TY  - JOUR
A1  - Gorenflot, Julien
A1  - Paulke, Andreas
A1  - Piersimoni, Fortunato
A1  - Wolf, Jannic
A1  - Kan, Zhipeng
A1  - Cruciani, Federico
A1  - El Labban, Abdulrahman
A1  - Neher, Dieter
A1  - Beaujuge, Pierre M.
A1  - Laquai, Frederic
T1  - From recombination dynamics to device performance
BT  - quantifying the efficiency of exciton dissociation, charge separation, and extraction in bulk heterojunction solar cells with Fluorine-Substituted polymer donors
JF  - dvanced energy materials
N2  - An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC71BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher VOC) during charge generation compared to P3HT:PCBM.
KW  - bulk heterojunction
KW  - charge generation yield
KW  - charge recombination yield
KW  - polymer solar cells
KW  - transient absorption spectroscopy
Y1  - 2018
U6  - https://doi.org/10.1002/aenm.201701678
SN  - 1614-6832
SN  - 1614-6840
VL  - 8
IS  - 4
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Benduhn, Johannes
A1  - Tvingstedt, Kristofer
A1  - Piersimoni, Fortunato
A1  - Ullbrich, Sascha
A1  - Fan, Yeli
A1  - Tropiano, Manuel
A1  - McGarry, Kathryn A.
A1  - Zeika, Olaf
A1  - Riede, Moritz K.
A1  - Douglas, Christopher J.
A1  - Barlow, Stephen
A1  - Marder, Seth R.
A1  - Neher, Dieter
A1  - Spoltore, Donato
A1  - Vandewal, Koen
T1  - Intrinsic non-radiative voltage losses in fullerene-based organic solar cells
JF  - Nature Energy
N2  - Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single-junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to (1.45-1.65) eV, that is, (0.2-0.3) eV higher than for technologies with minimized non-radiative voltage losses.
Y1  - 2017
U6  - https://doi.org/10.1038/nenergy.2017.53
SN  - 2058-7546
VL  - 2
PB  - Nature Publ. Group
CY  - London
ER  - 
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  - 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  - 
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  - Cardinaletti, Ilaria
A1  - Kesters, Jurgen
A1  - Bertho, Sabine
A1  - Conings, Bert
A1  - Piersimoni, Fortunato
A1  - Lutsen, Laurence
A1  - Nesladek, Milos
A1  - Van Mele, Bruno
A1  - Van Assche, Guy
A1  - Vandewal, Koen
A1  - Salleo, Alberto
A1  - Vanderzande, Dirk
A1  - Maes, Wouter
A1  - Manca, Jean V.
T1  - Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology
JF  - Journal of photonics for energy
N2  - When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.
KW  - organic photovoltaics
KW  - bulk heterojunction
KW  - thermal stability
KW  - phase separation
KW  - lifetime
Y1  - 2014
U6  - https://doi.org/10.1117/1.JPE.4.040997
SN  - 1947-7988
VL  - 4
PB  - SPIE
CY  - Bellingham
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  - Nikolis, Vasileios C.
A1  - Benduhn, Johannes
A1  - Holzmueller, Felix
A1  - Piersimoni, Fortunato
A1  - Lau, Matthias
A1  - Zeika, Olaf
A1  - Neher, Dieter
A1  - Koerner, Christian
A1  - Spoltore, Donato
A1  - Vandewal, Koen
T1  - Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies
JF  - dvanced energy materials
N2  - High photon energy losses limit the open-circuit voltage (V-OC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the V-OC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (alpha-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the V-OC of an alpha-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D-A interface. By accurately measuring the optical gap (E-opt) and the energy of the charge-transfer state (E-CT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. E-opt - qV(OC) losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the V-OC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices.
KW  - energy losses
KW  - nonradiative recombination
KW  - open-circuit voltage
KW  - organic solar cells
KW  - voltage losses
Y1  - 2017
U6  - https://doi.org/10.1002/aenm.201700855
SN  - 1614-6832
SN  - 1614-6840
VL  - 7
SP  - 122
EP  - 136
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Xu, Jingsan
A1  - Shalom, Menny
A1  - Piersimoni, Fortunato
A1  - Antonietti, Markus
A1  - Neher, Dieter
A1  - Brenner, Thomas J. K.
T1  - Color-Tunable Photoluminescence and NIR Electroluminescence in Carbon Nitride Thin Films and Light-Emitting Diodes
JF  - Advanced optical materials
Y1  - 2015
U6  - https://doi.org/10.1002/adom.201500019
SN  - 2195-1071
VL  - 3
IS  - 7
SP  - 913
EP  - 917
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Poelking, Carl
A1  - Benduhn, Johannes
A1  - Spoltore, Donato
A1  - Schwarze, Martin
A1  - Roland, Steffen
A1  - Piersimoni, Fortunato
A1  - Neher, Dieter
A1  - Leo, Karl
A1  - Vandewal, Koen
A1  - Andrienko, Denis
T1  - Open-circuit voltage of organic solar cells
BT  - interfacial roughness makes the difference
JF  - Communications physics
N2  - Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface. 
Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology.
Y1  - 2022
U6  - https://doi.org/10.1038/s42005-022-01084-x
SN  - 2399-3650
VL  - 5
IS  - 1
PB  - Nature portfolio
CY  - Berlin
ER  - 
TY  - JOUR
A1  - Lange, Ilja
A1  - Reiter, Sina
A1  - Kniepert, Juliane
A1  - Piersimoni, Fortunato
A1  - Paetzel, Michael
A1  - Hildebrandt, Jana
A1  - Brenner, Thomas J. K.
A1  - Hecht, Stefan
A1  - Neher, Dieter
T1  - Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures
JF  - Applied physics letters
N2  - An approach is presented to modify the work function of solution-processed sol-gel derived zinc oxide (ZnO) over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using poly(3-hexylthiophene): phenyl-C71-butyric acid methyl ester as the active layer. These devices compete with or even surpass the performance of the reference on indium tin oxide/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Our findings highlight the potential of properly modified ZnO as electron or hole extracting electrodes in hybrid optoelectronic devices. (C) 2015 AIP Publishing LLC.
Y1  - 2015
U6  - https://doi.org/10.1063/1.4916182
SN  - 0003-6951
SN  - 1077-3118
VL  - 106
IS  - 11
PB  - American Institute of Physics
CY  - Melville
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  - Hörmann, Ulrich
A1  - Zeiske, Stefan
A1  - Piersimoni, Fortunato
A1  - Hoffmann, Lukas
A1  - Schlesinger, Raphael
A1  - Koch, Norbert
A1  - Riedl, Thomas
A1  - Andrienko, Denis
A1  - Neher, Dieter
T1  - Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces
JF  - Physical review : B, Condensed matter and materials physics
N2  - We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.
Y1  - 2018
U6  - https://doi.org/10.1103/PhysRevB.98.155312
SN  - 2469-9950
SN  - 2469-9969
VL  - 98
IS  - 15
PB  - American Physical Society
CY  - College Park
ER  -