Alkali polishing additive for TOPCon 3.0 polyfinger process

TOPCon batteries have entered the 3.0 era, and the Polyfinger (rear-side laser patterning) process has become one of the key technical pathways for efficiency improvement. The microscopic pyramid base morphology formed during the alkali polishing step is directly inherited by the subsequently deposited tunneling oxide/Poly-Si stack, ultimately affecting the contact interface quality after laser grooving.

Conventional alkali polishing additives typically produce square platform structures with flat tops on the silicon substrate surface. However, these square pyramid bases have distinct steep steps in the transition region from the platform edge to the bottom. This stepped morphology during subsequent LPCVD deposition leads to non-uniform coverage of the tunneling oxide at the step corners and stress concentration during Poly-Si crystallization. After laser grooving, the exposed contact interface still exhibits microscopic morphology abrupt changes, affecting paste filling completeness and interface contact density, limiting further reduction in contact resistance.

This article introduces an alkali polishing additive specifically developed for the TOPCon 3.0 Polyfinger process. Its core advantage lies in inducing the formation of shaped pyramid bases with smooth transition characteristics (arc-shaped or rounded-top pyramid structures) on the silicon substrate surface. By optimizing the steep step-like edges of conventional square pyramid bases into smoothly graded arc profiles, it effectively improves the coverage uniformity and stress distribution of subsequent stack deposition, provides a flatter contact substrate for laser grooving, significantly enhances the quality of the metallization contact interface, and helps the Polyfinger process achieve higher efficiency gains.

Mechanism of Shaped Pyramid Bases in Enhancing the Polyfinger Process

In the Polyfinger process, the pyramid base morphology formed by rear-side alkali polishing serves as the "starting point" for all subsequent processes. The differences in impact on the subsequent process chain between conventional square platform bases and the shaped pyramid bases of this solution are as follows:

• Influence transfer process of conventional square platform bases:

Alkali polishing forms square platform bases with flat tops but steep step-like edges → During tunneling oxide/Poly-Si deposition, coverage at the step corners is thinner with stress concentration → After laser grooving, the contact surface in the step regions exhibits microscopic abrupt changes → Incomplete screen-printing paste filling → High contact resistance

• Influence transfer process of shaped pyramid bases (this solution):

Alkali polishing forms arc-shaped/rounded-top pyramid bases with smooth transitions from top to bottom and no steep steps → During tunneling oxide/Poly-Si deposition, the curved surface achieves uniform coverage and uniform stress distribution → After laser grooving, the contact surface is smooth and continuous → Complete paste filling → Reduced contact resistance and improved contact reliability

The specific enhancement mechanisms can be summarized in three points:

•Improved coverage uniformity: 

Regardless of whether the LP or PE route is adopted, the arc-shaped pyramid bases eliminate deposition weak zones at the step corners, ensuring the continuity and integrity of both the tunneling oxide and Poly-Si layers.

• Reduced Poly-Si crystallization stress: 

At the step corners of conventional square pyramid bases, stress concentration occurs during high-temperature deposition due to geometric discontinuities, affecting the crystallization quality of the Poly-Si layer. Arc-shaped pyramid bases eliminate stress concentration points through smooth transitions, resulting in more uniform internal stress distribution in the film, improved crystallization quality, and reduced interface defect state density.

• Improved metallization contact interface continuity:

 The flat and smoothly transitioning pyramid base morphology is inherited by the stack surface. After laser grooving, the exposed contact interface has no step discontinuities. During printing and sintering, the paste can more fully fill the microstructure of the contact surface with no voids or gaps, enhancing the metal-silicon contact density and significantly reducing contact resistance (Rs).

The above mechanisms ultimately manifest electrically as a significant reduction in series resistance (Rs), which in turn translates into improvements in FF and Eta.

Application Results

On a TOPCon 3.0 Polyfinger production line, a back-to-back comparison test was conducted between this additive and the conventional mass-production additive (210×210 mm² cells). The electrical performance advantages are as follows:

Note: BL refers to the conventional alkali polishing additive scheme on the production line; "+" indicates improvement, "–" indicates reduction.

Interpretation of results: 

Compared to the conventional additive, this solution achieves improved conversion efficiency (Eta), increased fill factor (FF), and reduced series resistance (Rs). The simultaneous improvement of these three parameters is driven by the enhanced contact interface quality provided by the shaped pyramid bases in the laser areas—the significant reduction in Rs directly contributes to the FF improvement, which in turn drives the overall efficiency gain.

This alkali polishing additive, by inducing the formation of shaped pyramid base structures, effectively increases the metal contact area, improves paste filling, and optimizes interface stress distribution, thereby achieving a systematic enhancement of contact performance. In the TOPCon 3.0 Polyfinger mass production process, this additive provides a viable solution for contact optimization in high-efficiency TOPCon batteries compared to conventional schemes.