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1. Breaking the angular dispersion limit in thin film optics by ultra-strong light-matter coupling

Author

Mischok, Andreas, Siegmund, Bernhard, Roux, Florian Le, Hillebrandt, Sabina, Vandewal, Koen, and Gather, Malte C.

Subjects
Physics - Optics, Condensed Matter - Soft Condensed Matter
Abstract

Thin film interference is integral to modern photonics and optoelectronics, e.g. allowing for precise design of high performance optical filters1, efficiency enhancements in photovoltaics and light-emitting devices, as well as the realization of microlasers and high-performance photodetectors. However, interference inevitably leads to a change of spectral characteristics with angle, which is generally undesired and can limit the usefulness of thin-film coatings and devices. Here, we introduce a strategy to overcome this fundamental limit in optics by utilizing and tuning the exciton-polariton modes arising in ultra-strongly coupled microcavities. We demonstrate optical filters with narrow pass bands that shift by less than their half width (<15 nm) even at extreme angles. Our filters cover the entire visible range and surpass comparable metal-dielectric-metal filters in all relevant metrics. By expanding this strategy to strong coupling with the photonic sidebands of dielectric multilayer stacks, we also obtain filters with high extinction ratios. Based on these findings, we realize ultrathin and flexible narrowband filter films, monolithically integrate our filters with organic photodiodes, and demonstrate polarization-sensitive polariton filters. These results illustrate how strong coupling provides additional degrees of freedom in thin film optics that will enable a multitude of exciting new applications in micro-optics, sensing, and biophotonics.

Published
2024

2. On the Role of Charge Transfer Excitations in Non-Fullerene Acceptors for Organic Photovoltaics

Author

Giannini, Samuele, Sowood, Daniel J. C., Cerda, Jesus, Frederix, Siebe, Grune, Jeannine, Londi, Giacomo, Marsh, Thomas, Ghosh, Pratyush, Duchemin, Ivan, Greenham, Neil C., Vandewal, Koen, D'Avino, Gabriele, Gillett, Alexander J., and Beljonne, David

Subjects
Condensed Matter - Materials Science
Abstract

Through the development of new non-fullerene electron acceptor (NFA) materials, such as Y6 and its molecular derivatives, the power conversion efficiencies of organic photovoltaics (OPVs) have now exceeded 19%. However, despite this rapid progress, our fundamental understanding of the unique optical and electronic properties of these Y-series NFAs is lacking, and this currently limits progress in material design. In this work, we provide a detailed computational-experimental characterisation of the archetypal NFA, Y6. To explain the significant broadening and red shift of the absorption spectrum observed when moving from the solution phase to the solid state, we first rule out more typical causes, such as J-aggregation. Instead, by considering the role of charge transfer (CT) excitations and their mixing with Frenkel exciton (FE) states, we can computationally reproduce the experimental absorption spectra of Y6 with excellent accuracy. Using transient absorption spectroscopy, we provide evidence for this dense manifold of FE-CT hybrid electronic excitations in Y6 through the prominent sub-picosecond relaxation events following supra band gap excitation. Furthermore, through sub band gap excitation, we also find states with polaronic character in Y6 that are in a dynamic equilibrium with the FE-CT hybrid states. Magnetic resonance spectroscopies reveal that these polaronic states are polaron pairs, most likely located on neighbouring Y6 molecules, not free charge carriers, as has been previously suggested. Thus, this new understanding of how the solid-state packing motif directly controls the optical and electronic properties of Y-series NFAs opens the door to intelligently design NFA materials to further increase OPV performance., Comment: 31 pages, 7 figures

Published
2023

4. Structural order promotes efficient separation of delocalized charges at molecular heterojunctions

Author

Jia, Xiangkun, Soprani, Lorenzo, Londi, Giacomo, Hosseini, Seyed Mehrdad, Talnack, Felix, Mannsfeld, Stefan, Shoaee, Safa, Neher, Dieter, Reineke, Sebastian, Muccioli, Luca, D'Avino, Gabriele, Vandewal, Koen, Beljonne, David, Jia, Donato Spoltore X., Reineke, S., Soprani, L., Muccioli, L., Londi, G., Beljonne, D., Hosseini, S. M., Shoaee, S., Neher, D., Talnack, F., Mannsfeld, S., D'Avino, G., Vandewal, K., and Spoltore, D.

Subjects
Physics - Applied Physics, Condensed Matter - Materials Science
Abstract

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer states (CTS) into free charge carriers in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by structural ordering. We experimentally determine the CTS binding energy of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors over 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher order of the B4PYMPM molecules provides more delocalized CTS. Furthermore, we find no correlation between the quantum efficiency of radiative free charge carrier recombination and the bound or unbound nature of the CTS. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient free carrier generation and shows that more ordering and less bound CT states do not preclude efficient radiative recombination.

Published
2022

8. Charge-Transfer States at the Fullerene Interface Cause Nonradiative Recombination Losses in Sn-Based Perovskite Solar Cells

Author

Zargar, Fatemeh, primary, Desta, Derese, additional, Mertens, Sigurd, additional, Van Landeghem, Melissa, additional, Hamtaei, Sarallah, additional, Prooth, Jeroen, additional, Zillner, Julia, additional, Hajhemati, Javid, additional, Safari, Mohammdhosein, additional, Hardy, An, additional, Schulz, Philip, additional, Ahlswede, Erik, additional, Vermang, Bart, additional, Vandewal, Koen, additional, and Boyen, Hans-Gerd, additional

Published
2024
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10. Redefining near-unity luminescence in quantum dots with photothermal threshold quantum yield

Author

Hanifi, David A, Bronstein, Noah D, Koscher, Brent A, Nett, Zach, Swabeck, Joseph K, Takano, Kaori, Schwartzberg, Adam M, Maserati, Lorenzo, Vandewal, Koen, van de Burgt, Yoeri, Salleo, Alberto, and Alivisatos, A Paul

Subjects
Quantum Physics, Physical Sciences, General Science & Technology
Abstract

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

Published
2019

15. Toward Sustainability in All‐Printed Accumulation Mode Organic Electrochemical Transistors.

Author

Makhinia, Anatolii, Bynens, Lize, Goossens, Arwin, Deckers, Jasper, Lutsen, Laurence, Vandewal, Koen, Maes, Wouter, Beni, Valerio, and Andersson Ersman, Peter

Subjects
TRANSISTORS, LOGIC circuits, SCREEN process printing, ELECTRONIC equipment, SUSTAINABILITY, ORGANIC field-effect transistors
Abstract

This study reports on the first all‐printed vertically stacked organic electrochemical transistors (OECTs) operating in accumulation mode; the devices, relying on poly([4,4′‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)‐2,2′‐bithiophen‐5,5′‐diyl]‐alt‐[thieno[3,2‐b]thiophene‐2,5‐diyl]) (pgBTTT) as the active channel material, are fabricated via a combination of screen and inkjet printing technologies. The resulting OECTs (W/L ≈5) demonstrate good switching performance; gm, norm ≈13 mS cm−1, µC* ≈21 F cm−1 V−1 s−1, ON–OFF ratio > 104 and good cycling stability upon continuous operation for 2 h. The inkjet printing process of pgBTTT is established by first solubilizing the polymer in dihydrolevoglucosenone (Cyrene), a non‐toxic, cellulose‐derived, and biodegradable solvent. The resulting ink formulations exhibit good jettability, thereby providing reproducible and stable p‐type accumulation mode all‐printed OECTs with high performance. Besides the environmental and safety benefits of this solvent, this study also demonstrates the assessment of how the solvent affects the performance of spin‐coated OECTs, which justifies the choice of Cyrene as an alternative to commonly used harmful solvents such as chloroform, also from a device perspective. Hence, this approach shows a new possibility of obtaining more sustainable printed electronic devices, which will eventually result in all‐printed OECT‐based logic circuits operating in complementary mode. [ABSTRACT FROM AUTHOR]

Published
2024
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28. Small Molecule Solar Cells

Author

Moench, Tobias, Koerner, Christian, Murawski, Caroline, Murawski, Jan, Nikolis, Vasileios Christos, Vandewal, Koen, Leo, Karl, He, Liang-Nian, Series editor, Rogers, Robin D., Series editor, Su, Dangsheng, Series editor, Tundo, Pietro, Series editor, Zhang, Z. Conrad, Series editor, Tian, Haining, editor, Boschloo, Gerrit, editor, and Hagfeldt, Anders, editor

Published
2018
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31. Exploring the High‐Temperature Window of Operation for Organic Photovoltaics: A Combined Experimental and Simulations Study.

Author

Negash, Asfaw, Hustings, Jeroen, Robert, Allyson, Genene, Zewdneh, Yilma, Desalegn, Schreurs, Dieter, Mathijs, Michiel, Liesenborgs, Jori, Van Reeth, Frank, Vandewal, Koen, Mammo, Wendimagegn, Admassie, Shimelis, Maes, Wouter, and Manca, Jean V.

Subjects
PHOTOVOLTAIC power generation, SUMMER, CLIMATE change, CURRENT-voltage characteristics, DESERTS, SOLAR technology, FULLERENES
Abstract

The global climate change negatively affects the photovoltaic performance of traditional solar cell technologies. This article investigates the potential of organic photovoltaics (OPV) for high‐temperature environments, ranging from urban hot summers (30—40 °C) and desert regions (65 °C) up to (aero) space conditions (130 °C), the thermal window in which OPV can operate. The approach is based on a combination of experiments and simulations up to 180 °C, moving significantly beyond the conventional temperature ranges reported in the literature. New 2H‐benzo[d][1,2,3]triazole‐5,6‐dicarboxylic imide‐based copolymers with decomposition onset temperatures above 340 °C are used for this study, in combination with non‐fullerene acceptors. Contrary to their inorganic counterparts, OPV devices show a positive temperature coefficient up to ≈90 °C. At temperatures of 150 °C, they are still operational, retaining their room temperature efficiency. Complementary simulations are performed using an in‐house developed software package that numerically solves the drift‐diffusion equations to understand the general trends in the obtained current–voltage characteristics and the materials' intrinsic behavior as a function of temperature. The presented methodology of combined high‐temperature experiments and simulations can be further applied to investigate the thermal window of operation for other OPV material systems, opening novel high‐temperature application routes. [ABSTRACT FROM AUTHOR]

Published
2024
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33. On the Importance of Chemical Precision in Organic Electronics: Fullerene Intercalation in Perfectly Alternating Conjugated Polymers

Author

Vanderspikken, Jochen, primary, Liu, Zhen, additional, Wu, Xiaocui, additional, Beckers, Omar, additional, Moro, Stefania, additional, Quill, Tyler James, additional, Liu, Quan, additional, Goossens, Arwin, additional, Marks, Adam, additional, Weaver, Karrie, additional, Hamid, Mouna, additional, Goderis, Bart, additional, Nies, Erik, additional, Lemaur, Vincent, additional, Beljonne, David, additional, Salleo, Alberto, additional, Lutsen, Laurence, additional, Vandewal, Koen, additional, Van Mele, Bruno, additional, Costantini, Giovanni, additional, Van den Brande, Niko, additional, and Maes, Wouter, additional

Published
2023
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35. Chalcohalide Antiperovskite Thin Films with Visible Light Absorption and High Charge-Carrier Mobility Processed by Solvent-Free and Low-Temperature Methods

Author

Sebastiá-Luna, Paz, primary, Rodkey, Nathan, additional, Mirza, Adeem Saeed, additional, Mertens, Sigurd, additional, Lal, Snigdha, additional, Gaona Carranza, Axel Melchor, additional, Calbo, Joaquín, additional, Righetto, Marcello, additional, Sessolo, Michele, additional, Herz, Laura M., additional, Vandewal, Koen, additional, Ortí, Enrique, additional, Morales-Masis, Mónica, additional, Bolink, Henk J., additional, and Palazon, Francisco, additional

Published
2023
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37. Balanced Energy Gaps as a Key Design Rule for Solution‐Phase Organic Room Temperature Phosphorescence

Author

PAREDIS, Simon, CARDEYNAELS, Tom, Kuila, Suman, DECKERS, Jasper, VAN LANDEGHEM, Melissa, VANDEWAL, Koen, Danos, Andrew, Monkman, Andrew P., Champagne, Benoit, MAES, Wouter, PAREDIS, Simon, CARDEYNAELS, Tom, Kuila, Suman, DECKERS, Jasper, VAN LANDEGHEM, Melissa, VANDEWAL, Koen, Danos, Andrew, Monkman, Andrew P., Champagne, Benoit, and MAES, Wouter

Subjects
room temperature phosphorescence, Organic Chemistry, energy gap tuning, General Chemistry, donor-acceptor fluorophores, time-resolved spectroscopy, Catalysis
Abstract

Metal-free organic emitters that display solution-phase room temperature phosphorescence (sRTP) remain exceedingly rare. Here, we investigate the structural and photophysical properties that support sRTP by comparing a recently reported sRTP compound (BTaz-Th-PXZ) to two novel analogous materials, replacing the donor group by either acridine or phenothiazine. The emissive triplet excited state remains fixed in all three cases, while the emissive charge-transfer singlet states (and the calculated paired charge-transfer T-2 state) vary with the donor unit. While all three materials show dominant RTP in film, in solution different singlet-triplet and triplet-triplet energy gaps give rise to triplet-triplet annihilation followed by weak sRTP for the new compounds, compared to dominant sRTP throughout for the original PXZ material. Engineering both the sRTP state and higher charge-transfer states therefore emerges as a crucial element in designing emitters capable of sRTP. The authors thank the Research Foundation – Flanders (FWO Vlaanderen) for financial support (projects G087718N, G0D1521N, I006320N, GOH3816NAUHL, the Scientific Research Community ‘Supramolecular Chemistry and Materials’ (W000620N), and Ph.D. scholarship S. Paredis). The calculations were performed on the computers of the ‘Consortium des équipements de Calcul Intensif (CÉCI)’ (http://www.ceci-hpc.be), including those of the ‘UNamur Technological Platform of HighPerformance Computing (PTCI)’ (http://www.ptci.unamur.be), for which we gratefully acknowledge financial support from the FNRS-FRFC, the Walloon Region, and the University of Namur (Conventions No. GEQ U.G006.15, U.G018.19, U.G011.22, RW/ GEQ2016, RW1610468, and RW2110213). A.P. Monkman is supported by EPSRC grant EP/T02240X

Published
2023

43. Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

Author

Zhong, Yufei, Causa’, Martina, Moore, Gareth John, Krauspe, Philipp, Xiao, Bo, Günther, Florian, Kublitski, Jonas, Shivhare, Rishi, Benduhn, Johannes, BarOr, Eyal, Mukherjee, Subhrangsu, Yallum, Kaila M., Réhault, Julien, Mannsfeld, Stefan C. B., Neher, Dieter, Richter, Lee J., DeLongchamp, Dean M., Ortmann, Frank, Vandewal, Koen, Zhou, Erjun, and Banerji, Natalie

Published
2020
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