Elucidation of photovoltage origin and charge transport in Cu2O heterojunctions for solar energy conversion

Heterojunctions between p-type cuprous oxide (Cu2O) and suitable n-type layers stand out as some of the best performing and abundant oxide photoabsorbers currently available for the generation of hydrogen with photoelectrochemical cells or conversion of solar energy to electricity. In this contribution, we used drift-diffusion semiconductor modeling to investigate the mechanism governing the charge transport in TiO2/Ga2O3/Cu2O and TiO2/Al:ZnO/Cu2O heterojunctions. The simulated photovoltage 0.9 V for TiO2/Ga2O3/Cu2O agrees well with the measured value of 1.0 V and the governing mechanism is identified to be thermionic emission of electrons across the Ga2O3/TiO2 interface. By modeling an optimized increase in Ga2O3 donor concentration, a photovoltage improvement of only 0.1 V is achievable, whereas further optimizing the electron affinity of Ga2O3 may lead to more significant improvement approaching 0.7 V. The optimized electron affinity of Ga2O3 enabled a simulated photovoltage of 1.6 V, close to the theoretical limit for Cu2O with a 2.17 eV bandgap energy. Additionally, we find that simulations can reproduce the measured photovoltage of TiO2/Al:ZnO/Cu2O only when an interface recombination layer at the Al:ZnO/Cu2O interface is included in the model. Our findings enable detailed understanding of the charge transport mechanism in Cu2O heterojunctions and offer various design directions for further photovoltage improvement.

On the Texturing and Nanostructuring of Iron Oxide Photoanodes for Solar Water Splitting

Maurin Cornuz

The world needs to think about the after fossil fuel era and while the sun baths the earth with a tremendous amount of energy, man must learn how to harvest and store it in order to dispose of a continuous supply meeting his needs. Water photo-electrolysis is a route toward the conversion of solar energy and its storage into chemical energy. Iron oxide, in the form of hematite, has been identified as a promising material, showing an excellent bandgap coupled with a high stability and abundance. However, its use is impeded by a very short charge carrier lifetime and poor oxygen evolving reaction kinetics. The former point creates the incentive for a fine nanostructuring, allowing the absorption of light close enough to the semiconductor-liquid interface for the photo-generated hole to be able to diffuse and react with water. The kinetics limitations can be overcome by mean of catalysis. In this thesis, I use a technique called atmospheric pressure chemical vapor deposition (APCVD), developed by Kay et al. in 2006 in order to fabricate hematite photoanodes that are highly nanostructured and show record solar-to-hydrogen conversion efficiencies. My work focuses on the improvement of the nanostructure in order to reduce the energy loss by deep light absorption in the material. First, the deposition parameters of the APCVD are optimized in order to improve the crystallinity of the photoanodes. The key parameter modified is the residence time of the precursor in the gas stream above the substrate. The process results in an improved overall solar energy conversion and a better electric conductivity of the material. The further application of the known best oxygen evolving catalyst – Iridium oxide – results in a record photocurrent of 3.0 mA cm−2 at 1.23 V bias vs. RHE for this class of material. In a second phase, the electrodes prepared using the newly developed conditions are analyzed by X-Ray diffraction and I find that the reduction of the residence time leads to an increased orientation of the iron oxide crystal planes. Since hematite presents a strong conduction anisotropy, and the better conducting basal planes are found perpendicular to the substrate, a correlation is made between the improved conductivity of the photoanodes prepared by reducing the residence and the preferential orientation. In order to better control the morphology of the hematite photoanodes, I present strategies of directed nucleation for the particle-assisted chemical vapor deposition. A flat model substrate is prepared as a platform for the elaboration of directing patterns. Gold nanoparticles are homogeneously deposited on the substrate with a controlled surface density. However, the chosen material reveals to be unable to initiate the iron oxide precursor decomposition and the nanoparticles show no effect on the morphology. Another patterning technique – the nanosphere lithography – is employed to create highly ordered nano-islands of a FePt alloy on the flat substrate. The dimensions of the patterning nanospheres used for the lithography controls the size and spacing of the imprints. The deposition of hematite by APCVD is successfully affected by the pattern and the size of the iron oxide nanostructures is controlled by the imprinted substrate. Finally, I combine the knowledge acquired on the directed nucleation and modified deposition conditions in order to use the APCVD in the so-called host-guest strategy. A transparent, conductive and macroporous host is built using templating TiO2 nanoparticles and a conductive niobium doped tin oxide scaffold. The host is then infiltrated by hematite using different combinations of parameters in the APCVD setup. A maximum photocurrent – equivalent to the state-of-the-art hematite host-guest strategy – is reached with a hybrid nanostructure. A dendritic formation growing atop of the macroporous host while Nb-SnO2 is conformally coated with a thin layer of hematite. By mean of parameter tuning in the APCVD system, a selective improvement of the conformal hematite layer is achieved, showing a better performance when the sample is illuminated from the substrate side as well as a modified surface morphology.

EPFL2013

Solar-driven reduction of CO₂

Marcel Roland Schreier

Rapidly increasing levels of atmospheric carbon dioxide and their damaging impact on the global climate system raise doubts about the sustainability of the fossil resource based energy system. Meanwhile, raising living standards and increasing global population lead to an ever growing need for energy. Renewable energy sources are believed to present a solution to these problems with the sheer abundance of solar energy showing particular promise to fulfill the world's energy needs. However, for large scale application of solar energy to be possible, the problem of its storage has to be addressed. The insufficient flexibility of present-day storage technologies has led to the quest for producing solar fuels, centering on hydrogen as a fuel in a prospective hydrogen economy. Nevertheless, the gaseous state, low volumetric energy density and explosive nature of hydrogen makes it a challenging fuel for practical applications. Using solar energy to produce carbon-based liquid fuels solves these challenges, closes the anthropogenic carbon cycle and allows for the continued utilization of existing infrastructures. A promising method for the production of such fuels consists in the photoelectrochemical and electrochemical conversion of carbon dioxide. In this thesis, both methods are investigated using molecular homogeneous catalysts and heterogeneous systems. The photoelectrochemical reduction of carbon dioxide on TiO2-protected Cu2O photocathodes was investigated using a rhenium bipyridyl catalyst in solution. Important charge transport limitations were encountered, which could be overcome by the addition of protic additives to the electrolyte. Improving on this result, the molecular catalyst was covalently immobilized on the TiO2 surface of the photocathode by modifying the bipyridyl ligand with a phosphonate binding group. A nanostructure of TiO2 was needed to support sufficient catalyst to sustain the photocurrent generated by the Cu2O photoelectrode. The complete device showed photocurrents exceeding 2.5 mA cm-2 and large faradaic efficiency for the production of CO. Moving toward heterogeneous catalysis, the promotion of the CO2 to CO conversion reaction on silver surfaces by imidazolium cations was investigated. Replacing the imidazolium C2 proton with a phenyl substituent led to an enhancement of the co-catalytic effect. Replacing the C4 and C5 protons with methyl groups, however, suppressed the catalysis-promoting effect of the imidazolium salt for different C2 substituents and led to new insights into the role of imidazolium. The unassisted solar-driven splitting of CO2 into CO and O2 was demonstrated using water as electron source. This was achieved by the use of a porous gold cathode and an IrO2 anode, driven by three methylammonium lead iodide perovskite solar cells in series. Extended operation over 18 h was shown, achieving a solar to CO efficiency exceeding 6.5 %. Atomic layer deposition (ALD) modification of CuO nanowire cathodes with SnO2 was investigated, leading to striking impacts on the catalytic selectivity of this system. In an aqueous electrolyte, bare CuO led to the production of a wide spectrum of products, which was modified to the production of CO with high selectivity upon ALD modification. By exploiting the oxygen evolving activity of SnO2-coated CuO, a low cost bifunctional system was constructed, achieving sustained solar-driven production of CO with up to 13.4% efficiency.

EPFL2017

Solar-driven reduction of CO₂

Marcel Roland Schreier

EPFL2017

On the Texturing and Nanostructuring of Iron Oxide Photoanodes for Solar Water Splitting

Maurin Cornuz

EPFL2013

Elucidation of photovoltage origin and charge transport in Cu2O heterojunctions for solar energy conversion

On the Texturing and Nanostructuring of Iron Oxide Photoanodes for Solar Water Splitting

Solar-driven reduction of CO₂

Solar Energy and Building Physics Laboratory - Activity Report 2012

Solar-driven reduction of CO₂

Solar Energy and Building Physics Laboratory - Activity Report 2012

On the Texturing and Nanostructuring of Iron Oxide Photoanodes for Solar Water Splitting