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Understanding and Controlling the Morphology of Organic Thin Films.

Record Type:	Language materials, printed : Monograph/item
Title/Author:	Understanding and Controlling the Morphology of Organic Thin Films./
Author:	Kim, Jongchan.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2021,
Description:	204 p.
Notes:	Source: Dissertations Abstracts International, Volume: 83-05, Section: B.
Contained By:	Dissertations Abstracts International83-05B.
Subject:	Materials science. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=28845640
ISBN:	9798471105584

Understanding and Controlling the Morphology of Organic Thin Films.
Kim, Jongchan.

Understanding and Controlling the Morphology of Organic Thin Films. - Ann Arbor : ProQuest Dissertations & Theses, 2021 - 204 p.

Source: Dissertations Abstracts International, Volume: 83-05, Section: B.

Thesis (Ph.D.)--University of Michigan, 2021.

This item must not be sold to any third party vendors.

Display panels today open the door to the repository of knowledge, and their use is expanding from conventional home appliances to transparent, wearable and mobile devices. Organic semiconductors are a perfect candidate for this ubiquitous use of display panels since organics are flexible, transparent with high brightness and capable of good color rendering. The expanding outdoor use of organic devices poses questions whether: i) the devices are bright enough to be visible under daylight, and ii) the devices can withstand extreme conditions such as the interior of an automobile in the summer. Organic light emitting devices (OLEDs) achieved ~100% internal quantum efficiency with phosphorescence in 1998 [1], and since then the bottle neck for OLED brightness has been the outcoupling efficiency. The capability of organics to withstand severe conditions is closely related to their morphological stability. Thus, improving the outcoupling efficiency and controlling the morphology of OLEDs are the two crucial aspects in the future display technology. In this sense, this thesis mainly deals with the methods to improve the light outcoupling of OLEDs by morphological control. Also, methods for understanding organic film morphology are discussed. In this thesis, we demonstrate a measurement technology to obtain precise nanoscale information about the morphologies of several organic thin film structures using Fourier plane imaging microscopy (FIM). We use FIM to detect the orientation of molecular transition dipole moments from an extremely low density (i.e. small fractions of a monolayer) of luminescent dye molecules, which we call “morphology sensors.” The orientation of the sensor molecules is driven by the local film structure, and thus can be used to determine details of the host morphology without influencing it. We use symmetric planar phosphorescent dye molecules as the sensors that are deposited into the bulk of organic film hosts during the growth. Furthermore, we monitor morphological changes arising from thermal annealing of metastable organic films that are commonly employed in photonic devices. Methods to control the organic film morphology to improve the light outcoupling are also demonstrated. Here we control the orientation of Pt complex molecules during the growth of emissive layers by two different methods: modifying the molecular structure, and using structural templating. Pt complex dopant molecules whose structures are modified by adjusting the ligands show an approximately 20% increased fraction of horizontally aligned transition dipole moments compared to the original molecule while being diluted in the host matrix. Alternatively, we pre-deposit a highly ordered structural template layer, which results in a 60% increase in horizontally aligned transition dipole moments compared to the film deposited in the absence of the template. Finally, we employ a 2-dimensional transition metal dichalcogenides as the active luminescent layer due to its optimum emission profile for efficient outcoupling. Therefore, a hybrid light emitting device (LED) is fabricated employing a chemical-vapor-deposition grown, centimeter-scale monolayer of WS2 (mWS2), embedded within conductive organic layers. As a result, LEDs with an average external quantum efficiency of 0.3 ± 0.3% and with the highest efficiency of 1% were achieved. Also, we show that negatively charged excitons, also known as trions, are generated in the mWS2 with the injected current, causing an efficiency roll-off at high current densities.

ISBN: 9798471105584Subjects--Topical Terms:

557839
Materials science.
Subjects--Index Terms:

Organic electronics

Understanding and Controlling the Morphology of Organic Thin Films.
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520 $a Display panels today open the door to the repository of knowledge, and their use is expanding from conventional home appliances to transparent, wearable and mobile devices. Organic semiconductors are a perfect candidate for this ubiquitous use of display panels since organics are flexible, transparent with high brightness and capable of good color rendering. The expanding outdoor use of organic devices poses questions whether: i) the devices are bright enough to be visible under daylight, and ii) the devices can withstand extreme conditions such as the interior of an automobile in the summer. Organic light emitting devices (OLEDs) achieved ~100% internal quantum efficiency with phosphorescence in 1998 [1], and since then the bottle neck for OLED brightness has been the outcoupling efficiency. The capability of organics to withstand severe conditions is closely related to their morphological stability. Thus, improving the outcoupling efficiency and controlling the morphology of OLEDs are the two crucial aspects in the future display technology. In this sense, this thesis mainly deals with the methods to improve the light outcoupling of OLEDs by morphological control. Also, methods for understanding organic film morphology are discussed. In this thesis, we demonstrate a measurement technology to obtain precise nanoscale information about the morphologies of several organic thin film structures using Fourier plane imaging microscopy (FIM). We use FIM to detect the orientation of molecular transition dipole moments from an extremely low density (i.e. small fractions of a monolayer) of luminescent dye molecules, which we call “morphology sensors.” The orientation of the sensor molecules is driven by the local film structure, and thus can be used to determine details of the host morphology without influencing it. We use symmetric planar phosphorescent dye molecules as the sensors that are deposited into the bulk of organic film hosts during the growth. Furthermore, we monitor morphological changes arising from thermal annealing of metastable organic films that are commonly employed in photonic devices. Methods to control the organic film morphology to improve the light outcoupling are also demonstrated. Here we control the orientation of Pt complex molecules during the growth of emissive layers by two different methods: modifying the molecular structure, and using structural templating. Pt complex dopant molecules whose structures are modified by adjusting the ligands show an approximately 20% increased fraction of horizontally aligned transition dipole moments compared to the original molecule while being diluted in the host matrix. Alternatively, we pre-deposit a highly ordered structural template layer, which results in a 60% increase in horizontally aligned transition dipole moments compared to the film deposited in the absence of the template. Finally, we employ a 2-dimensional transition metal dichalcogenides as the active luminescent layer due to its optimum emission profile for efficient outcoupling. Therefore, a hybrid light emitting device (LED) is fabricated employing a chemical-vapor-deposition grown, centimeter-scale monolayer of WS2 (mWS2), embedded within conductive organic layers. As a result, LEDs with an average external quantum efficiency of 0.3 ± 0.3% and with the highest efficiency of 1% were achieved. Also, we show that negatively charged excitons, also known as trions, are generated in the mWS2 with the injected current, causing an efficiency roll-off at high current densities.
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