Ray Tracing Isotextured Solar Cells

We describe a ray-tracing approach for isotextured solar cells. The approach is founded on the spherical cap model for isotexture, where the spherical caps are contained in cylindrical unit cells. The rays that intercept the sides of unit cells are partially randomized onto the sides of neighboring cylinders, and the rays that intercept the bottom of the unit cells are transferred to the bulk of the solar cell, which is represented by a rectangular prism. This approach introduces randomness to the isotexture model and couples the texture to other 2D or 3D features of the cell, such as fingers, contacts and glass texture. Simulations of 500,000 rays are typically solved in less than five minutes, even for modules with textured glass and thin films. The approach is demonstrated by evaluating isotextured wafers in terms of (i) escape reflection and light trapping, (ii) reflection vs the angle of incidence, and (iii) their behavior after encapsulation. The simulations indicate that relative to random pyramids, isotexture provides similar light trapping when the rear of the cell is Lambertian, and superior light trapping when the rear is planar and specular, but this additional light trapping does not compensate for its poorer front reflection. We also conclude that for an increasing incident angle, the relative advantage of random pyramids over isotexture decreases for unencapsulated cells, and increases slightly for encapsulated cells.