A solar cell with a PN junction is equivalent to a diode when it is not exposed to light. In the absence of light, the current formed when a reverse bias voltage is applied to both ends of the PN junction is called dark current. The photo-generated current is the current formed by the PN junction generating carriers when illuminated. The solar cell photogenerated current equation expressed by the short-circuit current, the saturated dark current and the forward bias voltage is:
In the formula, Isc is the short-circuit current, I0 is the total current, q is the electronic charge, and γ is the ideal coefficient, which is the parameter of the PN junction characteristic, usually 1~2, k is the Boltzmann constant, and T is the temperature.
When the output terminal of the solar cell is open circuit, I=0, the open circuit voltage can be obtained:
When the solar cell is connected to the load R, the resulting volt-ampere characteristic curve is shown in Figure 1, and the load R can be from zero to infinity. When the load maximizes the power output of the solar cell, the corresponding maximum power is:
In the formula: Im and Vm are the best working current and the best working voltage respectively. The ratio of the maximum power Pm to the product of the open circuit voltage and the short circuit current is defined as the fill factor FF:
The fill factor FF is an important parameter for evaluating the performance of solar cells. The factors that affect the fill factor are not only related to internal parameters such as the PN junction curve factor constant, series resistance, and parallel resistance of the battery material, but are also affected by external conditions such as the operating temperature and light intensity of the solar cell. Generally FF<1, the higher the value, the closer the output characteristic of the solar cell is to the rectangle, the higher the photoelectric conversion efficiency of the cell.
Under a certain sunshine intensity and temperature, the solar cell has a unique maximum output power point, and the solar cell can only maximize its output power when it works at the maximum power point.
The solar cell can be represented by a circuit composed of a PN junction diode, a constant current source Iph, a series resistance Rs, and a parallel resistance Rsh. This circuit is called the equivalent circuit of the solar cell, as shown in Figure 2. From Figure 2, it can be concluded that the relationship between the current and voltage at both ends of the solar cell is:
It can be seen from the above formula that in order to make the solar cell output more power, the series resistance Rs must be reduced as much as possible, and the parallel resistance Rsh must be increased.
The photoelectric conversion efficiency of a solar cell refers to the ratio of the maximum output power Pm when the cell is exposed to light to the power Pin of the human light irradiated on the cell, expressed as:
It can be seen from the previous formula that the main electrical performance parameters that affect the conversion efficiency of the cell are: open-circuit voltage Voc, short-circuit current Isc and fill factor FF.
The main reasons for the loss of solar cell conversion efficiency include electrical loss and optical loss.