TY  - JOUR
A1  - Shoaee, Safa
A1  - Armin, Ardalan
A1  - Stolterfoht, Martin
A1  - Hosseini, Seyed Mehrdad
A1  - Kurpiers, Jona
A1  - Neher, Dieter
T1  - Decoding Charge Recombination through Charge Generation in Organic Solar Cells
JF  - Solar RRL
N2  - The in-depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter-related via the kinetics of charge transfer states-being singlet or triplet states. Although high-charge-photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low-mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta-analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field-dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a "spin-related factor" that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge-generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin-related factor.
KW  - charge generation
KW  - charge transfers
KW  - non-Langevin recombination
KW  - spin-related factors
Y1  - 2019
U6  - https://doi.org/10.1002/solr.201900184
SN  - 2367-198X
VL  - 3
IS  - 11
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Krückemeier, Lisa
A1  - Rau, Uwe
A1  - Stolterfoht, Martin
A1  - Kirchartz, Thomas
T1  - How to report record open-circuit voltages in lead-halide perovskite solar cells
JF  - Advanced energy materials
N2  - Open-circuit voltages of lead-halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open-circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open-circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta-analysis of methods to determine the bandgap and a radiative limit for open-circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.
KW  - bandgap
KW  - fill factor losses
KW  - nonradiative voltage losses
KW  - photovoltaics
KW  - radiative limit
KW  - recombination
KW  - Shockley-Queisser model
Y1  - 2019
U6  - https://doi.org/10.1002/aenm.201902573
SN  - 1614-6832
SN  - 1614-6840
VL  - 10
IS  - 1
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Tockhorn, Philipp
A1  - Sutter, Johannes
A1  - Cruz Bournazou, Alexandros
A1  - Wagner, Philipp
A1  - Jäger, Klaus
A1  - Yoo, Danbi
A1  - Lang, Felix
A1  - Grischek, Max
A1  - Li, Bor
A1  - Li, Jinzhao
A1  - Shargaieva, Oleksandra
A1  - Unger, Eva
A1  - Al-Ashouri, Amran
A1  - Köhnen, Eike
A1  - Stolterfoht, Martin
A1  - Neher, Dieter
A1  - Schlatmann, Rutger
A1  - Rech, Bernd
A1  - Stannowski, Bernd
A1  - Albrecht, Steve
A1  - Becker, Christiane
T1  - Nano-optical designs for high-efficiency monolithic perovskite-silicon tandem solar cells
JF  - Nature nanotechnology
N2  - Designing gentle sinusoidal nanotextures enables the realization of high-efficiency perovskite-silicon solar cells <br /> Perovskite-silicon tandem solar cells offer the possibility of overcoming the power conversion efficiency limit of conventional silicon solar cells. Various textured tandem devices have been presented aiming at improved optical performance, but optimizing film growth on surface-textured wafers remains challenging. Here we present perovskite-silicon tandem solar cells with periodic nanotextures that offer various advantages without compromising the material quality of solution-processed perovskite layers. We show a reduction in reflection losses in comparison to planar tandems, with the new devices being less sensitive to deviations from optimum layer thicknesses. The nanotextures also enable a greatly increased fabrication yield from 50% to 95%. Moreover, the open-circuit voltage is improved by 15 mV due to the enhanced optoelectronic properties of the perovskite top cell. Our optically advanced rear reflector with a dielectric buffer layer results in reduced parasitic absorption at near-infrared wavelengths. As a result, we demonstrate a certified power conversion efficiency of 29.80%.
Y1  - 2022
U6  - https://doi.org/10.1038/s41565-022-01228-8
SN  - 1748-3387
SN  - 1748-3395
VL  - 17
IS  - 11
SP  - 1214
EP  - 1221
PB  - Nature Publishing Group
CY  - London [u.a.]
ER  - 
TY  - JOUR
A1  - Perdigón-Toro, Lorena
A1  - Zhang, Huotian
A1  - Markina, Anastaa si
A1  - Yuan, Jun
A1  - Hosseini, Seyed Mehrdad
A1  - Wolff, Christian Michael
A1  - Zuo, Guangzheng
A1  - Stolterfoht, Martin
A1  - Zou, Yingping
A1  - Gao, Feng
A1  - Andrienko, Denis
A1  - Shoaee, Safa
A1  - Neher, Dieter
T1  - Barrierless free charge generation in the high-performance PM6:Y6 bulk heterojunction non-fullerene solar cell
JF  - Advanced materials
N2  - Organic solar cells are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high-performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near-unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.
KW  - driving force
KW  - non-fullerene acceptors
KW  - organic solar cells
KW  - photocurrent generation
Y1  - 2020
U6  - https://doi.org/10.1002/adma.201906763
SN  - 0935-9648
SN  - 1521-4095
VL  - 32
IS  - 9
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Sandberg, Oskar J.
A1  - Kurpiers, Jona
A1  - Stolterfoht, Martin
A1  - Neher, Dieter
A1  - Meredith, Paul
A1  - Shoaee, Safa
A1  - Armin, Ardalan
T1  - On the question of the need for a built-in potential in Perovskite solar cells
JF  - Advanced materials interfaces
N2  - Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion controlled. A consensus along these lines is emerging in the field. However, the question as to whether the built-in potential plays any role is still of matter of some conjecture. This important question using phase-sensitive photocurrent measurements and theoretical device simulations based upon the drift-diffusion framework is addressed. In particular, the role of the built-in electric field and charge-selective transport layers in state-of-the-art p-i-n perovskite solar cells comparing experimental findings and simulation predictions is probed. It is found that while charge collection in the junction does not require a drift field per se, a built-in potential is still needed to avoid the formation of reverse electric fields inside the active layer, and to ensure efficient extraction through the charge transport layers.
KW  - built-in potential
KW  - charge collection
KW  - charge transport layers
KW  - perovskite solar cells
Y1  - 2020
U6  - https://doi.org/10.1002/admi.202000041
SN  - 2196-7350
VL  - 7
IS  - 10
PB  - Wiley
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Jiang, Wei
A1  - Tao, Chen
A1  - Stolterfoht, Martin
A1  - Jin, Hui
A1  - Stephen, Meera
A1  - Lin, Qianqian
A1  - Nagiri, Ravi C. R.
A1  - Burn, Paul L.
A1  - Gentle, Ian R.
T1  - Hole-transporting materials for low donor content organic solar cells
BT  - charge transport and device performance
JF  - Organic electronics : physics, materials and applications
N2  - Low donor content solar cells are an intriguing class of photovoltaic device about which there is still considerable discussion with respect to their mode of operation. We have synthesized a series of triphenylamine-based materials for use in low donor content devices with the electron accepting [6,6]-phenyl-C71-butyric acid methyl ester (PC(7)0BM). The triphenylamine-based materials absorb light in the near UV enabling the PC(7)0BM to be be the main light absorbing organic semiconducting material in the solar cell. It was found that the devices did not operate as classical Schottky junctions but rather photocurrent was generated by hole transfer from the photo-excited PC(7)0BM to the triphenylamine-based donors. We found that replacing the methoxy surface groups with methyl groups on the donor material led to a decrease in hole mobility for the neat films, which was due to the methyl substituted materials having the propensity to aggregate. The thermodynamic drive to aggregate was advantageous for the performance of the low donor content (6 wt%) films. It was found that the 6 wt% donor devices generally gave higher performance than devices containing 50 wt% of the donor.
KW  - photoexcited hole transfer
KW  - photocurrent generation
KW  - synthesis
KW  - hole
KW  - mobility
KW  - low donor content
KW  - Schottky junction
Y1  - 2020
U6  - https://doi.org/10.1016/j.orgel.2019.105480
SN  - 1566-1199
SN  - 1878-5530
VL  - 76
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - GEN
A1  - Wolff, Christian Michael
A1  - Canil, Laura
A1  - Rehermann, Carolin
A1  - Nguyen, Ngoc Linh
A1  - Zu, Fengshuo
A1  - Ralaiarisoa, Maryline
A1  - Caprioglio, Pietro
A1  - Fiedler, Lukas
A1  - Stolterfoht, Martin
A1  - Kogikoski, Junior, Sergio
A1  - Bald, Ilko
A1  - Koch, Norbert
A1  - Unger, Eva L.
A1  - Dittrich, Thomas
A1  - Abate, Antonio
A1  - Neher, Dieter
T1  - Correction to 'Perfluorinated self-assembled monolayers enhance the stability and efficiency of inverted perovskite solar cells' (2020, 14 (2), 1445−1456)
T2  - ACS nano
Y1  - 2020
U6  - https://doi.org/10.1021/acsnano.0c08081
SN  - 1936-0851
SN  - 1936-086X
VL  - 14
IS  - 11
SP  - 16156
EP  - 16156
PB  - American Chemical Society
CY  - Washington, DC
ER  - 
TY  - JOUR
A1  - Kirchartz, Thomas
A1  - Márquez, José A.
A1  - Stolterfoht, Martin
A1  - Unold, Thomas
T1  - Photoluminescence-based characterization of halide perovskites for photovoltaics
JF  - Advanced Energy Materials
N2  - Photoluminescence spectroscopy is a widely applied characterization technique for semiconductor materials in general and halide perovskite solar cell materials in particular. It can give direct information on the recombination kinetics and processes as well as the internal electrochemical potential of free charge carriers in single semiconductor layers, layer stacks with transport layers, and complete solar cells. The correct evaluation and interpretation of photoluminescence requires the consideration of proper excitation conditions, calibration and application of the appropriate approximations to the rather complex theory, which includes radiative recombination, non-radiative recombination, interface recombination, charge transfer, and photon recycling. In this article, an overview is given of the theory and application to specific halide perovskite compositions, illustrating the variables that should be considered when applying photoluminescence analysis in these materials.
KW  - metal halide perovskites
KW  - numerical simulations
KW  - photoluminescence
KW  - photon recycling
Y1  - 2020
U6  - https://doi.org/10.1002/aenm.201904134
SN  - 1614-6832
SN  - 1614-6840
VL  - 10
IS  - 26
SP  - 1
EP  - 21
PB  - Wiley
CY  - Weinheim
ER  - 
TY  - GEN
A1  - Kirchartz, Thomas
A1  - Márquez, José A.
A1  - Stolterfoht, Martin
A1  - Unold, Thomas
T1  - Photoluminescence-based characterization of halide perovskites for photovoltaics
T2  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - Photoluminescence spectroscopy is a widely applied characterization technique for semiconductor materials in general and halide perovskite solar cell materials in particular. It can give direct information on the recombination kinetics and processes as well as the internal electrochemical potential of free charge carriers in single semiconductor layers, layer stacks with transport layers, and complete solar cells. The correct evaluation and interpretation of photoluminescence requires the consideration of proper excitation conditions, calibration and application of the appropriate approximations to the rather complex theory, which includes radiative recombination, non-radiative recombination, interface recombination, charge transfer, and photon recycling. In this article, an overview is given of the theory and application to specific halide perovskite compositions, illustrating the variables that should be considered when applying photoluminescence analysis in these materials.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1419 
KW  - metal halide perovskites
KW  - numerical simulations
KW  - photoluminescence
KW  - photon recycling
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-519702
SN  - 1866-8372
IS  - 26
ER  - 
TY  - JOUR
A1  - Yazmaciyan, Aren
A1  - Stolterfoht, Martin
A1  - Burn, Paul L.
A1  - Lin, Qianqian
A1  - Meredith, Paul
A1  - Armin, Ardalan
T1  - Recombination losses above and below the transport percolation threshold in bulk heterojunction organic solar cells
JF  - Advanced energy materials
N2  - Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second-order recombination losses dominate the shape of the current density-voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric-field dependence of first-order losses, which includes electric-field-dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first- and second-order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.
KW  - bulk heterojunctions
KW  - charge transport
KW  - organic solar cells
KW  - percolation threshold
KW  - recombination losses
Y1  - 2018
U6  - https://doi.org/10.1002/aenm.201703339
SN  - 1614-6832
SN  - 1614-6840
VL  - 8
IS  - 18
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Wolff, Christian Michael
A1  - Amir, Yohai
A1  - Paulke, Andreas
A1  - Perdigón-Toro, Lorena
A1  - Caprioglio, Pietro
A1  - Neher, Dieter
T1  - Approaching the fill factor Shockley-Queisser limit in stable, dopant-free triple cation perovskite solar cells
JF  - Energy & Environmental Science
N2  - Perovskite solar cells now compete with their inorganic counterparts in terms of power conversion efficiency, not least because of their small open-circuit voltage (V-OC) losses. A key to surpass traditional thin-film solar cells is the fill factor (FF). Therefore, more insights into the physical mechanisms that define the bias dependence of the photocurrent are urgently required. In this work, we studied charge extraction and recombination in efficient triple cation perovskite solar cells with undoped organic electron/hole transport layers (ETL/HTL). Using integral time of flight we identify the transit time through the HTL as the key figure of merit for maximizing the fill factor (FF) and efficiency. Complementarily, intensity dependent photocurrent and V-OC measurements elucidate the role of the HTL on the bias dependence of non-radiative and transport-related loss channels. We show that charge transport losses can be completely avoided under certain conditions, yielding devices with FFs of up to 84%. Optimized cells exhibit power conversion efficiencies of above 20% for 6 mm(2) sized pixels and 18.9% for a device area of 1 cm(2). These are record efficiencies for hybrid perovskite devices with dopant-free transport layers, highlighting the potential of this device technology to avoid charge-transport limitations and to approach the Shockley-Queisser limit.
Y1  - 2017
U6  - https://doi.org/10.1039/c7ee00899f
SN  - 1754-5692
SN  - 1754-5706
VL  - 10
SP  - 1530
EP  - 1539
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Wang, Qiong
A1  - Smith, Joel A.
A1  - Skroblin, Dieter
A1  - Steele, Julian A.
A1  - Wolff, Christian Michael
A1  - Caprioglio, Pietro
A1  - Stolterfoht, Martin
A1  - Köbler, Hans
A1  - Turren-Cruz, Silver-Hamill
A1  - Li, Meng
A1  - Gollwitzer, Christian
A1  - Neher, Dieter
A1  - Abate, Antonio
T1  - Managing phase purities and crystal orientation for high-performance and photostable cesium lead halide perovskite solar cells
JF  - Solar RRL
N2  - Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, T-S80, of approximate to 300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 - xBrx perovskite solar cells.
KW  - cesium lead halides
KW  - crystal orientation
KW  - inorganic perovskites
KW  - ISOS-L-1I protocol
KW  - phase purity
KW  - photostability
Y1  - 2020
VL  - 4
IS  - 9
PB  - WILEY-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Lang, Felix
T1  - All-perovskite tandems get flexible
JF  - Nature energy
N2  - Flexible all-perovskite tandem photovoltaics open up new opportunities for application compared to rigid devices, yet their performance lags behind. Now, researchers show that molecule-bridged interfaces mitigate charge recombination and crack formation, improving the efficiency and mechanical reliability of flexible devices.
Y1  - 2022
U6  - https://doi.org/10.1038/s41560-022-01087-6
SN  - 2058-7546
VL  - 7
IS  - 8
SP  - 688
EP  - 689
PB  - Nature Publishing Group
CY  - London
ER  - 
TY  - GEN
A1  - Saliba, Michael
A1  - Stolterfoht, Martin
A1  - Wolff, Christian Michael
A1  - Neher, Dieter
A1  - Abate, Antonio
T1  - Measuring aging stability of perovskite solar cells
T2  - Joule
Y1  - 2018
U6  - https://doi.org/10.1016/j.joule.2018.05.005
SN  - 2542-4351
VL  - 2
IS  - 6
SP  - 1019
EP  - 1024
PB  - Cell Press
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Pisoni, Stefano
A1  - Stolterfoht, Martin
A1  - Lockinger, Johannes
A1  - Moser, Thierry
A1  - Jiang, Yan
A1  - Caprioglio, Pietro
A1  - Neher, Dieter
A1  - Buecheler, Stephan
A1  - Tiwari, Ayodhya N.
T1  - On the origin of open-circuit voltage losses in flexible n-i-p perovskite solar cells
JF  - Science and technology of advanced materials : STAM
N2  - The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in n-i-p configuration using vacuum deposited C-60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from similar to 1.23 eV for the bare absorber, just similar to 90 meV below the radiative limit, to similar to 1.10 eV when C-60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of similar to 30-40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure. [GRAPHICS] .
KW  - Perovskite solar cell
KW  - flexible
KW  - interface engineering
KW  - non-radiative recombination
KW  - quasi-Fermi level splitting
Y1  - 2019
U6  - https://doi.org/10.1080/14686996.2019.1633952
SN  - 1468-6996
SN  - 1878-5514
VL  - 20
SP  - 786
EP  - 795
PB  - Taylor & Francis
CY  - Abingdon
ER  - 
TY  - JOUR
A1  - Samson, Stephanie
A1  - Rech, Jeromy
A1  - Perdigón-Toro, Lorena
A1  - Peng, Zhengxing
A1  - Shoaee, Safa
A1  - Ade, Harald
A1  - Neher, Dieter
A1  - Stolterfoht, Martin
A1  - You, Wei
T1  - Organic solar cells with large insensitivity to donor polymer molar mass across all acceptor classes
JF  - ACS applied polymer materials
N2  - Donor polymer number-average molar mass (M-n) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction morphology. The exact nature of these M-n effects varies from system to system, although there is generally some intermediate M-n that results in optimal performance. Interestingly, our earlier work with the difluorobenzotriazole (FTAZ)-based donor polymer, paired with either N2200 (polymer acceptor) or PC61BM (fullerene acceptor), PcBm demonstrated <10% variation in power conversion efficiency and a consistent morphology over a large span of M-n (30 kg/mol to over 100 kg/mol). Would such insensitivity to polymer M-n still hold true when prevailing small molecular acceptors were used with FTAZ? To answer this question, we explored the impact of FTAZ on OPVs with ITIC, a high-performance small-molecule fused-ring electron acceptor (FREA). By probing the photovoltaic characteristics of the resulting OPVs, we show that a similar FTAZ mn insensitivity is also found in the FTAZ:ITIC system. This study highlights a single-donor polymer which, when paired with an archetypal fullerene, polymer, and FREA, results in systems that are largely insensitive to donor M. Our results may have implications in polymer batch-to-batch reproducibility, in particular, relaxing the need for tight M-n control during synthesis.
KW  - polymer solar cells
KW  - conjugated polymers
KW  - fullerenes
KW  - fluorination
KW  - molecular weight
KW  - non-fullerene acceptors
KW  - power conversion efficiency
Y1  - 2020
U6  - https://doi.org/10.1021/acsapm.0c01041
SN  - 2637-6105
VL  - 2
IS  - 11
SP  - 5300
EP  - 5308
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Le Corre, Vincent M.
A1  - Feuerstein, Markus
A1  - Caprioglio, Pietro
A1  - Koster, Lambert Jan Anton
A1  - Neher, Dieter
T1  - Voltage-Dependent Photoluminescence and How It Correlates with the Fill Factor and Open-Circuit Voltage in Perovskite Solar Cells
JF  - Acs energy letters
N2  - Optimizing the photoluminescence (PL) yield of a solar cell has long been recognized as a key principle to maximize the power conversion efficiency. While PL measurements are routinely applied to perovskite films and solar cells under open circuit conditions (V-OC), it remains unclear how the emission depends on the applied voltage. Here, we performed PL(V) measurements on perovskite cells with different hole transport layer thicknesses and doping concentrations, resulting in remarkably different fill factors (FFs). The results reveal that PL(V) mirrors the current-voltage (JV) characteristics in the power-generating regime, which highlights an interesting correlation between radiative and nonradiative recombination losses. In particular, high FF devices show a rapid quenching of PL(V) from open-circuit to the maximum power point. We conclude that, while the PL has to be maximized at V-OC at lower biases < V-OC the PL must be rapidly quenched as charges need to be extracted prior to recombination.
Y1  - 2019
U6  - https://doi.org/10.1021/acsenergylett.9b02262
SN  - 2380-8195
VL  - 4
IS  - 12
SP  - 2887
EP  - 2892
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - GEN
A1  - Schulze, Patricia S. C.
A1  - Bett, Alexander J.
A1  - Bivour, Martin
A1  - Caprioglio, Pietro
A1  - Gerspacher, Fabian M.
A1  - Kabaklı, Özde Ş.
A1  - Richter, Armin
A1  - Stolterfoht, Martin
A1  - Zhang, Qinxin
A1  - Neher, Dieter
A1  - Hermle, Martin
A1  - Hillebrecht, Harald
A1  - Glunz, Stefan W.
A1  - Goldschmidt, Jan Christoph
T1  - 25.1% high-efficiency monolithic perovskite silicon tandem solar cell with a high bandgap perovskite absorber
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p-i-n tandem configuration. A methylammonium-free FA(0.75)Cs(0.25)Pb(I0.8Br0.2)(3) perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in approximate to 75 nm thicker absorber layers and 0.7 mA cm(-2) higher short-circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1197 
KW  - heterojunction silicon solar cells
KW  - interfaces
KW  - perovskite solar cells
KW  - tandem solar cells
KW  - thin films
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-525668
SN  - 1866-8372
IS  - 7
ER  - 
TY  - GEN
A1  - Krückemeier, Lisa
A1  - Rau, Uwe
A1  - Stolterfoht, Martin
A1  - Kirchartz, Thomas
T1  - How to report record open-circuit voltages in lead-halide perovskite solar cells
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - Open-circuit voltages of lead-halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open-circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open-circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta-analysis of methods to determine the bandgap and a radiative limit for open-circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1194 
KW  - Shockley-Queisser model
KW  - bandgap
KW  - fill factor losses
KW  - nonradiative voltage losses
KW  - photovoltaics
KW  - radiative limit
KW  - recombination
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-525289
SN  - 1866-8372
IS  - 1
ER  - 
TY  - JOUR
A1  - Zhang, Shanshan
A1  - Stolterfoht, Martin
A1  - Armin, Ardalan
A1  - Lin, Qianqian
A1  - Zu, Fengshuo
A1  - Sobus, Jan
A1  - Jin, Hui
A1  - Koch, Norbert
A1  - Meredith, Paul
A1  - Burn, Paul L.
A1  - Neher, Dieter
T1  - Interface Engineering of Solution-Processed Hybrid Organohalide Perovskite Solar Cells
JF  - ACS applied materials & interfaces
N2  - Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage (V-OC) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI(3) perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.
KW  - organohalide lead perovskites
KW  - solar cells
KW  - surface wetting
KW  - work function
KW  - oxygen plasma
KW  - transport layer
Y1  - 2018
U6  - https://doi.org/10.1021/acsami.8b02503
SN  - 1944-8244
VL  - 10
IS  - 25
SP  - 21681
EP  - 21687
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Wolff, Christian Michael
A1  - Marquez, Jose A.
A1  - Zhang, Shanshan
A1  - Hages, Charles J.
A1  - Rothhardt, Daniel
A1  - Albrecht, Steve
A1  - Burn, Paul L.
A1  - Meredith, Paul
A1  - Unold, Thomas
A1  - Neher, Dieter
T1  - Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells
JF  - Nature Energy
N2  - The performance of perovskite solar cells is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here, we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pintype perovskite solar cells with undoped organic charge transport layers. We find significant quasi-Fermi-level splitting losses (135 meV) in the perovskite bulk, whereas interfacial recombination results in an additional free energy loss of 80 meV at each individual interface, which limits the open-circuit voltage (V-oc) of the complete cell to similar to 1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers leads to a substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm(2) perovskite solar cells surpassing 20% efficiency (19.83% certified) with stabilized power output, a high V-oc (1.17 V) and record fill factor (>81%).
KW  - Energy science and technology
KW  - Solar cells
Y1  - 2018
U6  - https://doi.org/10.1038/s41560-018-0219-8
SN  - 2058-7546
VL  - 3
IS  - 10
SP  - 847
EP  - 854
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Schulze, Patricia S. C.
A1  - Bett, Alexander J.
A1  - Bivour, Martin
A1  - Caprioglio, Pietro
A1  - Gerspacher, Fabian M.
A1  - Kabaklı, Özde Ş.
A1  - Richter, Armin
A1  - Stolterfoht, Martin
A1  - Zhang, Qinxin
A1  - Neher, Dieter
A1  - Hermle, Martin
A1  - Hillebrecht, Harald
A1  - Glunz, Stefan W.
A1  - Goldschmidt, Jan Christoph
T1  - 25.1% high-efficiency monolithic perovskite silicon tandem solar cell with a high bandgap perovskite absorber
JF  - Solar RRL
N2  - Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p-i-n tandem configuration. A methylammonium-free FA(0.75)Cs(0.25)Pb(I0.8Br0.2)(3) perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in approximate to 75 nm thicker absorber layers and 0.7 mA cm(-2) higher short-circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.
KW  - heterojunction silicon solar cells
KW  - interfaces
KW  - perovskite solar cells
KW  - tandem solar cells
KW  - thin films
Y1  - 2020
VL  - 4
IS  - 7
PB  - John Wiley & Sons, Inc.
CY  - New Jersey
ER  - 
TY  - JOUR
A1  - Shoaee, Safa
A1  - Stolterfoht, Martin
A1  - Neher, Dieter
T1  - The Role of Mobility on Charge Generation, Recombination, and Extraction in Polymer-Based Solar Cells
JF  - dvanced energy materials
N2  - Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability, highly scalable fabrication, and mechanical flexibility. In contrast to traditional inorganic semiconductors, organic semiconductors are intrinsically disordered systems and therefore exhibit much lower charge carrier mobilities-the Achilles heel of organic photovoltaic cells. In this progress review, the authors discuss recent important developments on the impact of charge carrier mobility on the charge transfer state dissociation, and the interplay of free charge extraction and recombination. By comparing the mobilities on different timescales obtained by different techniques, the authors highlight the dispersive nature of these materials and how this reflects on the key processes defining the efficiency of organic photovoltaics.
KW  - charge generation
KW  - charge recombination
KW  - extraction
KW  - mobility
KW  - organic solar cells
KW  - polymer:fullerene bulk heterojunction
Y1  - 2018
U6  - https://doi.org/10.1002/aenm.201703355
SN  - 1614-6832
SN  - 1614-6840
VL  - 8
IS  - 28
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - GEN
A1  - Reppert, Alexander von
A1  - Puddell, J.
A1  - Koc, A.
A1  - Reinhardt, M.
A1  - Leitenberger, Wolfram
A1  - Dumesnil, K.
A1  - Zamponi, Flavio
A1  - Bargheer, Matias
T1  - Persistent nonequilibrium dynamics of the thermal energies in the spin and phonon systems of an antiferromagnet
N2  - We present a temperature and fluence dependent Ultrafast X-Ray Diffraction study of a laser-heated antiferromagnetic dysprosium thin film. The loss of antiferromagnetic order is evidenced by a pronounced lattice contraction. We devise a method to determine the energy flow between the phonon and spin system from calibrated Bragg peak positions in thermal equilibrium. Reestablishing the magnetic order is much slower than the cooling of the lattice, especially around the Néel temperature. Despite the pronounced magnetostriction, the transfer of energy from the spin system to the phonons in Dy is slow after the spin-order is lost.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 272 
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-98710
ER  - 
TY  - JOUR
A1  - Zu, Fengshuo
A1  - Warby, Jonathan
A1  - Stolterfoht, Martin
A1  - Li, Jinzhao
A1  - Shin, Dongguen
A1  - Unger, Eva
A1  - Koch, Norbert
T1  - Photoinduced energy-level realignment at interfaces between organic semiconductors and metal-halide perovskites
JF  - Physical review letters
N2  - In contrast to the common conception that the interfacial energy-level alignment is affixed once the interface is formed, we demonstrate that heterojunctions between organic semiconductors and metal-halide perovskites exhibit huge energy-level realignment during photoexcitation. Importantly, the photoinduced level shifts occur in the organic component, including the first molecular layer in direct contact with the perovskite. This is caused by charge-carrier accumulation within the organic semiconductor under illumination and the weak electronic coupling between the junction components.
Y1  - 2021
U6  - https://doi.org/10.1103/PhysRevLett.127.246401
SN  - 0031-9007
SN  - 1079-7114
VL  - 127
IS  - 24
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Raoufi, Meysam
A1  - Hörmann, Ulrich
A1  - Ligorio, Giovanni
A1  - Hildebrandt, Jana
A1  - Pätzel, Michael
A1  - Schultz, Thorsten
A1  - Perdigón-Toro, Lorena
A1  - Koch, Norbert
A1  - List-Kratochvil, Emil
A1  - Hecht, Stefan
A1  - Neher, Dieter
T1  - Simultaneous effect of ultraviolet radiation and surface modification on the work function and hole injection properties of ZnO thin films
JF  - Physica Status Solidi.  A , Applications and materials science
N2  - The combined effect of ultraviolet (UV) light soaking and self-assembled monolayer deposition on the work function (WF) of thin ZnO layers and on the efficiency of hole injection into the prototypical conjugated polymer poly(3-hexylthiophen-2,5-diyl) (P3HT) is systematically investigated. It is shown that the WF and injection efficiency depend strongly on the history of UV light exposure. Proper treatment of the ZnO layer enables ohmic hole injection into P3HT, demonstrating ZnO as a potential anode material for organic optoelectronic devices. The results also suggest that valid conclusions on the energy-level alignment at the ZnO/organic interfaces may only be drawn if the illumination history is precisely known and controlled. This is inherently problematic when comparing electronic data from ultraviolet photoelectron spectroscopy (UPS) measurements carried out under different or ill-defined illumination conditions.
KW  - charge injection across hybrid interfaces
KW  - energy-level alignments
KW  - hybrid metal oxides
KW  - organic interfaces
Y1  - 2020
U6  - https://doi.org/10.1002/pssa.201900876
SN  - 1862-6300
SN  - 1862-6319
VL  - 217
IS  - 5
SP  - 1
EP  - 6
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Kaa, Johannes M.
A1  - Sternemann, Christian
A1  - Appel, Karen
A1  - Cerantola, Valerio
A1  - Preston, Thomas R.
A1  - Albers, Christian
A1  - Elbers, Mirko
A1  - Libon, Lelia
A1  - Makita, Mikako
A1  - Pelka, Alexander
A1  - Petitgirard, Sylvain
A1  - Plückthun, Christian
A1  - Roddatis, Vladimir
A1  - Sahle, Christoph J.
A1  - Spiekermann, Georg
A1  - Schmidt, Christian
A1  - Schreiber, Anja
A1  - Sakrowski, Robin
A1  - Tolan, Metin
A1  - Wilke, Max
A1  - Zastrau, Ulf
A1  - Konopkova, Zuzana
T1  - Structural and electron spin state changes in an x-ray heated iron carbonate system at the Earth's lower mantle pressures
JF  - Physical review research
N2  - The determination of the spin state of iron-bearing compounds at high pressure and temperature is crucial for our understanding of chemical and physical properties of the deep Earth. Studies on the relationship between the coordination of iron and its electronic spin structure in iron-bearing oxides, silicates, carbonates, iron alloys, and other minerals found in the Earth's mantle and core are scarce because of the technical challenges to simultaneously probe the sample at high pressures and temperatures. We used the unique properties of a pulsed and highly brilliant x-ray free electron laser (XFEL) beam at the High Energy Density (HED) instrument of the European XFEL to x-ray heat and probe samples contained in a diamond anvil cell. We heated and probed with the same x-ray pulse train and simultaneously measured x-ray emission and x-ray diffraction of an FeCO3 sample at a pressure of 51 GPa with up to melting temperatures. We collected spin state sensitive Fe K beta(1,3) fluorescence spectra and detected the sample's structural changes via diffraction, observing the inverse volume collapse across the spin transition. During x-ray heating, the carbonate transforms into orthorhombic Fe4C3O12 and iron oxides. Incipient melting was also observed. This approach to collect information about the electronic state and structural changes from samples contained in a diamond anvil cell at melting temperatures and above will considerably improve our understanding of the structure and dynamics of planetary and exoplanetary interiors.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevResearch.4.033042
SN  - 2643-1564
VL  - 4
IS  - 3
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - GEN
A1  - Zeuschner, Steffen Peer
A1  - Mattern, Maximilian
A1  - Pudell, Jan-Etienne
A1  - Reppert, Alexander von
A1  - Rössle, Matthias
A1  - Leitenberger, Wolfram
A1  - Schwarzkopf, Jutta
A1  - Boschker, Jos
A1  - Herzog, Marc
A1  - Bargheer, Matias
T1  - Reciprocal space slicing
BT  - a time-efficient approach to femtosecond x-ray diffraction
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - An experimental technique that allows faster assessment of out-of-plane strain dynamics of thin film heterostructures via x-ray diffraction is presented. In contrast to conventional high-speed reciprocal space-mapping setups, our approach reduces the measurement time drastically due to a fixed measurement geometry with a position-sensitive detector. This means that neither the incident (ω) nor the exit (2θ) diffraction angle is scanned during the strain assessment via x-ray diffraction. Shifts of diffraction peaks on the fixed x-ray area detector originate from an out-of-plane strain within the sample. Quantitative strain assessment requires the determination of a factor relating the observed shift to the change in the reciprocal lattice vector. The factor depends only on the widths of the peak along certain directions in reciprocal space, the diffraction angle of the studied reflection, and the resolution of the instrumental setup. We provide a full theoretical explanation and exemplify the concept with picosecond strain dynamics of a thin layer of NbO2.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1137 
Y1  - 2021
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-499761
SN  - 1866-8372
IS  - 1137
ER  - 
TY  - JOUR
A1  - Zeuschner, Steffen Peer
A1  - Mattern, Maximilian
A1  - Pudell, Jan-Etienne
A1  - Reppert, Alexander von
A1  - Rössle, Matthias
A1  - Leitenberger, Wolfram
A1  - Schwarzkopf, Jutta
A1  - Boschker, Jos
A1  - Herzog, Marc
A1  - Bargheer, Matias
T1  - Reciprocal space slicing
BT  - a time-efficient approach to femtosecond x-ray diffraction
JF  - Structural dynamics
N2  - An experimental technique that allows faster assessment of out-of-plane strain dynamics of thin film heterostructures via x-ray diffraction is presented. In contrast to conventional high-speed reciprocal space-mapping setups, our approach reduces the measurement time drastically due to a fixed measurement geometry with a position-sensitive detector. This means that neither the incident (ω) nor the exit (2θ) diffraction angle is scanned during the strain assessment via x-ray diffraction. Shifts of diffraction peaks on the fixed x-ray area detector originate from an out-of-plane strain within the sample. Quantitative strain assessment requires the determination of a factor relating the observed shift to the change in the reciprocal lattice vector. The factor depends only on the widths of the peak along certain directions in reciprocal space, the diffraction angle of the studied reflection, and the resolution of the instrumental setup. We provide a full theoretical explanation and exemplify the concept with picosecond strain dynamics of a thin layer of NbO2.
Y1  - 2020
U6  - https://doi.org/10.1063/4.0000040
SN  - 2329-7778
VL  - 8
IS  - 1
PB  - AIP Publishing LLC
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Kleinpeter, Erich
A1  - Koch, Andreas
T1  - The 13 C chemical shift and the anisotropy effect of the carbene electron-deficient centre
BT  - simple means to characterize the electron distribution of carbenes
JF  - Magnetic resonance in chemistry
N2  - Both the C-13 chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and C-13 chemical shifts were calculated and compared with the experimental delta(C-13)/ppm values and geometry parameters (as a quality criterion for obtained structures).
KW  - C-13 chemical shift
KW  - carbenes
KW  - zwitterions
KW  - carbene electron deficiency
KW  - nucleus-independent chemical shifts (NICS)
KW  - through space NMR shieldings
KW  - (TSNMRS)
Y1  - 2019
U6  - https://doi.org/10.1002/mrc.4979
SN  - 0749-1581
SN  - 1097-458X
VL  - 58
IS  - 3
SP  - 280
EP  - 292
PB  - Wiley
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Kleinpeter, Erich
A1  - Koch, Andreas
T1  - Quantification of sigma-acceptor and pi-donor stabilization in O, S and Hal analogues of N-heterocyclic carbenes (NHCs) on the magnetic criterion
JF  - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory
N2  - The spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of stable O, S and Hal analogues of N-heterocyclic carbenes (NHCs) have been calculated using the GIAO perturbation method employing the nucleus-independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSSs) of various sizes and directions. The TSNMRS values (actually the anisotropy effects measurable in H-1 NMR spectroscopy) are employed to qualify and quantify the position of the present mesomeric equilibria (carbenes <-> ylides). The results are confirmed by geometry (bond angles and bond lengths), IR spectra, UV spectra, and C-13 chemical shifts of the electron-deficient carbon centers.
Y1  - 2021
U6  - https://doi.org/10.1021/acs.jpca.1c05257
SN  - 1089-5639
SN  - 1520-5215
VL  - 125
IS  - 33
SP  - 7235
EP  - 7245
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - THES
A1  - Bojahr, Andre
T1  - Hypersound interaction studied by time-resolved inelastic light and x-ray scattering
T1  - Wechselwirkende Hyperschallwellen untersucht mittels zeitaufgelöster inelastischer Licht- und Röntgenstreuung
N2  - This publications-based thesis summarizes my contribution to the scientific field of ultrafast structural dynamics. It consists of 16 publications, about the generation, detection and coupling of coherent gigahertz longitudinal acoustic phonons, also called hypersonic waves. To generate such high frequency phonons, femtosecond near infrared laser pulses were used to heat nanostructures composed of perovskite oxides on an ultrashort timescale. As a consequence the heated regions of such a nanostructure expand and a high frequency acoustic phonon pulse is generated. To detect such coherent acoustic sound pulses I use ultrafast variants of optical Brillouin and x-ray scattering. Here an incident optical or x-ray photon is scattered by the excited sound wave in the sample. The scattered light intensity measures the occupation of the phonon modes. 
The central part of this work is the investigation of coherent high amplitude phonon wave packets which can behave nonlinearly, quite similar to shallow water waves which show a steepening of wave fronts or solitons well known as tsunamis. Due to the high amplitude of the acoustic wave packets in the solid, the acoustic properties can change significantly in the vicinity of the sound pulse. This may lead to a shape change of the pulse. I have observed by time-resolved Brillouin scattering, that a single cycle hypersound pulse shows a wavefront steepening. I excited hypersound pulses with strain amplitudes until 1% which I have calibrated by ultrafast x-ray diffraction (UXRD).
On the basis of this first experiment we developed the idea of the nonlinear mixing of narrowband phonon wave packets which we call "nonlinear phononics" in analogy with the nonlinear optics, which summarizes a kaleidoscope of surprising optical phenomena showing up at very high electric fields. Such phenomena are for instance Second Harmonic Generation, four-wave-mixing or solitons. But in case of excited coherent phonons the wave packets have usually very broad spectra which make it nearly impossible to look at elementary scattering processes between phonons with certain momentum and energy.
For that purpose I tested different techniques to excite narrowband phonon wave packets which mainly consist of phonons with a certain momentum and frequency. To this end epitaxially grown metal films on a dielectric substrate were excited with a train of laser pulses. These excitation pulses drive the metal film to oscillate with the frequency given by their inverse temporal displacement and send a hypersonic wave of this frequency into the substrate. The monochromaticity of these wave packets was proven by ultrafast optical Brillouin and x-ray scattering.
Using the excitation of such narrowband phonon wave packets I was able to observe the Second Harmonic Generation (SHG) of coherent phonons as a first example of nonlinear wave mixing of nanometric phonon wave packets.
N2  - Diese publikationsbasierte Dissertation fasst meinen Beitrag zum Forschungsgebiet der ultraschnellen Strukturdynamik zusammen.
Diese Arbeit besteht aus 16 Publikationen aus den Bereichen der Erzeugung, Detektion und Kopplung von kohärenten Gigahertz longitudinal-akustischen Phononen, auch Hyperschallwellen genannt. Um solch hochfrequente Phononen zu erzeugen, werden Femtosekunden nahinfrarot Laserpulse benutzt, um Nanostrukturen auf einer ultraschnellen Zeitskala zu erhitzen. Die aufgeheizten Regionen der Nanostruktur dehnen sich aufgrund der hohen Temperatur aus und ein hochfrequenter Schallpuls wird generiert. Um solche akustischen Pulse zu detektieren benutze ich ultraschnelle Varianten der Brillouin- und Röntgenstreuung. Dabei wird ein einfallendes optisches oder Röntgenphoton an der erzeugten Schallwelle gestreut. Die gemessene Streuintensität ist hierbei ein Maß für die Besetzung einzelner Phononenzustände.
Der zentrale Teil dieser Arbeit ist die Untersuchung von kohärenten Phonon-Wellenpaketen mit sehr hoher Amplitude. Diese Wellenpakete können sich nichtlinear verhalten, sehr ähnlich zu Flachwasserwellen bei denen nichtlineare Effekte in Form eines Aufsteilens der Wellenfronten oder der Existenz von Solitonen, bekannt als Tsunamis, äußern. Durch die hohe Amplitude der akustischen Wellenpakete können sich die akustischen Eigenschaften des Festkörpers in der Umgebung des Schallpulses signifikant ändern, welches sich dann in einer Formänderung des Schallpulses widerspiegelt. Ich konnte mittels zeitaufgelöster Brillouinstreuung das Aufsteilen der Wellenfronten eines Hyperschallpulses bestehend aus einem einzigen Oszillationszyklus beobachten. Hierbei wurden Hyperschallwellen mit einer Dehnungsamplitude von bis zu 1% angeregt, wobei ich diesen Wert mittels ultraschneller Röntgenbeugung kalibrieren konnte.
Mit diesem ersten Experiment als Basis entwickelten wir die Idee der nichtlinearen Wellenmischung von schmalbandigen Phonon-Wellenpaketen unter dem Titel "nichtlineare Phononik" in Analogie zur nichtlinearen Optik, welche sich aus einer Reihe von verblüffenden optischen Phänomenen bei sehr hohen elektrischen Feldstärken zusammensetzt. Solche Phänomene sind z. B. die optische Frequenzverdopplung, das Vier-Wellen-Mischen oder Solitone. Nur sind im Falle von kohärenten Phononen die erzeugten Spektren sehr breitbandig, was die Untersuchung von spezifischen Phononen mit festem Impuls und definierter Frequenz fast unmöglich macht.
Aus diesem Grund testete ich verschiedene Methoden um schmalbandige Phonon-Wellenpakete anzuregen, welche im Wesentlichen aus Phononen bestimmten Impulses und definierter Frequenz bestehen. Dafür wurden schließ lich epitaktisch auf ein dielektrisches Substrat aufgewachsene Metallfilme mit einen Laserpulszug angeregt. Hier sorgen die Lichtpulse für eine periodische Oszillation des Metalfilms, wobei die Anregefrequenz durch den inversen zeitlichen Abstand der Lichtpulse gegeben ist. Diese periodische Oszillation sendet dann ein Hyperschallwellenpaket eben dieser Frequenz ins Substrat. Die Monochromie dieser Wellenpakete konnte dabei mittels ultraschneller Brillouin- und Röntgenstreuung bestätigt werden.
Durch die Benutzung dieser schmalbandigen Phonon-Wellenpakete war es mir möglich, die Frequenzverdopplung (SHG) von kohärenten Phononen zu beobachten, was ein erstes Beispiel für die nichtlineare Wellenmischung von nanometrischen Phonon-Wellenpaketen ist.
KW  - hypersound
KW  - nonlinear acoustics
KW  - ultrafast
KW  - Brillouin scattering
KW  - x-ray diffraction
KW  - self-steepening
KW  - second-harmonic generation
KW  - Phononen
KW  - Wechselwirkung
KW  - Anharmonizität
KW  - nichtlineare Wellenmischung
KW  - zweite Harmonische
KW  - Phononenstreuung
KW  - nichlineare Phononik
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-93860
ER  - 
TY  - THES
A1  - Maerten, Lena
T1  - Spectroscopic perspectives on ultrafast coupling phenomena in perovskite oxides
T1  - Spektroskopische Untersuchung ultraschneller Kopplungsphänomene in Perowskit-Oxiden
N2  - In this thesis, I study ultrafast dynamics in perovskite oxides using time resolved broadband spectroscopy. I focus on the observation of coherent phonon propagation by time resolved Brillouin scattering: following the excition of metal transducer films with a femtosecond infrared pump pulse, coherent phonon dynamics in the GHz frequency range are triggered. Their propagation is monitored using a delayed white light probe pulse. The technique is illustrated on various thin films and multilayered samples. I apply the technique to investigate the linear and nonlinear acoustic response in bulk SrTiO_3, which displays a ferroelastic phase transition from a cubic to a tetragonal structural phase at T_a=105 K. In the linear regime, I observe a coupling of the observed acoustic phonon mode to the softening optic modes describing the phase transition. In the nonlinear regime, I find a giant slowing down of the sound velocity in the low temperature phase that is only observable for a strain amplitude exceeding the tetragonality of the material. It is attributed to a coupling of the high frequency phonons to ferroelastic domain walls in the material. I propose a new mechanism for the coupling of strain waves to the domain walls that is only effective for high amplitude strain. A detailed study of the phonon attenuation across a wide temperature range shows that the phonon attenuation at low temperatures is influenced by the domain configuration, which is determined by interface strain. Preliminary measurements on magnetic-ferroelectric multilayers reveal that the excitation fluence needs to be carefully controlled when dynamics at phase transitions are studied.
N2  - In dieser Doktorarbeit untersuche ich ultraschnelle Dynamik in perovskitischen Oxiden mittels zeitaufgelöster optischer Spektroskopie. Der Schwerpunkt liegt dabei auf Phononendynamik, die mithilfe von zeitaufgelöster Brillouin-Streuung sichtbar gemacht wird: durch die Anregung einer metallischen Transducer-Schicht mit einem ultrakurzen Anregepuls wird eine kohärente Phononendynamik im GHz Frequenzbereich erzeugt. Die Ausbreitung der Schallpulse wird mit einem Weißlicht-Abfragepuls aufgezeichnet. Diese Methode wird am Beispiel verschiedener Dünnschicht- und Übergitterproben illustriert. Die Methode und das gewonnene Verständnis wende ich an, um lineare und nichtlineare akustische Eigenschaften an einem SrTiO_3-Kristall zu untersuchen. Dieser weist einen ferroelastischen Phasenübergang von kubischer zu tetragonaler Kristallstruktur bei T_a=105 K auf. Im linearen Regime beobachte ich eine Kopplung der untersuchten akustischen Mode an eine weichwerdende optische Mode, welche den Phasenübergang charakterisiert. Im nichtlinearen Regime tritt eine gigantische Verlangsamung der Schallgeschwindigkeit unterhalb von T_a auf, wenn die induzierte Gitterverzerrung die Tetragonalität des Materials übersteigt. Dies kann auf eine Kopplung der hochfrequenten akustischen Mode an ferroelastische Domänenwände bei tiefen Temperaturen zurückgeführt werden. Ich entwickle einen neuen Mechanismus, der die Kopplung der Verzerrungswelle an die Domänenwände beschreibt. Eine detaillierte Untersuchung der Phononendämpfung in SrTiO_3 über einen weiten Temperaturbereich zeigt, dass diese bei tiefen Temperaturen durch die Domänenkonfiguration beeinflusst ist. Die Domänenkonfiguration ist durch Verzerrungen an der Kristall-Transducer Grenzfläche bestimmt. Erste Untersuchungen an magnetisch-ferroelektrischen Übergittern zeigen, dass die Anregungsfluenz vorsichtig eingestellt werden muss, um die Dynamik an Phasenübergängen zu untersuchen.
KW  - coherent phonons
KW  - phonon dynamics
KW  - time resolved
KW  - Brillouin scattering
KW  - perovskite oxides
KW  - optical spectroscopy
KW  - hypersound propagation
KW  - phonon damping
KW  - domain wall motion
KW  - phonon backfolding
KW  - superlattice dispersion
KW  - kohärente Phononen
KW  - Phononen Dynamik
KW  - zeitaufgelöst
KW  - Brillouin Streuung
KW  - Perowskit-Oxide
KW  - optische Spektroskopie
KW  - Hyperschall Propagation
KW  - Phononen Dämpfung
KW  - Domänenwandbewegung
KW  - Phononen Rückfaltung
KW  - Übergitter Dispersion
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-77623
ER  - 
TY  - JOUR
A1  - Shayduk, Roman
A1  - Hallmann, Jörg
A1  - Rodriguez-Fernandez, Angel
A1  - Scholz, Markus
A1  - Lu, Wei
A1  - Bösenberg, Ulrike
A1  - Möller, Johannes
A1  - Zozulya, Alexey
A1  - Jiang, Man
A1  - Wegner, Ulrike
A1  - Secareanu, Radu-Costin
A1  - Palmer, Guido
A1  - Emons, Moritz
A1  - Lederer, Max
A1  - Volkov, Sergey
A1  - Lindfors-Vrejoiu, Ionela
A1  - Schick, Daniel
A1  - Herzog, Marc
A1  - Bargheer, Matias
A1  - Madsen, Anders
T1  - Femtosecond x-ray diffraction study of multi-THz coherent phonons in SrTiO3
JF  - Applied physics letters
N2  - We report generation of ultra-broadband longitudinal acoustic coherent phonon wavepackets in SrTiO3 (STO) with frequency components extending throughout the first Brillouin zone. The wavepackets are efficiently generated in STO using femtosecond infrared laser excitation of an atomically flat 1.6 nm-thick epitaxial SrRuO3 film. We use femtosecond x-ray diffraction at the European X-Ray Free Electron Laser Facility to study the dispersion and damping of phonon wavepackets. The experimentally determined damping constants for multi-THz frequency phonons compare favorably to the extrapolation of a simple ultrasound damping model over several orders of magnitude.
Y1  - 2022
U6  - https://doi.org/10.1063/5.0083256
SN  - 0003-6951
SN  - 1077-3118
VL  - 120
IS  - 20
PB  - AIP Publishing
CY  - Melville
ER  - 
TY  - JOUR
A1  - Deb, Marwan
A1  - Popova, Elena
A1  - Jaffrès, Henri-Yves
A1  - Keller, Niels
A1  - Bargheer, Matias
T1  - Polarization-dependent subpicosecond demagnetization in iron garnets
JF  - Physical review : B, covering condensed matter and materials physics
N2  - Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators BixY3-xFe5O12 with 1 xBi 3. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of 0.6 ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevB.106.184416
SN  - 2469-9950
SN  - 2469-9969
VL  - 106
IS  - 18
PB  - American Institute of Physics, American Physical Society
CY  - Woodbury, NY
ER  - 
TY  - JOUR
A1  - Deb, Marwan
A1  - Popova, Elena
A1  - Jaffrès, Henri-Yves
A1  - Keller, Niels
A1  - Bargheer, Matias
T1  - Controlling high-frequency spin-wave dynamics using double-pulse laser excitation
JF  - Physical review applied
N2  - Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetiza-tion and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevApplied.18.044001
SN  - 2331-7019
VL  - 18
IS  - 4
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - GEN
A1  - Stete, Felix
A1  - Koopman, Wouter-Willem Adriaan
A1  - Bargheer, Matias
T1  - Signatures of strong coupling on nanoparticles
BT  - revealing absorption anticrossing by tuning the dielectric environment
T2  - Quantum Nano-Photonics
N2  - The electromagnetic coupling of molecular excitations to plasmonic nanoparticles offers a promising method to manipulate the light-matter interaction at the nanoscale. Plasmonic nanoparticles foster exceptionally high coupling strengths, due to their capacity to strongly concentrate the light-field to sub-wavelength mode volumes. A particularly interesting coupling regime occurs, if the coupling increases to a level such that the coupling strength surpasses all damping rates in the system. In this so-called strong-coupling regime hybrid light-matter states emerge, which can no more be divided into separate light and matter components. These hybrids unite the features of the original components and possess new resonances whose positions are separated by the Rabi splitting energy h Omega. Detuning the resonance of one of the components leads to an anticrossing of the two arising branches of the new resonances omega(+) and omega(-) with a minimal separation of Omega = omega(+) - omega(-).
Y1  - 2018
SN  - 978-94-024-1546-9
SN  - 978-94-024-1544-5
SN  - 978-94-024-1543-8
U6  - https://doi.org/10.1007/978-94-024-1544-5_53
SN  - 1871-465X
SP  - 445
EP  - 447
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - GEN
A1  - Stete, Felix
A1  - Schossau, Phillip Gerald
A1  - Koopman, Wouter-Willem Adriaan
A1  - Bargheer, Matias
T1  - Size Dependence of the Coupling Strength in Plasmon-Exciton Nanoparticles
T2  - Quantum Nano-Photonics
N2  - The coupling between molecular excitations and nanoparticles leads to promising applications. It is for example used to enhance the optical cross-section of molecules in surface enhanced Raman scattering, Purcell enhancement or plasmon enhanced dye lasers. In a coupled system new resonances emerge resulting from the original plasmon (ωpl) and exciton (ωex) resonances as
ω±=12(ωpl+ωex)±14(ωpl−ωex)2+g2−−−−−−−−−−−−−−−√,
(1)
where g is the coupling parameter. Hence, the new resonances show a separation of Δ = ω+ − ω− from which the coupling strength can be deduced from the minimum distance between the two resonances, Ω = Δ(ω+ = ω−).
Y1  - 2018
SN  - 978-94-024-1546-9
SN  - 978-94-024-1544-5
SN  - 978-94-024-1543-8
U6  - https://doi.org/10.1007/978-94-024-1544-5_26
SN  - 1871-465X
SP  - 381
EP  - 383
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - THES
A1  - Born, Artur
T1  - Electronic structure, quasi-particle interaction and relaxation in 3d-elements from X-ray spectroscopy
Y1  - 2021
ER  - 
TY  - THES
A1  - Schick, Daniel
T1  - Ultrafast lattice dynamics in photoexcited nanostructures : femtosecond X-ray diffraction with optimized evaluation schemes
T1  - Ultraschnelle Gitterdynamik in optisch angeregten Nanostrukturen : Femtosekunden-Röntgendiffraktion mit optimierten Auswerteroutinen
N2  - Within the course of this thesis, I have investigated the complex interplay between electron and lattice dynamics in nanostructures of perovskite oxides. Femtosecond hard X-ray pulses were utilized to probe the evolution of atomic rearrangement directly, which is driven by ultrafast optical excitation of electrons. The physics of complex materials with a large number of degrees of freedom can be interpreted once the exact fingerprint of ultrafast lattice dynamics in time-resolved X-ray diffraction experiments for a simple model system is well known. The motion of atoms in a crystal can be probed directly and in real-time by femtosecond pulses of hard X-ray radiation in a pump-probe scheme. In order to provide such ultrashort X-ray pulses, I have built up a laser-driven plasma X-ray source. The setup was extended by a stable goniometer, a two-dimensional X-ray detector and a cryogen-free cryostat. The data acquisition routines of the diffractometer for these ultrafast X-ray diffraction experiments were further improved in terms of signal-to-noise ratio and angular resolution. The implementation of a high-speed reciprocal-space mapping technique allowed for a two-dimensional structural analysis with femtosecond temporal resolution. I have studied the ultrafast lattice dynamics, namely the excitation and propagation of coherent phonons, in photoexcited thin films and superlattice structures of the metallic perovskite SrRuO3. Due to the quasi-instantaneous coupling of the lattice to the optically excited electrons in this material a spatially and temporally well-defined thermal stress profile is generated in SrRuO3. This enables understanding the effect of the resulting coherent lattice dynamics in time-resolved X-ray diffraction data in great detail, e.g. the appearance of a transient Bragg peak splitting in both thin films and superlattice structures of SrRuO3. In addition, a comprehensive simulation toolbox to calculate the ultrafast lattice dynamics and the resulting X-ray diffraction response in photoexcited one-dimensional crystalline structures was developed in this thesis work. With the powerful experimental and theoretical framework at hand, I have studied the excitation and propagation of coherent phonons in more complex material systems. In particular, I have revealed strongly localized charge carriers after above-bandgap femtosecond photoexcitation of the prototypical multiferroic BiFeO3, which are the origin of a quasi-instantaneous and spatially inhomogeneous stress that drives coherent phonons in a thin film of the multiferroic. In a structurally imperfect thin film of the ferroelectric Pb(Zr0.2Ti0.8)O3, the ultrafast reciprocal-space mapping technique was applied to follow a purely strain-induced change of mosaicity on a picosecond time scale. These results point to a strong coupling of in- and out-of-plane atomic motion exclusively mediated by structural defects.
N2  - Im Rahmen dieser Arbeit habe ich mich mit den komplexen Wechselwirkungen zwischen Elektronen- und Gitterdynamik in oxidischen Perowskit-Nanostrukturen beschäftigt. Dazu wurden verschiedene Proben mit intensiven, ultrakurzen Laserpulsen angeregt. Um die zeitliche Entwicklung der induzierten atomaren Umordnung zu untersuchen, wurden Femtosekunden-Pulse harter Röntgenstrahlung genutzt. Zunächst wurde die ultraschnelle Gitterdynamik in einfachen Modellsystemen mit zeitaufgelösten Röntgendiffraktionsexperimenten untersucht, um im Anschluss ähnliche Experimente an komplexeren Materialien mit mehreren Freiheitsgraden interpretieren zu können. Die Bewegung der Atome in einem Kristall kann über Anrege-Abtast-Verfahren direkt mit gepulster, harter Röntgenstrahlung gemessen werden. Die Dauer der Röntgenpulse muss dafür einige hundert Femtosekunden kurz sein. Um diese ultrakurzen Röntgenpulse zu erzeugen, habe ich eine lasergetriebene Plasma-Röntgenquelle aufgebaut. Der Aufbau wurde um ein stabiles Goniometer, einen zweidimensionalen Röntgendetektor und einen kryogenfreien Kryostat erweitert und in Bezug auf das Signal-zu-Rausch-Verhältnis und die Winkelauflösung optimiert. Durch die Entwicklung einer schnellen Methode zur Vermessung des reziproken Raums konnte erstmals an solch einer Quelle eine zweidimensionale Strukturanalyse mit Femtosekunden-Zeitauflösung realisiert werden. Die Anregung und Ausbreitung von kohärenten Phononen habe ich in optisch angeregten Dünnfilm- und Übergitterstrukturen untersucht. Eine entscheidende Rolle spielen dabei metallische SrRuO3 Schichten. Durch die quasi-instantane Kopplung des Gitters an die optisch angeregten Elektronen in SrRuO3 wird ein räumlich und zeitlich wohldefiniertes Druckprofil erzeugt. Dadurch kann der Einfluss der resultierenden kohärenten Gitterdynamik auf die zeitaufgelösten Röntgendiffraktionsdaten im Detail verstanden werden. Beobachtet wurde z.B. das Auftreten einer transienten Aufspaltung eines Bragg-Reflexes bei Dünnfilm- und Übergitterstrukturen aus SrRuO3. Außerdem wurde eine umfangreiche Simulationsumgebung entwickelt, mit deren Hilfe die ultraschnelle Dynamik und die dazugehörigen Röntgendiffraktionssignale in optisch angeregten eindimensionalen Kristallstrukturen berechnet werden können. Der von mir entwickelte experimentelle Aufbau sowie das Simulationspaket zur Datenanalyse und -interpretation wurden anschließend für die Untersuchung kohärenter Phononen in komplexeren Materialsystemen eingesetzt. Im Speziellen konnte ich in multiferroischem BiFeO3 eine stark lokalisierte Ladungsträgerverteilung nach einer optischen Femtosekunden-Anregung nachweisen. Sie ist die Ursache für einen quasi-instantanen und räumlich inhomogenen Druck, der die kohärenten Phononen in einem dünnen Film dieses Multiferroikums erzeugt. Außerdem habe ich die ultraschnelle Vermessung des reziproken Raums angewendet, um eine verzerrungsinduzierte Veränderung der Mosaizität in einem strukturell unvollkommenen Film aus ferroelektrischem Pb(Zr0.2Ti0.8)O3 zu verfolgen. Die Ergebnisse deuten auf eine ausschließlich durch strukturelle Defekte vermittelte Kopplung der atomaren Bewegungen parallel und senkrecht zur Flächennormalen des Filmes hin.
KW  - ultraschnelle Röntgendiffraktion
KW  - Gitterdynamik
KW  - Nanostruktur
KW  - optische Anregung
KW  - Perowskit
KW  - ultrafast X-ray diffraction
KW  - lattice dynamics
KW  - nanostructure
KW  - photoexcitation
KW  - perovskite
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-68827
ER  - 
TY  - THES
A1  - Haseeb, Haider
T1  - Charge and heat transport across interfaces in nanostructured porous silicon
T1  - Ladungs- und Wärmetransport über Grenzflächen in nanostrukturiertem porösem Silizium
N2  - This thesis discusses heat and charge transport phenomena in single-crystalline Silicon penetrated by nanometer-sized pores, known as mesoporous Silicon (pSi). Despite the extensive attention given to it as a thermoelectric material of interest, studies on microscopic thermal and electronic transport beyond its macroscopic characterizations are rarely reported. In contrast, this work reports the interplay of both.
PSi samples synthesized by electrochemical anodization display a temperature dependence of specific heat 𝐶𝑝 that deviates from the characteristic 𝑇^3 behaviour (at 𝑇<50𝐾). A thorough analysis reveals that both 3D and 2D Einstein and Debye modes contribute to this specific heat. Additional 2D Einstein modes (~3 𝑚𝑒𝑉) agree reasonably well with the boson peak of SiO2 in pSi pore walls. 2D Debye modes are proposed to account for surface acoustic modes causing a significant deviation from the well-known 𝑇^3 dependence of 𝐶𝑝 at 𝑇<50𝐾.
A novel theoretical model gives insights into the thermal conductivity of pSi in terms of porosity and phonon scattering on the nanoscale. The thermal conductivity analysis utilizes the peculiarities of the pSi phonon dispersion probed by the inelastic neutron scattering experiments. A phonon mean-free path of around 10 𝑛𝑚 extracted from the presented model is proposed to cause the reduced thermal conductivity of pSi by two orders of magnitude compared to p-doped bulk Silicon. Detailed analysis indicates that compound averaging may cause a further 10-50% reduction. The percolation threshold of 65% for thermal conductivity of pSi samples is subsequently determined by employing theoretical effective medium models.
Temperature-dependent electrical conductivity measurements reveal a thermally activated transport process. A detailed analysis of the activation energy 𝐸𝐴𝜎 in the thermally activated transport exhibits a Meyer Neldel compensation rule between different samples that originates in multi-phonon absorption upon carrier transport. Activation energies 𝐸𝐴𝑆 obtained from temperature-dependent thermopower measurements provide further evidence for multi-phonon assisted hopping between localized states as a dominant charge transport mechanism in pSi, as they systematically differ from the determined 𝐸𝐴𝜎 values.
N2  - Diese Dissertation befasst sich mit Wärme- und Ladungstransportphänomenen in mesoporösem Silizium (pSi) oder etwas genauer in einkristallinem Silizium, welches mit nanometergroßen Poren durchsetzt ist. Trotz der großen Aufmerksamkeit, die diesem thermoelektrischen Material zuteil wird, wird nur selten über Studien zum mikroskopischen thermischen und elektronischen Transport jenseits seiner makroskopischen Charakterisierung berichtet. Im Gegensatz dazu wird in dieser Studie das Zusammenspiel von beidem untersucht.
PSi-Proben, die durch elektrochemische Anodisierung synthetisiert wurden, zeigen eine Temperaturabhängigkeit der spezifischen Wärme 𝐶𝑝, die vom charakteristischen 𝑇3 Verhalten (bei 𝑇<50𝐾) abweicht. Eine gründliche Analyse zeigt, dass sowohl 3D- als auch 2D-Einstein- und Debye-Moden zu dieser spezifischen Wärme beitragen. Zusätzliche 2D-Einstein-Moden (~3 𝑚𝑒𝑉) stimmen gut mit dem Bosonen-Peak von SiO2 in teilweise oxidierten pSi-Porenwänden überein. 2D-Debye-Moden werden vorgeschlagen, um akustische Oberflächenmoden zu erklären, die eine signifikante Abweichung von der bekannten 𝑇3Abhängigkeit von 𝐶𝑝 bei 𝑇<50𝐾 verursachen.
Ein neuartiges theoretisches Modell gibt Einblicke in die Wärmeleitfähigkeit von pSi in Bezug auf Porosität und Phononenstreuung auf der Nanoskala. Die Analyse der Wärmeleitfähigkeit nutzt die Besonderheiten der pSi-Phononendispersion, die durch Experimente mit inelastischer Neutronenstreuung untersucht wurden. Ein mittlerer freier Weg der Phononen von etwa 10 𝑛𝑚, der aus dem vorgestellten Modell extrahiert wurde, wird als Ursache für die um zwei Größenordnungen geringere Wärmeleitfähigkeit von pSi im Vergleich zu p-dotiertem Silizium vorgeschlagen. Eine detaillierte Analyse zeigt, dass die Porosität selbst eine weitere Verringerung der Wärmeleitfähigkeit um 10-50% verursachen kann. Die Perkolationsschwelle von 65 % für die Wärmeleitfähigkeit von pSi-Proben wird anschließend mit Hilfe eines theoretischen Ansatzes für effektive Medien bestimmt.
Temperaturabhängige Messungen der elektrischen Leitfähigkeit lassen einen thermisch aktivierten Transportprozess erkennen. Eine detaillierte Analyse der Aktivierungsenergie 𝐸𝐴𝜎 im thermisch aktivierten Transport zeigt eine Meyer-Neldel-Kompensationsregel zwischen verschiedenen Proben, die auf Multiphononenabsorption beim Ladungsträgertransport zurückzuführen ist. Aktivierungsenergien 𝐸𝐴𝑆, die aus temperaturabhängigen Seebeck-Messungen gewonnen wurden, liefern weitere Beweise für Multiphononen-unterstütztes Springen zwischen lokalisierten Zuständen als dominanten Ladungstransportmechanismus in pSi, da sie sich systematisch von den ermittelten 𝐸𝐴𝜎 Werten unterscheiden.
KW  - mesoporous
KW  - silicon
KW  - Meyer-Neldel-rule
KW  - nanomaterials
KW  - Meyer-Neldel-Regel
KW  - mesoporös
KW  - Nanomaterialien
KW  - Silizium
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-611224
ER  - 
TY  - THES
A1  - Born, Artur
T1  - Electronic structure, quasi-particle interaction and relaxation in 3d-elements from X-ray spectroscopy
N2  - Any physical system can be described on the level of interacting particles, thus it is of fundamental importance to improve the scientific understanding of interacting many-body systems. This thesis experimentally addresses specific quasi-particle interactions, namely interactions be- tween electrons and between electrons and phonons. It describes the consequential effects of those processes on the electronic structure and the core-hole relaxation pathways in 3d metals. Despite the great amount of experimental and theoretical studies of these interactions and their impact on the behavior of solid-state matter, there are still open questions concerning the cor- responding physical, chemical and mechanical properties of solid-state matter. Especially, the study of 3d metals and their compounds is a great experimental challenge, since those exhibit a variety of spectral features originating from many-body effects such as multiplet splitting, shake up/off satellites, vibrationally excited states or more complex effects like superconductivity and ultrafast demagnetization. In X-ray spectroscopy, these effects often produce overlapping fea- tures, complicating the analysis and limiting the understanding. In this thesis, to overcome the limitations set by conventional X-ray spectroscopy, two different experimental approaches were successfully refined, namely Auger electron photoelectron coincidence spectroscopy (APECS) and temperature-dependent X-ray emission spectroscopy (tXES), which enabled the separation of different core-hole relaxation pathways and the isolation of the impact of specific many-body interactions in the experimental spectra. APECS was utilized at the new Coincidence electron spectroscopy for chemical analysis (Co- ESCA) station at BESSY II to study the core-hole decay and electron-correlation effects in single- crystal Ni, Cu and Co. The observation of photoelectrons in coincidence with Auger electrons allows for the separation of the initial and final state effects in the Auger electron spectra. The results show that a Cu LV V Auger spectrum can be represented by broadened atomic multiplets confirming the localized nature of the intermediate core-hole states. In contrast, the Co LV V Auger spectrum is band-like and can be represented by the self-convolution of the valence band. Ni behaves mixed, localized and itinerant. Thus, the Ni Auger spectrum can only be represented by a mixture of atomic multiplet peaks and the self-convoluted valence band. In the case of Ni, the LV V Auger electrons in coincidence with the 6 eV satellite photoelectrons were also stud- ied. Utilizing the core-hole clock method, the lifetime of the localized double-hole intermediate 2 p53d9 states of 1.8 fs could be determined. However, a fraction of these states delocalizes before the Auger decay contributing to the main peak. A similar delocalization was observed for the double-hole states produced by the L2L3M4,5 Coster-Kronig process. Additionally, the influence of surface oxidation on the Ni(111) 3p levels was studied with APECS. The Ni 3p PES spectrum is broad and featureless, due to overlapping many-body effects and gives little chance for exact analysis using conventional photoelectron spectroscopy. Utilizing APECS or precisely the final state selectivity of the method, the spectral width of the 3p levels could be narrowed and their positions and the spin-orbit splitting were determined. Moreover, due to the surface sensitivity of the method, the chemically shifted 3p photoelectron peaks originating from the oxidized surface and the bulk Ni were disentangled. For the study of the atomic electron-phonon spin-flip scattering in 3d metals as a spin-relaxation channel, the tXES method at the SolidFlexRIXS station was developed. The atomic spin-flip scat- tering was studied in single-crystal Ni, Cu, Co and in FeNi alloys, which show considerable dif- ferences in their behavior. The scattering rate in Ni increases with temperature, whereas the rate in Cu and Co remains constant within the measured temperature range up to 1000 K. In FeNi alloys, our results reveal that the spin-flip scattering is restricted by sublattice exchange energies J. The electron-phonon scattering driven spin-flips only appear in the case where the thermal energy ex- ceeds the exchange energy kT > J. This thresholding is an important microscopic process for the description of the sublattice dynamics in alloys, but as shown also relevant for elemental magnetic systems. Overall, the results strongly indicate that the spin-flip probability is correlated with the exchange energy, which might become an important parameter in the ultrafast demagnetization debate. Taken together, the applied experimental approaches allowed to study complex many-body effects in 3d metals. The results show that utilizing APECS enabled the distinction and clear assignment of otherwise overlapping features in AES or PES spectra of Ni, Cu, Co and NiO. This is of fundamental importance for the basic understanding of photoionization and core-hole decay processes but also for the chemical analysis in applied science. The measurement of the atomic electron-phonon spin-flip scattering rate utilizing tXES shows that the electron-phonon spin-flip scattering is a relevant atomic process for the macroscopic demagnetization process. Additionally, a temperature-dependent thresholding mechanism was discovered, which introduces an important dynamic factor into the electron-phonon spin-flip model.
KW  - X-ray spectroscopy
KW  - photoelectron spectroscopy
KW  - Auger electron spectroscop
KW  - X-ray absorption spectroscopy
KW  - X-ray emission spectroscopy
KW  - 3d metals
KW  - electronic structure
KW  - quasi-particle interaction
Y1  - 2021
ER  - 
TY  - JOUR
A1  - Deb, Marwan
A1  - Popova, Elena
A1  - Hehn, Michel
A1  - Keller, Niels
A1  - Petit-Watelot, Sebastien
A1  - Bargheer, Matias
A1  - Mangin, Stephane
A1  - Malinowski, Gregory
T1  - Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets
JF  - Physical review letters
N2  - We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (similar to 33 mT) by a spin dynamics with a frequency up to 15 GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics.
Y1  - 2019
U6  - https://doi.org/10.1103/PhysRevLett.123.027202
SN  - 0031-9007
SN  - 1079-7114
VL  - 123
IS  - 2
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - THES
A1  - Tchoumba Kwamen, Christelle Larodia
T1  - Investigating the dynamics of polarization reversal in ferroelectric thin films by time-resolved X-ray diffraction
T1  - Untersuchung der Dynamik der Polarisationsumkehr in ferroelektrischen Dünnschichten durch zeitaufgelöste Röntgenbeugung
N2  - Ferroic materials have attracted a lot of attention over the years due to their wide range of applications in sensors, actuators, and memory devices. Their technological applications originate from their unique properties such as ferroelectricity and piezoelectricity. In order to optimize these materials, it is necessary to understand the coupling between their nanoscale structure and transient response, which are related to the atomic structure of the unit cell.
In this thesis, synchrotron X-ray diffraction is used to investigate the structure of ferroelectric thin film capacitors during application of a periodic electric field. Combining electrical measurements with time-resolved X-ray diffraction on a working device allows for visualization of the interplay between charge flow and structural motion. This constitutes the core of this work. The first part of this thesis discusses the electrical and structural dynamics of a ferroelectric Pt/Pb(Zr0.2,Ti0.8)O3/SrRuO3 heterostructure during charging, discharging, and polarization reversal. After polarization reversal a non-linear piezoelectric response develops on a much longer time scale than the RC time constant of the device. The reversal process is inhomogeneous and induces a transient disordered domain state. The structural dynamics under sub-coercive field conditions show that this disordered domain state can be remanent and can be erased with an appropriate voltage pulse sequence. The frequency-dependent dynamic characterization of a Pb(Zr0.52,Ti0.48)O3 layer, at the morphotropic phase boundary, shows that at high frequency, the limited domain wall velocity causes a phase lag between the applied field and both the structural and electrical responses. An external modification of the RC time constant of the measurement delays the switching current and widens the electromechanical hysteresis loop while achieving a higher compressive piezoelectric strain within the crystal.
In the second part of this thesis, time-resolved reciprocal space maps of multiferroic BiFeO3 thin films were measured to identify the domain structure and investigate the development of an inhomogeneous piezoelectric response during the polarization reversal. The presence of 109° domains is evidenced by the splitting of the Bragg peak.
The last part of this work investigates the effect of an optically excited ultrafast strain or heat pulse propagating through a ferroelectric BaTiO3 layer, where we observed an additional current response due to the laser pulse excitation of the metallic bottom electrode of the heterostructure.
N2  - Ferroika haben aufgrund vielfältiger Anwendungsmöglichkeiten in Sensoren, Motoren und Speichermedien in den letzten Jahren viel Aufmerksamkeit erhalten. Das Interesse für technologische Anwendungen ist in ihren einzigartigen Eigenschaften wie Ferroelektrizität und Piezoelektrizität begründet. Um die Eigenschaften dieser Materialien zu optimieren ist es notwendig, die Kopplung zwischen ihrer Nanostruktur und der zeitabhängigen Antwort auf die Anregung zu verstehen, welcher von der Atomstruktur der Einheitszelle abhängig ist.
In dieser Arbeit wird Röntgenbeugung an einem Synchrotron verwendet, um die Struktur eines ferroelektrischen Dünnschichtkondensators während eines angelegten elektrischen Feld zu beobachten. Den Kern dieser Arbeit bildet die Kombination aus elektrischen zeitaufgelösten Röntgenbeugungsmessungen an einem betriebsfähigen Kondensator, was die Visualisierung des Zusammenspiels zwischen Ladungsbewegung und Strukturdynamik ermöglicht. Der erste Teil der Arbeit befasst sich mit der elektrischen und strukturellen Dynamik einer ferroelektrischen Pt/Pb(Zr0.2,Ti0.8)O3/SrRuO3 Heterostruktur während des Ladens, Entladens und der Polarisationsumkehr. Nach der Umkehr der Polarisation bildet sich auf einer längeren Zeitskala als die RC-Zeitkonstante der Probe ein nichtlineares piezoelektrisches Signal aus. Der Umkehrungsprozess ist inhomogen und induziert einen vorübergehenden Zustand ungeordneter Domänen. Die strukturelle Dynamik mit einem angelegten elektrischen Feld unterhalb des Koerzitivfelds zeigt, dass dieser ungeordnete Zustand remanent sein kann und mit einer entsprechenden Abfolge von Spannungspulsen wieder entfernt werden kann. Die frequenzabhängige Charakterisierung der Dynamik einer Pb(Zr0.52,Ti0.48)O3 Schicht mit einer Zusammensetzung, die der morphotropen Phasengrenze entspricht, zeigt, dass bei hohen Frequenzen die begrenzte Domänenwandgeschwindigkeit eine Phasenverzögerung zwischen dem angelegten Feld und dem strukturellen sowie dem elektrischen Signal verursacht. Eine externe Änderung der RC-Zeitkonstante verzögert den Schaltstrom und verbreitert die elektromechanische Hysteresekurve, während im Kristall eine höhere kompressive piezoeletrische Spannung erzeugt wird.
In dem zweiten Teil dieser Arbeit wurde der reziproke Raum von multiferroischen dünnen BiFeO3 Filmen vermessen, um die Domänenstruktur zu identifizieren und die Entwicklung eines inhomogenen piezoelektrischen Signals während der Polarisationsumkehr zu untersuchen. Das Aufspalten des Bragg Reflexes ist ein Hinweis auf die Existenz von 109° Domänen.
Der letzte Teil der Arbeit beschäftigt sich mit dem Effekt, den ein durch optische Anregung erzeugter ultraschneller Verspannungs- oder Wärmepuls hervorruft, der durch eine ferroelektrische BaTiO3 Schicht propagiert. Dabei wurde durch die Anregung der unteren metallischen Elektrode der Heterostruktur durch den Laserpuls ein zusätzliches Ladungssignal beobachtet.
KW  - ferroelectrics
KW  - X-ray diffraction
KW  - structural dynamics
KW  - Ferroelektrika
KW  - Röntgenbeugung
KW  - Strukturdynamik
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-427815
ER  - 
TY  - JOUR
A1  - Deb, Marwan
A1  - Popova, Elena
A1  - Hehn, Michel
A1  - Keller, Niels
A1  - Petit-Watelot, Sebastien
A1  - Bargheer, Matias
A1  - Mangin, Stephane
A1  - Malinowski, Gregory
T1  - Damping of Standing Spin Waves in Bismuth-Substituted Yttrium Iron Garnet as Seen via the Time-Resolved Magneto-Optical Kerr Effect
JF  - Physical review applied
N2  - We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.
Y1  - 2019
U6  - https://doi.org/10.1103/PhysRevApplied.12.044006
SN  - 2331-7019
VL  - 12
IS  - 4
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - THES
A1  - Jay, Raphael Martin
T1  - Principles of charge distribution and separation
BT  - the case of iron complexes probed by X-ray spectroscopy
N2  - The electronic charge distributions of transition metal complexes fundamentally determine their chemical reactivity. Experimental access to the local valence electronic structure is therefore crucial in order to determine how frontier orbitals are delocalized between different atomic sites and electronic charge is spread throughout the transition metal complex. To that end, X-ray spectroscopies are employed in this thesis to study a series of solution-phase iron complexes with respect to the response of their local electronic charge distributions to different external influences. Using resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) at the iron L-edge, changes in local charge densities are investigated at the iron center depending on different ligand cages as well as solvent environments. A varying degree of charge delocalization from the metal center onto the ligands is observed, which is governed by the capabilities of the ligands to accept charge density into their unoccupied orbitals. Specific solvents are furthermore shown to amplify this process. Solvent molecules of strong Lewis-acids withdraw charge from the ligand allowing in turn for more metal charge to be delocalized onto the ligand. The resulting local charge deficiencies at the metal center are, however, counteracted by competing electron-donation channels from the ligand towards the iron, which are additionally revealed. This is interpreted as a compensating effect which strives to maintain local charge densities at the iron center. This mechanism of charge density preservation is found to be of general nature. Using time-resolved RIXS and XAS at the iron L-edge, an analogous interplay of electron donation and back-donation channels is also revealed for the case of charge-transfer excited states. In such transient configurations, the electronic occupation of iron-centered frontier orbitals has been altered by an optical excitation. Changes in local charge densities that are expected to follow an increased or decreased population of iron-centered orbitals are, however, again counteracted. By scaling the degree of electron donation from the ligand onto the metal, local charge densities at the iron center can be efficiently maintained. Since charge-transfer excitations, however, often constitute the initial step in many electron transfer processes, these findings challenge common notions of charge-separation in transition metal dyes.
KW  - L-edge spectroscopy
KW  - charge-transfer excitations
KW  - transition metal complexes
KW  - electronic structure
KW  - photo-chemical pathways
Y1  - 2020
ER  - 
TY  - JOUR
A1  - Perdigón-Toro, Lorena
A1  - Le Quang Phuong, 
A1  - Eller, Fabian
A1  - Freychet, Guillaume
A1  - Saglamkaya, Elifnaz
A1  - Khan, Jafar
A1  - Wei, Qingya
A1  - Zeiske, Stefan
A1  - Kroh, Daniel
A1  - Wedler, Stefan
A1  - Koehler, Anna
A1  - Armin, Ardalan
A1  - Laquai, Frederic
A1  - Herzig, Eva M.
A1  - Zou, Yingping
A1  - Shoaee, Safa
A1  - Neher, Dieter
T1  - Understanding the role of order in Y-series non-fullerene solar cells to realize high open-circuit voltages
JF  - Advanced energy materials
N2  - 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.
KW  - energetic disorder
KW  - non-fullerene acceptors
KW  - open-circuit voltage
KW  - organic solar cells
Y1  - 2022
U6  - https://doi.org/10.1002/aenm.202103422
SN  - 1614-6832
SN  - 1614-6840
VL  - 12
IS  - 12
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - THES
A1  - Perdigón-Toro, Lorena
T1  - On the Generation and Fate of Free Carriers in Non-Fullerene Acceptor Organic Solar Cells
N2  - Organic solar cells offer an efficient and cost-effective alternative for solar energy harvesting. This type of photovoltaic cell typically consists of a blend of two organic semiconductors, an electron donating polymer and a low molecular weight electron acceptor to create what is known as a bulk heterojunction (BHJ) morphology. Traditionally, fullerene-based acceptors have been used for this purpose. In recent years, the development of new acceptor molecules, so-called non-fullerene acceptors (NFA), has breathed new life into organic solar cell research, enabling record efficiencies close to 19%. Today, NFA-based solar cells are approaching their inorganic competitors in terms of photocurrent generation, but lag in terms of open circuit voltage (V_OC). Interestingly, the V_OC of these cells benefits from small offsets of orbital energies at the donor-NFA interface, although previous knowledge considered large energy offsets to be critical for efficient charge carrier generation. In addition, there are several other electronic and structural features that distinguish NFAs from fullerenes.
My thesis focuses on understanding the interplay between the unique attributes of NFAs and the physical processes occurring in solar cells. By combining various experimental techniques with drift-diffusion simulations, the generation of free charge carriers as well as their recombination in state-of-the-art NFA-based solar cells is characterized. For this purpose, solar cells based on the donor polymer PM6 and the NFA Y6 have been investigated. The generation of free charge carriers in PM6:Y6 is efficient and independent of electric field and excitation energy. Temperature-dependent measurements show a very low activation energy for photocurrent generation (about 6 meV), indicating barrierless charge carrier separation. Theoretical modeling suggests that Y6 molecules have large quadrupole moments, leading to band bending at the donor-acceptor interface and thereby reducing the electrostatic Coulomb dissociation barrier. In this regard, this work identifies poor extraction of free charges in competition with nongeminate recombination as a dominant loss process in PM6:Y6 devices. Subsequently, the spectral characteristics of PM6:Y6 solar cells were investigated with respect to the dominant process of charge carrier recombination. It was found that the photon emission under open-circuit conditions can be almost entirely attributed to the occupation and recombination of Y6 singlet excitons. Nevertheless, the recombination pathway via the singlet state contributes only 1% to the total recombination, which is dominated by the charge transfer state (CT-state) at the donor-acceptor interface. Further V_OC gains can therefore only be expected if the density and/or recombination rate of these CT-states can be significantly reduced. Finally, the role of energetic disorder in NFA solar cells is investigated by comparing Y6 with a structurally related derivative, named N4. Layer morphology studies combined with temperature-dependent charge transport experiments show significantly lower structural and energetic disorder in the case of the PM6:Y6 blend. For both PM6:Y6 and PM6:N4, disorder determines the maximum achievable V_OC, with PM6:Y6 benefiting from improved morphological order. Overall, the obtained findings point to avenues for the realization of NFA-based solar cells with even smaller V_OC losses. Further reduction of nongeminate recombination and energetic disorder should result in organic solar cells with efficiencies above 20% in the future.
N2  - Organische Solarzellen bieten eine effiziente und kostengünstige Alternative für die Nutzung von Sonnenenergie. Bei dieser Art von Photovoltaikzellen werden in der Regel zwei organische Halbleiter, ein elektronenspendendes Polymer und ein niedermolekularer Elektronenakzeptor gemischt, um eine sogenannte „Bulk-Heterojunction“ (BHJ)-Morphologie zu erzeugen. Traditionell wurden hierfür Fulleren-basierte Akzeptoren verwendet. In den letzten Jahren hat die Entwicklung neuer Akzeptor-Moleküle, so genannter Nicht-Fulleren-Akzeptoren (NFA), der organischen Solarzellenforschung neues Leben eingehaucht und damit Rekordwirkungsgrade >19 % ermöglicht. Heutzutage nähern sich NFA-basierte Solarzellen ihren anorganischen Konkurrenten bezüglich der Photostromerzeugung an, nicht jedoch im Hinblick auf die Leerlaufspannung (V_OC). Interessanterweise profitiert der V_OC dieser Zellen von kleinen Offsets der Orbitalenergien an der Donor-NFA-Grenzfläche, obwohl nach bisherigem Wissen große Energieoffsets als entscheidend für die effiziente Ladungsträgergenerierung an der Heterogrenzfläche galten. Darüber hinaus gibt es eine Reihe weiterer elektronischer und struktureller Merkmale, die NFAs von Fullerenen unterscheiden.
Meine Dissertation konzentriert sich auf ein tiefgreifendes Verständnis des Zusammenspiels der einzigartigen Eigenschaften von NFAs und den physikalischen Prozessen in daraus hergestellten Solarzellen. Durch die Kombination verschiedener experimenteller Techniken mit Drift-Diffusions-Simulationen wird die Erzeugung freier Ladungsträger sowie deren Rekombination in modernen NFA-basierten Solarzellen charakterisiert. Zu diesem Zweck wurden Solarzellen auf Basis des Donor-Polymers PM6 und des NFA Y6 untersucht. Die Erzeugung freier Ladungsträger in PM6:Y6 erweist sich dabei als effizient und unabhängig von elektrischem Feld und Anregungsenergie. Temperaturabhängige Messungen zeigen eine sehr geringe Aktivierungsenergie für die Photostromerzeugung (ca. 6 meV), was auf eine barrierefreie Ladungsträgertrennung hinweist. Theoretische Modellierungen legen nahe, dass Y6-Moleküle große Quadrupolmomente aufweisen, was zu einer Bandverbiegung an der Donor-Akzeptor-Grenzfläche führt und dabei die elektrostatische Coulombsch-Dissoziationsbarriere reduziert. In dieser Hinsicht identifiziert diese Arbeit die schlechte Extraktion freier Ladungen in Konkurrenz zur „nongeminalen“ Rekombination als einen dominanten Verlustprozess in PM6:Y6 Zellen. In weiterer Folge wurden die spektralen Eigenschaften von PM6:Y6-Solarzellen im Hinblick auf den dominanten Prozess der Ladungsträgergenerierung und rekombination untersucht. Es zeigte sich, dass die Photonenemission unter Leerlaufbedingungen fast vollständig auf die Besetzung und Rekombination von Y6-Singlet-Exzitonen zurückgeführt werden kann. Trotzdem trägt der Rekombinationspfad über den Singlett-Zustand nur zu 1 % zur gesamten Rekombination bei, die über den Ladungstransfer-Zustand (CT-state) an der Donor-Akzeptor-Grenzfläche dominiert wird. Weitere V_OC Gewinne sind daher nur zu erwarten, wenn die Dichte und/oder die Rekombinationsrate dieser CT-Zustände erheblich reduziert werden kann. Schließlich wird die Rolle der energetischen Unordnung in NFA-Solarzellen durch den Vergleich von Y6 mit einem strukturverwandten Derivat, genannt N4, untersucht. Untersuchungen zur Schichtmorphologie in Kombination mit Experimenten zum temperaturabhängigen Ladungstransport zeigen eine deutlich geringere strukturelle und energetische Unordnung im Fall des PM6:Y6 Blends. Sowohl für PM6:Y6 als auch für PM6:N4 bestimmt die Unordnung den maximal erreichbaren V_OC, wobei PM6:Y6 von der verbesserten morphologischen Ordnung profitiert. Insgesamt weisen die gewonnenen Erkenntnisse Wege für die Realisierung von NFA-basierten Solarzellen mit noch kleineren V_OC-Verlusten auf. Durch die weitere Reduzierung der „nongeminaten“ Rekombination als auch der energetischen Unordnung sollten in Zukunft organische Solarzellen mit einem Wirkungsgrad von über 20 % möglich werden.
T2  - Über die Photogenerierung und Rekombination freier Ladungsträger in organischen Solarzellen mit Nicht-Fulleren-Akzeptoren
KW  - organic solar cells
KW  - non-fullerene acceptors
KW  - free charge generation
KW  - free charge recombination
KW  - energetic disorder
KW  - organic semiconductors
KW  - energetische Unordnung
KW  - Generierung freier Ladungsträger
KW  - freie Ladungsträger Rekombination
KW  - Nicht-Fulleren-Akzeptoren
KW  - organische Halbleiter
KW  - organische Solarzellen
Y1  - 2022
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-558072
ER  - 
TY  - JOUR
A1  - Marin-Beloqui, Jose
A1  - Zhang, Guanran
A1  - Guo, Junjun
A1  - Shaikh, Jordan
A1  - Wohrer, Thibaut
A1  - Hosseini, Seyed Mehrdad
A1  - Sun, Bowen
A1  - Shipp, James
A1  - Auty, Alexander J.
A1  - Chekulaev, Dimitri
A1  - Ye, Jun
A1  - Chin, Yi-Chun
A1  - Sullivan, Michael B.
A1  - Mozer, Attila J.
A1  - Kim, Ji-Seon
A1  - Shoaee, Safa
A1  - Clarke, Tracey M.
T1  - Insight into the origin of trapping in polymer/fullerene blends with a systematic alteration of the fullerene to higher adducts
JF  - Journal of physical chemistry C
N2  - The bimolecular recombination characteristics of conjugated polymer poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-y1 thiazolo 5,4-d thiazole)-2,5diy1] (PDTSiTTz) blended with the fullerene series PC60BM, ICMA, ICBA, and ICTA have been investigated using microsecond and femtosecond transient absorption spectroscopy, in conjunction with electroluminescence measurements and ambient photoemission spectroscopy. The non-Langevin polymer PDTSiTTz allows an inspection of intrinsic bimolecular recombination rates uninhibited by diffusion, while the low oscillator strengths of fullerenes allow polymer features to dominate, and we compare our results to those of the well-known polymer Si-PCPDTBT. Using mu s-TAS, we have shown that the trap -limited decay dynamics of the PDTSiTTz polaron becomes progressively slower across the fullerene series, while those of Si-PCPDTBT are invariant. Electroluminescence measurements showed an unusual double peak in pristine PDTSiTTz, attributed to a low energy intragap charge transfer state, likely interchain in nature. Furthermore, while the pristine PDTSiTTz showed a broad, low-intensity density of states, the ICBA and ICTA blends presented a virtually identical DOS to Si-PCPDTBT and its blends. This has been attributed to a shift from a delocalized, interchain highest occupied molecular orbital (HOMO) in the pristine material to a dithienosilole-centered HOMO in the blends, likely a result of the bulky fullerenes increasing interchain separation. This HOMO localization had a side effect of progressively shifting the polymer HOMO to shallower energies, which was correlated with the observed decrease in bimolecular recombination rate and increased "trap" depth. However, since the density of tail states remained the same, this suggests that the traditional viewpoint of "trapping" being dominated by tail states may not encompass the full picture and that the breadth of the DOS may also have a strong influence on bimolecular recombination.
Y1  - 2022
U6  - https://doi.org/10.1021/acs.jpcc.1c10378
SN  - 1932-7447
SN  - 1932-7455
VL  - 126
IS  - 5
SP  - 2708
EP  - 2719
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Cabalar, Pedro
A1  - Fandiño, Jorge
A1  - Lierler, Yuliya
T1  - Modular Answer Set Programming as a formal specification language
JF  - Theory and practice of logic programming
N2  - In this paper, we study the problem of formal verification for Answer Set Programming (ASP), namely, obtaining aformal proofshowing that the answer sets of a given (non-ground) logic programPcorrectly correspond to the solutions to the problem encoded byP, regardless of the problem instance. To this aim, we use a formal specification language based on ASP modules, so that each module can be proved to capture some informal aspect of the problem in an isolated way. This specification language relies on a novel definition of (possibly nested, first order)program modulesthat may incorporate local hidden atoms at different levels. Then,verifyingthe logic programPamounts to prove some kind of equivalence betweenPand its modular specification.
KW  - Answer Set Programming
KW  - formal specification
KW  - formal verification
KW  - modular logic programs
Y1  - 2020
U6  - https://doi.org/10.1017/S1471068420000265
SN  - 1471-0684
SN  - 1475-3081
VL  - 20
IS  - 5
SP  - 767
EP  - 782
PB  - Cambridge University Press
CY  - New York
ER  -