Strain Effects on the Work Function of an Organic Semiconductor

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important not only for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, but also for developing flexible electronic devices. Here we investigate tensile and compressive strain effects on the WF of rubrene single crystals. Mechanical strain induced by thermal expansion mismatch between the substrate and rubrene is quantified by X-ray diffraction. The corresponding WF change is measured by scanning Kelvin probe microscopy. The WF of rubrene increases (decreases) significantly with in-plane tensile (compressive) strain, which agrees qualitatively with density functional theory calculations. An elastic-to-plastic transition, characterized by a steep rise of the WF, occurs at ∼0.05% tensile strain along the rubrene π-stacking direction. The results provide the first concrete link between mechanical strain and WF of an organic semiconductor and have important implications for understanding the connection between structural and electronic disorder in soft organic electronic materials

Remarkable conductivity enhancement in P-doped polythiophenes via rational engineering of polymer-dopant interactions

Molecular doping is an effective approach to tune the charge density and optimize electrical performance of conjugated polymers. However, the introduction of dopants, on the other hand, may disturb the polymer microstructure and disrupt the charge transport path, often leading to a decrease of charge carrier mobility and deterioration of electrical conductivity of the doped films. Here we show that dopant-induced disorder can be overcome by rational engineering of polymer-dopant interactions, resulting in remarkable enhancement of electrical conductivity. Benchmark poly(3-hexylthiophene) (P3HT) and its analogous random polymers of 3-hexylthiophene and thiophene P[(3HT)1-x-stat-(T)x] were synthesized and doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Remarkably, random P[(3HT)1-x-stat-(T)x] was doped to a far superior electrical conductivity, that in the case of x ≥ 0.24, the conductivity of P[(3HT)1-x-stat-(T)x] is over 100 times higher than that of the doped P3HT, despite both P3HT and P[(3HT)1-x-stat-(T)x] exhibit comparable charge carrier mobility in their pristine state and in spite of their practically identical redox properties. This result can be traced back to the formation of π-stacked polymer-dopant-polymer co-crystals exhibiting extremely short packing distances of 3.13–3.15 \uc5. The mechanism behind these performances is based on a new role played by the dopant molecules that we name “bridging-gluing”. The results are coherently verified by the combination of optical absorption spectroscopy, X-ray diffraction, density functional theory calculations, and molecular dynamics simulations

On the molecular origin of charge separation at the donor-acceptor interface

C.R. thanks the University of Kentucky Vice President for Research and the Department of the Navy, Office of Naval Research (Award No. N00014-16-1-2985) for support. V.C. thanks the Department of the Navy, Office of Naval Research (Awards Nos. N00014-14-1-0580 and N00014-16-1-2520) for support. M.S. and D.D. acknowledge funding by the German Science Foundation through the SPP 1355 “Elementary Processes in Organic Photovoltaics.” The research data supporting this paper can be accessed at https://doi.org/10.17630/a6935caf-f7ed-48b2-b131-68ae72a26629.Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing but urgently needed to direct further material improvements. Here we use a combined experimental and theoretical approach to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. We use model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers and perform a detailed density functional theory (DFT) investigation. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.PostprintPeer reviewe

Diagrammes de correlation ab-initio Valence Bond pour l'attaque nucleophile sur des derives du carbone et du silicium

SIGLEINIST T 74814 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine‐based Covalent Organic Frameworks

International audienc

Evolution of the Nature of Excitons and Electronic Couplings in Hybrid 2D Perovskites as a Function of Organic Cation π‐Conjugation

International audienc

Why Is β-Me Elimination Only Observed in d⁰ Early Transition Metal Complexes? An Organometallic Hyperconjugation Effect with Consequences for the Termination Step in Ziegler–Natta Catalysis

Two processes, β-H and β-Me migration, have been observed to compete in the termination of alkene oligomerization (Ziegler–Natta catalysts) in certain d0 early transition metal systems. ECP ab initio calculations have been performed to study these processes. Models of intermediates (X2MRq: X = Cl, Cp; M = Zr, Hf, Ti, q = +1; Zr, Nb, q = 0) have been optimized at the HF level with additional single-point energy calculations at the MP2 level. It is shown that the β-Me elimination may be thermodynamically favored over β-H elimination for strongly electron-deficient metal centers. This preference is attributed to the presence of multiple bonding between a d0 transition metal and the methyl group, which behaves like a weak π donor via its occupied πCH3 orbitals. It is therefore analogous to the well documented hyperconjugation in organic chemistry.</p

Why is β-me elimination only observed in d⁰ early-transition-metal complexes? An organometallic hyperconjugation effect with consequences for the termination step in Ziegler-Natta catalysis

Two processes, beta-Hand beta- Me migration, have been observed to compete in the termination of alkene oligomerization (Ziegler-Natta catalysts) in certain d0 early transition metal systems. ECP ab initio calculations have been performed to study these processes. Models of intermediates (X2MR(q): X = Cl, Cp; M = Zr, Hf, Ti, q = +1; Zr, Nb, q = 0) have been optimized at the HF level with additional single-point energy calculations at the MP2 level. It is shown that the beta-Me elimination may be thermodynamically favored over beta-H elimination for strongly electron-deficient metal centers. This preference is attributed to the presence of multiple bonding between a d0 transition metal and the methyl group, which behaves like a weak pi donor via its occupied pi(CH3) orbitals. It is therefore analogous to the well documented hyperconjugation in organic chemistry