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TOPCon Cell Aluminum Contact Process: Contact Resistivity Optimization Achieves 23.7% Efficiency
Date : 2025-06-18Views : 200
As the market share of TOPCon solar cells exceeds 50%, its double-sided silver paste consumption (12-15 mg/W) leads to a surge in production cost. In this study, we propose to replace the back-side silver contacts with aluminum paste to solve the over-alloying problem at the aluminum/polysilicon interface through material formulation innovation and process optimization. The study is based on numerical simulation combined with a Millennial TLM contact resistance tester to accurately quantify the evolution of contact resistivity (ρc), and numerical simulation to clarify the industrialization path.
Research Methodology
Samples: Industrial n-type TOPCon cells and precursors (including finished cells, unmetallized and front-side metallized precursors) with M6 dimensions (166 × 166 mm²) were used.
Laser Cored Opening (LCO): a femtosecond UV laser (λ = 257 nm) was used to selectively remove the backside SiNₓ layer to prepare the contact area for the non-fired-through (nFT) aluminum contacts.
Contact composite extraction: area-weighted method was used to calculate J0, metal.
Metallization process: screen-printing of conventional and special aluminum pastes to optimize the contact performance through nine firing profiles (peak temperature 620/680/760°C, speed slow/medium/fast).
Characterization and simulation: microstructure analysis using PL imaging, SEM, AFM; Quokka 3 software to simulate efficiency loss and optimization paths.
Technical bottlenecks and breakthroughs in aluminum contacts
When conventional aluminum paste comes into contact with the n⁺ polysilicon layer, it forms a eutectic layer as deep as 8 μm, which completely penetrates the SiO₂/polysilicon layer of the TOPCon structure, resulting in a high contact complex current density (J0, metal) of 391,735 fA/cm², which is much higher than that of the silver contact at 50 fA/cm², and makes it impossible to use the contact directly.
To address this issue, Specialty Aluminum Paste raises the melting point above 760°C by adding Al-Si alloy particles with 25-40 wt% Si content, inhibiting over-alloying with polysilicon, while optimizing the glass powder composition to improve ohmic contact with n⁺ polysilicon. Optimization of sintering conditions shows that at 680°C - medium thermal budget - the J0, metal of the special aluminum paste can be reduced to 41,000 fA/cm², with a contact resistivity (ρc) as low as 0.4 mΩ・cm², laying the foundation for aluminum contact applications.
Aluminum back contact TOPCon cell performance evaluation
A femtosecond UV laser (257 nm) was used for the LCO process to achieve contact opening without damaging the passivation layer, and combined with screen printing and co-firing processes, the aluminum back-contact TOPCon cell was successfully prepared. The experimental results show that the efficiency of the champion cell reaches 22.92%, and the efficiency gap with the backside silver reference cell (23.66%) is 0.8%. The efficiency loss is mainly due to the high contact composite of the aluminum contacts (resulting in 10mV VOC loss) and the increase of series resistance (2.5% FF loss), while there is no significant difference in the JSC due to the same design of the front side.
Quokka 3 numerical simulation improves efficiency
Quokka 3 simulations show that 51% of the losses in an aluminum-backed cell come from the back surface, 88% of which are attributed to contact compound and resistive losses of 0.50 mW/cm² and 0.57 mW/cm², respectively, which are more than 10 times that of silver contacts. To close the efficiency gap, the simulation identifies key optimization parameters:
contact complex current density J0, metal needs to be reduced to below 10,000 fA/cm²;
contact resistivity ρc below 1.1 mΩ・cm²;
target J0, metal needs to be 2,500 fA/cm² when ρc is 0.4 mΩ・cm², which, combined with the LCO dot spacing of 8 μm, enables a reference cell efficiency of 23.7%. The reference cell efficiency of 23.7% can be achieved with 8μm LCO dot spacing.
This study successfully demonstrates the feasibility of replacing silver in TOPCon batteries with aluminum back contacts. The special aluminum paste achieves a cell efficiency of 22.92% by suppressing alloying and optimizing sintering conditions, narrowing the efficiency gap to 0.8%. Numerical simulations point the way to further performance improvements by lowering J0, metal to 2,500 fA/cm² and optimizing contact resistance.
Millennial TLM Contact Resistance Tester
Contact number: 400 008 6690
Millennial TLM contact resistance tester is equipped with contact resistivity test function, which can realize fast, flexible and accurate detection.
Static test repeatability ≤1%, dynamic test repeatability ≤3%
Line resistance measurement accuracy up to 5% or 0.1Ω/cm
Switch between contact resistivity test and line resistance test at will
Customize a variety of probes for measurement and analysis
The study successfully demonstrated the feasibility of silver replacement in TOPCon batteries with aluminum back contacts by providing critical support with the MillennialTLM contact resistance tester, providing a practical path to reduce silver consumption in terawatt-scale manufacturing of TOPCon batteries.
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