This effort is founded on the modeling and simulation of the GaAsPN/GaP quantum dot (QD) solar cell. This quaternary alloy is one of the III-V semiconductors, which gained importance in the recent years for optoelectronic applications. This importance comes from the fact that the quaternary GaAsPN can be a well-grown lattice matched to GaP and Si substrates and to the bandgap that can be decreased drastically with the incorporation of nitrogen and arsenic into GaP, improving consequently the absorption and the wavelengths near the red part. These qualities make GaAsPN a good candidate for the growth on the Si substrate and low-cost solar cell fabrication. The optical properties of GaAsPN/GaP QDs, such as strain, critical thickness, bandgap energy, the external quantum efficiency, and absorption coefficient, have been reported. The heterostructures consist of GaAs0.18P0.814N0.006 QDs separated by GaP barrier layers. The width and thickness of QDs are about 5 and 5 nm, respectively. Our results have been shown that 20 GaAs0.18P0.814N0.006/GaP QD layers produce a short-circuit current of about 3.55 mA/cm2 and an efficiency of about 7.5%. In addition, we will be able to extend the absorption edge of a GaP solar cell from 0.48 μm to 0.5 μm for the same QD number layers inserted. The temperature effect on efficiency is considered.