The interplay between emitter conformality and field-effect passivation in pyramid structured silicon solar devices

Light management in crystalline silicon solar cells has conventionally been achieved by random micropyramid texturing but other non-conventional surface texturing methods, such as diffraction gratings, have also attracted attention with the objective of attaining near-ideal light trapping. Unlike previous investigations on the subject in which the focus was predominantly to minimize optical losses, we report on surface passivation difference between the micron and nanoscale pyramid textures. From simulations, we find that the conformality of thermally diffused emitters is impacted by downsizing the pyramid texture to the light wavelength scale, which, in turn, affects the field-effect passivation of the resulting homojunction. The investigation reveals that, for a conformal junction, the field-effect passivation is diminished due to the formation of weak electrical field zones beneath the pyramid vertices, which is usually the case when homojunction emitters are formed by thermal diffusion on micropyramid textured surfaces. In contrast, thermal diffusion in nanoscale pyramid textured surfaces typically leads to non-conformal dopant profiles, resulting in flatter junctions and yielding stronger field passivation effect by preventing the formation of these weak electrical field zones. Experimentally, enhanced field-effect passivation is indirectly observed in terms of improvement in the effective minority-carrier lifetimes when substituting the standard micropyramid texturing by nanoscale pyramid gratings. At the device level, we demonstrate that the enhancement in the field-effect passivation increases the open-circuit voltage with respect to the micropyramid texture under the conditions of comparable chemical passivation and surface-area increase.