This study reports on the thermoelectric properties of poly(3‐alkylchalcogenophene) thin films (500 nm) as a function of heteroatom (sulfur, selenium, tellurium), and how these properties change with ...dopant (ferric chloride) concentration. UV–vis–NIR spectroscopy shows that polaronic charge carriers are formed upon doping. Poly(3‐alkyltellurophene) (P3RTe) is most easily doped followed by poly(3‐alkylselenophene) (P3RSe) and poly(3‐alkylthiophene) (P3RT), where R = 3,7‐dimethyloctyl chain is the pendant alkyl group. Thermoelectric properties vary as functions of the heteroatom and doping level. At low dopant concentrations (≈1 × 10−3
m), P3RTe shows the highest power factor of 10 µW m−1 K−2, while, at higher dopant concentrations (≈5 × 10−3
m), P3RSe shows the highest power factor of 13 µW m−1 K−2. Most notably, it is found that the measured properties are consistent with Mott's polaron hopping model and not consistent with other transport models. Additionally, temperature‐dependent conductivity measurements show that for a given dopant concentration, the activation energies for electronic transport decrease as the heteroatom is changed from sulfur to selenium to tellurium. Overall, this work presents a systematic study of poly(chalcogenophenes) and indicates the potential of polymers beyond P3HT by tuning the heteroatom and doping level for optimized thermoelectric performance.
This work details the thermoelectric and charge transport properties of poly(3‐alkylchalcogenophene) thin‐films as a function of heteroatom (sulfur, selenium, tellurium), and how these properties change with dopant (ferric chloride) concentration. This systematic investigation correlates the identity of the heteroatom in polyheterocycles with the doping process, the resulting thermoelectric properties, and the charge transport mechanism.
The nanostructure morphology and electron donor performance of a poly(3-hexylselenophene)-block-poly(3-hexylthiophene) (P3HS-b-P3HT) copolymer was studied in a photovoltaic device with a 6,6-phenyl ...C61 butyric acid methyl ester (PCBM) acceptor. P3HS-b-P3HT forms fiberlike nanostructures spontaneously, which leads to an initial optimal device performance. Furthermore the nanostructure morphology is not greatly affected by annealing, which leads to a device stability that outperforms P3HT, P3HS, or a P3HS/P3HT mixture under identical conditions. External quantum efficiency, hole mobility, and current–voltage measurements show that the block copolymer also outperforms a ternary blend that consists of a physical mixture of P3HS, P3HT, and PCBM with the same overall composition. Overall, the observation of optimal device performance and morphology without annealing as well as enhanced thermal stability demonstrates the advantage of fully conjugated diblock copolymers in nanostructured devices.
Whereas organic–inorganic hybrid perovskite nanocrystals (PNCs) have remarkable potential in the development of optoelectronic materials, their relatively poor chemical and colloidal stability ...undermines their performance in optoelectronic devices. Herein, this issue is addressed by passivating PNCs with a class of chemically addressable ligands. The robust ligands effectively protect the PNC surfaces, enhance PNC solution processability, and can be chemically addressed by thermally induced crosslinking or radical‐induced polymerization. This thin polymer shield further enhances the photoluminescence quantum yields by removing surface trap states. Crosslinked methylammonium lead bromide (MAPbBr3) PNCs are applied as active materials to build light‐emitting diodes that have low turn‐on voltages and achieve a record luminance of over 7000 cd m−2, around threefold better than previous reported MA‐based PNC devices. These results indicate the great potential of this ligand passivation approach for long lifespan, highly efficient PNC light emitters.
A new approach to enhance perovskite nanocrystal (PNC) stability is developed through a class of intrinsically crosslinkable ligands. These ligands provide an opportunity for crosslinking between PNCs, which effectively improves the material stability and photoluminescent properties. The application of these crosslinked PNCs in light‐emitting diodes is successfully achieved, demonstrating the importance that ligand design has on PNC stability and device performance.
Sequence‐defined synthetic oligomers and polymers provide unprecedented opportunities for polymer chemists to finely control properties such as chain folding, self‐assembly, and optoelectronic ...performance of materials. However, absolute control over both chain‐length and monomer sequence has been a long‐standing “grand challenge” for decades. Herein, we report a novel strategy to synthesize monodisperse sequence‐defined conjugated oligomers in a homogeneous manner by temperature cycling, thereby achieving single‐monomer precision in conjugated polyheterocycles. A series of sequence‐defined oligomers with up to twelve repeating units, four different monomers, and various sequences were successfully synthesized. Monomer sequence was also proved to affect optical properties. We believe this strategy not only exhibits general applicability to the synthesis of group 16 conjugated oligomers and polymers, but also has far‐reaching potential for other polymer systems.
Synthesis of structurally well‐defined macromolecules has been one of the ultimate goals and challenges for polymer chemists for decades. This work reports the first homogeneous synthesis of monodisperse sequence‐defined conjugated oligomers. The synthesis is supported by catalyst transfer polymerization and a custom Python‐based automated program.
Whereas monodisperse polymers are ubiquitous in Nature, they remain elusive to synthetic chemists. Absolute control over polymer length and structure is essential to imparting chemical functionality, ...reproducible properties, and specific solid-state behavior. Precise polymer length has proven to be extremely difficult to control. The most successful examples are generally similar to solid-phase oligo nucleotide or peptide synthesis, wherein the polymer is built up one unit at a time with each sequential monomer addition requiring purification and deprotection (or other functional group activation) step. We have discovered a stepwise homogeneous catalyst-transfer polymerization to prepare monodisperse oligo(3-hexylthiophene) using temperature to limit additions to one unit per chain per cycle. This is the first reported example of a one-pot synthesis of monodisperse oligomers that requires no additional purification or intermediate steps. It is our hope that the strategy of temperature cycling to “freeze” intermediates will be generalizable to other living polymerization techniques, such as other catalyst-transfer polymerization systems, and those where a resting state involves an association between the catalyst and growing chain.
We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and ...kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.
The synthesis of π-conjugated polymers containing a degradable 1,2,4-oxadiazole linker by direct heteroaryl polymerization (DHAP) is reported. The 1,2,4-oxadiazole linker is stable to acidic and ...basic conditions, as well as thermally stable. The degradation of a small molecule model is achieved by reduction followed by acid hydrolysis. The polymerization is performed using phosphine-free and branching-suppressing DHAP conditions. DFT calculations are performed on the monomers to determine their affinity for β-branching defects. The synthesized polymers are photoluminescent and exhibit quantum yields of up to 0.40. Partial degradation of a representative polymer is observed using the same conditions for degradation as the small molecule model.
The synthesis of π-conjugated polymers containing a degradable 1,2,4-oxadiazole linker by direct heteroaryl polymerization (DHAP) is reported.
We present the reactivity and photochemistry of 2,5-diphenyltellurophene. A change in oxidation state from Te(ii) to Te(iv) occurs by oxidative addition of bromine, chlorine, and fluorine from ...appropriate halogen sources. Photoreductive halogen elimination is demonstrated using optical absorption spectroscopy and NMR spectroscopy. The photodebromination reaction occurs with 16.9% quantum yield, the highest value for any Te compound. Photoreductive elimination of chlorine and fluorine occurs with quantum yields of 1.6% and 2.3%, respectively, albeit with less efficient halogen trapping when an organic trap is used. Improved fluorine trapping was achieved using water, allowing for much cleaner photodefluorination. This is the first example of photodefluorination from a tellurium compound.
Heavy atom substitution in chalcogenophenes is a versatile strategy for tailoring and ultimately improving conjugated polymer properties. While thiophene monomers are commonly implemented in polymer ...designs, relatively little is known regarding the molecular properties of the heavier chalcogenophenes. Herein, we use density functional theory (DFT) calculations to examine how group 16 heteroatoms, including the radioactive polonium, affect polychalcogenophene properties including bond length, chain twisting, aromaticity, and optical properties. Heavier chalcogenophenes are more quinoidal in character and consequently have reduced band gaps and larger degrees of planarity. We consider both the neutral and radical cationic species. Upon p‐type doping, bond length rearrangement is indicative of a more delocalized electronic structure, which combined with optical calculations is consistent with the polaron‐model of charge storage on conjugated polymer chains. A better understanding of the properties of these materials at their molecular levels will inevitably be useful in material design as the polymer community continues to explore more main group containing polymers to tackle issues in electronic devices.
Density functional theory (DFT) calculations are utilized to assess polychalcogenophene properties with different chalcogen substitutions, including polonium, for both neutral and cationic species. Studied properties include bond length, chain twisting, aromaticity and optical properties. Heavier heteroatoms reduce band gaps and cause ring distortion. The inter‐ring bond indicates quinoidal character. Furans are consistent outliers. Doping causes diminishment of bond‐length alternation and further reduction of band gaps.
Copolymers with graft architectures possess interesting material properties distinct from their linear polymer counterparts. The effects of multidimensional architectures on the optoelectronic and ...physical properties of all-conjugated graft copolymers are not well-known, thus providing a large incentive for their study. In order to readily access these materials (hypothesized to have “comb” architectures), it is extremely important to investigate the methods used in their synthesis. Here we study the graft-to synthesis of comb copolymers composed of polythiophene backbones and polyselenophene side chains and identify the opportunities and challenges associated with copolymer formation. Azide-functionalized polythiophene “backbones” and acetylene-terminated polyselenophene “side chains” were synthesized in a controlled fashion using Kumada catalyst-transfer polycondensation (KCTP) polymerization and grafted together using copper-catalyzed azide–alkyne click chemistry (CuAAC). 1H NMR, GPC, and FTIR results confirm the attachment of polyselenophene side chains to the polythiophene backbone, resulting in comb copolymers with varying grafting densities. Low grafting density copolymers are readily synthesized using various backbone and side chain polymers. Midrange grafting density copolymers are more challenging but can be accessed when the availability of the graft sites on the polythiophene backbones is maximized. The synthesis of high grafting density combs remains challenging even when various modifications to the backbone and side chain polymers are implemented to improve the grafting efficiency. Problematic Glaser homocoupling of acetylene-terminated polyselenophenes was observed in certain conditions; however, this can be successfully prevented using an organic-soluble copper catalyst which is broadly applicable to many polymer–polymer CuAAC reactions. Ultimately, this investigation demonstrates a graft-to synthetic method that is useful for low- and midgrafting density all-conjugated comb copolymers, thus providing a means to further the study of these interesting multidimensional semiconducting materials.
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