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    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, Prantl S, Godin RP, Durrant JR, Reisner Eet 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.

    Godin RP, Wang Y, Zwijnenburg MA, Tang J, Durrant Jet 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: 1520-5126

    Carbon 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.

    Pont S, Bryant D, Lin CH, Aristidou N, Wheeler S, Ma X, Godin R, Haque S, Durrant JRet 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: 2050-7488

    The 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.

    Pastor E, Le Formal F, Mayer MT, Tilley SD, Gratzel M, Francas Forcada L, Mesa CA, Durrant JRet al., 2017,

    Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface

    , Nature Communications, Vol: 8, ISSN: 2041-1723

    Multi-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.

    Warnan J, Willkomm J, Ng J, Godin RP, Prantl S, Durrant J, Reisner Eet 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: 2041-6539

    A 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.

    Du T, Burgess C, Kim J, Zhang J, Durrant J, McLachlan MAet al., 2017,

    Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells

    , Sustainable Energy & Fuels, Vol: 1, Pages: 119-126, ISSN: 2398-4902

    Here 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.

    Chadwick NP, Kafizas A, Quesada-Cabrera R, Sotelo-Vazquez C, Bawaked SM, Mokhtar M, Al Thabaiti SA, Obaid AY, Basahel SN, Durrant JR, Carmalt CJ, Parkin IPet al., 2017,

    Ultraviolet Radiation Induced Dopant Loss in a TiO2 Photocatalyst

    , ACS CATALYSIS, Vol: 7, Pages: 1485-1490, ISSN: 2155-5435
    Collado Fregoso E, Deledalle F, Utzat H, Tuladhar PS, Dimitrov S, Gillett A, Tan C, Zhang W, McCulloch I, Durrant Jet 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: 1616-3028

    Diketopyrrolopyrrole (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.

    Baran D, Kirchartz T, Wheeler S, Dimitrov S, Abdelsamie M, Gorman J, Ashraf R, Holliday S, Wadsworth A, Gasparini N, Kaienburg P, Yan H, Amassian A, Brabec C, Durrant J, McCulloch Iet al., 2016,

    Reduced voltage losses yield 10% and >1V fullerene free organic solar cells

    , Energy & Environmental Science, Vol: 9, Pages: 3783-3793, ISSN: 1754-5706

    Optimization 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 limitthe 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 achieveVoc 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 equivalentdevices employing fullerene acceptors. Using the balance between reduced recombination losses and good photocurrent generation efficiencies achieved experimentally as a baseline for simulations of theefficiency potential of organic solar cells, we estimate that efficiencies of up to 20 % are achievable if band gaps and fill factors are further optimized.

    Sachs M, Pastor E, Kafizas A, Durrant JRet 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: 1948-7185

    In 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.

    Ma Y, Mesa CA, Pastor E, Kafizas A, Francas L, Le Formal F, Pendlebury SR, Durrant JRet al., 2016,

    Rate law analysis of water oxidation and hole scavenging on a BiVO4 photoanode

    , ACS Energy Letters, Vol: 1, Pages: 618-623, ISSN: 2380-8195

    Spectroelectrochemical 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.

    Sprick RS, Bonillo B, Sachs M, Clowes R, Durrant JR, Adams DJ, Cooper AIet al., 2016,

    Extended conjugated microporous polymers for photocatalytic hydrogen evolution from water

    , Chemical Communications, Vol: 52, Pages: 10008-10011, ISSN: 1364-548X

    Conjugated 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.

    Lee HKH, Li Z, Durrant JR, Tsoi WCet al., 2016,

    Is organic photovoltaics promising for indoor applications?

    , Applied Physics Letters, Vol: 108, ISSN: 1077-3118

    This 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.

    Holliday S, Ashraf RS, Wadsworth A, Baran D, Yousaf A, Nielsen CB, Tan C, Dimitrov S, Shang Z, Gasparini N, Alamoudi M, Laquai F, Brabec C, Salleo A, Durrant J, Mcculloch Iet al.,

    High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor

    , Nature Communications, Vol: 7, ISSN: 2041-1723
    Morris MR, Pendlebury S, Hong J, Dunn S, Durrant Jet 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: 0935-9648

    Spontaneous 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.

    Ma Y, Kafizas A, Pendlebury SR, Le Formal F, Durrant JRet 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: 1616-301X

    This 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.

    Bryant D, Aristidou N, Pont S, Sanchez-Molina I, Chotchunangatchaval T, Wheeler S, Durrant JR, Haque SAet 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: 1754-5692

    Here, 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.

    Brinkert K, Le formal F, Li X, Durrant J, Rutherford AW, Fantuzzi Aet 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: 0005-2728

    We 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.

    Lindquist RJ, Phelan BT, Reynal A, Margulies EA, Shoer LE, Durrant JR, Wasielewski MRet al., 2016,

    Strongly oxidizing perylene-3,4-dicarboximides for use in water oxidation photoelectrochemical cells

    , Journal of Materials Chemistry A, Vol: 4, Pages: 2880-2893, ISSN: 2050-7496

    Perylene-3,4-dicarboximide (PMI) based chromophores have demonstrated the ability to inject electrons into TiO2 for dye-sensitized solar cell applications and to accept electrons from metal complexes relevant to water oxidation, but they are nearly unexplored for use in photoelectrochemical cells (PECs) for solar fuels generation. A series of related PMIs with high oxidation potentials and carboxylate binding groups was synthesized and investigated for this purpose. Charge injection and recombination dynamics were measured using transient absorption (TA) spectroscopy on the picosecond to second timescales. The dynamics and electron injection yields were correlated with the PMI energetics and structures. Injection began in less than 1 ps for the dye with the best performance and a significant charge-separated state yield remained at long times. Finally, this chromophore was used to oxidize a covalently bound water oxidation precatalyst following electron injection into TiO2 to demonstrate the utility of the dyes for use in PECs.

    Morais A, Longo C, Araujo JR, Barroso M, Durrant JR, Nogueira AFet al., 2016,

    Nanocrystalline anatase TiO2/reduced graphene oxide composite films as photoanodes for photoelectrochemical water splitting studies: the role of reduced graphene oxide

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 18, Pages: 2608-2616, ISSN: 1463-9076
    Dimitrov SD, Schroeder BC, Nielsen CB, Bronstein H, Fei Z, McCulloch I, Heeney M, Durrant JRet al., 2016,

    Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells

    , Polymers, Vol: 8, ISSN: 2073-4360

    The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.

    Feckl JM, Dunn HK, Zehetmaier PM, Mueller A, Pendlebury SR, Zeller P, Fominykh K, Kondofersky I, Doeblinger M, Durrant JR, Scheu C, Peter L, Fattakhova-Rohlfing D, Bein Tet al., 2015,

    Ultrasmall Co3O4 Nanocrystals Strongly Enhance Solar Water Splitting on Mesoporous Hematite

    , Advanced Materials Interfaces, Vol: 2, ISSN: 2196-7350

    The synthesis of crystalline, nonagglomerated, and perfectly dispersible Co3O4 nanoparticles with an average size of 3–7 nm using a solvothermal reaction in tert-butanol is reported. The very small size and high dispersibility of the Co3O4 nanoparticles allow for their homogeneous deposition on mesoporous hematite layers serving as the photoactive absorber in the light-driven water splitting reaction. This surface treatment leads to a striking photocurrent increase. While the enhancement of hematite photoanode performance by cobalt oxides is known, the preformation and subsequent application of well-defined cobalt oxide nanoparticles are novel and allow for the treatment of arbitrarily complex hematite morphologies. Photoelectrochemical and transient absorption spectroscopy studies show that this enhanced performance is due to the suppression of surface electron–hole recombination on time scales of milliseconds to seconds.

    Willkomm J, Orchard KL, Reynal A, Pastor E, Durrant JR, Reisner Eet al., 2015,

    Dye-sensitised semiconductors modified with molecular catalysts for light-driven H-2 production

    , Chemical Society Reviews, Vol: 45, Pages: 9-23, ISSN: 1460-4744

    The development of synthetic systems for the conversion of solar energy into chemical fuels is aresearch goal that continues to attract growing interest owing to its potential to provide renewable andstorable energy in the form of a ‘solar fuel’. Dye-sensitised photocatalysis (DSP) with molecular catalystsis a relatively new approach to convert sunlight into a fuel such as H2 and is based on the self-assemblyof a molecular dye and electrocatalyst on a semiconductor nanoparticle. DSP systems combineadvantages of both homogenous and heterogeneous photocatalysis, with the molecular componentsproviding an excellent platform for tuning activity and understanding performance at defined catalyticsites, whereas the semiconductor bridge ensures favourable multi-electron transfer kinetics betweenthe dye and the fuel-forming electrocatalyst. In this tutorial review, strategies and challenges for theassembly of functional molecular DSP systems and experimental techniques for their evaluation areexplained. Current understanding of the factors governing electron transfer across inorganic-molecularinterfaces is described and future directions and challenges for this field are outlined.

    Collado Fregoso E, Boufflet P, Fei Z, Gann E, Ashraf S, Li Z, McNeill C, Durrant J, Heeney Met al., 2015,

    Increased Exciton Dipole Moment Translates into Charge-transfer Excitons in Thiophene-fluorinated Low-bandgap Polymers for Organic Photovoltaic Applications

    , Chemistry of Materials, Vol: 27, Pages: 7934-7944, ISSN: 1520-5002

    In this study, we investigate the role of thiophene fluorination in a low-bandgap polymer for organic photovoltaic applications. We use a combined theoretical and experimental approach to investigate charge generation and recombination dynamics, and their correlation with blend microstructure and polymer dipole moment. We find that fluorination results in an increased change in the dipole moment upon exciton formation, which is correlated with the appearance of charge-transfer excitons, as evidenced from ultrafast transient absorption studies. Fluorination also results in smaller yet purer domains, evidenced by atomic force microscopy and resonant soft X-ray scattering, and in agreement with photoluminescence quenching measurements. This change in film morphology is correlated with a modest retardation of nongeminate recombination losses. The efficient charge generation and slower recombination are likely to be partly responsible for the enhanced device efficiency in the fluorinated polymer/fullerene devices.

    Collado L, Reynal A, Coronado JM, Serrano DP, Durrant JR, de la Pena O'Shea VAet al., 2015,

    Effect of Au surface plasmon nanoparticles on the selective CO2 photoreduction to CH4

    , 8th European Meeting on Solar Chemistry and Photocatalysis - Environmental Applications (SPEA), Publisher: ELSEVIER SCIENCE BV, Pages: 177-185, ISSN: 0926-3373
    Li Z, Chiu K-H, Shahid RS, Fearn S, Dattani R, Wong HC, Tan C-H, Wu J, Cabral JT, Durrant JRet al., 2015,

    Toward Improved Lifetimes of Organic Solar Cells under Thermal Stress: Substrate-Dependent Morphological Stability of PCDTBT:PCBM Films and Devices

    , Scientific Reports, Vol: 5, ISSN: 2045-2322

    Morphological stability is a key requirement for outdoor operation of organic solar cells. We demonstrate that morphological stability and lifetime of polymer/fullerene based solar cells under thermal stress depend strongly on the substrate interface on which the active layer is deposited. In particular, we find that the stability of benchmark PCDTBT/PCBM solar cells under modest thermal stress is substantially increased in inverted solar cells employing a ZnO substrate compared to conventional devices employing a PEDOT:PSS substrate. This improved stability is observed to correlate with PCBM nucleation at the 50 nm scale, which is shown to be strongly influenced by different substrate interfaces. Employing this approach, we demonstrate remarkable thermal stability for inverted PCDTBT:PC70BM devices on ZnO substrates, with negligible (<2%) loss of power conversion efficiency over 160 h under 85 °C thermal stress and minimal thermally induced “burn-in” effect. We thus conclude that inverted organic solar cells, in addition to showing improved environmental stability against ambient humidity exposure as widely reported previously, can also demonstrate enhanced morphological stability. As such we show that the choice of suitable substrate interfaces may be a key factor in achieving prolonged lifetimes for organic solar cells under thermal stress conditions.

    Ma Y, Le Formal F, Kafizas A, Pendlebury S, Durrant Jet al., 2015,

    Efficient suppression of back electron/hole recombination in cobalt phosphate surface-modified undoped bismuth vanadate photoanodes

    , Journal of Materials Chemistry A, Vol: 3, Pages: 20649-20657, ISSN: 2050-7496

    In this paper, we compared for the first time the dynamics of photogenerated holes in BiVO4 photoanodes with and without CoPi surface modification, employing transient absorption and photocurrent measurements on microsecond to second timescales. CoPi surface modification is known to cathodically shift the water oxidation onset potential; however, the reason for this improvement has not until now been fully understood. The transient absorption and photocurrent data were analyzed using a simple kinetic model, which allows quantification of the competition between electron/hole recombination and water oxidation. The results of this model are shown to be in excellent agreement with the measured photocurrent data. We demonstrate that the origin of the improvement of photocurrent onset resulting from CoPi treatment is primarily due to retardation of back electron/hole recombination across the space charge layer; no evidence of catalytic water oxidation via CoPi was observed.

    Wheeler, Deledalle F, Tokmoldin N, Kirchartz T, Nelson J, Durrant Jet al., 2015,

    Influence of Surface Recombination on Charge-Carrier Kinetics in Organic Bulk Heterojunction Solar Cells with Nickel Oxide Interlayers

    , Physical review applied, Vol: 4, ISSN: 2331-7019

    The choice of electrode for organic photovoltaics is known to be of importance to both device stability and performance, especially regarding the open-circuit voltage (VOC). Here we show that the work function of a nickel oxide anode, varied using an O2 plasma treatment, has a considerable influence on the open-circuit voltage VOC of an organic solar cell. We probe recombination in the devices using transient photovoltage and charge extraction to determine the lifetime as a function of charge-carrier concentration and compare the experimental results with numerical drift-diffusion simulations. This combination of experiment and simulations allows us to conclude that the variations in VOC are due to a change in surface recombination, localized at the NiO anode, although only a small change in carrier lifetime is observed.

    Yue W, Ashraf RS, Nielsen C, Collado-Fregoso E, Niazi M, Yousaf S, Kirkus M, Chen H-Y, Amassian A, Durrant JAMES, McCulloch IAINet al., 2015,

    A Thieno[3,2-b][1]benzothiophene Isoindigo Building Block for Additive- and Annealing-Free High-Performance Polymer Solar Cells

    , Advanced Materials, Vol: 27, Pages: 4702-4707, ISSN: 1521-4095

    A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2-b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar-cell devices of the high-molecular-weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.

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