The measuring is a last step in solar cells production. At this operation every solar cell is connected to electronic load and illuminated with light spectrum AM1,5 to obtain the voltage-current characteristic. Then calculates solar cells parameters like shot circuit current, open circuit voltage, fill factor, maximum point power, efficiency, serial and shot circuit resistances and others. The measuring system determines the class of current solar cell and places it into corresponded box.
The silicon solar cell efficiency has strong dependence on temperature. It should be kept in mind that measured efficiency should be recalculated to 25оС before classifying.
Also the great influence on measurement has arrangement of probe contacts and its quantity. It happens because silver busbar has quite high resistance (much higher than copper ribbon which used in assembling modules) and thus can not support uniform load by few contacts. The compromise of this situation is a balanced location of probe contacts or using multiple contacting systems.
An additional information about measuring you can find
here.
Additionally the electroluminescence imaging should be done to obtain complete picture of solar cell quality. The electroluminescence makes it possible to detect and distinguish most defects of solar cell (cracks, micro-cracks, fingers interrupts, low efficiency regions, higher series resistivity, low lifetime regions, shot circuit etc.) This method is nondestructive which take picture of solar cells by principle “dark region - trouble region”. An additional information about electroluminescence you can find here.
The etching of edges by plasma-chemical reaction is a batch operation. A light squeezed stack of wafers moves to a vacuum chamber for plasma etching at room temperature in the fluorine contained atmosphere. In this case the fluorine ions bind to silicon atoms form thus gas SiF4 which is easily removed from surface to vacuum pump. It leads to disappearance parasitic n-type layer from edges.
The silicon wafer is placed onto roll system which moves the wafer to the etching solution for liquid etching of parasitic n-type layer. It is important that solution does not take front surface to prevent removing front side emitter. Thus only emitter from back side and edges can be removed completely. It results to very good isolation.
The laser beam is scanned at front (some times at back) surface near the edges to make grooves between edge and front emitters. These grooves are formed by explosion some part of material during laser pulses illumination. The depth of the groove is much more then depth of n-type layer so there is no n-channel between front and edge emitters after laser treatment. It also results to very good isolation but it cuts some active part from front side emitter which leads to bit decreasing of shot circuit current and efficiency of cell. Although efficiency lost the laser isolation becomes more popular cause its easy setup and not expensive usage.