Combining structures on different length scales in ray tracing: analysis of optical losses in solar cell modules

Solar cell modules consist of optically relevant geometric structures on very different length scales. While the whole module and the solar cells are on a scale of meters and centimeters, the pyramids etched on mono-crystalline Si cells (for enhancing light-trapping) have sizes in the micrometer range. The simulation domain cannot be reduced substantially to still capture module specific effects. Hence, these large differences in length scale have so far prohibited a detailed ray tracing analysis of entire modules. In this work, we developed a ray tracing approach that separates large and small scale geometries into different simulation domains; the simulated photon automatically switches between the different domains as needed. With this approach, it is possible to simulate whole modules on current desktop computers within reasonable time. We demonstrate the capabilities of this method by analyzing the optical losses in solar cell modules from mass production, as well as in modules under development that have no encapsulant. Our ray tracing method can be applied to any geometric structures containing different length scales.