In this paper, a design of a diffractive optical element to split the solar spectrum into two separate parts for a laterally arrayed InGaP/GaAs solar cell is presented. Optical simulation is done by using the three-dimensional finite-difference time-domain method and the results are demonstrated to evaluate the optical performance of the designed structure. Anti-reflection coating for the designed splitter is also put forth. In addition to the optical analysis, electrical simulations are performed and current density-voltage and power density-voltage curves are presented in order to explain the electrical performance of the InGaP/GaAs solar cell by implementing the designed spectrum splitter. The results of the electrical simulations show that the designed InGaP/GaAs solar cell's best efficiency is 34.7% under unconcentrated sunlight. Further improvement is feasible if the parameters of the spectral splitting structure are optimized by the incorporation of non-intuitive optimization algorithms. Lastly, light trapping strategies can also be considered to enhance the efficiency of the solar cells.