Wouter Vijselaar

Fabrication and doping methods for silicon nano- and micropillar arrays for solar cell applications: a review

Silicon is one of the main components of commercial solar cells and is used in many other solar-light-harvesting devices. The overall efficiency of these devices can be increased by the use of structured surfaces that contain nanometer- to micrometer-sized pillars with radial p/n junctions. High densities of such structures greatly enhance the light-absorbing properties of the device, whereas the 3D p/n junction geometry shortens the diffusion length of minority carriers and diminishes recombination. Due to the vast silicon nano- and microfabrication toolbox that exists nowadays, many versatile methods for the preparation of such highly structured samples are available. Furthermore, the formation of p/n junctions on structured surfaces is possible by a variety of doping techniques, in large part transferred from microelectronic circuit technology. The right choice of doping method, to achieve good control of junction depth and doping level, can contribute to an improvement of the overall efficiency that can be obtained in devices for energy applications. A review of the state-of-the-art of the fabrication and doping of silicon micro and nanopillars is presented here, as well as of the analysis of the properties and geometry of thus-formed 3D-structured p/n junctions.

Molecular Monolayers for Electrical Passivation and Functionalization of Silicon-Based Solar Energy Devices

Silicon-based solar fuel devices require passivation for optimal performance yet at the same time need functionalization with (photo)catalysts for efficient solar fuel production. Here, we use molecular monolayers to enable electrical passivation and simultaneous functionalization of silicon-based solar cells. Organic monolayers were coupled to silicon surfaces by hydrosilylation in order to avoid an insulating silicon oxide layer at the surface. Monolayers of 1-tetradecyne were shown to passivate silicon micropillar-based solar cells with radial junctions, by which the efficiency increased from 8.7% to 9.9% for n+/p junctions and from 7.8% to 8.8% for p+/n junctions. This electrical passivation of the surface, most likely by removal of dangling bonds, is reflected in a higher shunt resistance in the J-V measurements. Monolayers of 1,8-nonadiyne were still reactive for click chemistry with a model catalyst, thus enabling simultaneous passivation and future catalyst coupling.

Formation of Nanocrystalline Barium Titanate in Benzyl Alcohol at Room Temperature

Nanocrystalline barium titanate (8-10 nm crystallite size) was prepared at temperatures of 23-78 °C through reaction of a modified titanium alkoxide precursor in benzyl alcohol with barium hydroxide octahydrate. The room temperature formation of a perovskite phase from solution is associated with the use of benzyl alcohol as solvent medium. The formation mechanism was elucidated by studying the stability and interaction of each precursor with the solvent and with each other using various experimental characterization techniques. Density functional theory (DFT) computational models which agreed well with our experimental data could explain the formation of the solid phase. The stability of the Ti precursor was enhanced by steric hindrance exerted by phenylmethoxy ligands that originated from the benzyl alcohol solvent. Electron microscopy and X-ray diffraction indicated that the crystallite sizes were independent of the reaction temperature. Crystal growth was inhibited by the stabilizing phenylmethoxy groups present on the surface of the crystallites.

Spatial decoupling of light absorption and catalytic activity of Ni–Mo-loaded high-aspect-ratio silicon microwire photocathodes

A solar-driven photoelectrochemical cell provides a promising approach to enable the large-scale conversion and storage of solar energy, but requires the use of Earth-abundant materials. Earth-abundant catalysts for the hydrogen evolution reaction, for example nickel–molybdenum (Ni–Mo), are generally opaque and require high mass loading to obtain high catalytic activity, which in turn leads to parasitic light absorption for the underlying photoabsorber (for example silicon), thus limiting production of hydrogen. Here, we show the fabrication of a highly efficient photocathode by spatially and functionally decoupling light absorption and catalytic activity. Varying the fraction of catalyst coverage over the microwires, and the pitch between the microwires, makes it possible to deconvolute the contributions of catalytic activity and light absorption to the overall device performance. This approach provided a silicon microwire photocathode that exhibited a near-ideal short-circuit photocurrent density of 35.5 mA cm−2, a photovoltage of 495 mV and a fill factor of 62% under AM 1.5G illumination, resulting in an ideal regenerative cell efficiency of 10.8%.Catalysts are required to increase the rate of H2 evolution over silicon photocathodes, but their presence can lead to parasitic light absorption. Here, the authors explore the contributions of catalysts and light absorption to the overall performance of Si microwires, depositing Ni–Mo catalysts spatioselectively to optimize efficiency.

Bright ways to utilize the sun: towards solar-to-fuel devices

Silicon (Si) is an attractive semiconductor material for a wide range of applications. Particular advantages result from the larger surface area of silicon microwires, and from surface functionalization with different materials that can be employed to tune the functionality of the substrate towards a desired application. For example, silicon can be used in photovoltaic (PV) cells, or it can be employed as one of the materials in a so-called solar-to-fuel (S2F) device, solely by changing the surface functionalization. In a simplistic view, a S2F device is constructed by adding electrocatalysts to a PV cell. However, to produce an efficient S2F device, silicon has to be modified in several manners, both regarding its structure (e.g. shaping into microwires) and using additional materials (e.g. catalysts, protective layer, junction, anti-reflection coating, etc.). The thesis discusses the use of silicon as a base material for a S2F device, employing structuring and modification. In conclusion of the thesis, silicon microwires with radial p/n junctions were fabricated and optimized with respect to various parameters, i.e., junction depth, microwire height and pitch, and passivation and anti-reflection layers. The investigated microwires proved to be a very suitable template for the development of a hydrogen half-cell, i.e., a photocathode, in a silicon-based S2F device. The increased surface area of Si microwires is beneficial for both the light absorption capabilities and the catalyst activity. A strong dependence on the overall efficiency has been found on catalyst loading, both catalyst coverage in combination with microwire pitch. By careful design of the parameters mentioned above, a highly efficient, earth-abundant, Si-based photocathode has been fabricated that can function in both acidic and alkaline electrolytes. Lastly, a full simplistic wireless S2F was constructed, and experimentally validated. Micropores within a Si triple PV cell provided ionic short cuts, which decreased the overall ionic potential loss, and prevented a pH gradient within the device.

Selective Functionalization with PNA of Silicon Nanowires on Silicon Oxide Substrates

Silicon nanowire chips can function as sensors for cancer DNA detection, whereby selective functionalization of the Si sensing areas over the surrounding silicon oxide would prevent loss of analyte and thus increase the sensitivity. The thermal hydrosilylation of unsaturated carbon–carbon bonds onto H-terminated Si has been studied here to selectively functionalize the Si nanowires with a monolayer of 1,8-nonadiyne. The silicon oxide areas, however, appeared to be functionalized as well. The selectivity toward the Si–H regions was increased by introducing an extra HF treatment after the 1,8-nonadiyne monolayer formation. This step (partly) removed the monolayer from the silicon oxide regions, whereas the Si–C bonds at the Si areas remained intact. The alkyne headgroups of immobilized 1,8-nonadiyne were functionalized with PNA probes by coupling azido-PNA and thiol-PNA by click chemistry and thiol–yne chemistry, respectively. Although both functionalization routes were successful, hybridization could only be detected on the samples with thiol-PNA. No fluorescence was observed when introducing dye-labeled noncomplementary DNA, which indicates specific DNA hybridization. These results open up the possibilities for creating Si nanowire-based DNA sensors with improved selectivity and sensitivity.

Efficient and Stable Silicon Microwire Photocathodes with a Nickel Silicide Interlayer for Operation in Strongly Alkaline Solutions

Most photoanodes commonly applied in solar fuel research (e.g., of Fe2O3, BiVO4, TiO2, or WO3) are only active and stable in alkaline electrolytes. Silicon (Si)-based photocathodes on the other hand are mainly studied under acidic conditions due to their instability in alkaline electrolytes. Here, we show that the in-diffusion of nickel into a 3D Si structure, upon thermal annealing, yields a thin (sub-100 nm), defect-free nickel silicide (NiSi) layer. This has allowed us to design and fabricate a Si microwire photocathode with a NiSi interlayer between the catalyst and the Si microwires. Upon electrodeposition of the catalyst (here, nickel molybdenum) on top of the NiSi layer, an efficient, Si-based photocathode was obtained that is stable in strongly alkaline solutions (1 M KOH). The best-performing, all-earth-abundant microwire array devices exhibited, under AM 1.5G simulated solar illumination, an ideal regenerative cell efficiency of 10.1%.