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.

Crystalline Si Surface Passivation with Nafion for Bulk Defects Detection with Electron Paramagnetic Resonance

“…Temporary surface passivation with a Nafion film has several advantages when introduced to photovoltaic applications: (a) the Cz Si wafer quality such as the bulk lifetime can be measured at high throughput at any device fabrication stages for process development; (b) it is compatible with photoluminescence (PL) mapping in ambient conditions; (c) the process for passivation with Nafion is fast, poses fewer constraints on process complexity, and does not destroy pre-existing bulk defects compared to elevated-temperature passivation with SiO x , SiN x , Al 2 O 3 , and a -Si/H; and (d) it can be applied to a small fragment of a degraded module to investigate microscopic defect formation mechanisms. In this latter application, passivation of laser-cut cell edges of the module fragment is needed prior to characterization with techniques such as electron paramagnetic resonance (EPR), electrically detected magnetic resonance (EDMR), deep-level transient spectroscopy (DLTS), and cell current–voltage ( J – V ) and Suns– V oc measurements. , In such cases, the defective, laser-damaged cell edges will obscure the true degradation mechanisms, and Nafion can effectively passivate them without causing further degradation of the bulk due to elevated-temperature passivation processes. Previously, Ji et al attributed the high quality of passivation of c-Si with Nafion to a high density of negative fixed charges ( Q f ≈ 7 × 10 13 cm –2 ).…”