Publications
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Hong DP, Hayasake K, Kim J, et al., 2018, One step facile synthesis of a novel anthanthrone dye-based, dopant-free hole transporting material for efficient and stable perovskite solar cells, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 6, Pages: 3699-3708, ISSN: 2050-7526
Francàs L, Mesa Zamora C, Pastor E, et al., 2018, Rate law analysis of water splitting photoelectrodes, Advances in Photoelectrochemical Water Splitting Theory, Experiment and Systems Analysis, Editors: Tilley, Lany, van de Krol, Publisher: Royal Society of Chemistry, ISBN: 9781782629252
The book starts by addressing the challenges of water splitting followed by chapters on the theoretical design of PEC materials and their computational screening.
Zhang J, Tan CH, Du T, et al., 2018, ZnO-PCBM bilayers as electron transport layers in low-temperature processed perovskite solar cells, Science Bulletin, Vol: 63, Pages: 343-348, ISSN: 2095-9273
We investigate an electron transport bilayer fabricated at < 110 °C to form all low-temperature processed, thermally stable, efficient perovskite solar cells with negligible hysteresis. The components of the bilayer create a symbiosis that results in improved devices compared with either of the components being used in isolation. A sol-gel derived ZnO layer facilitates improved energy level alignment and enhanced charge carrier extraction and a [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) layer to reduce hysteresis and enhance perovskite thermal stability. The creation of a bilayer structure allows materials that are inherently unsuitable to be in contact with the perovskite active layer to be used in efficient devices through simple surface modification strategies.
Lee HKH, Telford AM, Rohr JA, et al., 2018, The role of fullerenes in the environmental stability of polymer: fullerene solar cells, Energy and Environmental Science, Vol: 11, Pages: 417-428, ISSN: 1754-5692
Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current–voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices.
Cha H, Wheeler S, Holliday S, et al., 2018, Influence of blend morphology and energetics on charge separation and recombination dynamics in organic solar cells incorporating a nonfullerene acceptor, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X
Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T-2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC71BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T-2OD:FBR blends. This is assigned to the smaller LUMO-LUMO offset of the PffBT4T-2OD:FBR blends relative to PffBT4T-2OD:PC71BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T-2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells.
Godin R, Ma X, González-Carrero S, et al., 2018, Tuning charge carrier dynamics and surface passivation in organolead halide perovskites with capping ligands and metal oxide interfaces, Advanced Optical Materials, Vol: 6, ISSN: 2195-1071
Organolead halide perovskites have emerged as exciting optoelectronic materials but a complete understanding of their photophysical properties is still lacking. Here, a morphological series of methylammonium lead bromide (MAPbBr 3 ) perovskites are studied by transient optical spectroscopies over eight orders of magnitude in timescale to investigate the effect of nanostructuring and surface states on the charge carrier dynamics. The sample preparation route and corresponding morphology changes influence the position of the optical features, recombination dynamics, excitation fluence dependence, and dramatically impact surface trap passivation. Growth of the perovskite layer in the presence of capping ligands or within mesoporous alumina increases the photoluminescence efficiency by multiple orders of magnitude, indicating that interfacing with metal oxides can lead to the passivation of surface nonradiative recombination centers. Nanoparticles (NPs) dispersed in solution show mixed behavior since they consist of NPs on nanoplatelets, while isolated NPs could be grown within mesoporous alumina with the addition of capping ligands. Investigation on the microsecond timescale suggests that an exponential distribution of states below the band edges results in long-lived charges. The investigations of the relationship between sample architecture and charge carrier dynamics will help in the appropriate choice of perovskite morphology for use in optoelectronic devices.
Tan CH, Gorman J, Wadsworth A, et al., 2018, Barbiturate end-capped non-Fullerene acceptors for organic solar cells: tuning acceptor energetics to suppress geminate recombination losses, Chemical Communications, Vol: 54, Pages: 2966-2969, ISSN: 1359-7345
We report the synthesis of two barbiturate end-capped non-fullerene acceptors and demonstrate their efficient function in high voltage output organic solar cells. The acceptor with the lower LUMO level is shown to exhibit suppressed geminate recombination losses, resulting in enhanced photocurrent generation and higher overall device efficiency.
Francàs L, Mesa CA, Pastor E, et al., 2018, Chapter 5: Rate Law Analysis of Water Splitting Photoelectrodes, RSC Energy and Environment Series, Pages: 128-162, ISBN: 9781782629252
In this chapter, we discuss how rate law analyses can shed light into the kinetics and reaction mechanisms of those processes involved in the production of solar fuels. We show that the key data necessary to elucidate rate laws can be easily obtained by combining photo-induced absorbance (PIA) and transient photocurrent (TPC) measurements. The chapter is structured as follows: in the first part, we give a theoretical background (Section 5.1.1) on the use of rate laws and introduce our methodology and experimental approach (Section 5.1.2). In the second part, we show the potential of this technique through several practical examples on state-of-the art systems which cover: oxygen evolution, on α-Fe2O3 (Section 5.2.1.1) and BiVO4 (Sections 5.2.1.2 and 5.2.1.3) as well as proton reduction on a multi-layer photocathode, Cu2O/AZO/TiO2/RuOx (Section 5.2.2). In addition, the role of the catalyst is also discussed in detail in the last two sections. The kinetic analysis of these systems demonstrates that our methodology is capable of yielding reaction orders and rate constants, both key experimental parameters needed to advance the rational design of photoelectrodes for solar fuels production.
Durrant J, Simpson A, 2018, Welcome to the second volume of Sustainable Energy & Fuels, SUSTAINABLE ENERGY & FUELS, Vol: 2, Pages: 12-12, ISSN: 2398-4902
Hong J, Ha YH, Cha H, et al., 2017, All-Small-Molecule Solar Cells Incorporating NDI-Based Acceptors: Synthesis and Full Characterization, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 44667-44677, ISSN: 1944-8244
McLachlan MA, Morbidoni M, Burgess CH, et al., 2017, Nanoscale structure-property relationships in low temperature solution-processed electron transport layers for organic photovoltaics, Crystal Growth and Design, Vol: 17, Pages: 6559-6564, ISSN: 1528-7483
Here we elucidate the nanostructure–property relationships in low-temperature, solution-processed ZnO based thin films employed as novel electron transport layers (ETLs) in organic photovoltaic (OPV) devices. Using a low-cost zinc precursor (zinc acetate) in a simple amine–alcohol solvent mix, high-quality ETL thin films are prepared. We show that at a processing temperature of 110 °C the films are composed of nanoparticles embedded in a continuous organic matrix consisting of ZnO precursor species and stabilizers. Using a combination of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), we study the thermally induced morphological and compositional changes in the ETLs. Transient optoelectronic probes reveal that the mixed nanocrystalline/amorphous nature of the films does not contribute to recombination losses in devices. We propose that charge transport in our low-temperature processed ETLs is facilitated by the network of ZnO nanoparticles, with the organic matrix serving to tune the work function of the ETL and to provide excellent resistance to current leakage. To demonstrate the performance of our ETLs we prepare inverted architecture OPVs utilizing Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) as active layer materials. The low-temperature ETL devices showed typical power conversion efficiencies (PCEs) of >7% with the champion devices achieving a PCE > 8%.
McGettrick JD, Speller E, Li Z, et al., 2017, Use of gas cluster ion source depth profiling to study the oxidation of fullerene thin films by XPS, ORGANIC ELECTRONICS, Vol: 49, Pages: 85-93, ISSN: 1566-1199
Collado-Fregoso E, Hood SN, Shoaee S, et al., 2017, Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited., Journal of Physical Chemistry Letters, Vol: 8, Pages: 4061-4068, ISSN: 1948-7185
In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.
Wade J, Wood S, Collado-Fregoso E, et al., 2017, Impact of Fullerene Intercalation on Structural and Thermal Properties of Organic Photovoltaic Blends, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 20976-20985, ISSN: 1932-7447
The performance of organic photovoltaic blend devices is critically dependent on the polymer:fullerene interface. These interfaces are expected to impact the structural and thermal properties of the polymer with regards to the conjugated backbone planarity and transition temperatures during annealing/cooling processes. Here, we report the impact of fullerene intercalation on structural and thermal properties of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT), a highly stable material known to exhibit liquid crystalline behavior. We undertake a detailed systematic study of the extent of intercalation in the PBTTT:fullerene blend, considering the use of four different fullerene derivatives and also varying the loading ratios. Resonant Raman spectroscopy allows direct observation of the interface morphology in situ during controlled heating and cooling. We find that small fullerene molecules readily intercalate into PBTTT crystallites, resulting in a planarization of the polymer backbone, but high fullerene loading ratios or larger fullerenes result in nonintercalated domains. During cooling from melt, nonintercalated blend films are found to return to their original morphology and reproduce all thermal transitions on cooling with minimal hysteresis. Intercalated blend films show significant hysteresis on cooling due to the crystallized fullerene attempting to reintercalate. The strongest hysteresis is for intercalated blend films with excess fullerene loading ratio, which form a distinct nanoribbon morphology and exhibit a reduced geminate recombination rate. These results reveal that careful consideration should be taken during device fabrication, as postdeposition thermal treatments significantly impact the charge generation and recombination dynamics.
Cha H, Wu J, Wadsworth A, et al., 2017, An efficient, "burn in" free organic solar cell employing a nonfullerene electron acceptor, Advanced Materials, Vol: 29, ISSN: 0935-9648
A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'″-di(2-octyldodecyl)-2,2';5',2″;5″,2'″-quaterthiophen-5,5'″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC71 BM solar cells show significant efficiency loss under simulated solar irradiation ("burn in" degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC71 BM devices.
Mesa Zamora CA, Kafizas A, Francàs L, et al., 2017, Kinetics of photoelectrochemical oxidation of methanol on hematite photoanodes, Journal of the American Chemical Society, Vol: 139, Pages: 11537-11543, ISSN: 1520-5126
The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe2O3 photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe2O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe2O3. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2O3 is retarded ∼20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde and concomitant hydrogen evolution.
Keiderling C, Dimitrov S, Durrant JR, 2017, Exciton and Charge Generation in PC60BM Thin Films, Journal of Physical Chemistry C, Vol: 121, Pages: 14470-14475, ISSN: 1932-7447
Transient absorption spectroscopy is employed to contrast the photophysics of [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) dispersed in a polystyrene matrix and as a neat film. For the dispersed PC60BM:polystyrene film, singlet excitons are observed that undergo intersystem crossing to triplet excitons. In contrast, in the neat PC60BM film, the transient absorption data indicate significant polaron generation, with photogenerated polarons exhibiting dispersive, bimolecular charge recombination on the nano- to microsecond time scales. These results are discussed in terms of their implications for charge generation from PC60BM light absorption in polymer/fullerene solar cells.
Speller EM, McGettrick JD, Rice B, et al., 2017, Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 22739-22747, ISSN: 1944-8244
Armin A, Durrant JR, Shoaee S, 2017, Interplay Between Triplet-, Singlet-Charge Transfer States and Free Charge Carriers Defining Bimolecular Recombination Rate Constant of Organic Solar Cells, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 13969-13976, ISSN: 1932-7447
Kafizas A, Ma Y, Pastor E, et al., 2017, Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis, ACS CATALYSIS, Vol: 7, Pages: 4896-4903, ISSN: 2155-5435
It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2 oxidize water. In this paper, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent and transient absorption spectroscopy, we measure both the kinetics of electron extraction (t50% ≈ 200 μs, 1.5VRHE) and the kinetics of hole oxidation of water (t50% ≈ 100 ms, 1.5VRHE) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water-splitting system. Photoinduced absorption spectroscopy measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photoelectrochemical water oxidation. Under these conditions, we find that the surface density of these holes increases nonlinearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is third order with respect to surface hole density, with a rate constant kWO = 22 ± 2 nm4·s–1. Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions, the rate law was second order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20–25 s–1 at ∼1 sun irradiance). This showed that the rate-limiting step does not involve OH– nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH.
Francas 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: 2155-5435
We 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.
Moss 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: 2045-2322
In 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.
Wheeler 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: 1932-7455
Transient 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.
Wadsworth 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: 2380-8195
With 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.
Martindale 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, Vol: 129, Pages: 6559-6563, ISSN: 0044-8249
<jats:title>Abstract</jats:title><jats:p>Single‐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) H<jats:sub>2</jats:sub> evolution using g‐N‐CD (7950 μmol<jats:sub>H2</jats:sub> (g<jats:sub>CD</jats:sub>)<jats:sup>−1</jats:sup> h<jats:sup>−1</jats:sup>) 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.</jats:p>
Godin R, Kafizas A, Durrant JR, 2017, Electron transfer dynamics in fuel producing photosystems, Current Opinion in Electrochemistry, Vol: 2, Pages: 136-143, ISSN: 2451-9103
An 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.
Martindale 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: 1433-7851
Single-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.
Utzat 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: 1932-7447
A 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.
Hermerschmidt 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
Sotelo-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: 1616-301X
Semiconductor 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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