Large-Scale Silicon Nanowire Arrays on 6‑inch Mono‑ and Multi‑Crystalline Solar Cells via Enhanced Metal‑Assisted Chemical Etching

Introduction

Silicon nanostructures offer superior light‑trapping and anti‑reflection compared to planar silicon, enabling higher solar‑cell efficiencies without expensive antireflective coatings. Metal‑assisted chemical etching (MacEtch) provides a cost‑effective, room‑temperature route to fabricate such nanostructures, yet uniform large‑area production has been limited. This study addresses that gap by refining MacEtch for 6‑inch wafers and validating the resulting SiNW arrays in industrial‑scale Al‑BSF solar cells.

Experimental Methods

MacEtch Mechanism

In an AgNO₃/HF solution, Ag⁺ oxidizes Si to SiO₂, while reduced Ag deposits on the surface. HF removes the oxide, leaving Ag‑protected regions etched anisotropically to form SiNWs. The process is highly controllable through oxidant concentration and metal deposition pattern.

SiNW Fabrication on 6‑inch Wafers

Monocrystalline (100) and multi‑crystalline p‑type wafers (180 µm thick, 0.5–3 Ω·cm) were cleaned, then immersed in 23 mM AgNO₃/6.4 M HF for 3 min 19 s at room temperature. Two etching protocols were compared: (1) traditional single‑batch immersion, and (2) a modified holder‑based method that ensures simultaneous exposure of the entire wafer to the etchant, yielding uniform SiNWs.

Al‑BSF Solar‑Cell Fabrication

After SiNW texturing, wafers underwent POCl₃ diffusion (850 °C, 30 min) to form an N⁺ emitter (sheet resistance ≈75 Ω/□), followed by HF etch to remove PSG. A 70 nm SiNx:H anti‑reflection layer was deposited by PECVD, and standard Ag‑paste (front) and Al‑paste (back) screen printing was performed. Two device sets—SiNW‑textured and acid‑textured references—were fabricated and tested under AM 1.5G illumination.

Results and Discussion

Uniform SiNW Arrays on 6‑inch Wafers

Method 2 produced SiNW arrays with excellent uniformity: SEM cross‑sections show 470 nm length and high density across the wafer. Method 1 exhibited pronounced edge‑to‑center non‑uniformity due to differential Ag⁺ concentration during immersion.

Surface Morphology

Figures (omitted) illustrate the transition from pyramid or as‑cut surfaces to dense SiNW arrays. Multi‑crystalline wafers reveal orientation‑dependent SiNW growth (<100>, <110>, <111>), affecting optical coloration and reflectance.

Optical Reflectance

Measured reflectance of mono‑crystalline SiNW wafers remains <6 % across 400–1000 nm, with a minimum of 3 % at 500 nm. Reflectance variation across the wafer is <22 %. Multi‑crystalline wafers exhibit <10 % reflectance (minimum 4 % at 400 nm). Uniformity is preserved between 1.5 × 1.5 cm and 6‑inch substrates (difference <1 %).

Minority Carrier Lifetime

Effective minority carrier lifetime decreases from 2.55 µs to 2.11 µs (mono‑crystalline) and from 1.51 µs to 1.37 µs (multi‑crystalline) after SiNW formation, attributable to increased surface recombination velocity (S_eff). Despite this, device performance improves due to reduced reflectance.

Device Performance

SiNW‑textured Al‑BSF cells deliver an average efficiency of 17.83 % versus 17.23 % for acid‑textured controls—a 0.6 % absolute gain. Short‑circuit current density increases by 1.2 %, open‑circuit voltage by 1.35 %, and fill factor improves, likely due to enhanced light trapping and larger electrode contact area.

Conclusions

The enhanced MacEtch process reliably produces uniform, low‑reflectance SiNW arrays on 6‑inch mono‑ and multi‑crystalline wafers. The resulting Al‑BSF cells achieve >17.8 % efficiency, confirming the technique’s commercial viability. Its low cost, simplicity, and scalability position it as a strong candidate for next‑generation photovoltaic manufacturing.