A DLTS Perspective on Electrically Active Defects in Plated Crystalline Silicon n+p Solar Cells

Abstract

Laser ablation (LA) has been compared with standard wet etching for contact opening in crystalline silicon n(+)p solar cells, from a perspective of electrically active defects, assessed by Deep-Level Transient Spectroscopy (DLTS). Copper metallization is employed, including a plated nickel diffusion barrier. It is shown that a hole trap around 0.17 eV above the valence band is systematically present in the depletion region of the junctions, irrespective of the contact opening method. This level could correspond with the substitutional nickel donor level in silicon and indicates that Ni in-diffusion occurs during the contact processing. No clear evidence for the presence of electrically active copper has been found. In addition, two other hole traps H2 and H3, belonging to point defects, have been observed after wet etching and standard LA, while for the highest laser power (hard LA) a broad band develops around 175 K, which is believed to be associated with dislocations, penetrating the p-type base region. Evidence will also be given for the impurity decoration of the dislocations, which enhances their electrical activity.