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Extremely Thin Absorbers for Solar Energy Conversion and Storage

Periodic Report Summary 2 - ETASECS (Extremely Thin Absorbers for Solar Energy Conversion and Storage)

ETASECS (Extremely Thin Absorbers for Solar Energy Conversion and Storage) investigates iron oxide (Fe2O3) photoanodes for photoelectrochemical (PEC) solar water splitting as a viable means for hydrogen production from renewable sources: water and sunlight. Building upon our previous studies that have demonstrated remarkable stability and potential to achieve high efficiency using thin (20-30 nm) films of iron oxide on specular back reflectors such as silver-coated glass substrates, ETASECS aims to improve the performance of these photoanodes and combine them with photovoltaic (PV) cells to create PEC-PV tandem cells that convert solar energy to hydrogen. To achieve this ambitious goal, we pursue a multidisciplinary research with experimental studies, theory and modeling that combine materials science and engineering, electrochemistry, surface and interface science, optics and nanophotonics, solid state physics, semiconductor device physics and photovoltaics to develop a new technology for solar energy conversion and storage in the form of hydrogen fuel. Toward this end, we seek to understand the underlying physical and electrochemical processes that limit the performance of iron oxide photoanodes and devise viable solutions to overcome these limitations through material design at the nanoscale and innovative device architectures designed for optimal light harvesting and charge separation, transport and collection with maximal efficiency for solar water splitting.
In this midterm report we report the main achievements obtained in the first half term of the project. Our first efforts were devoted to optimizing the deposition of iron oxide thin films on substrates with transparent electrodes or metallic reflectors for photoanode fabrication and testing. Using pulsed laser deposition (PLD) and sputtering to deposit metal-oxide and metal layers, respectively, we optimized the deposition process to obtain reproducible deposition of high quality films with controlled structure, morphology and composition. This enabled systematic studies of the effect of crystal structure and orientation, chemical composition (doping) and surface structure on the photoelectrochemical properties and performance of our devices. We explored different dopants and doping schemes and found that heterogeneous doping profiles with different dopants in different sections of the device give rise to enhanced charge separation and reduced recombination that enable to achieve higher photocurrent at lower applied bias than conventional homogenous doping profiles. This provides a new path to improve the performance of iron oxide photoanodes.
We developed new analytical methods to study the charge carrier dynamics in iron oxide photoanodes. These methods can also be applied to study other materials. They enable rigorous analysis of different losses such as surface recombination that degrade the device performance. This is important for rational optimization of the photoanodes by identifying the most critical losses and observing how they respond to different modifications in material compositions, process conditions and device architectures that aim to minimize these losses. We discovered that some of the photo-generated charge carriers can travel large distances – much larger than previously thought – and contribute to the photocurrent, whereas others are short-lived and they do not contribute to the photocurrent. Further work is ongoing to understand the difference between productive and non-productive excitations and try to maximize the productive ones and minimize the non-productive ones.
We explored light trapping schemes using gold nanoparticles and white diffuser layers and found them to be less effective than the resonant light trapping method that we developed several years ago (Dotan et al., Nature Materials, 2013). Therefore, we continue to put our main efforts in this promising direction. We found that concentrated solar radiation improves the performance of iron oxide photoanodes, up to concentrations of ~30 Suns. This provides a new path to high performance.
Besides working on material design and light trapping architectures, we also made important progress toward the integration of PEC cells with PV cells, and the design of large-area solar plants with millions of PEC-PV tandem cells. One of the greatest challenges in such solar plants arises from the distributed nature of the sunlight that necessitates a large number (millions) of solar cells to produce enough hydrogen. Collecting hydrogen gas from millions of cells is a major challenge that requires an immense gas piping construction, sealing, and special safety measures to prevent hydrogen intermixing with oxygen. These challenges put a heavy burden on the hydrogen production economy. We found a way to separate the hydrogen production from the oxygen production, thereby enabling the construction of PEC solar plants that produce only oxygen (and electrical power) whereas the hydrogen is produced elsewhere in a central hydrogen generator. This invention is a complete game change for large-area PEC solar water splitting plants. We will develop this concept further in the proof-of-concept follow-up research.