Abstract In this paper, a graded-index metamaterial (GIM) nanostructured waveguide is proposed to enhance the performance of solar cells via a tunable absorption spectrum. The proposed four-layer nanostructured waveguide consists of two GIM and SiN_x films which are squeezes between glass substrate and air. Using a transmission matrix method, the transmittances as well as the reflectance are calculated for different film thicknesses, refractive indices and incidence angles. We demonstrate that the reflectance is nearly zero where SiN_x refractive index is 2.2 in vicinity of 620 nm. As the incident angle increases, the minimum reflectance wavelength blueshifts and slightly increase in the value. In addition, the variation in the minimum reflectance due to a change in the thickness of SiN_x layer studied in detail. We show that the absorbance of a solar cell can be easily controlled by varying refractive index and/or thickness of SiN_x layer in the proposed nanostructure. The result shows that the best efficiency occurs at normal incidence, n₂ = 2.2, and d₂ = 30 nm.