Search for: All records

Abstract The relaxation of photoexcited polarons in doped conjugated polymers is studied with ultrafast transient absorption (TA) spectroscopy to examine the effect of polymer morphology and counterion size on polaron mobility. Processing conditions are first used to create F₄TCNQ‐doped (2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane) poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) films with different morphologies and thus free and trapped polarons in different ratios. We find that less crystalline films have a higher fraction of trapped polarons, but, remarkably, that free and trapped polarons have the same relaxation times in all films. Films doped with a large dodecaborane (DDB) cluster‐based dopant are then used to show that trapping is based on Coulomb interactions between polarons and counterions; no trapped polarons are observed in TA due to the reduced Coulomb interaction between the polarons and the DDB counterion. Indeed, the relaxation of polarons in these films is an order of magnitude faster than that in F₄TCNQ‐doped films, consistent with reduced trapping. Finally, the results are used to argue that counterion size has a greater effect on polaron mobility than polymer morphology and crystallinity. All of the experiments show that pump/probe spectroscopy provides a straightforward way to determine the local mobilities and degree of carrier trapping in doped conjugated polymer films. 

Rhenium diboride (ReB2) exhibits high differential strain due to its puckered boron sheets that impede shear deformation. Here, we demonstrate the use of solid solution formation to enhance the Vickers hardness and differential strain of ReB2. ReB2-structured solid solutions (Re0.98Os0.02B2 and Re0.98Ru0.02B2, noted as “ReOsB2” and “ReRuB2”) were synthesized via arc-melting from the pure elements. In-situ high-pressure radial x-ray diffraction was performed in the diamond anvil cell to study the incompressibility and lattice strain of ReOsB2 and ReRuB2 up to ∼56 GPa. Both solid solutions exhibit higher incompressibility and differential strain than pure ReB2. However, while all lattice planes are strengthened by doping osmium (Os) into the ReB2 structure, only the weakest ReB2 lattice plane is enhanced with ruthenium (Ru). These results are in agreement with the Vickers hardness measurements of the two systems, where higher hardness was observed in ReOsB2. The combination of high-pressure studies with experimentally observed hardness data provides lattice specific information about the strengthening mechanisms behind the intrinsic hardness enhancement of the ReB2 system. 

Abstract Addressing sustainable energy storage remains crucial for transitioning to renewable sources. While Li‐ion batteries have made significant contributions, enhancing their capacity through alternative materials remains a key challenge. Micro‐sized silicon is a promising anode material due to its tenfold higher theoretical capacity compared to conventional graphite. However, its substantial volumetric expansion during cycling impedes practical application due to mechanical failure and rapid capacity fading. A novel approach is proposed to mitigate this issue by incorporating trace amounts of aluminum into the micro‐sized silicon electrode using ball milling. Density functional theory (DFT) is employed to establish a theoretical framework elucidating how grain boundary sliding, a key mechanism involved in preventing mechanical failure is facilitated by the presence of trace aluminum at grain boundaries. This, in turn, reduces stress accumulation within the material, reducing the likelihood of failure. To validate the theoretical predictions, capacity retention experiments are conducted on undoped and Al‐doped micro‐sized silicon samples. The results demonstrate significantly reduced capacity fading in the doped sample, corroborating the theoretical framework and showcasing the potential of aluminum doping for improved Li‐ion battery performance. 

Abstract When an electron is removed from a conjugated polymer, such as poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), the remaining hole and associated change in the polymer backbone structure from aromatic to quinoidal are referred to as a polaron. Bipolarons are created by removing the unpaired electron from an already‐oxidized polymer segment. In electrochemically‐doped P3HT films, polarons, and bipolarons are readily observed, but in chemically‐doped P3HT films, bipolarons rarely form. This is explained by studying the effects of counterion position on the formation of polarons, strongly coupled polarons, and bipolarons using both spectroscopic and X‐ray diffraction experiments and time‐dependent density functional theory calculations. The counterion positions control whether two polarons spin‐pair to form a bipolaron or whether they strongly couple without spin‐pairing are found. When two counterions lie close to the same polymer segment, bipolarons can form, with an absorption spectrum that is blueshifted from that of a single polaron. Otherwise, polarons at high concentrations do not spin‐pair, but insteadJ‐couple, leading to a redshifted absorption spectrum. The counterion location needed for bipolaron formation is accompanied by a loss of polymer crystallinity. These results explain the observed formation order of single polarons, coupled single polarons, and singlet bipolarons in electrochemically‐ and chemically‐doped conjugated polymers. 

Abstract Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)‐based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox‐potential‐driven. Remarkably, X‐ray scattering shows that despite their large 2‐nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film. 

Abstract The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) into poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X‐ray data suggest that F₄TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π‐stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F₄TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm²V⁻¹s⁻¹in the most crystalline P3HT samples. Temperature‐dependent conductivity measurements show that polaron mobility in SqP‐doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature.