| 1. |
- Belova, Valentina, et al.
(författare)
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Evidence for Anisotropic Electronic Coupling of Charge Transfer States in Weakly Interacting Organic Semiconductor Mixtures
- 2017
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 139:25, s. 8474-8486
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Tidskriftsartikel (refereegranskat)abstract
- We present a comprehensive investigation of the charge-transfer (CT) effect in weakly interacting organic semiconductor mixtures. The donor-acceptor pair diindenoperylene (DIP) and N,N′-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4/9,10-bis(dicarboxyimide) (PDIR-CN2) has been chosen as a model system. A wide range of experimental methods was used in order to characterize the structural, optical, electronic, and device properties of the intermolecular interactions. By detailed analysis, we demonstrate that the partial CT in this weakly interacting mixture does not have a strong effect on the ground state and does not generate a hybrid orbital. We also find a strong CT transition in light absorption as well as in photo- and electroluminescence. By using different layer sequences and compositions, we are able to distinguish electronic coupling in-plane vs out-of-plane and, thus, characterize the anisotropy of the CT state. Finally, we discuss the impact of CT exciton generation on charge-carrier transport and on the efficiency of photovoltaic devices.
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| 2. |
- Brunova, Alica, et al.
(författare)
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Structural and Trap-State Density Enhancement in Flash Infrared Annealed Perovskite Layers
- 2021
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Ingår i: Advanced Materials Interfaces. - : WILEY. - 2196-7350. ; 8:14
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite solar cells are well-known for their high energy conversion efficiency, low-temperature processing, and cost-effective production. Flash infrared annealing (FIRA) of slot-die cast perovskite precursors offers an attractive manufacturing route using high-throughput roll-to-roll technology. Despite the recent progress in FIRA perovskite annealing, the optimal composition of the perovskite precursor is yet to be developed. Here, the effect of methylammonium chloride (MACI) on the perovskite structure and trap-state density as a function ofthe FIRA annealing time is investigated. In situ real-time grazingincidence wide-angle X-ray scattering (GIWAXS) is employed to monitor the perovskite layer formation during FIRA annealing with millisecond temporal resolution. In addition, the density of states in the bandgap is estimated using ex situ energy-resolved electrochemical impedance spectroscopy. Evidence is found that adding 10% MACI into the perovskite precursor solution significantly improves the crystallographic orientation of the perovskite layers while reducing the trap-state density by one order of magnitude. In addition, using time-resolved GIWAXS, the most favorable time window for the FIRA processing of perovskite films with the lowest mosaicity and trap-state density is identified. The results are of general importance for elucidating the appropriate temporal windows in complex and fast-evolving crystallization processes.
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| 3. |
- Hörmann, Ulrich, et al.
(författare)
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Voc from a Morphology Point of View : the Influence of Molecular Orientation on the Open Circuit Voltage of Organic Planar Heterojunction Solar Cells
- 2014
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Ingår i: Journal of physical chemistry C. - : American Chemical Society (ACS). - 1932-7455 .- 1932-7447. ; 118:46, s. 26462-26470
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Tidskriftsartikel (refereegranskat)abstract
- The film morphology and device performance of planar heterojunctionsolar cells based on the molecular donor material α-sexithiophene (6T) are investigated.Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene anddiindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottomlayer has been varied between room temperature and 100 °C for each acceptor. By meansof X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecularorientation of 6T is influenced by the preparation conditions and that the 6T filmtemplates the growth of the subsequent acceptor layer. These structural changes areaccompanied by changes in the characteristic parameters of the correspondingphotovoltaic cells. This is most prominently observed as a shift of the open circuitvoltage (Voc): In the case of 6T/C60 heterojunctions, Voc decreases from 0.4 to 0.3 V,approximately, if the growth temperature of 6T is increased from room temperature to 100°C. By contrast, Voc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solarcells under the same conditions. We attribute these changes upon substrate heating toincreased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.
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| 4. |
- Ahlawat, Paramvir, et al.
(författare)
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A combined molecular dynamics and experimental study of two-step process enabling low-temperature formation of phase-pure alpha-FAPbI3
- 2021
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 7:17
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Tidskriftsartikel (refereegranskat)abstract
- It is well established that the lack of understanding the crystallization process in a two-step sequential deposition has a direct impact on efficiency, stability, and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occurring during the two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, x-ray diffraction, and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable alpha phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that result in the low-temperature crystallization of phase-pure alpha-formamidinium lead iodide.
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| 5. |
- Hagara, Jakub, et al.
(författare)
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Novel highly substituted thiophene-based n-type organic semiconductor : structural study, optical anisotropy and molecular control
- 2020
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Ingår i: CrystEngComm. - : Royal Society of Chemistry. - 1466-8033 .- 1466-8033. ; 22:42, s. 7095-7103
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Tidskriftsartikel (refereegranskat)abstract
- Oligothiophenes and their functionalized derivatives have been shown to be a viable option for high-performance organic electronic devices. The functionalization of oligothiophene-based materials allows further tailoring of their properties for specific applications. We have synthesized a new thiophene-based molecule 1-[5'-(2-naphthyl)-2,2'-bithiophen-5-yl]hexan-1-one (NCOH), and we have studied the optical and structural properties of NCOH thin films. NCOH is a highly substituted member of the oligothiophene family, designed to improve its molecular stacking, where the presence of an electron-withdrawing group enhances its electron transport capabilities. Employing in situ and time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, we determined the NCOH thin film crystallographic structure and its evolution starting from the early stages of the film growth. We observed strong optical anisotropy resulting from a highly oriented crystallographic structure. Additionally, we investigated the substrate-induced changes of the molecular orientation utilizing the few-layer MoS2 with different orientations of the atomic layers. This study, with its primary focus on the fundamentally important n-type molecular semiconductor, contributes to the field of organic-based (opto-)electronics.
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| 6. |
- Zhang, Hong, et al.
(författare)
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Multimodal host-guest complexation for efficient and stable perovskite photovoltaics
- 2021
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Formamidinium lead iodide perovskites are promising light-harvesting materials, yet stabilizing them under operating conditions without compromising optimal optoelectronic properties remains challenging. We report a multimodal host-guest complexation strategy to overcome this challenge using a crown ether, dibenzo-21-crown-7, which acts as a vehicle that assembles at the interface and delivers Cs+ ions into the interior while modulating the material. This provides a local gradient of doping at the nanoscale that assists in photoinduced charge separation while passivating surface and bulk defects, stabilizing the perovskite phase through a synergistic effect of the host, guest, and host-guest complex. The resulting solar cells show power conversion efficiencies exceeding 24% and enhanced operational stability, maintaining over 95% of their performance without encapsulation for 500h under continuous operation. Moreover, the host contributes to binding lead ions, reducing their environmental impact. This supramolecular strategy illustrates the broad implications of host-guest chemistry in photovoltaics. It remains a challenge to achieve a balance between performance and stability, as well as addressing the environmental impact of perovskite solar cells. Here, the authors propose a multimodal host-guest complexation strategy enabling these shortcomings to be addressed simultaneously.
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