Analysis for High-Efficiency GaAs Solar Cells on Si Substrates

The authors review some recent developments in GaAs thin-film solar cells fabricated on Si substrates by MOCVD (metal-organic chemical vapor deposition). High-efficiency GaAs solar cells with an efficiency of 18% have been successfully fabricated on Si substrates by reducing the dislocation density of GaAs thin films on Si to less than 3-5 × 10⁶ cm^-2. Reduced dislocation density was attained by combining strained-layer superlattice insertion and thermal cycle growth. A simple model was used to analyze the dislocation density dependence of GaAs/Si solar cell properties. The model assumed nonuniform dislocation distribution and that dislocations act as majority-carrier trapping centers as well as recombination centers. Good agreement between experimental and theoretical values for GaAs/Si cell properties indicates the validity of this analytical model. The lower open-circuit voltage for GaAs/Si cells with higher dislocation densities was attributed to a larger space-charge layer recombination current accompanied by majority-carrier trapping due to dislocation.