The integration of metal nanoparticles in an organic buffer matrix for plasmonic organic solar cells (OSCs) has been explored as a route for improving the photovoltaic performance, with localized electromagnetic field enhancement around nanoparticles. We investigate the optical behavior of gold-silica core-shell nanospheres (Au@SiO2 NSs) with different shell thicknesses integrated into a 30 nm-thick poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) layer which is traditionally used as a buffer layer in OSCs. The morphology and size of the chemically synthesized Au@SiO2 NSs are determined by TEM, indicating that the average diameter of the Au core is about 50 nm, while the thickness of the dielectric shell can be adjusted to around 5 or 10 nm. The effect of Au@SiO2 NSs on the surrounding electromagnetic field in such a heterogeneous matrix and subsequent multilayers is examined using a numerical simulation based on a 3D-FDTD method. Furthermore, a broadband absorption enhancement in the films, which can be primarily attributed to far-field scattering and also to the localized surface plasmon resonance around the wavelength of 530 nm, is observed in the simulated and measured absorption spectra. The analysis of the electromagnetic field between NSs and the active layer using Raman spectroscopy is also presented. The Raman spectra confirm that a plasmon effect occurs and induces a strong field enhancement; this does not change the Raman peak position but increases its signal intensity depending on the silica shell’s thickness. As a result, plasmonic devices including Au@SiO2 NSs with a 5 nm-shell thickness present the best optical behavior compared to bare NSs or 10 nm-thick shell Au@SiO2 NSs.