I16-Materials and Magnetism
I22-Small angle scattering & Diffraction
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Open Access
Abstract: Helical structures continue to inspire, prompted by examples such as DNA double-helix and alpha-helix in proteins. Most synthetic polymers also crystallize as helices, which relieves steric clashes by twisting, while keeping the molecules straight for their ordered packing. In columnar liquid crystals, which often display useful optoelectronic properties, overall helical chirality can be induced by inclusion of chiral chemical groups or dopants; these bias molecular twist to either left or right, analogous to a magnetic field aligning the spins in a paramagnet. In this work, however, we show that liquid-crystalline columns with long-range helical order can form by spontaneous self-assembly of straight- or bent-rod molecules without inclusion of any chiral moiety. A complex lattice with Fddd symmetry and 8 columns per unit cell (4 right-, 4 left-handed) characterizes this “antiferrochiral” structure. In selected compounds it allows close packing of their fluorescent groups reducing their bandgap and giving them promising light-emitting properties.
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Jan 2022
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I07-Surface & interface diffraction
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Jeroen
Royakkers
,
Kunping
Guo
,
Daniel T. W.
Toolan
,
Liang-Wen
Feng
,
Alessandro
Minotto
,
Daniel G.
Congrave
,
Magda
Danowska
,
Weixuan
Zeng
,
Andrew
Bond
,
Mohammed
Al-Hashimi
,
Tobin J.
Marks
,
Antonio
Facchetti
,
Franco
Cacialli
,
Hugo
Bronstein
Diamond Proposal Number(s):
[23587]
Open Access
Abstract: Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non-radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide-based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect interpolymer interactions. We reveal that the non-encapsulated analogue P(NDI-2OD-T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing p-interchain stacking distance. This suggests that decreasing p-stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.
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Sep 2021
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I07-Surface & interface diffraction
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Shuai
Yuan
,
Lin-Song
Cui
,
Linjie
Dai
,
Yun
Liu
,
Qing-Weii
Liu
,
Yu-Qi
Sun
,
Florian
Auras
,
Miguel
Anaya
,
Xiaopeng
Zheng
,
Edoardo
Ruggeri
,
You-Jun
Yu
,
Yang-Kun
Qu
,
Mojtaba
Abdi-Jalebi
,
Osman M.
Bakr
,
Zhao-Kui
Wang
,
Samuel D.
Stranks
,
Neil C.
Greenham
,
Liang-Sheng
Liao
,
Richard H.
Friend
Diamond Proposal Number(s):
[17223]
Open Access
Abstract: Metal halide perovskite semiconductors have demonstrated remarkable potentials in solution-processed blue light-emitting diodes (LEDs). However, the unsatisfied efficiency and spectral stability responsible for trap-mediated non-radiative losses and halide phase segregation remain the primary unsolved challenges for blue perovskite LEDs. In this study, it is reported that a fluorene-based π-conjugated cationic polymer can be blended with the perovskite semiconductor to control film formation and optoelectronic properties. As a result, sky-blue and true-blue perovskite LEDs with Commission Internationale de l'Eclairage coordinates of (0.08, 0.22) and (0.12, 0.13) at the record external quantum efficiencies of 11.2% and 8.0% were achieved. In addition, the mixed halide perovskites with the conjugated cationic polymer exhibit excellent spectral stability under external bias. This result illustrates that π-conjugated cationic polymers have a great potential to realize efficient blue mixed-halide perovskite LEDs with stable electroluminescence.
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Sep 2021
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[20419]
Abstract: Organic solar cells can generate low-cost renewable energy. These cells usually use a blend of carbon-based materials that can absorb light and create an electrical current. However, not all combinations work efficiently. The way that the molecules order themselves is fundamentally important in determining device efficiency. Researchers at Wuhan University of Technology, the National Center for Nanoscience and Technology in Beijing and the University of Sheffield explored what occurs in blends at the scale of a single molecule to determine device efficiency. They were interested in understanding the structure of a new type of solar cell, based on a blend of three different organic semiconductors. Although adding a third component can improve a solar cell’s ability to absorb light, it does not always enhance device efficiency. The I07 beamline at Diamond Light Source offers high-resolution X-ray investigations of the structure of surfaces and interfaces. The research team carried out X-ray scattering experiments on I07, combined with other techniques, to determine why adding a third component can positively or negatively affect organic solar cell efficiency.
Their measurements provided critical information on how adding a third component changes the molecular ordering of the blend. They were then able to correlate the thin film morphology with the electronic properties of the solar cell. Their results also allow predictions of whether a third component is likely to enhance device efficiency. Their work will help develop high-efficiency solar cells and is another step towards commercialisation of this technology.
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Jul 2021
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I07-Surface & interface diffraction
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Alberto
Privitera
,
Ross
Warren
,
Giacomo
Londi
,
Pascal
Kaienburg
,
Junjie
Liu
,
Andreas
Sperlich
,
Andreas E.
Lauritzen
,
Oliver
Thimm
,
Arzhang
Ardavan
,
David
Beljonne
,
Moritz
Riede
Diamond Proposal Number(s):
[20426]
Open Access
Abstract: We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ). Electron paramagnetic resonance (EPR) spectroscopy discloses the presence of two main paramagnetic species distinguished by two different g-tensors, which are assigned based on density functional theory calculations to the formation of a positive polaron on the host and a radical anion on the dopant. Close inspection of the EPR spectra shows that radical anions on the dopants couple in an antiferromagnetic manner at device-relevant doping concentrations, thereby suggesting the presence of dopant clustering, and that positive polarons on the molecular host move by polaron hopping with an activation energy of 5 meV. This activation energy is substantially smaller than that inferred from electrical conductivity measurements (∼233 meV), as the latter also includes a (major) contribution from charge-transfer state dissociation. It emerges from this study that probing the electron spin can provide rich information on the nature and dynamics of charge carriers generated upon doping molecular semiconductors, which could serve as a basis for the design of the next generation of dopant and host materials.
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Feb 2021
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[18563]
Open Access
Abstract: The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices. Previous work on the use of potassium iodide (KI) in active layers to passivate defects in triple-cation mixed-halide perovskites has been shown to enhance their luminescence efficiency and reduce current–voltage hysteresis. However, the operational stability of KI passivated perovskite solar cells under ambient conditions remains largely unexplored. By investigating perovskite solar cell performance with SnO2 or TiO2 electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite–ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and – at concentrations >5 mol% in the precursor solution – modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.
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Nov 2020
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I07-Surface & interface diffraction
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Baodan
Zhao
,
Yaxiao
Lian
,
Linsong
Cui
,
Giorgio
Divitini
,
Gunnar
Kusch
,
Edoardo
Ruggeri
,
Florian
Auras
,
Weiwei
Li
,
Dexin
Yang
,
Bonan
Zhu
,
Rachel A.
Oliver
,
Judith L.
Macmanus-Driscoll
,
Samuel D.
Stranks
,
Dawei
Di
,
Richard H.
Friend
Diamond Proposal Number(s):
[17223]
Abstract: Light-emitting diodes based on halide perovskites have recently reached external quantum efficiencies of over 20%. However, the performance of visible perovskite light-emitting diodes has been hindered by non-radiative recombination losses and limited options for charge-transport materials that are compatible with perovskite deposition. Here, we report efficient, green electroluminescence from mixed-dimensional perovskites deposited on a thin (~1 nm) lithium fluoride layer on an organic semiconductor hole-transport layer. The highly polar dielectric interface acts as an effective template for forming high-quality bromide perovskites on otherwise incompatible hydrophobic charge-transport layers. The control of crystallinity and dimensionality of the perovskite layer is achieved by using tetraphenylphosphonium chloride as an additive, leading to external photoluminescence quantum efficiencies of around 65%. With this approach, we obtain light-emitting diodes with external quantum efficiencies of up to 19.1% at high brightness (>1,500 cd m−2).
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Oct 2020
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I07-Surface & interface diffraction
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Michèle
Chevrier
,
Jurgen
Kesters
,
Judith E.
Houston
,
Niko
Van Den Brande
,
Sylvain
Chambon
,
Sébastien
Richeter
,
Bruno
Van Mele
,
Thomas
Arnold
,
Ahmad
Mehdi
,
Roberto
Lazzaroni
,
Philippe
Dubois
,
Rachel C.
Evans
,
Wouter
Maes
,
Sébastien
Clément
Diamond Proposal Number(s):
[13868]
Abstract: Phosphonium‐based polythiophene conjugated polyelectrolytes (CPEs) with three different counterions (dodecylsulfate (DS), octylsulfate (OS) and perfluorooctylsulfonate (PFOS)) are synthesized to determine how the nature of the counterion affects the thermal properties, the self‐assembly in thin films and the performance as cathode interfacial layer in polymer solar cells (PSCs). The counterion has a significant effect on the thermal properties of the CPEs, affecting both their glass transition and crystalline behavior. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) studies also indicate that changing the nature of the counterion influences the microstructural organization in thin films (face‐on vs . edge‐on orientation). The affinity of the CPEs with the underlying photoactive layer in PSCs is highly correlated with the counterion species. Finally, in addition to an increase of the power conversion efficiency of ~15% when using these CPEs as cathode interfacial layers in PSCs, a higher device stability is noted, as compared to a reference device with a calcium interlayer.
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Jul 2020
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[18563]
Abstract: Solvent vapour annealing (SVA) is a common post-processing technique used to increase the average grain size of lead halide perovskite films and thus enhance device performance. The prevailing wisdom is that large grain perovskite films lead to enhanced stability, however, we observed the reverse in CH3NH3PbI3 (MAPbI3) with dimethylformamide vapour treatment compared to non-SVA controls. Using a range of microstructural characterisation techniques, we reveal that SVA is not a chemically benign grain-growth process, but leads to substantial stoichiometric changes in the perovskite films. Intrinsic material degradation is investigated under external loading with in situ X-ray scattering, and combined with lifetime testing on full devices. We show that the operational stability of SVA devices greatly depends on the initial stoichiometry of the MAPbI3 with PbI2-excess compositions being least stable. However, the incorporation of excess organic-halides in the precursor solution helps to mitigate the deleterious effects of SVA on device stability. This work critically re-evaluates current thinking around grain structure and stoichiometry in achieving long-term stability for perovskite solar cells.
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Jun 2020
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I16-Materials and Magnetism
I22-Small angle scattering & Diffraction
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Open Access
Abstract: We introduce a new class of mesogens that are bird-like in shape and form honeycomb-type supramolecular liquid crystals. They have a bent pi-conjugated aromatic core as wings, a linear or branched chain as the tail and a selection of functional headgroups. Honeycombs of non-centrosymmetric trigonal type (p3m1) are obtained, along with two different complex honeycomb superlattices (p31m and p2gg) and a randomized hexagonal mesophase (p6mm). The key determinant of the self-assembled structure is the nature of interaction of the headgroup with the glycerols at the ends of the wings. The structure depends on whether the sub-columns lying along the edges of the prismatic cells contain pure or mixed headgroups and wing-end hydrogen-bonding groups. Its assembly is further controlled by reducing the tail-chain volume, inducing out-of-plane buckling of the honeycomb. These two modes of symmetry breaking lead to structural polarity both in- and out-of-plane, opening the way to applications in devices relying on properties such as ferroelectricity and second harmonic generation.
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May 2020
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