TY  - JOUR
A1  - Vandewal, Koen
A1  - Benduhn, Johannes
A1  - Schellhammer, Karl Sebastian
A1  - Vangerven, Tim
A1  - Rückert, Janna E.
A1  - Piersimoni, Fortunato
A1  - Scholz, Reinhard
A1  - Zeika, Olaf
A1  - Fan, Yeli
A1  - Barlow, Stephen
A1  - Neher, Dieter
A1  - Marder, Seth R.
A1  - Manca, Jean
A1  - Spoltore, Donato
A1  - Cuniberti, Gianaurelio
A1  - Ortmann, Frank
T1  - Absorption Tails of Donor
BT  - C-60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation
JF  - Journal of the American Chemical Society
N2  - In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D A complexes occur at photon energies below the optical gaps of both the donors and the C-60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C-60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.
Y1  - 2017
U6  - https://doi.org/10.1021/jacs.6b12857
SN  - 0002-7863
VL  - 139
IS  - 4
SP  - 1699
EP  - 1704
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Shivhare, Rishi
A1  - Erdmann, Tim
A1  - Hoermann, Ulrich
A1  - Collado-Fregoso, Elisa
A1  - Zeiske, Stefan
A1  - Benduhn, Johannes
A1  - Ullbrich, Sascha
A1  - Huebner, Rene
A1  - Hambsch, Mike
A1  - Kiriy, Anton
A1  - Voit, Brigitte
A1  - Neher, Dieter
A1  - Vandewal, Koen
A1  - Mannsfeld, Stefan C. B.
T1  - Alkyl Branching Position in Diketopyrrolopyrrole Polymers
BT  - Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells
JF  - Chemistry of materials : a publication of the American Chemical Society
N2  - Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.
Y1  - 2018
U6  - https://doi.org/10.1021/acs.chemmater.8b02739
SN  - 0897-4756
SN  - 1520-5002
VL  - 30
IS  - 19
SP  - 6801
EP  - 6809
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Li, Tian-yi
A1  - Benduhn, Johannes
A1  - Li, Yue
A1  - Jaiser, Frank
A1  - Spoltore, Donato
A1  - Zeika, Olaf
A1  - Ma, Zaifei
A1  - Neher, Dieter
A1  - Vandewal, Koen
A1  - Leo, Karl
T1  - Boron dipyrromethene (BODIPY) with meso-perfluorinated alkyl substituents as near infrared donors in organic solar cells
JF  - Journal of materials chemistry : A, Materials for energy and sustainability
N2  - Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at similar to 800 nm with absorption coefficient of over 280 000 cm(-1). Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at similar to 790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm(-2). Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V-oc of 0.76 V.
Y1  - 2018
U6  - https://doi.org/10.1039/c8ta06261g
SN  - 2050-7488
SN  - 2050-7496
VL  - 6
IS  - 38
SP  - 18583
EP  - 18591
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Piersimoni, Fortunato
A1  - Schlesinger, Raphael
A1  - Benduhn, Johannes
A1  - Spoltore, Donato
A1  - Reiter, Sina
A1  - Lange, Ilja
A1  - Koch, Norbert
A1  - Vandewal, Koen
A1  - Neher, Dieter
T1  - Charge Transfer Absorption and Emission at ZnO/Organic Interfaces
JF  - The journal of physical chemistry letters
N2  - We investigate hybrid charge transfer states (HCTS) at the planar interface between a-NPD and ZnO by spectrally resolved electroluminescence (EL) and external quantum efficiency (EQE) measurements. Radiative decay of HCTSs is proven by distinct emission peaks in the EL spectra of such bilayer devices in the NIR at energies well below the bulk a-NPD or ZnO emission. The EQE spectra display low energy contributions clearly red-shifted with respect to the a-NPD photocurrent and partially overlapping with the EL emission. Tuning of the energy gap between the ZnO conduction band and a-NPD HOMO level (E-int) was achieved by modifying the ZnO surface with self-assembled monolayers based on phosphonic acids. We find a linear dependence of the peak position of the NIR EL on E-int, which unambiguously attributes the origin of this emission to radiative recombination between an electron on the ZnO and a hole on a-NPD. In accordance with this interpretation, we find a strictly linear relation between the open-circuit voltage and the energy of the charge state for such hybrid organicinorganic interfaces.
Y1  - 2015
U6  - https://doi.org/10.1021/jz502657z
SN  - 1948-7185
VL  - 6
IS  - 3
SP  - 500
EP  - 504
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Li, Tian-yi
A1  - Benduhn, Johannes
A1  - Qiao, Zhi
A1  - Liu, Yuan
A1  - Li, Yue
A1  - Shivhare, Rishi
A1  - Jaiser, Frank
A1  - Wang, Pei
A1  - Ma, Jie
A1  - Zeika, Olaf
A1  - Neher, Dieter
A1  - Mannsfeld, Stefan C. B.
A1  - Ma, Zaifei
A1  - Vandewal, Koen
A1  - Leo, Karl
T1  - Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells
JF  - The journal of physical chemistry letters
N2  - An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm.
Y1  - 2019
U6  - https://doi.org/10.1021/acs.jpclett.9b01222
SN  - 1948-7185
VL  - 10
IS  - 11
SP  - 2684
EP  - 2691
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Ullbrich, Sascha
A1  - Benduhn, Johannes
A1  - Jia, Xiangkun
A1  - Nikolis, Vasileios C.
A1  - Tvingstedt, Kristofer
A1  - Piersimoni, Fortunato
A1  - Roland, Steffen
A1  - Liu, Yuan
A1  - Wu, Jinhan
A1  - Fischer, Axel
A1  - Neher, Dieter
A1  - Reineke, Sebastian
A1  - Spoltore, Donato
A1  - Vandewal, Koen
T1  - Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses
JF  - Nature materials
N2  - Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11).
KW  - Electronics, photonics and device physics
KW  - Optoelectronic devices and components
KW  - Photonic devices
KW  - Solar energy and photovoltaic technology
Y1  - 2019
U6  - https://doi.org/10.1038/s41563-019-0324-5
SN  - 1476-1122
SN  - 1476-4660
VL  - 18
IS  - 5
SP  - 459
EP  - 464
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Collado-Fregoso, Elisa
A1  - Pugliese, Silvina N.
A1  - Wojcik, Mariusz
A1  - Benduhn, Johannes
A1  - Bar-Or, Eyal
A1  - Perdigón-Toro, Lorena
A1  - Hörmann, Ulrich
A1  - Spoltore, Donato
A1  - Vandewal, Koen
A1  - Hodgkiss, Justin M.
A1  - Neher, Dieter
T1  - Energy-gap law for photocurrent generation in fullerene-based organic solar cells
BT  - the case of low-donor-content blends
JF  - Journal of the American Chemical Society
N2  - The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C-60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.
Y1  - 2019
U6  - https://doi.org/10.1021/jacs.8b09820
SN  - 0002-7863
VL  - 141
IS  - 6
SP  - 2329
EP  - 2341
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Fang, Lijia
A1  - Holzmueller, Felix
A1  - Matulaitis, Tomas
A1  - Baasner, Anne
A1  - Hauenstein, Christoph
A1  - Benduhn, Johannes
A1  - Schwarze, Martin
A1  - Petrich, Annett
A1  - Piersimoni, Fortunato
A1  - Scholz, Reinhard
A1  - Zeika, Olaf
A1  - Koerner, Christian
A1  - Neher, Dieter
A1  - Vandewal, Koen
A1  - Leo, Karl
T1  - Fluorine-containing low-energy-gap organic dyes with low voltage losses for organic solar cells
JF  - Synthetic metals : the journal of electronic polymers and electronic molecular materials
N2  - Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum-deposited organic solar cells. These molecules comprise a donor acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-donating and electron-withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with, similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C-60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8%. Moreover, the voltage loss for the PRTF device due to the non radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34V). (C) 2016 Published by Elsevier B.V.
KW  - (Z)-isomer
KW  - Donor materials
KW  - CH center dot center dot center dot F hydrogen bonds
KW  - Sublimation with good yield
KW  - Low voltage losses
Y1  - 2016
U6  - https://doi.org/10.1016/j.synthmet.2016.10.025
SN  - 0379-6779
VL  - 222
SP  - 232
EP  - 239
PB  - Elsevier
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Schwarze, Martin
A1  - Schellhammer, Karl Sebastian
A1  - Ortstein, Katrin
A1  - Benduhn, Johannes
A1  - Gaul, Christopher
A1  - Hinderhofer, Alexander
A1  - Perdigón-Toro, Lorena
A1  - Scholz, Reinhard
A1  - Kublitski, Jonas
A1  - Roland, Steffen
A1  - Lau, Matthias
A1  - Poelking, Carl
A1  - Andrienko, Denis
A1  - Cuniberti, Gianaurelio
A1  - Schreiber, Frank
A1  - Neher, Dieter
A1  - Vandewal, Koen
A1  - Ortmann, Frank
A1  - Leo, Karl
T1  - Impact of molecular quadrupole moments on the energy levels at organic heterojunctions
JF  - Nature Communications
N2  - The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the p-p-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor-acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.
Y1  - 2019
U6  - https://doi.org/10.1038/s41467-019-10435-2
SN  - 2041-1723
VL  - 10
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Benduhn, Johannes
A1  - Piersimoni, Fortunato
A1  - Londi, Giacomo
A1  - Kirch, Anton
A1  - Widmer, Johannes
A1  - Koerner, Christian
A1  - Beljonne, David
A1  - Neher, Dieter
A1  - Spoltore, Donato
A1  - Vandewal, Koen
T1  - Impact of triplet excited states on the open-circuit voltage of organic solar cells
JF  - dvanced energy materials
N2  - The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses.
KW  - charge-transfer states
KW  - nonradiative voltage losses
KW  - organic solar cells
KW  - triplet excited states
Y1  - 2018
U6  - https://doi.org/10.1002/aenm.201800451
SN  - 1614-6832
SN  - 1614-6840
VL  - 8
IS  - 21
PB  - Wiley-VCH
CY  - Weinheim
ER  -