Effect of Film Morphology and Thickness on Charge Transport in Ta₃N₅/Ta Photoanodes for Solar Water Splitting

Photoelectrochemical water splitting is one of many approaches being studied to harvest sunlight and produce renewable H₂. Tantalum nitride (Ta₃N₅) 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 Ta₃N₅ 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 Ta₃N₅ (<i>E</i>_g = 2.1 eV) on Ta foils were synthesized by oxidation of Ta metal in air at 550 °C and subsequent nitridation in NH₃ 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 Ta₃N₅ films

Using TiO₂ as a Conductive Protective Layer for Photocathodic H₂ 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 TiO₂ layer on top of a thin Ti metal layer may be used to protect an n⁺p Si photocathode during photocatalytic H₂ evolution. Although TiO₂ is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H₂ evolution conditions. This behavior is due to the fortunate alignment of the TiO₂ 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 H₂ 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 (H₂ evolution) was also significantly enhanced by the antireflective properties of the TiO₂ layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe²⁺/Fe³⁺ redox couple was used to help elucidate details of the band diagram

Protection of p⁺‑n-Si Photoanodes by Sputter-Deposited Ir/IrO_<i>x</i> Thin Films

Sputter deposition of Ir/IrO_<i>x</i> on p⁺-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H₂SO₄). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm² 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_<i>x</i> films were stable, enabling OER at a current density of 1 mA/cm² 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/cm² (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrO_<i>x</i>/Ir/p⁺-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⁺p‑Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H₂(g)

The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n⁺p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H₂SO₄(aq). The CoP-coated (2.0 mg cm^–2) n⁺p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (<i>V</i>_oc) of 0.48 V, a light-limited photocurrent density (<i>J</i>_ph) of 17 mA cm^–2, a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (η_IRC) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm^–2) n⁺p-Si MW-array photocathodes produced <i>V</i>_oc = 0.44 V, <i>J</i>_ph = 14 mA cm^–2, 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