Publications


Femtosecond Dynamics of Photoexcited C60 Films.

The journal of physical chemistry letters 9 (2018) 1885-1892

M Causa', I Ramirez, JF Martinez Hardigree, M Riede, N Banerji

The well known organic semiconductor C60 is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C60) charge-transfer (CT) excitons in neat and diluted C60 films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.


Key Tradeoffs Limiting the Performance of Organic Photovoltaics

Advanced Energy Materials (2018)

I Ramirez, M Causa', Y Zhong, N Banerji, M Riede

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 2017 saw the publication of several new material systems that challenge the long-held notion that a driving force is necessary for efficient exciton dissociation in organic photovoltaics (OPVs) and that a loss of ≈0.6 eV between the energy of the charge transfer state E ct and the energy corresponding to open circuit is general. In light of these developments, the authors combine insights from device physics and spectroscopy to review the two key tradeoffs limiting OPV performances. These are the tradeoff between the charge carrier generation efficiency and the achievable open circuit voltage (V oc ) and the tradeoff between device thickness (light absorption) and fill factor. The emergence of several competitive nonfullerene acceptors (NFAs) is exciting for both of these. The authors analyze what makes these materials compare favorably to fullerenes, including the potential role of molecular vibrations, and discuss both design criteria for new molecules and the achievable power conversion efficiencies.


Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

B Siegmund, MT Sajjad, J Widmer, D Ray, C Koerner, M Riede, K Leo, IDW Samuel, K Vandewal

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.


MICROSTRUCTURAL CHARACTERIZATION FOR EMERGING PHOTOVOLTAIC MATERIALS Introduction

in , 32 (2017) 1797-1797

D Delongchamp, C Nicklin, M Riede


MINERVA: A facility to study Microstructure and INterface Evolution in Realtime under VAcuum.

The Review of scientific instruments 88 (2017) 103901-

C Nicklin, J Martinez-Hardigree, A Warne, S Green, M Burt, J Naylor, A Dorman, D Wicks, S Din, M Riede

A sample environment to enable real-time X-ray scattering measurements to be recorded during the growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include studying microstructure development with time or during exposure to different environmental conditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop, to reduce the background scattering from the X-rays passing through the entrance and exit windows, together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used in organic solar cells and organic light emitting diodes, including the development of the microstructure with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room temperature under vacuum, highlighting the importance of using real time measurements to understand the as-deposited pristine film structure and its development with time. More generally, this sample environment is versatile and can be used for investigation of structure-property relationships in a wide range of vacuum deposited materials and their applications in, for example, optoelectronic devices and energy storage.


The Potential of Multijunction Perovskite Solar Cells

ACS ENERGY LETTERS 2 (2017) 2506-2513

MT Horantner, T Leijtens, ME Ziffer, GE Eperon, MG Christoforo, MD McGehee, HJ Snaith


Intrinsic non-radiative voltage losses in fullerene-based organic solar cells

NATURE ENERGY 2 (2017) ARTN 17053

J Benduhn, K Tvingstedt, F Piersimoni, S Ullbrich, Y Fan, M Tropiano, KA McGarry, O Zeika, MK Riede, CJ Douglas, S Barlow, SR Marder, D Neher, D Spoltore, K Vandewal


Dicyanovinylene-substituted oligothiophenes for organic solar cells

in Advances in Polymer Science, 272 (2017) 51-75

C Koerner, H Ziehlke, R Fitzner, M Riede, A Mishra, P Bï uerle, K Leo

� Springer International Publishing Switzerland 2017. We investigate dicyanovinyl-substituted oligothiophene derivatives as absorber materials in organic solar cells. We determine structure–property relationships, which are important for materials design. We demonstrate the influence of those structural changes on the processing ability, energy levels, optical properties, thin-film morphology, and charge transport. Furthermore, we give a detailed picture of the microscopic processes between photon absorption and charge carrier generation, in particular, the importance of triplet exciton losses and a relationship between the yield of charge carrier generation and macroscopic charge-transport properties.


In-situ observation of stacking fault evolution in vacuum-deposited C-60

APPLIED PHYSICS LETTERS 111 (2017) ARTN 233305

JFM Hardigree, IR Ramirez, G Mazzotta, C Nicklin, M Riede


Hybrid Organic/Inorganic Perovskite-Polymer Nanocomposites: Toward the Enhancement of Structural and Electrical Properties.

The journal of physical chemistry letters 8 (2017) 5981-5986

A Privitera, M Righetto, M De Bastiani, F Carraro, M Rancan, L Armelao, G Granozzi, R Bozio, L Franco

Hybrid organic/inorganic perovskite nanoparticles (NPs) have garnered remarkable research attention because of their promising photophysical properties. New and interesting properties emerge after combining perovskite NPs with semiconducting materials. Here, we report the synthesis and investigation of a composite material obtained by mixing CH3NH3PbBr3 nanocrystals with the semiconducting polymer poly(3-hexylthiophene) (P3HT). By the combination of structural techniques and optical and magnetic spectroscopies we observed multiple effects of the perovskite NPs on the P3HT: (i) an enlargement of P3HT crystalline domains, (ii) a strong p-doping of the P3HT, and (iii) an enhancement of interchain order typical of H-aggregates. These observations open a new avenue toward innovative perovskite NP-based applications.


Tuning Biocompatible Block Copolymer Micelles by Varying Solvent Composition: Core/Corona Structure and Solvent Uptake

Macromolecules 50 (2017) 4322-4334

TJ Cooksey, A Singh, KM Le, S Wang, EG Kelley, L He, S Vajjala Kesava, ED Gomez, BE Kidd, LA Madsen, ML Robertson

© 2017 American Chemical Society. Block copolymer micelles enable the formation of widely tunable self-assembled structures in liquid phases, with applications ranging from drug delivery to personal care products to nanoreactors. In order to understand fundamental aspects of micelle assembly and dynamics, the structural properties and solvent uptake of biocompatible poly(ethylene oxide-b-ϵ-caprolactone) (PEO-PCL) diblock copolymers in deuterated water (D 2 O)/tetrahydrofuran (THF-d 8 ) mixtures were investigated with a combination of small-angle neutron scattering, nuclear magnetic resonance, and transmission electron microscopy. PEO-PCL block copolymers, of varying molecular weight yet constant block ratio, formed spherical micelles through a wide range of solvent compositions. Varying the solvent composition from 10 to 60 vol % THF-d 8 in D 2 O/THF-d 8 mixtures was a convenient means of varying the core-corona interfacial tension in the micelle system. An increase in THF-d 8 content in the bulk solvent increased the solvent uptake within the micelle core, which was comparable for the two series, irrespective of the polymer molecular weight. Whereas the smaller molecular weight micelle series exhibited a decrease in aggregation number with increasing THF-d 8 content in the bulk solvent, as anticipated due to changes in the core-corona interfacial tension, the aggregation number of the larger molecular weight series was surprisingly invariant with bulk solvent composition. Differences in the dependencies of the micelle size parameters (core radius and overall micelle radius) on the solvent composition originated from the differing trends in aggregation number for the two micelle series. Incorporation of the known unimer content determined from NMR (described in the companion paper), and directly accounting for impacts of solvent swelling of the micelle core on the neutron scattering length density of the core, allowed refinement of and increased confidence in extracted micelle parameters. In summary, the two micelle series showed similar solvent uptake that was independent of the polymer molecular weight yet significantly different dependencies of their aggregation number and size parameters on the solvent composition.


Reply to 'Tandem organic solar cells revisited'

NATURE PHOTONICS 10 (2016) 355-355

R Timmreck, T Meyer, J Gilot, H Seifert, T Mueller, A Furlan, MM Wienk, D Wynands, J Hohl-Ebinger, W Warta, RAJ Janssen, M Riede, K Leo


Close-Packed Spherical Morphology in an ABA Triblock Copolymer Aligned with Large-Amplitude Oscillatory Shear

Macromolecules 49 (2016) 4875-4888

S Wang, R Xie, S Vajjala Kesava, ED Gomez, EW Cochran, ML Robertson

© 2016 American Chemical Society. A microphase-separated poly(styrene-b-(lauryl-co-stearyl acrylate)-b-styrene) (SAS) triblock copolymer exhibiting a disordered spherical microstructure with randomly oriented grains was aligned through the application of large-amplitude oscillatory shear (LAOS) at a temperature below the order-disorder transition temperature of the triblock copolymer, yet above the glass transition temperature of the polystyrene spherical domains. The thermoplastic elastomeric behavior of the SAS triblock copolymer provided a convenient means to observe the aligned morphology. Following application of LAOS, the specimen was quenched to room temperature (below the glass transition temperature of polystyrene), and small-angle X-ray scattering data were obtained in the three principal shear directions: shear gradient, velocity, and vorticity directions. The analysis revealed that the SAS triblock copolymer formed coexisting face-centered cubic and hexagonally close-packed spherical microstructures. The presence of a close-packed microstructure is in stark contrast to an extensive body of literature on sphere-forming bulk block copolymers that favor body-centered cubic systems under quiescent conditions and under shear. The aligned microstructure observed in this bulk block copolymer was reminiscent of that observed in various spherical soft material systems such as colloidal spheres, sphere-forming block copolymer solutions, and star polymer solutions. The highly unanticipated observation of close-packed spherical microstructures in a neat block copolymer under shear is hypothesized to originate from the dispersity of the block copolymer.


EU COST Action MP1307-Unravelling the Degradation Mechanisms of Emerging Solar Cell Technologies

PROCEEDINGS OF THE 18TH MEDITERRANEAN ELECTROTECHNICAL CONFERENCE MELECON 2016 (2016)

T Aernouts, E Katz, F Brunetti, M Ramos, J de la Fuente, M Riede, N Espinosa, A Urbina, K Vandewal, M Fonrodona, M Lira-Cantu, S Veenstra, Y Galagan, E von Hauff, H Hoppe


Influence of Meso and Nanoscale Structure on the Properties of Highly Efficient Small Molecule Solar Cells

Advanced Energy Materials 6 (2016) 1501280-1501280

T Moench, P Friederich, F Holzmueller, B Rutkowski, J Benduhn, T Strunk, C Koerner, K Vandewal, A Czyrska-Filemonowicz, W Wenzel, K Leo


Structured Organic-Inorganic Perovskite toward a Distributed Feedback Laser.

Advanced materials (Deerfield Beach, Fla.) 28 (2016) 923-929

M Saliba, SM Wood, JB Patel, PK Nayak, J Huang, JA Alexander-Webber, B Wenger, SD Stranks, MT Hörantner, JT-W Wang, RJ Nicholas, LM Herz, MB Johnston, SM Morris, HJ Snaith, MK Riede

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.


Cross-Linkable Fullerene Derivatives for Solution-Processed n-i-p Perovskite Solar Cells

ACS ENERGY LETTERS 1 (2016) 648-653

K Wojciechowski, I Ramirez, T Gorisse, O Dautel, R Dasari, N Sakai, JM Hardigree, S Song, S Marder, M Riede, G Wantz, HJ Snaith


A charge carrier transport model for donor-acceptor blend layers

JOURNAL OF APPLIED PHYSICS 117 (2015) ARTN 045501

J Fischer, J Widmer, H Kleemann, W Tress, C Koerner, M Riede, K Vandewal, K Leo


Optical properties and limiting photocurrent of thin-film perovskite solar cells

ENERGY & ENVIRONMENTAL SCIENCE 8 (2015) 602-609

JM Ball, SD Stranks, MT Hoerantner, S Huettner, W Zhang, EJW Crossland, I Ramirez, M Riede, MB Johnston, RH Friend, HJ Snaith


Mixed interlayers at the interface between PEDOT:PSS and conjugated polymers provide charge transport control

Journal of Materials Chemistry C 3 (2015) 2664-2676

AJ Moulé, MC Jung, CW Rochester, W Tress, D Lagrange, IE Jacobs, J Li, SA Mauger, MD Rail, O Lin, DJ Bilsky, Y Qi, P Stroeve, LA Berben, M Riede

© The Royal Society of Chemistry.2015. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulphonate) (PEDOT:PSS) is the most used organic hole injecting or hole transporting material. The hole carrying matrix PEDOT is highly doped by the acidic dopant PSS. When coated onto a substrate, PEDOT:PSS makes a highly uniform conductive layer and a thin (<5 nm) overlayer of PSS covers the air interface. Semiconducting polymer layers for organic photovoltaics or light emitting diodes are coated on top. In this article, we demonstrate that the PSS layer will mix with almost all conjugated polymers upon thermal annealing. Depending on the Fermi energy of the polymer an electrochemical reaction can take place, p-type doping the polymer at the interface between the PEDOT:PSS and the semiconducting polymer. We use chemical and spectroscopic analysis to characterize the polymer/PSS interlayer. We show that the stable and insoluble interlayer has a great effect on the charge injection and extraction from the interface. Finally we demonstrate and electronically model organic photovoltaic devices that are fabricated using these mixed interlayers.

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