In this paper, an emerging material GaAs_1-xBi_x has been introduced in different layers of multijunction solar cells. By modifying the spectral p-n junction model, the theoretical efficiencies of two-, three- and four-junction III-V bismide multijunction solar cells have been estimated to be 36.5%, 44.0%, and 52.2% respectively for AM1.5G under 1 sun condition. The effects of the material bandgap, sun concentration, and surface recombination velocity on the solar cell have been studied extensively. Simulation results revealed that an increment up to ~10% on the overall cell efficiency can be achieved by concentrating the solar radiation from 1 sun to 500 sun; and a 3 – 4 % increment on the overall cell efficiency by tuning the bandgap of the bismide layer as well as modeling the surface recombination velocity involved.  Simulation results could be utilized for better understanding the materials as well as in realization of highly efficient bismide based multijunction solar cells.