A Study on Charge Selective Transport for Highly Efficient Polymer Based Optoelectronic Devices

Department of Materials Science EngineeringPolymer based optoelectronic devices including polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) have been recently focused for display, energy source and flexible electronic applications because of their advantages such as low cost, light weight, easy solution process fabrication and mechanical flexibility. Moreover, so much effort has been made to maximize their device performance through optimization of device configuration and charge selective transport. In particular, balanced charge transport via charge selective interfacial engineering or surface modification is promising for optimized device performance. According to the device configuration, interfacial engineering can improve the minority carrier transport with well-matched energy level, passivate the charge trap sites and enhance the materials compatibility. It can also block abundant majority carrier and reduce the exciton quenching, leading to improving the recombination rate of balanced charges in PLEDs while disrupting bimolecular recombination in PSCs. Here, I present variety interfacial engineering strategies employing modified charge transport layer such as graphene oxide (GO) as a hole transport layer (HTL) in conventional PLEDs and surface modified zinc oxide (ZnO) as an electron transport layer (ETL) using ionic liquid molecules (ILMs), conjugated polyelectrolyte (CPE) and amine-based polar solvents in inverted polymer light-emitting diodes (iPLEDs) and polymer solar cells (iPSCs). A GO layer with a wide band gap blocks transport of electrons from an emissive layer to an indium tin oxide (ITO) anode while reduces the exciton quenching between the GO layer and the emissive layer. As a result, the GO layer maximizes hole-electron recombination within the emissive layer leading to improvement of device performance in PLEDs. In addition, surface modified ZnO layers with various interfacial layers such as ILMs, CPE and amine-based polar solvents remarkably enhance the devices performance by introducing spontaneously oriented interfacial dipoles between the ZnO layer and active layer in iPLEDs and iPSCs. This charge selective interfacial engineering is a promising way for organic optoelectronic devices such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic laser diodes (OLDs).ope

Characterization and device physics of polymer semiconducting devices with metal oxide contacts

Dit proefschrift beschrijft de fabricatie en karakterisatie van organische elektronische devices met metaal oxide contacten. Voornamelijk zijn zink oxide en vanadium pentoxide onderzocht. Manieren om op lage temperatuur dunne lagen te maken van deze metaal oxides zijn onderzocht om ze verenigbaar te maken met organische half-geleiders, die gevoelig zijn voor hoge temperaturen. De karakteristieken van organische licht-emitterende diodes and zonnecellen in verschillende structuren met deze contacten zijn onderzocht. De belangrijkste prestatie kenmerken van de verschillende types devices komen overeen met devices met een meer conventionele contact structuur. Ook zijn de metaal oxide contacten beter bestand tegen degradatie onder normale atmosferische omstandigheden dan de reactieve metalen die vaak gebruikt worden als cathode materiaal. De klassieke vergelijkingen voor de diffusie stroom in halfgeleider devices zijn aangepast aan de specifieke randvoorwaarden van contacten aan organische halfgeleiders. In het bijzonder is de rol van de Gaussische toestandsdichtheid van organische halfgeleiders en de hoogte van de energetische barriere in het geval een een non-ohmisch contact aan de organische halfgeleider uitgelicht. Deze resultaten zijn vervolgens gebruikt om een vergelijking voor de injectie stroomdichtheid vanuit een non-ohmisch contact in een organische halfgeleider op te stellen. Deze vergelijkingen voor de diffusie- en injectiestroomdichtheid in organische halfgeleiders zijn experimenteel gevalideerd voor verschillende materialen en devices

Magnetic, Optical and Dielectric Effects on Photovoltaic Processes in Organic Solar Cells

Organic bulk heterojunction photovoltaics have attracted extensive attention during the past decade due to the global energy crisis, and it had been nominated as one of the most promising substitution for the next generation of green energy. Organic Photovoltaics, also named as “plastic solar cells”, have many advantages including super-low cost, flexibility, and compatibility with the ink printing fabrication technique, etc. Although the photovoltaic efficiency of the organic bulk heterojunction is still not as high as that of the inorganic ones, its great potential makes it the most promising solar cells in the future. In this dissertation, Chapter 1 presents a basic introduction to the concepts of conjugated polymers, the widely utilized materials in photovoltaic devices, and the fundamental device physics. Meanwhile, some basic spintronics was also discussed in this chapter. Finally, the peer publications review is briefly discussed in order to cover the academic progress in this field. Chapter 2 and Chapter 3 systematically study the origin of open circuit voltage in organic photovoltaics. Chapter 4 and Chapter 5 study the magnetic field effect on photocurrent change of bulk heterojunction and double layer photovoltaics, respectively. Chapter 6 focuses on the “intra-molecular” interaction effect on internal photovoltaic processes in new low band gap materials based on magnetic field effect and photoassisted dielectric response techniques. Finally, Chapter 7 gives a short conclusion for the entire dissertation

Interfacial Engineering and Surface Plasmon Resonance Effect for High-Performance Polymer Solar Cells

Energy EngineeringPolymer solar cells (PSCs) have attracted great attention because of their many advantages including flexibility, light weight, and low fabrication cost. Among various strategies, the interfacial engineering and surface plasmon resonance (SPR) effect of metal nanoparticles (NPs) are promising and efficient ways to maximize performance of PSCs. Interfacial engineering can passivate charge trap sites, control energy level alignment, enhance charge extraction, guide active layer morphology, improve materials compatibility, alter work functions of anode and cathode. In addition, SPR effect of metal NPs can be an effective way to store the incident light energy in localized surface plasmon modes and enhance the photogeneration of excitons. Here, I present various interfacial engineering strategies employing novel charge transport layer, such as combined layer of metal oxide/ionic liquids (IL) and metal oxide/conjugated polyelectrolyte (CPE). Ionic dipoles within IL layer effectively influenced the work function of the metal oxide and thus the electron injection/transport barrier between the conduction band of metal oxide and the LUMO of active layer could be efficiently reduced. In addition, spontaneously oriented interfacial dipoles within the CPE layer lower the energy barrier for electron injection/transport and reduce the interfacial contact resistance and inherent incompatibility between the hydrophilic metal oxide and hydrophobic active layers. Surface modification of metal oxide with fullerene-based self-assembled monolayer (FSAM) reduced the contact resistance and inherent incompatibility at the metal oxide/active layer interface, resulting in an improved device performance. I also present various plasmonic materials, such as carbon dot-supported silver NPs (CD-Ag NPs), silica-coated Ag NPs (Ag@SiO2), and solvent-mediated Ag NPs (Ag@NMP) for high-performance PSCs. Compared to previous plasmonic materials, CD-Ag NPs led to broad light absorption originating from the ensemble of plasmon coupling effect caused by clustering Ag NPs in CD-Ag NPs. Furthermore, incorporating Ag@SiO2 between hole transport layer and active layer led to remarkable improvement in device efficiency caused by increased light absorption and scattering via enhanced electric field distribution. These versatile and effective methods using interfacial engineering and plasmonic materials may offer possibility to commercialize organic optoelectronic devices.ope

On the influence of physical and chemical structure on charge transport in disordered semiconducting materials and devices

Achieving fast charge carrier transport in disordered organic semiconductors is of great importance for the development of organic electronic devices. Disordered organic materials generally show low charge carrier mobilities due to their inherent energetic and configurational disorder, and the presence of chemical and physical defects. Efforts to improve mobility typically involve chemical design and materials processing to control macromolecular conformation and/or induce greater crystalline or liquid crystalline order. Whilst in many cases fruitful, these approaches have not always translated into higher bulk mobilities in devices. Addressing the adverse effect on mobility of specific types of disorder or specific defects has proven difficult due to problems distinguishing the many such features spectroscopically and controlling their formation in isolation. In the three experimental Chapters following, we attempt to make clear links between the charge carrier mobility and the presence of specific structural defects or sources of energetic or configurational disorder. In the first experimental study, we investigate hole transport in a family of polyfluorenes based on poly(9,9-dioctylfluorene) (PFO). By controlling the phase formation of the materials through processing and by virtue of their chemical design, we examine the effect on transport of distinct material phases. Remarkably, we are able to isolate the effect of the single chain conformation of PFO known as the beta-phase and show that when embedded in a glassy PFO matrix it acts as a strong hole trap, reducing the mobility of the bulk material by over two orders of magnitude. By fabricating a device with negligible beta-phase, we demonstrate the highest time-of-flight mobility in PFO to date, at over 3 10-2 cm2/Vs. This study provides the first clear and unambiguous example of the effect on transport of a distinct conformational defect in a conjugated polymer. We also demonstrate the adverse effect on mobility of crystallinity in the polyfluorenes. We suggest that our findings may generalise to other systems in the sense that the mobility may be limited by a minority population of structural traps, which may include highly ordered, crystalline regions. Significant mobility improvements may then be more easily achieved by removing the minority ordered phases than by increasing their concentration. We believe that this approach offers an alternative paradigm by which higher mobilities may be obtained in general, and in particular in systems where crystallinity is undesirable. In the second experimental study, we study charge transport in the fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), bis-PCBM and tris-PCBM. The fullerene multi-adducts bis-PCBM and tris-PCBM are of interest as alternative OPV acceptor materials with the potential to increase open-circuit voltage. However, most OPV blends employing the multi-adducts have failed to improve upon those employing PCBM. This is thought to be a result of the inferior electron transport properties of the multi-adducts, due to either (i) higher energetic disorder in the multiadducts due to the presence of isomers with varying LUMO energies or (ii) higher con gurational disorder due to a lower degree of order in molecular packing in the multi-adducts than in PCBM. We distinguish the e ects of energetic and con gurational disorder using temperature-dependent ToF and FET measurements. We find that differences in configurational disorder appear negligible, and that the reduced mobility in the multi-adducts is due predominantly to the energetic disorder resulting from the presence of a mixture of isomers with varying LUMO energies. In the third and final experimental study, we examine the charge transport properties of polymer: PCBM blends for OPV, focusing on the PTB7:PCBM and P3HT:PCBM systems. In particular, we address the question of why state-of-the-art OPV systems such as PTB7:PCBM perform so much worse at large active layer thicknesses than P3HT:PCBM. We find that low electron mobility is the main cause of this di erence. The electron mobility in PTB7:PCBM blends, at 10-5 { 10-4 cm2/Vs, is 1-2 orders of magnitude lower than the electron mobility in annealed P3HT:PCBM, at over 10-3 cm2/Vs. The hole mobility, in contrast, is the same to within a factor of approximately three. We hypothesise that the low tendency of PTB7 to order leads to a low degree of phase separation in the blend and to a poorly connected, disordered PCBM phase. We find that increasing the PCBM fraction is very effective in improving electron transport and electrical Fill Factor, but strongly reduces absorption. We suggest that a key challenge for OPV researchers is thus to achieve better connectivity and ordering in the fullerene phase in blends without relying on either (i) a large excess of fullerene or (ii) strong crystallisation of the polymer

Fabrication and Characterization of Hybrid Metal-Oxide/Polymer Light-Emitting Diodes

Hybrid metal-oxide/polymer light-emitting diodes (HyLEDs) are a novel class of electronic devices based on a combination of electroluminescent organic and charge-injecting metal-oxide components. These devices employ air-stable electrodes, such as ITO and Au, and are therefore well suited for fabrication of encapsulation-free light-emitting devices. The current work is intended to provide an insight into operating mechanisms and limitations of the HyLEDs, and, on the basis of this knowledge, aims at modifying the device architecture in order to improve the performance. The choice of optically transparent metal-oxide charge-injection layers appears to be critical in this respect in order to optimize the electron-hole balance within the polymer layer. Starting from the original device architecture, ITO/TiO2/F8BT/MoO3/Au, which uses ITO as a cathode and Au as an anode, we follow different approaches, such as the use of dipolar self-assembled monolayers and nanoscale structuring of the electron-injecting interface, pursuing the goal of enhancing electron injection into the emissive layer. However, substitution of the electron-injecting layer of TiO2 with ZrO2 is demonstrated to be the most efficient of the approaches employed herein. Further, optimization of the device utilizing the latter metal oxide is demonstrated in terms of deposition and post-deposition treatment of the electron-injecting and electroluminescent layers. Substrate temperature during spray pyrolysis deposition of the electron-injecting layer is found to have a strong influence on the HyLED performance, as well as the precursor solution spraying rate and the layer thickness. On the other hand, post-deposition annealing of the polymer layer is shown to improve the device efficiency and brightness significantly, possible explanations lying in enhancement in polymer luminescence efficiency and formation of a more intimate contact between the electron-injecting and the active polymer layers. Combining electron-transporting (TiO2 and ZnO) and hole-blocking (Al2O3 and ZrO2) materials into a single electron-injecting layer is demonstrated to be an effective strategy of enhancing efficiency in the HyLEDs. The search for a hole-injecting electrode alternative to the conventionally used MoO3/Au leads to the device employing the PEDOT:PSS/VPP-PEDOT system, which though resulting in a poorer device efficiency, provides route for fabrication of vacuum deposition-free organic light-emitting devices. Finally, the HyLED architecture is demonstrated to offer better stability than the conventional architecture using LiF/Al as a cathode. It is hoped that the current work provides a better understanding of the requirements for fabrication of encapsulation-free organic light-emitting devices