| 1. |
- Abdalla, Hassan, 1987-
(författare)
-
Charge and Energy Transport in Disordered Organic Semiconductors
- 2018
-
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.
|
|
| 2. |
- Andersson, Peter, 1976-
(författare)
-
Charge Transport Modulation and Optical Absorption Switching in Organic Electronic Devices
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic electronics has evolved into a well-established research field thanks to major progresses in material sciences during recent decades. More attention was paid to this research field when “the discovery and development of conductive polymers” was awarded the Nobel Prize in Chemistry in 2000. Electronic devices that rely on tailor-made material functionalities, the ability of solution processing and low-cost manufacturing on flexible substrates by traditional printing techniques are among the key features in organic electronics. The common theme while exploring organic electronics, and the focus of this thesis, is that (semi-)conducting polymers serve as active materials to define the principle of operation in devices.This thesis reviews two kinds of organic electronic devices. The first part describes electrochemical devices based on conducting polymers. Active matrix addressed displays that are printed on flexible substrates have been obtained by arranging electrochemical smart pixels, based on the combination of electrochemical transistors and electrochromic display cells, into cross-point matrices. The resulting polymer-based active-matrix displays are operated at low voltages and the same active material is used in the electrochemical transistors as well as in the electrochromic display cells, simply by employing the opto-electronic properties of the material. In addition to this first part, a switchable optical polarizer based on electrochromism in a stretch-aligned conducting polymer is described. The second part reports switchable charge traps in polymer diodes. Here, a device based on a solid-state blend of a conjugated polymer and a photochromic molecule has been demonstrated. The solid state blend, sandwiched between two electrodes, provide a polymer diode that allows reversible current modulation between two different charge transport mechanisms via externally triggered switching of the charge trap density.
|
|
| 3. |
- Asadpoordarvish, Amir, 1984-, et al.
(författare)
-
Light-Emitting Paper
- 2015
-
Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 25:21, s. 3238-3245
-
Tidskriftsartikel (refereegranskat)abstract
- A solution-based fabrication of flexible and light-weight light-emitting devices on paper substrates is reported. Two different types of paper substrates are coated with a surface-emitting light-emitting electrochemical cell (LEC) device: a multilayer-coated specialty paper with an intermediate surface roughness of 0.4 μm and a low-end and low-cost copy paper with a large surface roughness of 5 μm. The entire device fabrication is executed using a handheld airbrush, and it is notable that all of the constituent layers are deposited from solution under ambient air. The top-emitting paper-LECs are highly flexible, and display a uniform light emission with a luminance of 200 cd m−2 at a current conversion efficacy of 1.4 cd A−1.
|
|
| 4. |
- Carlegrim, Elin, 1979-
(författare)
-
Development of Organic-Based Thin Film Magnets for Spintronics
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the growing field of spintronics, development of semiconducting magnets is a high priority. Organic-based molecular magnets are attractive candidates since their properties can be tailor-made by organic chemistry. Other advantages include low weight and low temperature processing. Vanadium tetracyanoethylene, V(TCNE)x, x~2, is particularly interesting since it is one of very few semiconducting magnets with magnetic ordering above room temperature.The aim of the research presented in this thesis was to prepare and characterize thin film organic-based magnets with focus on V(TCNE)x. Photoelectron and absorption spectroscopy studies were performed leading to a more complete picture of the electronic and chemical structure of the material. Depending on the preparation method of V(TCNE)x, the material contains varying amounts of disorder which among other things makes it very air sensitive. In this thesis, a new preparation method for organic-based magnets based on physical vapor deposition is presented and the first result shows that it generates less air sensitive V(TCNE)x than previous methods reported. A new spin valve design based on V(TCNE)x was proposed where the material delivers both spin-filtering and spin-transporting functionality, making use of its fully spin-polarized transport levels. In such devices, the interface of V(TCNE)x with ferromagnetic metals is of great importance and was hence studied. As vanadium ions always are very reactive towards oxygen, substituting vanadium by a less reactive ion would be desirable from both an interface engineering and device packaging perspective. Very few alternatives exist however that orders magnetically above room temperature. In order to find out what are the key design criteria for preparing thin film semiconducting room temperature magnets, we have begun to study systems which order magnetically much below room temperature and compared them with V(TCNE)x.
|
|
| 5. |
- Chinga-Carrasco, Gary, et al.
(författare)
-
Inkjet-printed silver nanoparticles on nano-engineered cellulose films for electrically conducting structures and organic transistors: : concept and challenges
- 2012
-
Ingår i: Journal of nanoparticle research. - : Springer Science and Business Media LLC. - 1388-0764 .- 1572-896X. ; 14:11
-
Tidskriftsartikel (refereegranskat)abstract
- This study explores the suitability of microfibrillated cellulose (MFC) films as a substrate for printing electrically conductive structures and multilayer electronic structures such as organic field effect transistors. Various MFC qualities were tested, including mechanically produced MFC, 2,2,6,6-tetramethylpiperidinyl- 1-oxyl pre-treated MFC and carboxymethylated- MFC. The films differed significantly with respect to the surface structure. In addition, the carboxymethylated-MFC films were surface modified with hexamethyldisilazane (HMDS) to reduce the water-wettability of the films, and thus, improve the print resolution of the inkjet-printed silver (Ag) nanoparticles. The Ag-particles (diameter>50 nm) were printed on the HMDS-modified films, which were mainly composed of nanofibrils with diameters >20 nm. The effect of surface roughness and surface chemical characteristics on the ink spreading and print resolution of the Ag-structures was explored. It was demonstrated that organic transistors operating at low voltages can be fabricated on nano-engineered MFC films.
|
|
| 6. |
- Sandström, Andreas, 1985-
(författare)
-
Design and Fabrication of Light-Emitting Electrochemical Cells
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Glödlampan, en gång symbolen för mänsklig uppfinningsförmåga, är idag på väg att försvinna. Lysdioder och lågenergilampor har istället tagit över då dessa har betydligt längre livstid och högre effektivitet. Den tidigare så hyllade glödlampan anses numera vara en miljöbov, och förbud och restriktioner mot den blir allt vanligare. Trots detta så är de nya alternativen bara att betrakta som provisoriska steg på vägen mot en ideal ljuskälla, som idag tyvärr inte existerar. Lågenergilampor innehåller exempelvis kvicksilver, och utgör därmed ett direkt hot mot en användares hälsa. Både lysdioder och lågenergilampor består även av höga halter av andra tungmetaller, och är väldigt komplicerade att tillverka. Återvinning är därför ett måste, och en fullödig energibesparingsanalys måste ta hänsyn till den betydande energin som går åt vid tillverkningen. Till viss del kan detta lösas genom att göra komponenterna små och ljusstarka, men för att göra en sådan belysning angenäm används istället utrymmeskrävande och ofta energislukande lampskärmar. Lysdioder och lågenergilampor är helt enkelt bra, men långt ifrån perfekta.All elektronisk utrustning är idag beroende av metaller och inorganiska halvledare, vilket gör återvinning viktig och tillverkning komplicerad. Detta är kanske på väg att ändras då även organiska material, t.ex. plast, har visat sig kunna ha elektroniska egenskaper. Idag är organisk elektronik ett hett forskningsområde där material med liknande egenskaper som plast, fast med funktionella elektroniska egenskaper, undersöks och appliceras. Något som gör organiska material extra intressanta är att många kan lösas upp i vätskor, vilket möjliggör för skapandet av bläck. Detta leder i sin tur till möjligheter för användandet av storskaliga trycktekniker, t.ex. tidningspressar och bläckstråleskrivare, vilka leder till en stor kostnadsreduktion och förenklad tillverkning av lysande komponenter. Idag har plast redan ersatt många andra material i en mängd olika tillämpningar. Plastflaskor är vanligare än glasflaskor, och ylletröjor konkurerar idag med kläder gjorda av fleece och andra syntetiska fibrer. Med ljusemitterande plast finns det helt klart en möjlighet att en liknande utveckling kan ske även för lampor.Den här avhandlingen fokuserar på den fortsatta utvecklingen av den ljusemitterande elektrokemiska cellen (LEC), som 1995 uppfanns av Pei et al. LEC-tekniken använder sig av organiska halvledare för att konvertera elektrisk ström till ljus, men även en elektrolyt som möjliggör elektrokemisk dopning. Detta förbättrar den organiska halvledarens elektroniska egenskaper signifikant, vilket leder till mindre resistans och högre effektivitet hos den färdiga lysande komponenten.Visionen för denna och besläktade tekniker har sedan länge varit förverkligandet av en lysande tapet. Den här avhandlingen har försökt närma sig denna vision genom att visa hur en LEC kan uppnå hög effektivitet och lång livslängd, och samtidigt tillverkas i luft med storskaliga produktionsmetoder. Orsaker till en tidigare begränsad livslängd har identifierats och minimerats med hjälp av nya komponentstrukturer och materialformuleringar. En inkapslingsmetod presenteras också, vilken skyddar komponenten från syre och vatten som annars lätt reagerar med det dopade organiska materialet. Detta resulterar i en signifikant förbättring av livslängden.Genom att använda slot-die bestrykning och sprayning, båda kompatibla med rulle-till-rulle tillverkning, har möjligheter för storskalig produktion demonstrerats. Slutligen har en speciell metod för spraymålning av stora lysande ytor utvecklats.
|
|
| 7. |
- Tewari, Amit, et al.
(författare)
-
Low-cost dielectric sheets for large-area floor sensing applications
- 2022
-
Ingår i: Flexible and Printed Electronics. - : Institute of Physics. - 2058-8585. ; 7:4
-
Tidskriftsartikel (refereegranskat)abstract
- Sensitivity response is a critical parameter that decides the domain of dielectric materials to be implemented as piezocapacitive sensors for low- or high-pressure sensing applications. Here, we have clarified the sensitivity response behavior of three low-cost dielectric materials, two biodegradable paperboards, and one acoustic polymeric foam. The devices are fabricated in the form of a metal-insulator-metal structure, and the capacitive response of the devices is measured using the charge extraction by linearly increasing voltage technique. The sensitivity response curve (ΔC/C o vs. pressure) reveals that the paperboard materials are sensitive enough to detect low-pressure regimes (45 kPa), whereas the acoustic foam is quite promising for high-pressure monitoring (above 150 kPa). Using a multiplexer circuit, we demonstrated the sensitivity response via 2 by 2 matrix structure both as a steady-state and transient response. Our results show that the passive matrix structure interference between different pixels can be minimized after increasing the spacing between electrodes strip. Finally, a full-scale demonstrator (dimension 120 cm × 400 cm) with a 2 × 8 matrix structure laminated under floor tiling has been demonstrated. We show how such a floor sensor utilizing the low-cost substrates can be used to recognize single-stepping, walking, and falling.
|
|
| 8. |
- Wilken, Sebastian, et al.
(författare)
-
Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current–Voltage Curve
- 2020
-
Ingår i: Solar RRL. - : Wiley. - 2367-198X. ; 4:6
-
Tidskriftsartikel (refereegranskat)abstract
- Kinetic Monte Carlo (KMC) simulations are a powerful tool to study the dynamics of charge carriers in organic photovoltaics. However, the key characteristic of any photovoltaic device, its current–voltage (J–V) curve under solar illumination, has proven challenging to simulate using KMC. The main challenges arise from the presence of injecting contacts and the importance of charge recombination when the internal electric field is low, i.e., close to open-circuit conditions. Herein, an experimentally calibrated KMC model is presented that can fully predict the J–V curve of a disordered organic solar cell. It is shown that it is crucial to make experimentally justified assumptions on the injection barriers, the blend morphology, and the kinetics of the charge transfer state involved in geminate and nongeminate recombination. All of these properties are independently calibrated using charge extraction, electron microscopy, and transient absorption measurements, respectively. Clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.
|
|
| 9. |
- Zabihipour, Marzieh, 1985-
(författare)
-
Organic Electrochemical Transistors for Printed Digital Circuits
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic electronics enables cost-effective production of flexible electronic devices with high throughput and easy processing compared to the conventional electronics. Organic electronics, therefore, has the potential to realize various innovative applications on a large scale, for example, flexible displays, smart windows, solar cells, electronic skin and implantable medical devices.Many of the materials employed in the field of organic electronics can be processed from chemical solutions. This allows for making various types of inks and hence the possibility to use the traditional high-volume printing methods such as screen printing, inkjet printing and gravure printing for fabricating organic electronic devices on different surfaces. Screen printing has advantages over the other methods in terms of the range of ink viscosity, resolution, and controllable thickness of dry ink film.For various applications envisioned for an integration of printed organic electronics with other technology platforms, a prolonged lifetime and low power consumption are desired. This requires an optimized design of the electronic components and circuits so that they can operate at reduced voltages to guarantee both the long lifetime and the low power consumption. This thesis focuses on designing fully screen printed vertically stacked organic electrochemical transistors (OECTs) and OECT-based circuits operating at low supply voltages and at the same time delivering high gain and low power consumption with long lifetime. The OECTs and OECT-based circuits employ poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) as the organic polymer in their channel. The multi-layered OECTs have a small footprint with a high manufacturing yield and performance uniformity across the printed area, making them suitable for complex printed circuits. Furthermore, various inverter designs based on the reliable and reproducible OECTs are developed and explored to target circuits that can perform at relatively low supply voltages, yet offering improved performance.
|
|
| 10. |
- Zhang, Qilun, 1992-
(författare)
-
Materials and interfaces for sustainable organic solar cells
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photovoltaics, the apparatus utilizing green solar power to generate electricity, is one of the efficient measures to the continuously increasing energy demand and exacerbated carbon emission of the human civilization. As a candidate with the potential of providing the cheapest and greenest form of electricity, organic solar cells (OSCs) received tremendous scientific interest, resulting in a significant power conversion efficiency (PCE) boosting to above 19% recently. Despite the impressive achievements in PCE, it’s alone not enough for the commercialization of OSCs. Sufficient lifetime and scalability at competitive cost are necessary as well. A better understanding of the interface energetic properties and materials electronic structures in the multilayer stacked OSCs are needed to improve the inherent instability of the devices. In addition, exploration of the sustainable and low-cost materials for OSCs are crucial. In this thesis, we carefully investigated dipole-induced energy level matching at the OSC interfaces, and the oxygen/water caused electronic structure evolution of the OSC materials via various of spectroscopic characterizations, and introduced natural wood-based materials to achieve highly efficient and stable OSCs.We employed ultraviolet photoelectron spectroscopy (UPS) to investigate the interface energetic properties of a commercially available cathode interface layer (CIL) material polyvinylpyrrolidone (PVP) in OSCs and proposed a "double dipole" model to explain the work function modification properties of PVP on several substrates. Then we used the large-area compatible immersion method to obtain the ultrathin PVP layer on the ITO substrate, the fabricated OSCs have a comparable efficiency to the traditional Zinc Oxide (ZnO) CIL based devices. We further use photoelectron spectroscopy (PES) to investigate the electronic structures of advanced OSC materials, i.e., PM6 and Y6. To better understand the degradation mechanism caused by water and oxygen in these materials, the electronic structures of the materials were in-situ characterized in near-ambient pressure with controllable water and oxygen dosing. We carefully analyzed the evolution of the PES spectra during the water and oxygen dosing, and unveiled that oxygen affected backbone sulfur in PM6 and a weak interaction between cyano groups in Y6 with water. Furthermore, the enhanced stability of the Y6 was observed in the blend films as the electronic structures in the PES spectra, which matched the device results of PM6 and Y6 based OSCs that the blend photoactive layers show better stability in air atmosphere than bilayer.Lastly, we presented the application feasibility of the natural wood-based materials in state of art OSCs to achieve better stability and lower cost. We firstly introduced an insulating polymer of natural betulin into the active layer, following the “filler strategy”, resulting in an improved open circuit voltage (Voc) in donor-acceptor-insulator ternary OSCs. We attribute this improvement to the decreased trap-assisted recombination, however, we simultaneously found reduced charge collection in the devices caused by the penetration of the filler materials at the bottom, forming insulator interface to block the charge transfer. The present work expands the range of filler materials in OSCs to include biomass, with the aim of developing highly efficient, environmentally friendly, and cost-effective OSCs. We further extended the utilization of natural wood-based materials to cathode interface layer (CIL). Kraft lignin (KL), the most abundant natural source of aromatic material constituents, has potential compatibility to various of traditional CIL materials, owing to the chemical activity of phenolic functionalities. In this work, we successfully combined the traditional CIL materials, i.e., PFN-Br and bathocuproine (BCP), with large ratio (30%-50% in weight ratio) of industrial solvent fractionated KL, obtained binary CILs with tuneable WF. The binary CILs with suitable KL ratio worked well in OSCs, exhibited equivalent or even higher efficiency to the traditional CILs. In addition, the combination of KL and BCP significantly enhanced the stability of the devices, which mainly ascribed to the protection from KL to block the reaction between BCP and fused-ring electron acceptors.The author hopes the findings in this thesis can contribute to the industrialization of OSCs, especially from the aspect of the sustainability, cost and stability.
|
|
