Requirements for a 20%-Efficient Polycrystalline GaAs Solar Cell

Based on a literature review, we explore the material and performance requirements for high-efficiency, potentially low-cost, GaAs solar cells. The goal is a GaAs solar cell, on a low-cost substrate, having an efficiency greater than 20%. An important issue limiting efficiency for polycrystalline GaAs cells is recombination through deep levels associated with grain boundaries, specifically at thepart of a grain boundary that intersects the p-n junction. The effect of this junction recombination on cell efficiency is shown as a function of grain size. We explore the potential impact of grain size, grain-boundary passivation, intragrain defects, impurities, and crystal orientation. The impact of intragrain defects on minority-carrier lifetime and cell efficiency is also discussed. Weconclude that two critical parameters or achieving high efficiency are the mitigation of intragrain-defect density and perimeter junction recombination. To achieve over 20% efficiency, dislocation densities need to be reduced to less than 5 x 10(6)/cm2 for very large-grain material. Assuming that the intragrain and surface-recombination properties are state-of-the-art, grain sizes of 20-50 ..mu..m are needed to reach 20%. Once these conditions are met for GaAs cells fabricated on low-cost substrates, both low cost and high efficiency would be; possible.