Analysis of randomly textured interface in solar cells is an ambitious effort considering the numerous structures and variation in the texture. We use the Fourier transform based method, which takes randomly textured interface as initial input and manipulates its frequency spectrum to synthesize interface texture with the desired texture behavior. Afterward, the synthesized interfaces are applied to simulate flexible thin-film silicon tandem () solar cells. Using the simulation, we have shown how interface between the back and front areas of the solar cell evolves, and the effect of the back contact; the front area roughness is analyzed and interference fringes observed on experiments are discussed. For the simulations, a finite integration technique with time-harmonic inverse iterative method is used. All simulations are performed on high-performance computers, which allows simulation of a big simulation domain () with fine mesh size (). The analysis performed shows that the interface roughness at the front contact remains similar to the initial back contact roughness. Furthermore, a solar cell with flat back contact can be as efficient as a solar cell with rough back contact when the front area roughness is well optimized.