Small Molecule Organic Optoelectronic Devices

abstract: Organic optoelectronics include a class of devices synthesized from carbon containing ‘small molecule’ thin films without long range order crystalline or polymer structure. Novel properties such as low modulus and flexibility as well as excellent device performance such as photon emission approaching 100% internal quantum efficiency have accelerated research in this area substantially. While optoelectronic organic light emitting devices have already realized commercial application, challenges to obtain extended lifetime for the high energy visible spectrum and the ability to reproduce natural white light with a simple architecture have limited the value of this technology for some display and lighting applications. In this research, novel materials discovered from a systematic analysis of empirical device data are shown to produce high quality white light through combination of monomer and excimer emission from a single molecule: platinum(II) bis(methyl-imidazolyl)toluene chloride (Pt-17). Illumination quality achieved Commission Internationale de L’Éclairage (CIE) chromaticity coordinates (x = 0.31, y = 0.38) and color rendering index (CRI) > 75. Further optimization of a device containing Pt-17 resulted in a maximum forward viewing power efficiency of 37.8 lm/W on a plain glass substrate. In addition, accelerated aging tests suggest high energy blue emission from a halogen-free cyclometalated platinum complex could demonstrate degradation rates comparable to known stable emitters. Finally, a buckling based metrology is applied to characterize the mechanical properties of small molecule organic thin films towards understanding the deposition kinetics responsible for an elastic modulus that is both temperature and thickness dependent. These results could contribute to the viability of organic electronic technology in potentially flexible display and lighting applications. The results also provide insight to organic film growth kinetics responsible for optical, mechanical, and water uptake properties relevant to engineering the next generation of optoelectronic devices.Dissertation/ThesisDoctoral Dissertation Chemical Engineering 201

Organic additive engineering toward efficient perovskite light‐emitting diodes

Perovskite materials with excellent optical and electrical properties are promising for light‐emitting diodes. In the field of perovskite light‐emitting diodes (PeLEDs), organic materials additive engineering has been proved to be an effective scheme for enhancing efficiency and stability in PeLEDs. Most impressively, the reported external quantum efficiency of PeLEDs based on perovskite‐organic composite has reached over 20%. Herein, we will review the important progress of the organic materials\u27 additive‐modified PeLEDs and discuss the remaining problems and challenges and the key research direction in the near future

MOLECULAR DESIGN OF ORGANIC SEMICONDUCTORS FOR INTERFACIAL AND EMISSIVE MATERIAL APPLICATIONS

This dissertation describes the synthesis and characterization of functional optoelectronically active materials. Synthetic techniques were used to prepare polymers containing perylene diimide (PDI) or tetraphenylethylene (TPE) moieties in the polymer backbone. PDI-based structures were prepared with embedded cationic or zwitterionic moieties intended to tailor organic/inorganic interfaces in thin film photovoltaic devices. The aggregation-induced emission (AIE)-active TPE polymers were synthesized to study how AIE properties evolve in π-conjugated polymers. The syntheses discussed here focused on modulation of molecular architecture to give rise to materials with tailored optoelectronic properties. Chapter 1 provides a brief overview of the field of organic electronics and the key concepts underpinning this thesis research. Chapter 2 describes the synthesis of PDI-containing polyionenes and linear polymer zwitterions. Dual-functional PDI monomers containing tertiary amines at the imide position, and bulky bromide or phenyl groups at the aromatic core, afforded reactive and high solubility monomers. Polymers with ammonium bromide and sulfobetaine functionality in the backbone were prepared by reacting PDI monomers with the appropriate electrophiles. By controlling PDI content, macromolecular structures with tunable PDI-PDI interactions were achieved and studied spectroscopically. Chapter 3 focuses on integration of novel PDI-based materials into organic and perovskite solar cells as interfacial layers. The interfacial properties, morphology, and device enhancement were studied as a function of PDI incorporation in the polymer backbone. Trends in electronic properties and device performance were correlated to polymer structure and revealed a strong dependence on the selection of cationic vs. zwitterionic functionality. The PDI-containing polymers were found to enhance photovoltaic device performance, despite not being continuous conjugated structures, but rather having conjugated molecular segments in the polymer backbone. The effective work function modification of metal cathodes and energy level overlap with perovskite active layer permitted enhanced device performance when tertiary amine-functionalized PDI small molecules were incorporated into devices. Chapter 4 centers on the synthesis and characterization of conjugated polymers containing TPE. The optical properties of these materials were adjusted by controlling extent of vinylene groups in the polymer backbone. These vinylene-containing TPE polymers exhibited similar optical and electronic properties to poly(phenylene vinylene) (PPV) while maintaining the desirable AIE properties of TPE. Moreover, by controlling the mole percent of TPE in PPV copolymers aggregation-caused quenching (ACQ) was attenuated without perturbation of PPV’s optical properties. Finally, Chapter 5 projects an outlook of the discussed research. Emphasis is given to where research focus should be oriented to advance the technology beyond the academic space. The aim of this chapter is to highlight the impact of this work and its relationship to bringing the science closer to the general public. The experimental procedures of the materials synthesis, characterization, and device fabrication are then detailed in Chapter 6

Advances in All-Inorganic Perovskite Nanocrystal-Based White Light Emitting Devices

Metal halide perovskites (MHPs) are exceptional semiconductors best known for their intriguing properties, such as high absorption coefficients, tunable bandgaps, excellent charge transport, and high luminescence yields. Among various MHPs, all-inorganic perovskites exhibit benefits over hybrid compositions. Notably, critical properties, including chemical and structural stability, could be improved by employing organic-cation-free MHPs in optoelectronic devices such as solar cells and light-emitting devices (LEDs). Due to their enticing features, including spectral tunability over the entire visible spectrum with high color purity, all-inorganic perovskites have become a focus of intense research for LEDs. This Review explores and discusses the application of all-inorganic CsPbX3 nanocrystals (NCs) in developing blue and white LEDs. We discuss the challenges perovskite-based LEDs (PLEDs) face and the potential strategies adopted to establish state-of-the-art synthetic routes to obtain rational control over dimensions and shape symmetry without compromising the optoelectronic properties. Finally, we emphasize the significance of matching the driving currents of different LED chips and balancing the aging and temperature of individual chips to realize efficient, uniform, and stable white electroluminescence

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School of Energy and Chemical Engineering (Energy Engineering)My MS research has focused on light-emitting diodes (LEDs) devices based on perovskite nanocrystals (PeNCs). All-inorganic PeNCs are promising material for optoelectronic devices due to their easily tunable bandgap, narrow full width at half maximum (FWHM), and high photoluminescence (PL) quantum yield. Moreover, all-inorganic perovskite material has better stability than the other perovskite material that employs hybrid or organic part at A site of perovskite. These outstanding physical properties caused lot of interest in PeNCs for LEDs devices. To make highly efficient LEDs devices, there are several important factors. Those are smooth film morphology, energy band alignment for low charge injection barrier, and charge balance between hole and electron. Especially, perovskite material has low exciton binding energy (Eb), therefore, confining electron and hole into nanocrystals is very important to make exciton. By considering these factors, a lot of research is going on. However, blue-emitting PeNCs have resulted lower quantum yield compared to red or green-emitting PeNCs due to their large bandgap and anion segregation. To achieve commercialization of PeNCs for future display, white LEDs are indispensable, therefore, great development of blue-emitting LEDs is required. From this point of view for high-efficiency blue LEDs devices, 1,8-diiodooctane (DIO) additive has been used for our research. DIO is added to poly[bis(4-phenyl)(4-butylphenyl)amine] (poly-TPD) solution that is employed in hole transporting layer (HTL) to improve hole injection and passivate trap states between poly-TPD layer and perovskite NC layer. By adopting DIO additive to HTL, external quantum efficiency (EQE) of green PeNCs LEDs was enhanced from 2.89 % to 5.17 % compared to the reference device, and EQE of blue PeNCs LEDs was enhanced from 0.58 % to 1.34 % compared to the reference device.ope

Fabrication and characterization of organic light emitting diodes for display applications

Organic Light Emitting Diodes (OLEDs) based on the principle of electroluminescence, constitute a new and exciting emissive display technology for flat panel displays. In order to attain high quantum efficiency for electroluminescence, it is necessary to achieve three attributes: efficient charge injection from the electrodes at low drive voltage, good charge balance, and confinement of the injected charge carriers within the emitting layers. The purpose of this research work was to fabricate, measure and analyze OLEDs based on these fundamental principles using different cathode materials, injection layers and buffer layers in order to determine the best possible configuration. Starting from a simple bi-layered device, multilayered heterojunction OLEDs were built by employing energy band engineering. Since it was the first time that these imaging devices were being built in our Laboratory, developing tools and techniques to get reproducible OLEDs was a prerequisite to the realization of this goal. Thus, through this process, the Lab\u27s capability was realized from the fabrication and characterization perspective, and fundamental knowledge regarding the operation of OLEDs was gained. The OLEDs fabricated were of high efficiency and brightness, and their properties match well with the published literature

Novel processing approaches for thin film solar and related technologies.

A growing population along with developing nations are increasing the demand for energy. The International Energy Agency forecasts a global electricity demand increase of 70 percent by 2040. This is an increase from nearly 18 TW to over 30 TW. The sun can be a great clean source of achieving this energy demand. Despite the large solar industry development, the market is still growing as solar energy only accounted for 0.87% of the global energy production in 2013. The opportunity exists to manufacture more affordable solar energy that can penetrate more of the global energy market. In this dissertation, a photonic-based manufacturing technique called intense pulsed light (IPL) was investigated to enhance the photovoltaic properties of CdTe, better understand the CdCl2 treatment used to create higher efficiency CdTe solar devices, enable the first sintering and efficiency enhancement of perovskite solar cell (PSC), and study the possible conversion of a stable 2D perovskite to a 3D perovskite. CdTe thin films grown by low temperature electrodeposition were treated for the first time with IPL. The low temperature electrodeposition growth resulted in films consisting of nanoparticles, with reduced melting point temperatures. In combination with the high temperature rise produced by the pulses of light, the lower melting temperature resulted in pores/voids being filled as well as enhanced grain growth. As a result, pin-holes and grain boundary recombination were diminished. Subsequently the fill factors of PV devices created using this technology significantly increased. In addition, the IPL also successfully improved the crystallinity in the CdTe films by photonically initiating the popular CdCl2 treatment. To understand the mystery behind the mechanism of the CdCl2 treatment, low temperature PL was utilized and new electrodeposition precursors resulting from the study improved device efficiencies. Photoactive perovskite CH3NH3PbI3 layers were successfully sintered with a novel IPL treatment with efficiencies exceeding 12%. The processing time was reduced to 2 ms, which was significantly faster than those from previous reports. Additionally, the average performance of the IPL-processed samples showed an improvement compared to the hot- plate-processed samples. This advance creates an exciting new method to quickly create dense layers of perovskite, eliminating the rate-limiting annealing step detrimental to industry adoption, and shows the first known occurrence of sintering in CH3NH3PbI3 perovskite particles. Lastly, the fast photonic processing of the IPL enabled the first conversion of a stable 2D perovskite structure into a 3D structure. This caused an band gap shift from 2.0 eV to 1.6 eV and showed the capabilities of band gap tuning enabled by the IPL. While this work is the first documentation of band gap tuning enabled by a photonic effect, it presents a possible inexpensive manufacturing technique that could use one material to create several different colors for the future development of pixel-based LED displays