Imperial College London

ProfessorJamesDurrant

Faculty of Natural SciencesDepartment of Chemistry

Professor of Photochemistry
 
 
 
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Contact

 

+44 (0)20 7594 5321j.durrant Website

 
 
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Assistant

 

Miss Lisa Benbow +44 (0)20 7594 5883

 
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Location

 

G22CMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
to

534 results found

Bakulin A, Pastor E, Park J-S, Steier L, Kim S, Grätzel M, Durrant J, Walsh Aet al., In situ observation of picosecond polaron self-localisation in α-Fe2O3 photoelectrochemical cells, Nature Communications, ISSN: 2041-1723

Hematite (α-Fe2O3) is the most studied artificial oxygen-evolving photo-anode and yet its efficiency limitations and their origin remain unknown. A sub-picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self-localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device-based optical-control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes (<100 cm-1). Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly-studied hematite devices.

Journal article

Wang Y, Vogel A, Sachs M, Sprick RS, Wilbraham L, Moniz SJA, Godin R, Zwijnenburg MA, Durrant JR, Cooper AI, Tang Jet al., 2019, Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts, Nature Energy, Vol: 4, Pages: 746-760

Journal article

Corby S, Pastor E, Dong Y, Zheng X, Francàs L, Sachs M, Selim S, Kafizas A, Bakulin AA, Durrant JRet al., 2019, Charge Separation, Band-Bending, and Recombination in WO3 Photoanodes., J Phys Chem Lett, Pages: 5395-5401

In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.

Journal article

Way A, Luke J, Evans AD, Li Z, Kim J-S, Durrant JR, Hin Lee HK, Tsoi WCet al., 2019, Fluorine doped tin oxide as an alternative of indium tin oxide for bottom electrode of semi-transparent organic photovoltaic devices, AIP Advances, Vol: 9, Pages: 085220-1-085220-5, ISSN: 2158-3226

Indium tin oxide (ITO) is commonly used as the transparent bottom electrode for organic solar cells. However, it is known that the cost ofthe ITO is quite high due to the indium element, and in some studies ITO coated glass substrate is found to be the most expensive componentof device fabrication. Moreover, indium migration from ITO can cause stability issues in organic solar cells. Nevertheless, the use of ITO asthe bottom electrode is still dominating in the field. Here, we explore the possibility of using fluorine doped tin oxide (FTO) as an alternativeto ITO for the bottom electrode of organic solar cells particularly on semi-transparent cells. We present side-by-side comparisons on theiroptical, morphological and device properties and suggest that FTO could be more suitable than ITO as the bottom electrode for glass substratebased organic photovoltaic devices.

Journal article

Luo H, Papaioannou N, Salvadori E, Roessler MM, Ploenes G, van Eck ERH, Tanase LC, Feng J, Sun Y, Yang Y, Danaie M, Belen Jorge A, Sapelkin A, Durrant J, Dimitrov SD, Titirici M-Met al., 2019, Manipulating the Optical Properties of Carbon Dots by Fine-Tuning their Structural Features., ChemSusChem

As a new class of sustainable carbon material, "carbon dots" is an umbrella term covering many types of materials. Herein, a broad range of techniques was used to develop the understanding of hydrothermally synthesized carbon dots, and it is shown how fine-tuning the structural features by simple reduction/oxidation reactions can drastically affect their excited-state properties. Structural and spectroscopic studies found that photoluminescence originates from direct excitation of localized fluorophores involving oxygen functional groups, whereas excitation at graphene-like features leads to ultrafast phonon-assisted relaxation and largely quenches the fluorescent quantum yields. This is arguably the first study to identify the dynamics of photoluminescence including Stokes shift and allow the relaxation pathways in these carbon dots to be fully resolved. This comprehensive investigation sheds light on how understanding the excited-state relaxation processes in different carbon structures is crucial for tuning the optical properties for any potential commercial applications.

Journal article

Jain SM, Edvinsson T, Durrant JR, 2019, Green fabrication of stable lead-free bismuth based perovskite solar cells using a non-toxic solvent, Communications Chemistry, Vol: 2, Pages: 1-7, ISSN: 2399-3669

The very fast evolution in certified efficiency of lead-halide organic-inorganic perovskite solar cells to 24.2%, on par and even surpassing the record for polycrystalline silicon solar cells (22.3%), bears the promise of a new era in photovoltaics and revitalisation of thin film solar cell technologies. However, the presence of toxic lead and particularly toxic solvents during the fabrication process makes large-scale manufacturing of perovskite solar cells challenging due to legislation and environment issues. For lead-free alternatives, non-toxic tin, antimony and bismuth based solar cells still rely on up-scalable fabrication processes that employ toxic solvents. Here we employ non-toxic methyl-acetate solution processed (CH3NH3)3Bi2I9 films to fabricate lead-free, bismuth based (CH3NH3)3Bi2I9 perovskites on mesoporous TiO2 architecture using a sustainable route. Optoelectronic characterization, X-ray diffraction and electron microscopy show that the route can provide homogeneous and good quality (CH3NH3)3Bi2I9 films. Fine-tuning the perovskite/hole transport layer interface by the use of conventional 2,2′,7,7′-tetrakis (N,N′-di-p-methoxyphenylamino)−9,9′-spirbiuorene, known as Spiro-OMeTAD, and poly(3-hexylthiophene-2,5-diyl - P3HT as hole transporting materials, yields power conversion efficiencies of 1.12% and 1.62% under 1 sun illumination. Devices prepared using poly(3-hexylthiophene-2,5-diyl hole transport layer shown 300 h of stability under continuous 1 sun illumination, without the use of an ultra violet-filter.

Journal article

He Q, Shahid M, Wu J, Jiao X, Eisner FD, Hodsden T, Fei Z, Anthopoulos TD, McNeill CR, Durrant JR, Heeney Met al., 2019, Fused Cyclopentadithienothiophene acceptor enables ultrahigh short‐circuit current and high efficiency >11% in as‐cast organic solar cells, Advanced Functional Materials, Pages: 1904956-1904956, ISSN: 1616-301X

A new method to synthesize an electron‐rich building block cyclopentadithienothiophene (9H‐thieno‐[3,2‐b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2‐d]thiophene, CDTT) via a facile aromatic extension strategy is reported. By combining CDTT with 1,1‐dicyanomethylene‐3‐indanone endgroups, a promising nonfullerene small molecule acceptor (CDTTIC) is prepared. As‐cast, single‐junction nonfullerene organic solar cells based on PFBDB‐T: CDTTIC blends exhibit very high short‐circuit currents up to 26.2 mA cm−2 in combination with power conversion efficiencies over 11% without any additional processing treatments. The high photocurrent results from the near‐infrared absorption of the CDTTIC acceptor and the well‐intermixed blend morphology of polymer donor PFBDB‐T and CDTTIC. This work demonstrates a useful fused ring extension strategy and promising solar cell results, indicating the great potential of the CDTT derivatives as electron‐rich building blocks for constructing high‐performance small molecule acceptors in organic solar cells.

Journal article

Speller EM, Clarke AJ, Luke J, Lee HKH, Durrant JR, Li N, Wang T, Wong HC, Kim J-S, Tsoi WC, Li Zet al., 2019, From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells, Journal of Materials Chemistry A, ISSN: 2050-7488

<p>This review highlights the opportunities and challenges in stability of organic solar cells arising from the emergence of non-fullerene acceptors.</p>

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

Du T, Xu W, Daboczi M, Kim J, Xu S, Lin C-T, Kang H, Lee K, Heeney MJ, Kim J-S, Durrant JR, McLachlan MAet al., 2019, p-Doping of organic hole transport layers in p–i–n perovskite solar cells: correlating open-circuit voltage and photoluminescence quenching, Journal of Materials Chemistry A, Vol: 7, Pages: 18971-18979, ISSN: 2050-7488

Doping is a widely implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic (PV) devices. Here, in direct contrast to existing understanding, we find that a reduction in p-doping of the organic hole transport layer (HTL) leads to substantial improvements in PV performance in planar p–i–n perovskite solar cells (PSCs), driven by improvements in open circuit voltage (VOC). Employing a range of transient and steady state characterisation tools, we find that the improvements of VOC correlate with reduced surface recombination losses in less p-doped HTLs. A simple device model including screening of bulk electric fields in the perovskite layer is used to explain this observation. In particular, photoluminescence (PL) emission of complete solar cells shows that efficient performance is correlated to a high PL intensity at open circuit and a low PL intensity at short circuit. We conclude that desirable transport layers for p–i–n PSCs should be charge selective contacts with low doping densities.

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

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

Aitchison CM, Andrei V, Antón-García D, Apfel U-P, Badiani V, Beller M, Bocarsly AB, Bonnet S, Brueggeller P, Caputo CA, Cassiola F, Clausing ST, Cooper AI, Creissen CE, de la Peña O'Shea VA, Domcke W, Durrant JR, Grätzel M, Hammarström L, Hankin A, Hatzell MC, Karadas F, König B, Kuehnel MF, Lamaison S, Lin C-Y, Maneiro M, Minteer SD, R Paris A, Pastor E, Pornrungroj C, Reek JNH, Reisner E, Roy S, Sahm C, Shankar R, Shaw WJ, Shylin SI, Smith WA, Sokol K, Soo HS, Sprick RS, Viertl W, Vogel A, Wagner A, Wakerley D, Wang Q, Wielend D, Zwijnenburg MAet al., 2019, Synthetic approaches to artificial photosynthesis: general discussion., Faraday Discuss, Vol: 215, Pages: 242-281

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

Bozal-Ginesta C, Durrant JR, 2019, Artificial Photosynthesis - concluding remarks, Faraday Discussions, Vol: 215, Pages: 439-451, ISSN: 1359-6640

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 data bases. As such, we hope it provides helpful insights to researchersin the field.

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

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, ISSN: 0897-4756

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

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

Durrant J, 2019, Time resolved in operando optical spectroscopy of (photo)electrochemical water oxidation, National Meeting of the American-Chemical-Society (ACS), Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727

Conference paper

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

Dong Y, Cha H, Zhang J, Pastor E, Tuladhar PS, McCulloch I, Durrant JR, Bakulin AAet al., 2019, The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation, Journal of Chemical Physics, Vol: 150, ISSN: 0021-9606

Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PC71BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.

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

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, 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

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