I22-Small angle scattering & Diffraction
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Peter
Kohn
,
Hartmut
Komber
,
Volodymyr
Senkovskyy
,
Roman
Tkachov
,
Anton
Kiriy
,
Richard H.
Friend
,
Ullrich
Steiner
,
Wilhelm T. S.
Huck
,
Jens-uwe
Sommer
,
Michael
Sommer
,
Sven
Huettner
Diamond Proposal Number(s):
[7205]
Abstract: Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl2 as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ?70%. For Ni(dppp)Cl2-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.
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Mar 2012
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I22-Small angle scattering & Diffraction
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Kerr
Johnson
,
Ya-shih
Huang
,
Sven
Huettner
,
Michael
Sommer
,
Martin
Brinkmann
,
Rhiannon Clare
Mulherin
,
Dorota
Niedzialek
,
David
Beljonne
,
Jenny
Clark
,
Wilhelm T. S.
Huck
,
Richard H.
Friend
Abstract: We report the electronic properties of the conjugated coupling between a donor polymer and an acceptor segment serving as a model for the coupling in conjugated donor–acceptor block copolymers. These structures allow the study of possible intrachain photoinduced charge separation, in contrast to the interchain separation achieved in conventional donor–acceptor blends. Depending on the nature of the conjugated linkage, we observe varying degrees of modification of the excited states, including the formation of intrachain charge transfer excitons. The polymers comprise a block (typically 18 repeat units) of P3HT, poly(3-hexyl thiophene), coupled to a single unit of F8-TBT (where F8 is dioctylfluorene, and TBT is thiophene-benzothiadiazole-thiophene). When the P3HT chain is linked to the TBT unit, we observe formation of a localized charge transfer state, with red-shifted absorption and emission. Independent of the excitation energy, this state is formed very rapidly (<40 fs) and efficiently. Because there is only a single TBT unit present, there is little scope for long-range charge separation and it is relatively short-lived, <1 ns. In contrast, when the P3HT chain and TBT unit are separated by the wider bandgap F8 unit, there is little indication for modification of either ground or excited electronic states, and longer-lived charge separated states are observed.
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Mar 2013
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I22-Small angle scattering & Diffraction
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Abstract: The miscibility and aggregation of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) in a polymer matrix is of great importance for the development of fullerene-based organic photovoltaic cells (OPVs). In this study we have systematically investigated the loading of PCBM in regioregular and regiorandom P3HT (poly(3-hexylthiophene-2,5-diyl). Using optical microscopy, we demonstrate the partial miscibility of PCBM in thermally annealed P3HT films and relate it to the relative crystallinity of P3HT. The low polydispersity and the nearly 100% regioregularity of a self-synthesized P3HT allowed a detailed X-ray characterization as a function of PCBM content, revealing a superstructure of periodic amorphous and crystalline lamellar domains of fully chain extended polymer chains. PCBM dissolves in the amorphous interlamellar P3HT regions (partially index-matching the X-ray scattering contrast) up to a threshold, above which PCBM aggregates start to form. These results show that crystallization of P3HT into 10-nm-wide lamellar domains sets the main length scale in P3HT/PCBM structure formation. PCBM is displaced into the amorphous intralamellar regions, swelling the lamellar stack. This structure formation by crystallization, which is intrinsic to most semicrystalline polymers, followed by the enrichment, segregation, and crystallization of PCBM provides an interdigitated structure, which is conceptually ideal for excitonic solar cells.
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May 2013
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I07-Surface & interface diffraction
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Demet
Asil
,
Brian J.
Walker
,
Bruno
Ehrler
,
Yana
Vaynzof
,
Alessandro
Sepe
,
Sam
Bayliss
,
Aditya
Sadhanala
,
Philip C. Y.
Chow
,
Paul E.
Hopkinson
,
Ullrich
Steiner
,
Neil C.
Greenham
,
Richard H.
Friend
Diamond Proposal Number(s):
[8657]
Abstract: Semiconductor nanocrystals are promising materials for printed optoelectronic devices, but their high surface areas are susceptible to forming defects that hinder charge carrier transport. Furthermore, correlation of chalcogenide nanocrystal (NC) material properties with solar cell operation is not straightforward due to the disorder often induced into NC films during processing. Here, an improvement in long-range ordering of PbSe NCs symmetry that results from halide surface passivation is described, and the effects on chemical, optical, and photovoltaic device properties are investigated. Notably, this passivation method leads to a nanometer-scale rearrangement of PbSe NCs during ligand exchange, improving the long-range ordering of nanocrystal symmetry entirely with inorganic surface chemistry. Solar cells constructed with a variety of architectures show varying improvement and suggest that triplet formation and ionization, rather than carrier transport, is the limiting factor in singlet fission solar cells. Compared to existing protocols, our synthesis leads to PbSe nanocrystals with surface-bound chloride ions, reduced sub-bandgap absorption and robust materials and devices that retain performance characteristics many hours longer than their unpassivated counterparts.
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Dec 2014
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I22-Small angle scattering & Diffraction
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Sandeep
Pathak
,
Alessandro
Sepe
,
Aditya
Sadhanala
,
Felix
Deschler
,
Amir
Haghighirad
,
Nobuya
Sakai
,
Karl C.
Goedel
,
Samuel D.
Stranks
,
Nakita
Noel
,
Michael
Price
,
Sven
Hüttner
,
Nicholas A.
Hawkins
,
Richard H.
Friend
,
Ullrich
Steiner
,
Henry J.
Snaith
Diamond Proposal Number(s):
[8459]
Abstract: Recently, solution-processable organic–inorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance, one of the critical factors is the processing conditions of the perovskite film, which directly influences the photophysical properties and hence the device performance. Here we study the effect of annealing parameters on the crystal structure of the perovskite films and correlate these changes with its photophysical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces an improved crystallinity and large grain domains as compared to nitrogen. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. We note that the limiting nonradiative pathways (i.e., maximizing PLQE) is important for obtaining the highest device efficiency. This indicates a critical impact of the atmosphere upon crystallization and the ultimate device performance.
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Mar 2015
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I07-Surface & interface diffraction
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Baodan
Zhao
,
Mojtaba
Abdi-jalebi
,
Maxim
Tabachnyk
,
Hugh
Glass
,
Varun S.
Kamboj
,
Wanyi Andrew
Nie
,
J.
Pearson
,
Yuttapoom
Puttisong
,
Karl C.
Gödel
,
Harvey E.
Beere
,
David A.
Ritchie
,
Aditya D.
Mohite
,
Sian E.
Dutton
,
Richard H.
Friend
,
Aditya
Sadhanala
Diamond Proposal Number(s):
[12436]
Open Access
Abstract: Low-bandgap CH3NH3(Pb x Sn1– x )I3 (0 ≤ x ≤ 1) hybrid perovskites (e.g., ≈1.5–1.1 eV) demonstrating high surface coverage and superior optoelectronic properties have been fabricated. State-of-the-art photovoltaic (PV) performance is reported with power conversion efficiencies approaching 10% in planar heterojunction architecture with small (<450 meV) energy loss compared to the bandgap and high (>100 cm2 V−1 s−1) intrinsic carrier mobilities.
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Nov 2016
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Maxim
Tabachnyk
,
Arfa H.
Karani
,
Katharina
Broch
,
Luis M.
Pazos-outón
,
James
Xiao
,
Tom C.
Jellicoe
,
Jiri
Novak
,
David
Harkin
,
Andrew J.
Pearson
,
Akshay
Rao
,
Neil C.
Greenham
,
Marcus L.
Böhm
,
Richard H.
Friend
Open Access
Abstract: Carrier multiplication using singlet exciton fission (SF) to generate a pair of spin-triplet excitons from a single optical excitation has been highlighted as a promising approach to boost the photocurrent in photovoltaics (PVs) thereby allowing PV operation beyond the Shockley-Queisser limit. The applicability of many efficient fission materials, however, is limited due to their poor solubility. For instance, while acene-based organics such as pentacene (Pc) show high SF yields (up to200%), the plain acene backbone renders the organic molecule insoluble in common organic solvents. Previous approaches adding solubilizing side groups such as bis(tri-iso-propylsilylethynyl) to the Pc core resulted in low vertical carrier mobilities due to reduction of the transfer integrals via steric hindrance, which prevented high efficiencies in PVs. Here we show how to achieve good solubility while retaining the advantages of molecular Pc by using a soluble precursor route. The precursor fully converts into molecular Pc through thermal removal of the solubilizing side groups upon annealing above 150 °C in the solid state. The annealed precursor shows small differences in the crystallinity compared to evaporated thin films of Pc, indicating that the Pc adopts the bulk rather than surface polytype. Furthermore, we identify identical SF properties such as sub-100 fs fission time and equally long triplet lifetimes in both samples.
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Nov 2016
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I07-Surface & interface diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
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Alex J.
Barker
,
Aditya
Sadhanala
,
Felix
Deschler
,
Marina
Gandini
,
Satyaprasad P.
Senanayak
,
Phoebe M.
Pearce
,
Edoardo
Mosconi
,
Andrew
Pearson
,
Yue
Wu
,
Ajay Ram
Srimath Kandada
,
Tomas
Leitjens
,
Filippo
De Angelis
,
Sian E.
Dutton
,
Annamaria
Petrozza
,
Richard H.
Friend
Diamond Proposal Number(s):
[11454, 12436]
Abstract: Solution processable lead halide perovskites show immense promise for use in photovoltaics and other optoelectronic applications. The ability to tune their bandgap by alloying various halide anions (for example in CH3NH3Pb(I1-xBrx)3, 0
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May 2017
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I07-Surface & interface diffraction
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Hannah L.
Stern
,
Alexandre
Cheminal
,
Shane R.
Yost
,
Katharina
Broch
,
Sam L.
Bayliss
,
Kai
Chen
,
Maxim
Tabachnyk
,
Karl
Thorley
,
Neil
Greenham
,
Justin M.
Hodgkiss
,
John
Anthony
,
Martin
Head-gordon
,
Andrew J.
Musser
,
Akshay
Rao
,
Richard H.
Friend
Diamond Proposal Number(s):
[11220]
Abstract: Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S1) into two spin-triplet excitons (T1), could provide a means to overcome the Shockley–Queisser limit in photovoltaics. SF as measured by the decay of S1 has been shown to occur efficiently and independently of temperature, even when the energy of S1 is as much as 200 meV less than that of 2T1. Here we study films of triisopropylsilyltetracene using transient optical spectroscopy and show that the triplet pair state (TT), which has been proposed to mediate singlet fission, forms on ultrafast timescales (in 300 fs) and that its formation is mediated by the strong coupling of electronic and vibrational degrees of freedom. This is followed by a slower loss of singlet character as the excitation evolves to become only TT. We observe the TT to be thermally dissociated on 10–100 ns timescales to form free triplets. This provides a model for ‘temperature-independent’ efficient TT formation and thermally activated TT separation.
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Sep 2017
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I09-Surface and Interface Structural Analysis
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Mojtaba
Abdi-Jalebi
,
Zahra
Andaji-Garmaroudi
,
Stefania
Cacovich
,
Camille
Stavrakas
,
Bertrand
Philippe
,
Johannes M.
Richter
,
Mejd
Alsari
,
Edward P.
Booker
,
Eline M.
Hutter
,
Andrew J.
Pearson
,
Samuele
Lilliu
,
Tom J.
Savenije
,
Hakan
Rensmo
,
Giorgio
Divitini
,
Caterina
Ducati
,
Richard H.
Friend
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[15841]
Abstract: Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield—a quantity that must be maximized to obtain high efficiency—remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.
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Mar 2018
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