Analyzing emitter dopant inhomogeneities at textured Si surfaces by using 3D process and device simulations in combination with SEM imaging

The lowering of the phosphorus dopant density in the emitter of Si solar cells is a current topic in the photovoltaic industry. In lowly-doped emitters, diffusion inhomogeneities between the tops of the pyramids and the valleys affect the saturation current density J₀. We quantify diffusion inhomogeneities by means of 3D process simulations, and we evaluate J₀ by means of 3D device simulations. Finally, we compare the simulated diffusion results with a 2D dopant-contrast analysis obtained with a scanning electron microscope (SEM). Both methods show a deeper in-diffusion at the top of the pyramid and a shallower in-diffusion in the valley regions. Within the pyramidal faces, the diffusion is between both of these extreme points and comparable with in-diffusion at planar structures. The results of the device simulations indicate that the increase of J₀ from planar to textured surfaces depends on the dopant profile and the surface passivation, but that a factor of about 5 is observed in our example, as is observed experimentally.