| 1. |
- Jan Anton Koster, L., et al.
(författare)
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Quantifying Bimolecular Recombination Losses in Organic Bulk Heterojunction Solar Cells
- 2011
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Ingår i: Advanced Materials. - : Wiley-VCH Verlag. - 0935-9648 .- 1521-4095. ; 23:14, s. 1670-
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Tidskriftsartikel (refereegranskat)abstract
- We present a new experimental technique that affords direct quantification of the fraction of charge carriers lost in poly(3-hexylthiophene): fullerene solar cells by bimolecular recombination. Depending on annealing conditions up to 17% of carriers recombine bimolecularly under solar illumination. We explain our findings with a closed analytical expression for the photocurrent generated by an organic solar cell.
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| 2. |
- Kemerink, Martijn, et al.
(författare)
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Characteristics of a First-Generation X-Ray System
- 2011
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Ingår i: Radiology. - : Radiological Society of North America (RSNA). - 0033-8419 .- 1527-1315. ; 259:2, s. 534-539
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To compare the antiquated x-ray system of Hoffmans and van Kleef (circa 1896) with modern x-ray equipment in terms of radiation dose, x-ray beam properties, image quality, and electrical parameters. Materials and Methods: The antiquated x-ray system consisted of a Ruhmkorff inductor, battery, and Crookes tube. The radiation dose rate, x-ray beam properties, and electrical characteristics of this system were determined. A modern computed radiography plate was used to compare images of a hand specimen obtained by using the antiquated system with images obtained by using the modern system. Results: A peak voltage of 73 kV was obtained with an 8-V battery. With Crookes tube number 9, the half-value layer of the generated x-rays was 0.56 mm Al. Pinhole images showed that the x-rays originated from an extended area of the glass wall, causing image blurring. When measured on the skin of a hand specimen, the radiation dose of the antiquated system was about 10 times greater than that of the modern system for the same detector signal. The estimated skin dose was about 74 mGy for the antiquated system and 0.05 mGy for the modern system. The corresponding exposure times were 90 minutes and 21 msec. Conclusion: Radiation dose and exposure time of the antiquated system were greater than those of the modern system by about three and five orders of magnitude, respectively. Images of the hand specimen obtained with the antiquated system were severely blurred but were still awe inspiring, considering the simplicity of the system. (C)RSNA, 2011
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| 3. |
- Kemerink, Martijn, et al.
(författare)
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Editorial Material: The Application of X-Rays in Radiology: From Difficult and Dangerous to Simple and Safe in AMERICAN JOURNAL OF ROENTGENOLOGY, vol 198, issue 4, pp 754-759
- 2012
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Ingår i: American Journal of Roentgenology. - : AMER ROENTGEN RAY SOC. - 0361-803X .- 1546-3141. ; 198:4, s. 754-759
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- OBJECTIVE. This article will provide an assessment of the application of x-rays in the early days of radiology, which is an excellent way to come to value the convenience and safety of modern x-ray systems. CONCLUSION. The gas tubes that were originally applied for x-ray production were very unstable because of variations in the tubes vacuum. In an effort to understand some of the problems of these tubes and the high occupational exposure that was indirectly caused by the tubes erratic behavior, we measured x-ray output rates as a function of the gas pressure inside the tube. The pressure range for the optimal production of x-rays, using an original Ruhmkorff inductor as a high-voltage generator, was found to be narrow. With the vacuum changing over time, this might explain the many photographs from the first years of radiology with operators watching their unshielded tube, either with bare eyes or with a fluoroscope, and their own hand as a test object. This practice often led to severe damage of the hands and to many early deaths due to cancer. Today, after a century of technologic development of x-ray tubes and associated equipment, the total average effective dose of workers in radiology can be close to natural background levels.
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| 4. |
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| 5. |
- Maturova, Klara, et al.
(författare)
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Description of the Morphology Dependent Charge Transport and Performance of Polymer:Fullerene Bulk Heterojunction Solar Cells
- 2011
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Ingår i: Advanced Functional Materials. - : Wiley-VCH Verlag. - 1616-301X .- 1616-3028. ; 21:2, s. 261-269
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Tidskriftsartikel (refereegranskat)abstract
- We present a combined numerical charge transport and morphology model to describe the current density-voltage (j-V) characteristics of three different, benchmark polymer: fullerene bulk heterojunction organic solar cells in which the device performance critically depends on the processing conditions or composition of the active layer. We find that an accurate description of the j-V characteristics over a broad bias range can be obtained when the actual complex, three-dimensional (3D) phase separation is represented by a simplified 2D or even 1D description. The morphological device model allows predicting the potential for increasing device performance by further optimizing the morphology. The optimal simplified morphology consists of two, relatively thin alternating vertically oriented slabs, that allow for fast lateral separation of photocreated holes and electrons. This morphology can effectively be described as 1D.
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| 6. |
- Maturova, Klara, et al.
(författare)
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Scanning Kelvin Probe Microscopy on Bulk Heterojunction Polymer Blends
- 2009
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Ingår i: Advanced Functional Materials. - : Wiley-VCH Verlag. - 1616-301X .- 1616-3028. ; 19:9, s. 1379-1386
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Tidskriftsartikel (refereegranskat)abstract
- Here, correlated AFM and scanning Kelvin probe microscopy measurements with sub-100 nm resolution on the phase-separated active layer of polymer-fullerene (MDMO-PPV:PCBM) bulk heterojunction solar cells in the dark and under illumination are described. Using numerical modeling a fully quantitative explanation for the contrast and shifts of the surface potential in dark and light is provided. Under illumination an excess of photogenerated electrons is present in both the donor and acceptor phases. From the time evolution of the surface potential after switching off the light the contributions of free and trapped electrons can be identified. Based on these measurements the relative 3D energy level shifts of the sample are calculated. Moreover, by comparing devices with fine and coarse phase separation, it is found that the inferior performance of the latter devices is, at least partially, due to poor electron transport.
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| 7. |
- van Reenen, Stephan, et al.
(författare)
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Origin of Work Function Modification by Ionic and Amine-Based Interface Layers
- 2014
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Ingår i: Advanced Materials Interfaces. - : Wiley-VCH Verlagsgesellschaft. - 2196-7350. ; 1:8, s. 1400189-
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Tidskriftsartikel (refereegranskat)abstract
- Work function modification by polyelectrolytes and tertiary aliphatic amines is found to be due to the formation of a net dipole at the electrode interface, induced by interaction with its own image dipole in the electrode. In polyelectrolytes differences in size and side groups between the moving ions lead to differences in approach distance towards the surface. These differences determine magnitude and direction of the resulting dipole. In tertiary aliphatic amines the lone pairs of electrons are anticipated to shift towards their image when close to the interface rather than the nitrogen nuclei, which are sterically hindered by the alkyl side chains. Data supporting this model is from scanning Kelvin probe microscopy, used to determine the work function modification by thin layers of such materials on different substrates. Both reductions and increases in work function by different materials are found to follow a general mechanism. Work function modification is found to only take place when the work function modification layer (WML) is deposited on conductors or semiconductors. On insulators no effect is observed. Additionally, the work function modification is independent of the WML thickness or the substrate work function in the range of 3 to 5 eV. Based on these results charge transfer, doping, and spontaneous dipole orientation are excluded as possible mechanisms. This understanding of the work function modification by polyelectrolytes and amines facilitates design of new air-stable and solution-processable WMLs for organic electronics.
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| 8. |
- Abdalla, Hassan, 1987-
(författare)
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Charge and Energy Transport in Disordered Organic Semiconductors
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Improvement of the performance of organic disordered semiconductors (OSC) is driven by the understanding of the underlying charge transport mechanisms and systematic exploitation thereof. There exists a multitude of materials and material systems based on polymers and small molecules with promising performance for use in organic light emitting diodes, photovoltaics, organic field-effect transistors and thermoelectrics. However, universal understanding of many classes of these materials has eluded researchers, due to their broad spectrum of morphologies, molecular structures and electrical properties. Building on the large body of existing models, this thesis deals with charge transport phenomena from the perspective of transport energetics, by studying the interplay between a few but important concepts commonly accepted to play a crucial role in all OSC materials; energetic disorder, charge carrier hopping and Coulomb interactions. The influence of these concepts on the energetic landscape through which charge carriers move and how this translates to experimentally observed transport phenomena are studied by a combination of experimental work, kinetic Monte Carlo (MC) simulations and empirical and analytical models.The universal scaling and collapse of the temperature and electric field dependence of the conductivity of PEDOT:PSS to a single curve is shown to be functionally equivalent to the scaling of the effective temperature, which describes the effect of field heating as a broadening of the charge carrier distribution. From numerical investigation of the energy relaxation, an empirical model is developed that relates the physical meaning behind both concepts to the heat balance between Joule heating of the carrier distribution via the effective temperature and energy loss to the lattice. For this universal description to be applicable a strongly energy- dependent density of states (DOS) as well as Coulomb interactions and large carrier concentrations are needed.Chemical doping is a common way of improving charge transport in OSC and is also beneficial for energy transport, which combined leads to an increased thermoelectric power factor. The ensuing thermoelectric investigations not only showed the potential of these materials for use in thermoelectric generators, but are also helpful in unraveling charge transport mechanism as they give direct insight into the energetics of a material. Interestingly, doped OSC exhibit the same universal power-law relationship between thermopower and conductivity, independent of material system or doping method, pointing towards a common energy and charge transport mechanism. In this thesis an analytical model is presented, which reproduces said universal power-law behavior and is able to attribute it to Variable Range Hopping (VRH) or a transition between Nearest Neighbour Hopping (NNH) and VRH at higher concentrations. This model builds on an existing three- dimensional hopping formalism that includes the effect of the attractive Coulomb potential of ionized dopants that leads to a broadening of the DOS. Here, this model is extended by including the energy offset between host and dopant material and is positively tested against MC simulations and a set of thermoelectric measurements covering different material groups and doping mechanisms.Organic field effect transistors (OFETs) have become increasingly comparable in electrical mobility to their inorganic (silicon) counterparts. The spatial extent of charge transport in OFETs has been subject to debate since their inception with many experimental, numerical and analytical studies having been undertaken. Here it is shown that the common way of analyzing the dimensionality of charge transport in OFETs may be prone to misinterpretations. Instead, the results in this thesis suggest that charge transport in OFETs is, in fact, quasi- two-dimensional (2D) due to the confinement of the gate field in addition to a morphology-induced preferred in-plane direction of the transport. The inherently large charge carrier concentrations in OFETs in addition to the quasi-2D confinement leads to increased Coulomb interaction between charge carriers as compared to bulk material, leading to a thermoelectric behavior that deviates from doped organic systems. At very large concentrations interesting charge transport phenomena are observed, including an unexpected simultaneous increase of the concentration dependence and the magnitude of the mobility, the appearance of a negative transconductance, indicating a transition to an insulating Mott-Hubbard phase. The experimental and numerical results in this thesis relate these phenomena the intricacies of the interplay between Coulomb interactions, energetic disorder and charge carrier hopping.
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| 9. |
- Abdalla, Hassan, et al.
(författare)
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Effective Temperature and Universal Conductivity Scaling in Organic Semiconductors
- 2015
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the scalability of the temperature-and electric field-dependence of the conductivity of disordered organic semiconductors to universal curves by two different but commonly employed methods; by so-called universal scaling and by using the effective temperature concept. Experimentally both scaling methods were found to be equally applicable to the out-of-plane charge transport in PEDOT: PSS thin films of various compositions. Both methods are shown to be equivalent in terms of functional dependence and to have identical limiting behavior. The experimentally observed scaling behavior can be reproduced by a numerical nearest-neighbor hopping model, accounting for the Coulomb interaction, the high charge carrier concentration and the energetic disorder. The underlying physics can be captured in a simple empirical model, describing the effective temperature of the charge carrier distribution as the outcome of a heat balance between Joule heating and (effective) temperature-dependent energy loss to the lattice.
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| 10. |
- Abdalla, Hassan, et al.
(författare)
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Investigation of the dimensionality of charge transport in organic field effect transistors
- 2017
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 95:8
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Tidskriftsartikel (refereegranskat)abstract
- Ever since the first experimental investigations of organic field effect transistors (OFETs) the dimensionality of charge transport has alternately been described as two dimensional (2D) and three dimensional (3D). More recently, researchers have turned to an analytical analysis of the temperature-dependent transfer characteristics to classify the dimensionality as either 2D or 3D as well as to determine the disorder of the system, thereby greatly simplifying dimensionality investigations. We applied said analytical analysis to the experimental results of our OFETs comprising molecularly well-defined polymeric layers as the active material as well as to results obtained from kinetic Monte Carlo simulations and found that it was not able to correctly distinguish between 2D and 3D transports or give meaningful values for the disorder and should only be used for quasiquantitative and comparative analysis. We conclude to show that the dimensionality of charge transport in OFETs is a function of the interplay between transistor physics and morphology of the organic material.
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