1,720,989 research outputs found
Impact of the donor polymer on recombination via triplet excitons in a fullerene-free organic solar cell
No full textThe greater chemical tunability of non-fullerene acceptors enables fine-tuning of the donor-acceptor energy level offsets, a promising strategy towards increasing the open-circuit voltage in organic solar cells. Unfortunately, this approach could open an additional recombination channel for the charge-transfer (CT) state via a lower-lying donor or acceptor triplet level. In this work we investigate such electron and hole back-transfer mechanisms in fullerene-free solar cells incorporating the novel molecular acceptor 2,4-diCN-Ph-DTTzTz. The transition to the low-driving force regime is studied by comparing blends with well-established donor polymers P3HT and MDMO-PPV, which allows for variation of the energetic offsets at the donor-acceptor interface. Combining various optical spectroscopic techniques, the CT process and subsequent triplet formation are systematically investigated. Although both back-transfer mechanisms are found to be energetically feasible in both blends, markedly different triplet-mediated recombination processes are observed for the two systems. The kinetic suppression of electron back-transfer in the blend with P3HT suggests that energy losses due to triplet formation on the polymer can be avoided, regardless of favorable energetic alignment.Research Foundation Flanders (FWO - Vlaanderen)FWO [G0B6715N
Disentangling overlapping high-field EPR spectra of organic radicals: Identification of light-induced polarons in the record fullerene-free solar cell blend PBDB-T:ITIC
No full textWe present a combined high-field EPR and DFT study of light-induced radicals in the bulk heterojunction blend of PBDB-T:ITIC, currently one of the highest efficiency non-fullerene donor:acceptor combinations in organic photovoltaics. We demonstrate two different approaches for disentangling the strongly overlapping high-field EPR spectra of the positive and negative polarons after charge separation: (1) relaxation-filtered field-swept EPR based on the difference in T-1 spin-relaxation times and (2) field-swept EDNMR-induced EPR by exploiting the presence of N-14 hyperfine couplings in only one of the radical species, the small molecule acceptor radical. The approach is validated by light-induced EPR spectra on related blends and the spectral assignment is underpinned by DFT computations. The broader applicability of the spectral disentangling methods is discussed. (C) 2018 Elsevier Inc. All rights reserved.The authors want to acknowledge the Research Foundation Flanders (FWO - Vlaanderen) for support of this work through the project G0B6715N and the PhD fellowship of M. Van Landeghem
EPR Characterization of the Light-Induced Negative Polaron in a Functionalized Dithienylthiazolo[5,4-d]thiazole Acceptor for Organic Photovoltaics
No full textFunctionalized 2,5-dithienylthiazolo[5,4-d]thiazole (DTTzTz) derivatives have attracted interest towards application as non-fullerene acceptors in solution-processed organic solar cells. Here, we present a combined high-field electron paramagnetic resonance and density functional theory study of the light-induced negative polaron on the novel acceptor 2,4-diCN-Ph-DTTzTz formed after charge transfer in bulk heterojunction blends with a donor polymer. Despite spectral overlap with the polymer cation, the g-anisotropy of the acceptor radical could be directly confirmed through detection of its unique N-14 hyperfine couplings using electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (EDNMR) for spectral filtering. The spectral assignment is further underpinned by quantum-chemical calculations, which also provide detailed information about the spin density and charge distribution of the polaron in the DTTzTz acceptor
The Interplay of Stability between Donor and Acceptor Materials in a Fullerene‐Free Bulk Heterojunction Solar Cell Blend
No full textWith rapid advances in material synthesis and device performance, the long-term stability of organic solar cells has become the main remaining challenge toward commercialization. An investigation of photodegradation in blend films of the donor polymer poly(3-hexylthiophene) (P3HT) and the rhodanine-flanked small molecule acceptor 5,5 '-[(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(2,1,3-benzothiadiazole-7,4-diylmethylidyne)]bis[3-ethyl-2-thioxo-4-thiazolidinone] (FBR) is presented in an ambient atmosphere. The photobleaching kinetics of the pure materials and their blends is correlated with the generation of radicals and triplet excitons using optical and magnetic resonance techniques. In addition, spin-trapping methods are employed to identify reactive oxygen species (ROS). In films of P3HT, FBR, and the P3HT:FBR blend, superoxide is generated by electron transfer to molecular oxygen. However, it is found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the P3HT:FBR blend. In the early stage of degradation of the neat blend, it is protected from singlet oxygen by the fast donor-acceptor charge transfer, which competes with triplet exciton formation. These results provide initial input for a rational design of donor and acceptor materials through tuning the molecular singlet and triplet energy levels to prevent ROS-related photodegradation.This work was supported by the Research Foundation - Flanders (FWO Vlaanderen) through group project Project G.0B67.15N, the Hercules project GOH3816NAUHL, the SB Ph.D. fellowship of I.S. (Grant No. 1S53317N), and the Ph.D. fellowship of M.V.L.Van Doorslaer, S (corresponding author), Univ Antwerp, Dept Chem, Univ Pl 1, B-2610 Antwerp, Belgium.
Goovaerts, E (corresponding author), Univ Antwerp, Dept Phys, Univ Pl 1, B-2610 Antwerp, Belgium.
Maes, W (corresponding author), Hasselt Univ, Inst Mat Res, Agoralaan 1,Bldg D, B-3590 Diepenbeek, Belgium ; IMEC Vzw, Div IMOMEC, Wetenschapspk 1, B-3590 Diepenbeek, Belgium.
[email protected]; [email protected]; [email protected]
Extracting disorder parameters from optical spectra of non-fullerene acceptors
Full text linkOrganic solar cells have seen significant advancements through the use of non-fullerene acceptors, yet understanding the impact of molecular design on energetic disorder remains critical for optimizing material performance. In this work, we investigate three methodologies for quantifying static and dynamic excitonic disorder by analysing the temperature dependence of spectral features in thin film absorption and photoluminescence spectra. Our results demonstrate that fitting the temperature dependence of the first emission peak energy is the most reliable approach for assessing static disorder, while linewidth fitting of absorption spectra is best suited for quantifying dynamic disorder. Comparative case studies reveal that linear n-octyl side chains (e.g., in O-IDTBR and IDIC-4Cl) improve aggregation and induce the lowest static disorder, whereas bulkier side chains (e.g., 2-ethylhexyl and phenylhexyl) result in static disorder parameters which are approximately 50% larger in magnitude. For materials exhibiting strong aggregation, such as Y6, the limitations of current models underscore the need for caution when interpreting disorder metrics. These findings highlight the importance of side chain engineering in controlling the excitonic energetic landscape and provide guidance for the accurate assessment of the related disorder parameters in organic semiconductors.This work was funded via the European Research Council (ERC, grant agreement 864625). S. G. thanks the financial support from ICSC– Centro Nazionale di Ricerca in High Performance Computing, Big Data and Quantum Computing, funded by European Union– NextGenerationEU– PNRR, Missione 4 Componente 2 Investimento 1.4. M. V. acknowledges funding from the Research Foundation– Flanders (FWO) through Postdoctoral Fellowship Number 1270123N
Light-Induced Charge Transfer in Two-Dimensional Hybrid Lead Halide Perovskites
No full text2D hybrid perovskites consisting of layers of lead halide octahedra separated by long organic linker cations have great potential for functional material design toward tailored optical and electronic properties. Here, we report the first direct observation of light-induced charge transfer (CT) in a series of novel 2D perovskites incorporating conjugated linkers based on the carbazole molecule. Dedicated electron paramagnetic resonance experiments and supporting quantum-chemical calculations reveal that excitons generated in the lead halide layer undergo CT at the organic-inorganic interface, resulting in a positive polaron delocalized over several carbazole moieties and a partner electron residing in the inorganic layer. The occurrence of such CT processes in these materials not only offers interesting new perspectives for the functionalization of future 2D perovskites but could also lead to a better understanding of the unusual broadband emission reported for this material class, which has been directly related to polaronic effects.The authors want to acknowledge the Research Foundation Flanders (FWO Vlaanderen) for support of this work through the Ph.D. fellowship of M.V.L. (grant number 1185019N), the SB Ph.D. fellowship of W.V.G. (project number 1S17516N), the
funding of the SBO project called PROCEED (S002019N), the senior research project with grant number G043320N, and the M-ERA.NET project called PROMISES. Interreg VlaanderenNederland is acknowledged for the funding of the PVopMAAT
project
Ultrasonic spray coating of polyethylenimine (ethoxylated) as electron injection and transport layer for organic light emitting diodes: The influence of layer morphology and thickness on the interface physics between polyethylenimine (ethoxylated) and the Al cathode
No full textModern lighting is expected to be light weight, flexible, efficient, non-expensive and environmentally friendly fabricated. Organic light emitting diodes (OLEDs) meet all these requirements and can be manufactured using inexpensive and roll-to-roll compatible printing techniques. They, however, often use low work function, highly reactive metals, such as barium and calcium to facilitate electron injection, deposited using expensive and non-continuous vacuum techniques. Efficient and stable alternatives can be found in the aliphatic amines, polyethylenimine (PEI) and polyethylenimine(ethoxylated) (PEIE), that shift the work function of aluminum favorably for electron injection. This work demonstrates ultrasonic spray coating of PEI(E) as electron injection and transport layer for OLEDs, reducing the work function of the aluminum cathode by 0.355 eV allowing a luminous efficacy comparable to that of the OLEDs using calcium/aluminum electrodes. Slightly higher luminous results are noted for the OLEDs with spin coated PEI(E), indicating that the surface morphology and thickness of the PEI(E) layer are crucial factors: ultrasonic spray coated PEI(E) layers have an increased overall thickness and surface roughness. This study shows the potential of ultrasonic spray coating and the suitability of PEI(E) as excellent electron injection and transport layer for OLEDs and paves the way towards fully spray coated OLEDs. K E Y W O R D S electron injection and transport layer, organic light emitting diodes, printing techniques, ultrasonic spray coating This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.BOF UHasselt, Grant/Award Number: BOF20DCOV09; FWO-Vlaanderen, Grant/Award Number: G043320
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Relaxation mechanisms in ultra-low damping Fe80Co20 thin films
Full text linkFerromagnetic thin films with ultra-low damping constants (α) are intensively studied for applications in spintronic devices. We report here the influence of the sputtering conditions (deposition power and temperature) on the magnetic properties of Fe80Co20/Ta bilayers deposited on MgO (0 0 1) and Si (0 0 1) which have been studied using ferromagnetic resonance (FMR) and magneto-optic Kerr-effect (MOKE) techniques. We have found that for the studied fabrication conditions the samples deposited on MgO (0 0 1) have [1 0 0] epitaxial growth with a cubic anisotropy field Hc ~ 300 Oe, while those sputtered on Si (0 0 1) grow polycrystalline with a small uniaxial anisotropy due to the deposition conditions. With measurements at different excitation frequencies we have determined exceptionally low damping values for ferromagnetic conductors (α ~ 3 × 10−3), which makes the Fe80Co20/Ta system an excellent candidate for future applications in spintronic devices. From the analysis of the dependence of the FMR linewidth on the orientation of the applied magnetic field and the excitation frequency in FMR experiments, we were able to separate the contribution of the different relaxation mechanisms (Gilbert damping, two magnon scattering and mosaicity) to the linewidth and to explain quantitatively the observed behavior.Fil: Velázquez Rodriguez, Daniel. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Gomez, Javier Enrique. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Alejandro, Gabriela. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Avilés Félix, Luis. Universite Grenoble Alpes; FranciaFil: Van Landeghem, Melissa. Universite Libre de Bruxelles. Universiteit Antwerpen; BélgicaFil: Goovaerts, Etienne. Universite Libre de Bruxelles. Universiteit Antwerpen; BélgicaFil: Butera, Alejandro Ricardo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin
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