Ordered assembly of fluorinated graphene with nano fuels for coupling reaction and application in microenergetic devices

Electrosynthesis and characterization of an electrochromic material from poly(1,4-bis(2-thienyl)-benzene) and its application in electrochromic devices

A series of fused thiophenes composed of fused alpha-oligothiophene units as building blocks, end-capped with either styrene or 1-pentyl-4-vinylbenzene groups, has been synthesized through Stille coupling reactions. The compounds have been fully characterized by means of (1)H NMR spectrometry, high-resolution mass spectrometry, and elemental analysis. The molecules present a trans-trans configuration between their double bonds, which has been verified and confirmed by Fourier-transform infrared spectroscopy and single-crystal X-ray diffraction analysis. The X-ray crystal structures showed pi-pi overlap and sulfur-sulfur interactions between the adjacent molecules. The decomposition temperatures were all found to be above 300 degrees C, indicating that compounds of this series possess excellent thermal stability. The fact that no phase transition occurs at low temperature indicates that they should be well-suited for application in devices. Moreover, they possess low HOMO energy levels, based on cyclic voltammetry measurements, and suitable energy gaps, as determined from their thin-film UV/Vis spectra. Thin-film X-ray diffraction analysis and atomic force microscopy revealed high crystallinity on supporting substrates. In addition, as the substrate temperature has a significant influence on the morphology and the degree of crystallinity, the device performance could be optimized by varying the substrate temperature. These materials were found to exhibit optimal field-effect performance, with a mobility of 0.17 cm(2) V(-1) s(-1) and an on/off ratio of 10(5), at a substrate temperature of 70 degrees C.

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Semiconducting polymers have been paid great attention because their intensive application of light-emitting diodes, thin film transistors, and photovoltaic devices. The conjugated polymer is a class of semiconducting polymers under intensive research, because it owns tunable optoelectronic properties and good processibility, leading to great potential of device application. The main objective of this thesis is to synthesize a series of conjugated polymers composed of different electron donors and acceptor (mainly quinoxaline) for the fabrication and measurement of field effect transistors and photovoltaic cells. In the first part of this thesis (Chapter 2), five side-chain modified quinoxaline-containing conjugated copolymers with donor-acceptor or donor-acceptor -donor structures were synthesized and characterized, including poly[2,5-didecyloxy- 1,4-phenylene-alt-2,3–bis(4-(2-ethylhexyloxy)phenyl)-5,8–dithien-2–yl-quinoxaline] (POC10DTQ(EHP)),poly[2,5-didecyloxy-1,4-phenylene-alt-2,3-bis(4-(2-ethylhexyl oxy)phenyl)-5,8–dithien-2–yl-quinoxaline] (PFODTQ(EHP)), poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-quinoxaline] (PThQx(EHP)), poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethyl hexyloxy)phenyl)-5,8-dithien-2-yl-quinoxaline] (PThDTQ(EHP)), and poly[2,3bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl- quinoxaline] (PDTQ(EHP)). The hybridization of the high-lying HOMO energy level of the donor and low-lying LUMO energy level of the acceptor could produce a relatively small band gap polymer. The synthesized polymers (except PThDTQ(EHP)) are soluble in common organic solvents. Their band gaps are in the range of 1.57 ~ 1.92 eV and 1.93 ~ 2.14 eV measured by optical spectra and cyclic voltammetry, respectively. PThQx(EHP) has the smallest band gap because it is composed of the donor (thiophene) with higher electron-donating ability with the balanced percentage of the acceptor among the analogous polymers. These copolymers were fabricated into thin film transistors by their chlorobenzene solutions and measured their hole mobility in the range of 4.34×10-3 ~ 4.70×10-5 cm2/(Vs) with on/off ratios in the order of 103 ~104. The smooth and homogeneous surface of PDTQ(EHP) film may be a main factor for the highest charge carrier mobility. Furthermore, the solar cell power conversion efficiencies based on polymers PFODTQ(EHP), POC10DTQ(EHP) and PThQx(EHP) achieved 1.76%, 0.64% and 0.92%, respectively. In the second part of this thesis (Chapter 3), five different carbazole-based copolymers were synthesized by palladium(0)-catalyzed Suzuki coupling reaction, including poly[3,6-(2-ethylhexyl)carbazole-alt-2,1,3-benzothiadiazole] (PCzBTD), poly [3,6-(2-ethylhexyl)carbazole-alt-2,3-dimethyl -5,8-quinoxaline] (PCzDQ), poly[3,6-(2-ethylhexyl)carbazole -alt-2,3-dimethyl-5,8–dithien-2–yl-quinoxaline] (PCzDTQ), poly[3,6-(2-ethylhexyl)carbazole-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-quinoxaline] (PCzQx(EHP)) and poly[3,6-(2-ethylhexyl)carbazole-alt-2,3-bis (4-(2-ethylhexyloxy) phenyl)-5,8-dithien-2-yl-quinoxaline] (PCzDTQ(EHP)). The polymers containing quinoxaline with long alkoxy-phenylene side chains have better solubility, resulting larger molecular weights and meanwhile soluble in common organic solvents. Their optical and electrochemical band gaps were measured in the range of 1.91 ~ 2.37 eV and 1.63 ~ 2.34 eV, and the effective intramolecular charge transfer should account for the smallest band gap of PCzDTQ(EHP). Furthermore, by the red shifts of UV-visible spectra, we found that the alkoxy-phenylene side groups of quinoxaline both improve the solubility and enhance the effective conjugation length. In sum, the present study suggests the importance of donor/acceptor strength and backbone structure on the electronic and optoelectronic properties of conjugated polymers, and also demonstrated the potential on optoelectronic device application of these polymers.

We report the synthesis of various thiophene/phenylene co‐oligomers with a total number of thiophene and benzene (phenylene) rings of 5 and 6 with various terminal groups. Those terminal groups have been chosen from among alkyl groups, methoxy groups, trifluoromethyl groups, and cyano groups. The molecular backbone of these compounds comprises phenyl‐ or biphenylyl‐capped thiophene (or oligothiophene) or an alternating co‐oligomer. The synthesis is based on either the Suzuki coupling reaction or the Negishi coupling reaction. These reaction schemes enabled us to obtain the target compounds in high quality. In particular, the latter coupling method turned out to produce the compounds at a high yield. The terminal groups are expected to produce various functionalities based upon their electron donating character (alkyl groups and methoxy groups) or electron withdrawing character (trifluoromethyl groups and cyano groups). Additionally some of these groups bring about enhanced solubility. This will lead to the production of a diversity of modified compounds of thiophene/phenylene co‐oligomers. To give an example that demonstrates usefulness of the target compounds, we present optoelectronic data that are associated with their device applications.

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.

Transformation of Cu(OH)2 to mesostructural copper-silicate in alkaline silicate solution

Graphene-based materials have recently attracted much attention due to their extraordinary physical and chemical properties, which make them attractive candidates for many technological applications in sensing, optoelectronics, catalysis, and energy storage. Their chemical functionalization is key to tuning their properties. Herein, a novel two-step synthetic approach, which enables a high degree of covalent functionalization of graphene oxide (GO) is devised, thereby making the facile attachment of various robust functional molecules possible. Such a process relies initially on the grafting of an ethylenediamine linker followed by a second step consisting of the condensation reaction between aldehyde and amine groups to form imine bonds. As test beds, two kinds of graphene-based functional systems, namely, porphyrin-modified GO and ferrocene-modified GO, are prepared. Such hybrid systems are characterized by various spectroscopic and microscopic techniques. The degree of functionalization is quantified as the attachment of one porphyrin or ferrocene unit to every 34 or 77 carbon atoms of the GO scaffold, respectively, which is much higher than that of values obtained upon using various established chemical approaches to functionalize GO, such as condensation, cycloaddition, or coupling reactions. For the first time, the reduced form of ferrocene-modified GO was employed as an electrode material in supercapacitors, showing a specific capacitance of 127 F g-1 at a current density of 1 A g-1 , with capacitance retention of about 93 % after 5000 cycles at the same current density; this demonstrates great potential for application in high-performance energy-storage devices.

Novel and well‐defined pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymers were successfully achieved by combination of esterification, atom transfer radical polymerization (ATRP), divergent reaction, ring‐opening polymerization (ROP), and coupling reaction on the basis of pentaerythritol. The reaction of pentaerythritol with 2‐bromopropionyl bromide permitted ATRP of styrene (St) to form four‐arm star‐shaped polymer (PSt‐Br)4. The molecular weights of these polymers could be adjusted by the variation of monomer conversion. Eight‐hydroxyl star‐shaped polymer (PSt‐(OH)2)4 was produced by the divergent reaction of (PSt‐Br)4 with diethanolamine. (PSt‐(OH)2)4 was used as the initiator for ROP of ε‐caprolactone (CL) to produce eight‐arm star‐shaped dendrimer‐like copolymer (PSt‐b‐(PCL)2)4. The molecular weights of (PSt‐b‐(PCL)2)4 increased linearly with the increase of monomer. After the coupling reaction of hydroxyl‐terminated (PSt‐b‐(PCL)2)4 with 1‐pyrenebutyric acid, pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymer (PSt‐b‐(PCL‐pyrene)2)4 was obtained. The eight‐arm star‐shaped dendrimer‐like copolymers presented unique thermal properties and crystalline morphologies, which were different from those of linear poly(ε‐caprolactone) (PCL). Fluorescence analysis indicated that (PSt‐b‐(PCL‐pyrene)2)4 presented slightly stronger fluorescence intensity than 1‐pyrenebutyric acid when the pyrene concentration of them was the same. The obtained pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymer has potential applications in biological fluorescent probe, photodynamic therapy, and optoelectronic devices. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2788–2798, 2008

For the past decade, two-dimensional graphene-based nanomaterials are at risk of global interest in scientists and incentives to detect various potential application in energy storage, sensor, electronic device, and catalysis due to their outstanding chemical and physical feature, which has quickly improved. Recently, researchers very much interested have also targeted on carbon-based materials such as graphene oxide or graphene-based nanocomposite, which lead to the progress of several applications in catalysis, e.g., coupling reaction, oxidation, reduction, and organic catalytic reaction. There is a significant interrelation between structural shape with the catalytic performance of the nanocomposite materials, which is a central focus to develop a high-performance catalyst in the research area, which is discussed. The challenges of the future developments of such associated materials are proposed and discussed. In this study, the effective approaches of the synthesized methods and their recent development of various catalytic applications of graphene or graphene oxide nanocomposites have been discussed.

To meet the requirement in the application of medical devices, composites, biomaterials, corrosion resistance, and selective adsorptions, rubber surface modification is usually indispensable. Grafting treatment is one of most significate treatment methods. In this paper, we focus on rubber surface grafting modification, including grafting techniques and applications. Different grafting methods—including monomer grafting polymerization and coupling reaction—are covered and compared briefly. The related applications of surface grafting modification techniques, such as improving compatibility of waste rubber as fillers, hydrophobicity and lipophilicity of sponge rubber for oil–water separation, biocompatibility of rubber in the medical field, and forming surface patterns, are demonstrated in detail. The new research directions of surface grafting techniques as well as main challenges in application are also discussed.

Carbon-based nanostructures have attracted tremendous interest because of their versatile and tunable properties, which depend on the bonding type of the constituting carbon atoms. Graphene, as the most prominent representative of the π-conjugated carbon-based materials, consists entirely of sp(2)-hybridized carbon atoms and exhibits a zero band gap. Recently, countless efforts were made to open and tune the band gap of graphene for its applications in semiconductor devices. One promising method is periodic perforation, resulting in a graphene nanomesh (GNM), which opens the band gap while maintaining the exceptional transport properties. However, the typically employed lithographic approach for graphene perforation is difficult to control at the atomic level. The complementary bottom-up method using surface-assisted carbon-carbon (C-C) covalent coupling between organic molecules has opened up new possibilities for atomically precise fabrication of conjugated nanostructures like GNM and graphene nanoribbons (GNR), although with limited maturity. A general drawback of the bottom-up approach is that the desired structure usually does not represent the global thermodynamic minimum. It is therefore impossible to improve the long-range order by postannealing, because once the C-C bond formation becomes reversible, graphene as the thermodynamically most stable structure will be formed. This means that only carefully chosen precursors and reaction conditions can lead to the desired (non-graphene) material. One of the most popular and frequently used organic reactions for on-surface C-C coupling is the Ullmann reaction of aromatic halides. While experimentally simple to perform, the irreversibility of the C-C bond formation makes it a challenge to obtain long-range ordered nanostructures. With no postreaction structural improvement possible, the assembly process must be optimized to result in defect-free nanostructures during the initial reaction, requiring complete reaction of the precursors in the right positions. Incomplete connections typically result when mobile precursor monomers are blocked from reaching unsaturated reaction sites of the preformed nanostructures. For example, monomers may not be able to reach a randomly formed internal cavity of a two-dimensional (2D) nanostructure island due to steric hindrance in 2D confinement, leaving reaction sites in the internal cavity unsaturated. Wrong connections between precursor monomers, here defined as intermolecular C-C bonds forcing the monomer into a nonideal position within the structure, are usually irreversible and can induce further structural defects. The relative conformational flexibility of the monomer backbones permits connections between deformed monomers when they encounter strong steric hindrance. This, however, usually leads to heterogeneous structural motifs in the formed nanostructures. This Account reviews some of the latest developments regarding on-surface C-C coupling strategies toward the synthesis of carbon-based nanostructures by addressing the above-mentioned issues. The strategies include Ullmann coupling and other, "cleaner" alternative C-C coupling reactions like Glaser coupling, cyclo-dehydrogenation, and dehydrogenative coupling. The choice of substrate materials and precursor designs is crucial for optimizing substrate reactivity and precursor diffusion rates, and to reduce events of wrong linkage. Hierarchical polymerization is employed to steer the coupling route, which effectively improves the completeness of the reaction. Effects of byproducts on nanostructure formation is comprehended with both experimental and theoretical studies.

Although the synthesisof silacycles is of significant importance,it remains far less explored than that of N-, O-, and S-heterocycles.The past few years have seen growing attention toward the synthesisof these silacycles, owing to their applications in light-emittingmaterials, thin-film transistors, electroluminescent devices, solarcells, and notable photophysical and biological activities. In thisreview, we have discussed the recent developments in transition-metal-catalyzedsynthesis of π-conjugated silacycles by C–H activation,1,n-metal migration and cycloaddition reactions,chemo- and regio-divergent reactions, coupling reactions, and β-hydrideeliminations. The discussion of the key components of each strategy,with mechanistic study and the role of phosphine ligands are outlinedwith key achievements, limitations, and major applications.

Polymer solar cells (PSCs) have attracted much attention because of their potential application in flexible, light-weight, and low-cost large-area devices through roll-to-roll printing. The bulk heterojunction PSCs showed advanced features in realizing high efficiencies and solution-processible devices. The active layer in this kind of device consists of an interpenetrating network formed by an electron-donor material blended with an electron-acceptor material. 3] Typically, conjugated polymers are used as electron donors and fullerene derivatives are used as the electron acceptors in the PSCs. Recently, power conversion efficiencies (PCEs) of 6–7% have been realized by using new conjugated polymer donors or new fullerene-derived acceptors. Short circuit current density (Jsc), open circuit voltage (Voc), and fill factors (FF) are key parameters for a PSC device, because the PCE of the device is proportional to the values of the three parameters. To broaden the response wavelength range of a PSC device by using conjugated side chains or narrowband-gap conjugated polymers is an effective way to realize high Jsc values. Conjugated polymers with lower HOMO levels are helpful in realizing high Voc and PCE values, as the Voc value of PSCs is directly proportional to the offset between the HOMO level of electron donor and the LUMO level of electron acceptor. PSiFDTBT, PFDTBT, and PCDTBT are three excellent examples for this concept. Consequently, by using conjugated polymers with lower HOMO levels and also narrow band gaps, high PCEs were realized in different families of conjugated polymers. Conjugated polymers based on benzo[1,2-b :4,5-b’]dithiophene (BDT) units have attracted interest as electron donors in the PSC field in recent years, since the report of Hou and Yang et al. on the synthesis and photovoltaic properties of a series of copolymers based on BDT. Many copolymers of BDT with different conjugated units, such as thieno[3,4b]thiophene (TT), 4,7-dithiophene-2-yl-2,1,3-benzothiadiazole (DTBT), N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD), and bithiazole, etc. were synthesized, and the copolymers showed promising photovoltaic properties. In these BDT-based polymers, the alternative copolymers of BDT and TT, namely PBDTTTs, are an important family of photovoltaic materials. For additional improvements in the photovoltaic performance of the PBDTTTs, structural modifications brought about by using different substituents on BDT, or the copolymerized moieties is of great importance. For example, Liang et al. introduced a fluorine atom into the TT unit of the PBDTTTs, and the HOMO level of the resulting polymer was successfully lowered by approximately 0.12 eV, and thus a higher Voc value was achieved, resulting in a great improvement of PCE. Hou et al. optimized PBDTTTs further by replacing the alkoxycarbonyl group on the TT unit with the alkylcarbonyl groups. The structural modification can also be carried out on the BDT units. In this work, we designed an 5-alkylthiophene-2-yl-substituted BDT monomer and synthesized two new PBDTTT-based polymers having either the thienylsubstituted BDT with alkoxycarbonyl-substituted thieno[3,4b]thiophene (TT-E) or the alkylcarbonyl-substituted thieno[3,4-b]thiophene (TT-C); that is PBDTTT-E-Tand PBDTTTC-T, respectively (Scheme 1). To fully investigate the effect of the thienyl-substituted BDTon the photovoltaic properties of the polymers, two corresponding PBDTTT polymers based on the alkoxy-substituted BDT (BDT-O), PBDTTT-E and PBDTTT-C (Scheme 1), were also prepared. The synthetic route of the thienyl-substituted BDT monomer (BDT-T) is shown in Scheme 1. The branched alkyl group 2-ethylhexyl was employed as the side chain on the thiophene to guarantee high solubility of the target polymers. The TT-E and TT-C monomers are commercially available. The polymers were prepared through a Stille coupling reaction between the bis(trimethyltin) BDT monomers (BDT-T and BDT-O) and the bromides (TT-E and TTC) as shown in Scheme 1. All the polymers are soluble in chloroform (CHCl3), chlorobenzene, and dichlorobenzene. Thermogravimetric analysis (TGA) measurements were employed to evaluate the thermal stability of the polymers. We found that the two-dimentional (2D) conjugated polymers based on alkylthienyl-substituted BDTs are much more stable than their analogues, the alkoxy-substituted BDTs. The TGA plots of these four polymers are shown in Figure 1. It can be seen that the decomposition temperatures [*] Dr. L. Huo, S. Zhang, F. Xu, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences Beijing 100190 (China) E-mail: hjhzlz@iccas.ac.cn

Control of polymer-packing orientation in thin films through chemical structure of D-A type polymers and its application in efficient photovoltaic devices