Publications


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.


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 Bäuerle, 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.


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


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


Transient response of organo-metal-halide solar cells analyzed by time-resolved current-voltage measurements

Photonics 2 (2015) 1101-1115

MG Christoforo, ET Hoke, MD McGehee, EL Unger

© 2015 by the authors. The determination of the power conversion efficiency of solar cells based on organo-metal-halides is subject to an ongoing debate. As solar cell devices may exhibit very slow t ransient response, current-voltage scans in different directions may not be congruent, which is an effect often referred to as hysteresis. We here discuss time-resolved current-voltage measurements as a means to evaluate appropriate delay times (voltage settling times) to be used in current-voltage measurements of solar cells. Furthermore, this method allows the analysis of transient current response to extract time constants that can be used to compare characteristic differences between devices of varying architecture types, selective contacts and changes in devices due to storage or degradation conditions.


Experimental and theoretical study of phase separation in ZnPc:C60 blends

Organic Electronics: physics, materials, applications 27 (2015) 183-191

T Mönch, TS Sherkar, LJ Anton Koster, P Friederich, M Riede, P Formanek, C Koerner, K Vandewal, W Wenzel, K Leo

© 2015 Published by Elsevier B.V.Understanding the relationship between the absorber layer microstructure and the power conversion efficiency is of paramount importance to further improve the efficiency of organic solar cells. Utilizing transmission electron microscopy (TEM), (photo-)conductive atomic force microscopy ((p)cAFM), 3D drift-diffusion simulations, and density functional theory, we reveal the microscopic origins of phase-separation in a blend of zinc phthalocyanine (ZnPc) and C<inf>60</inf>, used as an absorber layer in organic solar cells. By means of (p)cAFM, we observe photoconductive, circular structures at the surface, which are identified as α-ZnPc islands. Moreover, in agreement with TEM investigations, we observe photoconductive, nanorod-like structures close to the surface which we assign to β-ZnPc. Finally, we apply a 3D drift-diffusion simulation based on the morphology determined by TEM to provide a link between the different contrasts observed in pcAFM and TEM.


Passive Parity-Time Symmetry in Organic Thin Film Waveguides

ACS Photonics 2 (2015) 319-325

Y Jia, Y Yan, SV Kesava, ED Gomez, NC Giebink


Measurement of Small Molecular Dopant F4TCNQ and C60F36 Diffusion in Organic Bilayer Architectures.

ACS applied materials & interfaces 7 (2015) 28420-28428

J Li, CW Rochester, IE Jacobs, S Friedrich, P Stroeve, M Riede, AJ Moulé

The diffusion of molecules through and between organic layers is a serious stability concern in organic electronic devices. In this work, the temperature-dependent diffusion of molecular dopants through small molecule hole transport layers is observed. Specifically we investigate bilayer stacks of small molecules used for hole transport (MeO-TPD) and p-type dopants (F4TCNQ and C60F36) used in hole injection layers for organic light emitting diodes and hole collection electrodes for organic photovoltaics. With the use of absorbance spectroscopy, photoluminescence spectroscopy, neutron reflectometry, and near-edge X-ray absorption fine structure spectroscopy, we are able to obtain a comprehensive picture of the diffusion of fluorinated small molecules through MeO-TPD layers. F4TCNQ spontaneously diffuses into the MeO-TPD material even at room temperature, while C60F36, a much bulkier molecule, is shown to have a substantially higher morphological stability. This study highlights that the differences in size/geometry and thermal properties of small molecular dopants can have a significant impact on their diffusion in organic device architectures.


Nanowire apparatuses and methods

(2015) 61708432

EC Garnett, ML Brongersma, Y Cui, MD McGehee, MG Christoforo, W Cai

Aspects of the present disclosure are directed to apparatuses and methods involving nanowires having junctions therebetween. As consistent with one or more embodiments, an apparatus includes first and second sets of nanowires, in which the second set overlaps the first set. The apparatus further includes a plurality of nanowire joining recrystallization junctions, each junction including material from a nanowire of the first set that is recrystallized into an overlapping nanowire of the second set.


Deposition and post-processing techniques for transparent conductive films

(2015) 61433105

MG Christoforo, S Mehra, A Salleo, P Peumans

In one embodiment, a method is provided for fabrication of a semitransparent conductive mesh. A first solution having conductive nanowires suspended therein and a second solution having nanoparticles suspended therein are sprayed toward a substrate, the spraying forming a mist. The mist is processed, while on the substrate, to provide a semitransparent conductive material in the form of a mesh having the conductive nanowires and nanoparticles. The nanoparticles are configured and arranged to direct light passing through the mesh. Connections between the nanowires provide conductivity through the mesh.


Efficiency enhancement of gallium arsenide photovoltaics using solution-processed zinc oxide nanoparticle light scattering layers

Journal of Nanomaterials 2015 (2015)

Y Kang, D Liang, S Mehra, Y Huo, Y Chen, MG Christoforo, A Salleo, JS Harris

© 2015 Yangsen Kang et al. We demonstrate a high-throughput, solution-based process for subwavelength surface texturing of a III-V compound solar cell. A zinc oxide (ZnO) nanoparticle ink is spray-coated directly on top of a gallium arsenide (GaAs) solar cell. The nanostructured ZnO films have demonstrated antireflection and light scattering properties over the visible/near-infrared (NIR) spectrum. The results show a broadband spectral enhancement of the solar cell external quantum efficiency (EQE), a 16% enhancement of short circuit current, and a 10% increase in photovoltaic efficiency.


Characterization of tandem organic solar cells comprising subcells of identical absorber material

Progress in Photovoltaics: Research and Applications 23 (2015) 1353-1356

R Timmreck, K Leo, M Riede

Copyright © 2014 John Wiley & Sons, Ltd. Recently organic tandem solar cells with record efficiency had been shown comprising identical absorber materials in both subcells. Such structures pose new challenges for characterization. The standard test methods for measuring spectral response of tandem solar cells can not be applied. The standard procedures demand for different bias illumination during measuring spectral response allowing to select the subcell being current limiting. With subcells comprising identical absorber materials, thus having identical absorption spectra, such a selection is not trivial. In this paper, we show that with the help of detailed optical simulations of such tandem organic solar cells, their characterization is possible, and we apply the proposed method to a sample structure.


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.


Miscibility and Acid Strength Govern Contact Doping of Organic Photovoltaics with Strong Polyelectrolytes

Macromolecules 48 (2015) 5162-5171

TP Le, Z Shang, L Wang, N Li, S Vajjala Kesava, JW O'Connor, Y Chang, C Bae, C Zhu, A Hexemer, EW Gomez, A Salleo, MA Hickner, ED Gomez

© 2015 American Chemical Society. Interfacial barriers at electrode-semiconductor contacts can greatly limit charge collection efficiency and hamper device performance. Doping of the semiconductor near the interface can mitigate charge extraction or injection problems by allowing charge tunneling through barriers with reduced width. Here we demonstrate that polymer acids can act as p-type dopants near electrode interfaces for active layers containing poly(3-hexylthiophene-2,5-diyl) (P3HT). By varying the pendant acidic groups between aromatic sulfonic acid, trifluoromethane sulfonimide, and perfluorosulfonic acid, we find the effectiveness of doping the conjugated polymer at the interface depends on the strength of the pendant acid group with stronger acid moieties being capable of creating more carriers in the doped system. Deposition of acidic polymeric dopants at the anode allows high carrier densities, of order 10 < sup > 20 < /sup > cm < sup > -3 < /sup > , to be obtained in polymer semiconductors near the electrode interface. The charge carrier density also depends on the miscibility between polymeric dopants and conjugated polymers. The overall doping efficacy near electrodes therefore depends on the interplay between the strength of pendant acid groups and miscibility between polymeric dopants and conjugated polymers.

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