Influence of surface defect density on the ultrafast hot carrier relaxation and transport in $${\hbox {Cu}}_2 {\hbox {O}}$$ photoelectrodes

Abstract

Cuprous oxide (Cu 2 O) is a promising material for photoelectrochemical energy conversion due to its small direct band gap, high absorbance, and its Earth-abundant constituents. High conversion efficiencies require transport of photoexcited charges to the interface without energy loss. We studied the electron dynamics in Cu 2 O(111) by time-resolved two-photon photoemission for different surface defect densities in order to elucidate the influence on charge carrier transport. On the pristine bulk terminated surface, the principal conduction bands could be resolved, and ultrafast, elastic transport of electrons to the surface was observed. On a reconstructed surface the carrier transport is strongly suppressed and defect states dominate the spectra. Evidence for surface oxygen vacancies acting as efficient carrier traps is provided, what is important for further engineering of Cu 2 O based photoelectrodes. The conversion of solar energy into fuel in photoelectrochemical cells (PEC) represents a sustainable way for energy conversion and storage, a typical route being the production of hydrogen via water splitting 1-3. Upon light absorption in some semiconducting electrode material, photoexcited charges are generated that are separated and transported to the solid-electrolyte interface. Here they drive some catalyst-promoted redox chemistry that stores their energy in chemical bonds 4. This concept promises to be both ecologically and economically attractive if electrodes with high conversion efficiencies can be built from cheap, Earth-abundant materials that are stable in the aggressive chemical environment of a PEC cell 5. Cuprous oxide (Cu 2 O) has attracted much attention recently as electrode material for photo-electrochemical water splitting 6-10. It is a p-type semiconductor with a band gap of 2.1 eV and has a maximum theoretical solarto-hydrogen (STH) conversion efficiency of 18% 8. The bare material is unstable towards reduction to metallic Cu under electrochemical hydrogen evolution conditions, but protective nanolayers of n-type TiO 2 or ZnO can stabilize the electrode. The alignment of the conduction bands and the band bending near the p-n junction make for the charge separation and transport of photoexcited electrons to the oxide-electrolyte interface 6. Ga 2 O 3 nanolayers showed an even better band alignment for charge transport due to the very small conduction band offset. Recently a nanostructured photocathode design with Cu 2 O/Ga 2 O 3 /TiO 2 buried junctions and with NiMo as hydrogen evolution catalyst was demonstrated 9. It was implemented conformally in a coaxial nanowire structure to match the light absorption depth to the much shorter minority carrier diffusion length. It combines efficient light absorption with high positive onset voltage and shows high photocurrent density, paving the way towards efficient water splitting devices based entirely on Earth-abundant materials. Electrode performance is very sensitive to the presence of interfacial or bulk defects, which typically form states wit...

Probing the Role of Atomic Defects in Photocatalytic Systems through Photoinduced Enhanced Raman Scattering

“…An interesting approach toward favorable charge transport processes follows the introduction of oxygen vacancies (V O ) within the material, ,− as desired properties of the substrate, such as toxicity and biocompatibility, are not drastically changed with partial reduction/oxidization. , V O are some of the most common reactive defect sites in MOSs, considerably impacting material properties even in a few ppm . Nevertheless, the role vacancies play in photocatalytic behavior of MOSs remains unclear in the literature, with contradictory results over their beneficial or detrimental impact. ,,,,− The presence of V O has been suggested to enhance electron donor density, improving charge carrier transport. , Local charge induced by defects can greatly influence carrier transport . Furthermore, V O create electron/hole donor levels within the MOS band gap, promoting light adsorption and charge carrier photogeneration. , On the other hand, V O can act as recombination centers, impacting photocatalytic efficiency…”

Photoexcited charge carrier dynamics in Sb2Se3(100)

“…1(d). It was already successfully applied to study Cu 2 O photocathodes [18]. The probing depth of tr-2PPE is much more shallow than the light absorption length and the depth of the surface depletion region.…”

“…In Ref. [18], tr-2PPE measurements on Cu 2 O(111) single crystals are presented for different surface defect densities.…”

Quasi‐Single Crystalline Cuprous Oxide Wafers via Stress‐Assisted Thermal Oxidation for Optoelectronic Devices

“…[2] Different from highly efficient polycrystalline thin-film solar cells comprising rare or toxic constituent elements [such as gallium arsenide (GaAs), cadmium telluride (CdTe), and lead halide perovskite], [3][4][5] Cu 2 O is an earth-abundant nontoxic p-type semiconductor with a direct bandgap of ≈2 eV, which is more suitable for sustainable development of energy conversion. [6][7][8][9] However, Cu 2 O-based solar cells deliver far lower power conversion efficiencies (PCEs) than their Shockley-Queisser limit of ≈20% because of insufficient crystallization quality and misaligned energy levels. So far, several methods have been employed to fabricate polycrystalline Cu 2 O films, including thermal oxidation of copper foil, [10] electrodeposition, [1,11] sputtering, [12] chemical vapor deposition, [13] and pulsed laser deposition (PLD).…”