Donor-Acceptor Conjugated Organic Materials for Thin-Film Transistors and Solar cells
Date Issued
2009
Date
2009
Author(s)
Lee, Wen-Ya
Abstract
Optoelectronic properties of donor-Acceptor conjugated organic materials determine the performances of organic electronics. In this thesis, a systematic study on the synthesis, optoelectronic properties and device characterizations of donor-acceptor conjugated organic materials was investigated. The goals of this thesis is to address the following issues: (1) designing and synthesizing new donor-acceptor organic materials, (2) clarifying the relationship of charge transport and chemical structures, and (3) characterizing the performances of organic electronics, e.g. thin film transistors, photovoltaic cells or light-emitting diodes, based on these studied materials. The objectives and the research findings are summarized as follows:. Synthesis, optoelectronic properties, and device applications of new fluorene-based donor-acceptor conjugated polymers (chapter 2): four fluorene-based conjugated polymers, including PFTT, PFDDTQ, PFDTBT, and PFDDTTP, with various acceptor structures, quinoxaline (Q), 2,1,3-benzothiadiazole (BT) and thieno[3,4-b]pyrazine (TP) were explored. It was found that the order of the value in the band gap energy was PFDDTTP < PFDTBT < PFDDTQ < PFTT, which was on the opposite trend of emission maximum and their charge carrier mobility due to the strong electron-withdrawing strength of the TP moiety and the coplanar backbone in PFDDTTP, leading to the highest intramolecular charge transfer and the highest hole mobility among all four polymers. Furthermore, PFDDTQ, PFDTBT, and PFDDTTP were used to fabricate in copolymer/[6,6]-phenyl-C61-butyricacid methyl ester (PCBM) bulk-heterojunction photovoltaic cells. The order in the short-circuit current density (JSC) and power-conversion efficiency (PCE) of the photovoltaic cells was PFDTBT > PFDDTQ > PFDDTTP, which contradicted the order in the band gap energy and mobility. However, the JSC and PCE coincided instead with the order in the mobility of the copolymer/PCBM blend, where the mobility was found to increase in PFDTBT and PFDDTQ devices owing to charge transfer with PCBM, but the mobility deceased in PFDDTTP due to inefficient phase separation resulting from the strong intermole-cular interactions between the polymer chains of PFDDTTP. With its high blended mobility and low band gap, PFDTBT achieved a PCE of 1.1 %.. The charge transport characteristics and surface morphology of three thiophene-thieno[3,4-b]pyrazine alternating copolymers (PTHTP-C7, PTHTP-C12, and PBTHTP-C7) (chapter 3). Long alkyl side chains promoted PTHTP-C12 to become a fibrillar-like structure on the hexamethyldisilazane (HMDS)-modified surface and resulted in better charge transport properties than those of the other two copolymers. However, a nodule-like morphology on the octyltrichlorosilane (OTS)-modified surface was observed due to the strong interaction between the non-polar alkyl chains of PTHTP-C12 and highly hydrophobic surface. By further annealing at a higher temperature, a densely packed grain morphology on octyltrichlorosilane (OTS) modified SiO2 surface was observed and led to the field effect mobility of 1.1 × 10-2 cm2V-1s-1. The present study suggests that the thiophene based donor-acceptor conjugated polymers could have a high FET mobility through the manipulation of their morphology.. Synthesis, characterization of new alternating copolymers of fluorene and 3,9- or 2,8- substituted di(2-ethylhexyl)-indolo[3,2-b]carbazole (chapter 4). poly(fluorene)-alt-3,9-(indolo[3,2-b]carbazole)) (PF-p-In) and poly(fluorene)-alt-2,8-(indolo[3,2-b]carbazole)) (PF-m-In) were synthesized by palladium-catalyzed Suzuki coupling polymerization and characterized for the applications of light-emitting diodes and field effect transistors (FET). The para-linkage, PF-p-In, facilitates π-electron delocalization and thus has a lower optical band gap and a higher emission maximum than those of the meta linkage, PF-m-In. The electroluminescence devices based on PF-p-In and PF-m-In as the emissive layer show a similar maximum luminance but with different emissive colors of green and blue, respectively. The FET hole mobilities of PF-p-In and PF-m-In are 6.73×10-5 and 1.50×10-4 cm2/V.s, respectively, which are significantly higher than that of polyfluorene. The present study demonstrates the electronic and optoelectronic properties of polyfluorene enhanced by incorporating hole transporting indolocarbazole with different linkages. . The solution-processing OTFT characterizations of new core-chlorinated Naphthalene diimide (NDI) derivatives with fluorinated alkyl chains (chapter 5). Four core-chlorinated NDI derivatives, including N,N’-bis(heptafluorobutyl) naphthalene diimide (1), N,N’-bis(heptafluorobutyl)-2,6-dichloro-naphthalene diimide (2), N,N’-bis(nonafluoropentyl)-2,3,6,7-tetrachloro-naphthalene diimide (3) and N,N’-bis(nonafluoropentyl)-2,6-dichloro-naphthalene diimide (4), were studied. A high electron average mobility of 0.17 cm-2V-1s-1 and an excellent ambient stability are observed in OTFT devices made from 2 through solution-sheared deposition method. From charge transport anisotropy measurement, a high mobility of 0.5 cm-2V-1s-1 of 2 was achieved from a perpendicular direction of OTFT. The direction of channel length in the device is vertical to the shearing direction. Furthermore, the threshold voltage of 1 increases dramatically upon exposure to the air because the grain boundaries of 1 provide channels for diffusion of oxygen and moisture, which act as electron traps. From XRD measurement result, it is concluded that the solution-sheared method affect the orientation of crystalline thin films and lower the d-spacing distance, as compared with that of the vapor deposition. This study affords the promising n-type high-performance core-chlorinated NDI derivatives for potential organic electronics.
Subjects
Donor-Acceptor Conjugated Polymer
Thin-Film Transistors
Solar cells
SDGs
Type
thesis
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