A copper nickel mixed oxide hole selective layer for Au-free transparent cuprous oxide photocathodes

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 costefficient 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.

Fabrication and Characterization of Functional ALD Metal Oxide Thin Films for Solar Applications

Ludmilla Steier

Motivated to revolutionize our today's fossil fuels based energy production, my work concentrated on the investigation of promising low-cost materials for photoelectrochemical hydrogen production and photovoltaic electricity generation. Hydrogen presents a fully scalable energy storage solution while photovoltaics have the biggest potential for clean electricity generation. Both are combined in the hydrogen-based economy that will be introduced in Chapter 1. A clear way to achieve this revolutionary technological and societal goal is through fundamental understanding of the complex electronic properties of the most promising low-cost semiconductors offering strong visible light absorption. The modern fields of photoelectrochemical (PEC) water splitting and photovoltaics have a lot in common: materials, scientific concepts and theoretical background. In other words, their complementarity was a strong motivation for the interdisciplinary work presented in this thesis. The main focus in this thesis is on hematite, which is a promising low-cost material offering visible light absorption and the chemical robustness for photoelectrochemical water oxidation. However, it has two major drawbacks: firstly, for a semiconductor, hematite has extremely low electron and hole mobilities. This makes it challenging to collect charges that are photo-generated deep within the hematite layer and far away from the surface. Secondly, water oxidation appears to be limited by trap states located in the mid band gap region. Chapter 3 addresses these drawbacks showing that doping of hematite from the underlayer, surface passivation from annealing treatments and/or overlayers are all key parameters to consider for the design of more efficient iron oxide electrodes. By better understanding the underlying principles of over- and underlayers, I was able to design multilayered hematite photoanodes comprised of functional thin films to obtain a significant reduction in the water oxidation overpotential. Whereas hematite thin film electrodes were fabricated by ultrasonic spray pyrolysis in Chapter 3, I introduce a new atomic layer deposition (ALD) route towards crystalline, highly photoactive, phase pure and impurity-free hematite films in Chapter 4. With this thin film model system I could precisely demonstrate that only the 10 nm thick space charge region of hematite is photoactive, which presents a major challenge when considering that around 60-70 nm are needed to achieve sufficient light absorption as shown in Chapter 5. In light of this charge transport limitation, I propose and demonstrate new host-guest electrode designs that would be indispensible for the realization of high performance ALD hematite photoanodes. To complement these studies, I demonstrate in Chapter 5 the basis for an optoelectronic modeling of the hematite PEC device that helps identifying and eliminating major optical and electronic losses in the PEC cell. In Chapter 6 my work on ALD SnO2 as an electron selective layer (ESL) led to major advances in low-cost flat hybrid organic-inorganic perovskite solar cells. The tin oxide layer is found to resolve the problem of an energy band misalignment encountered with the previously used ALD TiO2 ESL in addition to stabilizing the photovoltaic performance. Hysteresis-free solar conversion efficiencies of up to 18% have been achieved for flat devices paving the way towards low-cost flat film perovskite solar cells that will easily surpass the photovoltaic performance of polycrystalline silicon solar cells in the near future.

EPFL2016

On the Morphology and Interfaces of Nanostructured Hematite Photoanodes for Solar-Driven Water Splitting

Florian Le Formal

A sustainable route to store the energy provided by the Sun, is to directly convert sunlight into molecular hydrogen using a semiconductor performing water photolysis. Hematite (α-Fe2O3) is promising for this application due to its ample abundance, chemical stability and significant light absorption (with a band gap of 2.0 - 2.2 eV). Despite these advantageous properties, several drawbacks restrain the utilization of iron oxide for photoelectrochemical water splitting. The first limitation, namely the conduction band edge lower than the water reduction potential, can be straightforwardly overcome by adding a second solar system in tandem, which can absorb a complementary part of the solar spectrum and bring the electron at an energetic level higher than the hydrogen evolution potential. The second drawback arises from the disaccord between the short charge carrier diffusion length and the large light penetration depth. It is therefore necessary to control the hematite morphology on a length scale similar to the hole transport length. To further enhance the photoelectrochemical performance, a new concept for water splitting is introduced in this thesis. The host-guest approach consists in decoupling the different tasks of the photoanode (on one side light harvesting and water oxidation center, on the other side electron conduction to the substrate) by depositing a thin layer of hematite onto a mesoporous host (WO3 in this study). This concept has been demonstrated to increase the photocurrent by ca. 20% due to enhanced quantum efficiencies at long wavelengths. This demonstration has been nonetheless limited by the iron oxide thin films overall efficiency. Thin films photoactivity is then investigated by two means: first by controlling their nucleation on a modified substrate and secondly by incorporating plasmonic nanoparticles aimed to localize absorption in the thin film. The formation of a SiOx buffer layer on the substrate prior to deposition of hematite by Fe(acac)3 spray is shown to modify the film formation mode and its physical properties. These films exhibit photoactivity from an optical thickness of 12.5 nm (as compared to 25 nm without underlayer). The study of hematite photoanodes with gold nanoparticles, embedded or deposited on its surface, establish that charge transfer from metal nanoparticles is occurring only at overlapping wavelengths between the plasmonic resonance and the semiconductor absorption. Nevertheless, photoelectrochemical performances are reduced because of high recombination rate at the metal/semiconductor interface. Finally the third limitation, i.e. the large overpotential required to observe the onset of water splitting photocurrent, is tackled in the last part. The onset potential of photocurrent is decreased by a very thin coating of Al2O3 (0.1 - 2 nm), deposited by ALD, on the nanostructured photoanode. The subsequent application of the Co2+ catalyst further reduces the overpotential and results in a record photocurrent at 0.9 VRHE of over 0.4 mA cm-2. This investigation clearly distinguishes two causes for this energy loss: surface traps and slow oxidation kinetics. The charge accumulation and the Fermi level pinning, observed at low bias potential and assigned to these surface states were further rationalized in an investigation on photocurrent and photovoltage transients.

EPFL2011

Fabrication and Characterization of Functional ALD Metal Oxide Thin Films for Solar Applications

Ludmilla Steier

EPFL2016

On the Morphology and Interfaces of Nanostructured Hematite Photoanodes for Solar-Driven Water Splitting

Florian Le Formal

EPFL2011