Controlling Thin-film Morphology and Incorporating Novel Semiconducting Molecules toward High Performance Organic Optoelectronic Devices

Show simple item record Song, Byeongseop 2018-06-07T17:53:12Z 2018-06-07T17:53:12Z 2018 2018
dc.description.abstract Organic optoelectronic devices have been widely used in display, energy-storage, and consumer electronics. Insightful understanding on material properties, device architecture, and fabrication processes is inevitable to improve the performance of organic optoelectronic devices. My PhD research focuses on improving the performance of organic photovoltaics (OPV) and organic light-emitting diode (OLED) through the systematic processing and material design. The first part of the dissertation describes how to construct a highly conductive morphology of mixed donor:acceptor heterojunction. Organic vapor phase deposition (OVPD) was utilized to enhance crystallinity of C70 acceptor in the mixed tetraphenyldibenzoperiflanthen (DBP):C70 thin-film. Forming the face-center-cubic (fcc) structure of C70 facilitated charge extraction, thereby improving fill factor (FF) of the corresponding OPVs. The second part presents the study on the morphological stability and reliability of OPVs. The cathode buffer, bathophenanthroline (BCP), undergoes significant morphological degradation. This morphological degradation was successfully suppressed by making the underlying DBP:C70 layer rougher via the moving N2 carrier gas in OVPD. The open-circuit voltage (Voc) of the obtained heterojunction OPVs of DBP:C70 grown by OVPD experienced a negligible drop (< 3 % change) while the equivalent OPVs grown by VTE showed a significant decrease in Voc from 0.91±0.01 V to 0.74±0.01 after 1 Sun illumination for 250 h. The third part explains a more precise way to control the morphology of organic mixed layer. It was found that increase in the growth pressure of OVPD induced reorganization of molecules to form the equilibrium morphology. The morphology of the electron-filtering buffer layer of 3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (BP4mPy):C60 was optimized to achieve the highest electron mobility by means of the control of the growth pressure. Consequently, the resulting OPVs with optimized BP4mPy:C60 buffer showed FF = 0.65±0.01 and a much higher PCE = 8.0±0.2 % compared to PCE = 6.6±0.2 % of the equivalent OPVs with the same composition buffer layer grown by VTE. The fourth part summarizes the effects of the inclusion of novel block-copolymers on the performance of the polymer bulk-heterojunction photovoltaic cells. The block-copolymers were composed of thiophene units with and without a dangling phenyl-C61-butyric acid methyl ester (PCBM) side chain. The added copolymer into the poly(3-hexylthiophene) (P3HT): PCBM active layer resulted in greatly improved thermal stability of P3HT:PCBM. Furthermore, electron conductivity also increased since the fullerene units of the copolymers contribute to the formation of a percolation pathway for electron transport. While PCE of conventional P3HT:PCBM bulk-heterojunction solar cells decreases significantly from 2.6±0.2 to 1.2±0.2% after 90-min of thermal annealing, the equivalent OPVs with the copolymer shows a much smaller decrease in PCE from 3.1±0.2% to 2.7±0.2%. The last section of this dissertation covers the design of phosphorescent OLED employing a metal-free purely organic phosphor. Owing to their much longer triplet lifetime in the millisecond regime compared to microseconds of organometallics, a more careful consideration should be given in the device design. The requirements for the host materials in metal-free purely organic phosphor OLEDs are identified to be a high triplet energy, suitable HOMO and LUMO energy levels, and large spectral overlap with the absorption of the phosphors. Systematic investigation on various host molecules, electron transporting molecules, and the layer thickness of each layer allows us to demonstrate an optimized phosphorescent OLED having an external quantum efficiency (EQE) of 2.5 % at 1 mA/cm2.
dc.language.iso en_US
dc.subject Organic optoelectronic devices (OLED, OPV)
dc.subject Performance and reliability of organic semiconductor devices
dc.subject Organic thin-film growth
dc.subject crystallinity of organic thin-film
dc.subject purely organic phosphor
dc.title Controlling Thin-film Morphology and Incorporating Novel Semiconducting Molecules toward High Performance Organic Optoelectronic Devices
dc.type Thesis en_US
dc.description.thesisdegreename PHD
dc.description.thesisdegreediscipline Electrical Engineering
dc.description.thesisdegreegrantor University of Michigan, Horace H. Rackham School of Graduate Studies
dc.contributor.committeemember Guo, L Jay
dc.contributor.committeemember Kim, Jinsang
dc.contributor.committeemember McNeil, Anne Jennifer
dc.contributor.committeemember Phillips, Jamie Dean
dc.subject.hlbsecondlevel Electrical Engineering
dc.subject.hlbtoplevel Engineering
dc.identifier.orcid 0000-0002-4172-4457
dc.owningcollname Dissertations and Theses (Ph.D. and Master's)
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