As shown in Figure 1, it is the equivalent circuit of an ideal photovoltaic cell made by using the PN junction photovoltaic effect. Figure 2 shows the actual equivalent circuit of the solar cell. In the figure, the PN junction under illumination is regarded as an ideal diode and a constant current source in parallel. The current of the constant current source is the photo-generated current IL, Rl is the external load, and the junction current Ip passing through the PN junction is represented by a diode. The physical meaning of the equivalent circuit of ID is: the solar cell generates a certain photocurrent I after being illuminated, part of which is used to offset the junction current ID (the reason is that the direction of this current is opposite to the direction of the photogenerated current), and the other part is the supply Load current IR. The size of its terminal voltage U and working current I are related to the load resistance R, but the load resistance is not the only determining factor.
In actual solar cells, due to the contact between the front and back electrodes and the cell (that is, there is a contact resistance), and the electrode material itself has a certain resistivity, the material itself of the base region and the top layer inevitably has resistance. Introducing additional resistances, in the equivalent circuit, their total effect can be represented by a series resistance Rs. When the current flowing through the load passes through Rs, it will inevitably cause losses. Due to the leakage at the edge of the battery and the leakage of the metal bridge formed at the micro-cracks, scratches, etc. of the battery during the production of metallized electrodes, a part of the current that should pass through the load is short-circuited, forming a current passing through Rs (the effect of this effect). The size can be equivalent with a parallel resistor RsH). It can be seen from the above analysis that the larger the parallel resistance and the smaller the series resistance, the closer to the ideal photovoltaic cell, and the better the performance of the cell.
The most important application of the PN junction photovoltaic effect is for solar cells. The light energy of solar radiation has a spectral distribution. The narrower the band gap of the semiconductor, the wider the spectrum that can be used. However, if the band gap is too small, the corresponding photoelectromotive force that can be generated will be relatively small; on the contrary, for semiconductors with a large band gap, although the photoelectromotive force can be increased, the available solar spectrum range will be relatively small, that is, Said that the open circuit voltage increases with the increase of the forbidden band width. In addition, the short-circuit current density decreases with the increase of the forbidden band width. Therefore, there is a forbidden band width at a certain point, which can make the solar cell efficiency reach its maximum value (ie, peak value). Therefore, how to make full and reasonable use of solar energy resources is a key technical problem faced by scientific and technological workers engaged in solar cell research.
