Development of a self-aligned etch-back process for selectively doped silicon solar cells

Selectively doped (SD) solar cells have advantages of lower front diffusion recombination, lower contact resistivity and better blue response. In this work, a simple, self-aligned SD solar cell process is presented, eliminating the critical mask alignment step in current SD solar cells. In the new process, the front metal grid is deposited on top of a heavy phosphorus diffusion before the anti-reflection coating. The metal grid, formed in this experiment by evaporating aluminum through a shadow mask, acts as mask during a subsequent etch-back step, which therefore results in a heavily doped region (~18 Ω/sq) under the metal fingers and a lightly doped region (~100 Ω/sq) elsewhere. This etch-back process has been successfully demonstrated on laboratory-size p-type n⁺pp⁺ SD solar cells. On planar substrates the SD process led to a ~6 mV gain in V_oc, a ~0.6 mA/cm² gain in J_sc and a 0.7% (absolute) gain in efficiency, compared to control devices with a homogenous light diffusion. The best planar cell has a conversion efficiency of 16.4%. We have also implemented this process on textured surfaces, leading to a conversion efficiency of 17.5%. Although these prototype solar cells are still limited by recombination at the rear side, they serve as a proof of concept for a process that can be very attractive for industrial SD solar cell production.