Publications by Moritz Riede


Controlling energy levels and Fermi level en route to fully tailored energetics in organic semiconductors.

Nature communications 10 (2019) 5538-

R Warren, A Privitera, P Kaienburg, AE Lauritzen, O Thimm, J Nelson, MK Riede

Simultaneous control over both the energy levels and Fermi level, a key breakthrough for inorganic electronics, has yet to be shown for organic semiconductors. Here, energy level tuning and molecular doping are combined to demonstrate controlled shifts in ionisation potential and Fermi level of an organic thin film. This is achieved by p-doping a blend of two host molecules, zinc phthalocyanine and its eight-times fluorinated derivative, with tunable energy levels based on mixing ratio. The doping efficiency is found to depend on host mixing ratio, which is explained using a statistical model that includes both shifts of the host's ionisation potentials and, importantly, the electron affinity of the dopant. Therefore, the energy level tuning effect has a crucial impact on the molecular doping process. The practice of comparing host and dopant energy levels must consider the long-range electrostatic shifts to consistently explain the doping mechanism in organic semiconductors.


Solubilization of Carbon Nanotubes with Ethylene-Vinyl Acetate for Solution-Processed Conductive Films and Charge Extraction Layers in Perovskite Solar Cells.

ACS applied materials & interfaces 11 (2019) 1185-1191

G Mazzotta, M Dollmann, SN Habisreutinger, MG Christoforo, Z Wang, HJ Snaith, MK Riede, RJ Nicholas

Carbon nanotube (CNT) solubilization via non-covalent wrapping of conjugated semiconducting polymers is a common technique used to produce stable dispersions for depositing CNTs from solution. Here, we report the use of a non-conjugated insulating polymer, ethylene vinyl acetate (EVA), to disperse multi- and single-walled CNTs (MWCNT and SWCNT) in organic solvents. We demonstrate that despite the insulating nature of the EVA, we can produce semitransparent films with conductivities of up to 34 S/cm. We show, using photoluminescence spectroscopy, that the EVA strongly binds to individual CNTs, thus making them soluble, preventing aggregation, and facilitating the deposition of high-quality films. To prove the good electronic properties of this composite, we have fabricated perovskite solar cells using EVA/SWCNTs and EVA/MWCNTs as selective hole contact, obtaining power conversion efficiencies of up to 17.1%, demonstrating that the insulating polymer does not prevent the charge transfer from the active material to the CNTs.


Tuning the ambipolar behaviour of organic field effect transistors via band engineering

AIP ADVANCES 9 (2019) ARTN 035202

PR Warren, JFM Hardigree, AE Lauritzen, J Nelson, M Riede


Hole Transport in Low-Donor-Content Organic Solar Cells.

The journal of physical chemistry letters (2018) 5496-5501

D Spoltore, A Hofacker, J Benduhn, S Ullbrich, M Nyman, O Zeika, S Schellhammer, Y Fan, I Ramirez, S Barlow, M Riede, SR Marder, F Ortmann, K Vandewal

Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at ∼6 mol % in C60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well.


Modification of the fluorinated tin oxide/electron-transporting material interface by a strong reductant and its effect on perovskite solar cell efficiency

MOLECULAR SYSTEMS DESIGN & ENGINEERING 3 (2018) 741-747

F Pulvirenti, B Wegner, NK Noel, G Mazzotta, R Hill, JB Patel, LM Herz, MB Johnston, MK Riede, HJ Snaith, N Koch, S Barlow, SR Marder


Naphthalenetetracarboxylic Diimide Derivatives: Molecular Structure, Thin Film Properties and Solar Cell Applications

Zeitschrift fur Physikalische Chemie (2018)

C Falkenberg, M Hummert, R Meerheim, C Schünemann, S Olthof, C Körner, MK Riede, K Leo

© 2018 Walter de Gruyter GmbH, Berlin/Boston 2018. The effciency of organic solar cells is not only determined by their absorber system, but also strongly dependent on the performance of numerous interlayers and charge transport layers. In order to establish new custom-made materials, the study of structure-properties relationships is of great importance. This publication examines a series of naphthalenetetracarboxylic diimide molecules (NTCDI) with varying side-chain length intended for the use as n-dopable electron transport materials in organic solar cells. While all compounds basically share very similar absorption spectra and energy level positions in the desired range, the introduction of alkyl chains has a large impact on thin film growth and charge transport properties: both crystallization and the increase of conductivity by molecular doping are suppressed. This has a direct influence on the series resistance of corresponding solar cells comprising an NTCDI derivative as electron transport material (ETM) as it lowers the power conversion efficiency to 1%. In contrast, using the side-chain free compound it is possible to achive an efficiency of 6.5%, which is higher than the efficiency of a comparable device comprising n-doped C60as standard ETM.


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.


Carbon Nanotubes for Quantum Dot Photovoltaics with Enhanced Light Management and Charge Transport

ACS PHOTONICS 5 (2018) 4854-4863

Y Tazawa, SN Habisreutinger, N Zhang, DAF Gregory, G Nagamine, SV Kesava, G Mazzotta, HE Assender, M Riede, LA Padilha, RJ Nicholas, AAR Watt


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.


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 ELEMENTARY PROCESSES IN ORGANIC PHOTOVOLTAICS, 272 (2017) 51-75

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


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


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


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

ADVANCED ENERGY MATERIALS 6 (2016) ARTN 1501280

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


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


Managing BHJ microstructural evolution for long-term photoconversion efficiency (Conference Presentation)

Organic Photovoltaics XVII SPIE (2016)

J Martinez Hardigree, GE Morse, P Tiwana, G Mazzotta, IR Ramirez, CL Nicklin, OR Lozman, MK Riede


Plenary session 1: Engineering leadership & cognitive computing

2016 18th Mediterranean Electrotechnical Conference (MELECON) IEEE (2016)

NM Timotheou, M Reumann, JV Milanovic, LG Kornegay, R Stubner, J Hernandez-Castro, PA Amado Assuncao, J Georgiou, M Riede

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