Bithiophene Units Research Articles

Triarylamine/Bithiophene Copolymer with Enhanced Quinoidal Character as Hole‐Transporting Material for Perovskite Solar Cells

AbstractPolytriarylamine is a popular hole‐transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron‐catalyzed C−H/C−H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high‐performance in methylammonium lead iodide perovskite (MAPbI3) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole‐carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI3. We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open‐circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine (PTAA) and Spiro‐OMeTAD.

Reassessing the Significance of Reduced Aggregation and Crystallinity of Naphthalene Diimide-Based Copolymer Acceptors in All-Polymer Solar Cells

Synthesizing copolymer acceptors based on a mix of three co-monomers is a facile and effective strategy to control the aggregation and crystallinity of semiconducting polymers which has been exploited to improve the photovoltaic performance of all-polymer solar cells (all-PSCs). Applying this strategy to the well-studied electron-transporting polymer acceptor PNDI2OD-T2, different amounts of 3-octylthiophene (OT) are used to partially replace the bithiophene (T2) unit, resulting in three copolymer acceptors PNDI-OTx where x = 5, 10, or 15%. Another polymer, namely PNDI2OD-C8T2, consisting of naphthalene diimide (NDI) polymerized with 3-octyl-2,2′-bithiophene (C8T2) is also synthesized for comparison. It is found that the solution aggregation and thin-film crystallinity of PNDI-OTx are systematically tuned by varying x, evidenced by temperature-dependent UV–vis and grazing incidence wide-angle X-ray scattering measurements. PNDI2OD-C8T2 is also found to have reduced solution aggregation and thin-film crystallinity relative to PNDI2OD-T2. However, the photovoltaic performance of all-PSCs based on J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends are much lower than that of the reference J71:PNDI2OD-T2 system. Extensive morphological studies indicate that reduced aggregation and crystallinity do not guarantee a more favorable blend morphology, with coarser phase separation found in J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends compared to J71:PNDI2OD-T2 blends. Furthermore, the OT-modified copolymers with reduced crystallinity are found to have reduced electron mobilities. The results here suggest that reduced aggregation and less crystallinity of random copolymer acceptors do not always produce favorable morphology in polymer/polymer blends and do not guarantee for improvement in the photovoltaic performance.

Substitution effects in distyryl BODIPYs for near infrared organic photovoltaics

In this research, four new conjugated Bodipy (boron dipyrromethene) structures absorbing in the near IR region have been synthesized and their photophysical, electrochemical, morphological properties and their potential for organic photovoltaics (OPV) have been investigated. All the compounds were substituted with bithiophene units in their 3- and 5- positions. Besides, the meso (8-) and 2-, 6- positions were substituted with different groups, and it allowed analysis of substitution effects in these positions on the performance of organic photovoltaics. Among the synthesized compounds, BTT has better photovoltaic performance than others with 1.3% power conversion efficiency (PCE) value when an organic solar cell with PC71BM acceptor compound was formed under the optimized conditions. Experimental and theoretical studies demonstrated that photovoltaic performance is largely affected by the units on the meso position under the compounds studied and the electronic distributions of the frontier molecular orbitals which are presumably dictated by the orientation of the side groups around the Bodipy core. We believe, with the light of the studies performed novel substitution transformations would enhance implementation of these types of compounds in near infrared photovoltaics.

Diphenylamine Substituted High-performance Fully Nonfused Ring Electron Acceptors: The Effect of Isomerism

We have designed and synthesized two fully nonfused ring electron acceptors LW-in-2F and LW-out-2F with two diphenylamine side chains at different positions of the bithiophene core. The two diphenylamine side chains of LW-in-2F and LW-out-2F are at the outside and inside positions of the central bithiophene unit, respectively. The performance of LW-in-2F and LW-out-2F-based devices is very different, the photoelectric conversion efficiency (PCE) of LW-out-2F-based device is as high as 12.83%, while the efficiency of LW-in-2F-based device is only 0.43%. By GIWAXS test, we found that the change of diphenylamine position affects the way of molecular arrangement. When the diphenylamine side chain is located on the outer side of the central core, the molecular arrangement shows face-on orientation, which is beneficial for perpendicular charge transport, resulting in high and balanced charge mobilities and high PCE.

Highly stable bulk heterojunction organic solar cells based on asymmetric benzoselenadiazole‐oriented organic chromophores

A new asymmetrically designed D-A-D organic chromophore 4-(7-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]selenadiazol-4-yl)-N,N-diphenylaniline (RSeT) with benzoselenadiazole acceptor unit has been synthesized using two different electron donor materials of triphenylamine (TPA) and n-hexyl bithiophene unit. Attachment of nonplanar electron-donating TPA unit subsequently heightened the optoelectronic properties and altered the solubility in organic solvents. RSeT of the optical bandgap of ~1.95 eV with the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of −5.31 and −3.36 eV was applied as an electron donor material for stable bulk heterojunction organic solar cells. In this work, two different buffer layers of poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate acid (PEDOT:PSS) and compact titanium oxide (c-TiO2) were used and their influences on the device performance were investigated. Buffer layer of c-TiO2 with RSeT:PC61BM (1:3, w/w) active layer achieved a high open-circuit voltage (Voc) of ~0.985 V, short-circuit current density (Jsc) of ~11.26 mA/cm2 and improved fill factor of ~0.62 which resulted in a high power conversion efficiency (PCE) of ~6.88%. After 30 days, interestingly, RSeT:PC61BM (1:3, w/w) devices with a c-TiO2 buffer layer demonstrated good stability with the PCE value remaining at 85% of its initial value.

Guanine-Based G-Quadruplexes Templated by Various Cations toward Potential Use as Single-Ion Conductors.

Inspired by the atomic-sized, shape-regulated features of G-quadruplexes comprising guanine motifs with a monovalent metal cation, the G-quadruplex-forming ability, and properties of a guanine-based π-conjugated Y2 molecule containing bithiophene and peripheral dodecyl chain units in the presence of various cation salts (Li+ , Na+ , K+ , and Mg2+ ) were exploited. A series of structural characterization revealed that Y2 yielded desirable G-quadruplexes with all the tested cations as a consequence of the combination of a hydrogen-bonded cyclic G-quartet, π-stacking, and cation-dipole interactions. The radius and nature of the coordinating cations crucially affected the structural characteristics of G-quadruplexes, leading to variations in the ion migration ability inside the cavity of the G-quadruplex (Li+ >Na+ >K+ >Mg2+ ), as characterized through theoretical and experimental investigations. These results not only improve the understanding of G-quadruplex self-assemblies based on guanine but also provide an impetus for their diverse potential applications, especially in the field of Li batteries.

Fluorination effects on bithiophene unit in benzodithiophene-4,8-dione based D-A type alternating copolymers for highly efficient polymer solar cells.

In this study, two D-A polymers consisting of benzodithiophene-4,8-dione and tetrathiophene with or without fluorination were synthesized to reveal the photovoltaic properties of fluorination effect on the polymer backbone. Polymer PDFTB with two fluorine atoms substituted on the backbone exhibits an enhanced π-π stacking effect, deeper HOMO energy level and better backbone planarity than PDTB without fluorine atom substitution. Devices based on PDFTB:ITIC show a power conversion efficiency of 4.39%, which is 15% higher than that of PDTB-based devices due to the higher hole mobility, optimized surface morphology and homogeneous phase separation of the active layer. These results suggest that the fluorination strategy is a facile way to design polymeric donors for solvent-processed polymer solar cells.

End-capped modification of dithienosilole based small donor molecules for high performance organic solar cells using DFT approach

Considering the worth of molecular designing, A-D-A type four small donor (SM1-SM4) photoactive materials comprised of DTS core, substituted with push-pull acceptors at terminal sites linked through bithiophene units were investigated. In present project, detailed theoretical consideration under DFT and TD-DFT methods (CAM functional/basis set 6-31G) was given-to explore wavelength dependent (absorption maxima, first excitation energy, light harvesting efficiency), electronic (FMO, DOS, TDM), reactivity (IP, EA, MEP), and charge transfer parameters (Voc, FF) of selected molecules (SM1-SM4) in systematic way. All novel molecules revealed broadening of absorption maxima and lessening of band gaps comparative to absorption coefficient and energy difference between HOMO/LUMO of RM due to electron pulling nature of acceptor groups. Greater dipole moment of all designed chromophores in excited state suggests their good solubility in solvent medium (chloroform) as compared to gaseous medium. Furthermore, charge transfer dynamics in terms of internal hole and electron reorganization energies give lower values which indicate higher transfer abilities for SM2 and SM1 respectively. Excellent output results for modelled molecules interpreted through open circuit voltage (with PC61BM) and fill factor, predict rise in PCE value than already reported molecule. Out of class, SM4 molecule emerged as an ideal photovoltaic material on account of better optical properties, easy electronic transitions, enhanced chemical reactivity, fast charge conductivity, and high estimated PCE. Eventually, these encouraging results favors the structural modeling of small molecules achieved through effective combination of end-capped moieties. We look forward that this entire work may provide strong theoretical guidelines, aids in designing and amplifying the optoelectronic features of proposed frameworks on vast scale than reference.

Unveiling Mechanism of Organic Photogenerator for Hydroxyl Radicals Generation by Molecular Modulation.

Photodynamic therapy (PDT) with organic photosensitizers generally goes through the oxygen-dependent process, generating singlet oxygen and/or superoxide anion. However, the generation of reactive oxygen species is often suppressed as a result of hypoxia, one of the common features in tumors, therefore limiting the effectiveness of the tumor treatments. Consequently, it is urgent and significant to develop an oxygen-independent hydroxyl radical photogenerator and unveil the mechanism. In this work, a hydroxyl radical (·OH) photogenerator originating from the electron transfer process is engineered. Detailed mechanism studies reveal that the optimized photosensitizer, WS2D, which contains a bithiophene unit, could both promote charge carrier generation and accelerate reaction efficiency, resulting in the efficient production of ·OH. In addition, WS2D nanoparticles are constructed to improve the polydispersity and stability in aqueous solution, which exhibit excellent biocompatibility and mitochondrial targeting. Bearing the above advantages, WS2D is employed in phototheranostics, which could release ·OH effectively and damage mitochondria precisely, achieving high PDT efficiency in vitro and in vivo. Overall, this work successfully provides valuable insights into the structural design of a hydroxyl radicals (·OH) photogenerator with great practical perspectives.

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

Solution Processable Pentafluorophenyl End‐Capped Dithienothiophene Organic Semiconductors for Hole‐Transporting Organic Field Effect Transistors

AbstractTwo solution‐processable organic semiconductors, DFPT‐DTTR (1) and DFPbT‐DTTR (2), composed of pentafluorophenyl (FP) end‐capped 3,5‐dialkyl dithienothiophene (DTTR) core with thiophene (T) or bithiophene (bT) as π‐bridged spacers are developed and investigated for their optical, electrochemical, microstructural, and electrical properties. With more conjugated bithiophene units, compound 2 exhibits a red‐shifted UV–vis absorption band and upshifted HOMO/downshifted LUMO energy levels. According to the density functional theory, compound 2 features a more twisted molecular structure due to the intrinsic non‐coplanar blocks in the π‐backbones. Compound 1‐based organic field effect transistors exhibit efficient hole transport with mobility up to 0.48 cm2 V−1 s−1. This is one of the high mobility organic semiconductors exhibiting p‐channel characteristics based on solution‐processable small molecular FP end‐capped fused/oligothiophenes. With large and interconnected crystalline morphologies, decreased π–π stacking distance, and less steric hindrance, compound 1 exhibits two orders of magnitude higher mobility than the more distorted 2, which exhibits lower hole mobility of 1.82 × 10−3 cm2 V−1 s−1.

Regiochemistry-Driven Organic Electrochemical Transistor Performance Enhancement in Ethylene Glycol-Functionalized Polythiophenes.

Novel p-type semiconducting polymers that can facilitate ion penetration, and operate in accumulation mode are much desired in bioelectronics. Glycol side chains have proven to be an efficient method to increase bulk electrochemical doping and optimize aqueous swelling. One early polymer which exemplifies these design approaches was p(g2T-TT), employing a bithiophene-co-thienothiophene backbone with glycol side chains in the 3,3' positions of the bithiophene repeat unit. In this paper, the analogous regioisomeric polymer, namely pgBTTT, was synthesized by relocating the glycol side chains position on the bithiophene unit of p(g2T-TT) from the 3,3' to the 4,4' positions and compared with the original p(g2T-TT). By changing the regio-positioning of the side chains, the planarizing effects of the S-O interactions were redistributed along the backbone, and the influence on the polymer's microstructure organization was investigated using grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The newly designed pgBTTT exhibited lower backbone disorder, closer π-stacking, and higher scattering intensity in both the in-plane and out-of-plane GIWAXS measurements. The effect of the improved planarity of pgBTTT manifested as higher hole mobility (μ) of 3.44 ± 0.13 cm2 V-1 s-1. Scanning tunneling microscopy (STM) was in agreement with the GIWAXS measurements and demonstrated, for the first time, that glycol side chains can also facilitate intermolecular interdigitation analogous to that of pBTTT. Electrochemical quartz crystal microbalance with dissipation of energy (eQCM-D) measurements revealed that pgBTTT maintains a more rigid structure than p(g2T-TT) during doping, minimizing molecular packing disruption and maintaining higher hole mobility in operation mode.

Modulation of energy levels and vertical charge transport in polythiophene through copolymerization of non-fluorinated and fluorinated units for organic indoor photovoltaics

Organic indoor photovoltaics (OIPVs) for the development of a wireless power supplier that allows the portable operation of Internet-of-things and low-energy consumption devices have received tremendous interest. Particularly, polythiophene represented by poly(3-hexylthiophene) has been considered as a promising photoactive material for OIPVs owing to their desirable optoelectrical properties and power conversion efficiencies (PCEs) that exceed Si-based PVs under low-intensity illumination. However, the polythiophene-based OIPVs suffer from an inadequate charge transporting ability in the out-of-plane direction and a low open-circuit voltage (VOC), which currently hinder the further improvement of OIPVs. Herein, we designed and synthesized a new polythiophene derivative by combining fluorination and random copolymerization strategies. The optimized polymer obtained by tuning the ratio of fluorinated and non-fluorinated bi-thiophene units showed an increased population of face-on oriented crystallites, a denser packing, and a deeper highest occupied molecule orbital energy level compared with its homopolymer analogue. The optimized polymer was also revealed to provide improved vertical charge transport than homopolymer analogue. As a result, when fabricated using the phenyl-C71-butyric acid methyl ester as an electron-acceptor, the OIPVs with the optimized polymer showed high PCEs up to 13.4% with VOC of 0.68 V under 1000 lux white light-emitting diode illumination, which were improved values compared with the efficiencies observed in the devices with homopolymer (PCE = 5.6% and VOC = 0.57 V).

13C NMR investigations and molecular order of nematogens with biphenyl and bithiophene at terminus

ABSTRACT Four rod-like mesogens in which biphenyl and bithiophene units are located at one terminus are synthesised and the occurrence of enantiotropic nematic phase is established by hot-stage optical polarising microscopy and differential scanning calorimetry. For biphenyl based mesogens (BP-1 and BP-2), the mesophase is observed irrespective of number of phenyl rings in the amine moiety. For bithiophene (BT-1), on the other hand, the mesophase is only noted when the amine component has two rings. The mesophase range for BP-1 is found to be 163.7◦C and for BT-1 and BP-2, the values are 84.7◦C and 11.1◦C, respectively. BP-1 and BT-1 are subjected to 13C NMR investigation in nematic phase to determine the local order and accordingly, oriented 1D 13C and 2D SAMPI-4 NMR experiments are accomplished. The remarkable feature of the SAMPI-4 spectrum of BP-1 measured at 145◦C in nematic phase is observation of the 13C-1H dipolar coupling value of 16.67 kHz for terminal carbon (C18). For BT-1, however, the dipolar coupling value for C18 is found to be 8.60 kHz in nematic phase (138◦C). Such large values of 13C-1H dipolar couplings for the terminal ring carbons of the nematogens offer orientation of the rings of the core unit directly.

Random copolymerization of polythiophene for simultaneous enhancement of in‐plane and out‐of‐plane charge transport for organic transistors and perovskite solar cells

High-performance conjugated polymers for electronic applications can be developed by modulating an appropriate chemical structure that optimizes their crystal characteristics and charge-transport behavior. Herein, we demonstrated the simultaneous enhancement of the in-plane and out-of-plane charge transport of polythiophenes by random polymerization. We synthesized a polythiophene polymer by varying the ratio of two different dialkyl-substituted bi-thiophene and triethylene glycol-substituted mono-thiophene units; this polymer exhibited weakened orientation preferences of polymer crystallite films, a denser packing, and a more homogeneous surface morphology in comparison with its homopolymer analogue. Furthermore, this optimized random polymer afforded an enhanced in-plane mobility of 7.72 cm2 V−1 second−1, measured by field-effect transistor, and out-of-plane mobility of 8.86 × 10−4 cm2 V−1 second−1, measured by space-charge-limited-current device. These are respectively 2.4 times and 10 times higher than the mobilities of the homopolymer (field-effect mobility = 3.25 cm2 V−1 second−1 and space-charge-limited-current mobility = 8.73 × 10−5 cm2 V−1 second−1). The enhanced charge transport in out-of-plane direction was also confirmed by fabricating perovskite solar cells using optimized polythiophene as a hole-transporting material, which exhibited a higher efficiency of nearly 16.2% than the device with homopolymer analogue (12.0%).

Benzodithiophene-based small molecules with various termini as hole transporting materials in efficient planar perovskite solar cells

In this study we prepared four benzodithiophene (BDT)-based small organic molecules presenting bithiophene (TT), thiophene (FT), carbazole (CB), and triphenylamine (TPA) units, respectively, as termini, and used them as hole transporting materials for perovskite solar cells (PSCs). The high degrees of planarity of these BDT-based small molecules imparted them with high degrees of stacking and charge transport. These small molecules had suitable optical properties and energy level alignments for use in PSCs based on MAPbI 3 , with compact-TiO 2 as the electron transporting layer and a BDT-based material as the hole transporting layer, in a n–i–p structure. Among our tested BDT-based materials, the PSC incorporating BDT-TT had the best performance, with an average power conversion efficiency of 13.63%. The benzodithiophene (BDT) derivatives presenting bithiophene (TT), carbazole (CB), thiophene (FT), and triphenylamine (TPA) units based small molecules used them as hole transporting materials to fabricate planar perovskite solar cells. • We have prepared benzodithiophene (BDT) derivatives presenting bithiophene (TT), carbazole (CB), thiophene (FT), and triphenylamine (TPA) units and then used them as hole transporting materials to fabricate n–i–p PSCs. The power conversion efficiency of the device incorporating BDT-TT was the highest among our tested PSCs. • The film of the small molecule BDT-TT had the best morphology among our tested BDT-based HTMs, suggesting that TT units might be useful components when designing new hole transporting materials for high-performance PSCs. • Additives improved the performance of our PSCs by increasing the mobilities in the hole transporting materials, thereby achieving higher power conversion efficiencies.

Synthesis, Structure, and Optical Properties of Di-m-benzihexaphyrins (1.1.0.0.0.0) and Di-m-benziheptaphyrins (1.0.1.0.0.0.0): Blackening of m-Phenylene-Linked Dicarbaporphyrinoids by Simple π-Expansion

Acid-catalyzed condensation of a newly prepared di-m-benzipentapyrrane with appropriate mono- and diheterocyclic dialcohols selectively produced stable di-m-benzihexaphyrins and di-m-benziheptaphyrins with only two meso-carbon bridges. Single-crystal X-ray diffraction analyses reveal planar conformation with slight distortion of bridged phenylene rings. Despite the presence of m-phenylene units interrupting the global delocalization, the presence of bithiophene units in di-m-benziheptaphyrins 3a-b exhibits altered optical features covering the entire visible region (ca. 250-720 nm), exhibiting a black dye property as a "metal-free" porphyrinoid.

Synthesis and optical properties of compounds via platinum-catalyzed hydrosilylation of triethynyltriazine and silyl-substituted oligothiophenes

New types of starlike compounds and branched polymers with a triazine core and silylbithienyl or silylterthienyl arms were prepared by the use of platinum-catalyzed hydrosilylation. The UV–Vis absorption and fluorescence properties of these molecules have been investigated. In the fluorescence spectra of the silylbithienyl substituted starlike compound, their emission maxima derived from the bithiophene units were observed. In the branched polymer with silylbithienyl units, a peak derived from the interaction between the triazine and bithienyl units was observed at about 500 nm. No significant difference between the starlike compound and branched polymer with silylterthienyl units was observed in their UV–Vis absorption and fluorescence spectra. DFT calculations were performed to understand the photophysical properties of the starlike molecules with silylbithienyl and silylterthienyl arms.

Halogenation effect promoted low bandgap polymers based on asymmetric isoindigo unit with low energy loss

AbstractTo optimize the energy levels of the structural framework of isoindigo polymers, a series of asymmetric isoindigo based low bandgap polymers with chlorine, fluorine and thiazole substituents was constructed and their optical, electrochemical and photovoltaic properties were comparatively evaluated for the impact of different substitutions. In comparison with the polymer based on 2,2'‐bithiophene and isoindigo unit (PTi) with non‐substituted bithiophene as the donor moiety, the highest occupied molecular orbital energy level for the newly synthesized polymers is significantly decreased, and in turn an improvement of the open‐circuit voltage (VOC) is noted in the corresponding photovoltaic devices. More importantly, combined with a low bandgap of 1.32 eV, the energy losses (Eloss) could be reduced to 0.61 eV for polymer based on chlorinated 2,2'‐bithiophene and isoindigo unit (PCl). In addition, the halogen moieties are observed to be superior in device fabrication and give better values than the thiazole substituent. Both fluorinated and chlorinated polymer donors exhibited improved performance compared with the original polymer PTi. Consequently, this work not only presents the influence of different electron withdrawing substituents on the physicochemical and photovoltaic performance, but also backs the concept of how to reduce the energy loss via the heteroatom effect. © 2020 Society of Chemical Industry

Single-material organic solar cells with fully conjugated electron-donor alkoxy-substituted bithiophene units and electron-acceptor benzothiadiazole moieties alternating in the main chain

Main chain conjugated linear polymers, constituted by alternating electron-donor (D) and -acceptor (A) moieties, have been prepared with the aim of testing their performances as photoactive components in single material organic solar cells (SMOSCs).