Cost‑Effective PDMS Nanocone Cluster for Ultra‑Low Solar‑Cell Reflectance and Superhydrophobic Self‑Cleaning

Introduction

Solar photovoltaics are central to the transition toward renewable energy, yet their efficiency is often limited by sub‑optimal light absorption and surface fouling. Anti‑reflection (AR) coatings mitigate reflection losses by manipulating the refractive index gradient across the device surface [2,3]. Micro‑ and nanostructured AR layers—such as nanoholes, nanowires, nanoparticles, and nanocones—have been explored extensively [4–12]. Concurrently, superhydrophobicity is crucial for maintaining device performance, as dust accumulation can reduce efficiency by up to 50 % annually [4,13]. A hydrophobic surface that self‑cleans removes contaminants economically [14]. However, integrating both high‑efficiency AR and robust superhydrophobicity in a single, flexible film remains a challenge, because roughness that promotes wettability often introduces scattering losses.

Previous work has demonstrated AR and superhydrophobic nanostructures on rigid substrates (e.g., quartz) using reactive ion etching (RIE) [14], or achieved superior hydrophobicity with limited AR performance in the long‑wave regime [16]. There is a clear need for an environmentally friendly, flexible, and low‑cost approach that delivers both functions. In this study, we report a PDMS‑based nanocone cluster fabricated via a straightforward template process that meets these criteria.

Methods

Preparation of Nanocone Cluster Microstructures

We employed anodized aluminum oxide (AAO) templates produced by multistep anodization and wet etching to define the nanocone geometry. Three templates with aspect ratios (AR = height/pitch) of 1, 2, and 3 were fabricated, corresponding to heights of 450 nm, 900 nm, and 1350 nm and a constant pitch of 450 nm. Smaller pitches encourage aggregation during curing, yielding the desired cluster morphology.

After cleaning the AAO with acetone, ethanol, and distilled water, an anti‑sticking layer (GL‑AAC, GermanLitho) was spin‑coated. PDMS (GL‑ML CURE/GL‑ML BASE, 10:1 ratio) was drop‑cast onto the template, degassed in a vacuum chamber, and cured at 75 °C for 4 h (Fig. 1b,c). Upon cooling, the PDMS film was peeled from the template, producing a 0.3 mm thick film that exhibited inclined, aggregating nanocones—forming the cluster microstructure—due to surface energy minimization during drying (Fig. 1e).

a–e The schematic fabrication process of nanocone cluster microstructures

Characterizations

Surface morphology was examined by scanning electron microscopy (SEM, FEI NanoSEM650). Hydrophobicity was quantified with a JC2000D water contact angle tester (Zhongchen Digital Technic Apparatus). Optical performance was measured on a Varian Cary5E spectrophotometer over 400–1100 nm.

Results and Discussion

Figure 1 illustrates the fabrication steps, confirming the vertical orientation of nanocones before aggregation. The aggregation mechanism follows fractal percolation and Brownian motion, wherein individual cones coalesce into clusters to lower surface energy [22].

SEM images of a V‑shape AAO template and b–d PDMS nanocones with aspect ratios of 1, 2, and 3

The aspect‑ratio‑2 template produced clusters of 6–8 cones (≈950 nm diameter, 650 nm height), while the aspect‑ratio‑3 template yielded larger clusters (>10 cones). The aggregation is driven by increased side‑wall angles as height grows, encouraging neighboring cones to lean toward one another.

Optical measurements (Fig. 3) reveal that the patterned film outperforms flat PDMS across the visible spectrum. The aspect‑ratio‑2 clusters achieve reflectance below 3.5 % from 400 to 1100 nm, whereas aspect‑ratio‑1 and –3 clusters show <5 % and <4.5 % reflectance, respectively. The graded refractive index created by the clusters effectively suppresses front‑side reflection [23,24]. Transmittance is also higher for the aspect‑ratio‑2 film at long wavelengths, due to a smoother index gradient that balances scattering and transmission.

Reflectance and transmittance measurements of the PDMS films with and without nanocone cluster microstructures

Wettability tests (Fig. 4) show that flat PDMS is moderately hydrophobic (105°). Introducing nanostructures increases the contact angle, peaking at 151° for the aspect‑ratio‑2 clusters, satisfying the 150° threshold for superhydrophobicity. Aggregated clusters outperform isolated cones because they trap more air (f₂ = 0.831) and reduce solid contact fraction (f₁ = 0.169) per Cassie’s model [20,26–28]. The resultant self‑cleaning effect is evident in Fig. 5, where droplets roll off cleanly, lifting dust particles.

The water contact angles of PDMS films with different aspect ratios

Water droplets on a large surface of the superhydrophobic PDMS film

These results confirm that aggregated nanocone clusters provide superior optical and wetting performance compared to isolated nanocones. Although transfer to rigid substrates (silicon, sapphire) is currently limited by cluster stability, advances in nanoimprint and electron‑beam lithography could enable broader application.

Conclusions

We have demonstrated a facile, template‑based fabrication of PDMS nanocone clusters that simultaneously delivers sub‑3.5 % reflectance across the visible band and a 151° water contact angle. These dual attributes—ultra‑low optical loss and self‑cleaning superhydrophobicity—position the nanocone cluster as a cost‑effective, flexible coating for high‑efficiency photovoltaic and optoelectronic devices [29].

Abbreviations

3D

Three‑dimensional

AAO

Anodized aluminum oxide

AR

Aspect ratio

CA

Contact angle

PDMS

Polydimethylsiloxane

SEM

Scanning electron microscopy