Syntheses, Morphology, and Properties of New Donor-Acceptor Conjugated Polymers for Field Effect Transistor and Solar Cell Applications
Date Issued
2010
Date
2010
Author(s)
Tsai, Jung-Hsun
Abstract
Donor-acceptor (D-A) copolymers have attracted significant scientific interest recently as their electronic and optoelectronic properties can be manipulated through intramolecular charge transfer (ICT). Such polymers may have potential applications in various organic electronic devices, especially for field effect transistors and polymer solar cells. In order to obtain high-performance of these devices, it is essential to design and synthesize conjugated polymers with desirable properties, such as high carrier mobility, ordered and tunable morphology, and appropriate molecular energy levels. Utilizing highly electron-donating and coplanar aromatic fused-rings as a building block into D-A based system could probably meet above-methioned requirements, contructing ordered molecular packing and high charge transporting of conjugated polymers.
Herein we summarized the systematic studies on the syntheses, optoelectronic properties, morphology, and their device characterizations of the three D-A conjugated systems embedded with coplanar donors, including: (1) indolocarbazole-based, (2) thiophene-phenylene-thiophene (TPT)-based, (3) two-dimensional thiophene (2D)-based alternating or random copolymers. In addition, the final subject is to introduce P3HT-based coil-rod-coil copolymer into bulk-heterojunction solar cell as a surfactant, leading to the improved efficiency and device stability. The details of each topic are summarized as below:
1. Synthesis of New Indolocarbazole-Acceptor Alternating Conjugated Copolymers and Their Applications to Thin Film Transistors and Photovoltaic Cells (Chapter 2): we report the synthesis, properties, and optoelectronic device characteristics of six new indolocarbazole-acceptor conjugated copolymers prepared by Suzuki coupling reaction. Two different linkages of indolocarbazole (28IC and 39IC) and four acceptors of 2,3-didodecylthieno[3,4-b]pyrazine (TP12), 2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine (TPO), 2,1,3-benzothiadiazole (BT), and 2,3-bis(4-(2-ethylhexyloxy)phenyl)quinoxaline (QO) were used to explore the effects of acceptor structure, linkage, and side group on the electronic and optoelectronic properties. The hole mobility and on-off ratios of the studied copolymers were in the range of 1.66×10-5 ~ 4×10-4 cm2 V–1 s–1 and 40~46900, respectively. It basically depended on the degree of intromolecular charge transfer (ICT) between indolocarbazole and acceptor as well as the HOMO level. The power conversion efficiency (PCE) of the indolocarbazole-acceptor polymer/PC61BM or PC71BM based photovoltaic cells were in the range of 0.14-1.40% under the illumination of AM 1.5G (100 mW/cm2). P28IC-QO showed the best PCE among the studied copolymers because of its suitable HOMO/LUMO energy level, high molecular weight, good hole mobility, efficient PL quenching, and large Voc.
2. New Thiophene-Phenylene-Thiophene Acceptor Random Conjugated Copolymers for Optoelectronic Applications (Chapter 3): New low band-gap thiophene-phenylene-thiophene (TPT)-based donor-acceptor-donor random copolymers were synthesized for optoelectronic device applications by a palladium-catalyzed Stille coupling reaction under microwave heating. The acceptors included 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[5’-bromo-dithien-2-yl-quinoxa- lines] (DTQ) and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethyl-hexyl)pyrrolo[3,4-c] -pyrrole-1,4-dione (DPP). The prepared the random copolymers were named as PTPTDTQ0.55, PTPTDTQ0.34DPP0.14, and PTPTDTQ0.26DPP0.34 depending on the copolymer ratio and their corresponding Egopt (eV) were 1.74, 1.56, and 1.48 eV, respectively. The hole mobility obtained from the field effect transistor devices prepared from PTPTDTQ0.55, PTPTDTQ0.34DPP0.14, and PTPTDTQ0.26DPP0.34 were 2.2×10–3, 2.4×10–3, and 4.7×10–3 cm2 V–1s–1, respectively, with the on-off ratios of 4.0×104, 4.0×104, and 5.3×104. It suggested that the significant ICT effect between the TPT and acceptor led to the band gap reduction and hole mobility enhancement. Polymer solar cells of these TPT-based copolymers blended PC71BM exhibited power conversion efficiencies (PCEs) as high as 3.71 %. Besides, the near-infrared photodetector device prepared from PTPTDTQ0.26DPP0.34 showed a high external quantum efficiency exceeding 32% at 700 nm (under –3V bias) and fast-speed response. The present study suggests that the prepared TPT-based donor-acceptor random copolymers exhibited promising and versatile applications on optoelectronic devices.
3. New Two-Dimensional Thiophene-Acceptor Conjugated Copolymers for Field Effect Transistor and Photovoltaic Cell Applications (Chapter 4): we report the synthesis, properties and optoelectronic device applications of two-dimensional (2D) like conjugated copolymers, P4TBT, P4TDTBT, P4TDTQ, and P4TDPP, consisted of 2’,5’’-bis(trimethystannyl)-5,5’’’-di-(2-ethylhexyl) -[2,3’;5’,2’’;4’’,2’’’]quarterthiophene (4T) with the following four acceptors of BT, 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT), DTQ, and DPP. The 2D-like conjugated copolymers exhibited high hole mobilities in the range of 10-1 ~ 10-4 cm2 V–1s–1. Moreover, the FET electron mobilities were observed for P4TDTBT and P4TDPP, due to their relatively low-lying LUMO level suitable for electron injection. In particular, P4TDPP showed the ambipolar characteristics with the hole and electron mobilities of 0.115 cm2V–1s–1 (on/ff ratio : 2.49×104) and 3.08×10-3 cm2V-1s-1 (on/off ratio : 7.34×102), respectively, which was strongly related to its order intermolecular chain packing based on the DSC and XRD studies. The PCE could be reached to 2.43 % of P4TDPP/PC71BM (1:2) based device, due to the balanced hole/electron mobility. The above results indicate that these two-dimensional 4T-acceptor conjugated copolymers could enhance the charge-transport characteristics and are promising materials for organic optoelectronic devices.
4. Enhancement of P3HT/PCBM Photovoltaic Efficiency Using the Surfactant of Triblock Copolymer Containing Poly(3-hexylthiophene) and Poly(4-vinylphenylamine) Segments (Chapter 5): the well-defined coil-rod-coil triblock copolymer, PTPA-P3HT-PTPA, has been served as a surfactant for P3HT/PCBM (1:1) based solar cells. The device performance is enhanced in the presence of the 1.5% PTPA-P3HT-PTPA with optimized devices showing a power conversion efficiency of 4.4%. With the surfactant ratios in the range of 0% to 1% and 2% to 5%, the fiber-like structure could be observed. As the critical ratio of 1.5% is added, the sphere-like nanostructure were obtained resulting to smaller domain size and increasing the interfacial area for charge separation as compared to fibrous structure. On the other hand, the increasing hole mobility with the addition of surfactant maybe due to the donor characteristic of PTPA-P3HT-PTPA, leading to the balanced hole and electron mobility. Hence, the significant enhanced PCE of the 1.5% PTPA-P3HT-PTPA blended system as compared to the pristine P3HT/PCBM system (3.9%) could be attributed to the sphere-like structure formed and much more balanced mobility (μe/μh ~1.7). Additionally, the introduction of PTPA-P3HT-PTPA as a surfactant not only extents the life-time of solar cells but also reduces the PCBM aggregation upon annealing, resulting in better thermal stability. The surfactant effect also could be confirmed by DSC measurement revealing that the selective miscibility of coil segment with PCBM. These results indicate the superior compatibilizing effect for the triblock copolymer in solar cell application.
Subjects
conjugated polymer
donor-acceptor
organic solar cell
field effect transistor
SDGs
Type
thesis
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