External characteristics of solar cells

The performance of solar cells is very easily affected by the working environment, and irradiance, ambient temperature, particle radiation, etc. will all have an impact on the performance indicators of the battery. The influence of temperature and the influence of irradiance often exist at the same time. In order to ensure that the solar cell has higher working efficiency and more stable performance, the problem of improving the external characteristics of the solar cell must be considered when its manufacturing process, combination installation, and the design of a supporting control system.

Spectral response
The spectral energy of the solar spectrum decreases with the increase of the wavelength, and the distribution of the solar spectrum also changes with the wavelength, so different solar cells will produce different photon numbers when receiving the same light. Therefore, in actual use, the spectral response characteristics of solar cells should be considered, and the relative spectral response of solar cells is usually discussed. The definition is that when various wavelengths are incident on the solar cell with a certain amount of radiation photon beams, the short circuit produced The current is compared with the maximum short-circuit current, and the ratio change curve is calculated according to the wavelength distribution, which is the relative spectral response. The absolute spectral response means that when the unit radiant light energy of various wavelengths or the corresponding photons are incident on the solar cell, different short-circuit currents will be generated, and the corresponding short-circuit current change curve is obtained according to the wavelength distribution.

From the relative spectral response curve of a certain silicon solar cell shown in Figure 1, it can be seen that for different wavelength light components of different wavelengths of incident sunlight, silicon solar cells have different sensitivities, which can produce solar photovoltaic effects. The radiation wavelength range is generally in the range of 0.4~1.2μm, and the solar component radiation with a wavelength of less than 0.4μm and a wavelength of greater than 1.2μm cannot cause the silicon solar cell to generate photocurrent. The peak value of the spectral response of silicon solar cells is between 0.8 and 0.95 μm, which is determined by the solar cell manufacturing process and material resistance. When the resistivity is low, the peak value of the spectral response will reach 0.95 μm. Essentially, the long-wavelength spectral response mainly depends on the lifetime and diffusion length of the minority carrier in the matrix, and the short-wavelength response mainly depends on the lifetime of the minority carrier in the diffusion layer and the front surface recombination speed.

Figure 1 Relative spectral response curve of silicon solar cell

Temperature characteristics and light characteristics
The temperature characteristics of solar cells refer to the impact of the working environment temperature of the solar cell and the increase of its own temperature after the battery absorbs photons on the battery performance; because many parameters inside the solar cell material are functions of temperature and irradiance, such as this Carrier concentration, carrier diffusion length, photon absorption coefficient, etc., so reflected in the light characteristics refers to the relationship between the electrical performance of silicon solar cells and irradiance. Figure 2 shows the change curve of temperature versus photovoltage and photocurrent. The increase in temperature improves the diffusion length of carriers and the long-wave spectral response, so that the short-circuit current Isc has a positive temperature coefficient, but its rate of change with temperature is very small. The relationship between open circuit voltage Uoc and temperature is approximately linear, and Uoc has a negative temperature coefficient. When the temperature rises by 1°C, Uoc drops by about 2mV. Since the photoelectric conversion efficiency of solar cells is mainly affected by the fill factor and open circuit voltage, the photoelectric conversion efficiency also shows a tendency to decrease with increasing temperature. The ambient temperature for normal use of silicon solar cells is generally between ﹣65~+125C.

Figure 2 Variation curve of temperature versus photovoltage and photocurrent

Figure 3 lists the open-circuit voltage and short-circuit current of a certain silicon solar cell and the curve of the incident light irradiance. Among them, the open circuit voltage Uoc increases logarithmically with the increase in irradiance; the short-circuit current Isc is proportional to the irradiance. Figure 4 lists the change curve between the output power of a silicon solar cell and the incident light irradiance, and the output power is directly proportional to the irradiance.

Figure 3 The influence of irradiance on the current and voltage of solar cells
Figure 4 The influence of incident light irradiance on the output power of solar cells

Load characteristics
The output characteristic of the solar cell is greatly affected by the load. Figure 5 shows the volt-ampere characteristic curve of the solar cell circuit without an external bias voltage under different irradiance. It can be seen from the figure that for the same load RL, under different incident light, the current and voltage output by the solar cell are different. Under the same irradiance, changing the load size can also make the output circuit voltage change with the load. The output voltage and output current of the solar cell are related to the size of the load resistance RL. Figure 6 lists the relationship curves between the various electrical parameters of the solar cell and the load. As shown in the figure, there is no linear relationship between the output current IL of the solar cell, the output voltage UL, and the load resistance RL. The former decreases and the latter increases. To obtain the highest photoelectric conversion efficiency, the load must be matched at the maximum power output point.