High‑Efficiency (20.19 %) Inverted‑Pyramid Single‑Crystalline Silicon Solar Cell Fabricated via Optimized Metal‑Assisted Chemical Etching

Abstract

We report a single‑crystalline silicon (sc‑Si) solar cell featuring an inverted‑pyramid microstructure that achieves a record conversion efficiency of 20.19 % on a standard 156.75 × 156.75 mm² wafer. The structure is produced through a two‑step process: (1) metal‑assisted chemical etching (MACE) employing an ultra‑low silver‑ion concentration, and (2) optimized alkaline anisotropic texturing. By carefully tuning MACE and texturing parameters, we control the pyramid dimensions to a 1 µm width, yielding a normal reflectivity of 9.2 %. Compared with conventional upright‑pyramid wafers, the inverted design delivers a 0.19 % boost in efficiency and an additional 0.22 mA cm⁻² in short‑circuit current density (J_sc), surpassing previously reported structures. This approach offers a scalable pathway to high‑efficiency sc‑Si photovoltaics.

Background

Single‑crystalline silicon (sc‑Si) remains the benchmark material for commercial photovoltaics, owing to its superior photo‑conversion efficiency and proven reliability. However, recent advances in diamond‑wire sawing, passivation layers, and alternative crystalline configurations have narrowed the performance gap, forcing a re‑examination of surface light‑trapping strategies. Traditional upright‑pyramid texturing, limited by one‑step alkaline chemistry, achieves a reflectivity of 10–12 %—a plateau that yields diminishing returns on efficiency gains.

Black silicon, renowned for its sub‑0.3 % reflectivity across the UV–NIR spectrum, has emerged as a compelling alternative. While femtosecond laser ablation, reactive ion etching (RIE), and metal‑assisted chemical etching (MACE) can produce black silicon, MACE offers the best balance of compatibility, cost, and scalability for sc‑Si wafers.

Key challenges include managing the trade‑off between deep nanostructures (which enhance light trapping) and surface recombination (which degrades carrier lifetimes). Recent work has demonstrated that optimized MACE can generate high‑density nanoporous layers without excessive defect introduction. In this study, we extend that approach by integrating a low‑concentration Ag‑based MACE step with a controlled alkaline texturing stage, producing uniform inverted‑pyramid microstructures suitable for mass production.

Methods

Commercially sourced (100)‑oriented, p‑type sc‑Si wafers (200 ± 20 µm thick, 1–3 Ω cm) were pre‑cleaned in a NaOH/H₂O₂ bath (30 wt.%) and rinsed with ultrapure water. For MACE, wafers were first immersed in 0.2 M HF/3 × 10⁻⁵ M AgNO₃ at 25 °C to deposit Ag nanoparticles. Subsequent etching in a mixed acid (H₂O₂ 3.13 M/HF 2.46 M) with a 0.1 % commercial additive (C) for 3 min produced a nanoporous silicon layer. Ag residues were removed by a 5‑minute rinse in 0.1 M NH₃/0.1 M H₂O₂ and a final ultrapure water rinse.

The nanoporous wafers were then subjected to alkaline anisotropic texturing: a 0.003 M NaOH solution containing 0.4 % additive (A) at 60 °C for 7 min. This step converts the porous network into uniform inverted pyramids with a 1 µm base width. Subsequent industrial processing—p‑n junction formation by phosphorus diffusion, phosphoric‑silicate glass removal, SiNx antireflection coating via PECVD, and electrode metallization—completed the device fabrication.

Characterization encompassed SEM (Hitachi S‑4800), 3D metrology for micro‑feature sizing, UV–VIS–NIR reflectance measurements (UV‑3101PC with integrating sphere), SiNx thickness and refractive index (Filmetrics F20‑UV), and photovoltaic testing (Enlitech QE‑R and PVIV‑411V). FDTD simulations (λ = 631.57 nm) assessed optical field distribution in the inverted versus upright geometries.

Results and Discussion

Ag nanoparticle deposition was optimized to 5 ppm AgNO₃ for 2 min at room temperature, yielding a dense, uniform 15 nm‑diameter layer (Fig. 1). Higher concentrations or longer dwell times produced irregular, elongated particles that compromise surface uniformity.

High‑Efficiency (20.19 %) Inverted‑Pyramid Single‑Crystalline Silicon Solar Cell Fabricated via Optimized Metal‑Assisted Chemical Etching

Following MACE, nanoporous Si exhibited diameters ranging from 20 nm (1 min) to 110 nm (5 min) and depths of 1.3–3 µm (Fig. 2). Optimal conditions were 3 min at 35 °C, balancing minimal surface roughness and maximal light trapping (Fig. 3). Longer etching (>4 min) introduced curved cylindrical pores that increased reflectivity.

Alkaline anisotropic texturing transformed the porous network into inverted pyramids. Process times of 5–9 min produced pyramids with widths of 500 nm to 1 µm and depths of 350 nm to 400 nm (Fig. 4). At 7 min, the pyramids were uniformly distributed, 1 µm wide, and exhibited a 9.2 % average reflectivity—an improvement over standard upright textures.

Electrical evaluation revealed the inverted‑pyramid cell outperformed its upright counterpart: a 20.19 % efficiency versus 19.95 %, J_sc of 38.47 mA cm⁻² versus 38.25 mA cm⁻², V_oc of 647 mV versus 645 mV, and a 0.05 % higher fill factor (Table 2). IQE remained comparable, while EQE showed a 5–10 % gain in the 300–600 nm range, consistent with the reduced surface reflectivity (Fig. 6). FDTD simulations confirmed that the inverted geometry concentrates the electromagnetic field within the silicon bulk, enhancing photon absorption relative to upright pyramids (Fig. 7).

Conclusions

The integration of ultra‑low‑concentration MACE with a finely tuned alkaline texturing step yields a uniform 1 µm inverted‑pyramid microstructure that reduces surface reflectivity to 9.2 % and boosts conversion efficiency to 20.19 %. The resulting J_sc of 38.47 mA cm⁻² demonstrates the potential of this approach for scalable, high‑efficiency sc‑Si photovoltaic manufacturing. Further optimization of passivation layers and light‑trapping parameters could push efficiencies beyond 21 %.