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1. Spectroscopic analysis of vibrational coupling in multi-molecular excited states

Author

Hammer, Sebastian, Linderl, Theresa, Tvingstedt, Kristofer, Brütting, Wolfgang, and Pflaum, Jens

Subjects
Condensed Matter - Materials Science
Abstract

Multi-molecular excited states accompanied by an intra- and inter-molecular geometric relaxation are commonly encountered in optical and electrooptical studies and applications of organic semiconductors as, for example excimers or charge transfer states. Understanding the dynamics of these states is crucial to improve organic devices such as light emitting diodes and solar cells. Their full microscopic description, however, demands for sophisticated tools such as ab-initio quantum chemical calculations which come at the expenses of high computational costs and are prone to errors by assumptions as well as iterative algorithmic procedures. Hence, the analysis of spectroscopic data is often conducted on a phenomenological level only. Here, we present a toolkit to analyze temperature dependent luminescence data and gain first insights into the relevant microscopic parameters of the molecular system at hand. By means of a Franck-Condon based approach considering a single effective inter-molecular vibrational mode and different potentials for the ground and excited state we are able to explain the luminescence spectra of such multi-molecular states. We demonstrate that by applying certain reasonable simplifications the luminescence of charge transfer states as well as excimers can be satisfactorily reproduced for temperatures ranging from cryogenics to above room temperature. We present a semi-classical and a quantum-mechanical description of our model and, for both cases, demonstrate its applicability by analyzing the temperature depended luminescence of the amorphous donor-acceptor heterojunction tetraphenyldibenzoperiflanthene:C$_{60}$ as well as polycrystalline zinc-phthalocyanine to reproduce the luminescence spectra and extract relevant system parameters such as the excimer binding energy., Comment: manuscript: 16 pages, 6 figures; supplementary information: 12 pages, 7 figures

Published
2022
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2. On the Absence of Triplet Exciton Loss Pathways in Non-Fullerene Acceptor based Organic Solar Cells

Author

Kotova, Maria S., Londi, Giacomo, Junker, Johannes, Dietz, Stefanie, Privitera, Alberto, Tvingstedt, Kristofer, Beljonne, David, Sperlich, Andreas, and Dyakonov, Vladimir

Subjects
Physics - Chemical Physics
Abstract

We investigate the viability of highly efficient organic solar cells (OSCs) based on non-fullerene acceptors (NFA) by taking into consideration efficiency loss channels and stability issues caused by triplet excitons (TE) formation. OSCs based on a blend of the conjugated donor polymer PBDB-T and ITIC as acceptor were fabricated and investigated with electrical, optical and spin-sensitive methods. The spin-Hamiltonian parameters of molecular TEs and charge transfer TEs in ITIC e.g., zero-field splitting and charge distribution, were calculated by Density Functional Theory (DFT) modelling. In addition, the energetic model describing the photophysical processes in the donor-acceptor blend was derived. Spin-sensitive photoluminescence measurements prove the formation of charge transfer (CT) states in the blend and the formation of TEs in the pure materials and the blend. However, no molecular TE signal is observed in the completed devices under working conditions by spin-sensitive electrical measurements. The absence of a molecular triplet state population allows to eliminate a charge carrier loss channel and irreversible photooxidation facilitated by long-lived triplet states. These results correlate well with the high power conversion efficiency of the PBDB-T:ITIC-based OSCs and their high stability.

Published
2020
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3. A Theoretical Perspective on Transient Photovoltage and Charge Extraction Techniques

Author

Sandberg, Oskar J., Tvingstedt, Kristofer, Meredith, Paul, and Armin, Ardalan

Subjects
Physics - Applied Physics
Abstract

Transient photovoltage (TPV) is a technique frequently used to determine charge carrier lifetimes in thin-film solar cells such as organic, dye sensitized and perovskite solar cells. As this lifetime is often incident light intensity dependent, its relevance to understanding the intrinsic properties of a photoactive material system as a material or device figure of merit has been questioned. To extract complete information on recombination dynamics, the TPV measurements are often performed in conjunction with charge extraction (CE) measurements, employed to determine the photo-generated charge carrier density and thereby the recombination rate constant and its order. In this communication, the underlying theory of TPV and CE is reviewed and expanded. Our theoretical findings are further solidified by numerical simulations and experiments on organic solar cells. We identify regimes of the open-circuit voltage within which accurate lifetimes and carrier densities can be determined with TPV and CE experiments. A wide range of steady-state light intensities is required in performing these experiments in order to identify their 'working dynamic range' from which the recombination kinetics in thin-film solar cells can be determined.

Published
2019
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14. Optoelectronic Properties of Cs2AgBiBr6 Thin Films: The Influence of Precursor Stoichiometry

Author

Sirtl, Maximilian T., primary, Armer, Melina, additional, Reb, Lennart K., additional, Hooijer, Rik, additional, Dörflinger, Patrick, additional, Scheel, Manuel A., additional, Tvingstedt, Kristofer, additional, Rieder, Philipp, additional, Glück, Nadja, additional, Pandit, Pallavi, additional, Roth, Stephan V., additional, Müller-Buschbaum, Peter, additional, Dyakonov, Vladimir, additional, and Bein, Thomas, additional

Published
2020
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19. Influence of crystallisation on the structural and optical properties of lead-free Cs2AgBiBr6 perovskite crystals.

Author

Armer, Melina, Höcker, Julian, Büchner, Carsten, Häfele, Sophie, Dörflinger, Patrick, Sirtl, Maximilian T., Tvingstedt, Kristofer, Bein, Thomas, and Dyakonov, Vladimir

Subjects
OPTICAL properties, CRYSTAL optics, PEROVSKITE, CRYSTALLIZATION, CRYSTALS
Abstract

By slowly evaporating organic solvents, we succeeded in growing lead-free double perovskite Cs2AgBiBr6 crystals. Solubility curves prove the advantage of this growth technique and allow comparison with the controlled cooling method. The growth temperature significantly affects the optical properties of the crystals, which enables us to better understand the material structure–property relationships. [ABSTRACT FROM AUTHOR]

Published
2021
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24. How far does the defect tolerance of lead-halide perovskites range? The example of Bi impurities introducing efficient recombination centers

Author

Yavari, Mozhgan, primary, Ebadi, Firouzeh, additional, Meloni, Simone, additional, Wang, Zi Shuai, additional, Yang, Terry Chien-Jen, additional, Sun, Shijing, additional, Schwartz, Heidi, additional, Wang, Zaiwei, additional, Niesen, Bjoern, additional, Durantini, Javier, additional, Rieder, Philipp, additional, Tvingstedt, Kristofer, additional, Buonassisi, Tonio, additional, Choy, Wallace C. H., additional, Filippetti, Alessio, additional, Dittrich, Thomas, additional, Olthof, Selina, additional, Correa-Baena, Juan-Pablo, additional, and Tress, Wolfgang, additional

Published
2019
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28. Direct observation of spin states involved in organic electroluminescence based on thermally activated delayed fluorescence

Author

Väth, Stefan, Tvingstedt, Kristofer, Auth, Michael, Sperlich, Andreas, Dabulienė, Asta, Gražulevičius, Juozas Vidas, Stakhira, Pavlo, Cherpak, Vladyslav, Dyakonov, Vladimir, and „Wiley' grupė

Subjects
thermally activated delayed fluorescence, Materials science, Spin states, Physics::Instrumentation and Detectors, 02 engineering and technology, Electroluminescence, 010402 general chemistry, Photochemistry, 01 natural sciences, law.invention, electroluminescence, Condensed Matter::Materials Science, law, OLED, Electron paramagnetic resonance, Organic electroluminescence, business.industry, Condensed Matter::Other, Direct observation, 021001 nanoscience & nanotechnology, Fluorescence, Atomic and Molecular Physics, and Optics, organic light emitting diodes, 0104 chemical sciences, Electronic, Optical and Magnetic Materials, electron paramagnetic resonance, exciplexes, Optoelectronics, 0210 nano-technology, business
Abstract

The first application of electroluminescence detected electron spin resonance technique on electroluminescent thermally activated delayed fluorescence (TADF) devices is shown. It indubitably proves the involvement of spins in the emergence of TADF electroluminescence and reveals the temperature dependency of such spin-dependent processes on bilayer devices that consist of two different material systems.

Published
2017

30. Perovskite Solar Cells: Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States, Charge Transport, and Recombination (Adv. Energy Mater. 16/2018)

Author

Hu, Yinghong, primary, Hutter, Eline M., additional, Rieder, Philipp, additional, Grill, Irene, additional, Hanisch, Jonas, additional, Aygüler, Meltem F., additional, Hufnagel, Alexander G., additional, Handloser, Matthias, additional, Bein, Thomas, additional, Hartschuh, Achim, additional, Tvingstedt, Kristofer, additional, Dyakonov, Vladimir, additional, Baumann, Andreas, additional, Savenije, Tom J., additional, Petrus, Michiel L., additional, and Docampo, Pablo, additional

Published
2018
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31. Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States and Charge Carrier Mobility

Author

Hu, Yinghong, primary, Docampo, Pablo, additional, Rieder, Philipp, additional, Grill, Irene, additional, Hanisch, Jonas, additional, Aygüler, Meltem F., additional, Hufnagel, Alexander G., additional, Handloser, Matthias, additional, Hutter, Eline M., additional, Hartschuh, Achim, additional, Tvingstedt, Kristofer, additional, Dyakonov, Vladimir, additional, Baumann, Andreas, additional, Savenije, Tom J., additional, Petrus, Michiel L., additional, and Bein, Thomas, additional

Published
2018
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34. Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States, Charge Transport, and Recombination

Author

Hu, Yinghong, primary, Hutter, Eline M., additional, Rieder, Philipp, additional, Grill, Irene, additional, Hanisch, Jonas, additional, Aygüler, Meltem F., additional, Hufnagel, Alexander G., additional, Handloser, Matthias, additional, Bein, Thomas, additional, Hartschuh, Achim, additional, Tvingstedt, Kristofer, additional, Dyakonov, Vladimir, additional, Baumann, Andreas, additional, Savenije, Tom J., additional, Petrus, Michiel L., additional, and Docampo, Pablo, additional

Published
2018
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36. Optical modeling of a folded organic solar cell.

Author

Andersson, Viktor, Tvingstedt, Kristofer, and Inganäs, Olle

Subjects
*SOLAR cells, *FINITE element method, *ENERGY dissipation, *WAVE equation, *ELECTROMAGNETIC waves, *MATHEMATICAL models
Abstract

The optical behavior of a reflective tandem solar cell (V cell) is modeled by means of finite element method (FEM) simulations. The absorption of solar light in the active material as well as in both electrode layers is calculated. The FEM solves the electromagnetic wave equation on the entire defined geometry, resulting in the consideration of interference effects, as well as effects of refraction and reflection. Both single cells and tandem cells are modeled and confirmed to be in accordance with reflectance measurements. Energy dissipation in the active layers is studied as a function of layer thickness and folding angle, and the simulations clearly display the advantage of the light trapping feature of folded cells. This is especially prominent in cells with thinner active layers, where folding induces absorption in the active layer equivalent to that of much thicker cells. [ABSTRACT FROM AUTHOR]

Published
2008
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37. Electroluminescence from charge transfer states in polymer solar cells

Author

Tvingstedt, Kristofer, Vandewal, Koen, Gadisa, Abay, Fengling Zhang, Manca, Jean, and Inganas, Olle

Subjects
Charge transfer -- Analysis, Electroluminescence -- Evaluation, Polymers -- Chemical properties, Polymers -- Optical properties, Polymers -- Electric properties, Solar batteries -- Electric properties, Solar batteries -- Optical properties, Solar cells -- Electric properties, Solar cells -- Optical properties, Chemistry
Abstract

The omnipresent electroluminescence (EL) emitted from the charge transfer states in several different types of polymer solar cells is detected. The correlation of different voltage emission and the lower energy blend of EL are also discussed.

Published
2009

39. Intrinsic non-radiative voltage losses in fullerene-based organic solar cells

Author

Benduhn, Johannes, primary, Tvingstedt, Kristofer, additional, Piersimoni, Fortunato, additional, Ullbrich, Sascha, additional, Fan, Yeli, additional, Tropiano, Manuel, additional, McGarry, Kathryn A., additional, Zeika, Olaf, additional, Riede, Moritz K., additional, Douglas, Christopher J., additional, Barlow, Stephen, additional, Marder, Seth R., additional, Neher, Dieter, additional, Spoltore, Donato, additional, and Vandewal, Koen, additional

Published
2017
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44. Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; static vs.dynamic disorderElectronic supplementary information (ESI) available. See DOI: 10.1039/d0mh00385a

Author

Tvingstedt, Kristofer, Benduhn, Johannes, and Vandewal, Koen

Abstract

The origin of energetic disorder in organic semiconductors and its impact on opto-electronic properties remains a topic of intense controversy. Particularly the disorder at electron donor–acceptor interfaces for organic photovoltaics is pivotal to understand as it is expected to affect photo-carrier generation, recombination and consequently device efficiency parameters. In this work we evaluate the temperature dependence of the line-shape of the photoluminescence (PL) and electroluminescence (EL) spectra of small molecule:fullerene blend devices, with the ambition to disentangle dynamic and static disorder contributions. The EL emission spectra are dominated by charge-transfer (CT) state emission and are confirmed to be of Gaussian character and almost completely voltage independent. More importantly, a strong line-width narrowing is persistently observed upon cooling, down to a certain material specific low temperature, below which the line-width remains constant. It is consequently clear that the main portion of the line-width measured at operating conditions of room temperature or higher, is originating from thermally activated, or dynamic, disorder. The observed temperature dependence of the high-energy emission tail can be fully described by taking into account high and low frequency molecular vibrational modes, without having to rely on static disorder. The presence of low frequency molecular modes with large Huang–Rhys factors results in a Gaussian line-shape, which is additionally broadened at high temperature by thermal population of high frequency intra-molecular modes. We therefore cast strong doubts regarding the commonly used assumption that single temperature optical measurements of absorption or emission tails are able to provide meaningful information regarding the shape of a static density of states tail.

Published
2020
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45. Efficient Solution Processed CH3NH3PbI3 Perovskite Solar Cells with PolyTPD Hole Transport Layer.

Author

Höcker, Julian, Kiermasch, David, Rieder, Philipp, Tvingstedt, Kristofer, Baumann, Andreas, and Dyakonov, Vladimir

Subjects
PEROVSKITE, SILICON solar cells, SOLAR cells, FRONTIER orbitals, VALENCE bands
Abstract

The organic and hydrophobic polymer poly[N, N′-bis(4-butilphenyl)-N, N′-bis(phenyl)-benzidine] (polyTPD) represents a promising hole transport layer (HTL) for perovskite photovoltaics due to its suitable energy levels, whereby its highest occupied molecular orbital level matches well with the valence band level of methylammonium lead triiodide (CH3NH3PbI3, MAPbI3) perovskite. However, processing a perovskite layer from the solution on the surface of this organic material, is found to be difficult due to the surface properties of the latter. In this study, we evaluate efficient p-i-n type MAPbI3 perovskite solar cells employing differently processed polyTPD layers. We found that the surface coverage of the MAPbI3 perovskite layer strongly depends on the preparation method of the underlying polyTPD layer. By varying the solvents for the polyTPD precursor, its concentration, and by applying an optimised two-step perovskite deposition technique we increased both the surface coverage of the perovskite layer as well as the power conversion efficiency (PCE) of the corresponding solar cell devices. Our simple solvent-engineering approach demonstrates that no further interface modifications are needed for a successful preparation of efficient planar photovoltaic devices with PCEs in the range of 15 %–16 %. [ABSTRACT FROM AUTHOR]

Published
2019
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47. Efficient Solution Processed CH3NH3PbI3Perovskite Solar Cells with PolyTPD Hole Transport Layer

Author

Höcker, Julian, Kiermasch, David, Rieder, Philipp, Tvingstedt, Kristofer, Baumann, Andreas, and Dyakonov, Vladimir

Abstract

The organic and hydrophobic polymer poly[N, N′-bis(4-butilphenyl)-N, N′-bis(phenyl)-benzidine] (polyTPD) represents a promising hole transport layer (HTL) for perovskite photovoltaics due to its suitable energy levels, whereby its highest occupied molecular orbital level matches well with the valence band level of methylammonium lead triiodide (CH3NH3PbI3, MAPbI3) perovskite. However, processing a perovskite layer from the solution on the surface of this organic material, is found to be difficult due to the surface properties of the latter. In this study, we evaluate efficient p-i-n type MAPbI3perovskite solar cells employing differently processed polyTPD layers. We found that the surface coverage of the MAPbI3perovskite layer strongly depends on the preparation method of the underlying polyTPD layer. By varying the solvents for the polyTPD precursor, its concentration, and by applying an optimised two-step perovskite deposition technique we increased both the surface coverage of the perovskite layer as well as the power conversion efficiency (PCE) of the corresponding solar cell devices. Our simple solvent-engineering approach demonstrates that no further interface modifications are needed for a successful preparation of efficient planar photovoltaic devices with PCEs in the range of 15 %–16 %.

Published
2019
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49. Light Trapping and Alternative Electrodes for Organic Photovoltaic Devices

Author

Tvingstedt, Kristofer

Subjects
Light trapping, Organic solar cells, Naturvetenskap, Polymer solar cells, Plastsolceller, Natural Sciences, Tandem cells
Abstract

Organic materials, such as conjugated polymers, have emerged as a promising alternative for the production of inexpensive and flexible photovoltaic cells. As conjugated polymers are soluble, liquid based printing techniques enable production on large scale to a price much lower than that for inorganic based solar cells. Present day state of the art conjugated polymer photovoltaic cells are comprised by blends of a semiconducting polymer and a soluble derivative of fullerene molecules. Such bulk heterojunction solar cells now show power conversion efficiencies of up to 4-6%. The quantum efficiency of thin film organic solar cells is however still limited by several processes, of which the most prominent limitations are the comparatively low mobility and the high level of charge recombination. Hence organic cells do not yet perform as well as their more expensive inorganic counterparts. In order to overcome this present drawback of conjugated polymer photovoltaics, efforts are continuously devoted to developing materials or devices with increased absorption or with better charge carrier transporting properties. The latter can be facilitated by increasing the mobility of the pure material or by introducing beneficial morphology to prevent carrier recombination. Minimizing the active layer film thickness is an alternative route to collect more of the generated free charge carriers. However, a minimum film thickness is always required for sufficient photon absorption. A further limitation for low cost large scale production has been the dependence on expensive transparent electrodes such as indium tin oxide. The development of cheaper electrodes compatible with fast processing is therefore of high importance. The primary aim of this work has been to increase the absorption in solar cells made from thin films of organic materials. Device construction, deploying new geometries, and evaluation of different methods to provide for light trapping and photon recycling have been strived for. Different routes to construct and incorporate light trapping structures that enable higher photon absorption in a thinner film are presented. By recycling the reflected photons and enhancing the optical path length within a thinner cell, the absorption rate, as well as the collection of more charge carriers, is provided for. Attempts have been performed by utilizing a range of different structures with feature sizes ranging from nanometers up to centimeters. Surface plasmons, Lambertian scatterers, micro lenses, tandem cells as well as larger folded cell structures have been evaluated. Naturally, some of these methods have turned out to be more successful than others. From this work it can nevertheless be concluded that proper light trapping, in thin films of organic materials for photovoltaic energy conversion, is a technique capable of improving the cell performance. In addition to the study of light trapping, two new alternative electrodes for polymer photovoltaic devices are suggested and evaluated.

Published
2007

50. Triplet Excitons in Highly Efficient Solar Cells Based on the Soluble Small Molecule p-DTS(FBTTh2)2.

Author

Väth, Stefan, Tvingstedt, Kristofer, Baumann, Andreas, Heiber, Michael C., Sperlich, Andreas, Love, John A., Nguyen, Thuc‐Quyen, and Dyakonov, Vladimir

Subjects
*SOLAR cells, *EXCITON theory, *BUTYRIC acid, *CHARGE transfer, *ELECTRON paramagnetic resonance
Abstract

Triplet exciton formation in neat 7,7-(4,4- bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl) bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and blends with [6,6]-Phenyl C70 butyric acid methyl ester (PC70BM), with and without the selective solvent additive 1,8-diiodooctane, is investigated by means of spin sensitive photoluminescence measurements. For all three material systems, a significant amount of long living triplet excitons is detected, situated on the p-DTS(FBTTh2)2 molecules. The characteristic zero-field splitting parameters for this state are identified to be D = 42 mT (1177 MHz) and E = 5 mT (140 MHz). However, no triplet excitons located on PC70BM are detectable. Using electrically detected spin resonance, the presence of these triplet excitons is confirmed even at room temperature, highlighting that triplet excitons form during solar cell operation and influence the photocurrent and photovoltage. Surprisingly, the superior performing blend is found to have the largest triplet population. It is concluded, that the formation of triplet excitons from charge transfer states via electron back transfer has no crucial impact on device performance in p-DTS(FBTTh2)2:PC70BM based solar cells. [ABSTRACT FROM AUTHOR]

Published
2017
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