Design and Synthesis of π-Conjugated Polymers for Organic Optoelectronics
Date Issued
2011
Date
2011
Author(s)
Chen, Hung-Yang
Abstract
Polymer-based organic light-emitting diode (OLED) and organic photovoltaic (OPV) are two most extensively studied fields in polymer-based optoelectronics. In this thesis, three different series of π-conjugated copolymers have been designed and synthesized for OLED and OPV applications, respectively. The correlations between chemical structures and device performances have been studied and discussed in the context.
For OLED application, a series of new 9,10-diphenylanthracene-based, 2,6-linked blue-light-emitting copolymers bearing hole- or electron-transporter as well as bulky substituent have been successfully synthesized. Photophysical, thermal, electrochemical, and electroluminescent properties of these copolymers have studied and characterized. Bright and efficient blue fluorescence in the solid state have achieved by incorporating bulky substituent into the copolymer backbone. Both hole- and electron-transport-substituted copolymers apparently enhance the electroluminescent performance of their OLEDs. A better-balanced hole/electron charge carrier has ascribed to electron transporter-bearing copolymer OLED with a very mild luminous efficiency rolls off under a continuous operating voltages.
For OPV application, two series of low-band-gap copolymers have been designed and synthesized, first for a comparison study of thiophene- or selenophene-bridged donor-acceptor copolymers. The replacement of thiophene with selenophene in the backbone of these copolymers has found a significant reduction of energy band gaps, principally due to the lowering energy level of LUMO than the raising energy level of HOMO in these copolymers. Density functional theory (DFT) calculations are performed on these copolymers to assist explaining the heteroatom substitution on the variation of HOMO/LUMO energy levels. Organic field-effect-transistors (OFETs) have also been fabricated to study the heteroatom effect on the hole mobility of these copolymers. Largely increased short-circuit current density (JSC) and slightly decreased open-circuit voltage (VOC) in OPVs using [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor have been demonstrated for selenophene-bridged donor-acceptor copolymers when compared with those of thiophene-bridged copolymers.
Second, a series of donor-acceptor π-conjugated copolymers based on electron-rich benzo[1,2-b:4,5-b’]dithiophene (BDT) and electron-deficient cyanovinylene (CNV) or fumaronitrile (FN) moieties have been newly designed and synthesized. These copolymers are structurally tailored by anchoring alkyl chains on the backbone of the copolymer main chain to increase the solubility and molecular weights of copolymers. Having cyano- (CN) substituent on the copolymer chain, a considerable reduction of energy band-gap (Eg) has been found. It also deepens the HOMO energy level of copolymers, which increases VOC of OPVs. OFETs based on the CNV-containing copolymers and its composites with PC61BM have demonstrated hole mobilities close to those of non-annealed regioregular poly(3-hexylthiophene) (rrP3HT) and rrP3HT/PC61BM OFETs reported in literature, indicating similar charge transporting characteristics of the CNV-containing copolymer in fullerene-based OPVs. The CNV-containing copolymers are found solvent annealing effective in OPV fabrication for enhancing power conversion efficiency (PCE). This is rarely seen among BDT-based low band-gap donor-acceptor copolymers. High VOC (> 0.80 V) and PCE (~5.0 %) bulk-heterojunction [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)-based OPVs have achieved by using the copolymers as the electron-donor material.
Subjects
π-Conjugated Polymers
Organic Light-Emitting Diodes
Organic Photovoltaics
Selenophene
Benzo[1,2-b:4,5-b’]dithiophene
Cyanovinylene
Fumaronitrile
Solvent Annealing
SDGs
Type
thesis
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