565 results found
Lee J, Cha H, Yao H, et al., 2020, Toward Visibly Transparent Organic Photovoltaic Cells Based on a Near-Infrared Harvesting Bulk Heterojunction Blend., ACS Appl Mater Interfaces, Vol: 12, Pages: 32764-32770
Wavelength-selective harvesting by organic solar cells (OSCs) has attracted significant research attention due to the unique potential of these materials for smart photovoltaic window applications. Here, a visibly transparent OSC is demonstrated by utilizing both near-infrared (NIR)-absorbing polymer donor and nonfullerene acceptor (NFA) materials with narrow optical band gaps of less than 1.4 eV. Despite the substantial overlap in absorption spectra between the donor and acceptor, sufficient lowest unoccupied molecular orbital (LUMO) and highest occupied molecule orbital (HOMO) energy offsets for efficient charge separation with concurrent very low voltage losses yield a power conversion efficiency (PCE) of 9.13%. Moreover, with the introduction of an ultrathin Ag film (8 nm) as a transparent top electrode, semitransparent OSCs exhibit an excellent dual-side photovoltaic performance of 5.7 and 3.9% under bottom and top illumination, respectively, with high transmittance reaching 60% at wavelengths from 400 to 600 nm. This approach is expected to provide a new perspective in developing the highly efficient and transparent OSCs.
Wu J, Lee J, Chin Y-C, et al., 2020, Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells, Energy & Environmental Science, ISSN: 1754-5692
In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement.
Lin Y-H, Sakai N, Da P, et al., 2020, A piperidinium salt stabilizes efficient metal-halide perovskite solar cells., Science, Vol: 369, Pages: 96-102
Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.
Wang Y, Vogel A, Sachs M, et al., 2020, Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts (vol 4, pg 746, 2019), NATURE ENERGY, ISSN: 2058-7546
Daboczi M, Kim J, Lee J, et al., 2020, Towards efficient integrated perovskite/organic bulk heterojunction solar cells: interfacial energetic requirement to reduce charge carrier recombination losses, Advanced Functional Materials, Vol: 30, Pages: 1-8, ISSN: 1616-301X
Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.
Wang Y, Liu X, Han X, et al., 2020, Unique hole-accepting carbon-dots promoting selective carbon dioxide reduction nearly 100% to methanol by pure water, NATURE COMMUNICATIONS, Vol: 11, ISSN: 2041-1723
Lin C-T, Ngiam J, Xu S, et al., 2020, Enhancing the operational stability of unencapsulated perovskite solar cells through Cu-Ag bilayer electrode incorporation, Journal of Materials Chemistry A, Vol: 8, Pages: 8684-8691, ISSN: 2050-7488
We identify a facile strategy that significantly reduces electrode corrosion and device degradation in unencapsulated perovskite solar cells (PSCs) operating in ambient air. By employing Cu-Ag bilayer top electrodes PSCs, we show enhanced operational lifetime compared with devices prepared from single metal (Al, Ag and Cu) analogues. Time-of-flight secondary ion mass spectrometry depth profiles indicate that the insertion of the thin layer of Cu (10nm) below the Ag (100nm) electrode significantly reduces diffusion of species originating in the perovskite active layer into the electron transport layer and electrode. X-ray diffraction (XRD) analysis reveals the mutually beneficial relationship between the bilayer metals, whereby the thermally evaporated Ag inhibits Cu oxidation and the Cu prevents interfacial reactions between the perovskite and Ag. The results here not only demonstrate a simple approach to prevent the electrode and device degradation that enhance lifetime and stability but also give an insight into ageing related ion migration and structural reorganisation.
Woods DJ, Hillman S, Pearce D, et al., 2020, Side-chain tuning in conjugated polymer photocatalysts for improved hydrogen production from water, Energy & Environmental Science, Vol: 13, Pages: 1843-1855, ISSN: 1754-5692
Structure–property–activity relationships in solution processable polymer photocatalysts for hydrogen production from water were probed by varying the chemical structure of both the polymer side-chains and the polymer backbone. In both cases, the photocatalytic performance depends strongly on the inclusion of more polar groups, such as dibenzo[b,d]thiophene sulfone backbone units or oligo(ethylene glycol) side-chains. We used optical, spectroscopic, and structural characterisation techniques to understand the different catalytic activities of these systems. We find that although polar groups improve the wettability of the material with water in all cases, backbone and side-chain modifications affect photocatalytic performance in different ways: the inclusion of dibenzo[b,d]thiophene sulfone backbone units improves the thermodynamic driving force for hole transfer to the sacrificial donor, while the inclusion of oligo ethylene glycol side-chains aids the degree of polymer swelling and also extends the electron polaron lifetime. The best performing material, FS-TEG, exhibits a HER of 72.5 μmol h−1 for 25 mg photocatalyst (2.9 mmol g−1 h−1) when dispersed in the presence of a sacrificial donor and illuminated with λ > 420 nm light, corresponding to a hydrogen evolution EQE of 10% at 420 nm. When cast as a thin film, this HER was further boosted to 13.9 mmol g−1 h−1 (3.0 mmol m−2 h−1), which is among the highest rates in this field.
Lee J, Lee J-H, Yao H, et al., 2020, Efficient and photostable ternary organic solar cells with a narrow band gap non-fullerene acceptor and fullerene additive, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 8, Pages: 6682-6691, ISSN: 2050-7488
Spitler MT, Modestino MA, Deutsch TG, et al., 2020, Practical challenges in the development of photoelectrochemical solar fuels production, SUSTAINABLE ENERGY & FUELS, Vol: 4, Pages: 985-995, ISSN: 2398-4902
Back H, Kim G, Kim H, et al., 2020, Highly stable inverted methylammonium lead tri-iodide perovskite solar cells achieved by surface re-crystallization, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 13, Pages: 840-847, ISSN: 1754-5692
Lin C-T, Lee J, Kim J, et al., 2020, Origin of open-circuit voltage enhancements in planar Perovskite solar cells induced by addition of bulky organic cations, Advanced Functional Materials, Vol: 30, ISSN: 1616-301X
The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (Voc) of methylammonium lead iodide (MAPbI3) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both Voc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (Voc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing Voc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.
Corby S, Francàs L, Kafizas A, et al., 2020, Determining the role of oxygen vacancies in the photoelectrocatalytic performance of WO3 for water oxidation, Chemical Science, Vol: 11, Pages: 2907-2914, ISSN: 2041-6520
Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO3 photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (∼2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer J–V performances. Our conclusion, that an optimal – neither too high nor too low – concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the competing beneficial and detrimental impact these defects have on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation.
Kosco J, Bidwell M, Cha H, et al., 2020, Enhanced photocatalytic hydrogen evolution from organic semiconductor heterojunction nanoparticles, NATURE MATERIALS, Vol: 19, Pages: 559-+, ISSN: 1476-1122
Hamid Z, Wadsworth A, Rezasoltani E, et al., 2020, Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T-2DT/Nonfullerene Organic Solar Cells, ADVANCED ENERGY MATERIALS, Vol: 10, ISSN: 1614-6832
Francas L, Corby S, Selim S, et al., 2020, Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts (vol 10, 5208, 2019), NATURE COMMUNICATIONS, Vol: 11, ISSN: 2041-1723
Hong DP, Jain SM, Li M, et al., 2020, All-Rounder Low-Cost Dopant-Free D-A-D Hole-Transporting Materials for Efficient Indoor and Outdoor Performance of Perovskite Solar Cells, ADVANCED ELECTRONIC MATERIALS, Vol: 6, ISSN: 2199-160X
Mesa Zamora C, Francas Forcada L, Yang KR, et al., 2020, Multihole water oxidation catalysis on hematite photoanodes revealed by operando spectroelectrochemistry and density functional theory, Nature Chemistry, Vol: 12, Pages: 82-89, ISSN: 1755-4330
Water oxidation is the key kinetic bottle neck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reactionon metal-oxidephotoanodes remains a significant experimental and theoretical challenge. Here we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly upon hematite.We observe that hematite is able toaccess a reaction mechanism third order in surface hole density, assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits a remarkably low activation energy (Ea~ 60 meV). Density functional theory is employedto determine the energetics of charge accumulation and O-O bond formation on a modelhematite 110 surface. The proposed mechanism shows parallels with the function of oxygen evolving complex of photosystem II,and provides new insights to the mechanism of heterogeneous water oxidation on a metal oxide surface.
Selim S, Pastor E, García-Tecedor M, et al., 2019, Impact of oxygen vacancy occupancy on charge carrier dynamics in BiVO4 photoanodes, Journal of the American Chemical Society, Vol: 141, Pages: 18791-18798, ISSN: 0002-7863
Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications, remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionisation of these states. We obtain an activation energy of ̴ 0.2 eV for this ionisation process, with thermally activated electron de-trapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionised states however, function as deep hole traps, with such trapped holes being energetically unable to drive water oxidation. These observations help address recent controversies in the literature over oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.
Ambroz F, Xu W, Gadipelli S, et al., 2019, Room Temperature Synthesis of Phosphine-Capped Lead Bromide Perovskite Nanocrystals without Coordinating Solvents, PARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Vol: 37, ISSN: 0934-0866
Daboczi M, Hamilton I, Xu S, et al., 2019, The origin of open-circuit voltage losses in perovskite solar cells investigated by surface photovoltage measurement, ACS Applied Materials & Interfaces, Vol: 11, Pages: 46808-46817, ISSN: 1944-8244
Increasing the open circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction and their loss mechanisms inside a device upon illumination. Herein we report the important origin of Voc losses in methylammonium lead iodide perovskite (MAPI) based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the PEDOT:PSS hole transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50-nm-thick device compared to 230-nm-thick one). In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices (70 mV increase in 50-nm-thick device compared to 230-nm-thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.
Francàs L, Corby S, Selim S, et al., 2019, Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts, Nature Communications, Vol: 10, ISSN: 2041-1723
Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.
Wu J, Luke J, Lee HKH, et al., 2019, Tail state limited photocurrent collection of thick photoactive layers in organic solar cells, Nature Communications, Vol: 10, ISSN: 2041-1723
Weanalyseorganic solar cells with four differentphotoactive blends exhibiting differing dependencies ofshort-circuit current upon photoactive layer thickness.These blends and devices are analysedbytransient optoelectronic techniques ofcarrier kinetics and densities, airphotoemission spectroscopyof material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We concludethat,for the device series studied, the photocurrent losswith thick active layersis primarilyassociatedwith the accumulation of photo-generated charge carriers in intra-bandgap tail states.This charge accumulation screens the device internal electricalfield, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail statedistribution anddensity and increase the maximal photoactive thickness forefficient operation. Ourwork suggests that selectingorganic photoactive layerswith a narrow distribution of tail states isa keyrequirement for the fabrication of efficient, high photocurrent, thick organic solar cells.
Luo H, Papaioannou N, Salvadori E, et al., 2019, Manipulating the Optical Properties of Carbon Dots by Fine-Tuning their Structural Features, CHEMSUSCHEM, Vol: 12, Pages: 4432-4441, ISSN: 1864-5631
He Q, Shahid M, Wu J, et al., 2019, Fused Cyclopentadithienothiophene acceptor enables ultrahigh short‐circuit current and high efficiency >11% in as‐cast organic solar cells, Advanced Functional Materials, Vol: 29, Pages: 1-7, ISSN: 1616-301X
A new method to synthesize an electron‐rich building block cyclopentadithienothiophene (9H‐thieno‐[3,2‐b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2‐d]thiophene, CDTT) via a facile aromatic extension strategy is reported. By combining CDTT with 1,1‐dicyanomethylene‐3‐indanone endgroups, a promising nonfullerene small molecule acceptor (CDTTIC) is prepared. As‐cast, single‐junction nonfullerene organic solar cells based on PFBDB‐T: CDTTIC blends exhibit very high short‐circuit currents up to 26.2 mA cm−2 in combination with power conversion efficiencies over 11% without any additional processing treatments. The high photocurrent results from the near‐infrared absorption of the CDTTIC acceptor and the well‐intermixed blend morphology of polymer donor PFBDB‐T and CDTTIC. This work demonstrates a useful fused ring extension strategy and promising solar cell results, indicating the great potential of the CDTT derivatives as electron‐rich building blocks for constructing high‐performance small molecule acceptors in organic solar cells.
Macdonald TJ, Batmunkh M, Lin C-T, et al., 2019, Origin of performance enhancement in TiO2-carbon nanotube composite perovskite solar cells, Small Methods, Vol: 3, Pages: 1-10, ISSN: 2366-9608
Carbon nanotubes are shown to be beneficial additives to perovskite solar cells, and the inclusion of such nanomaterials will continue to play a crucial role in the push toward developing efficient and stable device architectures. Herein, titanium dioxide/carbon nanotube composite perovskite solar cells are fabricated, and device performance parameters are correlated with spectroscopic signatures of the materials to understand the origin of performance enhancement. By probing the charge carrier dynamics with photoluminescence and femtosecond transient absorption spectroscopy, the results indicate that charge transfer is not improved by the presence of the carbon nanotubes. Instead, carbon nanotubes are shown to passivate the electronic defect states within the titanium dioxide, which can lead to stronger radiative recombination in the titanium dioxide/carbon nanotube films. The defect passivation allows the perovskite solar cells made using an optimized titanium dioxide/carbon nanotube composite to achieve a peak power conversion efficiency of 20.4% (19% stabilized), which is one of the highest values reported for perovskite solar cells not incorporating a mixed cation light absorbing layer. The results discuss new fundamental understandings for the role of carbon nanomaterials in perovskite solar cells and present a significant step forward in advancing the field of high‐performance photovoltaics.
Sung MJ, Hong J, Cha H, et al., 2019, Acene-Modified Small-Molecule Donors for Organic Photovoltaics, Chemistry - A European Journal, Vol: 25, ISSN: 0947-6539
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Invited for the cover of this issue is the group of Tae Kyu An at the Korea National University of Transportation, Soon-Ki Kwon and Yun-Hi Kim at the Gyeongsang National University. The image depicts organic photovoltaics, in which fused acene cores have been used to modulate the conjugation lengths and the bulk heterojunction morphologies. Read the full text of the article at 10.1002/chem.201902177.
Corby S, Pastor E, Dong Y, et al., 2019, Charge separation, band-bending, and recombination in WO3 photoanodes, Journal of Physical Chemistry Letters, Vol: 10, Pages: 5395-5401, ISSN: 1948-7185
In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.
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.
Wang Y, Vogel A, Sachs M, et al., 2019, Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts, Nature Energy, Vol: 4, Pages: 746-760, ISSN: 2058-7546
The use of hydrogen as a fuel, when generated from water using semiconductor photocatalysts and driven by sunlight, is a sustainable alternative to fossil fuels. Polymeric photocatalysts are based on Earth-abundant elements and have the advantage over their inorganic counterparts in that their electronic properties are easily tuneable through molecular engineering. Polymeric photocatalysts have developed rapidly over the past decade, resulting in the discovery of many active materials. However, our understanding of the key properties underlying their photoinitiated redox processes has not kept pace, and this impedes further progress to generate cost-competitive technologies. Here, we discuss state-of-the-art polymeric photocatalysts and our microscopic understanding of their activities. We conclude with a discussion of five outstanding challenges in this field: non-standardized reporting of activities, limited photochemical stability, insufficient knowledge of reaction mechanisms, balancing charge carrier lifetimes with catalysis timescales and the use of unsustainable sacrificial reagents.
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