar cells

The circuit and equivalent circuit of the solar cell are shown in Figure 2(a) and Figure 2(b). Among them, RL is the external load resistance of the battery. At that time, the measured current was the short-circuit current ISC of the battery, that is, the current that flows through the two ends of the solar cell when the output terminal is short-circuited when the solar cell is placed under the illumination of a standard light source. 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 ISC is related to the area of the solar cell, the larger the area, the greater the ISC value. Generally speaking, the ISC value of a 1cm2 solar cell is about 16~30mA. The ISC value of the same solar cell is proportional to the irradiance of the incident light; when the ambient temperature rises, the ISC value rises slightly. When the temperature is increased by 1°C, the ISC value will increase by about 784A. When RiL-00 is used, the measured voltage is the open circuit voltage Voc of the battery (the solar battery is placed under the light source of 100mW/cm2, and when the two ends are open, the solar battery The output voltage value is called the open circuit voltage of the solar cell), and its value can be measured by a DC millivoltmeter with high internal resistance. The open circuit voltage of a solar cell is related to the spectral irradiance and has nothing to do with the size of the cell area. Under the solar spectral irradiance of 100mW/cm2, the open circuit voltage of the single crystal silicon solar cell is 450~600mV, and the highest is 690mV. When the incident spectral irradiance changes, the open circuit voltage of the solar cell is related to the incident spectral irradiance. When the ambient temperature increases, the open circuit voltage value of the solar cell will decrease. Generally, the Voc value will decrease by about 2~3MV for every 1°C increase in the temperature. ID (diode current) is the total diffusion current through the P-N junction, in the opposite direction to the ISC. Rs is the series resistance.

It is mainly composed of the volume resistance, surface resistance, electrode conductor resistance and contact resistance between the electrode and the silicon surface of the battery. Rsh is the shunt resistor, caused by unclean edges of the silicon wafer or defects in the body. An ideal solar cell. Rs is small. Rsh is large. Since Rs and Rsh are connected in series and parallel respectively in the circuit, they are both negligible when doing ideal circuit calculations. At this time, the current IL flowing through the load is:

In the formula, I0 is the saturation current (reverse saturation current) of the solar cell in the absence of light; q is the electron charge; k is the Boltzmann constant; A is the diode curve factor.

When IL=0, the voltage V is Voc, which can be expressed by the following formula:

According to the above two formulas, the current-voltage relationship curve of the solar cell can be obtained. This curve is simply referred to as the I-V curve or the voltammetry curve, as shown in Figure 3. In the figure, curve (a) is the dark volt-ampere characteristic curve of the diode, that is, the I-V curve of the solar cell when there is no light: curve (b) is the I-V curve of the battery after being illuminated, which can be changed from the I-V curve when there is no light to the first. The N-quadrant displacement Isc amount is obtained. After coordinate transformation, the commonly used illumination I-V curve can be finally obtained.

Iop is the optimum load current, and Vop is the optimum load voltage. Under this load condition, the output power of the solar cell is the maximum.

In the current-voltage coordinate system, the load corresponding to this point is called the optimal load.

(3) Fill factor of solar cells

To evaluate the output characteristics of solar cells, there is another important parameter, called fill factor (FF).

Fill factor is an important parameter to evaluate the output characteristics of solar cells. The higher its value, the closer the output characteristic curve of solar cells is to a rectangle, and the higher the conversion efficiency of the cells.

The series and parallel resistances have a great influence on the fill factor. The larger the series resistance is, the more the short-circuit current drops, and the fill factor is also greatly reduced; the smaller the parallel resistance, the greater the current and the much lower the open circuit voltage. The fill factor is also much lower, so the fill factor of high-quality solar cells is usually greater than 0.7