Characterization of height-selective emitters

In this work we characterize height-selective emitters. The concept of a height-selective emitter is similar to the classical selective emitter but applied on a much smaller scale. Concretely, we masked textured wafers by a spin coating oxide. Scanning electron microscopy (SEM) analysis revealed that only the valleys between the tips of the pyramids are covered by the oxide while the tips are open. Thus, subsequent phosphorus diffusion leads to a highly doped emitter on the tips of the pyramids, and a lowly doped emitter at the sides and valleys in between. This was demonstrated by SEM measurements on cross section prepared samples. Further characterization consisted of IV, spectral response, and contact resistivity measurements as well as the determination of the silver crystallites’ distribution, both on height-selective and standard emitter cells with different emitter sheet resistances. In contrast to homogeneous emitter cells, height-selective emitter cells, although exhibiting a high macroscopical sheet resistance, could still be well contacted by standard silver pastes. Just a few silver crystallites at the tips of the pyramids allowed a good contact. Up to 110 Ω/sq fill factors higher than 77.5% could be achieved. The last part of the characterization consisted of carrier lifetime measurements on standard and height-selective emitter samples with symmetric structures to determine the implied open circuit voltage, emitter saturation current density and effective lifetime to identify limiting factors such as emitter, front, and back surface recombination. These measurements demonstrated that height-selective emitters benefit from a lower recombination in the emitter allowing a higher open circuit voltage.