4 research outputs found
Effect of Film Morphology and Thickness on Charge Transport in Ta<sub>3</sub>N<sub>5</sub>/Ta Photoanodes for Solar Water Splitting
Photoelectrochemical water splitting is one of many approaches
being studied to harvest sunlight and produce renewable H<sub>2</sub>. Tantalum nitride (Ta<sub>3</sub>N<sub>5</sub>) is a promising photoanode
candidate as its band edges straddle the water redox potentials and
it absorbs a large portion of the solar spectrum. However, reported
photocurrents for this material remain far from the theoretical maximum.
Previous results indicate Ta<sub>3</sub>N<sub>5</sub> may be hindered
by charge transport limitations attributed to poor bulk charge transport,
charge transport across grain boundaries, and/or charge transfer across
the interface at the back contact. The primary goal of this work was
to study these mechanisms, especially bulk hole and electron transport,
to determine which processes limit device efficiency. Crystalline
thin films (60–780 nm) of Ta<sub>3</sub>N<sub>5</sub> (<i>E</i><sub>g</sub> = 2.1 eV) on Ta foils were synthesized by
oxidation of Ta metal in air at 550 °C and subsequent nitridation
in NH<sub>3</sub> at 900 °C. Scanning electron microscopy revealed
that thermal stresses and differences in the density of the phases
resulted in the formation of porous, textured films with high surface
area. Films were characterized by their photon absorption, crystal
grain size, and electrochemically active surface area. Trends in photoactivity
as a function of film thickness under broadband illumination as well
as in the incident photon-to-current efficiency revealed that minority
charge carrier (hole) and majority carrier (electron) transport both
play important roles in dictating photoconversion efficiency in Ta<sub>3</sub>N<sub>5</sub> films
Using TiO<sub>2</sub> as a Conductive Protective Layer for Photocathodic H<sub>2</sub> Evolution
Surface passivation is a general issue for Si-based photoelectrodes
because it progressively hinders electron conduction at the semiconductor/electrolyte
interface. In this work, we show that a sputtered 100 nm TiO<sub>2</sub> layer on top of a thin Ti metal layer may be used to protect an
n<sup>+</sup>p Si photocathode during photocatalytic H<sub>2</sub> evolution. Although TiO<sub>2</sub> is a semiconductor, we show
that it behaves like a metallic conductor would under photocathodic
H<sub>2</sub> evolution conditions. This behavior is due to the fortunate
alignment of the TiO<sub>2</sub> conduction band with respect to the
hydrogen evolution potential, which allows it to conduct electrons
from the Si while simultaneously protecting the Si from surface passivation.
By using a Pt catalyst the electrode achieves an H<sub>2</sub> evolution
onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated
by the red part (λ > 635 nm) of the AM 1.5 spectrum. The
saturation
photocurrent (H<sub>2</sub> evolution) was also significantly enhanced
by the antireflective properties of the TiO<sub>2</sub> layer. It
was shown that with proper annealing conditions these electrodes could
run 72 h without significant degradation. An Fe<sup>2+</sup>/Fe<sup>3+</sup> redox couple was used to help elucidate details of the band
diagram
Protection of p<sup>+</sup>‑n-Si Photoanodes by Sputter-Deposited Ir/IrO<sub><i>x</i></sub> Thin Films
Sputter deposition of Ir/IrO<sub><i>x</i></sub> on p<sup>+</sup>-n-Si without interfacial
corrosion protection layers yielded
photoanodes capable of efficient water oxidation (OER) in acidic media
(1 M H<sub>2</sub>SO<sub>4</sub>). Stability of at least 18 h was
shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen
electrode) under 38.6 mW/cm<sup>2</sup> simulated sunlight irradiation
(λ > 635 nm, AM 1.5G) and measurements with quartz crystal
microbalances.
Films exceeding a thickness of 4 nm were shown to be highly active
though metastable due to an amorphous character. By contrast, 2 nm
IrO<sub><i>x</i></sub> films were stable, enabling OER at
a current density of 1 mA/cm<sup>2</sup> at 1.05 V vs. RHE. Further
improvement by heat treatment resulted in a cathodic shift of 40 mV
and enabled a current density of 10 mA/cm<sup>2</sup> (requirements
for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation.
Thus, the simple IrO<sub><i>x</i></sub>/Ir/p<sup>+</sup>-n-Si structures not only provide the necessary overpotential for
OER at realistic device current, but also harvest ∼100 mV of
free energy (voltage) which makes them among the best-performing Si-based
photoanodes in low-pH media
Comparison of the Performance of CoP-Coated and Pt-Coated Radial Junction n<sup>+</sup>p‑Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H<sub>2</sub>(g)
The electrocatalytic performance
for hydrogen evolution has been
evaluated for radial-junction n<sup>+</sup>p-Si microwire (MW) arrays
with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact
with 0.50 M H<sub>2</sub>SO<sub>4</sub>(aq). The CoP-coated (2.0 mg
cm<sup>–2</sup>) n<sup>+</sup>p-Si MW photocathodes were stable
for over 12 h of continuous operation and produced an open-circuit
photovoltage (<i>V</i><sub>oc</sub>) of 0.48 V, a light-limited
photocurrent density (<i>J</i><sub>ph</sub>) of 17 mA cm<sup>–2</sup>, a fill factor (ff) of 0.24, and an ideal regenerative
cell efficiency (η<sub>IRC</sub>) of 1.9% under simulated 1
Sun illumination. Pt-coated (0.5 mg cm<sup>–2</sup>) n<sup>+</sup>p-Si MW-array photocathodes produced <i>V</i><sub>oc</sub> = 0.44 V, <i>J</i><sub>ph</sub> = 14 mA cm<sup>–2</sup>, ff = 0.46, and η = 2.9% under identical conditions.
Thus, the MW geometry allows the fabrication of photocathodes entirely
comprised of earth-abundant materials that exhibit performance comparable
to that of devices that contain Pt