Syntheses, Morphology, and Device Applications of Donor-Acceptor Semiconducting Polymer Systems
Other Title
電子施體/受體高分子系統之合成、形態及元件應用
Date Issued
2010
Author(s)
Abstract
Optoelectronic polymers are of wide interests for electronic and optoelectronic devices, such as thin film field effect transistors, photovoltaic cells, and polymer memory devices. However, systematic studies on the synthesis, optoelectronic properties, and device characterizations of the donor-acceptor conjugated polymer systems have not been fully explored yet, especially the acceptor based polymers or conjugated diblock copolymers and polyimide based systems. The goals of this thesis is to address the effect of donor-acceptor structure and morphology on the optoelectronic properties, carrier mobility, photovoltaic or memory characteristics, including: (1) acceptor-vinylene/ethynylene copolymers, (2) all-conjugated diblock poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene)/PCBM blends, (3) sulfur-donor containing polyimides, and (4) triphenylamine-based polyimides containing mono- or dual-mediated phenoxy linkages.
1. Syntheses, properties, and field effect transistors of small band gap quinoxaline- and thienopyrazine-vinylene/ethynylene conjugated polymers (Chapter 2): four new conjugated copolymers of quinoxaline (AQ) and thienopyrazine (ATP) with vinylene (V) or ethynylene (E), PAQV, PAQE, PATPV, and PATPE, were synthesized by Stille-coupling reaction. Both the optical and electrochemical band gaps of the PAQV, PAQE, PATPV, and PATPE were quite small around 1.00 to 2.00 eV, which arose from the reduced steric hindrance arising from the incorporation of the V or E linkage or the intramolecular charge transfer between the acceptor and the V or E linkages. Besides, the AQ/ATP-vinylene copolymers exhibited much higher vis/near infrared absorption intensity than the AQ/ATP-ethynylene ones, which suggested the stronger pi-pi transition intensity in the former and led to better charge-transporting characteristics. The saturation field effect hole mobilities of the PATPV were 2.1×10-3, 1.7×10-2, and 1.1×10-2 cm2V-1s-1 on bare, octyltrichlorosilane(OTS)-treated, and octadecyltrichlorosilane(ODTS)-treated SiO2, respectively, with on-off current ratios of 35, 6.02×102, and 7.56×102. On the other hand, the estimated FET hole mobility of the PATPE were in the range of 1.7×10-6 to 8.1×10-4 cm2V-1s-1, which were significantly smaller than that of the PATPV. The small band gaps and high charge carrier mobility of the prepared copolymers suggested their potential applications for near-infrared electronic and optoelectronic devices.
2. All-conjugated diblock copolymer of poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) for field-effect transistor and photovoltaic applications (Chapter 3): the electronic properties, morphology and optoelectronic device characteristics of all-conjugated diblock copolythiophene, poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), are reported. The polymer properties and structures were explored through different solvent mixtures of chloroform (CHCl3) and dichlorobenzene (DCB). The absorption maximum (λmax) of P3HT-b-P3PT prepared from DCB showed a shoulder peak indicative for the highly crystalline structure around 604 nm while that from CHCl3 did not show it. The field effect hole mobility of P3HT-b-P3PT increased from 6.0×10-3 to 2.0×10-2 cm2V-1s-1 as the DCB content in the solvent mixture increased. The AFM images suggested that CHCl3 processing solvent led to the amorphous surface structures while DCB resulted in the largely crystalline domain. Such difference on the morphology and hole mobility led to the varied power conversion efficiency (PCE) of the photovoltaic cells fabricated from the blend of P3HT-b-P3PT/PCBM (1:1, w/w). The PCE of polymer/PCBM could be improved to 2.80% while the ratio of polymer to PCBM goes to 1:0.7. The present studies suggested that the surface structures and optoelectronic device characteristics of all-conjugated diblock copolymers could be easily manipulated through the processing solvents, the block segment characteristic, and blend compositions.
3. Synthesis and memoy device characteristics of new sulfur-donor containing polyimides (Chapter 4): the synthesis and memory device characteristics of two new poly[2,7-bis(phenylenesulfanyl)thianthrenehexafluoro-isopropylidenediphthalimide] (APTT-6FDA) and poly[4,4’-thiobis(p-phenylene-sulfanyl)-hexafluoroisopropyliden-ediphthalimide] (3SDA-6FDA) are reported. The sulfur-containing APTT and 3SDA as electron-donor were designed to enhance the electron-donating and charge-transporting characteristics for device applications. The optical band gaps of APTT-6FDA and 3SDA-6FDA estimated from the absorption edges were 3.51 and 3.46 eV, which probably resulted from the difference on the structural coplanarity. The memory device with the configuration of ITO/Polymers/Al showed nonvolatile memory characteristics with low turn-on threshold voltages of 1.5 (APTT-6FDA) and 2.5 (3SDA-6FDA) V, probably resulted from the difference on the HOMO energy level. Also, the memory devices could be repeatedly written, read, and erased. The on/off current ratios of the devices were all around 104 in ambient atmosphere. The lower-lying HOMO energy level, larger band gap, and higher dipole moment of the sulfur-containing polyimide compared to the triphenylamine-based polyimide provided a stable CT complex for the flash memory device. The above electronic properties were further confirmed by the density functional theory (DFT) method at the B3LYP level with the 6-31G(d) basic set. The present study suggested that the new sulfur-containing polyimides would have potential applications for memory devices.
4. High performance volatile polymeric memory devices vased on novel triphenylamine-based polyimides containing mono- or dual-mediated phenoxy linkages (Chapter 5): two novel functional polyimides (PIs), PI(AAPT-TPA) and PI(APT-TPA), consisting of electron-donating 4-amino-4’-(p-aminophenoxy)-triphenylamine (AAPT) or 4,4’-bis(p-aminophenoxy)-triphenylamine (APT) and electron-accepting phthalimide moieties, were prepared for the memory device applications. The memory devices with the configuration of ITO/PIs/Al exhibited two conductivity states and could be swept positively or negatively with a high ON/OFF current ratio of 108~109. The PI(AAPT-6FDA) device relaxed from the ON state to the OFF state quickly after the applied voltages was removed, whereas the ON state of the PI(APT-6FDA) device could retain for around 4 min after the power was turned off. It suggested that dynamic random access memory (DRAM) was exhibited for the PI(AAPT-6FDA) device and static random access memory (SRAM) was for the PI(APT-6FDA) device. The volatile memory characteristics were probably attributed to the unstable charge transfer (CT) complex based on the weak theoretical dipole moments of the studied PIs. The dual-mediated phenoxy linkage of PI(APT-6FDA) led to the more twisted conformation compared to the mono-substituted PI(AAPT-6FDA) based on the theoretical analysis by the density functional theory (DFT) method. It thus produced a potential barrier for delaying the back CT process by the electric field and explained the SRAM characteristic. The present study suggested that the importance of the TPA structure on the memory characteristics. The fast switching and high ON/OFF characteristics also indicated the new TPA based polyimides for advanced memory technology.
Subjects
Transistor
Photovoltaic Cell
Polymeric Memory
Donor/Acceptor
SDGs
Type
thesis
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