TY  - JOUR
A1  - Bange, Sebastian
A1  - Schubert, Marcel
A1  - Neher, Dieter
T1  - Charge mobility determination by current extraction under linear increasing voltages : case of nonequilibrium charges and field-dependent mobilities
N2  - The method of current extraction under linear increasing voltages (CELIV) allows for the simultaneous determination of charge mobilities and charge densities directly in thin-film geometries as used in organic photovoltaic (OPV) cells. It has been specifically applied to investigate the interrelation of microstructure and charge-transport properties in such systems. Numerical and analytical calculations presented in this work show that the evaluation of CELIV transients with the commonly used analysis scheme is error prone once charge recombination and, possibly, field- dependent charge mobilities are taken into account. The most important effects are an apparent time dependence of charge mobilities and errors in the determined field dependencies. Our results implicate that reports on time-dependent mobility relaxation in OPV materials obtained by the CELIV technique should be carefully revisited and confirmed by other measurement methods.
Y1  - 2010
UR  - http://prb.aps.org/
U6  - https://doi.org/10.1103/Physrevb.81.035209
SN  - 1098-0121
ER  - 
TY  - JOUR
A1  - Blakesley, James C.
A1  - Schubert, Marcel
A1  - Steyrleuthner, Robert
A1  - Chen, Zhihua
A1  - Facchetti, Antonio
A1  - Neher, Dieter
T1  - Time-of-flight measurements and vertical transport in a high electron-mobility polymer
JF  - Applied physics letters
N2  - We investigate charge transport in a high-electron mobility polymer, poly(N, N-bis 2-octyldodecyl-naphthalene-1,4,5,8-bis dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene) [P(NDI2OD-T2), Polyera ActivInk (TM) N2200]. Time-of-flight measurements reveal electron mobilities approaching those measured in field-effect transistors, the highest ever recorded in a conjugated polymer using this technique. The modest temperature dependence and weak dispersion of the transients indicate low energetic disorder in this material. Steady-state electron-only current measurements reveal a barrier to injection of about 300 meV. We propose that this barrier is located within the P(NDI2OD-T2) film and arises from molecular orientation effects.
Y1  - 2011
U6  - https://doi.org/10.1063/1.3657827
SN  - 0003-6951
VL  - 99
IS  - 18
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Frisch, Johannes
A1  - Schubert, Marcel
A1  - Preis, Eduard
A1  - Rabe, Jürgen P.
A1  - Neher, Dieter
A1  - Scherf, Ullrich
A1  - Koch, Norbert
T1  - Full electronic structure across a polymer heterojunction solar cell
JF  - Journal of materials chemistry
N2  - We correlate the morphology and energy level alignment of bilayer structures comprising the donor poly(3-hexylthiophene) (P3HT) and the acceptor polyfluorene copolymer poly(9,90dialklylfluorene-alt-4,7-bis(2,5-thiendiyl)-2,1,3-benzothiadiazole) (PFTBTT) with the performance of these bilayers in organic photovoltaic cells (OPVCs). The conducting polymer poly(ethylenedioxythiophene): poly (styrenesulfonate) (PEDT:PSS) was used as the bottom electrode and Ca as the top electrode. Ultraviolet photoelectron spectroscopy (UPS) revealed that notable interface dipoles occur at all interfaces across the OPVC structure, highlighting that vacuum level alignment cannot reliably be used to estimate the electronic properties for device design. Particularly the effective electrode work function values (after contact formation with the conjugated polymers) differ significantly from those of the pristine electrode materials. Chemical reactions between PEDT: PSS and P3HT on the one hand and Ca and PFTBTT on the other hand are identified as cause for the measured interface dipoles. The vacuum level shift between P3HT and PFTBTT is related to mutual energy level pinning at gap states. Annealing induced morphological changes at the P3HT/PFTBTT interface increased the efficiency of OPVCs, while the electronic structure was not affected by thermal treatment.
Y1  - 2012
U6  - https://doi.org/10.1039/c1jm14968g
SN  - 0959-9428
VL  - 22
IS  - 10
SP  - 4418
EP  - 4424
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Kniepert, Juliane
A1  - Schubert, Marcel
A1  - Blakesley, James C.
A1  - Neher, Dieter
T1  - Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments
JF  - The journal of physical chemistry letters
N2  - Time-delayed collection field (TDCF) experiments are performed on bulk heterojunction solar cells comprised of a blend of poly(3-hexylthiophene) and [6,6]-phenyl C-71 butyric acid methyl ester. TDCF is analogous to a pump-probe experiment using optical excitation and an electrical probe with a resolution of < 100 ns. The number of free charge carriers extracted after a short delay is found to be independent of the electric field during illumination. Also, experiments performed with a variable delay between the optical excitation and the electrical probe do not reveal any evidence for the generation of charge via field-assisted dissociation of bound long-lived polaron pairs. Photocurrent transients are well fitted by computational drift diffusion simulations with only direct generation of free charge carriers. With increasing delay times between pump and probe, two loss mechanisms are identified; first, charge-carriers are swept out of the device by the internal electric field, and second, bimolecular recombination of the remaining carriers takes place with a reduced recombination coefficient.
Y1  - 2011
U6  - https://doi.org/10.1021/jz200155b
SN  - 1948-7185
VL  - 2
IS  - 7
SP  - 700
EP  - 705
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Roland, Steffen
A1  - Schubert, Marcel
A1  - Collins, Brian A.
A1  - Kurpiers, Jona
A1  - Chen, Zhihua
A1  - Facchetti, Antonio
A1  - Ade, Harald W.
A1  - Neher, Dieter
T1  - Fullerene-free polymer solar cells with highly reduced bimolecular recombination and field-independent charge carrier generation
JF  - The journal of physical chemistry letters
N2  - Photogeneration, recombination, and transport of free charge carriers in all-polymer bulk heterojunction solar cells incorporating poly(3-hexylthiophene) (P3HT) as donor and poly([N,N'-bis(2-octyldodecyl)-naphthelene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) (P(NDI2OD-T2)) as acceptor polymer have been investigated by the use of time delayed collection field (TDCF) and time-of-flight (TOF) measurements. Depending on the preparation procedure used to dry the active layers, these solar cells comprise high fill factors (FFs) of up to 67%. A strongly reduced bimolecular recombination (BMR), as well as a field-independent free charge carrier generation are observed, features that are common to high performance fullerene-based solar cells. Resonant soft X-ray measurements (R-SoXS) and photoluminescence quenching experiments (PQE) reveal that the BMR is related to domain purity. Our results elucidate the similarities of this polymeric acceptor with the superior recombination properties of fullerene acceptors.
Y1  - 2014
U6  - https://doi.org/10.1021/jz501506z
SN  - 1948-7185
VL  - 5
IS  - 16
SP  - 2815
EP  - 2822
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Schattauer, Sylvia
A1  - Reinhold, Beate
A1  - Albrecht, Steve
A1  - Fahrenson, Christoph
A1  - Schubert, Marcel
A1  - Janietz, Silvia
A1  - Neher, Dieter
T1  - Influence of sintering on the structural and electronic properties of TiO2 nanoporous layers prepared via a non-sol-gel approach
JF  - Colloid and polymer science : official journal of the Kolloid-Gesellschaft
N2  - In this work, a nonaqueous method is used to fabricate thin TiO2 layers. In contrast to the common aqueous sol-gel approach, our method yields layers of anatase nanocrystallites already at low temperature. Raman spectroscopy, electron microscopy and charge extraction by linearly increasing voltage are employed to study the effect of sintering temperature on the structural and electronic properties of the nanocrystalline TiO2 layer. Raising the sintering temperature from 120 to 600 A degrees C is found to alter the chemical composition, the layer's porosity and its surface but not the crystal phase. The room temperature mobility increases from 2 x 10(-6) to 3 x 10(-5) cm(2)/Vs when the sinter temperature is increased from 400 to 600 A degrees C, which is explained by a better interparticle connectivity. Solar cells comprising such nanoporous TiO2 layers and a soluble derivative of cyclohexylamino-poly(p-phenylene vinylene) were fabricated and studied with regard to their structural and photovoltaic properties. We found only weak polymer infiltration into the oxide layer for sintering temperatures up to 550 A degrees C, while the polymer penetrated deeply into titania layers that were sintered at 600 A degrees C. Best photovoltaic performance was reached with a nanoporous TiO2 film sintered at 550 A degrees C, which yielded a power conversion efficiency of 0.5 %. Noticeably, samples with the TiO2 layer dried at 120 A degrees C displayed short-circuit currents and open circuit voltages only about 15-20 % lower than for the most efficient devices, meaning that our nonaqueous route yields titania layers with reasonable transport properties even at low sintering temperatures.
KW  - Nonaqueous sol-gel
KW  - Thin nanocrystalline TiO2 layer
KW  - Solar cells
KW  - Effect of sintering
KW  - Linearly increasing voltage (CELIV)
KW  - Polymer infiltration
KW  - Transport properties titania
KW  - Transient fluorescence
Y1  - 2012
U6  - https://doi.org/10.1007/s00396-012-2708-9
SN  - 0303-402X
VL  - 290
IS  - 18
SP  - 1843
EP  - 1854
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Collins, Brian A.
A1  - Mangold, Hannah
A1  - Howard, Ian A.
A1  - Schindler, Wolfram
A1  - Vandewal, Koen
A1  - Roland, Steffen
A1  - Behrends, Jan
A1  - Kraffert, Felix
A1  - Steyrleuthner, Robert
A1  - Chen, Zhihua
A1  - Fostiropoulos, Konstantinos
A1  - Bittl, Robert
A1  - Salleo, Alberto
A1  - Facchetti, Antonio
A1  - Laquai, Frederic
A1  - Ade, Harald W.
A1  - Neher, Dieter
T1  - Correlated donor/acceptor crystal orientation controls photocurrent generation in all-polymer solar cells
JF  - Advanced functional materials
N2  - New polymers with high electron mobilities have spurred research in organic solar cells using polymeric rather than fullerene acceptors due to their potential of increased diversity, stability, and scalability. However, all-polymer solar cells have struggled to keep up with the steadily increasing power conversion efficiency of polymer: fullerene cells. The lack of knowledge about the dominant recombination process as well as the missing concluding picture on the role of the semi-crystalline microstructure of conjugated polymers in the free charge carrier generation process impede a systematic optimization of all-polymer solar cells. These issues are examined by combining structural and photo-physical characterization on a series of poly(3-hexylthiophene) (donor) and P(NDI2OD-T2) (acceptor) blend devices. These experiments reveal that geminate recombination is the major loss channel for photo-excited charge carriers. Advanced X-ray and electron-based studies reveal the effect of chloronaphthalene co-solvent in reducing domain size, altering domain purity, and reorienting the acceptor polymer crystals to be coincident with those of the donor. This reorientation correlates well with the increased photocurrent from these devices. Thus, effi cient split-up of geminate pairs at polymer/polymer interfaces may necessitate correlated donor/acceptor crystal orientation, which represents an additional requirement compared to the isotropic fullerene acceptors.
Y1  - 2014
U6  - https://doi.org/10.1002/adfm.201304216
SN  - 1616-301X
SN  - 1616-3028
VL  - 24
IS  - 26
SP  - 4068
EP  - 4081
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Dolfen, Daniel
A1  - Frisch, Johannes
A1  - Roland, Steffen
A1  - Steyrleuthner, Robert
A1  - Stiller, Burkhard
A1  - Chen, Zhihua
A1  - Scherf, Ullrich
A1  - Koch, Norbert
A1  - Facchetti, Antonio
A1  - Neher, Dieter
T1  - Influence of aggregation on the performance of All-Polymer Solar Cells containing Low-Bandgap Naphthalenediimide Copolymers
JF  - dvanced energy materials
N2  - The authors present efficient all-polymer solar cells comprising two different low-bandgap naphthalenediimide (NDI)-based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large-scale phase-separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donoracceptor heterojunction.
KW  - aggregation
KW  - morphology
KW  - naphthalenediimide
KW  - organic semiconductors
KW  - organic photovoltaics
Y1  - 2012
U6  - https://doi.org/10.1002/aenm.201100601
SN  - 1614-6832
VL  - 2
IS  - 3
SP  - 369
EP  - 380
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Frisch, Johannes
A1  - Allard, Sybille
A1  - Preis, Eduard
A1  - Scherf, Ullrich
A1  - Koch, Norbert
A1  - Neher, Dieter
T1  - Tuning side chain and main chain order in a prototypical donor-acceptor copolymer
BT  - implications for optical, electronic, and photovoltaic characteristics
JF  - Elementary Processes in Organic Photovoltaics
N2  - The recent development of donor–acceptor copolymers has led to an enormous improvement in the performance of organic solar cells and organic field-effect transistors. Here we describe the synthesis, detailed characterisation, and application of a series of structurally modified copolymers to investigate fundamental structure–property relationships in this class of conjugated polymers. The interplay between chemical structure and optoelectronic properties is investigated. These are further correlated to the charge transport and solar cell performance, which allows us to link their chemical structure to the observed physical properties.
KW  - Aggregate states
KW  - All-polymer heterojunctions
KW  - Alternating copolymers
KW  - Ambipolar charge transport
KW  - Ambipolar materials
KW  - Backbone modifications
KW  - Bilayer solar cells
KW  - Charge separation
KW  - Conformational disorder
KW  - Crystalline phases
KW  - Donor-acceptor copolymers
KW  - Electron traps
KW  - Energetic disorder
KW  - Energy-level alignment
KW  - Fermi-level alignment
KW  - Fermi-level pinning
KW  - Interface dipole
KW  - Interlayer
KW  - Intrachain order
KW  - Intragap states
KW  - Microscopic morphology
KW  - Mobility imbalance
KW  - Mobility relaxation
KW  - Monte Carlo simulation
KW  - Multiple trapping model
KW  - Nonradiative recombination
KW  - OFET
KW  - Open-circuit voltage
KW  - Optoelectronic properties
KW  - Partially alternating copolymers
KW  - Photo-CELIV
KW  - Photocurrent
KW  - Photovoltaic gap
KW  - Polymer intermixing
KW  - Recombination losses
KW  - Spectral diffusion
KW  - Statistical copolymers
KW  - Stille-type cross-coupling
KW  - Structure-property relationships
KW  - Time-dependent mobility
KW  - Time-of-flight (TOF)
KW  - Transient photocurrent
KW  - Ultraviolet photoelectron spectroscopy
KW  - Vacuum-level alignment
KW  - X-ray photoelectron spectroscopy
Y1  - 2016
SN  - 978-3-319-28338-8
SN  - 978-3-319-28336-4
U6  - https://doi.org/10.1007/978-3-319-28338-8_10
SN  - 0065-3195
VL  - 272
SP  - 243
EP  - 265
PB  - Springer
CY  - Berlin
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Preis, Eduard
A1  - Blakesley, James C.
A1  - Pingel, Patrick
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - Mobility relaxation and electron trapping in a donor/acceptor copolymer
JF  - Physical review : B, Condensed matter and materials physics
N2  - To address the nature of charge transport and the origin of severe (intrinsic) trapping in electron-transporting polymers, transient and steady-state charge transport measurements have been conducted on the prototype donor/acceptor copolymer poly[2,7-(9,9-dialkyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PFTBTT). A charge-generation layer technique is used to selectively address transport of the desired charge carrier type, to perform time-of-flight measurements on samples with < 200 nm thickness, and to combine the time-of-flight and the photocharge extraction by linearly increasing voltage (photo-CELIV) techniques to investigate charge carrier dynamics over a wide time range. Significant trapping of free electrons is observed in the bulk of dioctyl-substituted PFTBTT (alt-PF8TBTT), introducing a strong relaxation of the charge carrier mobility with time. We used Monte-Carlo simulation to simulate the measured transient data and found that all measurements can be modeled with a single parameter set, with the charge transport behavior determined by multiple trapping and detrapping of electrons in an exponential trap distribution. The influence of the concomitant mobility relaxation on the transient photocurrent characteristics in photo-CELIV experiments is discussed and shown to explain subtle features that were seen in former publications but were not yet assigned to electron trapping. Comparable studies on PFTBTT copolymers with chemical modifications of the side chains and backbone suggest that the observed electron trapping is not caused by a distinct chemical species but rather is related to interchain interactions.
Y1  - 2013
U6  - https://doi.org/10.1103/PhysRevB.87.024203
SN  - 1098-0121
VL  - 87
IS  - 2
PB  - American Physical Society
CY  - College Park
ER  -