# Parameters and working characteristics of solar cells

### Solar cell parameters

The main technical parameters of solar cells include open circuit voltage, short circuit current, maximum output power, energy conversion efficiency, power temperature coefficient, voltage temperature coefficient, current temperature coefficient, etc., among which:

(1) Open circuit voltage. When the solar cell is not connected to the load, the measured voltage between the positive and negative poles.

(2) Short-circuit current. The current measured by short-circuiting both ends of the solar cell.

(3) Maximum output power. Connect the solar battery to the load resistor, and there will be current flowing through the resistor. This current is called the battery’s working current or output current (I); the voltage at both ends of the load is called the battery’s working voltage (U). Multiplying the working voltage and working current is the output power P of the solar cell (P=UI). The working voltage and current of the solar cell change with the change of the load resistance. The working voltage and current value corresponding to the load with different resistance values are made into a curve, that is, the volt-ampere characteristic curve of the solar cell, also known as the working characteristic curve. Change the load from zero to infinity, and the working characteristic curve of the solar cell is shown in Figure 1. In Figure 1, Uoc is the open-circuit voltage, and Ik is the short-circuit current. Any point on the curve is called the operating point, and the horizontal and vertical coordinates corresponding to the operating point are the operating voltage and operating current, respectively. When the load resistance is adjusted to a certain value, the curve corresponds to the point M. At this time, the output power of the solar cell is the maximum, and this point is called the maximum power point. The corresponding voltage Um and current Im are respectively called the best working voltage and the best working current.

Since the output characteristics of solar cells are highly correlated with the external environment, the measurement of solar cells should be carried out under uniform standard conditions. This standard is defined by the European Commission as Standard No. 101, and its conditions are: the spectral irradiance is 1000W/m2, the air quality factor is AM1.5, and the solar cell temperature is 25°C. Under this condition, the maximum power output by the solar cell group is called the peak power, expressed as Wp (peak watt).

(4) Energy conversion efficiency. It is an important indicator for evaluating battery quality. It is defined as the ratio of the maximum output power of the solar cell to the solar power irradiated on the solar cell, expressed as a percentage, and the formula is as follows

In the formula,

Pm-the maximum output power of the solar cell;

Pin-the input power irradiated to the solar cell.

(5) Power temperature coefficient. The change of power with temperature, in %/K, is a negative value.

(6) Voltage temperature coefficient. The change of voltage with temperature, in %/K, is a negative value.

(7) Current temperature coefficient. The change of current with temperature, in %/K, is a positive value.

### Solar cell operating characteristics

(1) Temperature characteristics. When the temperature of the solar cell changes, its volt-ampere characteristics will also change accordingly, thereby affecting the conversion efficiency. Figure 2 shows the volt-ampere characteristic curve at different temperatures under the same light intensity. The open circuit voltage is greatly affected by temperature, and linearly decreases with the increase of temperature. The short-circuit current has little correlation with temperature, and it increases slightly with temperature rise. The corresponding power change is shown in Figure 3. Figure 3 shows the power and voltage curves of solar cells at different temperatures. It can be seen from the figure that as the temperature rises, the output power drops significantly. Generally, a temperature increase of 10°C reduces the efficiency of solar cells by about 5%. The internationally defined standard solar cell temperature is 25°C.