Updating the limit efficiency of silicon solar cells

Since the 1970 evaluation of the "limit efficiency" and design goals for the silicon solar, cell, performed under file auspices of the National Academy of Sciences, some performance limiting phenomina, such as Auger recombination and bandgap narrowing in heavily doped material, have been recognized, some improvements in material parameters, such as minority-carrier lifetimes and surface-recombination velocities, attained, and some remedial designs, such as surface texturizing, increasing the internal optical reflectance at the back surface of the cell, and applying "BSF" structures, evolved. A re-evaluation of the "limit efficiency"' under consideration of the new circumstances has, therefore, been carried out, following the previous approach of analyzing an idealized device structure by use of presently realistic-appearing material parameters, to obtain a conversion efficiency value which represents an upper limit to that technologically achievable in the near future. Removing the idealizations, "design goals" are established which lie near 90 percent of the limit efficiency. The new computations have shown that there exists a second design approach to obtaining high open-circuit and maximum power point voltages, after the first one, relying on high impurity concentration in both the front and the base regions of the cell, has been found limited by the "heavy doping effects." This second approach involves the "narrow-region" design for both the front and base regions of the solar cell, and relies heavily on low effective surface-recombination velocities front and back, as well as a textured front surface and an optical internally reflecting back surface. Using this design, the undesirable effects of high doping can be completely avoided, and the limit efficiency of the solar cell maintained at its earlier value near 25 percent. With the minority-carrier lifetime/impurity concentration model chosen as "realistic," the optimum cell design requires a thin cell, in the 50- to 150-µm range.