In this study, ultrathin CuInxGa1-xSe2 (CIGS) films with two distinct thicknesses were grown on n-Si substrates by pulsed laser deposition (PLD), with thickness controlled by varying the number of laser pulses. Au plasmonic nanoparticles were subsequently incorporated into the CIGS layers using the same PLD technique for photodetection applications. Owing to the localized surface plasmon resonance (LSPR) induced by the embedded Au nanoparticles, photon absorption within the CIGS layers was significantly enhanced across the visible and NIR spectral regions. Increasing the film thickness in the presence of Au nanoparticles promoted the formation of larger grains and yielded notable improvements in crystallinity. The dark electrical behavior of plasmonic and non-plasmonic p-CIGS/n-Si heterojunctions was analyzed using the conventional J–V method, as well as the Cheung–Cheung and Norde methods. Key diode parameters, including barrier height, ideality factor, and series resistance, were extracted and comparatively evaluated. Among the studied devices, the Au-CIGS2A heterojunction (based on Au-embedded CIGS thin film produced with 86,400 laser pulses) exhibited the most ideal diode characteristics, whereas the CIGS1A device (based CIGS thin film produced with 43,200 laser pulses) demonstrated the least favorable electrical performance. Under illumination, the combined effect of increased CIGS thickness and the LSPR-driven optical enhancement provided by the Au nanoparticles resulted in higher photovoltaic conversion efficiency in the corresponding heterojunction diodes.