The difference between dark current, reverse current and leakage current for solar panels
Oct 23, 2022
There are various currents inside the cell, such as dark current, reverse current, leakage current, etc.
Various currents have a greater or lesser impact on the power of solar panels. Distinguishing the characteristics of various currents can identify the cause of abnormal solar panel power and help solve the problem completely.
dark current
Dark current (DarkCurrent), also known as unilluminated current, refers to the reverse DC current generated when the P-N junction is under reverse bias conditions and there is no incident light. It is generally caused by the diffusion of carriers or defects on the surface and inside of the device, as well as harmful impurities. The principle of diffusion is that in the PN junction, there are more electrons in the N region and more holes in the P region. Because of the difference in concentration, the electrons in the N region will diffuse to the P region, and the holes in the P region will diffuse to the N region, although the PN junction The built-in electric field is to prevent this kind of diffusion, but in fact the diffusion has been going on all the time, just reaching a dynamic equilibrium, which is the formation of diffusion current. In addition, when there are defects on the surface and inside of the device, the defect energy level will play the role of a recombination center, which will capture electrons and holes to recombine at the defect energy level. When electrons and holes are captured at the defect energy level, Due to the movement of carriers to form a current, the same harmful impurities also play the role of recombination centers in the device, and the reason is the same as that of defects.
Dark current is generally considered when sorting silicon wafers. If the dark current is too large, it can indicate that the quality of the silicon wafer is unqualified. For example, there are many surface states, there are many defects in the lattice, there are harmful impurities, or the doping concentration is too high. , The cells made from such silicon wafers often have low minority carrier lifetime, which directly leads to low conversion efficiency!
For simple diodes, dark current is actually reverse saturation current, but for solar cells, dark current includes not only reverse saturation current, but also thin-layer leakage current and bulk leakage current.
Reverse saturation current
The reverse saturation current means that when a reverse bias voltage is applied to the PN junction, the applied voltage widens the depletion layer of the PN junction, the junction electric field (that is, the built-in electric field) becomes larger, and the potential energy of the electrons increases. It is difficult for the majority carriers (multi-dimensional holes in the P region and electrons in the N region) to cross the potential barrier, so the diffusion current tends to zero, but due to the increase of the junction electric field, the N region and the P region are The minority carriers are more prone to drift motion, so in this case the current in the PN junction is determined by the dominant drift current. The direction of the drift current is opposite to the direction of the diffusion current, which means that there is a reverse current left in the N region on the external circuit, which is formed by the drift movement of minority carriers. Since the minority carriers are generated by intrinsic excitation, under a certain temperature, the number of minority carriers generated by thermal excitation is constant, and the current tends to be constant.
leakage current
Solar cells can be divided into 3 layers, namely thin layer (ie N region), depletion layer (ie PN junction), body region (ie P region), for cells, there are always some harmful impurities and defects , some are in the silicon wafer itself, and some are formed in our process. These harmful impurities and defects can play the role of recombination center, which can capture holes and electrons and make them recombine. The process of recombination is always accompanied by The directional movement of carriers will inevitably generate tiny currents. These currents contribute to the dark current value obtained by the test. The part contributed by the thin layer is called the thin layer leakage current, and the part contributed by the body region It is called body leakage current.