We demonstrate the fabrication of H2-evolving photocathodes using p-n junction Si MW arrays and earth-abundant Ni-Mo alloy hydrogen evolution catalysts. 62 6.4 J−E data for a representative Ni–Mo-coated n+p Si MW array photocathode 65 6.5 J −E data for a representative Si photocathode without any catalyst and.
Solar energy conversion
Solar radiation can only be captured when the sun is shining, so a storage system is needed to provide a continuous, stable energy supply. The first two technologies discussed above cannot provide electricity at night without the use of expensive storage technology such as batteries or flywheels.
Requirements for a solar fuel system
Calculating practical device efficiencies
For a system capable of generating the full potential required to achieve water splitting, the conversion efficiency of solar energy to hydrogen (ηST H) can be calculated by taking the ratio of the chemical energy produced (the product of the operating current, Jop and the potential required to drive the response, 1.23 V) and the lighting power density using Eq. It is important to recognize that the photoelectrode efficiency, calculated by characterizing individual photoelectrodes based on the power they produce, represents only a portion of the Gibbs free energy required to fuel the overall water splitting reaction. while the rest of the energy is supplied by a potentiostat. .[13, 14].
Photocathodes for hydrogen evolution
By independently characterizing photoanodes and photocathodes, the expected performance of an integrated system without external electronics can be directly calculated. The overall water splitting efficiency for photoelectrolytic cells can be estimated by overlaying the individually tested J-E data for each photocathode/anode (Fig. The intersection of the two curves indicates the maximum operating current density (Jop) for the entire cell, which can then be are used to calculate the overall device efficiency based on Eq.
Coupling catalysts to photocathodes
Motivation for Si microwires
Radial geometry
Right: A highly structured substrate allows equal catalyst loading but does not prevent the semiconductor from absorbing all incident light. However, on a highly structured surface, the same catalyst loading can be achieved without blocking all the light (right of Figure 1.4).
Designing an artificial photosynthesis device
Overview
Several different geometries have been proposed to decouple the requirements for light absorption and carrier extraction in photovoltaic materials.[31, 32] The foundation of the current research effort at Caltech is built on the early modeling work of Kayes et al. All the devices discussed in this thesis are grown with Cu as the VLS catalyst.
Fabrication of substrates and wire growth
The (111) crystal orientation is the preferred direction of growth, so by using a (111) oriented substrate, this process leads to the formation of a wire of Si that grows perpendicular to the substrate. A positive photoresist layer (Shipley 1813, Microchem) was deposited on the oxide, with the photoresist exposed to produce a series of holes in the photoresist.
Flexible, polymer-embedded wire arrays
To remove this layer, the samples were etched for 20–30 s in 4 M KOH (aq) (semiconductor grade), and then rinsed with copious amounts of 18 MΩ-cm H2O. To address this issue, methods were developed to embed the wire arrays in a flexible polymer, such as PDMS (polydimethylsiloxane), and then to physically remove the wire/PDMS membrane from the substrate using a razor blade.[43] It was also demonstrated that after removing the wires, the original substrate could be reused to grow wires by selectively etching the base of the wires and then electrodepositing the new VLS metal catalyst into the holes in the patterned SiO2 (Fig.
Radial p-n junction Si microwire devices
Fabrication of solid-state p-n junction devices
The SiO2 was etched from the top of the wires using buffered HF, and then the PDMS was removed from the wires by etching in a mixture of tetrabutylammonium fluoride (TBAF) and 1-methyl-2-pyrrolidone (NMP) (3:1; v/v; NMP/ . 75% TBAF in water).[45] A piranha etch (4:1 H2SO4:H2O2) was used to remove any organic residue from the wires. The Si MW arrays were immersed in buffered HF for 30 s to remove the P2O5 glass formed during doping from the Si surface.
Characterization of solid-state p-n junction devices
The height of the PDMS layer was controlled by changing the amount of toluene and spinning speed. The PDMS was allowed to cure either on a hot plate at 150◦C for 30 min, or in a vacuum oven at 80◦C overnight.
Introduction
Methyl viologen
The MV2+/+ redox system usually forms a high barrier height with p-type semiconductors, and forms an ohmic contact with n-type systems.[57]. This redox couple has been used to look at the band energetics of functionalized Si surfaces [58, 59], to characterize porous Si [60], and to investigate the performance of p-type GaP as a photocathode.[61] A second reversible reduction to MV0 was reported at potentials ranging from -1.1 to -1.35 V vs .
Experimental setup
- Methyl viologen cell set-up
- Electrode fabrication
- Photoelectrochemical data collection
- Correction for series resistance and concentration overpotential
- External quantum efficiencies
- Calculating figures of merit
Right: After generation of MV+· species, carbon fabric electrodes are used as counter and reference electrodes. Data collected in "clear" MV2+/+ can be used to verify the actual density measured with both forms of the redox couple present.
Photoelectrochemical characterization of VLS Si MW arrays using MV 2+/+ 27
Large area VLS growth of Si MW arrays
After growth, the samples were cooled to 700 °C under H2 in about 15 min so that any Cu in the wires would diffuse to the surface where it could later be removed by chemical etching. The addition of the PDMS filler helped to improve the current density in the wires and was necessary to prevent shunting between the p-Si wires and the p+ substrate.
Processing of radial p-n junction Si MW arrays
The blue curve shows the data from the p-Si MW with oxide boots, and the red curve depicts the performance of the polymer-embedded n+-p junction in contact with MV2+/+. This behavior demonstrates that the top contact, and not the material quality of the Si MW array or doping profile, limited device performance.
Measurement of internal quantum yield
Internal quantum efficiency of a VLS-grown Si MW array: (blue) the EQY, measured in 10 mM MV2+/+ in pH=1 electrolyte; (black) the absorption measured for an array of peeled wires; and (red) the IQY calculated as the ratio of the EQY to the absorption.[66] (Image credit: M. Kelzenberg). Manufacturing good electrical contacts with highly structured photovoltaic materials can be challenging. Therefore, it is useful to be able to monitor the quality of semiconductor devices, regardless of the choice of contact material.
Conclusions
If the band edge positions of the Si/H+/H2(aq) contacts could be controlled to eliminate the dependence on pH, the photovoltage of Si/H+/H2(aq) contacts could be increased by simply increasing the pH of the aqueous electrolyte. The ability to decouple the photovoltage of a Si photocathode from the pH of the contacting aqueous solution should increase the versatility of these materials for use in photoelectrochemical fuel-forming systems.
Experimental setup
VLS wire growth and processing
The energy of the direct p-Si/MV2+/+ contact should change as a function of pH, due to the known dependence of the planar band potential of Si on the pH of the solution. In contrast, the introduction of an n+-doped emitter layer to create a buried junction should decouple the banding, and thus the photovoltage, of the photoelectrode from the energy of the semiconductor/liquid contact.
Fabrication of planar controls
The doping density of the wires was 2 × 1017 cm−3, as determined by measurements with four-point probes on single microwires. [29, 72] As discussed in section 3.3.3, an oxidizer was used to define the area of the wires. n+ emitter region. To create the n+emitter layer, the samples were then etched in BHF for 30 seconds, rinsed in H2O and dried with N2.
Photoelectrochemical measurements
Results
The diode quality factor, n, was also calculated for each type of electrode, from the slope of Voc vs. the logarithm of the photocurrent at different illumination intensities:. Planar p-Si electrodes and both planar and radial diffuse junctions exhibited while the diode quality factors for p-Si wire arrays were 1.5-1.6.
Discussion
The inset indicates that the surface energy of n+-Si is shifted as in fig. For an n+p buried junction, Φb and the open-circuit voltage during illumination are decoupled from the pH of the liquid electrolyte.
Conclusion
Most reports in the literature on p-Si photocathodes for solar water splitting use noble metal catalysts (Pt, Pd) in acidic electrolytes, but most HER catalysts found in Earth are not stable at low pH. However, the Si surface has poor kinetics for the hydrogen evolution reaction (HER) and requires a catalyst to achieve efficient solar energy conversion. There are several reports in the literature on Si-based photocathodes, but most devices are based on noble metal catalysts such as Pt or Pd, which are too rare and expensive to be viable options for a low-cost water splitting device. ].
Electrochemical characterization
Photoelectrochemical cell setup
We will then focus on the different catalyst systems investigated, and compare their performance on planar and structured Si electrodes.
HER photocathode efficiency calculations
The open circuit voltage (Voc) of the devices is the maximum free energy generated by the device relative to the thermodynamic HER potential under the specified experimental conditions. The short-circuit current density (Jsc) is the measured current density of the device at ERHE (Eq. 5.2).
Hydrogen evolution catalysts
Katalitiko a datos ti panangrugi ti HER para iti Pt, Ni, ken Ni–Mo kadagiti degenerate a Si MW a substrato. Ti dakdakkel a kalawa ti rabaw dagiti Si MW nga array ket mangipalubos ti nangatngato a panagkarga ti katalista, isu a ti Ni-Mo ken Ni ket sumayaat unay.
Catalyst deposition
The deposition was performed by immersing the electrode in the plating solution for 2 min increments and then testing the electrochemical performance of each electrode. Electrodeposition of Ni was performed by depositing Ni from a bath containing 1.3 M NiIIsulfamate and 0.5 M H3BO3.
Planar Si with HER catalysts
Due to the instantaneous nucleation method for Ni, the particle sizes were much smaller than those observed for Pt (center image Fig. Deposition was performed by immersing the Si sample in the electroless deposition bath for 4 min and then testing electrochemical performance.The samples were immersed in the deposition solution and illuminated at high intensity (~400 mW cm−2) with an ELH type tungsten-halogen lamp.
Right: J−Edata for planar p-Si electrodes with electroless Pt as HER catalyst in an aqueous solution pH = 2 (0.5 M K2SO4 adjusted with H2SO4). Left: J−E data for planar p-Si electrodes with Pt, Ni, or Ni-Mo as HER catalyst.
HER spectral response
Integrating sphere measurements have shown that tilting the angle of the Si MW array relative to a direct illumination source can maximize its absorption. This phenomenon was confirmed by measuring the Jsc while the sample was held at different angles to the illumination source. The spectral response data for the sample with enhanced absorption is shown in orange in Fig.
Conclusions
In this work, Ni-Mo has been deposited on n+p Si MW arrays with the goal of creating efficient H2-evolving photocathodes consisting only of earth-rich materials. We have also fabricated free-standing polymer-supported membranes based on Ni-Mo coated n+p Si MW microwire arrays and have compared the performance of such systems with the behavior of Si MW arrays physically connected to the crystalline Si growth substrate.
Experimental
- Photoelectrochemical characterization
- Characterization of the n + p junction
- Ni–Mo catalyst deposition
- Current density vs. potential behavior
- Stability
- Spectral response
- Free-standing polymer-supported photoactive membranes
HER J−E data for n+p Si MW arrays with Ni–Mo deposited by galvanostatic (red) or potentiostatic (blue) methods. Ni-Mo coated Si MW devices without scattering particles on the bases of Si MWs exhibited Jsc ~ 5 mA cm−2 (Table 6.1).
Discussion
Balancing catalytic activity and short-circuit current density
This finding is consistent with the expectation that the increased surface area of the wire arrays provides an advantage in the efficient catalytic switching of these devices. Polarization curves for a Pt disc in 0.5 M H2SO4 (green) and pH 4.5 KPH buffer (red). maximum power point potential, EP max).
Optical absorption in catalyst-coated Si MW arrays
Radial n+p electrodes coated with Si MW (closed symbols) and n+p-Si (open symbols) electrodes in H2 buffer pH 4.5 KHP over a range of light intensities.
Conclusion and outlook
One of the main motivating factors for the development of Si MW photovoltaic and solar fuel systems is the potential to use low-cost, high-power manufacturing techniques to produce flexible, single-crystal solar absorbers. Additionally, device physics modeling suggests that the optimal wire diameter for solar energy conversion is >10 µm, a size regime that was difficult to fabricate using traditional photolithography.[102] Electrodeposition of the VLS catalyst allows the growth of wires in this size regime, while also eliminating metal waste from evaporation and lift-off of the VLS catalyst.
Patterning the VLS growth substrate
Reactive stamping
A Si/SU-8 master was used to create a PDMS template, which can then be used to create multiple agarose stamps. The measured stamping speeds were much lower than reported in the literature (0.2 µm h−1 versus 2 µm h). Attempts were made to adjust the etch rate by varying the soak time of the stamp in HF, changing the mineral oil conditions, and by varying the HF concentration, but no significant improvement in the etch rate was observed.
Imprint lithography
The stamp was printed into sol-gel which was spun onto a wafer while the sol-gel cured. To keep the sol-gel pattern uniform over large areas, the master is designed.
Electrodeposition of the VLS catalyst
Controlling wire diameter
Electrodeposition provides the ability to control the amount of Cu VLS catalyst without worrying about thickness limitations due to the release of lithographically defined metal films, allowing the diameter of the wires to be increased more than was possible using evaporation of the catalyst. Here, a similar approach was taken to precisely control the amount of Cu deposited via electrolysis and ultimately correlate this with the diameter of the VLS-grown microwire arrays.
Electrodepositing on Si patterned with sol-gel
VLS wire growth
SEM of electrodeposited Cu with different thicknesses and geometries. tionship was developed as a useful tool for predicting the thickness of Cu in a wide range of pattern geometries, as the measured thicknesses differed systematically from the thicknesses predicted by a simple faradaic analysis of the charge required for electrode deposition.[111 ]. Top: SEM images of Cu electrodeposited with weak adhesion on the sol-gel patterned substrate.
Comparison of wires grown from electrodeposited and evaporated Cu
Single-wire measurements
The effective doping density for the wires grown from electrolytically deposited Cu was well over an order of magnitude higher than that of high-purity Cu, but not as large as the optimal doping range for photovoltaic devices. Left: MV2+/+ J−E data of VLS wires grown from electrolytically deposited Cu on microimprinted patterned substrate.
Regenerative photoelectrochemistry
MV2+/+ regenerative photoelectrochemistry was used to characterize the performance of p-type Si MW arrays grown from electrolytically deposited Cu.
Cu detection in VLS Si MW
Another benefit of fabricating wires with larger diameters is that it enabled the use of electron beam techniques such as microprobe analysis which have sample interaction volumes of several microns in diameter. The Cu content in the cross sections of the wire was analyzed using a JXA-8200 electronic probe and was not shown to be above the detection limit of the machine, even in interface regions likely to be rich in Cu impurities.
Predicting VLS wire diameter
The thickness and geometry of the electrolytically deposited catalyst before growing was measured by cross-sectional SEM and used to calculate Vcat. This factor is probably responsible for the volumetric expansion of the eutectic mixture, which contains both Si and Cu.
Conclusion
Motivation from experimental results
One of the highest efficiency photocathode devices ever reported consisted of p-type InP decorated with Pt group metals, which generated high conversion efficiency in acidic electrolyte. In our work we observed that when Pt is deposited as a thin film on clean p-Si using e-beam evaporation, the devices produce no photovoltage and the catalytic onset is almost identical to that of polished Pt (left side of figure 8.1). . XPS analysis of the Si 2p region showed that a significant amount of SiO2 is generated during the electroless Pt deposition.[79] More research is needed to understand whether the observed photovoltage is due to the different geometry of the catalyst particles (pinch-off), or to the formation of a metal/insulator/semiconductor junction due to a small amount of SiO2 between the Si and Pt -particle. , or both.
Pinch-off
For a simple geometry, such as a circular part, with low barrier height of radiusR0, the potential behind the point center can be calculated (in cylindrical coordinates) from Eq. A particle with a sufficiently small radius will be pinned, which means that the effective barrier height (Φef f) across a mixed contact junction will be much larger than that of a metal contact with a low barrier height.
Prior work on the n-Si/Ni system
In the limit of a very small particle, the patch can be approximated by a point dipole, and Eq.
Catalyst loading
To understand whether catalyst loading issues would limit the application of the pinch-off model, the effect of catalyst loading was investigated using Pt patterning on degenerate doped planar p-type Si. Modeling is needed to understand how higher catalyst loading and closely spaced metal particles affect band bending at the semiconductor/liquid interface.
Modeling inhomogeneous semiconductor contacts
Finite element device physics modeling
As expected, the conduction band energy reaches a maximum that increases as the size of the metal piece decreases. Interestingly, the magnitude of the change in conduction band maximum is less than 100 mV over this range of metal coverage.
Modeling the p-Si/Pt/HER system
Simulated band edge positions behind Ni circle on n-Si in contact with high barrier height electrolyte in the dark. Simulated edge positions of the valence band behind low barrier height Pt patches on p-Si in contact with aqueous pH=0 electrolyte.
Experimental design
Patterning the catalyst
Care was taken to place the samples perpendicular to the evaporation source to minimize deposition on the sidewalls of the resist material. The excess material on the sample is due to excess Ni on the photoresist sidewalls during evaporation that could not be removed during lift-off.
Electrochemistry
The exposed resist was removed with 1:3 MIBK:IPA, leaving a pattern of holes in the resist. The samples were briefly immersed in buffered HF to remove any native oxide from the Si surface, and then the metal was evaporated using an e-beam evaporator.
Future work
This system can be used to understand how the energy conversion efficiency of Si MW arrays is affected by array geometry, doping density, p-n junction formation and other processing steps. We demonstrated that it was possible to grow high-fidelity VLS Si MW arrays using electrodeposited Cu and substrates patterned with imprint lithography.
Future directions
Concurrent work has shown that Ag nanowires and PEDOT:PSS can be used to make electrical contact with Si MW arrays, but systems need to be developed to improve the ease of manipulation and handling of these membrane-based devices during processing.[65] . The VLS Si MW project demonstrated that it is possible to grow high-quality, single-crystalline Si material that can be embedded in a flexible substrate from an inexpensive precursor material using relatively simple technology.
Schottky contact definitions
Values used in simulations
- Overlaid J − E data for a photoanode and photocathode
- Schematic of planar vs. structured photo-catalytic electrodes
- Comparison of loading requirements for fast and slow catalysts
- Schematic of a membrane-based water splitting prototype design
- Schematic of VLS wire growth
- SEM images of VLS Si MW arrays grown from Cu and SiCl 4
- Schematic of peeling Si MW arrays and reusing the growth substrate
- Schematic of radial p-n junction formation
- Energetics of different semiconductor band edges and redox couples
- Methyl viologen: Structure and cyclic voltammetry
- Absorbance of methyl viologen
- Schematic of MV 2+/+ photoelectrochemical cell
- Raw and corrected J − E data for a planar p-Si electrode in MV 2+/+
- J − E data for planar p-Si and p-Si MW electrodes in MV 2+/+
- Large-area Cu-catalyzed VLS wire growth
- Methyl viologen characterization of Si MW arrays under different growth and
- J − V data for the same Si MW array at different stages of device processing. 32
- SEM images pf p-Si and n + p-Si silicon microwire arrays
- Modeling absorption is catalyst-coated Si MW arrays
- Schematic of reactive micro-contact printing
- Micro-contact stamps and patterned Si
- Fabrication schematic for micro-imprint lithography
- Sol gel patterned Si substrates
- SEM image of patterned sol-gel on Si after annealing
- Schematic of Cu electrodeposition
- Electrodeposited Cu on patterned SiO 2 before and after annealing
- Comparison of electrodeposited Cu thickness with charge density passed dur-
- Electrodeposited Cu in patterned sol-gel and resulting VLS wires
- VLS wires grown from electrodeposited Cu patterned with micro-imprint
- Single wire data for wires grown from evaporated and electrodeposited Cu . 86
- Large-diameter VLS wires before and after catalyst removal
- Comparison of predicted and actual wire diameters
- J − E data for Pt/p-Si photocathodes
- Schematic of the pinch-off effect for p-type semiconductors
- HER polarization data for evaporated Pt films of different thicknesses on p + Si 98
- Simulated band edge positions behind Ni circle on n-Si in contact with elec-
- Varying metal coverage for n-Si/Ni|electrolyte system
- Simulated J − V data for n-Si system with inhomogeneous contacts
- Simulated band edge positions behind Pt circles of various sizes and pitches
- Pinch-off criteria for p-Si/Pt system
Controlling the ph dependence of the band edges of si(111) surfaces using mixed methyl/allyl monolayers. Control of the stability, electron transfer kinetics, and pH-dependent energetics of Si/H2O interfaces by methyl termination of Si(111) surfaces.
