| 1. |
- Urbanaviciute, Indre, 1990-
(författare)
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Multifunctional Supramolecular Organic Ferroelectrics
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ferroelectric materials are known and valued for their multifunctionality arising from the possibility to perturb the remnant ferroelectric polarization by electric field, temperature and/or mechanical stimuli. While inorganic ferroelectrics dominate the current market, their organic counterparts may provide highly desired properties like eco-friendliness, easy processability and flexibility, concomitantly opening unique opportunities to combine multiple functionalities into a single compound that facilitates unprecedented device concepts and designs. Supramolecular organic ferroelectrics of columnar discotic type, that are the topic of this thesis, offer additional advantages related to their strong hierarchical self-assembly and easy tunability by molecular structure modifications, allowing optimization of ferroelectric characteristics and their hybridization with, e.g., semiconductivity. This not only leads to textbook ferroelectric materials that can be used as model systems to understand the general behaviour of ferroics, but also gives rise to previously unobserved effects stemming from the interplay of different functionalities.The core-shell structure of the molecules under the scope enables multiple pathways forrational design by molecular structure modification. This was firstly pursued via peripheral tail engineering on an archetypal self-assembling ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA). We found that by shortening the alkyl chain length all the ferroelectric properties can be continuously tuned. In particular, changing the tail from C18H37 to C6H13causes an increase in depolarization activation energy (~0.8 eV to ~1.55 eV), coercive field(~25 V/μm to ~50 V/μm) and remnant polarization (~20 mC/m2 to ~60 mC/m2). The combination of the mentioned characteristics resulted in a record polarization retention time of close to 3 months at room temperature for capacitor devices of the material having the shortest alkyl chain – BTA-C6, which at the time of writing was one of the best results for liquid-crystalline ferroelectrics.Taking one step further, we experimentally demonstrated how introduction of branched-tailsubstituents results in materials with a wide operating temperature range and a data retention time of more than 10 years in thin-film solution-processed capacitor devices already atelevated temperatures with no measurable depolarization at room temperature. The observed differences between linear- and branched-tail compounds were analysed using density functional theory (DFT) and molecular dynamics (MD) simulations. We concluded that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs. Synergistic effects upon blending of compounds with branched and linear sidechains were shown to further improve the materials’ characteristics.Exploiting the excellent ferroelectric performance and the well-defined nanostructure of BTAs, we experimentally determined the Preisach (hysteron) distribution of BTA and confronted it to the one obtained for the semi-crystalline P(VDF:TrFE). This allowed to elucidate how the broadening of the Preisach distribution relates to the materials’ morphology. We further connected the experimental Preisach distribution to the corresponding microscopic switching kinetics. We argue that the combination of the two underlays the macroscopic dispersive switching kinetics as commonly observed for practical ferroelectrics. These insights lead to guidelines for further advancement of ferroelectric materials both for conventional and multi-bit data storage applications.Although having strong differences in the Preisach distribution, BTA and P(VDF:TrFE) both demonstrate negative piezoelectricity – a rare anomalous phenomenon which is characteristic to two-phased materials and has never been observed in small-molecular ferroelectrics. We measured a pronounced negative piezoelectric effect in a whole family of BTAs and revealed its tunability by mesogenic tail substitution and structural disorder. While the large- and small-signal strain in highly ordered thin-film BTA capacitor devices are dominated by intrinsic contributions and originates from piezostriction, rising disorder introduces additional extrinsic factors that boost the large-signal d33 up to −20 pm/V in short-tailed molecules. Interestingly, homologues with longer mesogenic tails show a large-signal electromechanical response that is dominated by the quadratic Maxwell strain with significant mechanical softening upon polarization switching, whereas the small-signal strain remains piezostrictive. Molecular dynamics and DFT calculations both predict a positive d33 for defect-free BTA stacks. Hence, the measured negative macroscopic d33 is attributed to the presence of structural defects that enable the dimensional effect to dominate the piezoelectric response of BTA thin films.The true multifunctionality of supramolecular discotics manifests when large semiconducting cores surrounded by field-switchable strongly polar moieties are introduced in the structure. We showed how the combination of switchable dipolar side groups and the semiconducting core of the newly synthetized C3-symmetric benzotristhiophene molecule (BTTTA) leads to an ordered columnar material showing continuous tunability from injection- to bulk-limited conductivity modulation. Both these resistive switching mechanisms may lead to the next-generation high-density non-volatile rewritable memory devices with high on/off ratios and non-destructive data readout – the element that has been desperately sought after to enablefully organic flexible electronics.
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| 2. |
- Ji, Fuxiang, 1991-, et al.
(författare)
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Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2023
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Ingår i: Advanced Optical Materials. - : Wiley-Blackwell. - 2162-7568 .- 2195-1071.
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Tidskriftsartikel (refereegranskat)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 3. |
- Ji, Fuxiang, 1991-, et al.
(författare)
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Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2024
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Ingår i: Advanced Optical Materials. - : John Wiley & Sons. - 2162-7568 .- 2195-1071. ; 12:8
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Tidskriftsartikel (refereegranskat)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron-phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK(-1) based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III-V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 4. |
- Jin, Yingzhi, 1991-
(författare)
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Organic electronic devices for solar energy conversion and storage
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on two types of organic electronic devices: organic photovoltaic (OPV) devices for solar energy conversion, and photo-capacitors for energy storage.OPVs have been under the focus of research for decades as an effective technique to convert solar energy to electricity. So far, the efficiency of bulk heterojunction OPV consisting donor and acceptor materials is approaching to 18% with non-fullerene acceptor (NFA), which make it close to commercialization. The process of charge generation and recombination are two competing processes in OPVs, since their requirements for the active layer morphology are contradictory. Large donor/acceptor interfaces facilitate charge generation but hinder the transporting pathways for charge transportation. The simultaneously enhanced charge generation and transportation are achieved by using the ternary strategy in my first paper. The fully mixed donors and NFAs are beneficial for the charge generation and fullerene is introduced as an extra electron transport channel. The hierarchical morphology of the blend film is confirmed by the TEM results. The voltage loss analyses indicate that the hierarchical morphology could suppress unfavorable charge transfer state and non-radiative recombination loss. In my second paper, efficient charge generation with low voltage loss are achieved in the solar cells by rational designing a series of NFAs. The detailed voltage losses are discussed in these binary systems, revealing the critical relationship between radiative efficiency and device performance.To harvest photocurrent in OPVs, long lifetime triplet excitons are highly expected to be good candidates. The potential of triplet materials in OPVs has been explored since 1970s. However, the performance of the triplet materials-based OPVs is far behind. The voltage loss in triplet OPVs is intensively studied in my third work. A higher open circuit voltage (0.88 V) is observed for Ir(FOtbpa)3-based devices than those of Ir(Ftbpa)3 (0.80 V) despite a lower charge transfer state energy. To understand above result, the voltage losses through radiative and non-radiative recombination pathways in two devices are quantitively investigated, which indicate a reduced non-radiative recombination loss in the Ir(FOtbpa)3-based devices.The fluctuation of sun irradiation resulting the unstable output power of solar cells. Therefore, it is important to store electricity of solar cells for later use. Integrated photo-capacitor (IPC), combining a solar cell and a super-capacitor by sharing one common electrode, is able to simultaneously realize the energy harvesting and storage. Building upon this advantage, IPC devices received tremendous research attention. In my fourth and last papers, we introduced super-capacitors to construct IPC devices with OPV device or modules. A free standing thick- PEDOT:PSS film is successfully integrated into an all solution-processed IPC device as the common electrode. Resulting devices demonstrate good performance and outstanding stability. With solar PV modules, a higher voltage can be generated and stored by asymmetric supercapacitors, which could be used as a portable power unit.
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| 5. |
- Li, Xian'e, 1993-
(författare)
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Interfaces in Organic Solar Cells
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic semiconductors (OSs), the promising candidates for the next generation electronic devices, have shown their advantages of light weight, flexibility, semi-transparency, tunable optical gaps, and energy levels, in the development history of over 70 years. OSs have been widely used in versatile applications such as organic light-emitting diodes, organic field-effect transistors, organic photodetectors, and organic solar cells (OSCs) which are mainly studied in this thesis. Nowadays, OSCs have been developed rapidly and reached a new era of high efficiencies with new records close to 20%, since the development of non-fullerene-based donor (D) -acceptor (A) systems. Despite the impressive progress in the field of OSCs, fundamental understandings on the interface energetics are lagging far behind the development of materials and device engineering, which dampens the further material design, device optimization, and scalable production.Interface energetics take charge of many key electronic processes in organic electronic devices, such as charge injection or extraction at the electrode/ OS interface, charge generation or recombination at the D/A interface, and ambient optoelectronic stability resulted from the OS/air interface, all of which significantly affect the device performance. In this thesis, we systematically study the energy level alignment (ELA) at interfaces of inorganic/organic, organic/organic, and organic/air in OSCs, correlate the investigated energetic landscape with the device performance, and provide new understandings on the optoelectronic process in organic electronic devices.Firstly, we determine the pinning energies of a wide variety of newly developed donors and non-fullerene acceptors (NFAs) through ultraviolet photoelectron spectroscopy, to provide the critical characteristics of the material for ELA prediction at either electrode/OS or D/A interfaces. The ELA for inorganic/organic interfaces follows the predicted behavior based on integer charge transfer model, but for organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer, the pinning energies often underestimate the experimentally obtained interface vacuum level (VL) shift. To explore the origin of the extra VL shift, we map the ELA at a range of D/NFA interfaces by fabricating and characterizing D-A bilayer heterojunctions monolayer-by-monolayer with the Langmuir-Schäfer technique. We find that the abrupt and significant VL shifts at the D-A interfaces are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of D-NFA systems. Furthermore, we investigate the influence of H2O and O2 molecules from ambient air on the work functions (WFs) of OS films. We find that OS films generally show higher WFs measured in ambient air, but lower WFs measured in high vacuum, compared to the WFs measured in ultrahigh vacuum. Two mechanisms are proposed to explain this phenomenon: (1) Competition between p-doping induced by O2 or H2O/O2 complexes, and n-doping induced by H2O clusters; (2) Polar H2O molecules preferentially modifying the ionization energy of one of the frontier molecular orbitals over the other. Finally, we fabricate the charge-transport-layer (CTL) free OSCs based on a newly developed NFA molecule with minimum performance degradation. Based on the determined D-A composition and ELA at the Anode/OS and the Cathode/OS interface, we propose several interface design rules for the efficient CTL-free devices, shedding new light on the simplified device structure for achieving more efficient optoelectronic applications.
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| 6. |
- Wang, Qingqing, 1992-
(författare)
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Revealing Electronic Structures of 2D Molecular Crystals and Correlating Them with Optoelectronic Properties
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electronic band structure serves as the foundation for understanding the physics of semiconductors. The electronic band structure of inorganic semiconductors has been well understood based on ideal single crystal samples, thus actually laid the foundation for the prospering development of inorganic semiconductor devices.As for organic semiconductors, which are currently being pervasive in our daily life, however, the ‘physics’ of organic semiconductors is still far from complete understanding compared to their inorganic counter-parts which could hinder the rapid development of organic electronics. Only few organic single crystals (e.g., rubrene, pentacene) have been well investigated to probe the electronic structure and their relationship with their electrical properties, sufficient experimental evidence is still lacking for intrinsic properties of organic single crystal, which mainly hindered by the limited crystal size and low conductivity.Two-dimensional molecular crystals (2DMCs) of organic semiconductors are intriguing materials because their unique advantages, such as long-range molecular packing, low defect density and lack of grain boundaries, make 2DMCs an ideal platform for exploring the structure-property relationship, revealing the intrinsic properties and probable carrier transport mechanism, fabricating high-performance optoelectronic devices. Especially, unique optoelectronic properties exhibited in 2DMCs are not found in their bulk counterparts. With breakthrough in crystal engineering for producing large-area (e.g., millimeter or centi-meter even wafer-sized) 2DMCs and material engineering for designing novel organic semiconductors (e.g., C10-DNTT, C6-DPA), all provide a great opportunity to explore the physical origin behind the novel optoelectronic properties of kinds of organic semiconductors and their correlation with optoelectronic device performance, which further guiding material design and facilitating flourishing of organic electronics.The aim of this thesis is to investigate the electronic structure of 2DMCs and correlate them with their optoelectronic properties. The 2DMCs were mainly produced by space-confined strategy and layer-defining strategy, the produced 2DMCs could be transferred to any substrates for further characterization. One of selected organic materials is 2,6-Bis(4-hexylphenyl)anthracene (C6-DPA), which belongs to anthracene derivates family, typically known for their high luminescence efficiency and carrier mobility. All characterized 2DMCs of C6-DPA show a high quality. Firstly, we fabricated integrated organic effect field transistors e.g., organic phototransistors, organic memory phototransistors based on 2DMCs of C6-DPA to elucidate the high performance and potential applications. To clarify the physical origin of opto-electronic properties, some advanced surface science experimental techniques were used to determine the electronic structure of 2DMCs of C6-DPA. Resonant photoemission spectroscopy reveals a room temperature band dispersion of C6-DPA, which is well explained by calculated band structure. Angle-resolved photoemission spectroscopy results confirm the room temperature dispersion and exhibit anisotropic band dispersion in plane. The anisotropic charge carrier mobility is 2 in plane, where the highest mobility obtained along the molecular direction with obvious band dispersion, suggesting the electronic property and electrical property quite match well. We then investigated the in-fluence of degree of crystallinity on electronic structure by ultraviolet photoemission spectroscopy, all results indicate that high crystallinity help to overcome Coulomb interaction and facilities charge to be delocalized on whole 2D crystal, while in verse in less degree of thin film. We then selected perylene as the second material to explore the exciton band structure by electron energy-loss spectroscopy. The observed negative band dispersion is rationalized by effective inter-dimer coupling with an additional charge transfer contribution. This result could provide guidance for understanding the in-plane charge transport properties in 2D crystal of perylene.
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| 7. |
- Zhang, Qilun, 1992-
(författare)
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Materials and interfaces for sustainable organic solar cells
- 2023
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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.
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| 8. |
- Ajjan, Fátima, 1986-, et al.
(författare)
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Doped Conjugated Polymer Enclosing a Redox Polymer : Wiring Polyquinones with Poly(3,4‐Ethylenedioxythiophene)
- 2020
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Ingår i: Advanced Energy and Sustainability Research. - : John Wiley & Sons. - 2699-9412. ; 1:2
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Tidskriftsartikel (refereegranskat)abstract
- The mass implementation of renewable energies is limited by the absence of efficient and affordable technology to store electrical energy. Thus, the development of new materials is needed to improve the performance of actual devices such as batteries or supercapacitors. Herein, the facile consecutive chemically oxidative polymerization of poly(1-amino-5-chloroanthraquinone) (PACA) and poly(3,4-ethylenedioxythiophene (PEDOT) resulting in a water dispersible material PACA-PEDOT is shown. The water-based slurry made of PACA-PEDOT nanoparticles can be processed as film coated in ambient atmosphere, a critical feature for scaling up the electrode manufacturing. The novel redox polymer electrode is a nanocomposite that withstands rapid charging (16 A g−1) and delivers high power (5000 W kg−1). At lower current density its storage capacity is high (198 mAh g−1) and displays improved cycling stability (60% after 5000 cycles). Its great electrochemical performance results from the combination of the redox reversibility of the quinone groups in PACA that allows a high amount of charge storage via Faradaic reactions and the high electronic conductivity of PEDOT to access to the redox-active sites. These promising results demonstrate the potential of PACA-PEDOT to make easily organic electrodes from a water-coating process, without toxic metals, and operating in non-flammable aqueous electrolyte for large scale pseudocapacitors.
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| 9. |
- Chen, Yongzhen, 1990-
(författare)
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Image dipoles and polarons in organic semiconductors
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The rapid development of organic electronics depends on the synthesis of new π-conjugated molecules/polymers and the exploration of fundamental physics. However, most of the efforts have been concentrated on the former, leading to a lack of thorough understanding of many important concepts, which will become the ultimate limiting factor for overall performance and applications. Thereinto, the interface energetics in multilayer stacked optoelectronic devices and the electronic structure of doped organic semiconductors are two of the most complicated and yet inevitable topics in this field. A better understanding of them can provide needed additional insights into the operation of devices and give valuable guidance for device and molecule design. Hence, the aim of this thesis is to investigate these two fundamental issues using various spectroscopic characterizations and supported by computational modeling.The cathode/organic interface plays a critical role in achieving balanced charge transport and improved stability in organic electronic devices. Employing stable cathode materials, however, typically results in large electron injection barriers and sub-optimal devices. Using small-molecule electron transport materials (ETMs) as interlayers is an effective approach to reduce the electron injection barriers, but the working mechanism is still under debate. By studying the energy level alignment behavior of ETMs on different types of substrates, we find the work function of the substrate is reduced by an extra “image” dipole formed at the interface and within the first layer of the ETM film. The use of non-reactive substrates and the results from X-ray photoelectron spectroscopy core level analysis exclude the orbital hybridization theory, which states that an ETM-metal complex may form at the interface. The characterization results on molecular orientation disqualify an explanation using intrinsic molecular dipole moments. Instead, experiments demonstrate that the interface dipole depends on the areal density and direction of lone electron pairs on the heteroatoms in ETMs, which is similar to previous observations in tertiary aliphatic amines. This behavior is well described by the so-called “double dipole step” model, where one dipole formed by the nitrogen nuclei and the lone pairs in the organic side points from the substrate surface to the organic film and the other one formed by their image charges in the electrode side shows the same direction.The polaron charge carrier is another important concept involved in multiple (opto)electronic processes during device operation, such as charge transport, exciton recombination/dissociation. Although numerous experimental and theoretical efforts have been made, there is still a lack of comprehensive studies on the electronic structure of negative polarons due to their high air sensitivity, including correlation between the valence band structure measured by ultraviolet photoelectron spectroscopy (UPS) and the optical band gap derived from the UV−vis−NIR absorption. In the present work, we are able to integrate the optical and electrical measurements with photoelectron spectroscopy to collect all information without breaking an ultra-high vacuum. Negative polarons formed in alkali metal-doped polymers are detected with new polaronic states below the Fermi level and lower energy absorption bands arising from the excitation from polaronic states to unoccupied states. In addition, the Fermi level shifts toward the conduction band with increasing the doping ratio, and the doubly-occupied polaronic state shows slightly lower energy than the topmost valance band peak of the neutral polymer. These observations are supported by the density functional theory (DFT) simulations, from which we also demonstrate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. By comparison of different polymer and dopant systems, we find the polymer-dopant interaction and the polaron delocalization are dependent on the distortion of the polymer backbone and the size of the dopant, properties that in turn affect the conductivity and air stability of the n-doped materials.I hope that the findings presented in the thesis can greatly promote the understanding of the energetics of the ETMs/cathode interface and the electronic structures of negative polarons in alkali metal-doped polymers, contributing to further providing new guidelines for the molecular design and improve the device performance.
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| 10. |
- Cornelissen, Tim, 1993-
(författare)
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Switching Kinetics and Charge Transport in Organic Ferroelectrics
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The continued digitalization of our society means that more and more things are getting connected electronically. Since currently used inorganic electronics are not well suited for these new applications because of costs and environmental issues, organic electronics can play an important role here. These essentially plastic materials are cheap to produce and relatively easy to recycle. Unfortunately, their poor performance has so far hindered widespread application beyond displays.One key component of any electronic device is the memory. For organic electronics several technologies are being investigated that could serve as memories. One of these are the ferroelectrics, materials that have a spontaneous electrical polarization that can be reversed with an electric field. This bistable polarization which shows hysteresis makes these materials excellent candidates for use as memories.This thesis focuses on a specific type of organic ferroelectric, the supramolecular discotics. These materials consist of disk‐like molecules that form columns in which all dipolar groups are aligned, giving a macroscopic ferroelectric polarization. Of particular interest are the benzenetricarboxamides (BTA), which are used as a model system for the whole class of discotic ferroelectrics. BTA uses a core‐shell architecture which allows for easy modification of the molecular structure and thereby the ferroelectric properties. To gain a deeper understanding of the switching processes in this organic ferroelectric BTA, both microscopic and analytical modeling are used. This is supported by experimental data obtained through electrical characterization.The microscopic model reduces the material to a collection of dipoles and uses electrostatics to calculate the probability that these dipoles flip. These flipping rates are the input for a kinetic Monte Carlo simulation (kMC), which simulates the behavior of the dipoles over time. With this model we simulated three different switching processes on experimental time and length scales: hysteresis loops, spontaneous depolarization, and switching transients. The results of these simulations showed a good agreement with experiments and we can rationalize the obtained parameter dependencies in the framework of thermally activated nucleation limited switching (TA‐NLS).The microscopic character of the model allows for a unique insight into the nucleation process of the polarization switching. We found that nucleation happens at different locations for field driven polarization switching as compared to spontaneous polarization switching. Field‐driven nucleation happens at the contacts, whereas spontaneous depolarization starts at defects. This means that retention times in disordered ferroelectrics could be improved by reducing the disorder, without affecting the coercive field. Detailed analysis of the nucleation process also revealed a critical nucleation volume that decreases with applied field, which explains the Merz‐like field‐dependence of the switching time observed in experiments.In parallel to these microscopic simulations we developed an analytical framework based on the theory of TA‐NLS. This framework is mainly focused on describing the switching transients of disordered ferroelectrics. It can be combined with concepts of the Preisach model, which considers a non‐ideal ferroelectric as a collection of ideal hysterons. We were able to relate these hysterons and the distribution in their up‐ and down‐switching fields to the microscopic structure of the material and use the combined models to explain experimentally observed dispersive switching kinetics.Whereas ferroelectrics on their own could potentially serve as memories, the readout of ferroelectric memories becomes easier if they are combined with semiconductors. We have introduced several molecular materials following the same design principle of a core‐shell structure, which uniquely combine ferroelectricity and semiconductivity in one material. The experimental IV‐curves of these materials could be described using an asymmetric Marcus hopping model and show their potential as memories. The combination of modeling and experimental work in this thesis thereby provides an increased understanding of organic ferroelectrics, which is crucial for their application as memories.
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