| 1. |
- Tress, Wolfgang, et al.
(författare)
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Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance
- 2013
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Ingår i: Advanced Energy Materials. - : Wiley-VCH Verlag. - 1614-6832 .- 1614-6840. ; 3:5, s. 631-638
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge-carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non-geminate recombination is the dominating loss mechanism in these devices. Numerical drift-diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.
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| 2. |
- Gillett, Alexander J., et al.
(författare)
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The role of charge recombination to triplet excitons in organic solar cells
- 2021
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Ingår i: Nature. - : NATURE PORTFOLIO. - 0028-0836 .- 1476-4687. ; 597:7878, s. 666-
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Tidskriftsartikel (refereegranskat)abstract
- The use of non-fullerene acceptors (NFAs) in organic solar cells has led to power conversion efficiencies as high as 18%(1). However, organic solar cells are still less efficient than inorganic solar cells, which typically have power conversion efficiencies of more than 20%(2). A key reason for this difference is that organic solar cells have low open-circuit voltages relative to their optical bandgaps(3), owing to non-radiative recombination(4). For organic solar cells to compete with inorganic solar cells in terms of efficiency, non-radiative loss pathways must be identified and suppressed. Here we show that in most organic solar cells that use NFAs, the majority of charge recombination under open-circuit conditions proceeds via the formation of non-emissive NFA triplet excitons; in the benchmark PM6:Y6 blend(5), this fraction reaches 90%, reducing the open-circuit voltage by 60 mV. We prevent recombination via this non-radiative channel by engineering substantial hybridization between the NFA triplet excitons and the spin-triplet charge-transfer excitons. Modelling suggests that the rate of back charge transfer from spin-triplet charge-transfer excitons to molecular triplet excitons may be reduced by an order of magnitude, enabling re-dissociation of the spin-triplet charge-transfer exciton. We demonstrate NFA systems in which the formation of triplet excitons is suppressed. This work thus provides a design pathway for organic solar cells with power conversion efficiencies of 20% or more. A substantial pathway for energy loss in organic solar cells may be suppressed by engineering hybridization between non-fullerene acceptor triplet excitons and spin-triplet charge transfer excitons.
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| 3. |
- Moritz, J., et al.
(författare)
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Functional integration approaches via laser powder bed processing
- 2019
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Ingår i: Journal of laser applications. - : Laser Institute of America. - 1042-346X .- 1938-1387. ; 31:2
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing design rules are different from those of conventional fabrication techniques. These allow geometries that would not be possible to achieve otherwise. One example of application is the integration of functional parts as part of the manufacturing process. Conceivable applications range from mechanical functions like integration of moving parts or thermodynamic functions, for example, cooling channels or incorporation of electric circuits for electrical functionalization [J. Glasschroeder, E. Prager, and M. F. Zaeh, Rapid Prototyping J. 21, 207–215 (2015)]. Nevertheless, the potential of functional integration using powder-bed processes is far from being exhausted. The present approach addresses the generation of inner cavities and internal structures of titanium-based parts or components by the use of selective laser melting. This paper focusses on the investigation of voids and cavities regarding their capabilities to add new functions to the material. To this end, comprehensive characterization is performed using destructive as well as nondestructive testing methods. These include 3D scanning, computed tomography, and surface roughness measurements as well as microscopic analysis. Voids and cavities were filled with different thermoplastic materials, followed by the qualitative assessment of the mold filling and resulting material properties. Finally, applications are derived and evaluated with respect to the field of lightweight design or damping structures.
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| 4. |
- Moritz, Juliane, et al.
(författare)
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Hybrid manufacturing of titanium Ti-6Al-4V combining laser metal deposition and cryogenic milling
- 2020
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Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer. - 0268-3768 .- 1433-3015. ; 107:7-8, s. 2995-3009
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Tidskriftsartikel (refereegranskat)abstract
- Hybrid manufacturing, which, e.g., combines additive manufacturing with conventional machining processes, can be a way of overcoming limitations currently encountered in additive manufacturing. Cryogenic milling might be a viable option for hard-to-cut materials, as it leaves a contamination-free surface and can increase surface quality and tool life compared to conventional cooling concepts. In this study, the influence of cryogenic milling with carbon dioxide on titanium Ti-6Al-4V specimens manufactured with laser metal deposition (LMD) was investigated regarding tool wear and surface integrity in comparison to dry machining and machining with cooling lubricants. Moreover, additional layers of material were deposited on top of conventionally and cryogenically machined surfaces by means of LMD. The interface zone was then examined for defects. The milling process was closely monitored by means of thermal and high-speed imaging. Optical and tactile surface analysis provided evidence that lower roughness values and improved surface qualities could be obtained with cryogenic machining in comparison to dry machining. Moreover, significantly less tool wear was observed when a cryogenic cooling medium was applied. Although the utilization of conventional cooling lubricants resulted in satisfying surface qualities, substantial residual contamination on the milled surface was detected by means of fluorescence analysis. These contaminants are suspected to cause defects when the next layer of material is deposited. This is supported by the fact that pores were found in the weld bead applied on top of the milled specimens by means of LMD. Conversely, cryogenic machining resulted in very clean surfaces due to the residue-free evaporation of the coolant. Hence, a good metallurgical bonding between the weld bead and the milled substrate could be achieved. The results indicate the great potential of cryogenic milling in hybrid manufacturing, especially in terms of intermediate machining, as it provides residue-free surfaces for subsequent material deposition without an additional cleaning step and can significantly prolongate tool life.
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| 5. |
- O Reese, Matthew, et al.
(författare)
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Consensus stability testing protocols for organic photovoltaic materials and devices
- 2011
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Ingår i: SOLAR ENERGY MATERIALS AND SOLAR CELLS. - : Elsevier Science B.V., Amsterdam.. - 0927-0248. ; 95:5, s. 1253-1267
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Tidskriftsartikel (refereegranskat)abstract
- Procedures for testing organic solar cell devices and modules with respect to stability and operational lifetime are described. The descriptions represent a consensus of the discussion and conclusions reached during the first 3 years of the international summit on OPV stability (ISOS). The procedures include directions for shelf life testing, outdoor testing, laboratory weathering testing and thermal cycling testing, as well as guidelines for reporting data. These procedures are not meant to be qualification tests, but rather generally agreed test conditions and practices to allow ready comparison between laboratories and to help improving the reliability of reported values. Failure mechanisms and detailed degradation mechanisms are not covered in this report.
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| 6. |
- Polenz, S., et al.
(författare)
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Wavelength dependent laser material processing of ceramic materials
- 2019
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Ingår i: Journal of laser applications. - : Laser Institute of America. - 1042-346X .- 1938-1387. ; 31:2
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Tidskriftsartikel (refereegranskat)abstract
- In the future, ceramic materials will find even more applications in aerospace, energy, and drive technology. Reasons for this are the comparatively low density and good long-term stability at high temperatures for applications for components exposed to high temperatures, e.g., of engines. By using increasing combustion temperatures through the use of ceramics increases the efficiency of modern drive systems [Ohnabe, Masaki, Onozuka, Miyahara, and Sasa, Compos. Part A Appl. Sci. Manuf. 30, 489–496 (1999)]. Despite the high interest of the aviation industry to increase the use of ceramic materials, the time- and energy-consuming classical production of these materials and the concomitant limiting factors in terms of shape and size are still a drawback [Krenkel, Ceramic Matrix Composites Fiber Reinforced Ceramics and their Applications (WIY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008)]. This paper follows a new approach to producing ceramic matrix composites (CMCs). The laser material deposition (LMD) and selective laser melting techniques were used to investigate the coupling of different laser wavelengths into ceramic materials. By combining different energy sources and utilizing wavelength-dependent energy coupling, the additive manufacturing application of ceramic materials to metallic substrates was tested. With the knowledge gained from wavelength-dependent energy coupling, the potential for the production of CMCs should be demonstrated by means of LMD
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| 7. |
- Seidel, André, et al.
(författare)
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Hybrid Additive Manufacturing of Gamma Titanium Aluminide Space Hardware
- 2018
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Ingår i: Contributed Papers from Materials Science and Technology 2018 (MS&T18). - : Association for Iron and Steel Technology (AISTECH). ; , s. 13-21
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Konferensbidrag (refereegranskat)abstract
- A major part of laser additive manufacturing focuses on the fabrication of metallic parts for applications in the space and aerospace sector. Especially the processing of the very brittle titanium aluminides can be particularly challenging [1-2].In the present paper a gamma titanium aluminide (γ-TiAl) nozzle, manufactured via Electron Beam Melting (EBM), is extended and adapted via hybrid Laser Metal Deposition (LMD). The presented approach considers critical impacts like processing temperatures, temperature gradients and solidification conditions with particular regard to crucial material properties like the phenomena of lamellar interface cracking [3-6]. Furthermore, the potential of microstructural tailoring is going to be addressed by the process-specific manipulation of the composition and/or microstructure.In addition to this, selected destructive and non-destructive testing is performed in order to prove the material properties. Finally, post manufacturing and surface modification are briefly addressed.
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| 8. |
- Seidel, A., et al.
(författare)
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Surface modification of additively manufactured gamma titanium aluminide hardware
- 2019
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Ingår i: Journal of laser applications. - : Laser Institute of America. - 1042-346X .- 1938-1387. ; 31:2
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Tidskriftsartikel (refereegranskat)abstract
- A major part of additive manufacturing focuses on the fabrication of metallic parts in different fields of applications. Examples include components for jet engines and turbines and also implants in the medical sector. Titanium alloys represent a material group which is used cross-sectoral in a large number of applications. The present paper addresses the titanium aluminides in particular. These materials have a low density in combination with a comparatively high-temperature resistance [G. Sauthoff, Intermetallics (Wiley-VCH Verlag, Weinheim, Germany, 2008)]. Nevertheless, the laser material processing is rather challenging because of their distinct tendency to lamellar interface cracking. This requires tailored processing strategies and equipment [C. Leyens et al., in Ti-2015: The 13th World Conference on Titanium, Symposium 5. Intermetallics and MMCs, 16–20 August 2015, San Diego, CA (The Minerals, Metals & Materials Society, Pittsburgh, PA, 2016)]. This work focusses on tailored processing of titanium aluminides with focus on the process-dependent surface characteristics. This includes the as-built status for powder bed processing and direct laser metal deposition but also the surface modification via post and/or advanced machining. Finally, comprehensive characterization is performed using destructive as well as nondestructive testing methods. The latter includes 3D scanning, computed tomography, microscopic analysis, and, in particular, surface roughness measurements.
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| 9. |
- Tress, Wolfgang, et al.
(författare)
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Dominating recombination mechanisms in organic solar cells based on ZnPc and C-60
- 2013
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 102:16
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the dominating recombination mechanisms in bulk heterojunction solar cells, using a blend of ZnPc and C-60 as model system. Analyzing the open-circuit voltage (V-oc) as a function of illumination intensity, we find that trap-assisted recombination dominates for low light intensities, whereas at 1 sun, direct/bimolecular recombination becomes important. The recombination parameters are not significantly influenced by the blend mixing ratio and are also valid for injected charges. By changing the hole transport layer, recombination at the contact is separately identified as further mechanism reducing Voc at higher light intensities.
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| 10. |
- Tress, Wolfgang, et al.
(författare)
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Investigation of Driving Forces for Charge Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing S-Shaped Current-Voltage Characteristics
- 2013
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Ingår i: Advanced Energy Materials. - : Wiley-VCH Verlag. - 1614-6832 .- 1614-6840. ; 3:7, s. 873-880
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Tidskriftsartikel (refereegranskat)abstract
- The role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S-shaped current-voltage (J-V) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric field and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the s to ms regime. In combination with drift-diffusion simulation data, we demonstrate a pile-up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S-kinks in the J-V curve and can be distinguished from each other and from other reasons for S-kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S-shaped J-V curve close to open circuit to evaluate whether their electrodes are optimized.
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| 11. |
- Tress, Wolfgang, et al.
(författare)
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Photoconductivity as loss mechanism in organic solar cells
- 2013
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Ingår i: Physica Status Solidi. Rapid Research Letters. - : Wiley-VCH Verlag. - 1862-6254 .- 1862-6270. ; 7:6, s. 401-405
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Tidskriftsartikel (refereegranskat)abstract
- We observe that the reverse current under illumination in solar cells containing C60 and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current-voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C60 layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C60 absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power-generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices.
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