State-of-the-art methods in materials science such as artificial intelligence and data-driven techniques advance the investigation of photovoltaic materials.
}}, author = {{Mirhosseini, Hossein and Kormath Madam Raghupathy, Ramya and Sahoo, Sudhir K. and Wiebeler, Hendrik and Chugh, Manjusha and Kühne, Thomas D.}}, issn = {{1463-9076}}, journal = {{Physical Chemistry Chemical Physics}}, keywords = {{Physical and Theoretical Chemistry, General Physics and Astronomy}}, number = {{46}}, pages = {{26682--26701}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{In silico investigation of Cu(In,Ga)Se2-based solar cells}}}, doi = {{10.1039/d0cp04712k}}, volume = {{22}}, year = {{2020}}, } @unpublished{32242, abstract = {{We consider a resource-aware variant of the classical multi-armed bandit problem: In each round, the learner selects an arm and determines a resource limit. It then observes a corresponding (random) reward, provided the (random) amount of consumed resources remains below the limit. Otherwise, the observation is censored, i.e., no reward is obtained. For this problem setting, we introduce a measure of regret, which incorporates the actual amount of allocated resources of each learning round as well as the optimality of realizable rewards. Thus, to minimize regret, the learner needs to set a resource limit and choose an arm in such a way that the chance to realize a high reward within the predefined resource limit is high, while the resource limit itself should be kept as low as possible. We derive the theoretical lower bound on the cumulative regret and propose a learning algorithm having a regret upper bound that matches the lower bound. In a simulation study, we show that our learning algorithm outperforms straightforward extensions of standard multi-armed bandit algorithms.}}, author = {{Bengs, Viktor and Hüllermeier, Eyke}}, booktitle = {{arXiv:2011.00813}}, title = {{{Multi-Armed Bandits with Censored Consumption of Resources}}}, year = {{2020}}, } @article{19844, abstract = {{The defect-electronic properties of {112} microfaceted surfaces of epitaxially grown CuInSe2 thin films are investigated by scanning tunneling spectroscopy and photoelectron spectroscopy techniques after various surface treatments. The intrinsic CuInSe2 surface is found to be largely passivated in terms of electronic defect levels in the band-gap region. However, surface oxidation leads to an overall high density of defect levels in conjunction with a considerable net surface dipole, which persists even after oxide removal. Yet, a subsequent annealing under vacuum restores the initial condition. Such oxidation/reduction cycles are reversible for many times providing robust control of the surface and interface properties in these materials. Based on ab initio simulations, a mechanism where oxygen dissociatively adsorbs and subsequently diffuses to a subsurface site is proposed as the initial step of the observed dipole formation. Our results emphasize the relevance of oxidation-induced dipole effects at the thin film surface and provide a comprehensive understanding toward passivation strategies of these surfaces.}}, author = {{Elizabeth, Amala and Sahoo, Sudhir K. and Lockhorn, David and Timmer, Alexander and Aghdassi, Nabi and Zacharias, Helmut and Kühne, Thomas and Siebentritt, Susanne and Mirhosseini, Hossein and Mönig, Harry}}, journal = {{Phys. Rev. Materials}}, pages = {{063401}}, publisher = {{American Physical Society}}, title = {{{ Oxidation/reduction cycles and their reversible effect on the dipole formation at CuInSe2 surfaces}}}, doi = {{10.1103/PhysRevMaterials.4.063401}}, volume = {{4}}, year = {{2020}}, } @article{21112, abstract = {{Photovoltaics is one of the most promising and fastest-growing renewable energy technologies. Although the price-performance ratio of solar cells has improved significantly over recent years{,} further systematic investigations are needed to achieve higher performance and lower cost for future solar cells. In conjunction with experiments{,} computer simulations are powerful tools to investigate the thermodynamics and kinetics of solar cells. Over the last few years{,} we have developed and employed advanced computational techniques to gain a better understanding of solar cells based on copper indium gallium selenide (Cu(In{,}Ga)Se2). Furthermore{,} we have utilized state-of-the-art data-driven science and machine learning for the development of photovoltaic materials. In this Perspective{,} we review our results along with a survey of the field.}}, author = {{Mirhosseini, S. Hossein and Kormath Madam Raghupathy, Ramya and Sahoo, Sudhir K. and Wiebeler, Hendrik and Chugh, Manjusha and Kühne, Thomas}}, journal = {{Phys. Chem. Chem. Phys.}}, pages = {{26682--26701}}, publisher = {{The Royal Society of Chemistry}}, title = {{{In silico investigation of Cu(In,Ga)Se2-based solar cells}}}, doi = {{10.1039/D0CP04712K}}, volume = {{22}}, year = {{2020}}, } @article{21240, abstract = {{Rechargeable aqueous Zn-ion energy storage devices are promising candidates for next-generation energy storage technologies. However, the lack of highly reversible Zn2+-storage anode materials with low potential windows remains a primary concern. Here, we report a two-dimensional polyarylimide covalent organic framework (PI-COF) anode with high-kinetics Zn2+-storage capability. The well-organized pore channels of PI-COF allow the high accessibility of the build-in redox-active carbonyl groups and efficient ion diffusion with a low energy barrier. The constructed PI-COF anode exhibits a specific capacity (332 C g–1 or 92 mAh g–1 at 0.7 A g–1), a high rate capability (79.8% at 7 A g–1), and a long cycle life (85% over 4000 cycles). In situ Raman investigation and first-principle calculations clarify the two-step Zn2+-storage mechanism, in which imide carbonyl groups reversibly form negatively charged enolates. Dendrite-free full Zn-ion devices are fabricated by coupling PI-COF anodes with MnO2 cathodes, delivering excellent energy densities (23.9 ∼ 66.5 Wh kg–1) and supercapacitor-level power densities (133 ∼ 4782 W kg–1). This study demonstrates the feasibility of covalent organic framework as Zn2+-storage anodes and shows a promising prospect for constructing reliable aqueous energy storage devices.}}, author = {{Yu, Minghao and Chandrasekhar, Naisa and Kormath Madam Raghupathy, Ramya and Ly, Khoa Hoang and Zhang, Haozhe and Dmitrieva, Evgenia and Liang, Chaolun and Lu, Xihong and Kühne, Thomas and Mirhosseini, S. Hossein and Weidinger, Inez M. and Feng, Xinliang}}, issn = {{0002-7863}}, journal = {{Journal of the American Chemical Society}}, number = {{46}}, pages = {{19570--19578}}, publisher = {{American Chemical Society}}, title = {{{A High-Rate Two-Dimensional Polyarylimide Covalent Organic Framework Anode for Aqueous Zn-Ion Energy Storage Devices}}}, doi = {{10.1021/jacs.0c07992}}, volume = {{142}}, year = {{2020}}, } @article{17374, abstract = {{Lead halide perovskite semiconductors providing record efficiencies of solar cells have usually mixed compositions doped in A- and X-sites to enhance the phase stability. The cubic form of formamidinium (FA) lead iodide reveals excellent opto-electronic properties but transforms at room temperature (RT) into a hexagonal structure which does not effectively absorb visible light. This metastable form and the mechanism of its stabilization by Cs+ and Br− incorporation are poorly characterized and insufficiently understood. We report here the vibrational properties of cubic FAPbI3 investigated by DFT calculations on phonon frequencies and intensities, and micro-Raman spectroscopy. The effects of Cs+ and Br− partial substitution are discussed. We support our results with the study of FAPbBr3 which expands the identification of vibrational modes to the previously unpublished low frequency region (<500 cm−1). Our results show that the incorporation of Cs+ and Br− leads to the coupling of the displacement of the A-site components and weakens the bonds between FA+ and the PbX6 octahedra. We suggest that the enhancement of α-FAPbI3 stability can be a product of the release of tensile stresses in the Pb–X bond, which is reflected in a red-shift of the low frequency region of the Raman spectrum (<200 cm−1).}}, author = {{Ibaceta-Jaña, Josefa and Muydinov, Ruslan and Rosado, Pamela and Mirhosseini, Hossein and Chugh, Manjusha and Nazarenko, Olga and Dirin, Dmitry N. and Heinrich, Dirk and Wagner, Markus R. and Kühne, Thomas and Szyszka, Bernd and Kovalenko, Maksym V. and Hoffmann, Axel}}, journal = {{Phys. Chem. Chem. Phys.}}, pages = {{5604--5614}}, publisher = {{The Royal Society of Chemistry}}, title = {{{Vibrational dynamics in lead halide hybrid perovskites investigated by Raman spectroscopy}}}, doi = {{10.1039/C9CP06568G}}, volume = {{22}}, year = {{2020}}, } @article{17376, abstract = {{The record conversion efficiency of thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorbers has exceeded 23%. Such a high performance is currently only attainable by the incorporation of heavy alkali metals like Cs into the absorber through an alkali fluoride post-deposition treatment (PDT). As the effect of the incorporated heavy alkali metals is under discussion, we investigated the local composition and microstructure of high efficiency CIGS solar cells via various high-resolution techniques in a combinatory approach. An accumulation of Cs is clearly detected at the p-n junction along with variations in the local CIGS composition, showing the formation of a beneficial secondary phase with a laterally inhomogeneous distribution. Additionally, Cs accumulations were detected at grain boundaries with a random misorientation of the adjacent grains where a reduced Cu concentration and increased In and Se concentrations are detected. No accumulation was found at Σ3 twin boundaries as well as the grain interior. These experimental findings are in excellent agreement with complementary ab-initio calculations, demonstrating that the grain boundaries are passivated by the presence of Cs. Further, it is unlikely that Cs with its large ionic radius is incorporated into the CIGS grains where it would cause detrimental defects.}}, author = {{Schöppe, Philipp and Schönherr, Sven and Chugh, Manjusha and Mirhosseini, Hossein and Jackson, Philip and Wuerz, Roland and Ritzer, Maurizio and Johannes, Andreas and Martínez-Criado, Gema and Wisniewski, Wolfgang and Schwarz, Torsten and T. Plass, Christian and Hafermann, Martin and Kühne, Thomas and S. Schnohr, Claudia and Ronning, Carsten}}, issn = {{2211-2855}}, journal = {{Nano Energy}}, pages = {{104622}}, title = {{{Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells}}}, doi = {{https://doi.org/10.1016/j.nanoen.2020.104622}}, volume = {{71}}, year = {{2020}}, } @inproceedings{20504, abstract = {{In recent years time domain speech separation has excelled over frequency domain separation in single channel scenarios and noise-free environments. In this paper we dissect the gains of the time-domain audio separation network (TasNet) approach by gradually replacing components of an utterance-level permutation invariant training (u-PIT) based separation system in the frequency domain until the TasNet system is reached, thus blending components of frequency domain approaches with those of time domain approaches. Some of the intermediate variants achieve comparable signal-to-distortion ratio (SDR) gains to TasNet, but retain the advantage of frequency domain processing: compatibility with classic signal processing tools such as frequency-domain beamforming and the human interpretability of the masks. Furthermore, we show that the scale invariant signal-to-distortion ratio (si-SDR) criterion used as loss function in TasNet is related to a logarithmic mean square error criterion and that it is this criterion which contributes most reliable to the performance advantage of TasNet. Finally, we critically assess which gains in a noise-free single channel environment generalize to more realistic reverberant conditions.}}, author = {{Heitkaemper, Jens and Jakobeit, Darius and Boeddeker, Christoph and Drude, Lukas and Haeb-Umbach, Reinhold}}, booktitle = {{ICASSP 2020 Virtual Barcelona Spain}}, keywords = {{voice activity detection, speech activity detection, neural network, statistical speech processing}}, title = {{{Demystifying TasNet: A Dissecting Approach}}}, year = {{2020}}, } @inproceedings{20141, author = {{Heindorf, Stefan and Scholten, Yan and Wachsmuth, Henning and Ngonga Ngomo, Axel-Cyrille and Potthast, Martin}}, booktitle = {{Proceedings of the 28th ACM International Conference on Information and Knowledge Management (CIKM 2020)}}, pages = {{3023--3030}}, title = {{{CauseNet: Towards a Causality Graph Extracted from the Web}}}, doi = {{10.1145/3340531.3412763}}, year = {{2020}}, } @inproceedings{20505, abstract = {{Speech activity detection (SAD), which often rests on the fact that the noise is "more'' stationary than speech, is particularly challenging in non-stationary environments, because the time variance of the acoustic scene makes it difficult to discriminate speech from noise. We propose two approaches to SAD, where one is based on statistical signal processing, while the other utilizes neural networks. The former employs sophisticated signal processing to track the noise and speech energies and is meant to support the case for a resource efficient, unsupervised signal processing approach. The latter introduces a recurrent network layer that operates on short segments of the input speech to do temporal smoothing in the presence of non-stationary noise. The systems are tested on the Fearless Steps challenge database, which consists of the transmission data from the Apollo-11 space mission. The statistical SAD achieves comparable detection performance to earlier proposed neural network based SADs, while the neural network based approach leads to a decision cost function of 1.07% on the evaluation set of the 2020 Fearless Steps Challenge, which sets a new state of the art.}}, author = {{Heitkaemper, Jens and Schmalenstroeer, Joerg and Haeb-Umbach, Reinhold}}, booktitle = {{INTERSPEECH 2020 Virtual Shanghai China}}, keywords = {{voice activity detection, speech activity detection, neural network, statistical speech processing}}, title = {{{Statistical and Neural Network Based Speech Activity Detection in Non-Stationary Acoustic Environments}}}, year = {{2020}}, } @inproceedings{20762, abstract = {{The rising interest in single-channel multi-speaker speech separation sparked development of End-to-End (E2E) approaches to multispeaker speech recognition. However, up until now, state-of-theart neural network–based time domain source separation has not yet been combined with E2E speech recognition. We here demonstrate how to combine a separation module based on a Convolutional Time domain Audio Separation Network (Conv-TasNet) with an E2E speech recognizer and how to train such a model jointly by distributing it over multiple GPUs or by approximating truncated back-propagation for the convolutional front-end. To put this work into perspective and illustrate the complexity of the design space, we provide a compact overview of single-channel multi-speaker recognition systems. Our experiments show a word error rate of 11.0% on WSJ0-2mix and indicate that our joint time domain model can yield substantial improvements over cascade DNN-HMM and monolithic E2E frequency domain systems proposed so far.}}, author = {{von Neumann, Thilo and Kinoshita, Keisuke and Drude, Lukas and Boeddeker, Christoph and Delcroix, Marc and Nakatani, Tomohiro and Haeb-Umbach, Reinhold}}, booktitle = {{ICASSP 2020 - 2020 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP)}}, pages = {{7004--7008}}, title = {{{End-to-End Training of Time Domain Audio Separation and Recognition}}}, doi = {{10.1109/ICASSP40776.2020.9053461}}, year = {{2020}}, } @inproceedings{20764, abstract = {{Most approaches to multi-talker overlapped speech separation and recognition assume that the number of simultaneously active speakers is given, but in realistic situations, it is typically unknown. To cope with this, we extend an iterative speech extraction system with mechanisms to count the number of sources and combine it with a single-talker speech recognizer to form the first end-to-end multi-talker automatic speech recognition system for an unknown number of active speakers. Our experiments show very promising performance in counting accuracy, source separation and speech recognition on simulated clean mixtures from WSJ0-2mix and WSJ0-3mix. Among others, we set a new state-of-the-art word error rate on the WSJ0-2mix database. Furthermore, our system generalizes well to a larger number of speakers than it ever saw during training, as shown in experiments with the WSJ0-4mix database. }}, author = {{von Neumann, Thilo and Boeddeker, Christoph and Drude, Lukas and Kinoshita, Keisuke and Delcroix, Marc and Nakatani, Tomohiro and Haeb-Umbach, Reinhold}}, booktitle = {{Proc. Interspeech 2020}}, pages = {{3097--3101}}, title = {{{Multi-Talker ASR for an Unknown Number of Sources: Joint Training of Source Counting, Separation and ASR}}}, doi = {{10.21437/Interspeech.2020-2519}}, year = {{2020}}, } @inproceedings{20753, abstract = {{In this paper we present our system for the detection and classification of acoustic scenes and events (DCASE) 2020 Challenge Task 4: Sound event detection and separation in domestic environments. We introduce two new models: the forward-backward convolutional recurrent neural network (FBCRNN) and the tag-conditioned convolutional neural network (CNN). The FBCRNN employs two recurrent neural network (RNN) classifiers sharing the same CNN for preprocessing. With one RNN processing a recording in forward direction and the other in backward direction, the two networks are trained to jointly predict audio tags, i.e., weak labels, at each time step within a recording, given that at each time step they have jointly processed the whole recording. The proposed training encourages the classifiers to tag events as soon as possible. Therefore, after training, the networks can be applied to shorter audio segments of, e.g., 200ms, allowing sound event detection (SED). Further, we propose a tag-conditioned CNN to complement SED. It is trained to predict strong labels while using (predicted) tags, i.e., weak labels, as additional input. For training pseudo strong labels from a FBCRNN ensemble are used. The presented system scored the fourth and third place in the systems and teams rankings, respectively. Subsequent improvements allow our system to even outperform the challenge baseline and winner systems in average by, respectively, 18.0% and 2.2% event-based F1-score on the validation set. Source code is publicly available at https://github.com/fgnt/pb_sed.}}, author = {{Ebbers, Janek and Haeb-Umbach, Reinhold}}, booktitle = {{Proceedings of the Detection and Classification of Acoustic Scenes and Events 2020 Workshop (DCASE2020)}}, title = {{{Forward-Backward Convolutional Recurrent Neural Networks and Tag-Conditioned Convolutional Neural Networks for Weakly Labeled Semi-Supervised Sound Event Detection}}}, year = {{2020}}, } @article{16311, author = {{Burkhardt, Lukas and Vukadinovic, Yannik and Nowakowski, Michał and Kalinko, Aleksandr and Rudolph, Julian and Carlsson, Per-Anders and Jacob, Christoph R. and Bauer, Matthias}}, issn = {{0020-1669}}, journal = {{Inorganic Chemistry}}, pages = {{3551--3561}}, title = {{{Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]− Revisited by X-ray Emission and Absorption Spectroscopy}}}, doi = {{10.1021/acs.inorgchem.9b02092}}, year = {{2020}}, } @article{17322, author = {{Mukherjee, Amlan and Widhalm, Alex and Siebert, Dustin and Krehs, Sebastian and Sharma, Nandlal and Thiede, Andreas and Reuter, Dirk and Förstner, Jens and Zrenner, Artur}}, issn = {{0003-6951}}, journal = {{Applied Physics Letters}}, keywords = {{tet_topic_qd}}, pages = {{251103}}, title = {{{Electrically controlled rapid adiabatic passage in a single quantum dot}}}, doi = {{10.1063/5.0012257}}, volume = {{116}}, year = {{2020}}, } @article{40233, author = {{Meier, Lukas and Braun, Christian and Hannappel, Thomas and Schmidt, Wolf Gero}}, issn = {{0370-1972}}, journal = {{physica status solidi (b)}}, keywords = {{Condensed Matter Physics, Electronic, Optical and Magnetic Materials}}, number = {{2}}, publisher = {{Wiley}}, title = {{{Band Alignment at Ga x In 1– x P/Al y In 1– y P Alloy Interfaces from Hybrid Density Functional Theory Calculations}}}, doi = {{10.1002/pssb.202000463}}, volume = {{258}}, year = {{2020}}, } @article{17067, author = {{Speiser, Eugen and Esser, Norbert and Halbig, Benedikt and Geurts, Jean and Schmidt, Wolf Gero and Sanna, Simone}}, issn = {{0167-5729}}, journal = {{Surface Science Reports}}, number = {{1}}, title = {{{Vibrational Raman spectroscopy on adsorbate-induced low-dimensional surface structures}}}, doi = {{10.1016/j.surfrep.2020.100480}}, volume = {{75}}, year = {{2020}}, } @article{40435, abstract = {{Coulomb binding energy is reduced when a few-molecule integer charge transfer complex (ICTC) is formed.
}}, author = {{Dong, Chuan-Ding and Schumacher, Stefan}}, issn = {{2050-7526}}, journal = {{Journal of Materials Chemistry C}}, keywords = {{Materials Chemistry, General Chemistry}}, number = {{34}}, pages = {{11929--11935}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{Molecular doping in few-molecule polymer-dopant complexes shows reduced Coulomb binding}}}, doi = {{10.1039/d0tc02185g}}, volume = {{8}}, year = {{2020}}, } @article{19190, abstract = {{Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In this work, we use density-functional theory and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate (LiNbO3). Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at Nb_Li antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at Nb_Li antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models, and on the calculated charge-transition levels and potential-energy barriers, we propose two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of bipolarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.}}, author = {{Schmidt, Falko and Kozub, Agnieszka L. and Biktagirov, Timur and Eigner, Christof and Silberhorn, Christine and Schindlmayr, Arno and Schmidt, Wolf Gero and Gerstmann, Uwe}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{4}}, publisher = {{American Physical Society}}, title = {{{Free and defect-bound (bi)polarons in LiNbO3: Atomic structure and spectroscopic signatures from ab initio calculations}}}, doi = {{10.1103/PhysRevResearch.2.043002}}, volume = {{2}}, year = {{2020}}, }