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Journal articleLindquist RJ, Phelan BT, Reynal A, et al., 2016,
Strongly oxidizing perylene-3,4-dicarboximides for use in water oxidation photoelectrochemical cells
, Journal of Materials Chemistry A, Vol: 4, Pages: 2880-2893, ISSN: 2050-7496Perylene-3,4-dicarboximide (PMI) based chromophores have demonstrated the ability to inject electrons into TiO2 for dye-sensitized solar cell applications and to accept electrons from metal complexes relevant to water oxidation, but they are nearly unexplored for use in photoelectrochemical cells (PECs) for solar fuels generation. A series of related PMIs with high oxidation potentials and carboxylate binding groups was synthesized and investigated for this purpose. Charge injection and recombination dynamics were measured using transient absorption (TA) spectroscopy on the picosecond to second timescales. The dynamics and electron injection yields were correlated with the PMI energetics and structures. Injection began in less than 1 ps for the dye with the best performance and a significant charge-separated state yield remained at long times. Finally, this chromophore was used to oxidize a covalently bound water oxidation precatalyst following electron injection into TiO2 to demonstrate the utility of the dyes for use in PECs.
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Journal articleMorais A, Longo C, Araujo JR, et al., 2016,
Nanocrystalline anatase TiO<sub>2</sub>/reduced graphene oxide composite films as photoanodes for photoelectrochemical water splitting studies: the role of reduced graphene oxide
, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 18, Pages: 2608-2616, ISSN: 1463-9076- Cite
- Citations: 92
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Journal articleDimitrov SD, Schroeder BC, Nielsen CB, et al., 2016,
Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells
, Polymers, Vol: 8, ISSN: 2073-4360The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.
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Journal articleFeckl JM, Dunn HK, Zehetmaier PM, et al., 2015,
Ultrasmall Co3O4 Nanocrystals Strongly Enhance Solar Water Splitting on Mesoporous Hematite
, Advanced Materials Interfaces, Vol: 2, ISSN: 2196-7350The synthesis of crystalline, nonagglomerated, and perfectly dispersible Co3O4 nanoparticles with an average size of 3–7 nm using a solvothermal reaction in tert-butanol is reported. The very small size and high dispersibility of the Co3O4 nanoparticles allow for their homogeneous deposition on mesoporous hematite layers serving as the photoactive absorber in the light-driven water splitting reaction. This surface treatment leads to a striking photocurrent increase. While the enhancement of hematite photoanode performance by cobalt oxides is known, the preformation and subsequent application of well-defined cobalt oxide nanoparticles are novel and allow for the treatment of arbitrarily complex hematite morphologies. Photoelectrochemical and transient absorption spectroscopy studies show that this enhanced performance is due to the suppression of surface electron–hole recombination on time scales of milliseconds to seconds.
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Journal articleWillkomm J, Orchard KL, Reynal A, et al., 2015,
Dye-sensitised semiconductors modified with molecular catalysts for light-driven H-2 production
, Chemical Society Reviews, Vol: 45, Pages: 9-23, ISSN: 1460-4744The development of synthetic systems for the conversion of solar energy into chemical fuels is aresearch goal that continues to attract growing interest owing to its potential to provide renewable andstorable energy in the form of a ‘solar fuel’. Dye-sensitised photocatalysis (DSP) with molecular catalystsis a relatively new approach to convert sunlight into a fuel such as H2 and is based on the self-assemblyof a molecular dye and electrocatalyst on a semiconductor nanoparticle. DSP systems combineadvantages of both homogenous and heterogeneous photocatalysis, with the molecular componentsproviding an excellent platform for tuning activity and understanding performance at defined catalyticsites, whereas the semiconductor bridge ensures favourable multi-electron transfer kinetics betweenthe dye and the fuel-forming electrocatalyst. In this tutorial review, strategies and challenges for theassembly of functional molecular DSP systems and experimental techniques for their evaluation areexplained. Current understanding of the factors governing electron transfer across inorganic-molecularinterfaces is described and future directions and challenges for this field are outlined.
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Journal articleCollado Fregoso E, Boufflet P, Fei Z, et al., 2015,
Increased Exciton Dipole Moment Translates into Charge-transfer Excitons in Thiophene-fluorinated Low-bandgap Polymers for Organic Photovoltaic Applications
, Chemistry of Materials, Vol: 27, Pages: 7934-7944, ISSN: 1520-5002In this study, we investigate the role of thiophene fluorination in a low-bandgap polymer for organic photovoltaic applications. We use a combined theoretical and experimental approach to investigate charge generation and recombination dynamics, and their correlation with blend microstructure and polymer dipole moment. We find that fluorination results in an increased change in the dipole moment upon exciton formation, which is correlated with the appearance of charge-transfer excitons, as evidenced from ultrafast transient absorption studies. Fluorination also results in smaller yet purer domains, evidenced by atomic force microscopy and resonant soft X-ray scattering, and in agreement with photoluminescence quenching measurements. This change in film morphology is correlated with a modest retardation of nongeminate recombination losses. The efficient charge generation and slower recombination are likely to be partly responsible for the enhanced device efficiency in the fluorinated polymer/fullerene devices.
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Conference paperCollado L, Reynal A, Coronado JM, et al., 2015,
Effect of Au surface plasmon nanoparticles on the selective CO<sub>2</sub> photoreduction to CH<sub>4</sub>
, 8th European Meeting on Solar Chemistry and Photocatalysis - Environmental Applications (SPEA), Publisher: ELSEVIER, Pages: 177-185, ISSN: 0926-3373- Cite
- Citations: 96
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Journal articleLi Z, Chiu K-H, Shahid RS, et al., 2015,
Toward Improved Lifetimes of Organic Solar Cells under Thermal Stress: Substrate-Dependent Morphological Stability of PCDTBT:PCBM Films and Devices
, Scientific Reports, Vol: 5, ISSN: 2045-2322Morphological stability is a key requirement for outdoor operation of organic solar cells. We demonstrate that morphological stability and lifetime of polymer/fullerene based solar cells under thermal stress depend strongly on the substrate interface on which the active layer is deposited. In particular, we find that the stability of benchmark PCDTBT/PCBM solar cells under modest thermal stress is substantially increased in inverted solar cells employing a ZnO substrate compared to conventional devices employing a PEDOT:PSS substrate. This improved stability is observed to correlate with PCBM nucleation at the 50 nm scale, which is shown to be strongly influenced by different substrate interfaces. Employing this approach, we demonstrate remarkable thermal stability for inverted PCDTBT:PC70BM devices on ZnO substrates, with negligible (<2%) loss of power conversion efficiency over 160 h under 85 °C thermal stress and minimal thermally induced “burn-in” effect. We thus conclude that inverted organic solar cells, in addition to showing improved environmental stability against ambient humidity exposure as widely reported previously, can also demonstrate enhanced morphological stability. As such we show that the choice of suitable substrate interfaces may be a key factor in achieving prolonged lifetimes for organic solar cells under thermal stress conditions.
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Journal articleMa Y, Le Formal F, Kafizas A, et al., 2015,
Efficient suppression of back electron/hole recombination in cobalt phosphate surface-modified undoped bismuth vanadate photoanodes
, Journal of Materials Chemistry A, Vol: 3, Pages: 20649-20657, ISSN: 2050-7496In this paper, we compared for the first time the dynamics of photogenerated holes in BiVO4 photoanodes with and without CoPi surface modification, employing transient absorption and photocurrent measurements on microsecond to second timescales. CoPi surface modification is known to cathodically shift the water oxidation onset potential; however, the reason for this improvement has not until now been fully understood. The transient absorption and photocurrent data were analyzed using a simple kinetic model, which allows quantification of the competition between electron/hole recombination and water oxidation. The results of this model are shown to be in excellent agreement with the measured photocurrent data. We demonstrate that the origin of the improvement of photocurrent onset resulting from CoPi treatment is primarily due to retardation of back electron/hole recombination across the space charge layer; no evidence of catalytic water oxidation via CoPi was observed.
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Journal articleWheeler, Deledalle F, Tokmoldin N, et al., 2015,
Influence of Surface Recombination on Charge-Carrier Kinetics in Organic Bulk Heterojunction Solar Cells with Nickel Oxide Interlayers
, Physical review applied, Vol: 4, ISSN: 2331-7019The choice of electrode for organic photovoltaics is known to be of importance to both device stability and performance, especially regarding the open-circuit voltage (VOC). Here we show that the work function of a nickel oxide anode, varied using an O2 plasma treatment, has a considerable influence on the open-circuit voltage VOC of an organic solar cell. We probe recombination in the devices using transient photovoltage and charge extraction to determine the lifetime as a function of charge-carrier concentration and compare the experimental results with numerical drift-diffusion simulations. This combination of experiment and simulations allows us to conclude that the variations in VOC are due to a change in surface recombination, localized at the NiO anode, although only a small change in carrier lifetime is observed.
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Journal articleYue W, Ashraf RS, Nielsen C, et al., 2015,
A Thieno[3,2-b][1]benzothiophene Isoindigo Building Block for Additive- and Annealing-Free High-Performance Polymer Solar Cells
, Advanced Materials, Vol: 27, Pages: 4702-4707, ISSN: 1521-4095A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2-b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar-cell devices of the high-molecular-weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.
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Journal articleAndemach R, Utzat H, Dimitrov SD, et al., 2015,
Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers
, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 137, Pages: 10383-10390, ISSN: 0002-7863- Author Web Link
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- Citations: 41
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Journal articleTan CH, Wong HC, Li Z, et al., 2015,
Synergetic enhancement of organic solar cell thermal stability by wire bar coating and light processing
, Journal of Materials Chemistry C, Vol: 3, Pages: 9551-9558, ISSN: 2050-7534We demonstrate that organic solar cells can exhibit different morphological and performance stability under thermal stress depending upon the processing technique employed, without compromising initial device efficiency. In particular, we investigate benchmark PCDTBT:PC60BM solar cells fabricated by wire bar coating (a technique attractive for commercial manufacture) and the more widely employed, lab scale, technique of spin coating. For this system, wire bar deposition results in superior device stability, with lifetime improvements in excess of 20-fold compared to spun cast devices. Neutron reflectivity reveals that the enhanced PC60BM segregation to the top interface in the slower, wire bar, casting process is likely responsible for the hindered PC60BM nucleation at tens of nm length scale, characterized by atomic force microscopy (AFM), and thus enhanced morphological stability. Modest light exposure of the active layer (at approximately 10 mW cm−2), known to reversibly photo-oligomerize fullerenes and thus impart morphological stability, is found to further improve device stability by a factor of 10. The combined effects of wire bar coating and light processing are highly synergetic, resulting in solar cells which are overall 200 times more stable than devices prepared by spin casting without light processing.
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Journal articleBryant D, Wheeler S, ORegan BC, et al., 2015,
Observable Hysteresis at Low Temperature in “Hysteresis Free” Organic–Inorganic Lead Halide Perovskite Solar Cells
, The Journal of Physical Chemistry Letters, Vol: 6, Pages: 3190-3194, ISSN: 1948-7185 -
Journal articleWade J, Wood S, Beatrup D, et al., 2015,
Operational electrochemical stability of thiophene-thiazole copolymers probed by resonant Raman spectroscopy.
, Journal of Chemical Physics, Vol: 142, Pages: 244904-244904, ISSN: 1089-7690We report on the electrochemical stability of hole polarons in three conjugated polymers probed by resonant Raman spectroscopy. The materials considered are all isostructural to poly(3-hexyl)thiophene, where thiazole units have been included to systematically deepen the energy level of the highest occupied molecular orbital (HOMO). We demonstrate that increasing the thiazole content planarizes the main conjugated backbone of the polymer and improves the electrochemical stability in the ground state. However, these more planar thiazole containing polymers are increasingly susceptible to electrochemical degradation in the polaronic excited state. We identify the degradation mechanism, which targets the C=N bond in the thiazole units and results in disruption of the main polymer backbone conjugation. The introduction of thiazole units to deepen the HOMO energy level and increase the conjugated backbone planarity can be beneficial for the performance of certain optoelectronic devices, but the reduced electrochemical stability of the hole polaron may compromise their operational stability.
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Journal articleReynal A, Pastor E, Gross MA, et al., 2015,
Unravelling the pH-dependence of a molecular photocatalytic system for hydrogen production
, Chemical Science, Vol: 6, Pages: 4855-4859, ISSN: 2041-6539Photocatalytic systems for the reduction of aqueous protons are strongly pH-dependent, but the origin of this dependency is still not fully understood. We have studied the effect of different degrees of acidity on the electron transfer dynamics and catalysis taking place in a homogeneous photocatalytic system composed of a phosphonated ruthenium tris(bipyridine) dye (RuP) and a nickel bis(diphosphine) electrocatalyst (NiP) in an aqueous ascorbic acid solution. Our approach is based on transient absorption spectroscopy studies of the efficiency of photo-reduction of RuP and NiP correlated with pH-dependent photocatalytic H2 production and the degree of catalyst protonation. The influence of these factors results in an observed optimum photoactivity at pH 4.5 for the RuP–NiP system. The electron transfer from photo-reduced RuP to NiP is efficient and independent of the pH value of the medium. At pH <4.5, the efficiency of the system is limited by the yield of RuP photo-reduction by the sacrificial electron donor, ascorbic acid. At pH >4.5, the efficiency of the system is limited by the poor protonation of NiP, which inhibits its ability to reduce protons to hydrogen. We have therefore developed a rational strategy utilising transient absorption spectroscopy combined with bulk pH titration, electrocatalytic and photocatalytic experiments to disentangle the complex pH-dependent activity of the homogenous RuP–NiP photocatalytic system, which can be widely applied to other photocatalytic systems.
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Journal articleLe Formal F, Pastor E, Tilley SD, et al., 2015,
Rate law analysis of water oxidation on a hematite surface
, Journal of the American Chemical Society, Vol: 137, Pages: 6629-6637, ISSN: 1520-5126Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.
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Journal articleWang X, Kafizas A, Li X, et al., 2015,
Transient Absorption Spectroscopy of Anatase and Rutile: The Impact of Morphology and Phase on Photocatalytic Activity
, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 10439-10447, ISSN: 1932-7447- Cite
- Citations: 144
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Journal articleKafizas AG, Durrant JR,
Transient Absorption Spectroscopy of Anatase and Rutile: the Impact of Morphology and Phase on Photocatalytic Activity
, The Journal of Physical Chemistry C, ISSN: 1932-7447 -
Journal articleSotelo-Vazquez C, Noor N, Kafizas A, et al., 2015,
Multifunctional P-Doped TiO2 films: a new approach to self-cleaning, transparent conducting oxide materials
, Chemistry of Materials, Vol: 27, Pages: 3234-3242, ISSN: 1520-5002Multifunctional P-doped TiO2 thin films were synthesized by atmospheric pressure chemical vapor deposition (APCVD). This is the first example of P-doped TiO2 films with both P5+ and P3– states, with the relative proportion being determined by synthesis conditions. This technique to control the oxidation state of the impurities presents a new approach to achieve films with both self-cleaning and TCO properties. The origin of electrical conductivity in these materials was correlated to the incorporation of P5+ species, as suggested by Hall Effect probe measurements. The photocatalytic performance of the films was investigated using the model organic pollutant, stearic acid, with films containing predominately P3– states found to be vastly inferior photocatalysts compared to undoped TiO2 films. Transient absorption spectroscopy studies also showed that charge carrier concentrations increased by several orders of magnitude in films containing P5+ species only, whereas photogenerated carrier lifetimes—and thus photocatalytic activity—were severely reduced upon incorporation of P3– species. The results presented here provide important insights on the influence of dopant nature and location within a semiconductor structure. These new P-doped TiO2 films are a breakthrough in the development of multifunctional advanced materials with tuned properties for a wide range of applications.
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Journal articleDeledalle F, Kirchartz T, Vezie MS, et al., 2015,
Understanding the Effect of Unintentional Doping on Transport Optimization and Analysis in Efficient Organic Bulk-Heterojunction Solar Cells
, PHYSICAL REVIEW X, Vol: 5, ISSN: 2160-3308- Cite
- Citations: 54
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Journal articleO'Mahony FTF, Lee YH, Jellett C, et al., 2015,
Improved environmental stability of organic lead trihalide perovskite-based photoactive-layers in the presence of mesoporous TiO2
, Journal of Materials Chemistry A, Vol: 3, Pages: 7219-7223, ISSN: 2050-7496Impressive hybrid photovoltaic device performances have been realised with the methylammonium lead triiodide (MAPbI3) perovskite absorber in a wide range of device architectures. However, the question as to which of these systems represents the most commercially viable long-term prospect is yet to be answered conclusively. Here, we report on the photoinduced charge transfer processes in MAPbI3 based films measured under inert and ambient conditions. When exposed to ambient conditions, the coated mesoporous Al2O3 and bilayer systems show a rapid and significant degradation in the yield of long-lived charge separation. This process, which does not affect sensitized-mesoporous TiO2 films, is only found to occur when both light and oxygen are present. These observations indicate that the presence of a mesostructured TiO2 electron acceptor to rapidly extract the photoexcited electron from the perovskite sensitizer may be crucial for fundamental photovoltaic stability and significantly increases innate tolerance to environmental conditions. This work highlights a significant advantage of retaining mesoscale morphological control in the design of perovskite photovoltaics.
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Journal articleDimitrov SD, Wheeler S, Niedzialek D, et al., 2015,
Polaron pair mediated triplet generation in polymer/fullerene blends
, Nature Communications, Vol: 6, ISSN: 2041-1723Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.
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Journal articleWindle CD, Pastor E, Reynal A, et al., 2015,
Improving the Photocatalytic Reduction of CO<sub>2</sub> to CO through Immobilisation of a Molecular Re Catalyst on TiO<sub>2</sub>
, CHEMISTRY-A EUROPEAN JOURNAL, Vol: 21, Pages: 3746-3754, ISSN: 0947-6539- Cite
- Citations: 147
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Journal articleHolliday S, Ashraf RS, Nielsen CB, et al., 2015,
A rhodanine flanked nonfullerene acceptor for solution-processed organic photovoltaics
, Journal of the American Chemical Society, Vol: 137, Pages: 898-904, ISSN: 0002-7863A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a nonfullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing poly(3-hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave power conversion efficiencies (PCE) up to 4.1%. Transient and steady state optical spectroscopies indicated efficient, ultrafast charge generation and efficient photocurrent generation from both donor and acceptor. Ultrafast transient absorption spectroscopy was used to investigate polaron generation efficiency as well as recombination dynamics. It was determined that the P3HT:FBR blend is highly intermixed, leading to increased charge generation relative to comparative devices with P3HT:PC60BM, but also faster recombination due to a nonideal morphology in which, in contrast to P3HT:PC60BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better performance than P3HT:PC60BM devices, used as control, demonstrating that this acceptor shows great promise for further optimization.
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Journal articleShoaee S, Deledalle F, Tuladhar PS, et al., 2015,
A Comparison of Charge Separation Dynamics in Organic Blend Films Employing Fullerene and Perylene Diimide Electron Acceptors
, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 6, Pages: 201-205, ISSN: 1948-7185- Author Web Link
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- Citations: 34
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Journal articleWood S, Wade J, Shahid M, et al., 2015,
Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers
, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 8, Pages: 3222-3232, ISSN: 1754-5692- Cite
- Citations: 104
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Journal articleShoaee S, Durrant JR, 2015,
Oxygen diffusion dynamics in organic semiconductor films
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 10079-10084, ISSN: 2050-7526- Author Web Link
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- Citations: 14
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Journal articleNeri G, Walsh JJ, Wilson C, et al., 2015,
A functionalised nickel cyclam catalyst for CO<sub>2</sub> reduction: electrocatalysis, semiconductor surface immobilisation and light-driven electron transfer
, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 17, Pages: 1562-1566, ISSN: 1463-9076- Cite
- Citations: 59
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Journal articleKim JS, Wood S, Shoaee S, et al., 2015,
Morphology-performance relationships in polymer/fullerene blends probed by complementary characterisation techniques - effects of nanowire formation and subsequent thermal annealing
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 9224-9232, ISSN: 2050-7526- Author Web Link
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- Citations: 11
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