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Journal articleFrancas L, Matheu R, Pastor E, et al., 2017,
Kinetic Analysis of an Efficient Molecular Light-Driven Water Oxidation System
, ACS CATALYSIS, Vol: 7, Pages: 5142-5150, ISSN: 2155-5435We report an efficient molecular light-driven system to oxidize water to oxygen and a kinetic analysis of the factors determining the efficiency of the system. The system comprises a highly active molecular catalyst ([RuIV(tda)(py)2(O)]), [RuII(bpy)(bpy-COOEt)2]2+ (RuP), as sensitizer and Na2S2O8 as sacrificial electron acceptor. This combination exhibits a high quantum yield (25%) and chemical yield (93%) for photodriven oxygen evolution from water. The processes underlying this performance are identified using optical techniques, including transient absorption spectroscopy and photoluminescence quenching. A high catalyst concentration is found to be required to optimize the efficiency of electron transfer between the oxidized sensitizer and the catalyst, which also has the effect of improving sensitizer stability. The main limitation of the quantum yield is the relatively low efficiency of S2O82– as an electron scavenger to oxidize the photoexcited ruthenium sensitizer RuP* to 2 RuP+, mainly due to competing back electron transfers to the RuP ground state. The overall rate of light-driven oxygen generation is determined primarily by the rate of photon absorption by the molecular sensitizer under the incident photon flux. As such, the performance of this efficient light-driven system is limited not by the properties of the molecular water oxidation catalyst, which exhibits both good kinetics and stability, but rather by the light absorption and quantum efficiency properties of the sensitizer and electron scavenger. We conclude by discussing the implications of these results for further optimization of molecular light-driven systems for water oxidation.
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Journal articleMoss B, Lim KK, Beltram A, et al., 2017,
Comparing photoelectrochemical water oxidation, recombination kinetics and charge trapping in the three polymorphs of TiO2
, Scientific Reports, Vol: 7, ISSN: 2045-2322In this article we present the first comparative study of the transient decay dynamics of photo-generated charges for the three polymorphs of TiO2. To our knowledge, this is the first such study of the brookite phase of TiO2 over timescales relevant to the kinetics of water splitting. We find that the behavior of brookite, both in the dynamics of relaxation of photo-generated charges and in energetic distribution, is similar to the anatase phase of TiO2. Moreover, links between the rate of recombination of charge carriers, their energetic distribution and the mode of transport are made in light of our findings and used to account for the differences in water splitting efficiency observed across the three polymorphs.
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Journal articleWheeler SGM, Bryant D, Troughton J, et al., 2017,
Transient optoelectronic analysis of the impact of material energetics and recombination kinetics on the open-circuit voltage of hybrid perovskite solar cells
, Journal of Physical Chemistry C, Vol: 121, Pages: 13496-13506, ISSN: 1932-7455Transient optoelectronic measurements were used to evaluate the factors determining the open-circuit voltage of a series of planar photovoltaic devices based on hybrid perovskite layers with varying iodine/bromine ratios. Employing differential charging and transient photovoltage measurements, we used a simple device model based on the charge-carrier-density dependence of nongeminate recombination to re-create correctly not only the measured device open-circuit voltage (VOC) as a function of light intensity but also its dependence on bromine substitution. The 173 (±7) mV increase in device voltage observed with 20% bromine substitution is shown to result from a 227 (±8) mV increase in effective electronic band gap, which was offset in part by a 56 (±5) mV voltage loss due to faster carrier recombination. The faster recombination following 20% bromine substitution can be avoided by indene–C60 bisadduct (ICBA) substitution into the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) electron-collection layer, resulting in a further 73 (±7) mV increase in device VOC. These results are consistent with surface recombination losses at the perovskite/fullerene interface being the primary limitation on the VOC output of bromine-substituted devices. This study thus presents, and experimentally validates, a simple model for the device physics underlying voltage generation in such perovskite-based solar cells and demonstrates that this approach can provide key insights into factors limiting this voltage output as a function of material energetics.
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Journal articleWadsworth A, Ashraf RS, Abdelsamie M, et al., 2017,
Highly efficient and reproducible nonfullerene solar cells from hydrocarbon solvents
, ACS Energy Letters, Vol: 2, Pages: 1494-1500, ISSN: 2380-8195With chlorinated solvents unlikely to be permitted for use in solution-processed organic solar cells in industry, there must be a focus on developing nonchlorinated solvent systems. Here we report high-efficiency devices utilizing a low-bandgap donor polymer (PffBT4T-2DT) and a nonfullerene acceptor (EH-IDTBR) from hydrocarbon solvents and without using additives. When mesitylene was used as the solvent, rather than chlorobenzene, an improved power conversion efficiency (11.1%) was achieved without the need for pre- or post-treatments. Despite altering the processing conditions to environmentally friendly solvents and room-temperature coating, grazing incident X-ray measurements confirmed that active layers processed from hydrocarbon solvents retained the robust nanomorphology obtained with hot-processed chlorinated solvents. The main advantages of hydrocarbon solvent-processed devices, besides the improved efficiencies, were the reproducibility and storage lifetime of devices. Mesitylene devices showed better reproducibility and shelf life up to 4000 h with PCE dropping by only 8% of its initial value.
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Journal articleGodin R, Kafizas A, Durrant JR, 2017,
Electron transfer dynamics in fuel producing photosystems
, Current Opinion in Electrochemistry, Vol: 2, Pages: 136-143, ISSN: 2451-9103An often overlooked aspect of solar fuel production is the inherent mismatch between bulk charge carrier lifetimes and rates of charge transfer reactions. Considering water oxidation, interfacial charge transfer occurs on the millisecond to second timescales while bulk charge carrier lifetimes of metal oxides are typically in the fast picosecond–nanosecond regime. For charge transfer to efficiently compete with charge recombination, strategies that substantially increase the charge carrier lifetime need to be applied. In this chapter, we discuss the magnitude of the kinetic mismatch, overview common effective charge separation strategies that address this mismatch and highlight recent developments in our understanding of these processes. We also touch upon recent advances in determining the chemical nature of key reaction intermediates.
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Journal articleMartindale BCM, Hutton GAM, Caputo CA, et al., 2017,
Enhancing light absorption and charge transfer efficiency in carbon dots through graphitization and core nitrogen doping
, Angewandte Chemie - International Edition, Vol: 56, Pages: 6459-6463, ISSN: 1433-7851Single-source precursor syntheses have been devised for the preparation of structurally similar graphitic carbon dots (CDs), with (g-N-CD) and without (g-CD) core nitrogen doping for artificial photosynthesis. An order of magnitude improvement has been realized in the rate of solar (AM1.5G) H2 evolution using g-N-CD (7950 μmolH2 (gCD)−1 h−1) compared to undoped CDs. All graphitized CDs show significantly enhanced light absorption compared to amorphous CDs (a-CD) yet undoped g-CD display limited photosensitizer ability due to low extraction of photogenerated charges. Transient absorption spectroscopy showed that nitrogen doping in g-N-CD increases the efficiency of hole scavenging by the electron donor and thereby significantly extends the lifetime of the photogenerated electrons. Thus, nitrogen doping allows the high absorption coefficient of graphitic CDs to be translated into high charge extraction for efficient photocatalysis.
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Journal articleUtzat H, Dimitroy SD, Wheeler S, et al., 2017,
Charge-Separation in Intermixed Polymer:PC70BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition
, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 9790-9801, ISSN: 1932-7447A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics within molecularly intermixed donor–acceptor domains versus the dynamics between phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC70BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3–5 nm. At high fullerene compositions (≥67%), where the intermixed domains are 1–3 nm and the pure fullerene phases reach ∼4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase, making the fullerene domains accessible for electron escape.
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Journal articleHermerschmidt F, Savva A, Georgiou E, et al., 2017,
Influence of the Hole Transporting Layer on the Thermal Stability of Inverted Organic Photovoltaics Using Accelerated-Heat Lifetime Protocols
, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 14136-14144, ISSN: 1944-8244 -
Journal articleSotelo-Vazquez C, Quesada-Cabrera R, Ling M, et al., 2017,
Evidence and Effect of Photogenerated Charge Transfer for Enhanced Photocatalysis in WO3/TiO2 Heterojunction Films: A Computational and Experimental Study
, ADVANCED FUNCTIONAL MATERIALS, Vol: 27, ISSN: 1616-301XSemiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar-to-chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO3/TiO2 heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO3 layer of vertically aligned nanorods that is then coated with a conformal layer of TiO2. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO3 layer into TiO2. State-of-the-art hybrid density functional theory and hard X-ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO3/TiO2 interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.
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Journal articleGodin RP, Wang Y, Zwijnenburg MA, et al., 2017,
Time-resolved spectroscopic investigation of charge trapping in carbon nitrides photocatalysts for hydrogen generation
, Journal of the American Chemical Society, Vol: 139, Pages: 5216-5224, ISSN: 1520-5126Carbon nitride (g-C3N4) as a benchmark polymer photocatalyst is attracting significant research interest because of its visible light photocatalytic performance combined with good stability and facile synthesis. However, little is known about the fundamental photophysical processes of g-C3N4, which are key to explain and promote photoactivity. Using time-resolved absorption and photoluminescence spectroscopies, we have investigated the photophysics of a series of carbon nitrides on time scales ranging from femtoseconds to seconds. Free charge carriers form within a 200 fs excitation pulse, trap on the picosecond time scale with trap states in a range of energies, and then recombine with power law decays that are indicative of charge trapping–detrapping processes. Delayed photoluminescence is assigned to thermal excitation of trapped carriers back up to the conduction/valence bands. We develop a simple, quantitative model for the charge carrier dynamics in these photocatalysts, which includes carrier relaxation into an exponential tail of trap states extending up to 1.5 eV into the bandgap. This trapping reduces the efficiency of surface photocatalytic reactions. Deep trapped electrons observed on micro- to millisecond time scales are unable to reduce electron acceptors on the surface or in solution. Within a series of g-C3N4, the yield of these unreactive trapped electrons correlates inversely with H2 evolution rates. We conclude by arguing that the photophysics of these carbon nitride materials show closer parallels with inorganic semiconductors than conjugated polymers, and that the key challenge to optimize photocatalytic activity of these materials is to prevent electron trapping into deep, and photocatalytically inactive, electron trap states.
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Journal articlePont S, Bryant D, Lin CH, et al., 2017,
Tuning CH 3 NH 3 Pb(I 1-x Br x ) 3 Perovskite Oxygen Stability in Thin Films and Solar Cells
, Journal of Materials Chemistry A, Vol: 5, ISSN: 2050-7488The rapid development of organic–inorganic lead halide perovskites has resulted in high efficiency photovoltaic devices. However the susceptibility of these devices to degradation under environmental stress has so far hindered commercial development, requiring for example expensive device encapsulation. Herein, we have investigated the stability of CH3NH3Pb(I1−xBrx)3 [x = 0–1] thin films and solar cells under controlled humidity, light, and oxygen conditions. We show that higher bromide ratios increase tolerance to moisture, with x = 1 thin films being stable to 120 h of moisture stress. Under light and dry air, partial bromide (x < 1) substitution does not enhance film stability significantly, with the corresponding solar cells degrading within two hours. In contrast, CH3NH3PbBr3 films show excellent stability, with device stability being limited by the organic interlayer. For these x = 1 films, we show that charge carriers are quenched in the presence of oxygen and form superoxide; however in contrast to perovskites containing iodide, this superoxide does not degrade the crystal. Our observations show that iodide limits the oxygen and light stability of CH3NH3Pb(I1−xBrx)3 perovskites, but that CH3NH3PbBr3 provides an opportunity to develop inherently stable high voltage photovoltaic devices and 4-terminal tandem solar cells.
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Journal articleDu T, Burgess C, Kim J, et al., 2017,
Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells
, Sustainable Energy and Fuels, Vol: 1, Pages: 119-126, ISSN: 2398-4902Here we report the investigation of controlled PbI2 secondary phase formation in CH3NH3PbI3 (MAPI) photovoltaics through post-deposition thermal annealing, highlighting the beneficial role of PbI2 on device performance. Using high-resolution transmission electron microscopy we show the location of PbI2 within the active layer and propose a nucleation and growth mechanism. We discover that during the annealing that PbI2 forms mainly in the grain boundary regions of the MAPI films and that at certain temperatures the PbI2 formed can be highly beneficial to device performance – reducing current–voltage hysteresis and increasing the power conversion efficiency. Our analysis shows that the MAPI grain boundaries as susceptible areas that, under thermal loading, initiate the conversion of MAPI into PbI2.
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Journal articleDurrant J, Simpson A, Abe R, et al., 2017,
Welcome to the first issue of Sustainable Energy & Fuels
, SUSTAINABLE ENERGY & FUELS, Vol: 1, Pages: 10-13, ISSN: 2398-4902 -
Journal articlePastor E, Le Formal F, Mayer MT, et al., 2017,
Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface
, Nature Communications, Vol: 8, ISSN: 2041-1723Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a pectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to directly compare the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving thehomolytic reductive elimination of hydrogen.
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Journal articleWarnan J, Willkomm J, Ng J, et al., 2017,
Solar H₂ evolution in water with modified diketopyrrolopyrrole dyes immobilised on molecular Co and Ni catalyst-TiO₂ hybrids
, Chemical Science, Vol: 8, Pages: 3070-3079, ISSN: 2041-6539A series of diketopyrrolopyrrole (DPP) dyes with a terminal phosphonic acid group for attachment to metal oxide surfaces were synthesised and the effect of side chain modification on their properties investigated. The organic photosensitisers feature strong visible light absorption (λ = 400 to 575 nm) and electrochemical and fluorescence studies revealed that the excited state of all dyes provides sufficient driving force for electron injection into the TiO2 conduction band. The performance of the DPP chromophores attached to TiO2 nanoparticles for photocatalytic H2 evolution with co-immobilised molecular Co and Ni catalysts was subsequently studied, resulting in solar fuel generation with a dye-sensitised semiconductor nanoparticle system suspended in water without precious metal components. The performance of the DPP dyes in photocatalysis did not only depend on electronic parameters, but also on properties of the side chain such as polarity, steric hinderance and hydrophobicity as well as the specific experimental conditions and the nature of the sacrificial electron donor. In an aqueous pH 4.5 ascorbic acid solution with a phosphonated DuBois-type Ni catalyst, a DPP-based turnover number (TONDPP) of up to 205 was obtained during UV-free simulated solar light irradiation (100 mW cm−2, AM 1.5G, λ > 420 nm) after 1 day. DPP-sensitised TiO2 nanoparticles were also successfully used in combination with a hydrogenase or platinum instead of the synthetic H2 evolution catalysts and the platinum-based system achieved a TONDPP of up to 2660, which significantly outperforms an analogous system using a phosphonated Ru tris(bipyridine) dye (TONRu = 431). Finally, transient absorption spectroscopy was performed to study interfacial recombination and dye regeneration kinetics revealing that the different performances of the DPP dyes are most likely dictated by the different regeneration efficiencies of the oxidised chromophores.
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Journal articleChadwick NP, Kafizas A, Quesada-Cabrera R, et al., 2017,
Ultraviolet Radiation Induced Dopant Loss in a TiO2 Photocatalyst
, ACS CATALYSIS, Vol: 7, Pages: 1485-1490, ISSN: 2155-5435 -
Book chapterKafizas A, Godin R, Durrant JR, 2017,
Charge Carrier Dynamics in Metal Oxide Photoelectrodes for Water Oxidation
, SEMICONDUCTORS FOR PHOTOCATALYSIS, Editors: Mi, Wang, Jagadish, Publisher: ELSEVIER ACADEMIC PRESS INC, Pages: 3-46- Author Web Link
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- Citations: 28
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Journal articleCollado Fregoso E, Deledalle F, Utzat H, et al., 2016,
Photophysical study of DPPTT-T/PC70BM blends and solar devices as a function of fullerene loading: an insight into EQE limitations of DPP-based polymers
, Advanced Functional Materials, Vol: 27, ISSN: 1616-3028Diketopyrrolopyrrole (DPP)-based polymers have been consistently used for the fabrication of solar cell devices and transistors, due to the existence of intermolecular short contacts,resulting in high electron and hole mobilities. However, they also often show limited external quantum efficiencies (EQEs). In this contribution we analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation and recombination kinetics in thin films and solar devices of a DPP-based donor polymer, DPPTT-T(thieno[3,2-b]thiophene-diketopyrrolopyrrolecopolymer)blended with varying weight fractions of the fullerene acceptor PC70BM. From the correlations between photoluminescence quenching (PLQ), transient absorption studies and EQEmeasurements, we concludethat the main limitation of photon-to-charge conversion in DPPTT-T/PC70BM devices is poor exciton dissociation. This exciton quenching limit is related to the low affinity/miscibility of the materials, as confirmed by WAXDdiffraction and transmission electron microscopy data, but also to the relatively short DPPTT-T singlet exciton lifetime,possibly associated with highnon-radiative losses. A further strategy to improve EQE in this class of polymers without sacrificing the good extractionproperties in optimized blends is therefore to limit those non-radiative decay processes.
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Journal articleBaran D, Kirchartz T, Wheeler S, et al., 2016,
Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages
, Energy & Environmental Science, Vol: 9, Pages: 3783-3793, ISSN: 1754-5706Optimization of the energy levels at the donor–acceptor interface of organic solar cells has driven their efficiencies to above 10%. However, further improvements towards efficiencies comparable with inorganic solar cells remain challenging because of high recombination losses, which empirically limit the open-circuit voltage (Voc) to typically less than 1 V. Here we show that this empirical limit can be overcome using non-fullerene acceptors blended with the low band gap polymer PffBT4T-2DT leading to efficiencies approaching 10% (9.95%). We achieve Voc up to 1.12 V, which corresponds to a loss of only Eg/q − Voc = 0.5 ± 0.01 V between the optical bandgap Eg of the polymer and Voc. This high Voc is shown to be associated with the achievement of remarkably low non-geminate and non-radiative recombination losses in these devices. Suppression of non-radiative recombination implies high external electroluminescence quantum efficiencies which are orders of magnitude higher than those of equivalent devices employing fullerene acceptors. Using the balance between reduced recombination losses and good photocurrent generation efficiencies achieved experimentally as a baseline for simulations of the efficiency potential of organic solar cells, we estimate that efficiencies of up to 20% are achievable if band gaps and fill factors are further optimized.
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Journal articleBaran D, Ashraf RS, Hanifi DA, et al., 2016,
Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells
, Nature Materials, Vol: 16, Pages: 363-369, ISSN: 1476-4660Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.
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Journal articleFallon KJ, Wijeyasinghe N, Manley EF, et al., 2016,
Indolo-naphthyridine-6,13-dione Thiophene Building Block for Conjugated Polymer Electronics: Molecular Origin of Ultrahigh n-Type Mobility
, CHEMISTRY OF MATERIALS, Vol: 28, Pages: 8366-8378, ISSN: 0897-4756 -
Journal articleSachs M, Pastor E, Kafizas A, et al., 2016,
Evaluation of Surface State Mediated Charge Recombination in Anatase and Rutile TiO2
, Journal of Physical Chemistry Letters, Vol: 7, Pages: 3742-3746, ISSN: 1948-7185In nanostructured thin films, photogeneratedcharge carriers can access the surface more easily than indense films and thus react more readily. However, the highsurface area of these films has also been associated withenhanced recombination losses via surface states. We hereinuse transient absorption spectroscopy to compare the ultrafastcharge carrier kinetics in dense and nanostructured TiO2films for its two most widely used polymorphs: anatase andrutile. We find that nanostructuring does not enhance recombinationrates on ultrafast timescales, indicating thatsurface state mediated recombination is not a key loss pathwayfor either TiO2 polymorph. Rutile shows faster, and lessintensity-dependent recombination than anatase, which weassign to its higher doping density. For both polymorphs, weconclude that bulk rather than surface recombination is theprimary determinant of charge carrier lifetime.
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Journal articleMa Y, Mesa CA, Pastor E, et al., 2016,
Rate law analysis of water oxidation and hole scavenging on a BiVO4 photoanode
, ACS Energy Letters, Vol: 1, Pages: 618-623, ISSN: 2380-8195Spectroelectrochemical studies employing pulsed LED irradiation are used to investigate the kinetics of water oxidation on undoped dense bismuth vanadate (BiVO4) photoanodes under conditions of photoelectrochemical water oxidation and compare to those obtained for oxidation of a simple redox couple. These measurements are employed to determine the quasi-steady-state densities of surface-accumulated holes, ps, and correlate these with photocurrent density as a function of light intensity, allowing a rate law analysis of the water oxidation mechanism. The reaction order in surface hole density is found to be first order for ps < 1 nm–2 and third order for ps > 1 nm–2. The effective turnover frequency of each surface hole is estimated to be 14 s–1 at AM 1.5 conditions. Using a single-electron redox couple, potassium ferrocyanide, as the hole scavenger, only the first-order reaction is observed, with a higher rate constant than that for water oxidation. These results are discussed in terms of catalysis by BiVO4 and implications for material design strategies for efficient water oxidation.
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Journal articleSprick RS, Bonillo B, Sachs M, et al., 2016,
Extended conjugated microporous polymers for photocatalytic hydrogen evolution from water
, Chemical Communications, Vol: 52, Pages: 10008-10011, ISSN: 1364-548XConjugated microporous polymers (CMPs) have been used as photocatalysts for hydrogen production from water in the presence of a sacrificial electron donor. The relative importance of the linker geometry, the co-monomer linker length, and the degree of planarisation were studied with respect to the photocatalytic hydrogen evolution rate.
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Journal articleLee HKH, Li Z, Durrant JR, et al., 2016,
Is organic photovoltaics promising for indoor applications?
, Applied Physics Letters, Vol: 108, ISSN: 1077-3118This work utilizes organic photovoltaics (OPV) for indoor applications, such as powering smallelectronic devices or wireless connected Internet of Things. Three representative polymerbasedOPV systems, namely, poly(3-hexylthiophene-2,5-diyl), poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)], and poly[[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]],were selected as the donor materials in blend with fullerene derivatives for comparison under lowlight level condition using fluorescent lamps. PCDTBT based devices are found to be the best performingsystem, generating 13.9 lW/cm2 corresponding to 16.6% power conversion efficiency at300 lx, although PTB7 based devices show the highest efficiency under one sun conditions. Thishigh performance suggests that OPV is competitive to the other PV technologies under low lightcondition despite much lower performance under one sun condition. Different properties of thesedevices are studied to explain the competitive performance at low light level. A low energyconsuming method for maximum power point tracking is introduced for the operation of the OPVdevices. Finally, a 14 cm 14 cm OPV module with 100 cm2 active area is demonstrated for realapplications. These findings suggest that OPV, in particular, PCDTBT based devices, could be apromising candidate for indoor applications.
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Journal articleHolliday S, Ashraf RS, Wadsworth A, et al., 2016,
High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor
, Nature Communications, Vol: 7, Pages: 1-11, ISSN: 2041-1723Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.
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Journal articleMorris MR, Pendlebury S, Hong J, et al., 2016,
Effect of internal electric fields on charge carrier dynamics in a ferroelectric material for solar energy conversion
, Advanced Materials, Vol: 28, Pages: 7123-7128, ISSN: 0935-9648Spontaneous polarization is shown to enhance the lifetimes of photogenerated species in BaTiO3. This is attributed to polarization-induced surface band bending acting as a thermal barrier to electron/hole recombination. The study indicates that the efficiencies of solar cells and solar fuels devices can be enhanced by the use of ferroelectric materials.
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Journal articleMa Y, Kafizas A, Pendlebury SR, et al., 2016,
Photoinduced Absorption Spectroscopy of CoPi on BiVO4: The Function of CoPi during Water Oxidation
, Advanced Functional Materials, Vol: 26, Pages: 4951-4960, ISSN: 1616-301XThis paper employs photoinduced absorption and electrochemical techniques to analyze the charge carrier dynamics that drive photoelectrochemical water oxidation on bismuth vanadate (BiVO4), both with and without cobalt phosphate (CoPi) co-catalyst. These results are correlated with spectroelectrochemical measurements of CoII oxidation to CoIII in a CoPi/FTO (fluorine doped tin oxide) electrode during dark electrocatalytic water oxidation. Electrocatalytic water oxidation exhibits a non-linear dependence on CoIII density, with a sharp onset at 1 × 1017 CoIII cm−2. These results are compared quantitatively with the degree of CoPi oxidation observed under conditions of photoinduced water oxidation on CoPi–BiVO4 photoanodes. For the CoPi–BiVO4 photoanodes studied herein, ≤5% of water oxidation proceeds from CoPi sites, making the BiVO4 surface the predominant water oxidation site. This study highlights two key factors that limit the ability of CoPi to improve the catalytic performance of BiVO4: 1) the kinetics of hole transfer from the BiVO4 to the CoPi layer are too slow to effectively compete with direct water oxidation from BiVO4; 2) the slow water oxidation kinetics of CoPi result in a large accumulation of CoIII states, causing an increase in recombination. Addressing these factors will be essential for improving the performance of CoPi on photoanodes for solar-driven water oxidation.
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Journal articleBryant D, Aristidou N, Pont S, et al., 2016,
Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells
, Energy and Environmental Science, Vol: 9, Pages: 1655-1660, ISSN: 1754-5692Here, we demonstrate that light and oxygen-induced degradation is the main reason for the low operational stability of methylammonium lead triiodide (MeNH3PbI3) perovskite solar cells exposed to ambient conditions. When exposed to both light and dry air, unencapsulated MeNH3PbI3 solar cells rapidly degrade on timescales of minutes to a few hours. This rapid degradation is also observed under electrically bias driven current flow in the dark in the presence of O2. In contrast, significantly slower degradation is observed when the MeNH3PbI3 devices are exposed to moisture alone (e.g. 85% relative humidity in N2). We show that this light and oxygen induced degradation can be slowed down by the use of interlayers that are able to remove electrons from the perovskite film before they can react with oxygen to form O2-. These observations demonstrate that the operational stability of electronic and optoelectronic devices that exploit the electron transporting properties of MeNH3PbI3 will be critically dependent upon the use of suitable barrier layers and device configurations to mitigate the oxygen sensitivity of this remarkable material.
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Journal articleBrinkert K, Le formal F, Li X, et al., 2016,
Photocurrents from photosystem II in a metal oxide hybrid system: electron transfer pathways
, Biochimica et Biophysica Acta-Bioenergetics, Vol: 1857, Pages: 1497-1505, ISSN: 0005-2728We have investigated the nature of the photocurrent generated by Photosystem II (PSII), the water oxidising enzyme, isolated from Thermosynechococcus elongatus, when immobilized on nanostructured titanium dioxide on an indium tin oxide electrode (TiO2/ITO). We investigated the properties of the photocurrent from PSII when immobilized as a monolayer versus multilayers, in the presence and absence of an inhibitor that binds to the site of the exchangeable quinone (QB) and in the presence and absence exogenous mobile electron carriers (mediators). The findings indicate that electron transfer occurs from the first quinone (QA) directly to the electrode surface but that the electron transfer through the nanostructured metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the nanostructured semiconductor surface to the ITO electrode surface not from PSII. This is demonstrated by photocurrent enhancement using a mediator incapable of accepting electrons from PSII. This model for electron transfer also explains anomalies reported in the literature using similar and related systems. The slow rate of the electron transfer step in the TiO2 is due to the energy level of electron injection into the semiconducting material being below the conduction band. This limits the usefulness of the present hybrid electrode. Strategies to overcome this kinetic limitation are discussed.
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