The effects of surface plasmon polaritons (SPPs) on the efficiency, series resistance, and shunt resistance of thin-film Si solar cells are studied and analyzed in this work. Different SPP shapes and their effects on the optical and electrical properties and thereby the efficiency of thin-film solar cells are studied. Semiconductor and electromagnetic models are incorporated to study the electrical and optical behaviors of the thin-film solar cells, respectively, using COMSOL Multiphysics three-dimensional (3D) numerical simulation software. An efficiency of 14.76% is achieved for triangular SPPs, representing a 1.07% improvement compared with SPP-free solar cells. The solar cell electrical parameters are also extracted based on a single-diode equivalent model. The series resistance is decreased by 3% for solar cells having equilateral-triangle SPPs compared with SPP-free solar cells.