Introduction

From the previous study to a new breakthrough

Yesterday we discussed a paper from Jiangnan University on TOPCon copper plating: laser grooving damages silicon, the crystallinity drops by 30 percentage points, and annealing is required to repair it. That paper concluded that 750°C annealing + HF cleaning could restore the efficiency from 23.41% back to 24.85%.

But anyone on a production line knows that 750°C annealing itself carries a hydrogen-induced blister risk — the temperature window is extremely narrow. Above 775°C the rear passivation layer blisters, and at 800°C the result is even worse than no annealing at all.

Is there a better way?

A second paper just published in 2026 by Jiangnan University + Jiangsu Xianghuan + DR Laser offers a new answer: use LIF (Laser-Induced Firing) to replace traditional low-temperature sintering, while simultaneously repairing the laser damage.

The results: efficiency improvement of +0.45% abs., Voc gain of 0.86mV, and — a major improvement in contact resistance uniformity.

1. A quick recap: the TOPCon copper plating flow and its pain points

The standard process and where it hurts

The standard TOPCon Ni/Cu plating flow:

Laser grooving → High-temperature annealing for damage repair → HF cleaning → Ni plating → Low-temperature sintering → Cu plating

Two pain points:

• Laser grooving damages silicon: as discussed in the previous article, crystallinity drops from 99.3% to 69.8%, requiring high-temperature annealing for repair.

• Traditional low-temperature sintering is non-uniform: the furnace heats the entire cell, edges dissipate heat faster while the center stays hotter, causing contact resistance to be high at the edges and low at the center — non-uniform current collection hurts FF.

The core breakthrough of this new paper: inserting LIF into the plating flow kills two birds with one stone — it replaces the non-uniform low-temperature sintering and assists in repairing the laser damage.

2. What is LIF, and how is it different from traditional sintering?

Furnace heating vs. point-to-point welding

Traditional low-temperature sintering: place the whole cell in a furnace and bake at 200–400°C. The problem is uneven heating — edges cool faster, the center gets hotter, and contact resistance varies significantly across the cell.

LIF (Laser-Induced Firing): a 1064nm infrared laser rapidly scans the front of the cell while a reverse bias (2–18V) is applied. The laser excites photogenerated carriers, the reverse bias drives them directionally, producing precise localized Joule heating at the metal–silicon interface.

One-sentence difference: traditional sintering is "whole-cell baking", LIF is "point-to-point welding". LIF only heats the contact region under the gridlines, leaving everything else thermally untouched.

3. How well does LIF work on copper-plated cells?

Finding the sweet spot at 14V

The paper first runs a baseline experiment: apply LIF at different reverse bias voltages on cells that have already completed Ni/Cu plating.

Optimal parameters: 14V reverse bias, efficiency gain +0.401% abs., FF gain 1.22%, Rs reduction 23%.

Why does higher voltage make things worse?

The paper uses Suns-Voc to measure the dark saturation current densities J01 and J02:

• J01 (representing pn-junction recombination): little change with voltage

• J02 (representing metal–silicon interface recombination): lowest at 14V, soars at 16–18V

Translation: too much voltage means excessive Joule heating, and the interface gets "welded to death". The window sits right around 14V.

4. Why can LIF repair laser damage?

Raman spectroscopy reveals the secret

The paper ran a key experiment: strip the plated metal off and use Raman spectroscopy to measure the crystallinity of the silicon under the gridlines.

On top of high-temperature annealing, LIF further pushes crystallinity higher.

The mechanism: LIF generates a localized instantaneous high temperature (far above traditional annealing temperatures) that allows amorphous silicon to recrystallize more completely, and it only heats the regions under the gridlines, leaving the rear passivation layer untouched.

This solves the lingering concern from the previous article — the temperature window for high-T annealing is narrow, and above 775°C the rear passivation blisters. LIF is local heating; the rear is unaffected, so the temperature can go higher and the repair effect is better.

5. When should LIF be applied? Timing matters

Three candidates and a clear winner

The plating process has three steps: Ni plating → low-temperature sintering → Cu plating. Where should LIF be inserted?

The paper compares three timings:

Conclusion: LIF works best when placed at the very end — after Cu plating is complete.

Why?

After Cu plating, electrode resistance drops dramatically. When LIF applies voltage, current distribution is more uniform, Joule heating is more uniform, and interface contact is optimized more thoroughly.

If LIF is applied only on the Ni layer (before Cu plating), the resistance is high; the same voltage produces excessive Joule heating, which can easily "weld the interface to death".

6. A bigger discovery: LIF can fully replace low-temperature sintering

Skipping the furnace altogether

If LIF can optimize the Ni–Si contact, then can we simply skip the traditional low-temperature sintering step entirely?

The paper designed an experiment (Group D): Ni plating → LIF (8V) → direct Cu plating, skipping the low-temperature sintering step.

Results:

Group D's contact resistance uniformity crushes every group that includes traditional sintering.

Why?

Traditional sintering furnaces heat unevenly — edges dissipate heat fast, the center is hotter — causing contact resistance to be higher at the edges and lower at the center. LIF is a point scan; every point receives exactly the same energy, uniform by nature.

Further optimizing the LIF voltage to 6V, Group D reaches an efficiency of 24.74%, with Voc reaching 696.72mV — +0.45% abs. higher in efficiency and +0.86mV higher in Voc than the traditional sintering + no LIF baseline.

7. Production-line implications: is the mass-production threshold for copper plating lowered?

Three concrete advances

This paper delivers several tangible advances:

1. Voc damage can be repaired, and repaired better. The 750°C annealing from the previous article had a narrow temperature window and a risk of rear-side blistering. LIF heats locally, the rear stays safe, and the repair is more effective.

2. One process step is saved, but equipment investment must be weighed. Traditional flow: Ni plating → low-T sintering → Cu plating. LIF approach: Ni plating → LIF → Cu plating. Saves the sintering furnace and process time, but LIF equipment itself is more expensive, and integration with the plating line is more complex. The actual ROI depends on equipment quotations.

3. Contact resistance uniformity is the hidden bonus. Traditional sintering shows an edge-to-center contact resistance gap of 3.53Ω; the LIF approach cuts it down to 0.45Ω. Better uniformity means more uniform current collection, higher FF, and lower hot-spot risk at the module level.

But mass-production hurdles remain:

• LIF equipment investment: while replacing the sintering furnace, you add a laser + power supply + control system. Equipment vendor pricing decides the economics.

• Line integration complexity: LIF must seamlessly dock with the plating line, and cycle-time matching (the paper uses a 20 m/s scan speed) needs validation.

• GW-scale consistency: the paper is at lab/pilot level; yield stability at large-scale mass production still needs supporting data.

8. Comparison with Aiko ABC

Two paths, two stories

Aiko's BC architecture naturally avoids the laser-grooving pitfall. TOPCon cannot avoid it, but LIF offers a "fill-the-pit + optimize" combo solution — not only repairing damage, but also saving a process step and improving uniformity.

9. Summary

Where things stand

This new paper from Jiangnan University proves one thing: the laser damage in TOPCon copper plating can not only be repaired, but LIF repairs it better than traditional annealing — and along the way it also solves the uniformity problem of low-temperature sintering.

Efficiency gain of +0.45% abs., Voc gain of 0.86mV, and major improvement in contact resistance uniformity — these three numbers are worth a serious evaluation on any production line.

The mass-production threshold still exists, but the technical roadmap is becoming clearer.

Discussion topic: Is LIF replacing low-temperature sintering the "final kick" for mass-producing TOPCon copper plating, or just a "lab-side icing on the cake"?

Reference information:

• Title: Integration of laser-induced firing with Ni/Cu plating for TOPCon solar cell metallization

• Authors: Jingyun Zhang, Xi Xi, Jianbo Shao et al. (Jiangnan University + Jiangsu Xianghuan Technology + DR Laser)

• Journal: Solar Energy Materials and Solar Cells

• Year: 2026

• DOI: 10.1016/j.solmat.2026.114198

Ooitech's View

Ooitech believes: LIF turns laser-damage repair and sintering uniformity into a single step, making TOPCon copper plating a meaningfully more viable route to silver-free mass production.