Imperial College London

Dr Dimitrov

Faculty of Natural SciencesDepartment of Chemistry

 
 
 
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Contact

 

s.dimitrov

 
 
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Location

 

CO62ChemistrySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

46 results found

Machreki M, Tyuliev G, Zigon D, Guo Q, Chouki T, Sobrido ABJ, Dimitrov S, Emin Set al., 2024, Photoelectrochemical activation of peroxymonosulfate using Sn-doped a-Fe2O3 thin film for degradation of anti-inflammatory pharmaceutical drug, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, Vol: 446, ISSN: 1010-6030

Journal article

Marin-Beloqui JM, Congrave DG, Toolan DTW, Montanaro S, Guo J, Wright IA, Clarke TM, Bronstein H, Dimitrov SDet al., 2023, Generating Long-Lived Triplet Excited States in Narrow Bandgap Conjugated Polymers, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, ISSN: 0002-7863

Journal article

Guo Q, Zhao Q, Crespo-Otero R, Di Tommaso D, Tang J, Dimitrov SD, Titirici M-M, Li X, Sobrido ABJet al., 2023, Single-Atom Iridium on Hematite Photoanodes for Solar Water Splitting: Catalyst or Spectator?, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, ISSN: 0002-7863

Journal article

Hernandez FJ, Fei Z, Osborne C, Crespo-Otero R, Heeney M, Dimitrov SDet al., 2022, Triplet Generation Dynamics in Si- and Ge-Bridged Conjugated Copolymers, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 126, Pages: 1036-1045, ISSN: 1932-7447

Journal article

Ye Q, Xu X, Paghi A, Bamford T, Horrocks BR, Houlton A, Barillaro G, Dimitrov S, Palma Met al., 2021, Solution-Processable Carbon Nanotube Nanohybrids for Multiplexed Photoresponsive Devices, ADVANCED FUNCTIONAL MATERIALS, Vol: 31, ISSN: 1616-301X

Journal article

Pean E, Dimitrov S, De Castro CS, Davies MLet al., 2020, Interpreting time-resolved photoluminescence of perovskite materials, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 22, Pages: 28345-28358, ISSN: 1463-9076

Journal article

Hou B, Kim B-S, Lee HKH, Cho Y, Giraud P, Liu M, Zhang J, Davies ML, Durrant JR, Tsoi WC, Li Z, Dimitrov SD, Sohn JI, Cha S, Kim JMet al., 2020, Multiphoton Absorption Stimulated Metal Chalcogenide Quantum Dot Solar Cells under Ambient and Concentrated Irradiance, ADVANCED FUNCTIONAL MATERIALS, Vol: 30, ISSN: 1616-301X

Journal article

Luo H, Liu Y, Dimitrov SD, Steier L, Guo S, Li X, Feng J, Xie F, Fang Y, Sapelkin A, Wang X, Titirici M-Met al., 2020, Pt single-atoms supported on nitrogen-doped carbon dots for highly efficient photocatalytic hydrogen generation, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 8, Pages: 14690-14696, ISSN: 2050-7488

Journal article

Daboczi M, Kim J, Lee J, Kang H, Hamilton I, Lin C-T, Dimitrov SD, McLachlan MA, Lee K, Durrant JR, Kim J-Set al., 2020, Towards efficient integrated perovskite/organic bulk heterojunction solar cells: interfacial energetic requirement to reduce charge carrier recombination losses, Advanced Functional Materials, Vol: 30, Pages: 1-8, ISSN: 1616-301X

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.

Journal article

Luo H, Dimitrov S, Daboczi M, Kim J-S, Guo Q, Fang Y, Stoeckel M-A, Samori P, Fenwick O, Sobrido ABJ, Wang X, Titirici M-Met al., 2020, Nitrogen-Doped Carbon Dots/TiO<sub>2</sub> Nanoparticle Composites for Photoelectrochemical Water Oxidation, ACS APPLIED NANO MATERIALS, Vol: 3, Pages: 3371-3381, ISSN: 2574-0970

Journal article

Pean E, De Castro CS, Dimitrov S, De Rossi F, Meroni S, Baker J, Watson T, Davies MLet al., 2020, Investigating the Superoxide Formation and Stability in Mesoporous Carbon Perovskite Solar Cells with an Aminovaleric Acid Additive, ADVANCED FUNCTIONAL MATERIALS, Vol: 30, ISSN: 1616-301X

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, Jorge AB, 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, Vol: 12, Pages: 4432-4441, ISSN: 1864-5631

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

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, Journal of the American Chemical Society, Vol: 141, Pages: 4634-4643, ISSN: 0002-7863

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

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

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

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, Wheeler S, Holliday S, Dimitrov SD, Wadsworth A, Lee HH, Baran D, McCulloch I, Durrant JRet al., 2018, Influence of blend morphology and energetics on charge separation and recombination dynamics in organic solar cells incorporating a nonfullerene acceptor, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X

Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T-2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC71BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T-2OD:FBR blends. This is assigned to the smaller LUMO-LUMO offset of the PffBT4T-2OD:FBR blends relative to PffBT4T-2OD:PC71BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T-2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells.

Journal article

Keiderling C, Dimitrov S, Durrant JR, 2017, Exciton and Charge Generation in PC60BM Thin Films, Journal of Physical Chemistry C, Vol: 121, Pages: 14470-14475, ISSN: 1932-7447

Transient absorption spectroscopy is employed to contrast the photophysics of [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) dispersed in a polystyrene matrix and as a neat film. For the dispersed PC60BM:polystyrene film, singlet excitons are observed that undergo intersystem crossing to triplet excitons. In contrast, in the neat PC60BM film, the transient absorption data indicate significant polaron generation, with photogenerated polarons exhibiting dispersive, bimolecular charge recombination on the nano- to microsecond time scales. These results are discussed in terms of their implications for charge generation from PC60BM light absorption in polymer/fullerene solar cells.

Journal article

Utzat H, Dimitroy SD, Wheeler S, Collado-Fregoso E, Tuladhar PS, Schroeder BC, McCulloch I, Durrant JRet al., 2017, Charge-Separation in Intermixed Polymer:PC70BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 9790-9801, ISSN: 1932-7447

A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics within molecularly intermixed donor–acceptor domains versus the dynamics between phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC70BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3–5 nm. At high fullerene compositions (≥67%), where the intermixed domains are 1–3 nm and the pure fullerene phases reach ∼4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase, making the fullerene domains accessible for electron escape.

Journal article

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.

Journal article

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% efficient fullerene free organic solar cells with >1 V open circuit voltages, 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 limit the 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 achieve Voc 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 equivalent devices employing fullerene acceptors. Using the balance between reduced recombination losses and good photocurrent generation efficiencies achieved experimentally as a baseline for simulations of the efficiency potential of organic solar cells, we estimate that efficiencies of up to 20% are achievable if band gaps and fill factors are further optimized.

Journal article

Fallon KJ, Wijeyasinghe N, Manley EF, Dimitrov SD, Yousaf SA, Ashraf RS, Duffy W, Guilbert AAY, Freeman DME, Al-Hashimi M, Nelson J, Durrant JR, Chen LX, McCulloch I, Marks TJ, Clarke TM, Anthopoulos TD, Bronstein Het al., 2016, Indolo-naphthyridine-6,13-dione Thiophene Building Block for Conjugated Polymer Electronics: Molecular Origin of Ultrahigh n-Type Mobility, CHEMISTRY OF MATERIALS, Vol: 28, Pages: 8366-8378, ISSN: 0897-4756

Journal article

Casey A, Dimitrov SD, Shakya-Tuladhar P, Fei Z, Nguyen M, Han Y, Anthopoulos TD, Durrant JR, Heeney Met al., 2016, Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance, Chemistry of Materials, Vol: 28, Pages: 5110-5120, ISSN: 0897-4756

We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either 0, 1, or 2 cyano groups on the BT unit and their corresponding copolymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor–acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by ∼0.25 eV upon addition of each cyano group, while the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the monocyano substituted BT polymer. However, the PCE drops to less than 1% for the dicyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by 1 order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in the photocurrent was investigated by studying the charge generation properties of the photoactive polymer–fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measurements revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties while maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit optimized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer’s light harvesting properties were improved without sacrificing either the exciton dissociation yield or device VOC. We also identify that the dr

Journal article

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., 2016, High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor, Nature Communications, Vol: 7, Pages: 1-11, ISSN: 2041-1723

Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.

Journal article

Berndl S, Dimitrov SD, Menacher F, Fiebig T, Wagenknecht H-Aet al., 2016, Thiazole Orange Dimers in DNA: Fluorescent Base Substitutions with Hybridization Readout, CHEMISTRY-A EUROPEAN JOURNAL, Vol: 22, Pages: 2386-2395, ISSN: 0947-6539

Journal article

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.

Journal article

Fallon KJ, Dimitrov S, Durrant J, Bronstein H, Clarke TMet al., 2016, Transient absorption spectroscopy of ultra-low band gap polymers for organic electronic applications, Conference on Physical Chemistry of Interfaces and Nanomaterials XV, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X

Conference paper

Andemach R, Utzat H, Dimitrov SD, McCulloch I, Heeney M, Durrant JR, Bronstein Het al., 2015, Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 137, Pages: 10383-10390, ISSN: 0002-7863

Journal article

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