In this work a new method to determine the emitter-window surface recombination speed in GaAs based n-p junctions has been developed. Using this new technique, the relation between the said surface recombination speed and the emitter thickness and doping levels has been investigated experimentally by processing multiple sets of GaAs substrate solar cells. Based on the experimental results, a very first, approximated empirical model describing the surface recombination speed has been used, together with other empirical and analytical models, to run a MATLAB optimization program. This routine is based on a 1D model simulating solar cells which relies on a scattering matrix method to treat the optics inside the cell in an exact way and on the Hovel (or Shockley) model to describe the electrical behaviour of the active region.