| 1. |
- Gorenflot, Julien, et al.
(författare)
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Increasing the Ionization Energy Offset to Increase the Quantum Efficiency in Non-Fullerene Acceptor-Based Organic Solar Cells: How Far Can We Go?
- 2023
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Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- Molecular engineering of organic semiconductors provides a virtually unlimited number of possible structures, yet only a handful of combinations lead to state-of-the-art efficiencies in photovoltaic applications. Thus, design rules that guide material development are needed. One such design principle is that in a bulk heterojunction consisting of an electron donor and lower bandgap acceptor an offset (Delta IE) of at least 0.45 eV is required between both materials ionization energies to overcome energy level bending at the donor-acceptor interface, in turn maximizing the charge separation yield and the cell's internal quantum efficiency. The present work studies energy losses associated with Delta IE and, based on 24 blends, finds that losses are minimal up to a Delta IE of 0.6 eV. Electroluminescence spectroscopy shows that low energy losses are achieved when the charge transfer state energy (E-CT) is similar to the acceptor's optical bandgap (E-g(A)). Further Delta IE increase lowers E-CT with respect to E-g(A), thus decreasing V-OC. Within that 0.45-0.6 eV Delta IE sweet range, the fill factor FF, hence the power conversion efficiency, increases only marginally as the FF is often already close to maximal for Delta IE = 0.45 eV. The results are extended to 76 binary and ternary blends.
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| 2. |
- Han, Jianhua, et al.
(författare)
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Understanding photochemical degradation mechanisms in photoactive layer materials for organic solar cells
- 2024
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Ingår i: Chemical Society Reviews. - 1460-4744 .- 0306-0012. ; 53:14, s. 7426-7454
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Forskningsöversikt (refereegranskat)abstract
- Over the past decades, the field of organic solar cells (OSCs) has witnessed a significant evolution in materials chemistry, which has resulted in a remarkable enhancement of device performance, achieving efficiencies of over 19%. The photoactive layer materials in OSCs play a crucial role in light absorption, charge generation, transport and stability. To facilitate the scale-up of OSCs, it is imperative to address the photostability of these electron acceptor and donor materials, as their photochemical degradation process remains a challenge during the photo-to-electric conversion. In this review, we present an overview of the development of electron acceptor and donor materials, emphasizing the crucial aspects of their chemical stability behavior that are linked to the photostability of OSCs. Throughout each section, we highlight the photochemical degradation pathways for electron acceptor and donor materials, and their link to device degradation. We also discuss the existing interdisciplinary challenges and obstacles that impede the development of photostable materials. Finally, we offer insights into strategies aimed at enhancing photochemical stability and discuss future directions for developing photostable photo-active layers, facilitating the commercialization of OSCs.
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| 3. |
- Hultmark, Sandra, 1994, et al.
(författare)
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Suppressing Co-Crystallization of Halogenated Non-Fullerene Acceptors for Thermally Stable Ternary Solar Cells
- 2020
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 30:48
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Tidskriftsartikel (refereegranskat)abstract
- While photovoltaic blends based on non-fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene-based acceptors that readily co-crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine-grained ternary blend with nanometer-sized domains that do not coarsen due to a high Tg ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high-temperature processing protocols, which are needed for upscaling and high-throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.
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| 4. |
- Kim, Jongho, et al.
(författare)
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Remarkable conductivity enhancement in P-doped polythiophenes via rational engineering of polymer-dopant interactions
- 2023
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Ingår i: Materials Today Advances. - : Elsevier BV. - 2590-0498. ; 18
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Tidskriftsartikel (refereegranskat)abstract
- Molecular doping is an effective approach to tune the charge density and optimize electrical performance of conjugated polymers. However, the introduction of dopants, on the other hand, may disturb the polymer microstructure and disrupt the charge transport path, often leading to a decrease of charge carrier mobility and deterioration of electrical conductivity of the doped films. Here we show that dopant-induced disorder can be overcome by rational engineering of polymer-dopant interactions, resulting in remarkable enhancement of electrical conductivity. Benchmark poly(3-hexylthiophene) (P3HT) and its analogous random polymers of 3-hexylthiophene and thiophene P[(3HT)1-x-stat-(T)x] were synthesized and doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Remarkably, random P[(3HT)1-x-stat-(T)x] was doped to a far superior electrical conductivity, that in the case of x ≥ 0.24, the conductivity of P[(3HT)1-x-stat-(T)x] is over 100 times higher than that of the doped P3HT, despite both P3HT and P[(3HT)1-x-stat-(T)x] exhibit comparable charge carrier mobility in their pristine state and in spite of their practically identical redox properties. This result can be traced back to the formation of π-stacked polymer-dopant-polymer co-crystals exhibiting extremely short packing distances of 3.13–3.15 Å. The mechanism behind these performances is based on a new role played by the dopant molecules that we name “bridging-gluing”. The results are coherently verified by the combination of optical absorption spectroscopy, X-ray diffraction, density functional theory calculations, and molecular dynamics simulations.
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| 5. |
- Liu, Jian, 1985, et al.
(författare)
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Electrically Programmed Doping Gradients Optimize the Thermoelectric Power Factor of a Conjugated Polymer
- 2024
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Ingår i: Advanced Functional Materials. - 1616-3028 .- 1616-301X. ; 34:18
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Tidskriftsartikel (refereegranskat)abstract
- Functionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the thermoelectric properties of a chemically doped conjugated polymer is studied. The in-plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 µm wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight-fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll-off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices.
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| 6. |
- Paleti, Sri Harish Kumar, et al.
(författare)
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Correlating Acceptor Structure and Blend Nanostructure with the Photostability of Nonfullerene Organic Solar Cells
- 2022
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Ingår i: Solar RRL. - : Wiley. - 2367-198X. ; 6:10
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Tidskriftsartikel (refereegranskat)abstract
- The formation of photoinduced traps resulting in the loss of electron mobility deteriorates the performance of organic solar cells under continuous light soaking. The genesis of these loss mechanisms is elucidated by examining the structural stability of halogenated ITIC derivative films and the phase behavior of the respective binary systems by blending with the donor polymer PBDBT-2F. Under constant illumination, ITIC-4Cl is found to maintain its structural integrity, whereas fluorine on the peripheral moieties of ITIC-4F undergoes chemical substitution to form a mixture of ITIC and ITIC-4F. Thermal analysis of the light-soaked binary films reveals that ITIC-4Cl loses its crystalline phase while the crystallinity of ITIC-4F does not undergo changes. Further, it is shown that the addition of a small amount of ITIC-4F as a third component hinders the loss of ITIC-4Cl crystalline phase in bulk heterojunction blends through the formation of cocrystals. These results suggest that long-range ordering of NFAs does not necessarily improve the photostability of organic solar cells and that the addition of a third component, irrespective of the crystalline nature, can prevent changes in bulk heterojunction blend nanostructure.
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| 7. |
- Paleti, Sri Harish Kumar, et al.
(författare)
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Hexanary blends : a strategy towards thermally stable organic photovoltaics
- 2023
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 °C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells.
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| 8. |
- Paleti, Sri Harish Kumar, 1992, et al.
(författare)
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Impact of doping on the mechanical properties of conjugated polymers
- 2024
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Ingår i: Chemical Society Reviews. - 1460-4744 .- 0306-0012. ; 53:4, s. 1702-1729
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Forskningsöversikt (refereegranskat)abstract
- Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.
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| 9. |
- Xu, Han, et al.
(författare)
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Dissecting the structure-stability relationship of Y-series electron acceptors for real-world solar cell applications
- 2023
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Ingår i: Joule. - 2542-4351. ; 7:9, s. 2135-2151
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Tidskriftsartikel (refereegranskat)abstract
- Despite striking progress toward improving the photovoltaic (PV) performance of organic solar cells (OSCs) with recent Y-series non-fullerene acceptors (Y-NFAs), knowledge about their outdoor performance under real-world conditions and photodegradation mechanisms remains elusive, which is urgently needed to close the lab-to-fab gap of OSCs. Herein, for the first time, we study the structure-outdoor-stability relationship of Y-NFAs. We show that Y-NFAs with long internal side-chains exhibit high energy barriers for photoisomerization, and fluorinated end-groups can enhance the structural confinement to inhibit the photodegradation pathway and thereby improve device stability. Furthermore, the performance loss of Y-NFA-based OSCs under illumination is mainly driven by increased trap-assisted recombination over time. The structure-stability correlation and demonstration of outdoor performance of these state-of-the-art Y-NFA cells provided in this study highlight molecular engineering of device stability control to minimize power output losses in real-world climates.
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| 10. |
- Östergren, Ida, 1991, et al.
(författare)
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A surface passivated fluorinated polymer nanocomposite for carbon monoxide resistant plasmonic hydrogen sensing
- 2024
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Ingår i: Journal of Materials Chemistry A. - 2050-7488 .- 2050-7496. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- Plasmonic hydrogen sensors are promising safety monitoring devices for the emerging hydrogen economy provided a fast response time and poisoning resistance can be achieved. Nanocomposites composed of palladium nanoparticles embedded in a polymer matrix facilitate rapid hydrogen diffusion if a fluorinated polymer is used, while a denser polymer such as atactic poly(methyl methacrylate) (PMMA) facilitates a high degree of gas selectivity. However, nanocomposites that combine a fast response with poisoning resistance have not yet been realized. Here, these two properties are achieved simultaneously by modifying the surface of a fluorinated polymer nanocomposite with a thin PMMA coating, which functions as a molecular sieve that effectively blocks carbon monoxide. The resulting surface passivated nanocomposite shows a high degree of poisoning resistance without compromising a fast sensing response of 2-3 seconds upon exposure to 100 mbar of hydrogen. The sensor signal and response are preserved over 55 cycles of synthetic air containing 5% hydrogen and 500 ppm of carbon monoxide, indicating that nanocomposites are a viable approach for the realization of robust hydrogen sensors.
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