Optimal shape for optical absorption in organic thin film solar cells

The power conversion efficiency of organic solar cells can be increased by using light trapping geometries, which enhance the light absorption. In this paper, we analyze the optical performance of organic thin film solar cells using the finite element method solving the Maxwell equations. Shape optimization is then performed with the goal of maximizing the light absorption in the active layer, while keeping its thickness low. The optimization algorithm is based on the gradient of the objective function, where sensitivity is obtained from the adjoint approach. To avoid irregular shapes in the optimized structures, two different shape representation techniques, finite element node based curve representation in conjunction with the Helmholtz filter and B-spline curve representation with varying number of control points are used. Both are demonstrated being effective in smoothing the design shapes. Periodic grating structures are observed in the optimized shapes and significant increase in light absorption is achieved in the active layer with low thickness.