Fundamental Collection Efficiency
The fundamental collection efficiency is dependent on the silicon layer thickness and the sunlight illuminations. Typical fundamental collection efficiency for a silicon cell as a function of silicon layer thickness under AM1 sunlight illumination is depicted in Figure 3.10. Note higher silicon thickness will increase the cost and the complexity of the cell. When a photon with energy level greater than or equal to the bandgap energy (E ) enters a silicon wafer layer of finite thickness, it can either be absorbed to generate a hole-electron pair or to propagate through the silicon layer without undergoing a generative interaction, which defines the fundamental collection efficiency as mentioned above. It is evident from this figure that a silicon layer thickness of about 100 pm is required to achieve a fundamental collection efficiency of 90 percent under AM1 sunlight conditions.
The inactive area between the individual V-groove elements for electrical isolation must be kept to a minimum with proper control of the width of the openings in the V-groove etching masks. This particular area must not exceed 2 percent of
the illuminated area of the VGMJ cell to maintain high total collection efficiency. It is interesting to mention that the inactive area loss in the VGMJ solar cell is similar to the shadowing loss observed in a conventional planar silicon solar cell. However, the shadowing loss in a conventional silicon solar cell is much larger than the inactive area loss, which is typically around 10 percent or higher under concentrated sunlight conditions.