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Journal articleDu T, Kim J, Ngiam J, et al., 2018,
Elucidating the origins of subgap tail states and open-circuit voltage in methylammonium lead triiodide perovskite solar cells
, Advanced Functional Materials, Vol: 28, Pages: 1-11, ISSN: 1616-301XRecombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, the impact of active layer crystallinity on the accumulated charge and open‐circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI) is demonstrated. It is shown that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high‐performance devices.
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Journal articleDu T, Kim J, Ngiam J, et al., 2018,
Elucidating the origins of sub-gap tail states and open-circuit voltage in methylammonium lead triiodide perovskite solar cells
, Advanced Functional Materials, Vol: 28, ISSN: 1616-301XRecombination via sub-gap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, we demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI). We show MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV. Using in-situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap states correlates with faster carrier trapping and more non-radiative recombination pathways. We provide fundamental insights into the origin of Voc in perovskite photovoltaics and demonstrate why highly crystalline perovskite films are paramount for high-performance devices.
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Journal articleKasap H, Godin R, Jeay-Bizot C, et al., 2018,
Interfacial Engineering of a Carbon Nitride-Graphene Oxide-Molecular Ni Catalyst Hybrid for Enhanced Photocatalytic Activity
, ACS CATALYSIS, Vol: 8, Pages: 6914-6926, ISSN: 2155-5435 -
Journal articleChristoforidis KC, Syrgiannis Z, La Parola V, et al., 2018,
Metal-free dual-phase full organic carbon nanotubes/g-C3N4 heteroarchitectures for photocatalytic hydrogen production
, NANO ENERGY, Vol: 50, Pages: 468-478, ISSN: 2211-2855 -
Journal articleLin C, Pont S, Kim J, et al., 2018,
Passivation of oxygen and light induced degradation by the PCBM electron transport layer in planar perovskite solar cells
, Sustainable Energy and Fuels, Vol: 2, Pages: 1686-1692, ISSN: 2398-4902Herein, we investigate the causes of a 20 fold improved stability of inverted, planar structure (ITO/PTAA/CH3NH3PbI3/PCBM/BCP/Cu) compared to conventional structure devices (FTO/compact-TiO2/meso-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au) under oxygen and light stress. The PCBM layer is shown to function as an oxygen diffusion barrier and passivation layer against superoxide mediated degradation. The passivation properties of the PCBM layer are shown to depend on the electron affinity of fullerene acceptor, attributed to low LUMO level of PCBM energetically inhibiting superoxide generation. We also find the planar structure devices shows slower lateral oxygen diffusion rates than mesoporous scaffold devices, with these slower diffusion rates (days per 100 μm) also being a key factor in enhancing stability. Faster degradation is observed under voltage cycling, attributed to oxygen diffusion kinetics being ion motion dependent. We conclude by discussing the implications of these results for the design of perovskite solar cells with improved resistance to oxygen and light induced degradation.
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Journal articleLee HKH, Durrant JR, Li Z, et al., 2018,
Stability study of thermal cycling on organic solar cells
, JOURNAL OF MATERIALS RESEARCH, Vol: 33, Pages: 1902-1908, ISSN: 0884-2914 -
Journal articleJain SM, Phuyal D, Davies ML, et al., 2018,
An effective approach of vapour assisted morphological tailoring for reducing metal defect sites in lead-free, (CH3NH3)(3)Bi2I9 bismuth-based perovskite solar cells for improved performance and long-term stability
, NANO ENERGY, Vol: 49, Pages: 614-624, ISSN: 2211-2855 -
Journal articleHong DP, Thu TD, Kim J, et al., 2018,
Molecular Engineering Using an Anthanthrone Dye for Low-Cost Hole Transport Materials: A Strategy for Dopant-Free, High-Efficiency, and Stable Perovskite Solar Cells
, ADVANCED ENERGY MATERIALS, Vol: 8, ISSN: 1614-6832 -
Journal articleBaran D, Gasparini N, Wadsworth A, et al., 2018,
Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination
, Nature Communications, Vol: 9, ISSN: 2041-1723Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm−2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells.
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Journal articleFrancas Forcada L, Burns E, Steier L, et al., 2018,
Rational design of a neutral pH functional and stable organic photocathode.
, Chemical Communications, Vol: 2018, ISSN: 1359-7345In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.
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Journal articleLee HKH, Wu J, Barbe J, et al., 2018,
Organic photovoltaic cells - promising indoor light harvesters for self-sustainable electronics
, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 6, Pages: 5618-5626, ISSN: 2050-7488 -
Journal articleHong 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 -
Journal articleZhang 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: 2095-9273We 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.
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Journal articleLee 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: 1754-5692Environmental 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.
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Journal articleCha 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: 1616-301XNonfullerene 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.
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Journal articleGodin 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: 2195-1071Organolead 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.
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Journal articleTan 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: 1359-7345We 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.
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Book chapterFrancàs L, Mesa CA, Pastor E, et al., 2018,
Chapter 5: Rate Law Analysis of Water Splitting Photoelectrodes
, Rsc Energy and Environment Series, Pages: 128-162In 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-Fe<inf>2</inf>O<inf>3</inf> (Section 5.2.1.1) and BiVO<inf>4</inf> (Sections 5.2.1.2 and 5.2.1.3) as well as proton reduction on a multi-layer photocathode, Cu<inf>2</inf>O/AZO/TiO<inf>2</inf>/RuO<inf>x</inf> (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.
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Journal articleDurrant J, Simpson A, 2018,
Welcome to the second volume of Sustainable Energy & Fuels
, SUSTAINABLE ENERGY & FUELS, Vol: 2, Pages: 12-12, ISSN: 2398-4902 -
Journal articleHong 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 -
Journal articleMcLachlan 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: 1528-7483Here 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%.
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Journal articleMcGettrick 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 -
Journal articleCollado-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: 1948-7185In 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.
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Journal articleWade 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: 1932-7447The 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.
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Journal articleCha 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: 0935-9648A 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.
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Journal articleMesa 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: 1520-5126The 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.
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Journal articleKeiderling 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: 1932-7447Transient 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.
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Journal articleSpeller 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 -
Journal articleArmin 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 -
Journal articleKafizas 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: 2155-5435It 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.
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