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. In this work, we developed a ray tracing approach by separating large and small scale geometries into different simulation domains; the ray tracer 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 modules from mass production, and also in modules under development, having no encapsulant. For the first time, we are able to assess the optical properties under tilted incidence, and we show that the optical losses of modules are underestimated under standard testing conditions (normal incidence). We derived an average yearly light source from thirteen years of meteorological measurements, and reveal that reflection from the glass cover and absorption in the glass as well as in the encapsulant account for 58-76% of the optical losses in module power-while under normal incidence, these losses account for only 41-47%.