Protected Light-Trapping Silicon by a Simple Structuring Process for Sunlight-Assisted Water Splitting

Efficient Broadband Light-Trapping Structures on Thin-Film Silicon Fabricated by Laser, Chemical and Hybrid Chemical/Laser Treatments

Light-trapping structures formed on surfaces of various materials have attracted much attention in recent years due to their important role in many applications of science and technology. This article discusses various methods for manufacturing light-trapping "black" silicon, namely laser, chemical and hybrid chemical/laser ones. In addition to the widely explored laser texturing and chemical etching methods, we develop a hybrid chemical/laser texturing method, consisting in laser post-texturing of pyramidal structures obtained after chemical etching. After laser treatments the surface morphology was represented by a chaotic relief of microcones, while after chemical treatment it acquired a chaotic pyramidal relief. Moreover, laser texturing of preliminarily chemically microtextured silicon wafers is shown to take five-fold less time compared to bare flat silicon. In this case, the chemically/laser-treated samples exhibit average total reflectance in the spectral range of 250-1100 nm lower by 7-10% than after the purely chemical treatment.

Understanding the varying mechanisms between the conformal interlayer and overlayer in the silicon/hematite dual-absorber photoanode for solar water splitting

Dual-absorber photoelectrodes have been proved to have great potential in the photoelectrochemical (PEC) water splitting application due to their broadband absorption and suitable energy-band position, while the surface/interface issues are still not clearly resolved and understood. Here, during the preparation of a silicon/hematite dual-absorber photoanode achieved via synthesizing a Sn-doped hematite film on the silicon nanowire (SiNW) substrate, we separately introduced the conformal overlayer and interlayer of an Al₂O₃ thin film by atomic layer deposition. With the thickness-optimized interlayer (overlayer) of the Al₂O₃ thin film, the photocurrent density at 1.23V_RHE can be enhanced from 0.85 mA cm^-2 to 1.51 mA cm^-2 (1.25 mA cm^-2), and the on-set potential has a cathodic shift of ∼0.32 V. Although both the overlayer and interlayer modification can substantially improve the PEC performance, the underlying mechanisms are obviously different. The overlayer can only reduce the carrier recombination on the top surface and in the bulk of the hematite film; in contrast, the interlayer not only passivates the SiNW surface and bottom surface of the hematite film, but also the top surface of the photoanode due to Al³⁺ thermal diffusion from the bottom to the top surface of the hematite film and the resultant Al₂O₃ formation. This work deepens our understanding for the roles of the surface and interface engineering in the achievement of high-performance PEC systems based on dual or more absorbers.

Broad-Band Ultra-Low-Reflectivity Multiscale Micro-Nano Structures by the Combination of Femtosecond Laser Ablation and In Situ Deposition

Functional surfaces with broad-band ultralow optical reflection have many potential applications in areas like national defense and energy conversion. For efficient, high-quality manufacturing of material surfaces with antireflection features, a novel machining method for multiscale micro-nano structures is proposed. This method can enable the collaborative manufacturing of both microstructures via laser ablation and micro-nano structures with high porosity via in situ deposition, and it can simplify the fabrication process of multiscale micro-nano structures. As a result, substantially improved antireflection properties of the treated material surface can be realized by optimizing light trapping of the microstructures and enhancing the effective medium effect for the micro-nano structures with high porosity. In ultraviolet-visible-near-infrared regions, average reflectances, as low as 2.21 and 3.33%, are achieved for Si and Cu surfaces, respectively. Furthermore, the antireflection effect of the treated surface can also be extended to the mid-infrared wavelength range, where the average reflectances for the Si and Cu surfaces decrease to 5.28 and 5.18%, respectively. This novel collaborative manufacturing method is both simple and adaptable for different materials, which opens new doors for the preparation of broad-band ultra-low-reflectivity materials.

Quasi-hydrophilic black silicon photocathodes with inverted pyramid arrays for enhanced hydrogen generation

Micro-/nanostructured silicon (Si) photoelectrodes are promising for efficient solar-driven water splitting. In this work, an elaborate study on textured Si photocathodes is reported. Compared to conventional textured Si photocathodes, the well-designed Si photocathode with randomly-distributed inverted pyramid arrays (SiIPs) generates a larger photovoltage of 440 mV for its higher effective minority carrier density, and produces a higher photocurrent density at a high reverse bias voltage due to its quasi-hydrophilicity. With the help of cobalt disulfide (CoS₂) nanocrystals, sluggish charge kinetics of SiIP photocathodes can be further improved. The optimal SiIP/CoS₂ photocathode yields an onset potential of 0.22 V vs. reversible hydrogen electrode (RHE) and a saturated photocurrent density of 10.4 mA cm^-2 at -0.45 V (vs. RHE). Besides, this cathode produces a stable photocurrent density of ∼6.60 mA cm^-2 at 0 V (vs. RHE) for 12 000 s in acidic media. Notably, our work presents a facile and inexpensive method to fabricate efficient Si photoelectrodes, which may promote the evolution of textured Si-based electrodes for potential photoelectrochemical and photocatalytic applications.

All-in-One Derivatized Tandem p+n-Silicon-SnO2/TiO2 Water Splitting Photoelectrochemical Cell

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO₂ films with an outer 4.3 nm shell of TiO₂ added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p⁺n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru(bda)(4-O(CH₂)₃PO₃H₂)-pyr)₂], 1, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate), and chromophore, [Ru(4,4'-PO₃H₂-bpy) (bpy)₂]²⁺, RuP²⁺, (bpy = 2,2'-bipyridine), which form 2:1 RuP²⁺/1 assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm²; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm² with an 88% Faradaic efficiency (FE) for O₂ production at an applied bias of 600 mV versus RHE ( ACS Energy Lett. , 2016 , 1 , 231 - 236 ). The SnO₂/TiO₂-chromophore-catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO₂ (Ti/a-TiO₂) coating as an interconnect. In the integrated electrode, p⁺n-Si-Ti/a-TiO₂-SnO₂/TiO₂|-2RuP²⁺/1, the p⁺n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 μA/cm² were obtained for water splitting, 2H₂O → 2H₂ + O₂. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode.