A novel multi-junction GaP/InP/Si solar cells with improved p++AlGaAs/n++ AlGaAs tunnel junction model is designed using the Synopsys/RSoft/Solar cell utility software. The optical and electrical simulations of this cell are performed using the 2D full-wave and solar cell utility, resulting in an open circuit voltage of 3.02V, short circuit current density of 15.942mA/cm(2), fill factor of 82.7%, and power conversion efficiency of 44.23%. Then, an optimization study of a function of variation in thickness of different layers is performed. Simulation results of the optimized structure under AM 1.5G condition showed a small improvement in the short circuit current density by 0.38%, efficiency by 0.375%, whereas the fill factor and open circuit voltage are maintained the same. The proposed multi-junction solar cell has reported the highest power conversion efficiency of 44.397% among the III-V triple junction solar cells designed already. Copyright (c) 2017 John Wiley & Sons, Ltd.