Although a continued exponential growth of solar power generation over the world paves a path to a future in sustainable energy, development of photovoltaic (PV) technologies with low-cost and high-stability materials remains a challenge and has attracted tremendous attention to solar energy research. The prevalence of thin film solar cells substantially reduces the material costs. However, even in the presence of their band gap properties, a major issue faced by most thin film solar cells is the low output efficiency due to manufacturing variability and uncertain operating conditions. Thus, to ensure the reliability and performance robustness of thin film PV technologies, the design of the solar cell is studied. To represent the thin film PV technologies, a copper gallium (di)selenide (CIGS) solar cell model is developed and optimized with the Reliability-based Robust Design Optimization (RBRDO) method. The main contribution of this research is the development of a probabilistic thin film solar cell model that considers the presence of the uncertainties in the PV system. This model takes into account the variability of the structure and the material properties of the CIGS solar cells, and assumes operation in ideal-weather conditions. A general reliability-based methodology to optimize the design of the CIGS PV technologies is presented in this research and this approach also could be used to facilitate the development and assessment of new PV technologies with more robust performance in efficiency and stability.