## The basic characteristics of solar cells: polarity/current-voltage characteristics

**Polarity of solar cells**Silicon solar cells usually have two types, N

^{+}/P-type structure and P

^{+}/N-type structure. When the solar cell is exposed to light, the polarity of the output voltage is: the side of the P-type electrode is positive, and the side of the N-type electrode is negative. In actual use, it is necessary to keep the solar cell working in a positive state.

In the battery polarity model, the first symbol is accompanied by a positive sign, which indicates the conductivity type of the semiconductor material of the front light layer of the solar cell, that is, P+ indicates that the top layer is a P-type material and N^{+} indicates that the top layer is an N-type material; the second symbol is N and P indicate the conductivity type of the semiconductor material on the backside of the solar cell. The electrical performance of solar cells is related to the characteristics of the semiconductor materials used to manufacture the cells. When irradiated by sunlight or other light, 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, it must be ensured that the solar cell is working in a positive state. When the solar battery is used in combination with other power sources, if the positive pole of the external circuit is connected to the P electrode of the battery, and the negative pole is connected to the N electrode of the battery, the external power supply provides a forward bias to the solar cell: if the positive pole of the external power supply is connected to the battery’s When the N electrode is connected, and the negative electrode is connected to the P electrode, the external power supply provides reverse bias to the solar cell.

**Current-voltage characteristics of solar cells**As mentioned earlier, a solar cell is essentially a diode that can produce a photovoltaic effect. In order to better understand the electrical characteristics of solar cells, the equivalent circuit of a model composed of circuit elements with known characteristics can be used to approximate its characteristics. An ideal solar cell can be simply equivalent to a diode and a current source parallel circuit, but the actual solar cell also needs to consider the series resistance and parallel resistance in the circuit model. The equivalent circuit is shown in Figure 1. Among them, RL is the external load of the solar cell, Rs is the series resistance of the solar cell, and RsH is the parallel resistance of the solar cell.

Isc is the short-circuit current of the solar cell, that is, the current at both ends of the solar cell when the external load is zero. It is defined as the current that flows through both ends of the solar cell when the solar cell is exposed to a standard light source when the output terminal is short-circuited. The short-circuit current of the solar cell represents the power generation capacity of the solar cell. In the actual measurement, an ammeter with small internal resistance (<10) can be used to connect to the two ends of the solar cell. The value of short-circuit current Isc is related to the type of solar cell, and is also related to the area of the solar cell. The larger the area, the greater the value of short-circuit current Isc. Usually the short-circuit current Isc value of lcm2 silicon solar cell is 16~30mA. The short-circuit current Isc value of the solar cell is also proportional to the irradiance of the incident light, and is usually slightly lower than the nominal value in actual use. In addition, the short-circuit current Isc value of solar cells will also be affected by the ambient temperature. When the ambient temperature rises, the short-circuit current Isc value will increase slightly, generally 78μA/°C for silicon solar cells.

Another main indicator of solar cells is the open circuit voltage. The so-called nominal open circuit voltage Uoc of a solar cell is the output voltage value of the solar cell when the two ends of the cell are open under the condition of 10mW/cm^{2} of light radiation. In the actual measurement, the open circuit voltage of the battery can be measured with a high internal anode (>500Ω) DC millivoltmeter. The open circuit voltage of the solar cell has nothing to do with the size of the cell area, and its size is related to the spectral irradiance. Under the condition of 10mW/cm2 of light radiation, the open circuit voltage of silicon solar cells is 450~600mV and can reach up to 690mV. When the irradiance of the incident light changes, the open circuit voltage of the solar cell is proportional to the logarithm of the incident spectral irradiance. When the ambient temperature rises, the open circuit voltage of the solar cell will decrease, generally 2~3mV/° C.

Rs is the series resistance of the solar cell, which is mainly composed of the cell’s body resistance, surface resistance, electrode conductor resistance, and contact resistance between the electrode and the silicon surface. For an ideal solar cell, the series resistance Rs should be as small as possible. The equivalent series resistance of a silicon solar cell will affect its forward volt-ampere characteristics and short-circuit current, but has no effect on the open circuit voltage. In addition, the increase in series resistance will cause the solar cell’s The fill factor and photoelectric conversion efficiency are reduced.

RSH is the parallel resistance of solar cells, which is caused by unclean silicon wafers and defects in the substrate. The existence of parallel resistance causes leakage current of solar cells, mainly including PN junction leakage current, surface leakage current along the edge of the battery, and bridge leakage current formed along microscopic cracks or grain boundaries after metallization. The parallel resistance RsH affects the open circuit voltage of the solar cell, and the decrease of RSH will reduce the open circuit voltage, but has no effect on the short-circuit current. In actual production, it is always hoped that the parallel resistance RsH is as large as possible. For solar cells with a small area (2~15cm^{2}), the parallel resistance is usually very large, with a typical value of 103~105Ω. Therefore, the production process does not require much control. But for large area (70-100cm^{2}) solar cells, especially polysilicon solar cells, the parallel resistance RsH is relatively low, only a few ohms, so it will affect the open circuit voltage and output power of the solar cells.

The series resistance Rs affects the short-circuit current, and the increase of Rs will reduce the short-circuit current, but has no effect on the open circuit voltage; the decrease of RsH and the increase of RsH will reduce the fill factor and photoelectric conversion efficiency of the solar cell. The series resistance affects the forward volt-ampere characteristics of the solar cell, so that the current is greater than the ideal value when the forward bias voltage is low, and the volt-ampere characteristics deviate from the exponential relationship when the forward bias voltage is increased: the leakage current generated by the parallel resistance affects the reverse characteristic and the forward direction. The characteristic of low bias voltage makes the current greater than the ideal value when the forward bias voltage is low, so that the reverse current cannot be saturated. When the reverse bias voltage is large, the current and voltage deviate from the exponential relationship.

When the load is connected externally, because the parallel resistance RSH and series resistance Rs will have a relatively small effect on the circuit output, they can be ignored when calculating the ideal circuit, so that the current of the load can be simplified as

I_{L}=I_{sc}– I_{D}

In the formula, ID is the current flowing through the ideal diode, which can be calculated by the following formula

In the formula, I0 is the reverse saturation current of the equivalent ideal diode inside the solar cell, which is related to the nature of the solar cell’s own material, and represents the maximum recombination ability of the solar cell for photo-generated carriers. Usually the reverse saturation current is constant, and Not affected by changes in irradiance; UD is the terminal voltage of the equivalent diode of the solar cell; q is the electron charge, 1.6X10﹣19C; k is the Boltzmann constant, 0.86X10-4eV/K; T is the thermodynamic temperature , Unit K; A is the curve constant of the solar cell equivalent diode PN junction.

Therefore, the characteristic curve equation of the solar cell can be obtained

When IL=0, the open circuit voltage Uoc of the solar cell can be obtained, which can be expressed by the following formula

Under low light conditions, I_{sc}《I_{0}, the open circuit voltage Uoc of the solar cell can be approximated

Under strong light conditions, I_{sc}》I_{0}, the open circuit voltage Uoc of the solar cell can be approximated

When the sunlight is weak, the open circuit voltage of the solar cell changes linearly with the irradiance of the light, and when the sunlight is strong, it changes logarithmically. Usually the open circuit voltage of silicon solar cells is between 0.5 and 0.58V.

According to the relationship between the current and voltage characteristics of the solar cell, the dark characteristic and the volt-ampere characteristic curve of the light can be obtained . The upper curve is the volt-ampere characteristic curve of the solar cell when there is no light, which is called the dark characteristic curve of the solar cell; the lower curve is the volt-ampere characteristic curve of the solar cell when it is illuminated. Since the parallel resistance is usually relatively large, the load current is almost unaffected, and the short-circuit current Isc value can be approximately constant. Therefore, as long as the dark characteristic curve is moved down the current axis by the magnitude of the short-circuit current Isc, the output volt-ampere characteristic curve of the solar cell can be obtained. The volt-ampere characteristic curve during illumination moves to the fourth quadrant. Usually, in order to intuitively understand the characteristics of the solar cell, the coordinate transformation can be carried out. The original current value is inverted, that is, the image is flipped upward according to the voltage axis, so that the solar cell volt-ampere is obtained. The characteristic curve is more common. The intersection of the volt-ampere characteristic curve of such a solar cell with the current axis is the short-circuit current Isc, and the intersection with the voltage axis is the open circuit voltage Uoc.

Normally, the volt-ampere characteristic curve of changing coordinates, the output current of the solar cell under light is positive. Figure 2 shows the volt-ampere characteristic curve of the solar cell under different irradiance conditions. It can also be seen directly from the figure that the silicon solar cell The short-circuit current of the battery is proportional to the solar irradiance. Therefore, a simple method of measuring the short-circuit current of a standard solar battery can be used to determine the solar light conditions when the silicon solar battery is working, which is convenient for design and analysis.