Our efforts focus on developing a method to produce hydrogenated nanocrystalline silicon (nc-Si:H) with larger
crystallites to enhance carrier transport properties. A new PECVD methodology, called double pulsed PECVD (DPPECVD), employs alternating low frequency and high frequency discharge sub-cycles to sequentially grow and etch the evolving film, respectively. This confers enhanced process control compared to conventional methods, and provides a pathway to achieve our goal of enhanced carrier mobility. Preliminary results demonstrate nc-Si:H films possessing grains as large as 29 nm, with (220) preferred orientation, which is suitable for solar cell applications. Reactions between plasma species in a SiF4:H2:SiH4 glow discharge, which expectedly contribute to evolution of large grains, are also discussed. Our findings suggest the double pulse strategy is a valuable method for manipulating the microstructural evolution of PECVD grown thin film materials.
Photoelectrochemical (PEC) water dissociation into hydrogen and oxygen at a semiconductor-liquid interface offers an
environmentally benign approach to hydrogen production. We have developed an integrated PEC device using
hydrogenated amorphous silicon carbide (a-SiC or a-SiC:H) material as photoelectrode in conjunction with an
amorphous silicon (a-Si) tandem photovoltaic device. Such a "hybrid PV/a-SiC" PEC cell produces photocurrent of
about 1.3 mA/cm2 in a short-circuit configuration and is durable in a pH2 electrolyte. On the other hand, the
aforementioned structure finished with ITO contacts and measured as a solid-state device features a current density of 5
mA/cm2, indicating a potential solar-to-hydrogen (STH) conversion efficiency of about 6% in the hybrid PV/a-SiC PEC
cell. The much lower photocurrent measured in the hybrid PEC cell suggests that there exists an interfacial barrier
between the a-SiC and electrolyte, which hinders the photocurrent extraction. In order to mitigate against the interfacial
barrier and hence improve the photo-generated charge carrier transport through the a-SiC/electrolyte interface, we have
explored several surface modification techniques, namely the use of metallic nano-particles (such as platinum or
palladium) and the growth of an additional thin layer (a-SiNx, carbon-rich a-SiC, a-SiF, etc.) on the top of a-SiC by
PECVD. In the latter case, it is observed that the addition of a thin PECVD-fabricated layer does not significantly
improve the photocurrent, presumably due to a poor band alignment at the a-SiC/electrolyte interface. The use of lower
work function nanoparticles like titanium has led to promising results in terms of photocurrent enhancement and an
a nodic shift in the onset potential.
Amorphous silicon carbide alloys are being investigated as a possible top photovoltaic layer in photoelectochemical
(PEC) cells used for water splitting. In order to be used as such, it is important that the effect that varying carbon
concentration has on bonding, and thus the electronic and optical properties, is well understood. The samples being
studied are silicon rich films with between 6 and 11 atomic percent of carbon. Electron spin resonance (ESR)
experiments, including light-induced ESR (LESR), were performed to study defects from dangling bonds which occur
dominantly at the silicon atoms in these films. Spin densities resulting from silicon dangling bonds varied between 1016
and 1017 spins/cm3. Lastly, to test the validity of these materials being used for devices we prepared pin structured solar
cells with the films being studied used as the absorber layer.
Photoelectrochemical (PEC) water splitting at a semiconductor-electrolyte interface using sunlight is of considerable interest as it offers a clean approach to hydrogen production. PEC cells require semiconductor photoelectrode materials fulfilling a number of important requirements, such as band-edge alignment, corrosion resistance to electrolyte, and adequate current generation. We report the development of RF-PECVD-deposited hydrogenated amorphous silicon
carbide (a-SiC:H) photoelectrodes with improved durability, which, when combined with a-Si:H tandem photovoltaic devices, should produce hydrogen directly from water under sunlight. The a-SiC:H is commonly grown with a bandgap in excess of 2.0 eV and completes the PEC device by providing contact with the electrolyte, proper band-edge alignment,
and acts as a buffer for the a-Si:H tandem structure. Effects of the pH of electrolyte, type of substrates, and a platinum nanoparticle coating on the durability of a-SiC photoelectrodes will be presented. From these studies we surmise that corrosion or damage mechanism occurring on a-SiC:H layer could be divided into different aspects of physical and chemical. From the physical point of view, defects associated with spikes in textured TCO substrates, roughness of
stainless steel, or other sources of pinholes may initiate delamination as confirmed by SEM (Scanning Electron Microscopy) and EDS (Energy-Dispersive X-ray Spectroscopy) studies. Chemically, the production of hydrogen involves reactions that may etch the electrode, especially when physical defects are involved. We observe that reducing the acidity of the electrolyte (increasing the pH from 0 to 2) significantly reduces corrosion while the useful photocurrent
output of the a-SiC:H p/i structure is unaffected.
In this paper we describe the fabrication of amorphous SiC:H materials and using them as photoelectrodes in
photoelectrochemical cells (PEC). With the increase of CH4 flow (in SiH4 gas mixture) during growth, the bandgap, Eg,
increases from ~ 1.8eV to ~2.0eV, while the photoconductivity decreases from ~10-5 S/cm to ~10-8 S/cm. These high-quality
a-SiC:H materials with Eg of 2.0eV included into a solar cell configuration led to a conversion efficiency,η~7%
on textured Asahi U type SnO2 coated substrates, with the i-layer thickness of ~300nm. For a reduced i-layer thickness
of ~100 nm, a current density, Jsc ~8.45mA/cm2 has been achieved, Immersing the a-SiC:H(p)/a-SiC:H(i) structure in
0.33M H3PO4 electrolytes, produced a photocurrent of ~7mA/cm2. With a further optimization we expect that the
photocurrent could exceed 9mA/cm2. With the use of this configuration substrate/silicon tandem device (a-Si/a-Si or a-
Si/nc-Si)/a-SiC:H(p)/a-SiC:H(i), it may therefore be possible to increase the solar-to-hydrogen (STH) efficiencies to
beyond 10%.
We report the use of amorphous silicon (a-Si) tandem junctions as part of an integral "hybrid" photoelectrochemical
(PEC) cell to produce hydrogen directly from water using sunlight. The device configuration consists of stainless steel
(SS)/ni2pni1p/ZnO/WO3. When the device is immersed in an electrolyte and illuminated, O2 is evolved at the
WO3/electrolyte interface and H2 is produced at the counter electrode. A voltage >1.23V is required to split water;
typically 1.6-1.8V are needed, taking account of losses in a practical water-splitting system. We use a-Si tandem cells,
deposited by plasma-enhanced chemical vapor deposition, to supply this voltage. Current matching in the two a-Si
subcells is achieved by altering the thicknesses of the two layers (i1 and i2) while keeping their band gaps at ~1.75eV,
which results in a device with an open circuit voltage >1.6V, short circuit current density (Jsc) >6mA/cm2 (on SS
substrates), and a fill factor >0.6. Deposition on a textured SnO2 coated glass has resulted in Jsc >9mA/cm2. Photoactive
WO3 films, deposited using the RF sputtering technique, have achieved photocurrents >3mA/cm2 at 1.6V vs. saturated
calomel electrode (SCE). The PEC device operates at the point at which the WO3 photocurrent IV curve and the a-Si
(filtered by WO3) light IV curve cross, leading to operating currents of 2.5mA/cm2 and solar-to-hydrogen (STH)
conversion efficiency of >3%.
We have investigated electrical switching behavior of sol-gel derived SiO2 films in c-Si(p)/SiO2/metal structures. The SiO2 film is fabricated from silicafilm (a soluble Si polymer in dissolved in denatured alcohol) using spin-coating technique. The thickness of the film is in the range of 300~2000 Å which depends on both spinning speed and the degree of dilution. We find that, with Ag as the top contact, when the applied voltage reaches a critical value of 1.5 ~ 2.5 V, current increases rapidly many orders of magnitude higher, and an irreversible switching occurs. The switching is also found to be polarity-dependent. The switching occurs only when the top contact (Ag) is biased positively, indicating the switching involves diffusion or electromigration of Ag. Both switching voltage and current are found to scale with the top contact area of the device. The switching dynamics is studied using voltage-pulse over a duration of 300 ns to 500 ms and amplitude of 2 to 20 V. We find that the switching electrical field is strongly dependent of the delay time prior to switching, and not related to the film thickness.
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