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  • Journal article
    Pont S, Osella S, Smith A, Marsh AV, Li Z, Beljonne D, Cabral JT, Durrant JRet al., 2019,

    Evidence for strong and weak phenyl-C61-butyric acid methyl ester photodimer populations in organic solar cells

    , Chemistry of Materials, Vol: 31, Pages: 6076-6083, ISSN: 0897-4756

    In polymer/fullerene organic solar cells, the photochemical dimerization of phenyl-C61-butyric acid methyl ester (PCBM) was reported to have either a beneficial or a detrimental effect on device performance and stability. In this work, we investigate the behavior of such dimers by measuring the temperature dependence of the kinetics of PCBM de-dimerization as a function of prior light intensity and duration. Our data reveal the presence of both “weakly” and “strongly” bound dimers, with higher light intensities preferentially generating the latter. DFT simulations corroborate our experimental findings and suggest a distribution of dimer binding energies, correlated with the orientation of the fullerene tail with respect to the dimer bonds on the cage. These results provide a framework to rationalize the double-edged effects of PCBM dimerization on the stability of organic solar cells.

  • Journal article
    Cha H, Fish G, Luke J, Alraddadi A, Lee HH, Zhang W, Dong Y, Limbu S, Wadsworth A, Maria IP, Francas L, Sou HL, Du T, Kim J-S, McLachlan MA, McCulloch I, Durrant JRet al., 2019,

    Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity

    , ADVANCED ENERGY MATERIALS, Vol: 9, ISSN: 1614-6832
  • Journal article
    Yang W, Godin R, Kasap H, Moss B, Dong Y, Hillman SAJ, Steier L, Reisner E, Durrant JRet al., 2019,

    Electron accumulation induces efficiency bottleneck for hydrogen production in carbon nitride photocatalysts

    , Journal of the American Chemical Society, Vol: 141, Pages: 11219-11229, ISSN: 1520-5126

    This study addresses the light intensity dependence of charge accumulation in a photocatalyst suspension, and its impact on both charge recombination kinetics and steady-state H2 evolution efficiency. Cyanamide surface functionalized melon-type carbon nitride (NCNCNx) has been selected as an example of emerging carbon nitrides photocatalysts because of its excellent charge storage ability. Transient spectroscopic studies (from ps to s) show that the bimolecular recombination of photogenerated electrons and holes in NCNCNx can be well described by a random walk model. Remarkably, the addition of hole scavengers such as 4-methylbenzyl alcohol can lead to ∼400-fold faster recombination kinetics (lifetime shortening to ∼10 ps). We show that this acceleration is not the direct result of ultrafast hole extraction by the scavenger, but is rather caused by long-lived electron accumulation in NCNCNx after hole extraction. The dispersive pseudo-first order recombination kinetics become controlled by the density of accumulated electrons. H2 production and steady-state spectroscopic measurements indicate that the accelerated recombination caused by electron accumulation limits the H2 generation efficiency. The addition of a reversible electron acceptor and mediator, methyl viologen (MV2+), accelerates the extraction of electrons from the NCNCNx and increases the H2 production efficiency under one sun irradiation by more than 30%. These results demonstrate quantitatively that while long-lived electrons are essential to drive photoinduced H2 generation in many photocatalysts, excessive electron accumulation may result in accelerated recombination losses and lower performance, and thus highlight the importance of efficient electron and hole extraction in enabling efficient water splitting photocatalysts.

  • Journal article
    Bozal-Ginesta C, Durrant JR, 2019,

    Artificial photosynthesis - concluding remarks.

    , Faraday Discuss, Vol: 215, Pages: 439-451

    This paper follows on from the Concluding Remarks presentation of the 3rd Faraday Discussion Meeting on Artificial Photosynthesis, Cambridge, UK, 25-27th March 2019. It aims to discuss the context for the research discussed at this meeting, starting with an overview of the motivation for research on artificial photosynthesis. It then goes onto analysing the composition and trends in the field of artificial photosynthesis, and its scale relative to other related research areas, primarily using the results of searches of publication databases. As such, we hope it provides helpful insights to researchers in the field.

  • Journal article
    Abe R, Aitchison CM, Andrei V, Beller M, Cheung D, Creissen CE, O'Shea VADLP, Durrant JR, Gratzel M, Hammarstrom L, Haussener S, In S-I, Kalamaras E, Kudo A, Kuehnel MF, Kunturu PP, Lai Y-H, Lee C-Y, Maneiro M, Moore EE, Huu CN, Paris AR, Pornrungroj C, Reek JNH, Reisner E, Schreck M, Smith WA, Soo HS, Sprick RS, Venugopal A, Wang Q, Wielend D, Zwijnenburg MAet al., 2019,

    Demonstrator devices for artificial photosynthesis: general discussion

    , FARADAY DISCUSSIONS, Vol: 215, Pages: 345-363, ISSN: 1359-6640
  • Journal article
    Abe R, Bajada M, Beller M, Bocarsly AB, Butt JN, Cassiola F, Domcke W, Durrant JR, Gavrielides S, Gratzel M, Hammarstrom L, Hatzell MC, Konig B, Kudo A, Kuehnel MF, Lage A, Lee C-Y, Maneiro M, Minteer SD, Paris AR, Plumere N, Reek JNH, Reisner E, Roy S, Schnedermann C, Shankar R, Shylin SI, Smith WA, Soo HS, Wagner A, Wielend Det al., 2019,

    Beyond artificial photosynthesis: general discussion

    , FARADAY DISCUSSIONS, Vol: 215, Pages: 422-438, ISSN: 1359-6640
  • Journal article
    Sachs M, Park JS, Pastor E, Kafizas A, Wilson AA, Francàs L, Gul S, Ling M, Blackman C, Yano J, Walsh A, Durrant JRet al., 2019,

    Effect of oxygen deficiency on the excited state kinetics of WO3 and implications for photocatalysis

    , Chemical Science, Vol: 10, Pages: 5667-5677, ISSN: 2041-6520

    Oxygen vacancies are widely used to tune the light absorption of semiconducting metal oxides, but a photophysical framework describing the impact of such point defects on the dynamics of photogenerated charges, and ultimately on catalysis, is still missing. We herein use WO3 as a model material and investigate the impact of significantly different degrees of oxygen deficiency on its excited state kinetics. For highly oxygen-deficient films, photoelectron spectroscopy shows an over 2 eV broad distribution of oxygen vacancy states within the bandgap which gives rise to extended visible light absorption. We examine the nature of this distribution using first-principles defect calculations and find that defects aggregate to form clusters rather than isolated vacancy sites. Using transient absorption spectroscopy, we observe trapping of photogenerated holes within 200 fs after excitation at high degrees of oxygen deficiency, which increases their lifetime at the expense of oxidative driving force. This loss in driving force limits the use of metal oxides with significant degrees of sub-stoichiometry to photocatalytic reactions that require low oxidation power such as pollutant degradation, and highlights the need to fine-tune vacancy state distributions for specific target reactions.

  • Journal article
    Wadsworth A, Bristow H, Hamid Z, Babics M, Gasparini N, Boyle CW, Zhang W, Dong Y, Thorley KJ, Neophytou M, Ashraf RS, Durrant JR, Baran D, McCulloch Iet al.,

    End Group Tuning in Acceptor–Donor–Acceptor Nonfullerene Small Molecules for High Fill Factor Organic Solar Cells

    , Advanced Functional Materials, Pages: 1808429-1808429, ISSN: 1616-301X
  • Journal article
    Green JP, Cha H, Shahid M, Creamer A, Durrant JR, Heeney Met al., 2019,

    Dithieno[3,2-b:2,3-d]arsole-containing conjugated polymers in organic photovoltaic devices

    , Dalton Transactions, Vol: 48, Pages: 6676-6679, ISSN: 1477-9234

    Arsole-derived conjugated polymers are a relatively new class of materials in the field of organic electronics. Herein, we report the synthesis of two new donor polymers containing fused dithieno[3,2-b:2′,3′-d]arsole units and report their application in bulk heterojunction solar cells for the first time. Devices based upon blends with PC71BM display high open circuit voltages around 0.9 V and demonstrate power conversion efficiencies around 4%.

  • Journal article
    Pont S, Durrant JR, Cabral JT, 2019,

    Dynamic PCBM:dimer population in solar cells under light and temperature fluctuations

    , Advanced Energy Materials, Vol: 9, ISSN: 1614-6832

    Photoinduced dimerization of phenyl-C61-butyric acid methyl ester (PCBM) has a significant impact on the stability of polymer:PCBM organic solar cells (OSCs). This reaction is reversible, as dimers can be thermally decomposed at sufficiently elevated temperatures and both photodimerization and decomposition are temperature dependent. In operando conditions of OSCs evidently involve exposure to both light and heat, following periodic diurnal and seasonal profiles. In this work, the kinetics of dimer formation and decomposition are examined and quantified as a function of temperature, light intensity, blend composition, and time. The activation energy for photodimerization is estimated to be 0.021(3) eV, considerably smaller than that for decomposition (0.96 eV). The findings are benchmarked with a variety of conjugated polymer matrices to propose a descriptive dynamic model of PCBM:dimer population in OSCs, and a framework is proposed to rationalize its interplay with morphology evolution and charge quenching. The model and parameters enable the prediction of the dynamic and long-term PCBM:dimer populations, under variable temperature and light conditions, which impact the morphological stability of OSCs.

  • Journal article
    Wang Y, Daboczi M, Mesa CA, Ratnasingham SR, Kim JS, Durrant JR, Dunn S, Yan H, Briscoe J, Wang Y, Daboczi M, Mesa CA, Ratnasingham SR, Kim J-S, Durrant JR, Dunn S, Yan H, Briscoe Jet al., 2019,

    Bi₂Fe₄O₉ thin films as novel visible-light-active photoanodes for solar water splitting

    , Journal of Materials Chemistry A, Vol: 7, Pages: 9537-9541, ISSN: 2050-7496

    We report the chemical solution deposition (CSD) of a phase-pure Bi2Fe4O9 thin film for use as a photoanode in photoelectrochemical (PEC) water splitting. The energy levels of Bi2Fe4O9 films have been measured and n-type characteristics have been confirmed. With band gaps determined as 2.05 eV (indirect) and 2.80 eV (direct) and valence and conduction bands straddling the water oxidation and reduction potentials, this material is highly promising as a photocatalyst for solar water splitting. The photocurrent of a planar photoanode reached 0.1 mA cm−2 at 1.23 VNHE under AM1.5G illumination. The addition of H2O2 as a hole scavenger increased the photocurrent to 0.25 mA cm−2, indicating hole injection is one limiting factor to the performance. The performance was enhanced by nearly 5-fold when the Bi2Fe4O9 photoanode is coupled to a Co–Pi surface co-catalyst. The photoanode also shows excellent stability with no change in photocurrent over three hours of continuous illumination. These results indicate that this material represents a promising addition to the growing selection of low-cost, stable photocatalysts for use in solar water splitting.

  • Journal article
    Luke J, Speller EM, Wadsworth A, Wyatt MF, Dmiitrov S, Lee HKH, Li Z, Tsoi WC, McCulloch I, Bagnis D, Durrant JR, Kim J-Set al., 2019,

    Twist and degrade – Impact of molecular structure on the photostability of non-fullerene acceptors and their photovoltaic blends

    , Advanced Energy Materials, Vol: 9, Pages: 1-14, ISSN: 1614-6832

    Non-fullerene acceptors (NFAs) dominate organic photovoltaic (OPV) research due to their promising efficiencies and stabilities. However, there is very little investigation into the molecular processes of degradation, which is critical to guiding design of novel NFAs for long-lived, commercially viable OPVs. Here we investigate the important role of molecular structure and conformation on NFA photostability in air by comparing structurally similar but conformationally different promising NFAs; planar O-IDTBR and non-planar O-IDFBR. We identify a three-phase degradation process: (i) initial photo-induced conformational change (i.e. torsion about the Core-BT dihedral), induced by non-covalent interactions with environmental molecules, (ii) followed by photo-oxidation and fragmentation, leading to chromophore bleaching and degradation product formation, and (iii) finally complete chromophore bleaching.Initial conformational change is a critical prerequisite for further degradation, providing fundamental understanding of the relative stability of IDTBR and IDFBR, where the alreadytwisted IDFBR is more prone to degradation. When blended with the donor polymer P3HT, both NFAs exhibit improved photostability whilst the photostability of the polymer itself is significantly reduced by the more miscible twisted NFA. Our findings elucidate the important role of NFA molecular structure on photostability of OPV systems, and provide vital insights into molecular design rules for intrinsically photostable NFAs.

  • Journal article
    Speller EM, Clarke AJ, Aristidou N, Wyatt MF, Francàs L, Fish G, Cha H, Lee HKH, Luke J, Wadsworth A, Evans AD, McCulloch I, Kim JS, Haque SA, Durrant JR, Dimitrov SD, Tsoi WC, Li Zet al., 2019,

    Toward improved environmental stability of polymer:fullerene and polymer:non-fullerene organic solar cells: a common energetic origin of light and oxygen induced degradation

    , ACS Energy Letters, Vol: 4, Pages: 846-852, ISSN: 2380-8195

    With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

  • Journal article
    Kim J-S, 2019,

    Impact of initial bulk-heterojunction morphology on operational stability of polymer:fullerene photovoltaic cells

    , Advanced Materials Interfaces, Vol: 6, ISSN: 2196-7350

    Controlling initial bulk-heterojunction (BHJ) morphology is critical for device performance of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long-term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in-situ monitoring of devices at a molecular level. Here, we utilize thermal annealing preconditioning step to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in-situ degradation pathways. We report direct spectroscopic evidence for molecular-scale phase segregation temperature (TPS) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well-mixed blend morphology (no annealing) shows interface instability related to the hole-extracting PEDOT:PSS layer via de-doping. Likewise, initially phase-segregatedmorphology at a molecular level (annealed above TPS) shows instability in the photoactive layer via continuous phase segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. Our results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below TPS is critical for improved operational stability whilst maintaining high efficiency.

  • Journal article
    Dimitrov SD, Azzouzi M, Wu J, Yao J, Dong Y, Tuladhar PS, Schroeder BC, Bittner ER, McCulloch I, Nelson J, Durrant JRet al., 2019,

    Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

    , J Am Chem Soc, Vol: 141, Pages: 4634-4643

    Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer-fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be exp

  • Journal article
    Pham HD, Jain SM, Li M, Manzhos S, Feron K, Pitchaimuthu S, Liu Z, Motta N, Wang H, Durrant JR, Sonar Pet al., 2019,

    Dopant-free novel hole-transporting materials based on quinacridone dye for high-performance and humidity-stable mesoporous perovskite solar cells

    , Journal of Materials Chemistry A, Vol: 7, Pages: 5315-5323, ISSN: 2050-7488

    © 2019 The Royal Society of Chemistry. This study reports three newly developed dopant-free hole-transporting materials (HTMs) for perovskite solar cells. The design is based on a quinacridone (QA) dye as the core with three different extended end-capping moieties, namely, acenaphthylene (ACE), triphenylamine (TPA) and diphenylamine (DPA), attached to the QA core. These HTMs were synthesized and used to successfully fabricate in mesoscopic TiO 2 /CH 3 NH 3 PbI 3 /HTM perovskite devices. Under AM 1.5G illumination at 100 mW cm -2 , the devices achieved a maximum efficiency of 18.2% for ACE-QA-ACE, 16.6% for TPA-QA-TPA and 15.5% for DPA-QA-DPA without any additives, whereas reference devices with doped spiro-OMeTAD as the HTM achieved a PCE of 15.2%. Notably, the unencapsulated devices based on the novel dopant-free HTMs exhibited impressive stability in comparison with the devices based on doped spiro-OMeTAD under a relative humidity of 75% for 30 days. These linear symmetrical HTMs pave the way to a new class of organic hole-transporting materials for cost-efficient and large-area applications of printed perovskite solar cells.

  • Journal article
    Tan CH, Wadsworth A, Gasparini N, Wheeler S, Holliday S, Ashraf RS, Dimitrov SD, Baran D, McCulloch I, Durrant JRet al., 2019,

    Excitation Wavelength-Dependent Internal Quantum Efficiencies in a P3HT/Nonfullerene Acceptor Solar Cell

    , Journal of Physical Chemistry C, Vol: 123, Pages: 5826-5832, ISSN: 1932-7447

    © 2018 American Chemical Society. Solar cells based on blends of the donor polymer, P3HT, with the nonfullerene acceptor, O-IDTBR, have been shown to exhibit promising efficiencies and stabilities for low-cost organic photovoltaic devices. We focus herein on the charge separation and recombination dynamics in such devices. By employing selective wavelength excitations of P3HT and O-IDTBR, we show that photoexcitation of P3HT results in lower internal quantum efficiency (IQE) for photocurrent generation than that observed for photoexcitation of O-IDTBR. Transient absorption and photoluminescence quenching studies indicate that this lower IQE results primarily from higher geminate recombination losses of photogenerated charges following P3HT excitation compared with O-IDTBR excitation, rather than from differences in exciton separation efficiency. These higher geminate recombination losses result in lower photocurrent generation efficiency at short circuit upon selective excitation of the P3HT donor, when compared with O-IDTBR excitation.

  • Journal article
    Selim S, Francàs L, García-Tecedor M, Corby S, Blackman C, Gimenez S, Durrant JR, Kafizas Aet al., 2019,

    WO3/BiVO4: impact of charge separation at the timescale of water oxidation

    , Chemical Science, Vol: 10, Pages: 2643-2652, ISSN: 2041-6520

    The four hole oxidation of water has long been considered the kinetic bottleneck for overall solar-driven water splitting, and thus requires the formation of long-lived photogenerated holes to overcome this kinetic barrier. However, photogenerated charges are prone to recombination unless they can be spatially separated. This can be achieved by coupling materials with staggered conduction and valence band positions, providing a thermodynamic driving force for charge separation. This has most aptly been demonstrated in the WO3/BiVO4 junction, in which quantum efficiencies for the water oxidation reaction can approach near unity. However, the charge carrier dynamics in this system remain elusive over timescales relevant to water oxidation (μs–s). In this work, the effect of charge separation on carrier lifetime, and the voltage dependence of this process, is probed using transient absorption spectroscopy and transient photocurrent measurements, revealing sub-μs electron transfer from BiVO4 to WO3. The interface formed between BiVO4 and WO3 is shown to overcome the “dead-layer effect” encountered in BiVO4 alone. Moreover, our study sheds light on the role of the WO3/BiVO4 junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO3/BiVO4 junction improves both the yield and lifetime of holes present in the BiVO4 layer over timescales relevant to water oxidation.

  • Journal article
    Crake A, Christoforidis K, Godin R, Moss B, Kafizas A, Zafeiratos S, Durrant J, Petit Cet al., 2019,

    Titanium dioxide/carbon nitride nanosheet nanocomposites for gas phase CO2 photoreduction under UV-visible irradiation

    , Applied Catalysis B: Environmental, Vol: 242, Pages: 369-378, ISSN: 0926-3373

    In the field of photocatalysis and particularly that of CO2 photoreduction, the formulation of nanocomposites provids avenues to design a material platform with a unique set of structural, optoelectronic and chemical features thereby addressing shortcomings of single-phase materials and allowing synergistic effects. In this work, inorganic/organic composite photocatalysts for CO2 reduction comprised of titanium dioxide (TiO2) and carbon nitride nanosheets (CNNS) were synthesized using a hydrothermal in-situ growth method. Specifically, pre-formed CNNS were used to synthesize TiO2/CNNS heterostructures with control over the TiO2 facet formation. This synthesis approach improved the catalytic properties by increasing CO2 adsorption capacity and facilitating charge transfer. The materials were characterised by various spectroscopic, imaging, and analytical techniques to investigate their structural (from nano- to macroscale), chemical, and optical properties. TiO2 nanoparticles were efficiently grown on the CNNS. The CO2 adsorption capacity of the composites was measured, and they were tested for CO2 photoreduction under UV-Vis illumination with hydrogen as the reducing agent in a heterogeneous gas-solid system to combine CO2 capture and conversion into a single-step process. Catalytic tests were performed without adding any precious metal co-catalyst. The composites exhibited enhanced CO2 adsorption capacity and photocatalytic CO2 conversion compared to their constituent materials (> ten-fold increase) and outperformed the TiO2 P25 benchmark material. The TiO2/CNNS composite with more {001} TiO2 facets was the most catalytically active. Further investigations using transient absorption spectroscopy (TAS) revealed the control of facet formation improved interfacial transfer at the TiO2/CNNS junction. A photocatalytic mechanism was proposed based on the spectroscopic analyses as well as the CO2 adsorption, and CO2 conversion results.

  • Journal article
    Lin C-T, Rossi F, Kim J, Baker J, Ngiam J, Xu S, Pont S, Aristidou N, Haque S, Watson T, McLachlan M, Durrant Jet al., 2019,

    Evidence for surface defect passivation as the origin of the remarkable photostability of unencapsulated perovskite solar cells employing aminovaleric acid as a processing additive

    , Journal of Materials Chemistry A, Vol: 7, ISSN: 2050-7496

    This study addresses the cause of enhanced stability of methyl ammonium lead iodide when processed with aminovaleric acid additives (AVA-MAPbI3) in screen printed, hole transport layer free perovskite solar cells with carbon top electrodes (c-PSC). Employing AVA as an additive in the active layer caused a 40-fold increase in device lifetime measured under full sun illumination in ambient air (RH ~15%). This stability improvement with AVA was also observed in optical photobleaching studies of planar films on glass, indicating this improvement is intrinsic to the perovskite film. Employing low-energy ion scattering spectroscopy, photoluminescence studies as a function of AVA and oxygen exposure, and a molecular probe for superoxide generation, we conclude that even though superoxide is generated in both AVA-MAPbI3 and MAPbI3 films, AVA located at grain boundaries is able to passivate surface defect sites, resulting in enhanced resistivity to oxygen induced degradation. These results are discussed in terms of their implications for the design of environmentally stable perovskite solar cells.

  • Journal article
    Kafizas A, Xing X, Selim S, Mesa CA, Ma Y, Burgess C, McLachlan MA, Durrant JRet al., 2019,

    Ultra-thin Al<inf>2</inf>O<inf>3</inf>coatings on BiVO<inf>4</inf>photoanodes: Impact on performance and charge carrier dynamics

    , Catalysis Today, Vol: 321-322, Pages: 59-66, ISSN: 0920-5861

    Bismuth vanadate (BiVO 4 ) has emerged as one of the most promising photoanode materials for oxidising water due to its visible light activity and low cost. Recent studies have shown that the performance of BiVO 4 photoanodes can be remarkably improved when coated with ultra-thin passivation layers. In this article we investigate the use of ultra-thin Al 2 O 3 layers grown using atomic layer deposition (ALD). At an optimum thickness (~0.33nm, 3 ALD cycles), the Al 2 O 3 layer favourably shifted the onset potential by ~200mV and increased photocatalytic currents for the water oxidation reaction. When held at 1.23V RHE , we observe a remarkable increase in the theoretical solar photocurrent; from ~0.47mAcm -2 in uncoated BiVO 4 to ~3.0mAcm -2 in Al 2 O 3 -coated BiVO 4 . Using transient photocurrent (TPC) and transient absorption spectroscopy (TAS) the charge carrier dynamics in Al 2 O 3 -coated BiVO 4 photoanodes were examined for the first time. TPC showed that photogenerated electrons in the BiVO 4 layer were extracted within ~1ms. TAS showed that the remaining holes oxidised water from ~100ms to 1s. Ultra-thin Al 2 O 3 coatings did not improve the reaction kinetics towards water oxidation, but rather, suppressed bi-molecular recombination on the μs-ms timescale in BiVO 4 , and increased the yield of long-lived holes on the ms-s timescale required to oxidise water. This is attributed to an inhibition of surface recombination on BiVO 4 by Al 2 O 3 , which inhibited the early timescale recombination of charge carriers formed within the space charge layer.

  • Journal article
    Moss B, Hegner FS, Corby S, Selim S, Francas L, Lopez N, Gimenez S, Galan-Mascaros J-R, Durrant JRet al., 2019,

    Unraveling Charge Transfer in CoFe Prussian Blue Modified BiVO4 Photoanodes

    , ACS ENERGY LETTERS, Vol: 4, Pages: 337-342, ISSN: 2380-8195
  • Journal article
    Du T, Burgess C, Lin C-T, Eisner F, Kim J, Xu S, Kang H, Durrant J, McLachlan Met al., 2018,

    Probing and controlling intra-grain crystallinity for improved low-temperature processed perovskite solar cells

    , Advanced Functional Materials, Vol: 28, ISSN: 1616-301X

    Here, previously unobserved nanoscale defects residing within individual grains of solution‐processed methylammonium lead tri‐iodide (CH3NH3PbI3, MAPI) thin films are identified. Using scanning transmission electron microscopy (STEM), the defects inherently associated with the established solution‐processing methodology are identified, and a facile processing modification to eliminate these defects is introduced. Specifically, defect elimination is achieved by coannealing the as‐deposited MAPI layer with the electron transport layer (phenyl‐C61‐butyric acid methyl, PCBM) resulting in devices that significantly outperform devices prepared using the established methodology—with power conversion efficiencies increasing from 13.6% to 17.4%. The use of transmission electron microscopy allows the correlation of performance enhancements to improved intragrain crystallinity and shows that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intragrain crystallinity drives an improvement of charge collection and a reduction of PEDOT:PSS/perovskite interfacial recombination. The study suggests that the microstructural defects in MAPI, owing to a lack of structural coherence throughout the thickness of thin film, are a significant cause of interfacial recombination.

  • Journal article
    Kim J, Godin R, Dimitrov SD, Du T, Bryant D, McLachlan MA, Durrant JRet al., 2018,

    Excitation density dependent photoluminescence quenching and charge transfer efficiencies in hybrid perovskite/organic semiconductor bilayers

    , Advanced Energy Materials, Vol: 8, ISSN: 1614-6832

    This study addresses the dependence of charge transfer efficiency between bilayers of methylammonium lead iodide (MAPI3) with PC61BM or poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) charge transfer layers on excitation intensity. It analyzes the kinetic competition between interfacial electron/hole transfer and charge trapping and recombination within MAPI3 by employing a range of optical measurements including steady-state (SS) photoluminescence quenching (PLQ), and transient photoluminescence and absorption over a broad range of excitation densities. The results indicate that PLQ measurements with a typical photoluminescence spectrometer can yield significantly different transfer efficiencies to those measured under 1 Sun irradiation. Steady-state and pulsed measurements indicate low transfer efficiencies at low excitation conditions (<5E + 15 cm−3) due to rapid charge trapping and low transfer efficiencies at high excitation conditions (>5E + 17 cm−3) due to fast bimolecular recombination. Efficient transfer to PC61BM or PEDOT:PSS is only observed under intermediate excitation conditions (≈1 Sun irradiation) where electron and hole transfer times are determined to be 36 and 11 ns, respectively. The results are discussed in terms of their relevance to the excitation density dependence of device photocurrent generation, impact of charge trapping on this dependence, and appropriate methodologies to determine charge transfer efficiencies relevant to device performance.

  • Journal article
    Kosco J, Sachs M, Godin R, Kirkus M, Francas L, Bidwell M, Qureshi M, Anjum D, Durrant JR, McCulloch Iet al., 2018,

    The effect of residual palladium catalyst contamination on the photocatalytic hydrogen evolution activity of conjugated polymers

    , Advanced Energy Materials, Vol: 8, ISSN: 1614-6832

    The effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the hydrogen evolution reaction (HER) rate. The HER rate increases linearly with increasing Pd concentration from <1 ppm to approximately 100 ppm, at which point the rate begins to saturate. Transient absorption spectroscopy experiments support these conclusions, and suggest that residual Pd mediates electron transfer from the F8BT nanoparticles to protons in the aqueous medium.

  • Journal article
    Corby S, Francàs L, Selim S, Sachs M, Blackman C, Kafizas A, Durrant JRet al., 2018,

    Water oxidation and electron extraction kinetics in nanostructured tungsten trioxide photoanodes

    , Journal of the American Chemical Society, Vol: 140, Pages: 16168-16177, ISSN: 1520-5126

    A thorough understanding of the kinetic competition between desired water oxidation/electron extraction processes and any detrimental surface recombination is required to achieve high water oxidation efficiencies in transition-metal oxide systems. The kinetics of these processes in high Faradaic efficiency tungsten trioxide (WO3) photoanodes (>85%) are monitored herein by transient diffuse reflectance spectroscopy and correlated with transient photocurrent data for electron extraction. Under anodic bias, efficient hole transfer to the aqueous electrolyte is observed within a millisecond. In contrast, electron extraction is found to be comparatively slow (∼10 ms), increasing in duration with nanoneedle length. The relative rates of these water oxidation and electron extraction kinetics are shown to be reversed in comparison to other commonly examined metal oxides (e.g., TiO2, α-Fe2O3, and BiVO4). Studies conducted as a function of applied bias and film processing to modulate oxygen vacancy density indicate that slow electron extraction kinetics result from electron trapping in shallow WO3 trap states associated with oxygen vacancies. Despite these slow electron extraction kinetics, charge recombination losses on the microsecond to second time scales are observed to be modest compared to other oxides studied. We propose that the relative absence of such recombination losses, and the observation of a photocurrent onset potential close to flat-band, result directly from the faster water oxidation kinetics of WO3. We attribute these fast water oxidation kinetics to the highly oxidizing valence band position of WO3, thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.

  • Journal article
    Collado L, Reynal A, Fresno F, Barawi M, Escudero C, Perez-Dieste V, Coronado JM, Serrano DP, Durrant JR, de la Pena O'Shea VAet al., 2018,

    Unravelling the effect of charge dynamics at the plasmonic metal/semiconductor interface for CO2 photoreduction

    , Nature Communications, Vol: 9, ISSN: 2041-1723

    Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO2 reduction by artificial photosynthesis is one of the cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO2 reduction by Ag/TiO2 catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.

  • Journal article
    Sachs M, Sprick RS, Pearce D, Hillman SAJ, Monti A, Guilbert AAY, Brownbill NJ, Dimitrov S, Shi X, Blanc F, Zwijnenburg MA, Nelson J, Durrant JR, Cooper AIet al., 2018,

    Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution

    , Nature Communications, Vol: 9, ISSN: 2041-1723

    Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

  • Journal article
    Cha H, Tan C-H, Wu J, Dong Y, Zhang W, Chen H, Rajaram S, Narayan KS, McCulloch I, Durrant JRet al., 2018,

    An analysis of the factors determining the efficiency of photocurrent generation in polymer:nonfullerene acceptor solar cells

    , Advanced Energy Materials, Vol: 8, ISSN: 1614-6832

    Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔECS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔECS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.

  • Journal article
    Sorcar S, Thompson J, Hwang Y, Park YH, Majima T, Grimes CA, Durrant JR, In S-Iet al., 2018,

    High-rate solar-light photoconversion of CO2 to fuel: controllable transformation from C-1 to C-2 products

    , ENERGY & ENVIRONMENTAL SCIENCE, Vol: 11, Pages: 3183-3193, ISSN: 1754-5692

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