Ag plasmonic nanoparticles have been produced depending on laser pulse energy by applying pulsed laser deposition technique. Localized surface plasmon resonance (LSPR) peak of silver (Ag) plasmonic nanoparticles has been shifted towards longer wavelength region by increasing laser energy. By embedding Ag nanoparticles into copper zinc tin sulphide (CZTS) ultrathin film, the crystal structure of CZTS ultrathin film has been improved somewhat and its band gap has been decreased. CZTS and (Ag) plasmonic CZTS/n-Si heterojunction solar cells have been produced and their J–V characteristics have been obtained for darkness and light environments. The parameters of heterojunctions such as barrier height, ideality factor and serial resistance have been calculated under dark environment by applying the conventional J–V and Norde and Cheung–Cheung methods and found to be partially compatible with each other. The photocurrent, Jsc, open circuit voltage, Voc, and thus, η values for plasmonic CZTS solar cell have been increased, as hot electron formation occurred in CZTS ultrathin film due to LSPR property of Ag nanoparticles. In addition, the photovoltaic parameters of non-plasmonic and plasmonic CZTS solar cells have been calculated by SCAPS-1D simulation; thus, the experimental and calculated parameters have been compared and found to be compatible with each other.