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The structural rigidity of fused units in the polymer backbone, in addition to the resulting stabilizing effect of the quinoidal structure, and tunable electronic properties have played a key role in promoting highly-ordered π-stacking moieties, exhibiting promising charge carrier mobilities. The electron-deficient thiazole moiety shows high planarity and effective π–π stacking, which leads to the reduction in the energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO), and ideally enhances the electron charge mobility. Four heterocycle-based monomers BTzS, BTzSe, DTS, and DTG based on fused bithiazole and dithiophene units incorporated with sulfur, selenium, silicon, and germanium as the bridging atoms were synthesized and characterized. The monomers were copolymerized with the electron-rich alkylated thienylenevinylene (TV) unit to afford copolymers P1–P4. The thermal, optical, and electrochemical properties and crystallinity of the copolymers were thoroughly investigated. Extensive OFET device optimization using different solvents and annealing temperatures resulted in the best charge mobility of 0.09 cm2 V−1 s−1 for the electron-deficient bithiazole BTzS copolymer P1 and 0.36 cm2 V−1 s−1 for the DTS copolymer P3.

We report the synthesis and characterization of four novel small bandgap copolymers incorporating the electron-deficient thieno[2,3-d:5,4-d′]bisthiazole and selenopheno[2,3-d:5,4-d′]bisthiazole building blocks with a series of electron-deficient diketopyrrolopyrole units. The four resultant copolymers were synthesized via palladium Stille cross-coupling reaction, and their optical, thermal stability, electrochemical, and field-effect charge transport properties were investigated. All copolymers showed low optical bandgaps (1.53–1.56 eV); in addition, X-ray diffraction on solution-cast films revealed that the selenium-containing copolymers exhibit higher crystallinity compared to their thiophene counterparts.

New low band gap copolymers P1–P4, based on thienylenevinylene (TV) and pyrrolo[3,2-d:4,5-d′]bisthiazole (PBTz) units composed of different alkyl side chains, such as 2-octyldodecyl (OD), n-hexadecyl (HD), 2-ethylhexyl (EH), and 9-heptadecyl (HD) groups, respectively, have been synthesized and characterized. Electrochemical and optical studies of the copolymers indicated low energy band gaps in the range of 1.40–1.47 eV. Moreover, theoretical calculation with density functional theory (DFT) and time-dependent DFT calculations demonstrated that the energy band gaps, HOMO energy levels and maximum absorption values in the copolymers were in good agreement with the experimental results. The decomposition temperature of all copolymers was measured to be above 340 °C by thermogravimetric analysis (TGA), which indicates high thermal stability. Thermally annealed OTFT devices based on P1–P4 thin films demonstrated a range of hole mobilities; thus, the P2 based OTFT device exhibited the highest hole mobility of 0.062 cm2 V−1 s−1.

Novel low band gap polymers based on pyrrolo[3,2-d:4,5-d′]bisthiazole (PBTz) and thienylenevinylene (TV) For Organic Electronic Applications Dhananjaya Patra?,1 Hassan S. Bazzi,1 Lei Fang¬2 and Mohammed Al-Hashimi1 1Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar 2Department of Chemistry and Department of Materials Science and Engineering, Texas A&M University, College Station, TX. Corresponding author: dhananjaya.patra@qatar.tamu.edu π-Conjugated organic polymers have been intensively studied over the past two decades due to their potential applications in areas such as organic light-emitting diodes (OLEDs),1 organic thin-film transistors (OTFTs), organic photovoltaics (OPV), and organic laser devices. In addition, they possess excellent properties that enable them to be solution processed offering a path-way for fabrication on large-area printable and cost-effective flexible electronic devices. Several donor-acceptor (D-A) conjugated polymers and small molecules are reported recently with photovoltaic performance over 12%.2-4 Recently significant development has been made in the design and synthesis of high performance polymers with mobilities now easily surpassing μ > 1 cm2 V-1 s-1.5 Among the various conjugated electron-donating building blocks, thienylenevinylene (TV) unit has attracted much interest owing to its coplanarity and extended π-conjugation which are induced by the presence of a vinylene spacer between the two thiophene units. As a result this enhanced interchain interaction the polymers exhibit a higher hole mobility and reduced energy band gaps.4 Recently, the incorporation of vinylene unit into several electron-rich (i.e., donor) and electron deficient (i.e., acceptor) units has been widely studied, especially, targeting at the device application for n-channel and ambipolar OTFTs with promising device performance. Among these units are diketopyrrolopyrrole (DPP), benzoselenadiazole (BSe), phthalimide (PhI), naphthalenediimide (NDI), benzothiadiazole (BT), dithienothiophene (DTT), thienopyrroledione (TPD), and isoindigo (iI) as depicted in Figure 1. For instance, Yoo et al. reported the synthesis of various polymers based on DPP and (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene units by altering the number of thiophene in the repeat unit to induce strong π–π stacking and favorable molecular conformation. By replacing the thiophene unit with the selenophene vinylene selenophene Kang et al. reported an enhanced mobility. Several DPP and TV based polymers reported by Oh and Kim et al. via various side-chain engineering on the DDP units, obtained remarkable mobilities from 8.74 to 17.8 cm2 V − 1 s − 1.5 Reichmanis and co-workers also reported the synthesis of a series of BT oligothiophene and oligo-TV donor-acceptor (D − A) copolymers exhibiting mobilities of up to 0.75 cm2 V − 1 s − 1. Kim and co-workers reported the synthesis and characterization of highly soluble poly(thienylenevinylene) exhibiting carrier mobility of 1 cm2 V − 1 s − 1.6 Al-Hashimi, Heeney and co-workers systematically investigated the role of modifying the heteroatom, by synthesizing a series of vinylene copolymers containing 3-dodecylthiophene, selenophene and tellurophene.7,8 Figure 1. Structures of thiophene (Th), thiazole (Tz) diketopyrrolopyrrole (DPP), benzoselenadiazole (BSe), phthalimide (PhI), naphthalenediimide (NDI), benzothiadiazole (BT), dithienothiophene (DTT), thienopyrroledione (TPD), and isoindigo (iI). Another particularly promising class of building block for the development of high performing polymeric semiconductors for OTFTs is the electron-deficient pyrrolo[3,2-d:4,5-d′]bisthiazole (PBTz)-based heterocycle. In comparison to dithienothiophene (Figure 1), PBTz-unit is a weak acceptor having an electronegative nitrogen atom, which lowers the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies. Therefore, it is expected that the large polarity of the thiazole ring can enhance the intermolecular interaction. In addition, the bridging nitrogen (N) of PBTz offers the opportunity to include an additional solubilizing sidechain, which is not present in the analogous DTT. This potentially offer improved solubility and processability of the polymers. Nonetheless, to the best of our knowledge, there is no single report on the synthesis, polymer characterization, and charge carrier transport properties employing the fused PBTz containing TV units. In this work, we report the synthesis and characterization of a series of four PBTz-TV-based copolymers P1-P4 (Scheme 2) with various alkyl side chains for OTFT applications. Particularly, we have selected a dodecyl alkyl side chain on the thienylenevinylene unit, three branched alkyl side chains such as 2-octyldodecyl (OD), 9-heptadecyl (HD) and 2-ethylhexyl (EH), and a long straight n-hexadecyl (HD) chain on the nitrogen of the pyrrolobisthiazole unit, thus, for improving polymer solubility and effectively to promote π − π interchain interactions. The electrochemical redox properties and related electronic structures (HOMO/LUMO energy levels) were systematically investigated by cyclic voltammetry (CV). In addition, the microstructure and morphology of the polymer thin films were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). Finally, the PBTZ-copolymers show band gaps in the range of 1.40 − 1.46 eV and mobilities in the range of 0.002-0.062 cm2 V − 1 s − 1 in bottom-gate/top-contact OTFTs.9 References. 1. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature, 1990, 347, 539. 2. Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Nat. Mater.,2010, 9, 1015.3. Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. J. Am. Chem. Soc., 2017, 139, 7148. 4. Li, N.; Baran, D.; Forberich, K.; Machui, F.; Ameri, T.; Turbiez, M.; Carrasco-O., M.; Drees, M.; Facchetti, A.; Krebse, F.C.; Brabec, C. J. Energy Environ. Sci., 2013, 6, 3407. 5. Kim, J.; Lim, B.; Baeg, K. J.; Noh, Y. Y.; Khim, D.; Jeong, H. G.; Yun, J. M.; Kim, D. Y. Chem. Mater.,2011, 23, 4663. 6. Fu, B. Y.; Baltazar, J.; Hu, Z. K.; Chien, A. T.; Kumar, S.; Henderson, C. L.; Collard, D. M.; Reichmanis, E. Chem. Mater.,2012, 24, 4123 7.Al-Hashimi, M.; Han, Y.; Smith, J.; Bazzi, H. S.; Alqaradawi, S. Y. A.; Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Chem. Sci.,2016, 7, 1093. 8. Al-Hashimi, M.; Baklar, M. A.; Colleaux, F.; Watkins, S. E.; Anthopoulos, T. D.; Stingelin, N.; Heeney, M. Macromolecules, 2011, 44, 5194. 9. Patra, D.; Lee, J.; Lee, J.; Sredojevic, D. N.; White, A.J.P.; Bazzi, H. S.; Brothers, E. N.; Heeney, M.; Fang, L.; Yoon. M.-H.; Al-Hashimi, M. J. Mater. Chem. C., 2017, 5, 2247.

Global environmental and resource concerns dictate that future energy supply and security will become increasingly dependent upon the development of accessible, sustainable and scalable energy technologies. State-of-art polymer solar cells (PSCs) has been considered as one of the renewable important technologies which can harvest solar energy from sunlight to generate electricity. Intensive research efforts from both academia and industry have been dedicated into solution-processed organic solar cells due to development of the next-generation solar cells technology 1,2. Owing to the readily available carbon feedstock as well as the numerous and flexible synthetic pathways, polymer solar cells (PSCs) gained tremendous attentions over silicon solar cell in the past decay due to low- cost and quick energy pay-back, solution-processable, lightweight, and flexible/stretchable, large area photovoltaic panels. 1,2 So as to achieve the high performance solar cells it is very important to develop novel kinds of active materials, which have to cover entire solar spectrum i.e. from ultraviolet to infrared (IR) regions, suitable molecular energy levels morphologies and high mobilities. Several donor-acceptor (D-A) conjugated polymers are reported recently with photovoltaic performance over 10%. 3 However, in D-A PSC materials have high intrinsic torsional defects, which impacts the negative impact on performance of the OPV devices. The torsional defects partially break the conjugation pathways of the polymers, leading to shortened coherent lengths along the polymer chain and decreased carrier mobilities. Meanwhile, the torsional defects perturb the intermolecular packing of the polymer materials so that the electronic coupling between the polymer chains are interrupted, adding an energy barrier for the charge carriers and excitons to transport within the active layer. 4 Moreover, the torsional defects increase the band gap of conjugated polymers, hence to prevent their photo-absorption in longer wavelength region. Overall the torsional defects often lead to larger π–π stacking distances in the polymer thin film, making the thin film more susceptible to the permeation of oxygen and water, hence decreasing the stability of the overall OPV devices. Our approach looks into ways to overcome the drawbacks raised by torsional defects on a fundamental level. By definition, Ladder polymers consist of cyclic subunits, connected to each other by two links that are attached to different sites of the respective subunits, comparable to a graphene nanoribbon. Consequently, ladder polymers have two or more independent strands of bonds which are tied together regularly without merging to a single or double bond or crossing each other. 4 As a result, ladder polymers have large planar core structures with no torsional defects. Such defect-free feature grants them with rigid and hence highly conjugated core structures. On one hand, the highly conjugated cores not only afford low band gaps that allow strong optical absorption at long wavelength in terms of energy absorption, but also lead to low beta value for coherent tunneling and low activation energy for electron hopping, in terms of charge transport alongside the polymer chains. There were many carbazole-containing organic D-A polymer materials has been demonstrated for high performance solar cell applications and no such types of ladder polymers reported by utilizing carbazole core. 5 Herein, we report the synthesis of fully conjugated carbazole-based ladder polymer with low level of unreacted defects, by utilizing the controlled ring-closing olefin metathesis (RCM) reaction. The designed ladder polymer is well soluble in common organic solvents for solution processability. We also discussed the photo-physical, electrochemical and optoelectronic properties of torsional defect-free ladder polymers.References[1] L. Lu, T. Zheng, Q. Wu, A. M. Schneider, D. Zhao, L. Yu, Chem. Rev. 2015, DOI: 10.1021/acs.chemrev.5b00098.[2] L. Dou, Y. Liu, Ziruo Hong, Gang Li, Y. Yang, Chem. Rev. 2015, DOI: 10.1021/acs.chemrev.5b00165.[3] J-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li, J.-X. Tang, Adv. Mater. 2015, 27, 1035.[4] A.-D. Schluter Adv. Mater. 1991, 3, 283.[5] J. Lee, B. B. Rajeeva, T. Yuan, Z. Guo, Y. Lin, M. Al-Hashimi, Y. Zheng and L. Fang, Chem. Sci., 2015, DOI: 10.1039/C5SC02385H.

Two random (Zn(II)-based P1-P2) and two alternating (Ru(II)-based P3-P4) metallo-copolymers containing bis-terpyridyl ligands with various central donor (i.e., fluorene or carbazole) and acceptor (i.e., benzothiadiazole) moieties were synthesized. The effects of electron donor-acceptor interactions with metal (Zn(II) and Ru(II)) ions on their thermal, optical, and electrochemical properties were investigated. Because of the strong ICT transitions between donor and acceptor ligands in both Zn(II)- and Ru(II)-based metallo-coplymers and MLCT transitions in Ru(II)-based metallo-coplymers, the absorption spectra covered a broad range of 260-750 nm with the band gaps of 1.57-1.77 eV. In addition, the introduction of Ru(II)-based metallo-coplymer P4 mixed with PC(60)BM as an active layer of the BHJ solar cell device exhibited the highest PCE value up to 0.90%.

Five conjugated LBG polymers P1–P5 consisting of a DTP unit as an electron donor and various bithiazole units as electron acceptors are designed and synthesized for photovoltaic applications. The effects of the electron-deficient bithiazole derivatives on the thermal, optical, electrochemical, and photovoltaic properties are investigated. The molecular structures are confirmed and characterized. The resulting polymers have high thermal stabilities and broad absorption bands with low optical bandgaps. The hole and electron mobilities are calculated. PSC devices are fabricated utilizing the polymers as electron donors and PCBM as an electron acceptor. The PSC device with an active layer of P4:PCBM (1:1 by weight) exhibits the best power-conversion efficiency.

Two b-cyano-thiophenevinylene-based polymers containing cyclopentadithiophene (CPDT-CN) and dithienosilole (DTS-CN) units were synthesized via Stille coupling reaction with Pd(PPh3)4 as a catalyst. The effects of the bridged atoms (C and Si) and cyano-vinylene groups on their thermal, optical, electrochemical, charge transporting, and photovoltaic proper- ties were investigated. Both polymers possessed the highest occupied molecular orbital (HOMO) levels of about � 5.30 eV and the lowest unoccupied molecular orbital (LUMO) levels of about � 3.60 eV, and covered broad absorption ranges with narrow optical band gaps (ca. 1.6 eV). The bulk heterojunction polymer solar cell (PSC) devices containing an active layer of

A series of π-conjugated bis-terpyridyl ligands (M1–M3) bearing various benzodiazole cores and their corresponding main-chain Ru(II) metallo-polymers were designed and synthesized. The formation of metallo-polymers were confirmed by NMR, relative viscosity, and UV-visible titration measurements. The effects of electron donor and acceptor interactions on their thermal, optical, electrochemical, and photovoltaic properties were investigated. Due to the strong intramolecular charge transfer (ICT) interaction and metal to ligand charge transfer (MLCT) in Ru(II)-containing polymers, the absorption spectra covered a broad range of 260–750 nm with the optical band gaps of 1.77–1.63 eV. In addition, due to the broad sensitization areas of the metallo-polymers, their bulk heterojunction (BHJ) solar cell devices containing [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor exhibited a high short-circuit current (Jsc). An optimum PVC device based on the blended polymer P1:PCBM = 1:1 (w/w) achieved the maximum power conversion efficiency (PCE) value up to 0.45%, with Voc = 0.61 V, Jsc = 2.18 mA cm−2, and FF = 34.1% (under AM 1.5 G 100 mW cm−2), which demonstrated a novel family of conjugated polyelectrolytes with the highest PCE value comparable with BHJ solar cells fabricated from ionic polythiophene and C60.

A series of novel low-bandgap triphenylamine- based conjugated polymers (PCAZCN, PPTZCN, and PDTPCN) consisting of different electron-rich donor main chains (N-alkyl- 2,7-carbazole, phenothiazine, and cyclopentadithinopyrol, respectively) as well as cyano- and dicyano-vinyl electron- acceptor pendants were synthesized and developed for poly- mer solar cell applications. The polymers covered broad absorption spectra of 400-800 nm with narrow optical band- gaps ranging 1.66-1.72 eV. The highest occupied molecular or- bital and lowest unoccupied molecular orbital levels of the polymers measured by cyclic voltammetry were found in the range of � 5.12 to � 5.32 V and � 3.45 to � 3.55 eV, respectively. Under 100 mW/cm 2 of AM 1.5 white-light illumination, bulk heterojunction photovoltaic devices composing of an active

A series of low-band-gap (LBG) donor-accepor con- jugated main-chain copolymers (P1-P4) containing planar 2,7- carbazole as electron donors and bithiazole units (4,4 0 -dihexyl- 2,2 0 -bithiazole and 4,4 0 -dihexyl-5,5 0 -di(thiophen-2-yl)-2,2 0 -bithia- zole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1-P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with opti- cal band gaps in the range of 1.93-1.99 eV, which overlapped with the major region of the solar emission spectrum. More- over, carbazole-based polymers P1-P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which provided good air stability and high open circuit voltages (Voc) in the PV applications. The BHJ PV devices were fabricated using polymers P1-P4 as electron donors and (6,6)-phenyl-C61- butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC71BM (1:1.5 w/w) showed the best power conver- sion efficiency value of 1.01% with a short circuit current den- sity (Jsc) of 4.83 mA/cm 2 , a fill factor (FF) of 35%, and Voc ¼ 0.60 V under 100 mW/cm 2 of AM 1.5 white-light illumination.