Television control of electroluminescent defects of solar panels when performing the PID test

Abstract

The paper presents the results of the study of defects in silicon photovoltaic solar cells, which has an electroluminescent effect in the visible radiation range when applied to the battery of the reverse voltage. The experiments involve a specially designed laboratory test bench for television measurements, which can also be used when testing solar panels for a long acting high voltage (PID) test. As a result, the volt-brightness characteristics of individual defects were obtained and a map of visible electroluminescent defects of a serial solar cell with a power of 30 W on the basis of monocrystalline silicon was prepared.

It has been established that electroluminescent defects of crystalline silicon based solar cells that emit in the visible range of the spectrum have two different nature. Defects of the first type occur at reverse voltages close to the breakdown voltage. Such a glow is called pre-breakdown. After the occurrence of pre-break electroluminescence, further increase in voltage leads to thermal breakdown and destruction of the solar cell. The glow has a characteristic orange or red hue. The lumens are concentrated in zones bordering with current collection tanks. The geometric dimensions of these defects can reach one millimeter or more. The shape of defects is incorrect with an arbitrary contour. The glow occurs throughout the area of the defect. This defect has a rather narrow range of voltage: the threshold occurs at a value of 60-70% of the breakdown voltage. Pre-break electroluminescence of crystalline silicon is well studied, work on this topic originates in the middle of the 20th century.

The second type of defects is due to the emergence of a tunnel effect, which provides the small cell size of the defect, resulting in a huge electric field tension (up to 10 kV/cm). Defects are formed when aluminum drops fall during the formation of conductive tires on the surface of silicon of n-type for thick film technology in the process of manufacturing solar cells and through the diffusion of aluminum into a surface layer of silicon in the thickness of 300-500 nm. Defects have the correct shape, which is close to the circle. The geometric dimensions of the defects are small - from 1 to 20 microns. The defects are observed in a thin circle of 1 micron in width at the boundary of the defect zone, so they can be called micro-defects. The nature and physical properties of this type of defects are currently not sufficiently explored. The presented work adds a new tool for such studies on serially manufactured photovoltaic solar cells.

The main idea of the research submitted to consideration is the use of a television information and measurement system in the context of conducting a PID - solar battery test. A powerful optical system allows you to capture the presence of microscopes that glow, and a television camera and personal computer - to perform pixel brightness measurements in real time. Possibility of adjusting the high return voltage supplied during excitation of defects allows to obtain their volt-brightness and ampere-brightness characteristics.

The results of the work can be used in the production of photovoltaic solar cells, as well as in the control of defects during the life of the batteries.