Publications


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

ACS Appl Mater Interfaces (2018)

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


How Contact Layers Control Shunting Losses from Pinholes in Thin-Film Solar Cells

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 27263-27272

P Kaienburg, P Hartnage, BE Pieters, J Yu, D Grabowski, Z Liu, J Haddad, U Rau, T Kircharte


Engineering interactions in QDs-PCBM blends: a surface chemistry approach.

Nanoscale 10 (2018) 11913-11922

M Righetto, A Privitera, F Carraro, L Bolzonello, C Ferrante, L Franco, R Bozio

Here we present a comprehensive study on the photophysics of QDs-fullerene blends, aiming to elucidate the impact of ligands on the extraction of carriers from QDs. We investigated how three different ligands (oleylamine, octadecanethiol and propanethiol) influence the dynamics of charge generation, separation, and recombination in blends of CdSe/CdS core/shell QDs and PCBM. We accessed each relevant process directly by combining the results from both optical and magnetic spectroscopies. Transient absorption measurements revealed a faster interaction dynamics in thiol-capped ligands. Through phenomenological modeling of the interaction processes, i.e., energy transfer and electron transfer, we estimated the suppression of exciton migration and the enhancement of electron transfer processes when alkyl-thiols are employed as ligands. Contextually, we report the profound impact of the ligands' alkyl chain length, leading to strengthened interactions with PCBM acceptors. Quantitatively, we measured a 10-fold increase in the electron transfer rate when oleylamine ligands were exchanged with propanethiol ligands. EPR spectroscopy gave access to subtle details regarding both the enhanced charge generation and lower binding energy of charge-transfer states in blends compared to PCBM alone. Moreover, through pulsed EPR techniques, we inferred the localization of deep electron traps in localized sites close to QDs in the blends. Therefore, our thorough characterization evidenced the essential role of ligands in determining QD interactions. We believe that these discoveries will contribute to the efficient incorporation of QDs in existing organic PV technologies.


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.


High irradiance performance of metal halide perovskites for concentrator photovoltaics (vol 3, pg 855, 2018)

NATURE ENERGY 3 (2018) 1013-1013

Z Wang, Q Lin, B Wenger, MG Christoforo, Y-H Lin, MT Klug, MB Johnston, LM Herz, HJ Snaith


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.


Spin-coated planar Sb2S3 hybrid solar cells approaching 5% efficiency.

Beilstein journal of nanotechnology 9 (2018) 2114-2124

P Kaienburg, B Klingebiel, T Kirchartz

Antimony sulfide solar cells have demonstrated an efficiency exceeding 7% when assembled in an extremely thin absorber configuration deposited via chemical bath deposition. More recently, less complex, planar geometries were obtained from simple spin-coating approaches, but the device efficiency still lags behind. We compare two processing routes based on different precursors reported in the literature. By studying the film morphology, sub-bandgap absorption and solar cell performance, improved annealing procedures are found and the crystallization temperature is shown to be critical. In order to determine the optimized processing conditions, the role of the polymeric hole transport material is discussed. The efficiency of our best solar cells exceeds previous reports for each processing route, and our champion device displays one of the highest efficiencies reported for planar antimony sulfide solar cells.


Understanding Thermal Admittance Spectroscopy in Low-Mobility Semiconductors

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 9795-9803

S Wang, P Kaienburg, B Klingebiel, D Schillings, T Kirchartz


Figures of Merit Guiding Research on Organic Solar Cells

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 5829-5843

T Kirchartz, P Kaienburg, D Baran


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


High irradiance performance of metal halide perovskites for concentrator photovoltaics

NATURE ENERGY 3 (2018) 855-861

Z Wang, Q Lin, B Wenger, MG Christoforo, Y-H Lin, MT Klug, MB Johnston, LM Herz, HJ Snaith


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


The central role of ligands in electron transfer from perovskite nanocrystals

MRS Advances 2 (2017) 2327-2335

A Privitera, M Righetto, R Bozio, L Franco

© 2017 Materials Research Society. The nanoscale miniaturization of hybrid organic-inorganic perovskite has given rise to new functionalities, but the full understanding of the multifaceted properties of perovskite nanostructures is still incomplete. Using a combination of optical and magnetic resonance (EPR) spectroscopies, we focused our investigation on the photoinduced electron transfer process taking place in perovskite nanocrystals blended with the fullerene derivative PCBM. In particular we analyzed the different effect of two types of nanocrystal ligands, namely octylamine and oleylamine, on the photoinduced processes. The electron transfer process resulted in efficient fluorescence quenching in a mixed solution and in the formation of charges (PCBM anions) detected by EPR in the blends. Both the optical and EPR techniques revealed a stronger effect when the shorter ligand is present. Finally, pulsed EPR demonstrated the stabilization of the photogenerated charges in proximity of perovskite nanocrystals.


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


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.

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