Search

In crystalline silicon photovoltaics (c‐Si PV), a pulsed laser can be used as a substitute for a high‐temperature furnace dopant diffusion/activation step. In contrast to furnace‐based activation, lasers can be used to achieve highly localized doping with controlled dopant concentrations, useful in advanced architectures such as the interdigitated back contact (IBC) solar cell. In this study, a pulsed ultraviolet (UV) laser is utilized for phosphorus dopant activation within a low‐pressure chemical vapor deposited (LPCVD) polycrystalline silicon (poly‐Si) passivated contact layer. The highest implied open‐circuit voltage iVoc values achieved using this approach reach 726 mV. However, this comes at the expense of high specific contact resistivities ρc, which is attributed to a lower dopant concentration across the poly‐Si(n+)/SiOx/c‐Si interface. Regardless, the optimum iVoc, ρc combination is measured at a laser fluence of 0.78 J cm−2 producing values of 712 mV and 89 mΩ‐cm2, respectively. These values are still compatible with high‐efficiency solar cell designs, underscoring the feasibility and effectiveness of this approach. [ABSTRACT FROM AUTHOR]

Abstract In crystalline silicon photovoltaics (c‐Si PV), a pulsed laser can be used as a substitute for a high‐temperature furnace dopant diffusion/activation step. In contrast to furnace‐based activation, lasers can be used to achieve highly localized doping with controlled dopant concentrations, useful in advanced architectures such as the interdigitated back contact (IBC) solar cell. In this study, a pulsed ultraviolet (UV) laser is utilized for phosphorus dopant activation within a low‐pressure chemical vapor deposited (LPCVD) polycrystalline silicon (poly‐Si) passivated contact layer. The highest implied open‐circuit voltage iVoc values achieved using this approach reach 726 mV. However, this comes at the expense of high specific contact resistivities ρc, which is attributed to a lower dopant concentration across the poly‐Si(n+)/SiOx/c‐Si interface. Regardless, the optimum iVoc, ρc combination is measured at a laser fluence of 0.78 J cm−2 producing values of 712 mV and 89 mΩ‐cm2, respectively. These values are still compatible with high‐efficiency solar cell designs, underscoring the feasibility and effectiveness of this approach.

State-of-the-art monocrystalline Si (c -Si) solar cells require passivating contacts to achieve a high degree of charge-carrier separation and collection. In this work, we focus on boron-doped polycrystalline Si on locally etched silicon nitride/silicon oxide (PLENO) passivating contacts. In PLENO contacts, excellent surface passivation is provided by the ∼10 nm dielectric bilayer, while pinholes in the dielectric bilayer, that are filled with doped polycrystalline Si, provide charge-carrier selectivity and transport. During PLENO fabrication, etch undercut in the dielectric bilayer occurs. Using electrical characterization and microscopies, we show that undercut causes pinholes to be electrically resistive in PLENO. A processing sequence that eliminates the undercut in the final PLENO structure results in electrically conductive pinholes with low contact resistivity. [ABSTRACT FROM AUTHOR]

Passivating contacts based on poly-Si have enabled record-high c-Si solar cell efficiencies due to their excellent surface passivation quality and carrier selectivity. The eventual existence of pinholes within the ultra-thin SiO x layer is one of the key factors for carrier collection, beside the tunneling mechanism. However, pinholes are usually believed to have negative impact on the passivation quality of poly-Si passivating contacts. This work studied the influence of the pinhole density on the passivation quality of ion-implanted poly-Si passivating contacts by decoupling the pinhole generation from the dopants diffusion process by means of two annealing steps: (1) a pre -annealing step at high temperature after the intrinsic poly-Si deposition to visualize the formation of pinholes and (2) a post -annealing step for dopants activation/diffusion after ion-implantation. The pinhole density is quantified in the range of 1✕106 to 3✕108 cm2 by the TMAH selective etching approach. The passivation quality is discussed with respect to the pinhole density and the post-annealing thermal budget (TB) for dopants diffusion. The study shows that a moderate pinhole density does not induce doping profile variations that can be detectable by the coarse spatial resolution of ECV measurements. It is surprising that the existence of pinholes in a moderate density within our thickness fixed SiO x layer can effectively enhance the passivation qualities for both n + and p + poly-Si passivating contacts. We speculate the reason is due to the enhanced field-effect passivation at the pinhole surrounding. In fact, the variation of the passivation quality depends on the balance between a strengthened field-effect passivation and an excessive local Auger recombination, being both effects induced by the higher and deeper level of dopants diffused into the c-Si surface through the pinholes. • Decoupling of the pinhole formation and the dopant diffusion using two annealing processes. • Application of the thermal diffusion budget concept to the fabrication of the poly-Si passivating contacts. • The existence of pinholes within the SiO x layer enhances the localized dopant diffusion. • The dopant diffusion through the pinholes enhances the electrical field passivation locally. • A moderate pinhole density within SiO x induces obvious increase in passivation qualities for both n+ and p+ poly-Si passivating contacts, although the sensitivity of passivation on pinhole density is less for p + poly-Si. [ABSTRACT FROM AUTHOR]

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiO x ) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiO x film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells.

The diffusion of dopants from the poly-Si layer through the interfacial oxide into the crystalline silicon wafer is studied experimentally and via numerical simulation. It is found that phosphorus piles up at the poly-Si/SiO x interface but boron readily diffuses through the oxide into the silicon and thereby forms a less shallow junction which adversely affects surface passivation. Although boron diffusion was suppressed by using a nitrided oxide – realized by a plasma oxidation process – a reduction in passivation was observed due to deterioration of the interface. Via 1D and 3D process simulation it was established that diffusion through pinholes contributed little to the aggregate diffusion profiles measured by ToF-SIMS but direct diffusion through the oxide was the main diffusion mechanism. The segregation coefficient at the poly-Si/SiO x interface and the diffusivity in SiO x were determined. Using these parameters simulation-based predictions were conducted in order to pave the way for future optimization of hole-selective passivating contacts. [ABSTRACT FROM AUTHOR]