34 results found
Hernandez FJ, Fei Z, Osborne C, et 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
Luo H, Dimitrov S, Daboczi M, et al., 2020, Nitrogen-Doped Carbon Dots/TiO2 Nanoparticle Composites for Photoelectrochemical Water Oxidation, ACS APPLIED NANO MATERIALS, Vol: 3, Pages: 3371-3381, ISSN: 2574-0970
Speller EM, Clarke AJ, Aristidou N, et 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.
Dimitrov SD, Azzouzi M, Wu J, et 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
Tan CH, Wadsworth A, Gasparini N, et 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.
Kim J, Godin R, Dimitrov SD, et 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.
Sachs M, Sprick RS, Pearce D, et 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.
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.
Utzat H, Dimitroy SD, Wheeler S, et 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.
Collado Fregoso E, Deledalle F, Utzat H, et al., 2016, Photophysical study of DPPTT-T/PC70BM blends and solar devices as a function of fullerene loading: an insight into EQE limitations of DPP-based polymers, Advanced Functional Materials, Vol: 27, ISSN: 1616-3028
Diketopyrrolopyrrole (DPP)-based polymers have been consistently used for the fabrication of solar cell devices and transistors, due to the existence of intermolecular short contacts,resulting in high electron and hole mobilities. However, they also often show limited external quantum efficiencies (EQEs). In this contribution we analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation and recombination kinetics in thin films and solar devices of a DPP-based donor polymer, DPPTT-T(thieno[3,2-b]thiophene-diketopyrrolopyrrolecopolymer)blended with varying weight fractions of the fullerene acceptor PC70BM. From the correlations between photoluminescence quenching (PLQ), transient absorption studies and EQEmeasurements, we concludethat the main limitation of photon-to-charge conversion in DPPTT-T/PC70BM devices is poor exciton dissociation. This exciton quenching limit is related to the low affinity/miscibility of the materials, as confirmed by WAXDdiffraction and transmission electron microscopy data, but also to the relatively short DPPTT-T singlet exciton lifetime,possibly associated with highnon-radiative losses. A further strategy to improve EQE in this class of polymers without sacrificing the good extractionproperties in optimized blends is therefore to limit those non-radiative decay processes.
Baran D, Kirchartz T, Wheeler S, et 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.
Fallon KJ, Wijeyasinghe N, Manley EF, et 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
Casey A, Dimitrov SD, Shakya-Tuladhar P, et 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
Holliday S, Ashraf RS, Wadsworth A, et 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.
Berndl S, Dimitrov SD, Menacher F, et 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
Dimitrov SD, Schroeder BC, Nielsen CB, et 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.
Fallon KJ, Dimitrov S, Durrant J, et 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
Andemach R, Utzat H, Dimitrov SD, et 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
O'Mahony FTF, Lee YH, Jellett C, et al., 2015, Improved environmental stability of organic lead trihalide perovskite-based photoactive-layers in the presence of mesoporous TiO2, Journal of Materials Chemistry A, Vol: 3, Pages: 7219-7223, ISSN: 2050-7496
Impressive hybrid photovoltaic device performances have been realised with the methylammonium lead triiodide (MAPbI3) perovskite absorber in a wide range of device architectures. However, the question as to which of these systems represents the most commercially viable long-term prospect is yet to be answered conclusively. Here, we report on the photoinduced charge transfer processes in MAPbI3 based films measured under inert and ambient conditions. When exposed to ambient conditions, the coated mesoporous Al2O3 and bilayer systems show a rapid and significant degradation in the yield of long-lived charge separation. This process, which does not affect sensitized-mesoporous TiO2 films, is only found to occur when both light and oxygen are present. These observations indicate that the presence of a mesostructured TiO2 electron acceptor to rapidly extract the photoexcited electron from the perovskite sensitizer may be crucial for fundamental photovoltaic stability and significantly increases innate tolerance to environmental conditions. This work highlights a significant advantage of retaining mesoscale morphological control in the design of perovskite photovoltaics.
Dimitrov SD, Wheeler S, Niedzialek D, et al., 2015, Polaron pair mediated triplet generation in polymer/fullerene blends, Nature Communications, Vol: 6, ISSN: 2041-1723
Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.
Law C, Miseikis L, Dimitrov S, et al., 2014, Performance and Stability of Lead Perovskite/TiO2, Polymer/PCBM, and Dye Sensitized Solar Cells at Light Intensities up to 70 Suns, ADVANCED MATERIALS, Vol: 26, Pages: 6268-6273, ISSN: 0935-9648
Dimitrov SD, Huang Z, Deledalle F, et al., 2014, Towards optimisation of photocurrent from fullerene excitons in organic solar cells, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 7, Pages: 1037-1043, ISSN: 1754-5692
Dimitrov SD, Durrant JR, 2014, Materials Design Considerations for Charge Generation in Organic Solar Cells, CHEMISTRY OF MATERIALS, Vol: 26, Pages: 616-630, ISSN: 0897-4756
Huang Z, Fregoso EC, Dimitrov S, et al., 2014, Optimisation of diketopyrrolopyrrole:fullerene solar cell performance through control of polymer molecular weight and thermal annealing, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 2, Pages: 19282-19289, ISSN: 2050-7488
Shivanna R, Shoaee S, Dimitrov S, et al., 2014, Charge generation and transport in efficient organic bulk heterojunction solar cells with a perylene acceptor, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 7, Pages: 435-441, ISSN: 1754-5692
Cappel UB, Dowland SA, Reynolds LX, et al., 2013, Charge Generation Dynamics in CdS:P3HT Blends for Hybrid Solar Cells, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 4, Pages: 4253-4257, ISSN: 1948-7185
Bronstein H, Hurhangee M, Fregoso EC, et al., 2013, Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells, CHEMISTRY OF MATERIALS, Vol: 25, Pages: 4239-4249, ISSN: 0897-4756
Bakulin AA, Dimitrov SD, Rao A, et al., 2013, Charge-Transfer State Dynamics Following Hole and Electron Transfer in Organic Photovoltaic Devices, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 4, Pages: 209-215, ISSN: 1948-7185
Bronstein H, Collado-Fregoso E, Hadipour A, et al., 2013, Thieno [3, 2-b] thiophene-diketopyrrolopyrrole Containing Polymers for Inverted Solar Cells Devices with High Short Circuit Currents, Advanced Functional Materials
Juozapavicius M, Kaucikas M, Dimitrov SD, et al., 2013, Evidence for “Slow” Electron Injection in Commercially Relevant Dye-2 Sensitized Solar Cells by Vis−NIR and IR Pump−Probe Spectroscopy, J Phys Chem C
We present femtosecond to nanosecond transient absorption (TA)data on electron injection in dye-sensitized solar cells (DSSCs) fabricated with lowvolatility, commercially relevant electrolytes, with and without added lithium.Results are shown over an extended time range (300 fs−6.3 ns) and extendedwavelength range (800−1400 nm) for both N719 and C106 dyes. Kinetics weremeasured on both TiO2 and noninjecting ZrO2. Using the latter, we havedetermined the spectra and absorption coefficient of N719* across the wavelengthrange. We find an isosbestic point in the TA spectra on TiO2 near 900 nm for allcells, existing from <1 ps to >1 ns. We show how measurements near this isosbesticpoint can give a false impression of uniformly femtosecond injection dynamics inDSSCs. Comparison of dynamics measured at 1200 nm with mid-IR transient absorption measured at 5100 nm confirms amajority proportion of slow (>10 ps) electron injection in these commercially relevant cells. We also comment on a recentpublication which appears to directly contradict the results we present.
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