While this chapter provides much of the background simulation and process development work for investigation of pinch-off for the Pt/p-Si/HER system, equipment outages have limited the number of experimental samples that have been prepared. Once experimental nano-patterned samples have been fabricated, the J −E performance of the patterned electrodes should be investigated in a high barrier height redox couple such as methyl viologen, as well as for the HER. Work is currently underway to fabricate nano-imprint stamps from EBPG-patterned masters to increase the throughput of fabrication for nano- patterned samples. This should enable investigation of different semiconductor and catalyst materials, as well as controlled introduction of thin barrier layers using ALD, which have been shown to improve performance and stability of photoelectrochemical systems.[128]
Future modeling work will focus on simulating the performance of these patterned de- vices under illumination. Initial work will focus on using simple Lambert-Beer absorption in the Si to determine how the Voc values change with the effective barrier height of the junction. Eventually, the optical effects of the nano-patterned catalyst (which may intro- duce plasmonic resonances, significantly changing the absorption profile near the Si surface) can be coupled to the electrical simulation.
Chapter 9
Conclusion and outlook
9.1 Summary of work
The work presented in this thesis expands the existing body of knowledge on the develop- ment of Si MW arrays as absorber materials for solar energy conversion. The ability to grow a high quality semiconductor material from an inexpensive precursor (SiCl4) using high- throughput processing techniques makes this technology a viable alternative to the current industry standard of bulk crystal growth, wafering, and polishing. While a great deal of previous work in the field focused on the use of VLS grown Si MW arrays for photovoltaic applications, we have demonstrated that these materials also have potential application for solar fuel generation. The potential low cost, high surface area, and flexibility of Si MW array devices provide a platform for future efforts to build a membrane-based solar fuel generating system.
In Chapter 3, we demonstrated that regenerative photoelectrochemistry, using the methyl viologen redox couple, can be used to make conformal, high barrier height contacts to Si MW arrays to characterize their energy conversion properties. We discussed the photoelec- trochemical experimental setup, and provided examples of how MV2+/+ electrochemistry was used to quantify the material quality of p-type Si MW arrays grown by the VLS process.
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. In Chapter 4, we expanded the use of the MV2+/+ to investigate the pH-dependence of the energetics of Si photoelectrodes in aqueous environments. Even though the band edges of Si varied with pH due to changes in surface chemistry, the introduction of a p- n homojunction decoupled the energy conversion properties of the solid-state device from
that of the contacting electrolyte. The n+p-Si devices also had improved photoconversion efficiencies compared to the p-Si/liquid contacts investigated previously.
Chapters 5 and 6 discussed the integration of HER catalysts onto Si MW arrays for solar-driven hydrogen production. By comparing different catalysts on both planar and high surface Si MW electrodes, we demonstrated that Ni–Mo is a promising alternative to Pt as an HER photocatalyst under mildly acidic conditions. However, when p-Si absorbers were directly coupled to HER catalysts, the photocathodes were only able to generate a tiny portion of the photovoltage needed for the overall water splitting reaction. The introduc- tion of a n+ emitter layer to create a solid-state homojunction allowed us to maintain the high photovoltages observed using regenerative photoelectrochemistry while also driving the HER photosynthetic reaction. We first demonstrated that Pt/n+p-Si MW devices could drive the HER at 6% efficiency, and then developed fabrication techniques to drive the same reaction using earth-abundant Ni–Mo catalysts at ∼ 2% efficiency. Using a slower, earth- abundant catalyst presented several design challenges which were addressed by analyzing the catalyst morphology, p-n junction properties, wire geometry, and spectral response of the system.
Chapter 7 presented new fabrication techniques to lower the cost of manufacturing the wire arrays by using scalable technologies such as micro-imprint lithography and electrode- position. We demonstrated that it was possible to grow high-fidelity VLS Si MW arrays us- ing electrodeposited Cu and substrates patterned with imprint lithography. Using MV2+/+
photoelectrochemistry, we confirmed that the material quality and energy conversion prop- erties of these Si MW arrays was similar to those grown from evaporated high-purity Cu.
These techniques will allow for high-throughput processing and enable the fabrication of larger diameter VLS-grown Si MW arrays.
Finally, Chapter 8 investigated the physics of catalyst/Si surface by using finite-element device physics simulations to understand the factors controlling the band energetics at Si electrodes patterned with metal nanoparticles. Future work will determine if the “pinch-off”
effect can be exploited to create high-barrier height contacts for semiconductors patterned with low-barrier height metal catalysts. The use of a three-dimensional simulation allows the interactions between band-bending induced by adjacent particles to be examined over a wide range of geometries, which expands the capabilities of prior models that treated each particle separately. The model was compared to prior experimental work on the pinch-off effect using
samples of patterned Ni on n-Si that were characterized with regenerative electrochemistry, and then used to examine the energetics of the Pt/p-Si|HER system. We explored the effects of Pt catalyst loading, and found that it should be possible to achieve fast HER catalysis with loadings low enough to observe the pinch-off effect. Methods to pattern nanoscale patches of metal catalysts over large areas have been developed, and future work will examine how these nano-patterned samples will perform as HER photocathodes.