As solar cell technologies continue to evolve, innovations like laser-assisted firing, such as Laser-Enhanced Contact Optimization (LECO), are pushing the boundaries of efficiency and reliability. Laser-assisted firing, a post-firing treatment, optimises metal-semiconductor contacts in silicon solar cells, enhancing current flow, reducing recombination losses, and enabling higher efficiency. Let’s explore the technical underpinnings of laser-assisted firing and its impact on the solar industry.
What is laser-assisted firing?
laser-assisted firing is an advanced process applied to finished solar cells to improve the quality of metal-semiconductor contacts. It employs a laser and reverse bias voltage to selectively reduce contact resistance. Initially developed for Passivated Emitter and Rear Cells (PERC),[1] laser-assisted firing has shown success with newer architectures like Tunnel Oxide Passivated Contact (TOPCon) cells.[2]
Figure 1: Schematic illustration of the laser-assisted firing process. A solar cell is held under a reverse bias and a high-intensity light is scanned over the surface. This results a high local current that results in heating at the nano-scale that improves the contact between the metal and the underlying silicon.
How Does Laser-Assisted Firing Work?
1. Laser-Induced Charge Carrier Injection
A focused laser scans across the solar cell surface, locally exciting charge carriers (electrons and holes) within the silicon wafer. This excitation increases carrier mobility and creates a dense flow of charge near the metal-semiconductor interface.[3]
2. Reverse Bias Voltage Application
A reverse bias voltage (typically around 10 V) is applied across the solar cell during the laser scanning process. This voltage drives the injected charge carriers through the contact interface, significantly reducing the contact resistivity. The electrical current density in these areas is amplified, facilitating enhanced ohmic contact.[3]
3. Nano-Scale Joule Heating
The high current density generates localised heating (nano-scale Joule heating), which softens the interface and enables the diffusion of metal (e.g., silver) into the silicon lattice. This forms robust silver-silicon alloy micro-contacts. Studies using electron microscopy have confirmed these microstructural changes, with nano-scale silicide formation enhancing conductivity.[3, 4]
Key Advantages of Laser Assisted Firing
Improved Contact Resistivity
Laser-Assisted Firing reduces the contact resistivity by factors of up to 10x, especially in underfired cells where the initial resistivity is higher. This improvement directly enhances the fill factor (FF), a critical performance parameter.
Optimised Firing Profiles
The Laser-Assisted Firing process enables solar cells to achieve optimal electrical contact at lower peak firing temperatures. Reducing the thermal stress on the cells preserves the passivation quality of layers like SiNx and AlOx, minimising recombination losses.[3]
Recovery of Off-Spec Cells
Manufacturing imperfections can result in cells with suboptimal efficiency. Laser-assisted firing has been shown to recover these off-spec cells to standard or even higher performance levels, reducing scrap rates and enhancing production yield.
Compatibility with Advanced Architectures
Laser-assisted firing has demonstrated exceptional compatibility with modern architectures like TOPCon, which benefit from its ability to form low-resistance contacts on boron emitters and n+-polycrystalline silicon layers. In such cases, efficiency gains of up to 0.6% (absolute) have been achieved.[1, 2]
Improved Damp-Heat reliability
Laser-assisted firing has also been shown to improve the reliability of TOPCon solar cells. It allows the use of lower-temperature processes and Al-free pastes. Compared to the standard one-step firing process, this approach improves power conversion efficiency and enhances resistance to damp-heat degradation, supporting cost-effective materials while maintaining reliability.[5]
Technical Highlights
Microstructural Modifications
High-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) have revealed significant microstructural changes induced by laser-assisted firing. These include:
- Formation of silver-silicon alloyed regions at the contact interface.
- Reduction in interfacial glass layer thickness, improving charge carrier flow.
- Localized recrystallisation at the silicon surface, enhancing electrical conductance.
Energy Efficiency
Compared to traditional thermal processes, laser-assisted firing requires significantly less energy. Its short processing time (1-2 seconds per cell) and localised action make it a sustainable choice for industrial production.
Applications in the Solar Industry
Laser-assisted firing is being adopted at scale in industrial settings due to its compatibility with conveyor-belt-based processing systems. It has been used effectively to enhance yield and efficiency in gigawatt-scale manufacturing. Additionally, its ability to process cells with diverse metallisation schemes, including screen-printed and plated contacts, broadens its applicability.
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