Total Conversion Efficiency of a VGMJ Solar Cell
Note the overall conversion efficiency or the total conversion efficiency of a silicon solar cell is dependent on all the parameters and effects as illustrated in Figure 3.12. A prototype and optimized version of the VGMJ cell illustrated in Figure 3.7 measuring 4900 pm on a side and with 43 series connected diode elements was fabricated following the fabrication steps shown in Figure 3.8 . The optimized version of the device provided an open-circuit voltage of 0.702 V per diode and a short-circuit current of 44.3 mA. The optimized cell design configuration provided a fill factor of 0.8 and an open circuit of 30.2 V at 300 K and under 300 suns concentration conditions. Electrical performance characteristics and design parameters of the optimized version of the VGMJ silicon solar cell are summarized in Table 3.2.
It is important to point out that the spectral efficiency (nspec) of silicon is strictly a function of the solar radiation wavelength, the depth of the p-n junction below the surface, the wafer thickness, the minority carrier lifetime and mobility in the base region, the absorption coefficient, quality of surface finish, purity and type of the silicon material, and the surface recombination velocity on the back surface of the device. It is extremely difficult to measure the spectral efficiency accurately under so many variables. However, this spectral efficiency can vary anywhere from 45 to 54 percent, according to various publications. Therefore, the total conversion efficiency can vary from 25 percent at a spectral efficiency of 48 percent to 26 percent at a spectral efficiency of 50 percent to 27 percent at a spectral efficiency of 52 percent and finally to 28 percent at a spectral efficiency of 54 percent. These predicted total conversion efficiencies for the VGMJ solar cells are the highest reported to date.