Bipolaron

About: Bipolaron is a research topic. Over the lifetime, 1335 publications have been published within this topic receiving 29154 citations. The topic is also known as: bipolarons.

Exploring organic magnetoresistance : an investigation of microscopic and device properties

Abstract: Recently there has been much interest in combining the fields of organic electronics and spintronics. This has been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes. Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new phenomena which must be explained with novel physics, here we explore one of these phenomena, organic magnetoresistance (OMAR). OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a large change in resistance (up to 25%) at low magnetic fields ( 20mT). OMAR represents a scientific puzzle since no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of the sign change since understanding this unique property should be a major step in unraveling the microscopic origin of OMAR. We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule and polymer devices, in which we observed sign changes as functions of voltage and temperature. These devices showed a strong correlation between the sign change and the onset of minority charge carrier injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign. From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR. To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons. The models show, counter-intuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. We show that these models describe the observed MR(V) behavior very well, and if one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature. The device model also predicts how different device parameters affect the observed MR, to test its predictions we performed experiments in which we increased the charge recombination by dye doping the organic active layer, we also observed how changing the charge injection by altering the organic semiconductor/ metal contacts experimentally compared with the device model. The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR.

Lattice- and spin-polaronic defects in antiferromagnetic halogen-bridged transition-metal chain compounds.

Abstract: The results of many-body modeling are reported for polaronic defect excitations in the nickel-based halogen-bridged chain compounds, exemplified by NiBr. One-electron band structures and Raman scattering amplitudes, for electron and hole polarons and bipolarons, triplet excitons, negative, neutral, and positive solitons, are qualitatively described. A singlet exciton is found to be unstable with our model parameters, due to the large on-site Hubbard repulsion. Most significantly, the local defect excitations exhibit a substantial lattice distortion relative to the undistorted antiferromagnetic spin background. The stoichiometric lattice symmetry is therefore broken locally, resulting in Raman activity, which is unexpected on the basis of the stoichiometric ground-state symmetry

Electronic Structure of Processable Conducting Polymers

Abstract: The electronic structure of poly-3-hexylthiophene is studied in the solid state with photoelectron spectroscopy, as the polymer is gradually doped from NOPF6. The evolution of the core level binding energies is related to the modification of the electron density on the conjugated backbone, due to the creation of polaron and bipolaron defects. Upon doping, valence spectra show a shift in the Fermi level of the system, and at saturation doping a finite density of states is observed at the Fermi level. In the case of poly-para-phenylene vinylene, the experimental density of states of the neutral polymer is related to the results of Valence Effecttive Hamiltonian calculations. Comparison with gas phase data on styrene leads to an estimation of the polarization energy.

Periodic superstructures in tetrahedrally bonded homopolymers.

Abstract: Using a lattice sum over single bipolaron potentials displaced by periodicity d, we have analytically obtained a solution for a bipolaron lattice for a tetrahedrally bonded homopolymer within the continuum model of Rice and Phillpot. This solution is used to derive the band structure, which consists of two bipolaron bands symmetrically located about the middle of the band gap in addition to the conduction and valence bands. The electronic density of states, chemical potential \ensuremath{\mu}, and the energy of formation of a bipolaron lattice are also calculated as a function of the bipolaron density ${\ensuremath{\rho}}_{b}$. The bipolaron chemical potential lies between the conduction-band edge and the upper edge of the upper bipolaron band, indicating that the bipolaron lattice is energetically the most favorable charge configuration at low ${\ensuremath{\rho}}_{b}$. In the strict weak-coupling limit (infinite momentum cutoff \ensuremath{\Lambda}) the bipolaron-bipolaron interaction is found to be repulsive and varies with bipolaron density as (1/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$)exp(-2/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$${\ensuremath{\xi}}_{\mathit{p}}$), ${\ensuremath{\xi}}_{\mathit{p}}$ being the bipolaron characteristic length. Thus, the bipolaron lattice is stable only in the range 0${\ensuremath{\rho}}_{b}$2/${\ensuremath{\xi}}_{p}$. This suggests the possibility of a phase separation of a doped homopolymer into conducting bipolaron droplets at ${\ensuremath{\rho}}_{b}$=2/${\ensuremath{\xi}}_{p}$, while the rest of the system is insulating. Our results apply to polysilylenes, polygermylenes, and their derivatives, as well as to a wide class of carbon-based polymers.