### Characteristics of solar cells

1) Photoelectric characteristics of solar cells

When the light shines on the solar cell, the current IR and ij flow on the cell load resistance R and inside the cell respectively. Where ij is the forward current through the PN junction. When the illumination is constant, the photocurrent lp=ir+ij is also constant. The flow of photocurrent inside and outside the solar cell can be expressed by an equivalent circuit. The voltage VJ at the photoelectric cell end should be equal to the voltage VR on the resistance. The current ij of the photocell has an exponential relationship with the change of VJ:

Where, q is the electronic charge, taking 1.6 × 10-10°C； T is the absolute temperature, in K; K is Boltzmann constant, taking 1.380 × 10-23 j/k or 0.86 × 10-4eV/K； A is the effective area of solar cell, unit: mm2.

The relationship between current and voltage on load resistance R is

From formula (2-9), it can be concluded that the relationship between current and voltage on the load resistance is r=vj/i, the voltage drop on the load resistance is equal to the junction voltage, and the electrical power obtained on the load resistance R is ivj. In order to obtain high conversion efficiency, solar cells must output as much power ivj as possible under certain solar radiation.

2) Spectral characteristics of solar cells

The spectral characteristics of solar cells refer to the relationship between the photogenerated current of solar cells and the wavelength of radiation light when the solar radiation energy is the same. In solar cells, only those photons whose energy is greater than the “band gap” width of their material can produce electron hole pairs in photovoltaic materials when they are absorbed, while those photons whose energy is less than the “band gap” width can not produce electron hole pairs even if they are absorbed (they can only heat photovoltaic materials). There is a cut-off wavelength for the absorption of light by photovoltaic materials. Theoretical analysis shows that for sunlight, photovoltaic materials that can get the best working performance should have a “forbidden band” width of 1.5ev. When the “forbidden band” width increases, the total solar energy absorbed by photovoltaic materials will be less and less.

Each solar cell has its own spectral response curve to sunlight, which shows the sensitivity of solar cells to different wavelengths of light (photoelectric conversion ability). Table 2-1 shows the cut-off wavelength and solar energy absorption efficiency of different photovoltaic materials. When the sun shines on the solar cell, the light of a certain wavelength and the spectral sensitivity of the solar cell relative to the light of that wavelength determine the photogenerated current value of the light of that wavelength, and the total photogenerated current value of the solar cell is the sum of the photogenerated current values of the light of each wavelength.

3) I-V characteristics of solar cells

The electrical characteristics of solar cell modules mainly refer to I-V output characteristics, also known as V-I characteristic curve, as shown in Figure 2-12. The I-V characteristics of solar cells are similar to those of diodes, which are generally called I-V curve characteristics. In the solar I-V curve, short-circuit current (ISC), open circuit voltage (VOC) and maximum power (PMPP) are the main technical parameters of solar cells.

The V-I characteristic curve shows the relationship between the current im transmitted through the solar cell module and the voltage VM under a specific solar irradiance. IM is the maximum working current, that is, the current in the maximum output state; VM is the maximum working voltage, that is, the voltage in the maximum output state. If the solar cell module circuit is short circuited (v=0), the current at this time is called short-circuit current ISC; If the circuit is open (i=0), the voltage at this time is called open circuit voltage VOC. The output power of the solar cell module is equal to the product of the current flowing through the module and the voltage, that is, p=vi.

When the voltage of the solar cell module rises, for example, by increasing the resistance value of the load or when the voltage of the module increases from zero (under short-circuit conditions), the output power of the module also increases from 0; When the voltage reaches a certain value, the power can reach the maximum. At this time, if the resistance continues to increase, the power will jump over the maximum point and gradually reduce to zero, that is, the voltage reaches the open circuit voltage VOC. The internal resistance of solar cells shows strong nonlinearity. The maximum point of component output power is called the maximum power point; The voltage corresponding to this point is called the maximum power point voltage VM (also known as the maximum working voltage); The current corresponding to this point is called the maximum power point current im (also known as the maximum working current); The power at this point is called the maximum power PM.

With the increase of solar cell temperature, the open circuit voltage decreases. For every 1 ℃ increase in temperature, the voltage of each solar cell decreases by 5mv, which is equivalent to the typical temperature coefficient at the maximum power point of -0.4% / ℃. In other words, if the temperature of the solar cell increases by 1 ℃, the maximum power decreases by 0.4%. In the summer when the sun is direct, although the amount of solar radiation is relatively large, if the ventilation is not good, the temperature rise of the solar cell will be too high, and it may not output a lot of power. The temperature change and I-V curve of solar cells are shown in Figure 2-13.

The characteristic curve of solar cell sunshine intensity and maximum output is shown in Figure 2-14. The short-circuit current of solar cell is directly proportional to the sunshine intensity. The characteristic curve of solar cell temperature and maximum output is shown in Figure 2-15. The output power of solar cells decreases with the rise of the surface temperature of solar cells, and the output power changes with the seasonal temperature. Under the same sunshine intensity, the output power in winter is higher than that in summer.

Since the output power of the solar cell module depends on the solar irradiance, the distribution of the solar spectrum and the temperature of the solar cell, the measurement of the solar cell module should be carried out under standard conditions (STC). The standard measurement conditions are defined as Standard No. 101 by the European Commission. The test under the simulated light source conditions of 25 ℃ surface temperature of solar panels, 1000 w/m2 solar radiation intensity and AM1.5 spectral distribution is called the standard test state, as shown in Figure 2-16.

Under standard measurement conditions, the maximum power output of solar cell modules is called peak power, which is expressed by WP (peak Watt). In many cases, the peak power of components is usually measured by a solar simulator and compared with the standardized solar cells of international certification bodies.

It is difficult to measure the peak power of solar cell modules outdoors. Because the actual spectrum of sunlight received by solar cell modules depends on atmospheric conditions and the position of the sun; In addition, in the process of measurement, the temperature of solar cells is also changing. The error of outdoor measurement can easily reach 10% or more.

If the solar cell module is covered by other objects (such as bird droppings, tree shadows, etc.) for a long time, the covered solar cell module will be seriously heated, which is the “heat island effect”. This effect will cause serious damage to solar cells. Some or all of the energy generated by illuminated solar cells may be consumed by shaded solar cells. In order to prevent the solar cell from being damaged due to the “heat island effect”, a bypass diode needs to be connected in parallel between the positive and negative poles of the solar cell module to avoid the energy generated by the illuminated solar cell module being consumed by the shaded solar cell module. Its function is to provide current path when the module is open or shaded, so as not to make the whole string of solar cell modules fail.

Pay attention to the polarity when using the solar cell. The positive pole of the bypass diode is connected with the negative pole of the solar cell module, and the negative pole of the bypass diode is connected with the positive pole of the solar cell module. At ordinary times, the bypass diode is in the reverse bias state and basically does not consume electric energy. However, the withstand voltage and allowable forward current of the bypass diode should be greater than the working voltage and current of the component.

The connection box of the solar cell is a very important component, which protects the interface between the solar cell and the outside world and the wires and other system components connected inside each component. The connection box contains a junction box and 1 or 2 bypass diodes.

The main technical parameters of solar cells are as follows.

(1) Photoelectric conversion efficiency η。 Photoelectric conversion efficiency is an important index to evaluate the performance of solar cells. Photoelectric conversion efficiency of solar cells refers to the ratio of solar cells to convert received light energy into electric energy, that is

Where, η Is photoelectric conversion efficiency; P0 is the output power; E is the solar energy irradiated by the solar panel.

The photoelectric conversion efficiency of solar cell modules is an important factor to determine whether solar cells have use value. The theoretical photoelectric conversion efficiency limit of crystalline silicon solar cells is 29%, while the photoelectric conversion efficiency of current solar cells is 17% to 19%. Therefore, there is still much room for the development of solar cells in technology.

(2) The output voltage V of a single solar cell is 0.4~0.6v, which is determined by the physical properties of the material.

(3) Fill factor FF is an important index to evaluate the load capacity of solar cells, and

Where ISC is short-circuit current; VOC is the open circuit voltage; IM is the best working current; VM is the best working voltage.

The power output capacity of solar cells is closely related to its area. The larger the area, the greater the output power under the same lighting conditions. The advantages and disadvantages of solar cells are mainly measured by open circuit voltage and short-circuit current.

(4) The influence of temperature on the performance of solar cells. The ambient temperature and the temperature of solar cell modules directly affect the performance of solar cells. When the temperature rises, its open circuit voltage decreases linearly. Solar cells of different materials have their own operating temperature range. For a certain kind of solar cell, the optimal load required to obtain the maximum output power is also different at different temperatures. For example, under standard conditions, AM1.5 light intensity, t=25 ℃, the output power of a certain type of solar cell is 100wp. If the temperature of the solar cell rises to 45 ℃, the output power of the solar panel is less than 100 WP.