Role of oxygen in the UV-ps laser triggered amorphization of poly-Si for Si solar cells with local passivated contacts

Abstract

In recent years, poly-Si based passivated contacts elevated the conversion efficiencies of crystalline Si solar cells to levels of 26%abs due to their outstanding electrical surface passivation performance and current transport characteristics. A major associated challenge, however, is the large parasitic light absorption within the doped poly-Si, regardless if the contacts are applied on the front and/or on the rear side of the solar cell. It, therefore, might be beneficial to confine the passivated contacts to local regions underneath the metal contacts. We present an effective and flexible laser-based approach to structure the poly-Si layer after its full-area deposition. Laser pulses with a pulse duration of 9 ps and a wavelength of 355 nm trigger an amorphization of the poly-Si surface. The minimum threshold fluence for amorphization is between 89 and 129 mJ/cm2. The a-Si layer, which is laterally homogeneous and up to (33 ± 4) nm in thickness, works as an etch barrier in an alkaline solution. The most robust barrier corresponding to the maximum thickness of the a-Si layer is found for a fluence of (270 ± 30) mJ/cm2. Besides the impact of the laser fluence on the etch resistiveness of the modified poly-Si layer, we study the role of oxygen during the laser process. We find that oxygen becomes incorporated into the material for certain laser fluences, which results in a more robust etch barrier. The amount of oxygen incorporated is below 3 wt. %. Eventually, we present a phenomenological model of our findings.

Large area co‐plated bifacial n‐PERT cells with polysilicon passivating contacts on both sides

“…For this purpose, we compared two patterning methods: one being the traditional masking and etch approach using inkjet 14 and the other, a simple and cost‐effective patterning using a UV laser 15 . The process using UV laser involves surface modification by amorphization and incorporation of oxygen at the poly‐Si surface, thereby making the surface etch‐resistant against alkaline solutions 16 . Since the surface behaves similarly to an oxidized silicon layer, this process is referred to as “laser oxidation” in this work.…”

“…15 The process using UV laser involves surface modification by amorphization and incorporation of oxygen at the poly-Si surface, thereby making the surface etch-resistant against alkaline solutions. 16 Since the surface behaves similarly to an oxidized silicon layer, this process is referred to as "laser oxidation" in this work. We note that other alternatives are reported in the literature such as masked plasma-enhanced chemical vapor deposition (PECVD) deposition for PERC 17 or interdigitated back contact solar cells (IBC).…”

Investigation of the Defect Distribution of Laser Contact Opening Applied to Poly‐Si/SiNxStacks

“…[ 14–16 ] For laser processing of poly‐Si, it is known that such amorphization can take place as well. [ 17 ] However, for poly‐Si surface passivation, the interfacial oxide (SiO x ) plays a crucial role and pinholes can result in increased recombination rates. [ 18,19 ] While amorphization is reversible, damage to the SiO x by laser process could therefore lead to permanently reduced surface passivation performance.…”