Direct Heteroarylation Guidelines for Well-Defined Thiophene-Based Conjugated Molecules.

Direct (hetero)arylation polymerization (DHAP) shows great promise for simple, low cost, and benign preparation of conjugated polymers. However, coupling selectivity has always posed a problem. Herein, direct (hetero)arylation was studied on small molecule models to develop suitable conditions for C-C couplings between 2-methylthiophene acting as an electron-donating moiety and 2-thiophenecarbonitrile acting as an electron-withdrawing moiety, when one of the partners is brominated. We observed that the best conditions are obtained when the electron-withdrawing moiety is halogenated.

Biosourced Vanillin-Based Building Blocks for Organic Electronic Materials.

Forest biomass is viewed as a significant source of organic carbon and thus the ideal replacement of petroleum products. From the resources derived from biomass, lignocellulose is the most abundant biobased material on earth. One of the aromatic added value compounds obtained from the depolymerization of lignin is vanillin. Here, we report the preparation of new compounds having benzothiophene, indole, isatin, benzofuroxan, benzofurazan, benzothiadiazole, and phthalimide heteroaromatic ring structures. More precisely, our results show that vanillin can be used as a biosourced starting material for the preparation of a variety of aromatic dibrominated monomers. X-ray crystallography on single crystals was also performed to obtain meaningful information on their solid-state ordering. This work opens the way to new, sustainable, biosourced aromatic materials (small molecules or polymers) for organic electronics.

Structural and Photophysical Templating of Conjugated Polyelectrolytes with Single-Stranded DNA.

A promising approach to influence and control the photophysical properties of conjugated polymers is directing their molecular conformation by templating. We explore here the templating effect of single-stranded DNA oligomers (ssDNAs) on cationic polythiophenes with the goal to uncover the intermolecular interactions that direct the polymer backbone conformation. We have comprehensively characterized the optical behavior and structure of the polythiophenes in conformationally distinct complexes depending on the sequence of nucleic bases and addressed the effect on the ultrafast excited-state relaxation. This, in combination with molecular dynamics simulations, allowed us a detailed atomistic-level understanding of the structure-property correlations. We find that electrostatic and other noncovalent interactions direct the assembly with the polymer, and we identify that optimal templating is achieved with (ideally 10-20) consecutive cytosine bases through numerous π-stacking interactions with the thiophene rings and side groups of the polymer, leading to a rigid assembly with ssDNA, with highly ordered chains and unique optical signatures. Our insights are an important step forward in an effective approach to structural templating and optoelectronic control of conjugated polymers and organic materials in general.

Slot-Die-Coated Ternary Organic Photovoltaics for Indoor Light Recycling.

Efficient organic photovoltaics (OPVs) based on slot-die-coated (SD) ternary blends were developed for low-intensity indoor light harvesting. For active layers processed in air and from eco-friendly solvents, our device performances (under 1 sun and low light intensity) are the highest reported values for fluoro-dithiophenyl-benzothiadiazole donor polymer-based OPVs. The N-annulated perylene diimide dimer acceptor was incorporated into a blend of donor polymer (FBT) and fullerene acceptor (PC61BM) to give ternary bulk heterojunction blends. SD ternary-based devices under 1 sun illumination showed enhanced power conversion efficiency (PCE) from 6.8 to 7.7%. We observed enhancement in the short-circuit current density and open-circuit voltage of the devices. Under low light intensity light-emitting device illumination (ca. 2000 lux), the ternary-based devices achieved a PCE of 14.0% and a maximum power density of 79 µW/cm2 compared to a PCE of 12.0% and a maximum power density of 68 µW/cm2 for binary-based devices. Under the same illumination conditions, the spin-coated (SC) devices showed a PCE of 15.5% and a maximum power density of 88 µW/cm2. Collectively, these results demonstrate the exceptional promise of a SD ternary blend system for indoor light harvesting and the need to optimize active layers based on industry-relevant coating approaches toward mini modules.

Fused Benzothiadiazole: A Building Block for n-Type Organic Acceptor to Achieve High-Performance Organic Solar Cells.

Narrow bandgap n-type organic semiconductors (n-OS) have attracted great attention in recent years as acceptors in organic solar cells (OSCs), due to their easily tuned absorption and electronic energy levels in comparison with fullerene acceptors. Herein, a new n-OS acceptor, Y5, with an electron-deficient-core-based fused structure is designed and synthesized, which exhibits a strong absorption in the 600-900 nm region with an extinction coefficient of 1.24 × 105 cm-1 , and an electron mobility of 2.11 × 10-4 cm2 V-1 s-1 . By blending Y5 with three types of common medium-bandgap polymers (J61, PBDB-T, and TTFQx-T1) as donors, all devices exhibit high short-circuit current densities over 20 mA cm-2 . As a result, the power conversion efficiency of the Y5-based OSCs with J61, TTFQx-T1, and PBDB-T reaches 11.0%, 13.1%, and 14.1%, respectively. This indicates that Y5 is a universal and highly efficient n-OS acceptor for applications in organic solar cells.

Air-Processed, Stable Organic Solar Cells with High Power Conversion Efficiency of 7.41.

High efficiency, excellent stability, and air processability are all important factors to consider in endeavoring to push forward the real-world application of organic solar cells. Herein, an air-processed inverted photovoltaic device built upon a low-bandgap, air-stable, phenanthridinone-based ter-polymer (C150 H218 N6 O6 S4 )n (PDPPPTD) and [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM) without involving any additive engineering processes yields a high efficiency of 6.34%. The PDPPPTD/PC61 BM devices also exhibit superior thermal stability and photo-stability as well as long-term stability in ambient atmosphere without any device encapsulation, which show less performance decay as compared to most of the reported organic solar cells. In view of their great potential, solvent additive engineering via adding p-anisaldehyde (AA) is attempted, leading to a further improved efficiency of 7.41%, one of the highest efficiencies for all air-processed and stable organic photovoltaic devices. Moreover, the device stability under different ambient conditions is also further improved with the AA additive engineering. Various characterizations are conducted to probe the structural, morphology, and chemical information in order to correlate the structure with photovoltaic performance. This work paves a way for developing a new generation of air-processable organic solar cells for possible commercial application.

CH Activation as a Shortcut to Conjugated Polymer Synthesis.

Direct (hetero)arylation polymerization exploits the palladium-catalyzed activation of aromatic CH bonds for the atom-economical synthesis of conjugated polymers for a wide range of applications. This account outlines how direct arylation methodologies overcome many of the limitations of contemporary polymerization techniques at both the research and production scale, and explains how monomer design and reaction conditions must be tailored to ensure high polymer molecular weight, yield, and structural integrity. Current research aims to improve further this reaction's profile as a sustainable methodology while at the same time making it competitive with the Migita-Stille and Miyaura-Suzuki polymerizations both in scope of accessible structures and synthetic efficiency. This feature article charts the recent developments and future directions of CH activation research as it moves toward becoming at once an industrially feasible, environmentally friendly, and synthetically powerful polymerization technique.

Theoretical Calculations for Highly Selective Direct Heteroarylation Polymerization: New Nitrile-Substituted Dithienyl-Diketopyrrolopyrrole-Based Polymers.

Direct Heteroarylation Polymerization (DHAP) is becoming a valuable alternative to classical polymerization methods being used to synthesize π-conjugated polymers for organic electronics applications. In previous work, we showed that theoretical calculations on activation energy (Ea) of the C⁻H bonds were helpful to rationalize and predict the selectivity of the DHAP. For readers' convenience, we have gathered in this work all our previous theoretical calculations on Ea and performed new ones. Those theoretical calculations cover now most of the widely utilized electron-rich and electron-poor moieties studied in organic electronics like dithienyl-diketopyrrolopyrrole (DT-DPP) derivatives. Theoretical calculations reported herein show strong modulation of the Ea of C⁻H bond on DT-DPP when a bromine atom or strong electron withdrawing groups (such as fluorine or nitrile) are added to the thienyl moiety. Based on those theoretical calculations, new cyanated dithienyl-diketopyrrolopyrrole (CNDT-DPP) monomers and copolymers were prepared by DHAP and their electro-optical properties were compared with their non-fluorinated and fluorinated analogues.

Salt-induced thermochromism of a conjugated polyelectrolyte.

We report here the photophysical properties of a water-soluble conjugated polythiophene with cationic side-chains. When dissolved in aqueous buffer solution (PBS, phosphate buffered saline), there is ordering of the polymer chains due to the presence of the salts, in contrast to pure water, where a random-coil conformation is adopted at room temperature. The ordering leads to a pronounced colour change of the solution (the absorption maximum shifts from 400 nm to 525 nm). Combining resonance Raman spectroscopy with density functional theory computations, we show a significant backbone planarization in the ordered phase. Moreover, the ratio of ordered phase to random-coil phase in PBS solution, as well as the extent of intermolecular interactions in the ordered phase, can be tuned by varying the temperature. Femtosecond transient absorption spectroscopy reveals that the excited-state behaviour of the polyelectrolyte is strongly affected by the degree of ordering. While triplet state formation is favoured in the random-coil chains, the ordered chains show a weak yield of polarons, related to interchain interactions. The investigated polyelectrolyte has been previously used as a biological DNA sensor, based on optical transduction when the conformation of the polyelectrolyte changes during assembly with the biomolecule. Therefore, our results, by correlating the photophysical properties of the polyelectrolyte to backbone and intermolecular conformation in a biologically relevant buffer, provide a significant step forward in understanding the mechanism of the biological sensing.

Direct heteroarylation polymerization: guidelines for defect-free conjugated polymers.

Direct (hetero)arylation polymerization (DHAP) has emerged as a valuable and atom-economical alternative to traditional cross-coupling methods for the synthesis of low-cost and efficient conjugated polymers for organic electronics. However, when applied to the synthesis of certain (hetero)arene-based materials, a lack of C-H bond selectivity has been observed. To prevent such undesirable side-reactions, we report the design and synthesis of new, bulky, phosphine-based ligands that significantly enhance selectivity of the DHAP process for both halogenated and non-halogenated electron-rich and electron-deficient thiophene-based comonomers. To better understand the selectivity issues, density functional theory (DFT) calculations have been performed on various halogenated and non-halogenated electron-rich and electron-deficient thiophene-based comonomers. Calculations showed that the presence of bromine atoms decreases the energy of activation (Ea) of the adjacent C-H bonds, allowing undesirable ß-defects for some brominated aromatic units. Both calculations and the new ligands should lead to the rational design of monomers and methods for the preparation of defect-free conjugated polymers from DHAP.

Robust Direct (Hetero)arylation Polymerization in Biphasic Conditions.

The synthesis of conjugated polymers from direct (hetero)arylation polymerization (DHAP) has been achieved for the first time using biphasic water/toluene conditions. This protocol is robust enough to form polymers even when air is introduced in the system. General reactivity is demonstrated for a single set of polymerization conditions with thienyl- or phenyl-based substrates, whether they are electron-rich or electron-poor. Complete characterization from differential scanning calorimetry and 1H NMR and UV-vis-NIR spectroscopies is presented, demonstrating this DHAP protocol offers comparable or better properties than the very best values published thus far. High molecular weights are obtained, showcasing the perfect equilibrium of reactivity and selectivity attained with this method. Moreover, this efficient and versatile methodology, which also uses low-cost, "wet" reagents, is scalable and done at ambient pressure.

Direct (Hetero)arylation Polymerization: Simplicity for Conjugated Polymer Synthesis.

Direct (hetero)arylation polymerization (DHAP) has recently been established as an environmentally benign method for the preparation of conjugated polymers. This synthetic tool features the formation of C-C bonds between halogenated (hetero)arenes and simple (hetero)arenes with active C-H bonds, thereby circumventing the preparation of organometallic derivatives and decreasing the overall production cost of conjugated polymers. Since its inception, selectivity and reactivity of DHAP procedures have been improved tremendously through the careful scrutinity of polymerization outcomes and the fine-tuning of reaction conditions. A broad range of monomers, from simple arenes to complex functionalized heteroarenes, can now be readily polymerized. The successful application of DHAP now leads to nearly defect-free conjugated polymers possessing comparable, if not slightly better, characteristics than their counterparts prepared using classical cross-coupling methods. This comprehensive review describes the mechanisms involved in this process from experimental and theoretical standpoints, presents an up-to-date compendium of materials obtained by this means, and exposes its current limitations and challenges.

Direct (Hetero)arylation Polymerization: Trends and Perspectives.

Conjugated polymers have attracted much attention in recent years, as they can combine the best features of metals or inorganic semiconducting materials (excellent electrical and optical properties) with those of synthetic polymers (mechanical flexibility, simple processing, and low-cost production), thereby creating altogether new scientific synergies and technological opportunities. In the search for more efficient synthetic methods for the preparation of conjugated polymers, this Perspective reports advances in the field of direct (hetero)arylation polymerization. This recently developed polymerization method encompasses the formation of carbon-carbon bonds between simple (hetero)arenes and (hetero)aryl halides, reducing both the number of synthetic steps and the production of organometallic byproducts. Along these lines, we describe the most general and adaptable reaction conditions for the preparation of high-molecular-weight, defect-free conjugated polymers. We also discuss the bottleneck presented by the utilization of certain brominated thiophene units and propose some potential solutions. It is, however, firmly believed that this polymerization method will become a versatile tool in the field of conjugated polymers by providing a desirable atom-economical alternative to standard cross-coupling polymerization reactions.

Increasing Polymer Solar Cell Fill Factor by Trap-Filling with F4-TCNQ at Parts Per Thousand Concentration.

Intrinsic traps in organic semiconductors can be eliminated by trap-filling with F4-TCNQ. Photovoltaic tests show that devices with F4-TCNQ at parts per thousand concentration outperform control devices due to an improved fill factor. Further studies confirm the trap-filling pathway and demonstrate the general nature of this finding.

Small-bandgap polymer solar cells with unprecedented short-circuit current density and high fill factor.

Small-bandgap polymer solar cells (PSCs) with a thick bulk heterojunction film of 340 nm exhibit high power conversion efficiencies of 9.40% resulting from high short-circuit current density (JSC ) of 20.07 mA cm(-2) and fill factor of 0.70. This remarkable efficiency is attributed to maximized light absorption by the thick active layer and minimized recombination by the optimized lateral and vertical morphology through the processing additive.

Elucidating the impact of molecular packing and device architecture on the performance of nanostructured perylene diimide solar cells.

The performance of organic photovoltaic devices (OPV) with nanostructured polymer:perylene diimide (PDI) photoactive layers approaches the levels of the corresponding polymer:fullerene systems. Nevertheless, a coherent understanding of the difficulty for PDI-based OPV devices to deliver high power conversion efficiencies remains elusive. Here we perform a comparative study of a set of four different polymer:PDI OPV model systems. The different device performances observed are attributed to differences in the nanostructural motif of these composites, as determined by wide-angle X-ray scattering (WAXS) measurements. Long-range structural order in the PDI domain dictates (i) the stabilization energy and (ii) the concentration of the PDI excimers in the composites. The quenching of the PDI excimer photoluminescence (PL) is found to be insensitive to the former, but it depends on the latter. High PL quenching occurs for the low concentration of PDI excimers that are formed in PDI columns with a length comparable to the PDI excimer diffusion length. The stabilization of the PDI excimer state increases as the long-range order in the PDI domains improves. The structural order of the PDI domains primarily affects charge transport. Electron mobility reduces as the size of the PDI domain increases, suggesting that well-ordered PDI domains suffer from poor electronic connectivity. WAXS further reveals the presence of additional intermolecular PDI interactions, other than the direct face-to-face intermolecular coupling, that introduce a substantial energetic disorder in the polymer:PDI composites. Conventional device architectures with hole-collecting ITO/PEDOT:PSS bottom electrodes are compared with inverted device architectures bearing bottom electron-collecting electrodes of ITO/ZnO. In all cases the ZnO-functionalized devices surpass the performance of the conventional device analogues. X-ray photoelectron spectroscopy explains that in PEDOT: PSS-functionalized devices, the PDI component preferentially segregates closer to the hydrophilic PEDOT: PSS electrode, thus impeding the efficient charge extraction and limiting device photocurrent.

Photoinduced Dynamics of Charge Separation: From Photosynthesis to Polymer-Fullerene Bulk Heterojunctions.

Understanding charge separation and charge transport is crucial for improving the efficiency of organic solar cells. Their active media are based on organic molecules and polymers, serving as both light-absorbing and transport layers. The charge-transfer (CT) states play an important role, being intermediate for free carrier generation and charge recombination. Here, we use light-induced electron paramagnetic resonance spectroscopy to study the CT dynamics in blends of the polymers P3HT, PCDTBT, and PTB7 with the fullerene derivative C60-PCBM. Time-resolved EPR measurements show strong spin-polarization patterns for all polymer-fullerene blends, confirming predominant generation of singlet CT states and partial orientation ordering near the donor-acceptor interface. These observations allow a comparison with charge separation processes in molecular donor-acceptor systems and in natural and artificial photosynthetic assemblies, and thus the elucidation of the initial steps of sequential CT in organic photovoltaic materials.

Conjugated Polymers à la Carte from Time-Controlled Direct (Hetero)Arylation Polymerization.

Direct (hetero)arylation polymerization (DHAP) shows great promise for simple, cheap, and environmentally benign preparation of conjugated polymers, but seems to involve a lack of selectivity when different aromatic C-H bonds are present. We report that some time-controlled DHAP reactions can yield well-defined and processable semiconducting polymers. Following these procedures, various aromatic compounds have been efficiently polymerized, including 2,7-dibromofluorene, 2,7-dibromocarbazole, 1,4-dibromobenzene, bithiophene, dithienyl-benzothiadiazole, and diketopyrrolopyrrole derivatives. All resulting polymers have shown comparable, if not slightly better, properties than their Stille- and Suzuki-synthesized analogs. These findings (re)open the door for the low-cost production of many conjugated polymers for plastic electronics.