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Authors: Kunz, Alexander
Title: Controlling Charge Carrier Trapping in Polymeric Semiconductors : Connecting Morphology to Device Behavior
Online publication date: 8-Apr-2021
Year of first publication: 2021
Language: english
Abstract: Performance and efficiency of polymeric optoelectronic devices strongly depends on morphology, mixing behavior and purity of the conjugated polymers used. The transport behavior of electric charge carriers - holes and electrons - can be directly tuned by these properties. It also has great impact on loss processes in polymeric light emitting diodes (PLED) and polymeric photovoltaic devices (OPV). Typically, charge transport in a conjugated polymer is unbalanced, e.g. hole transport along the backbone of a conjugated polymer is of space-charge-limited nature and trap free. Electron transport in principle follows the same mechanism, but in contrast to hole transport it suffers trapping of electrons in localized energetic states within the energy band gap of the conjugated polymer, which leads to an overall reduction of electron transport by more than three orders of magnitude. The disparity of hole and electron transport leads to a severe loss process in both PLEDs and OPVs: excitons (bound electron-hole pairs) are generated close to the cathode contact and decay non-radiatively by transfer of their recombination energy to the metallic contact via dipole-dipole interaction mechanism. Additionally, non-radiative trap-assisted recombination occurs as another loss process. In order to address the loss processes in conjugated polymers, the work in this thesis is focused on tuning the charge carrier transport in conjugated polymers by blending them with other conjugated and non-conjugated polymers. The first chapter introduces the work and discusses on the physical and chemical background of conjugated polymers, electrical devices and the phase dynamics of polymer mixtures that are relevant for comprehension. In the second chapter a discussion on the role of morphology of MEH-PPV:PVK blends on charge transport properties in hole and electron only devices. It is demonstrated how morphology evolves from phase dynamics of the different polymer blends and how the intimacy of the blend constituents affects the charge transport properties in solution processed active layers of hole only and electron only devices. It is found that weak segregation of the blend components takes place into coexisting MEH-PPV-rich and PVK-rich phases. Ultimately, it is demonstrated that for MEH-PPV:PVK thin film devices with weight ratios of 1:9 equilibrated electron and hole transport is observed as the negative effect of trapped electrons is effectively eliminated due to dilution of the low band gap polymer MEH-PPV in the high band gap polymer PVK. To strengthen and broaden the findings made in chapter 2, in chapter 3 polystyrene is introduced as an insulating polymer into the blend system with MEH-PPV. Polystyrene is an insulator with an even higher band gap than PVK and available in a wide variety of molecular weights and suitable to prove the generality of the previously reported performance improvement by dilution of the conjugated polymer and its electronic trap states, also in comparison to the MEH-PPV:PVK blends mentioned in chapter 2. First, the third chapter focuses on calculating the phase dynamics in order to qualitatively predict mixing behavior of the MEH-PPV:PS blend when solution processed. Second, it compares the morphological outcome and electronic performance. Two distinct cases are presented and analyzed. The first case , a fully phase separating blend MEH-PPV with a molecular weight of 354~kg/mol and polystyrene with a molecular weight of 35~kg/mol, with no improvement in electronic performance of light emitting devices, independent from weight ratio between the two polymers. The second case, again the same MEH-PPV but in a blend with a low molecular weight polystyrene with 1.1~kg/mol, fully miscible when spin cast from solution and showing improved electron transport properties by three orders of magnitude. Ultimately, this chapter emphasizes the importance of phase dynamics that is directly related to performance of solution processed thin film devices. It also shows that MEH-PPV based PLEDs can be doubled in efficiency by using an insulating diluent polymer such as polystyrene in a weight ratio of 1:9. The fourth chapter concentrates on expanding the previous studies on other conjugated polymer blends such as BEH-PPV:PS. At first glance, BEH-PPV as compared to MEH-PPV looks very similar, but the substitution pattern of side chains is symmetric. The on chain order of BEH-PPV is increased and higher mobility of charge carriers is feasible when used in thin film devices. Lower operating voltages are possible and therefore of practical importance for improved device performance. As a first result, trap free electron transport in BEH-PPV:PS electron only devices is demonstrated successfully. However, BEH-PPV gives rise to different phase dynamics than MEH-PPV when blended with low molecular weight polystyrene. Indeed, though lateral phase separation is not observed experimentally, injection issues of positive charge carriers into BEH-PPV:PS thin films are observed. A systematic study of the origin of these injection issues on thin films of BEH-PPV:PS blends in different weight ratios is performed by Kelvin-Probe and ToF-SIMS analysis. It is revealed that not a mismatch of energetic levels, but rather a surface directed effect on the vertical composition of the blend creates preferential accumulation of insulating polystyrene on the PEDOT:PSS injection contact. Finally, by blending the PEDOT:PSS injection contact with perfluorooctane sulfonic acid (FOS) in excess a lowering of the surface energy of the PEDOT:PSS contact is achieved. Injection issues are eliminated successfully by fabrication of hole only devices made with PEDOT:PSS:FOS hole injection contacts and BEH-PPV:PS 1:9 as active thin film. Together with the demonstration of trap free electron transport in BEH-PPV:PS 1:9 thin films promising device performances are obtained. The fifth chapter discusses the role of density of structural defects of the conjugated polymer BEH-PPV on electron only device performance. More specifically, a study on electron only device performance as a function of chain end density is performed. The chain end density is seen as a critical origin of energetic trap sites for positive and negative charge carriers simultaneously. Therefore, most synthesis routes focus on creation of high molecular weight BEH-PPV in order to reduce the chain end density to rule out the possible negative effects, accepting the compromise of fairly poor solution processability. Here the electron transport properties of BEH-PPV polymers is studied in electron only devices. The molecular weight of the solution processed BEH-PPV thin films is varied in the range of two orders of magnitude in number average molecular weight, namely BEH-PPV with 52~kg/mol, 123~kg/mol and 521~kg/mol. Device performance is simulated with the extended Gaussian disorder model, where mobility, trap density and electronic disorder is analyzed. Within the chosen molecular weight range no significant change in trap density as well as transport behavior of electrons is observed. As a recommendation, the synthesis of BEH-PPV in the number molecular weight range around 100~kg/mol is proposed. An intermediate molecular weight of BEH-PPV can improve solution processability in terms of repeatability and preparation time without any effective disadvantage of increased structural defects due to increased chain end density.
Verlustprozesse limitieren die Effizienz polymerbasierter Leuchtdioden (PLED) und Solarzellen (OPV). Sie entstehen durch eine Disparität von Loch- und Elektronentransport, wobei letzterer typischerweise drei Größenordnungen kleiner ist, da lokalisierte energetische Fallenzustände innerhalb der Bandlücke eines halbleitenden konjugierten Polymers existieren. Dies hat zur Folge, dass Rekombination von Elektronen mit Löchern strahlungsfrei über Fallenzustände oder Relaxation von Excitonen in der Nähe des Kathodenkontaktes geschehen. Der Ladungstransport und die Verlustmechanismen können durch geeignete Wahl und Mischung halbleitender konjugierter Polymere kontrolliert und verbessert werden. Prinzipien der Soft-Matter Dynamics, wie Mischungsverhalten und Einstellen morphologischer Eigenschaften werden in dieser Arbeit dazu genutzt Verlustmechanismen in halbleitenden polymerbasierten Bauteilen zu identifizieren und zu reduzieren. Es wird dabei eine Verbindung zwischen Mischungsdynamik, Morphologie und Performance optoelektronischer Bauteile hergestellt. Im Besonderen wird die Mischungsdynamik Polyphenylenevinylene-basierter Polymere mit halbleitenden und isolierenden Polymeren, wie Polyvinylkarbazol und Polystyrol über Phasendiagramme vorausberechnet. Analysen über die Morphologie und Topologie der Dünnschichten werden hergestellt und herangezogen sowie in optoelektronischen Bauteilen praktisch erprobt. Abschließend kann durch die Verbindung von Mischungsdynamik und Bauteil-Performance gezeigt werden, dass energetische Fallenzustände innerhalb der Bandlücke halbleitender konjugierter Polymere in ihren negativen Eigenschaften neutralisiert werden können. Der Ladungstransport wird homogenisiert und die Effizienz der optoelektronischen Bauteile wird erhöht.
DDC: 333.7 Natürliche Ressourcen
333.7 Natural resources
500 Naturwissenschaften
500 Natural sciences and mathematics
530 Physik
530 Physics
540 Chemie
540 Chemistry and allied sciences
600 Technik
600 Technology (Applied sciences)
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-9c2be191-9cff-4595-8a0d-fcd023020ecd1
Version: Original work
Publication type: Dissertation
License: CC BY
Information on rights of use:
Extent: XVI, 99 Seiten Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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