State-of-the-art cuprous oxide (Cu2O) photocathodes for photoelectrochemical (PEC) water splitting have a long tradition of using gold (Au)-coated F-doped SnO2 (FTO) substrates for the improvement of ...Cu2O electrodeposition and overall PEC performance. Au is one of the best contact materials for Cu2O photocathodes due to its large work function enabling proper alignment with the valence band level of Cu2O. Due to its relatively large band gap (2.0 eV), Cu2O is preferentially used as the top-cell absorber in tandem with a photoanode or a photovoltaic (PV) cell for overall solar-driven water splitting. However, the Au contact poses a major issue due to its poor transparency. Moreover, Au is a precious metal, which increases the cost and can hinder the scalability of PEC devices. In this work, we propose an effective replacement of the Au layer with a transparent and cost-efficient copper-nickel mixed oxide (CuO/NiO) thin film, which is prepared by a facile sequential sputtering deposition combined with an annealing process in air. We successfully demonstrate that a thin layer of the CuO/NiO film shows better transparency as well as well-aligned energy levels for efficient hole collection leading to an improved PEC performance compared to the performance of a Au-contact based equivalent device in a pH 5 electrolyte biased at 0 V versus the reversible hydrogen electrode. This new transparent and efficient CuO/NiO layer paves the way for the development of efficient, yet inexpensive PEC-PV or photocathode-photoanode stacked tandem devices for a hydrogen fuel based economy.
Though Cu2O has demonstrated high performance as a photocathode for solar water splitting, its band gap is too large for efficient use as the bottom cell in tandem configurations. Accordingly, copper ...chalcopyrites have recently attracted much attention for solar water splitting due to their smaller and tunable band gaps. However, their fabrication is mainly based on vacuum evaporation, which is an expensive and energy consuming process. Here, we have developed a novel and low-cost solution fabrication method, and CuInS2 was chosen as a model material due to its smaller band gap compared to Cu2O and relatively simple composition. The nanostructured CuInS2 electrodes were synthesized at low temperature in crystalline form by solvothermal treatment of electrochemically deposited Cu2O films. Following the coating of overlayers and decoration with Pt catalyst, the as-fabricated CuInS2 electrode demonstrated water splitting photocurrents of 3.5 mA cm–2 under simulated solar illumination. To the best of our knowledge, this is the highest performance yet reported for a solution-processed copper chalcopyrite electrode for solar water splitting. Furthermore, the electrode showed good stability and had a broad incident photon-to-current efficiency (IPCE) response to wavelengths beyond 800 nm, consistent with the smaller bandgap of this material.
Single-atom catalysis has become the most active new frontier in energy conversion applications due to its remarkable catalytic activity and low material consumption. However, the issue of atom ...aggregation during the synthesis process or catalytic reaction must be overcome. In this work, we have developed a one-step photo-deposition process to fabricate Pt single-atom catalysts (SACs) on nitrogen doped carbon dots (NCDs). The Pt-NCDs were then hybridized with TiO
2
to achieve high hydrogen generation activity and to understand the fundamentals at the Pt/NCD/TiO
2
interface. The synergistic effect of Pt SAC and NCDs with maximized atomic efficiency of Pt and improved charge transfer capability provides a new strategy to rationally design a multi-scale photocatalyst structure to achieve high H
2
evolution efficiency. The facile synthesis process also holds great potential for various applications such as electrocatalysis, heterogeneous catalysis and drug delivery, providing a promising way to reduce the high cost of noble metals.
Pt single-atom catalysts prepared
via
photo-deposition on nitrogen-doped carbon dots exhibit high activity and stability towards photocatalytic H
2
production.
Abstract
Two key strategies for enhancing the efficiency of Cu(In,Ga)Se
2
solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of ...Na and Rb into the absorber has brought large efficiency gains compared to Na‐containing or alkali‐free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition‐graded Cu(In,Ga)Se
2
solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga‐rich region near the Mo‐back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga‐concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se
2
film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga‐concentrations).
This study reports a comparison of the kinetics of electrochemical (EC) versus photoelectrochemical (PEC) water oxidation on bismuth vanadate (BiVO4) photoanodes. Plots of current density versus ...surface hole density, determined from operando optical absorption analyses under EC and PEC conditions, are found to be indistinguishable. We thus conclude that EC water oxidation is driven by the Zener effect tunneling electrons from the valence to conduction band under strong bias, with the kinetics of both EC and PEC water oxidation being determined by the density of accumulated surface valence band holes. We further demonstrate that our combined optical absorption/current density analyses enable an operando quantification of the BiVO4 photovoltage as a function of light intensity.
Two key strategies for enhancing the efficiency of Cu(In,Ga)Se2 solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of Na and Rb ...into the absorber has brought large efficiency gains compared to Na‐containing or alkali‐free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition‐graded Cu(In,Ga)Se2 solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga‐rich region near the Mo‐back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga‐concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se2 film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga‐concentrations).
Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition‐graded Cu(In,Ga)Se2 solar cells. Electron lifetimes are found to be highly dependent on alkali PDT as well as on the film composition and morphology.
Abstract
Cu(In,Ga)Se
2
solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency ...progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post‐deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se
2
films, the role of the Ga‐gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga‐gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se
2
grains with a Ga/(Ga+In) ratio between 0.2–0.5 are found to drift‐diffuse across
≈
1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm
2
V
−1
s
−1
. In addition, the recombination times strongly depend on the Ga‐content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga‐content region near the Mo‐back contact. An analytical model, as well as drift‐diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition‐graded absorber structure, demonstrating the potential of TAS.
Optical losses in a photoelectrochemical (PEC) cell account for a substantial part of solar‐to‐hydrogen conversion losses, but limited attention is paid to the detailed investigation of optical ...losses in PEC cells. In this work, an optical model of combined coherent and incoherent light propagation in all layers of the PEC cell based on spectroscopic measurements is presented. Specifically, photoelectrodes using transparent conductive substrates such as F:SnO2 coated with thin absorber films are focused. The optical model is verified for hematite photoanodes fabricated by atomic layer deposition and successfully used to determine wavelength‐dependent reflection, transmission, layer absorptances, and charge generation rates. Furthermore, the calculated absorptances enable 20–30% more accurate calculations of the absorbed photon‐to‐current efficiency of PEC cells. Our optical model is a powerful tool for the optimization of the optical performance of PEC cells focusing on single absorber or tandem configurations and represents a cornerstone of a complete (optical and electrical) model for PEC water splitting cells.
An in‐depth analysis of wavelength‐dependent optical losses for a photoelectrochemical water splitting cell based on a transparent conducting substrate is presented. Specifically, ultrathin films with a thickness comparable to the roughness of the substrate are described conveniently with the graded layer approach. Our validated model enables extraction of absorptance and absorbed photon‐to‐current efficiency of the photoabsorber with improved accuracy over established methods.
Cu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are ...the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post‐deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se2 films, the role of the Ga‐gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga‐gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se2 grains with a Ga/(Ga+In) ratio between 0.2–0.5 are found to drift‐diffuse across ≈1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm2 V−1 s−1. In addition, the recombination times strongly depend on the Ga‐content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga‐content region near the Mo‐back contact. An analytical model, as well as drift‐diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition‐graded absorber structure, demonstrating the potential of TAS.
In Cu(In,Ga)Se2 solar cells, a Ga‐concentration dependent bandgap gradient has long been implemented as a strategy to achieve high device efficiencies. Here, employing transient absorption spectroscopy, minority carrier dynamics are tracked along the gradient allowing the development of a rigorous physical model that decouples charge carrier recombination and electron transport throughout the absorber layer.
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