@article{StolterfohtArminPhilippaetal.2016, author = {Stolterfoht, Martin and Armin, Ardalan and Philippa, Bronson and Neher, Dieter}, title = {The Role of Space Charge Effects on the Competition between Recombination and Extraction in Solar Cells with Low-Mobility Photoactive Layers}, series = {The journal of physical chemistry letters}, volume = {7}, journal = {The journal of physical chemistry letters}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.6b02106}, pages = {4716 -- 4721}, year = {2016}, abstract = {The competition between charge extraction and nongeminate recombination critically determines the current-voltage characteristics of organic solar cells (OSCs) and their fill factor. As a measure of this competition, several figures of merit (FOMs) have been put forward; however, the impact of space charge effects has been either neglected, or not specifically addressed. Here we revisit recently reported FOMs and discuss the role of space charge effects on the interplay between recombination and extraction. We find that space charge effects are the primary cause for the onset of recombination in so-called non-Langevin systems, which also depends on the slower carrier mobility and recombination coefficient. The conclusions are supported with numerical calculations and experimental results of 25 different donor/acceptor OSCs with different charge transport parameters, active layer thicknesses or composition ratios. The findings represent a conclusive understanding of bimolecular recombination for drift dominated photocurrents and allow one to minimize these losses for given device parameters.}, language = {en} } @article{SaphiannikovaNeher2005, author = {Saphiannikova, Marina and Neher, Dieter}, title = {Thermodynamic theory of light-induced material transport in amorphous azobenzene polymer films}, issn = {1520-6106}, year = {2005}, abstract = {It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements}, language = {en} } @article{BlakesleySchubertSteyrleuthneretal.2011, author = {Blakesley, James C. and Schubert, Marcel and Steyrleuthner, Robert and Chen, Zhihua and Facchetti, Antonio and Neher, Dieter}, title = {Time-of-flight measurements and vertical transport in a high electron-mobility polymer}, series = {Applied physics letters}, volume = {99}, journal = {Applied physics letters}, number = {18}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.3657827}, pages = {3}, year = {2011}, abstract = {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.}, language = {en} } @article{YinSchubertStilleretal.2008, author = {Yin, Chunhong and Schubert, Marcel and Stiller, Burkhard and Castellani, Mauro and Neher, Dieter and Kumke, Michael Uwe and H{\"o}rhold, Hans-Heinrich}, title = {Tuning of the excited-state properties and photovoltaic performance in PPV-based polymer blends}, doi = {10.1021/Jp803977k}, year = {2008}, language = {en} } @article{SchubertFrischAllardetal.2017, author = {Schubert, Marcel and Frisch, Johannes and Allard, Sybille and Preis, Eduard and Scherf, Ullrich and Koch, Norbert and Neher, Dieter}, title = {Tuning side chain and main chain order in a prototypical donor-acceptor copolymer}, series = {Elementary Processes in Organic Photovoltaics}, volume = {272}, journal = {Elementary Processes in Organic Photovoltaics}, publisher = {Springer}, address = {Berlin}, isbn = {978-3-319-28338-8}, issn = {0065-3195}, doi = {10.1007/978-3-319-28338-8_10}, pages = {243 -- 265}, year = {2017}, abstract = {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.}, language = {en} } @article{LangeReiterPaetzeletal.2014, author = {Lange, Ilja and Reiter, Sina and Paetzel, Michael and Zykov, Anton and Nefedov, Alexei and Hildebrandt, Jana and Hecht, Stefan and Kowarik, Stefan and Woell, Christof and Heimel, Georg and Neher, Dieter}, title = {Tuning the work function of polar zinc oxide surfaces using modified phosphonic acid self-assembled monolayers}, series = {Advanced functional materials}, volume = {24}, journal = {Advanced functional materials}, number = {44}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201401493}, pages = {7014 -- 7024}, year = {2014}, abstract = {Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self-assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole-injecting and electron-injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well-defined molecular monolayers.}, language = {en} } @article{PerdigonToroLeQuangPhuongElleretal.2022, author = {Perdigon-Toro, Lorena and Le Quang Phuong, and Eller, Fabian and Freychet, Guillaume and Saglamkaya, Elifnaz and Khan, Jafar and Wei, Qingya and Zeiske, Stefan and Kroh, Daniel and Wedler, Stefan and Koehler, Anna and Armin, Ardalan and Laquai, Frederic and Herzig, Eva M. and Zou, Yingping and Shoaee, Safa and Neher, Dieter}, title = {Understanding the role of order in Y-series non-fullerene solar cells to realize high open-circuit voltages}, series = {Advanced energy materials}, volume = {12}, journal = {Advanced energy materials}, number = {12}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.202103422}, pages = {13}, year = {2022}, abstract = {Non-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing temperature-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage V-OC on temperature and illumination intensity. Disorder is found to influence the value of the V-OC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend.}, language = {en} } @article{PingelZenNeheretal.2009, author = {Pingel, Patrick and Zen, Achmad and Neher, Dieter and Lieberwirth, Ingo and Wegner, Gerhard and Allard, Sybille and Scherf, Ullrich}, title = {Unexpectedly high field-effect mobility of a soluble, low molecular weight oligoquaterthiophene fraction with low polydispersity}, issn = {0947-8396}, doi = {10.1007/s00339-008-4994-0}, year = {2009}, abstract = {Layers made from soluble low molecular weight polythiophene PQT-12 with low polydispersity exhibit a highly ordered structure and charge-carrier mobilities of the order of 10(-3) cm(2)/(V s), which we attribute to its proximity to monodispersity. We propose that polydispersity is a decisive factor with regard to structure formation and transport properties of soluble low molecular weight polythiophenes.}, language = {en} } @article{ZuWolffRalaiarisoaetal.2019, author = {Zu, Fengshuo and Wolff, Christian Michael and Ralaiarisoa, Maryline and Amsalem, Patrick and Neher, Dieter and Koch, Norbert}, title = {Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies}, series = {ACS applied materials \& interfaces}, volume = {11}, journal = {ACS applied materials \& interfaces}, number = {24}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.9b05293}, pages = {21578 -- 21583}, year = {2019}, abstract = {The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites' photophysical properties.}, language = {en} } @article{XuBrennerChenetal.2014, author = {Xu, Jingsan and Brenner, Thomas J. K. and Chen, Zupeng and Neher, Dieter and Antonietti, Markus and Shalom, Menny}, title = {Upconversion-agent induced improvement of g-C3N4 photocatalyst under visible light}, series = {ACS applied materials \& interfaces}, volume = {6}, journal = {ACS applied materials \& interfaces}, number = {19}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/am5051263}, pages = {16481 -- 16486}, year = {2014}, abstract = {Herein, we report the use of upconversion agents to modify graphite carbon nitride (g-C3N4) by direct thermal condensation of a mixture of ErCl3 center dot 6H(2)O and the supramolecular precursor cyanuric acid-melamine. We show the enhancement of g-C3N4 photoactivity after Er3+ doping by monitoring the photodegradation of Rhodamine B dye under visible light. The contribution of the upconversion agent is demonstrated by measurements using only a red laser. The Er3+ doping alters both the electronic and the chemical properties of g-C3N4. The Er3+ doping reduces emission intensity and lifetime, indicating the formation of new, nonradiative deactivation pathways, probably involving charge-transfer processes.}, language = {en} }