29 results found
Corby S, Rao R, Steier L, et al., 2021, The kinetics of metal oxide photoanodesfrom charge generation to catalysis, Nature Reviews Materials, ISSN: 2058-8437
Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for both photoelectrochemical and photocatalytic systems and a key determinant of their efficiency. This review addresses the charge carrier dynamics underlying the performance of metal oxides as photoanodes and their ability to drive photoelectrochemical water oxidation, alongside wider comparison with metal oxide function in photocatalytic and electrocatalytic systems. We start by highlighting the disparity between the ps–ns lifetimes of electron and holes photoexcited in bulk metal oxides versus the ms –s timescale of water oxidation catalysis. We go onto review recent literature of the dominant kinetic processes determining photoanode performance, namely charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and finally the kinetics of water oxidation catalysis. With each topic, we review current understanding and note areas of remaining uncertainty or controversy. We discuss the potential for material selection and examine approaches such as doping, nanostructuring, junction formation and/or co-catalyst deposition to enhance performance. Critically, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimisation.
Chang Y-H, Carron R, Ochoa M, et al., 2021, Impact of RbF and NaF Postdeposition Treatments on Charge Carrier Transport and Recombination in Ga-Graded Cu(In,Ga)Se-2 Solar Cells, ADVANCED FUNCTIONAL MATERIALS, ISSN: 1616-301X
Moss B, Wang Q, Butler K, et al., 2021, Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts, Nature Materials, Vol: 20, Pages: 511-517, ISSN: 1476-1122
Recently, high solar-to-hydrogen efficiencies were demonstrated using La and Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low-cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 V versus the reversible hydrogen electrode) and a Rh oxidation state dependent reorganization of the electronic structure, involving the loss of a vacant Rh 4d mid-gap state. This reorganization enables Rh:SrTiO3 to be reduced by co-doping without compromising the p-type character. In situ time-resolved spectroscopies show that the electronic structure reorganization induced by Rh reduction controls the electron lifetime in photocatalyst sheets. In Rh:SrTiO3, enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, which results in long-lived photogenerated electrons even at very positive potentials (+1 V versus the reversible hydrogen electrode), in which both components of the complete device operate effectively. This understanding of the role of co-dopants provides a new insight into the design principles for water-splitting devices based on bandgap-engineered metal oxides.
Chang Y-H, Carron R, Ochoa M, et al., 2021, Insights from transient absorption spectroscopy into electron dynamics along the Ga-gradient in Cu(In,Ga)Se2 solar cells, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832
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.
Mesa CA, Steier L, Moss B, et al., 2020, Impact of synthesis route on the water oxidation kinetics of hematite photoanodes, The Journal of Physical Chemistry Letters, Vol: 11, Pages: 7285-7290, ISSN: 1948-7185
Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. Whilst these photoanodes exhibit very different current / voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential towards less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intraband-gap states, indicating these intraband-gap states do not contribute directly to water oxidation.
Luo H, Liu Y, Dimitrov SD, et al., 2020, Pt single-atoms supported on nitrogen-doped carbon dots for highly efficient photocatalytic hydrogen generation, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 8, Pages: 14690-14696, ISSN: 2050-7488
Mesa CA, Steier L, Moss B, et al., 2020, Impact Of Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes, Publisher: American Chemical Society (ACS)
<jats:p><p><i>Operando</i> spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. Whilst these photoanodes exhibit very different current / voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Lower photoanode performance was found to correlate with the observation of optical signals indicative of charge accumulation in mid-gap oxygen vacancy states, indicating these states do not contribute directly to water oxidation.</p></jats:p>
Bakulin A, Pastor E, Park J-S, et al., 2019, In situ observation of picosecond polaron self-localisation in α-Fe2O3 photoelectrochemical cells, Nature Communications, Vol: 10, ISSN: 2041-1723
Hematite (α-Fe2O3) is the most studied artificial oxygen-evolving photo-anode and yet its efficiency limitations and their origin remain unknown. A sub-picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self-localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device-based optical-control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes (<100 cm-1). Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly-studied hematite devices.
Yang W, Godin R, Kasap H, et al., 2019, Electron accumulation induces efficiency bottleneck for hydrogen production in carbon nitride photocatalysts, Journal of the American Chemical Society, Vol: 141, Pages: 11219-11229, ISSN: 1520-5126
This study addresses the light intensity dependence of charge accumulation in a photocatalyst suspension, and its impact on both charge recombination kinetics and steady-state H2 evolution efficiency. Cyanamide surface functionalized melon-type carbon nitride (NCNCNx) has been selected as an example of emerging carbon nitrides photocatalysts because of its excellent charge storage ability. Transient spectroscopic studies (from ps to s) show that the bimolecular recombination of photogenerated electrons and holes in NCNCNx can be well described by a random walk model. Remarkably, the addition of hole scavengers such as 4-methylbenzyl alcohol can lead to ∼400-fold faster recombination kinetics (lifetime shortening to ∼10 ps). We show that this acceleration is not the direct result of ultrafast hole extraction by the scavenger, but is rather caused by long-lived electron accumulation in NCNCNx after hole extraction. The dispersive pseudo-first order recombination kinetics become controlled by the density of accumulated electrons. H2 production and steady-state spectroscopic measurements indicate that the accelerated recombination caused by electron accumulation limits the H2 generation efficiency. The addition of a reversible electron acceptor and mediator, methyl viologen (MV2+), accelerates the extraction of electrons from the NCNCNx and increases the H2 production efficiency under one sun irradiation by more than 30%. These results demonstrate quantitatively that while long-lived electrons are essential to drive photoinduced H2 generation in many photocatalysts, excessive electron accumulation may result in accelerated recombination losses and lower performance, and thus highlight the importance of efficient electron and hole extraction in enabling efficient water splitting photocatalysts.
Alqahtani M, Sathasivam S, Cui F, et al., 2019, Heteroepitaxy of GaP on silicon for efficient and cost-effective photoelectrochemical water splitting, Journal of Materials Chemistry A, Vol: 7, Pages: 8550-8558, ISSN: 2050-7488
Photoelectrochemical production of hydrogen by using sunlight to split water offers a sustainable approach for clean energy generation. III-V semiconductors have shown the highest efficiencies for photoelectrochemical water splitting but the prohibitive cost of commercial single-crystalline GaP wafers limit practical use and large-scale application. Here, we report a high-quality GaP photocathode directly grown on a silicon substrate by solid-source molecular beam epitaxy. The photocathode can be stabilized under acidic electrolyte 1 M HClO 4 (pH 0) by combining an amorphous TiO 2 layer coated with a molybdenum sulphide MoS 2 hydrogen evolution catalyst by atomic layer deposition (ALD). Under simulated AM 1.5G solar illumination, the Si/GaP photocathode yielded a maximum photocurrent density of 0.95 (mA cm -2 ) with a proton reduction onset potential of 467 mV versus the reversible hydrogen electrode. The average faradaic efficiency of the Si/GaP photocathode was measured to be over 73.4 ± 20.2% for over 100 minutes. The photoelectrochemical studies for the Si/GaP photocathode show the potential for widespread deployment of cost-effective photoelectrodes for hydrogen generation.
Steier L, Holliday S, 2018, A bright outlook on organic photoelectrochemical cells for water splitting, Journal of Materials Chemistry A, Vol: 6, Pages: 21809-21826, ISSN: 2050-7496
Photoelectrochemcial (PEC) water splitting is increasingly attracting attention as a means to generate clean, renewable hydrogen fuel from solar energy. This Highlight article covers the key advances that have been made over recent years in organic photoelectrochemical (OPEC) cell research, and identifies a pathway forwards combining state-of-the-art materials and device engineering from both the organic photovoltaics and inorganic PEC water splitting communities. We discuss the advantages of using buried junction device architectures in OPEC photoelectrodes and identify the need for new materials for OPEC water splitting cells in order to improve their efficiency and operating stability. We present an overview on promising new absorbers and device architectures employed in the parallel field of organic photovoltaics with a critical view on requirements for OPEC water splitting. We also elaborate on progress made with organic multijunction cells that we see as promising photocathode designs for voltage-unassisted water splitting. Finally, we see the urgent need for benchmarking rules, in terms of performance and stability parameters, as this emerging field of OPEC water splitting progresses.
Francas Forcada L, Burns E, Steier L, et al., 2018, Rational design of a neutral pH functional and stable organic photocathode., Chemical Communications, Vol: 2018, ISSN: 1359-7345
In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.
Anaraki EH, Kermanpur A, Mayer MT, et al., 2018, Low-Temperature Nb-Doped SnO2 Electron-Selective Contact Yields over 20% Efficiency in Planar Perovskite Solar Cells, ACS ENERGY LETTERS, Vol: 3, Pages: 773-778, ISSN: 2380-8195
Cendula P, Steier L, Losio P, et al., 2017, Analysis of Optical Losses in a Photoelectrochemical Cell: A Tool for Precise Absorptance Estimation, Advanced Functional Materials, ISSN: 1616-301X
Steier L, Bellani S, Rojas HC, et al., 2017, Stabilizing organic photocathodes by low-temperature atomic layer deposition of TiO 2, Sustainable Energy Fuels, Vol: 1, Pages: 1915-1920
Schreier M, Héroguel F, Steier L, et al., 2017, Solar conversion of CO₂ to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO, Nature Energy, Vol: 2, ISSN: 2058-7546
The solar-driven electrochemical reduction of CO2 to fuels and chemicals provides a promising way for closing the anthropogenic carbon cycle. However, the lack of selective and Earth-abundant catalysts able to achieve the desired transformation reactions in an aqueous matrix presents a substantial impediment as of today. Here we introduce atomic layer deposition of SnO2 on CuO nanowires as a means for changing the wide product distribution of CuO-derived CO2 reduction electrocatalysts to yield predominantly CO. The activity of this catalyst towards oxygen evolution enables us to use it both as the cathode and anode for complete CO2 electrolysis. In the resulting device, the electrodes are separated by a bipolar membrane, allowing each half-reaction to run in its optimal electrolyte environment. Using a GaInP/GaInAs/Ge photovoltaic we achieve the solar-driven splitting of CO2 into CO and oxygen with a bifunctional, sustainable and all Earth-abundant system at an efficiency of 13.4%.
Ummadisingu A, Steier L, Seo J-Y, et al., 2017, The effect of illumination on the formation of metal halide perovskite films, NATURE, Vol: 545, Pages: 208-+, ISSN: 0028-0836
Son M-K, Steier L, Schreier M, et al., 2017, A copper nickel mixed oxide hole selective layer for Au-free transparent cuprous oxide photocathodes, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 10, Pages: 912-918, ISSN: 1754-5692
Kavan L, Steier L, Grätzel M, 2017, Ultrathin Buffer Layers of SnO 2 by Atomic Layer Deposition: Perfect Blocking Function and Thermal Stability, The Journal of Physical Chemistry C, Vol: 121, Pages: 342-350, ISSN: 1932-7447
Anaraki EH, Kermanpur A, Steier L, et al., 2016, Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide, Energy and Environmental Science, Vol: 9, Pages: 3128-3134, ISSN: 1754-5692
Perovskite solar cells (PSCs) are one of the most promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature planar PSCs are particularly suited for large-scale manufacturing. Here, we propose a simple, solution-processed technological approach for depositing SnO2 layers. The use of these layers in planar PSCs yields a high stabilized power conversion efficiency close to 21%, exhibiting stable performance under real operating conditions for over 60 hours. In addition, this method yielded remarkable voltages of 1214 mV at a band gap of 1.62 eV (approaching the thermodynamic limit of 1.32 V) confirming the high selectivity of the solution-processed layers. PSCs aged under 1 sun illumination and maximum power point tracking showed a final PCE of 20.7% after ageing and dark storage, which is slightly higher than the original efficiency. This approach represents an advancement in the understanding of the role of electron selective layers on the efficiency and stability of PSCs. Therefore, the newly proposed approach constitutes a simple, scalable method paving the way for industrialization of perovskite solar cells.
Gholipour S, Correa-Baena J-P, Domanski K, et al., 2016, Highly Efficient and Stable Perovskite Solar Cells based on a Low-Cost Carbon Cloth, Advanced Energy Materials, Pages: 1601116-1601116, ISSN: 1614-6832
A low-cost carbon cloth is applied in perovskite solar cells (PSC) as a collector composite and degradation inhibitor. This study incorporates carbon fibers as a back contact in perovskite solar cells, which results in enhancement in all photovoltaic parameters. This material is suitable for large-scale fabrication of PSCs as it has shown an improved long-term stability when compared to the gold counterpart under elevated temperatures.
Luo J, Steier L, Son M-K, et al., 2016, Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting, NANO LETTERS, Vol: 16, Pages: 1848-1857, ISSN: 1530-6984
Albrecht S, Saliba M, Correa Baena JP, et al., 2016, Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature, Energy and Environmental Science, Vol: 9, Pages: 81-88, ISSN: 1754-5692
Tandem solar cells combining silicon and perovskite absorbers have the potential to outperform state-of-the-art high efficiency silicon single junction devices. However, the practical fabrication of monolithic silicon/perovskite tandem solar cells is challenging as material properties and processing requirements such as temperature restrict the device design. Here, we fabricate an 18% efficient monolithic tandem cell formed by a silicon heterojunction bottom- and a perovskite top-cell enabling a very high open circuit voltage of 1.78 V. The monolithic integration was realized via low temperature processing of the semitransparent perovskite sub-cell where an energetically aligned electron selective contact was fabricated by atomic layer deposition of tin oxide. The hole selective, transparent top contact was formed by a stack of the organic hole transport material spiro-OMeTAD, molybdenum oxide and sputtered indium tin oxide. The tandem cell design is currently limited by the photocurrent generated in the silicon bottom cell that is reduced due to reflectance losses. Based on optical modelling and first experiments, we show that these losses can be significantly reduced by combining optical optimization of the device architecture including light trapping approaches.
Steier L, Luo J, Schreier M, et al., 2015, Low-Temperature Atomic Layer Deposition of Crystalline and Photoactive Ultrathin Hematite Films for Solar Water Splitting, ACS NANO, Vol: 9, Pages: 11775-11783, ISSN: 1936-0851
Correa Baena JP, Steier L, Tress W, et al., 2015, Highly efficient planar perovskite solar cells through band alignment engineering, Energy & Environmental Science, Vol: 8, Pages: 2928-2934, ISSN: 1754-5706
The simplification of perovskite solar cells (PSCs), by replacing the mesoporous electron selective layer (ESL) with a planar one, is advantageous for large-scale manufacturing. PSCs with a planar TiO2 ESL have been demonstrated, but these exhibit unstabilized power conversion efficiencies (PCEs). Herein we show that planar PSCs using TiO2 are inherently limited due to conduction band misalignment and demonstrate, with a variety of characterization techniques, for the first time that SnO2 achieves a barrier-free energetic configuration, obtaining almost hysteresis-free PCEs of over 18% with record high voltages of up to 1.19 V.
Schreier M, Curvat L, Giordano F, et al., 2015, Efficient photosynthesis of carbon monoxide from CO2 using perovskite photovoltaics, Nature Communications, Vol: 6, ISSN: 2041-1723
Artificial photosynthesis, mimicking nature in its efforts to store solar energy, has received considerable attention from the research community. Most of these attempts target the production of H2 as a fuel and our group recently demonstrated solar-to-hydrogen conversion at 12.3% efficiency. Here, in an effort to take this approach closer to real photosynthesis, which is based on the conversion of CO2, we demonstrate the efficient reduction of CO2 to carbon monoxide driven solely by simulated sunlight using water as the electron source. Employing series-connected perovskite photovoltaics and high-performance catalyst electrodes, we reach a solar-to-CO efficiency exceeding 6.5%, which represents a new benchmark in sunlight-driven CO2 conversion. Considering hydrogen as a secondary product, an efficiency exceeding 7% is observed. Furthermore, this study represents one of the first demonstrations of extended, stable operation of perovskite photovoltaics, whose large open-circuit voltage is shown to be particularly suited for this process.
Luo J, Tilley SD, Steier L, et al., 2015, Solution Transformation of Cu2O into CuInS2 for Solar Water Splitting, NANO LETTERS, Vol: 15, Pages: 1395-1402, ISSN: 1530-6984
Steier L, Herraiz-Cardona I, Gimenez S, et al., 2014, Understanding the Role of Underlayers and Overlayers in Thin Film Hematite Photoanodes, ADVANCED FUNCTIONAL MATERIALS, Vol: 24, Pages: 7681-7688, ISSN: 1616-301X
Azevedo J, Steier L, Dias P, et al., 2014, On the stability enhancement of cuprous oxide water splitting photocathodes by low temperature steam annealing, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 7, Pages: 4044-4052, ISSN: 1754-5692
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