Designing a hybrid thin-film/wafer silicon triple photovoltaic junction for solar water splitting

Solar fuels are a promising way to store solar energy seasonally. This paper proposes an earth-abundant heterostructure to split water using a photovoltaic-electrochemical device (PV-EC). The heterostructure is based on a hybrid architecture of a thin-film (TF) silicon tandem on top of a c-Si wafer (W) heterojunction solar cell (a-Si:H (TF)/nc-Si:H (TF)/c-Si(W)) The multijunction approach allows to reach enough photovoltage for water splitting, while maximizing the spectrum utilization. However, this unique approach also poses challenges, including the design of effective tunneling recombination junctions (TRJ) and the light management of the cell. Regarding the TRJs, the solar cell performance is improved by increasing the n-layer doping of the middle cell. The light management can be improved by using hydrogenated indium oxide (IOH) as transparent conductive oxide (TCO). Finally, other light management techniques such as substrate texturing or absorber bandgap engineering were applied to enhance the current density. A correlation was observed between improvements in light management by conventional surface texturing and a reduced nc-Si:H absorber material quality. The final cell developed in this work is a flat structure, using a top absorber layer consisting of a high bandgap a-Si:H. This triple junction cell achieved a PV efficiency of 10.57%, with a fill factor of 0.60, an open-circuit voltage of 2.03 V and a short-circuit current density of 8.65 mA/cm2. When this cell was connected to an IrOx/Pt electrolyser, a stable solar-to-hydrogen (STH) efficiency of 8.3% was achieved and maintained for 10 hours.</p

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.</p

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate-gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions

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

Tandem Cuprous Oxide/Silicon Microwire Hydrogen-Evolving Photocathode with Photovoltage Exceeding 1.3 V

Large research efforts have been devoted to optimizing the output of earth-abundant photoabsorbers in solar-to-fuel (S2F) devices. Here, we report a Cu2O/Ga2O3 heterojunction/Si microwire photocathode with an underlying buried radial Si p–n junction, which achieves efficient light harvesting across the visible spectrum to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%, with a photocurrent onset above +1.35 V vs RHE, a photocurrent density of ∼10 mA/cm2 at 0 V vs RHE, and an ideal regenerative efficiency of 5.51%. We show step-by-step the effects of every photocathode design element (i.e., Si p–n junction, Cu2O layer thickness, microwire length, microwire pitch, etc.) on the overall efficiency of our final microwire Si/Cu2O photocathode by comparing every addition to a baseline Cu2O photocathode. Lastly, we show a stable operation exceeding 200 h at a bias potential of +1.0 V vs RHE, with an average current density of 4.5 mA/cm2

Integration of Molybdenum-Doped, Hydrogen-Annealed BiVO 4 with Silicon Microwires for Photoelectrochemical Applications

H-BiVO 4-x :Mo was successfully deposited on microwire-structured silicon substrates, using indium tin oxide (ITO) as an interlayer and BiOI prepared by electrodeposition as precursor. Electrodeposition of BiOI, induced by the electrochemical reduction of p-benzoquinone, appeared to proceed through three stages, being nucleation of particles at the base and bottom of the microwire arrays, followed by rapid (homogeneous) growth, and termination by increasing interfacial resistances. Variations in charge density and morphology as a function of spacing of the microwires are explained by (a) variations in mass transfer limitations, most likely associated with the electrochemical reduction of p-benzoquinone, and (b) inhomogeneity in ITO deposition. Unexpectedly, H-BiVO 4-x :Mo on microwire substrates (4 μm radius, 4 to 20 μm spacing, and 5 to 16 μm length) underperformed compared to H-BiVO 4-x :Mo on flat surfaces in photocatalytic tests employing sulfite (SO 3 2- ) oxidation in a KPi buffer solution at pH 7.0. While we cannot exclude optical effects, or differences in material properties on the nanoscale, we predominantly attribute this to detrimental diffusion limitations of the redox species within the internal volume of the microwire arrays, in agreement with existing literature and the observations regarding the electrodeposition of BiOI. Our results may assist in developing high-efficiency PEC devices

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

Tandem Cuprous Oxide/Silicon Microwire Hydrogen-Evolving Photocathode with Photovoltage Exceeding 1.3 v

Large research efforts have been devoted to optimizing the output of earth-abundant photoabsorbers in solar-to-fuel (S2F) devices. Here, we report a Cu2O/Ga2O3 heterojunction/Si microwire photocathode with an underlying buried radial Si p-n junction, which achieves efficient light harvesting across the visible spectrum to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%, with a photocurrent onset above +1.35 V vs RHE, a photocurrent density of ∼10 mA/cm2 at 0 V vs RHE, and an ideal regenerative efficiency of 5.51%. We show step-by-step the effects of every photocathode design element (i.e., Si p-n junction, Cu2O layer thickness, microwire length, microwire pitch, etc.) on the overall efficiency of our final microwire Si/Cu2O photocathode by comparing every addition to a baseline Cu2O photocathode. Lastly, we show a stable operation exceeding 200 h at a bias potential of +1.0 V vs RHE, with an average current density of 4.5 mA/cm

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%