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%.
Copper chalcopyrite films exhibit properties suitable for solar energy conversion processes such as direct bandgap, and
excellent carrier transport. To explore the possibilities of solar-powered hydrogen production by photoelectrolysis using
these materials, we have synthesized p-type polycrystalline CuGaSe2 films by vacuum co-evaporation of the elemental
constituents, and performed physical and electrochemical characterizations of the resulting films and electrodes. Based
on CuGaSe2 material with 1.65 eV bandgap, a 2.2 micron thick electrode exhibited an outdoor 1-sun photocurrent of 16
mA/cm2, while a 0.9 micron thin device still produced 12.6 mA/cm2 in conjunction with vigorous gas evolution.
Flatband potential measurements and bias voltage requirements for saturation photocurrents indicate a valence band
position to high for practical device implementation. Future photoelectrolysis devices may be based on copper
chalcopyrites with lower valence band maximum in conjunction with a suitable auxiliary junction.
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%.
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