This study pinpoints the advantages of ultrathin electron selective layers (ESL) of SnO2 fabricated by atomic layer deposition (ALD). These layers recently caught attention in planar perovskite solar ...cells and appear as powerful alternatives to other oxides such as TiO2. Here, we carry out a thorough characterization of the nature of these ultrathin ALD SnO2 layers providing a novel physical insight for the design of various photoelectrodes in perovskite and dye-sensitized solar cells and in photoelectrochemical water splitting. We use a combination of cyclic voltammetry, electrochemical impedance spectroscopy, Hall measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and electron microscopy to analyze the blocking behavior and energetics of as-deposited (low-temperature) and also calcined ALD SnO2 layers. First, we find that the low-temperature ALD-grown SnO2 layers are amorphous and perfectly pinhole-free for thicknesses down to 2 nm. This exceptional blocking behavior of thin ALD SnO2 layers allows photoelectrode designs with even thinner electron selective layers, thus potentially minimizing resistance losses. The compact nature and blocking function of thin SnO2 films are not perturbed by annealing at 450 °C, which is a significant benefit compared to other amorphous ALD oxides. Further on, we show that amorphous and crystalline ALD SnO2 films substantially differ in their flatband (and conduction band) positionsa finding to be taken into account when considering band alignment engineering in solar devices using these high-quality blocking layers.
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.
A review on organic photoelectrochemical water splitting cells giving perspective on promising device architectures and materials.
Due to its abundance, scalability, and nontoxicity, Cu2O has attracted extensive attention toward solar energy conversion, and it is the best performing metal oxide material. Until now, the high ...efficiency devices are all planar in structure, and their photocurrent densities still fall well below the theoretical value of 14.5 mA cm–2 due to the incompatible light absorption and charge carrier diffusion lengths. Nanowire structures have been considered as a rational and promising approach to solve this issue, but due to various challenges, performance improvements through the use of nanowires have rarely been achieved. In this work, we develop a new synthetic method to grow Cu2O nanowire arrays on conductive fluorine-doped tin oxide substrates with well-controlled phase and excellent electronic and photonic properties. Also, we introduce an innovative blocking layer strategy to enable high performance. Further, through material engineering by combining a conformal nanoscale p–n junction, durable protective overlayer, and uniform catalyst decoration, we have successfully fabricated Cu2O nanowire array photocathodes for hydrogen generation from solar water splitting delivering unprecedentedly high photocurrent densities of 10 mA cm–2 and stable operation beyond 50 h, establishing a new benchmark for metal oxide based photoelectrodes.
Optimizing the morphology of metal halide perovskite films is an important way to improve the performance of solar cells when these materials are used as light harvesters, because film homogeneity is ...correlated with photovoltaic performance. Many device architectures and processing techniques have been explored with the aim of achieving high-performance devices, including single-step deposition, sequential deposition and anti-solvent methods. Earlier studies have looked at the influence of reaction conditions on film quality, such as the concentration of the reactants and the reaction temperature. However, the precise mechanism of the reaction and the main factors that govern it are poorly understood. The consequent lack of control is the main reason for the large variability observed in perovskite morphology and the related solar-cell performance. Here we show that light has a strong influence on the rate of perovskite formation and on film morphology in both of the main deposition methods currently used: sequential deposition and the anti-solvent method. We study the reaction of a metal halide (lead iodide) with an organic compound (methylammonium iodide) using confocal laser scanning fluorescence microscopy and scanning electron microscopy. The lead iodide crystallizes before the intercalation of methylammonium iodide commences, producing the methylammonium lead iodide perovskite. We find that the formation of perovskite via such a sequential deposition is much accelerated by light. The influence of light on morphology is reflected in a doubling of solar-cell efficiency. Conversely, using the anti-solvent method to form methyl ammonium lead iodide perovskite in a single step from the same starting materials, we find that the best photovoltaic performance is obtained when films are produced in the dark. The discovery of light-activated crystallization not only identifies a previously unknown source of variability in opto-electronic properties, but also opens up new ways of tuning morphology and structuring perovskites for various applications.
Biomass is recognized as an ideal CO2 neutral, abundant, and renewable resource substitute to fossil fuels. The rich proton content in most biomass derived materials, such as ethanol, ...5‐hydroxymethylfurfural (HMF) and glycerol allows it to be an effective hydrogen carrier. The oxidation derivatives, such as 2,5‐difurandicarboxylic acid from HMF, glyceric acid from glycerol are valuable products to be used in biodegradable polymers and pharmaceuticals. Therefore, combining biomass‐derived compound oxidation at the anode and hydrogen evolution reaction at the cathode in a biomass electrolysis or photo‐reforming reactor would present a promising strategy for coproducing high value chemicals and hydrogen with low energy consumption and CO2 emissions. This review aims to combine fundamental knowledge on photo and electro‐assisted catalysis to provide a comprehensive understanding of the general reaction mechanisms of different biomass‐derived molecule oxidation. At the same time, catalyst requirements and recent advances for various feedstock compounds are also reviewed in detail. Technoeconomic assessment and life cycle analysis are performed on various feedstocks to assess the relative benefits of various processes, and finally critical prospects are given on the challenges and opportunities for technology development to meet the sustainability requirement of the future global energy economy.
This review summarizes recent research progress in photo‐ and electrochemical oxidation of biomass feedstocks for the production of chemicals and hydrogen. High‐level technoeconomic assessment and life cycle analysis are performed to evaluate the economic value and environmental impact of biomass electrolysis. Based on the outcomes, perspectives on current technical challenges and future opportunities are provided.
Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for photoelectrochemical and photocatalytic systems, and a key determinant of their efficiency. Metal ...oxides are widely explored as photoanodes for photoelectrochemical water oxidation. However, their application is limited by the disparity between the picosecond–nanosecond lifetimes of electrons and holes photoexcited in bulk metal oxides versus the millisecond–second timescale of water oxidation catalysis. This Review addresses the charge-carrier dynamics underlying the performance of metal oxide photoanodes and their ability to drive photoelectrochemical water oxidation, alongside comparison with metal oxide function in photocatalytic and electrocatalytic systems. We assess 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. We examine approaches to enhance performance, including material selection, doping, nanostructuring, junction formation and/or co-catalyst deposition. Crucially, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimization.Metal oxides are widely used in photoelectrochemical and photocatalytic systems for fuel synthesis and environmental remediation. In this Review, we examine the kinetic challenges, from charge generation to water oxidation catalysis, that determine the performance of metal oxide photo(electro)catalysts.
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.
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 (NCNCN x ) 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 NCNCN x 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 NCNCN x 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 NCNCN x 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.
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 vialow 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.
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 ...TiO sub(2) ESL have been demonstrated, but these exhibit unstabilized power conversion efficiencies (PCEs). Herein we show that planar PSCs using TiO sub(2) are inherently limited due to conduction band misalignment and demonstrate, with a variety of characterization techniques, for the first time that SnO sub(2) achieves a barrier-free energetic configuration, obtaining almost hysteresis-free PCEs of over 18% with record high voltages of up to 1.19 V.
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