Realization of Micropatterned, Narrow Line-Width Ni–Cu–Sn Front Contact Grid Pattern Using Maskless Direct-Write Lithography for Industrial Silicon Solar Cells

Abstract

An industry-ready strategic process for the fabrication of cost-effective, micropatterned Ni–Cu–Sn front contact metallization has been demonstrated using maskless direct-write lithography, which could effectively reduce the shadow loss and thereby enhance the efficiency of silicon solar cells by increasing the active area. This investigation also addresses the challenging issues in Ni–Cu–Sn metallization, such as adhesion of the seed layer, low-ohmic contact formation, background plating, and cell processing complications. An eco-friendly aluminum paste with a sheet resistivity of 35 mΩ/cm2 has been developed to fabricate the rear contact on silicon solar cells. A front contact metallization grid with an optimal narrow finger width of 20 μm with an interfinger spacing of 1000 μm has been micropatterned using maskless direct-write lithography for the metallization process. To improve the electrical and mechanical properties of the nickel seed layer, the thickness was optimized as ∼100 nm with a contact resistivity of 6.87 μΩ cm2, which exhibited an adhesion strength of 2.5 N/mm. A low ohmic contact intermediate silicide layer has been created at the Ni–Si interface by the rapid thermal annealing process at 420 °C for 90 s with subsequent copper and tin electroplating to form the Ni–Cu–Sn contacts. An average cell efficiency of 18.5% is achieved for silicon solar cells with a micropatterned Ni–Cu–Sn-based narrow line-width front contact grid design, which could exhibit an ∼1% cell efficiency enhancement as compared to commercial Ag screen-printed solar cells. An ∼6% improvement in cell performance is achieved by reducing the shadow loss with the Ni–Cu–Sn-based front contact metallization as compared to the commercial Ag screen-printed metallization.

Narrow Line Width Ni–Cu–Sn Front Contact Metallization Patterns for Low‐Cost High‐Efficiency Crystalline Silicon Solar Cells using Nano‐Imprint Lithography

“…In this investigation, an eco‐friendly back contact aluminum paste was used for creating rear side metallization of the solar cell as demonstrated in our previous report. [ 26 ] Before the imprinting process, ANSYS 2D FEM‐based simulation tool was used to optimize the load/pressure for damage‐less imprinting process on poly (methyl methacrylate) (PMMA) coated solar cells. Based on the simulation studies, a cost‐effective process flow was used for fabricating the NIL Ni hard stamps using electroless deposition.…”

“…The sheet resistance of the aluminum layer after annealing process was estimated to be 0.45 Ω □ À1 , which shows better conductivity compared to commercial silicon solar cells. [26] Scheme 1. Process flow adopted for fabrication of Ni-Cu-Sn front contact metallization using nanoimprint lithography technique.…”

“…The imprinted solar cell showed %6% enhancement in cell performance by reducing the shadow loss which is also in good agreement with simulation results predicted in our previous report. [26] In order to compare the overall manufacturing cost of solar cells with commercial silver screen printed and NIL fabricated narrow finger width front contact metallization, a detailed cost analysis also was carried out as summarized in Table S1, Supporting information. Assuming the three-busbar case, the cost of ownership of Ag-screen printed and Ni/Cu/Sn metallization has been compared.…”

“…Recently, we have demonstrated a new process flow for fabrication of silver‐free, micropatterned Ni−Cu−Sn front contact metallization using mask‐less direct‐write lithography, which could effectively reduce the shadow loss and thereby enhance the efficiency of silicon solar cells by increasing the active area. [ 26 ] In order to eliminate the usage of expensive photoresists during the micropatterning, present investigation focuses on the development of inexpensive advanced pre‐ and post‐metallization processes using an in‐house‐built nanoimprint lithography (NIL) tool to achieve fine line width metallization pattern for silicon solar cells. This approach can pave way for a low‐cost industry‐ready Ni/Cu/Sn‐based narrow‐linewidth metallization process for enhancing solar cell performance.…”

Copper Outdiffusion from Copper-Plated Solar Cell Contacts during Damp Heat Exposure

“…In homojunction c-Si cells with direct contact between the metal and Si, this barrier layer is usually nickel (Ni) and is usually annealed to form an interfacial nickel silicide layer (NiSi X ). The annealed Ni layer is also understood to improve adhesion and reduce contact resistance. − A number of studies have tested for the downward diffusion of Cu (into cells) through the Ni barrier by means of accelerated testing at elevated temperatures (usually 200 °C), with detrimental degradation noted only when the Ni layer is insufficiently thick (<∼200 nm). , It is assumed that with sufficiently thick Ni barriers, the downward ingress of Cu is prevented both by the formation of NiSi X layer at the bottom of the stack and alloying at the Ni–Cu interface. ,, However, one study showed detrimental Cu precipitates underneath Ni diffusion barriers after 200 °C heat treatments and rapid cooling, suggesting that downward Cu may somehow defeat Ni barriers at moderate temperatures but that slow cooling inhibits the formation of performance-limiting extended Cu defects …”