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  - Chen, Zupeng
A1  - Savateev, Aleksandr
A1  - Pronkin, Sergey
A1  - Papaefthimiou, Vasiliki
A1  - Wolff, Christian Michael
A1  - Willinger, Marc Georg
A1  - Willinger, Elena
A1  - Neher, Dieter
A1  - Antonietti, Markus
A1  - Dontsova, Dariya
T1  - "The Easier the Better" Preparation of Efficient Photocatalysts-Metastable Poly(heptazine imide) Salts
JF  - Advanced materials
N2  - Cost-efficient, visible-light-driven hydrogen production from water is an attractive potential source of clean, sustainable fuel. Here, it is shown that thermal solid state reactions of traditional carbon nitride precursors (cyanamide, melamine) with NaCl, KCl, or CsCl are a cheap and straightforward way to prepare poly(heptazine imide) alkali metal salts, whose thermodynamic stability decreases upon the increase of the metal atom size. The chemical structure of the prepared salts is confirmed by the results of X-ray photoelectron and infrared spectroscopies, powder X-ray diffraction and electron microscopy studies, and, in the case of sodium poly(heptazine imide), additionally by atomic pair distribution function analysis and 2D powder X-ray diffraction pattern simulations. In contrast, reactions with LiCl yield thermodynamically stable poly(triazine imides). Owing to the metastability and high structural order, the obtained heptazine imide salts are found to be highly active photo-catalysts in Rhodamine B and 4-chlorophenol degradation, and Pt-assisted sacrificial water reduction reactions under visible light irradiation. The measured hydrogen evolution rates are up to four times higher than those provided by a benchmark photocatalyst, mesoporous graphitic carbon nitride. Moreover, the products are able to photocatalytically reduce water with considerable reaction rates, even when glycerol is used as a sacrificial hole scavenger.
KW  - carbon nitride
KW  - glycerol oxidation
KW  - mesocrystals
KW  - poly(heptazine imide)
KW  - water reduction reactions
Y1  - 2017
U6  - https://doi.org/10.1002/adma.201700555
SN  - 0935-9648
SN  - 1521-4095
VL  - 29
SP  - 21800
EP  - 21806
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Collado-Fregoso, Elisa
A1  - Hood, Samantha N.
A1  - Shoaee, Safa
A1  - Schröder, Bob C.
A1  - McCulloch, Iain
A1  - Kassal, Ivan
A1  - Neher, Dieter
A1  - Durrant, James R.
T1  - Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited
JF  - The journal of physical chemistry letters
N2  - In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.
Y1  - 2017
U6  - https://doi.org/10.1021/acs.jpclett.7b01571
SN  - 1948-7185
VL  - 8
SP  - 4061
EP  - 4068
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Di Pietro, Riccardo
A1  - Erdmann, Tim
A1  - Carpenter, Joshua H.
A1  - Wang, Naixiang
A1  - Shivhare, Rishi Ramdas
A1  - Formanek, Petr
A1  - Heintze, Cornelia
A1  - Voit, Brigitte
A1  - Neher, Dieter
A1  - Ade, Harald W.
A1  - Kiriy, Anton
T1  - Synthesis of High-Crystallinity DPP Polymers with Balanced Electron and Hole Mobility
JF  - Chemistry of materials : a publication of the American Chemical Society
Y1  - 2017
U6  - https://doi.org/10.1021/acs.chemmater.7b04423
SN  - 0897-4756
SN  - 1520-5002
VL  - 29
SP  - 10220
EP  - 10232
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Hofacker, Andreas
A1  - Neher, Dieter
T1  - Dispersive and steady-state recombination in organic disordered semiconductors
JF  - Physical review : B, Condensed matter and materials physics
N2  - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevB.96.245204
SN  - 2469-9950
SN  - 2469-9969
VL  - 96
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Hofacker, Andreas
A1  - Neher, Dieter
T1  - Dispersive and steady-state recombination in organic disordered semiconductors
JF  - Physical review : B, Condensed matter and materials physics
N2  - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevB.96.245204
SN  - 2469-9950
SN  - 2469-9969
VL  - 96
SP  - 5640
EP  - 5649
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Jošt, Marko
A1  - Albrecht, Steve
A1  - Kegelmann, Lukas
A1  - Wolff, Christian Michael
A1  - Lang, Felix
A1  - Lipovšek, Benjamin
A1  - Krč, Janez
A1  - Korte, Lars
A1  - Neher, Dieter
A1  - Rech, Bernd
A1  - Topič, Marko
T1  - Efficient light management by textured nanoimprinted layers for perovskite solar cells
JF  - ACS photonics
N2  - Inorganic-organic perovskites like methylammonium-lead-iodide have proven to be an effective class of 17 materials for fabricating efficient solar cells. To improve their performance, light management techniques using textured surfaces, similar to those used in established solar cell technologies, should be considered. Here, we apply a light management foil created by UV nanoimprint lithography on the glass side of an inverted (p-i-n) perovskite solar cell with 16.3% efficiency. The obtained 1 mA cm(-2) increase in the short-circuit current density translates to a relative improvement in cell performance of 5%, which results in a power conversion efficiency of 17.1%. Optical 3D simulations based on experimentally obtained parameters were used to support the experimental findings. A good match between the simulated and experimental data was obtained, validating the model. Optical simulations reveal that the main improvement in device performance is due to a reduction in total reflection and that relative improvement in the short-circuit current density of up to 10% is possible for large-area devices. Therefore, our results present the potential of light management foils for improving the device performance of perovskite solar cells and pave the way for further use of optical simulations in the field of perovskite solar cells.
KW  - perovskite solar cells
KW  - antireflection
KW  - light management
KW  - UV nanoimprint lithography
KW  - optical simulations
Y1  - 2017
U6  - https://doi.org/10.1021/acsphotonics.7b00138
SN  - 2330-4022
VL  - 4
SP  - 1232
EP  - 1239
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Kegelmann, Lukas
A1  - Wolff, Christian Michael
A1  - Awino, Celline
A1  - Lang, Felix
A1  - Unger, Eva L.
A1  - Korte, Lars
A1  - Dittrich, Thomas
A1  - Neher, Dieter
A1  - Rech, Bernd
A1  - Albrecht, Steve
T1  - It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis
JF  - ACS applied materials & interfaces
N2  - Solar cells made from inorganic organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 degrees C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C-60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop, the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.
KW  - perovskite solar cell
KW  - electron contact
KW  - double-layer
KW  - regular planar architecture
KW  - hysteresis
KW  - fullerene
KW  - metal oxide
Y1  - 2017
U6  - https://doi.org/10.1021/acsami.7b00900
SN  - 1944-8244
VL  - 9
SP  - 17246
EP  - 17256
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Kelly, Mary Allison
A1  - Roland, Steffen
A1  - Zhang, Qianqian
A1  - Lee, Youngmin
A1  - Kabius, Bernd
A1  - Wang, Qing
A1  - Gomez, Enrique D.
A1  - Neher, Dieter
A1  - You, Wei
T1  - Incorporating Fluorine Substitution into Conjugated Polymers for Solar Cells
BT  - three Different Means, Same Results
JF  - The journal of physical chemistry : C, Nanomaterials and interfaces
N2  - Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies have investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%) yet via different methods (chemically vs physically, random vs regular). We show that these three systems have equivalent photovoltaic behavior:,similar to 5.2% efficiency with a short-circuit current (J(sc)) at,similar to 11 mA cm(-2), an open-circuit voltage (v(oc)) at 0.77 V, and a fill factor (FF) of similar to 60%. Further investigation of these three systems demonstrates that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that it is possible to improve performance not only via regular or random fluorination but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance.
Y1  - 2017
U6  - https://doi.org/10.1021/acs.jpcc.6b10993
SN  - 1932-7447
VL  - 121
IS  - 4
SP  - 2059
EP  - 2068
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Love, John A.
A1  - Feuerstein, Markus
A1  - Wolff, Christian Michael
A1  - Facchetti, Antonio
A1  - Neher, Dieter
T1  - Lead Halide Perovskites as Charge Generation Layers for Electron Mobility Measurement in Organic Semiconductors
JF  - ACS applied materials & interfaces
N2  - Hybrid lead halide perovskites are introduced as charge generation layers (CGLs) for the accurate determination of electron mobilities in thin organic semiconductors. Such hybrid perovskites have become a widely studied photovoltaic material in their own right, for their high efficiencies, ease of processing from solution, strong absorption, and efficient photogeneration of charge. Time-of-flight (ToF) measurements on bilayer samples consisting of the perovskite CGL and an organic semiconductor layer of different thickness are shown to be determined by the carrier motion through the organic material, consistent with the much higher charge carrier mobility in the perovskite. Together with the efficient photon-to-electron conversion in the perovskite, this high mobility imbalance enables electron-only mobility measurement on relatively thin application-relevant organic films, which would not be possible with traditional ToF measurements. This architecture enables electron-selective mobility measurements in single components as well as bulk-heterojunction films as demonstrated in the prototypical polymer/fullerene blends. To further demonstrate the potential of this approach, electron mobilities were measured as a function of electric field and temperature in an only 127 nm thick layer of a prototypical electron-transporting perylene diimide-based polymer, and found to be consistent with an exponential trap distribution of ca. 60 meV. Our study furthermore highlights the importance of high mobility charge transporting layers when designing perovskite solar cells.
KW  - mobility
KW  - bulk heterojunction
KW  - time of flight
KW  - lead halide perovskites
KW  - charge generation layers
Y1  - 2017
U6  - https://doi.org/10.1021/acsami.7b10361
SN  - 1944-8244
VL  - 9
SP  - 42011
EP  - 42019
PB  - American Chemical Society
CY  - Washington
ER  -