Session Y17: Focus Session: Organic Electronics and Photonics - Electronic, Optical, Magnetic Properties

Photoinduced absorption studies of triplet-enhanced conjugated polymer-based diodes

Trace concentration of metallic impurities present at the ppm level in conjugated polymers allow transition between the singlet and triplet manifolds, thus permitting photophysical studies of the triplet states at room temperature. We present photoinduced absorption (PIA) studies of diphenyl-substituted ladder-type poly paraphenylene (PhLPPP) containing a trace concentration of covalently bound Pd atoms. In the bulk polymer sample, the T$_1$-T$_N$ peak is observed at $\sim$ 1.3 eV, which is also known to blue shift with increasing temperatures. In this work we present PIA studies as a function of an applied electric field from PhLPPP-based diodes. In addition to the T$_1$-T$_N$ peak at 1.3 eV, we also observe a strong signal at 1.35 eV, which is traced to the interface between the polymer and the hole transport layer. Field-induced PIA from polymer devices is thus a promising tool for probing weak electronic states, such as the triplet and charge transfer states, and further provides a mechanism for disentangling bulk and interface properties.   

Structure and optical properties of self-assembled distyrylbenzene electrolytes

Conjugated Oligoelectrolytes (COEs) are of interest as optical components in solution based biosensors and as electron transport layers in organic light-emitting diodes (OLEDs). These applications require solution processing of the COEs where the packing of oligoelectrolyte aggregates (directed by intermolecular interactions) governs their optical and electronic properties. The structural organization of distyrylbenzene derivatives dissolved in water is systematically studied as a function of the oligoelectrolyte concentration. Small angle neutron scattering (SANS) shows that above a critical micelle concentration these oligoelectrolytes self-assemble into cylindrical micelles with diameter $\sim $ 3.5 nm. The inter micelle distance decreases with increasing concentration and finally these organization arrange themselves into a cluster with a characteristic length scale $\sim $ 50 nm as derived from Guinier analysis of the SANS. Complementary structural studies have also been performed using liquid atomic force microscopy and cryogenic transmission electron microscopy techniques. The optical properties of this self-assembled oligoelectrolyte system will also be presented. We acknowledge NCNR NIST for facilitating the SANS experiments.   

Mode delocalization in all-polymer distributed Bragg lasers

Photon localization in photonic crystals has received considerable attention during the last decade. Such wave localization arises from disorder inside the structure. At the same time, roll-to-roll processing for new generations of electronic and photonic devices and systems is currently a major motivation for research into organic and polymeric materials. An all-polymer distributed Bragg laser (DBR) includes two co-extruded multilayer polymer mirrors that exhibit considerable variation in layer thickness. It is well-known that the spacing of the longitudinal modes in a laser is related to the cavity length. Here, we report on measurements of mode penetration into the multilayer film within the bandgap. From transfer matrix simulations, we can determine how the localization length depends on lattice disorder. Simulations and studies of the ensemble average were able to connect the gap-delocalized modes to localized modes outside of the gap arising from Anderson localization, as well as identify the controlling parameters. In addition, FDTD simulations confirmed our results.   

Reversible photodegradation of Amplifield Spontaneous Emmission of Disperse Orange 11 dye doped in PMMA Polymer

We use amplified spontaneous emission (ASE) and linear absorption spectroscopy to study the mechanisms of reversible photodegradation of 1-amino-2-methylanthraquinone (disperse orange11-DO11) in solid poly(methyl methacrylate). Chemical and physical damage of the dye doped polymer recovers to its initial state after few hours. Linear absorption measurements as a function of decay of the ASE intensity reveals that there is a damaged state that is populated as the sample damages. Linear absorption measurements at elevated temperatures confirms the presence of a damaged state, but its character is not identical to the damaged population induced by high-intensity light. A preliminary energy level diagram is determined, from measurements which agrees with independent data and theory. In this presentation I will discuss temperature dependent measurements which support our theory and qualitative explanations of the mechanisms.   

Using Scale Invariance to Identify Universal Properties of Quantum Systems With Exceptionally Large Nonlinear-Optical Response

The invention of the laser led to the discovery of a broad range of nonlinear-optical phenomena,[1] which have applications in optical computing, high-speed telecommunications, photo-dynamic cancer therapies, optical data storage, 3D photolithography, etc. At the heart of these applications is the requirement of a large nonlinear susceptibility. Intense effort has therefore focused on understanding and optimizing the nonlinearity of organic molecules. Using fundamental limits of the nonlinear-optical response, the scale invariant intrinsic hyperpolarizabilities - which account for size/energy scaling - can be determined. Most molecules measured over three decades of research have remained a factor of 30 below the fundamental limit.[2] Using several theoretical techniques, we have identified the properties required of a quantum system for its response to be at the fundamental limit. Interestingly, while there are many quantum systems with a maximal intrinsic hyperpolarizability, they all share certain universal properties. I will discuss how this may lead to better materials and new physics. [1] www.NLOsource.com [2] M. G. Kuzyk, J. Mater. Chem. 19, 7444 (2009).   

A Multifunctional Organic Bistable Device

Bias-controlled spin valve effect is observed in an organic bistable device using rubrene (C$_{42}$H$_{28}$) as an organic semiconductor channel, half-metallic LSMO and Fe as the two ferromagnetic electrodes. The device displays reproducible switching between a low-impedance (ON) state and a high- impedance (OFF) state by applying different polarities of high biases. In the ON state, the device shows a spin valve effect with magnetoresistance (MR) values up to 3.75 \%. The observed spin valve effect disappears when the device recovers to the initial OFF state. This bias induced switching of the conductive states, as well as the spin valve effect, promise multiple functionalities for organic bistable devices.   

Electrical Conductivity of Polyaniline Thin Films

We use infrared spectroscopy measurements to estimate the electrical conductivity of highly conductive polyaniline at different temperatures. The reflectance data were fitted to a model that includes a number of contributions to the electrical conductivity, such as Drude response of the free carriers, vibrational contributions, and interband transitions. We use classical models for these contributions and an exact model for the optical properties of thin films on the thick substrates. The results give the effective charge-carrier density and the transport lifetime of these charge carriers. The far infrared measurements will be compared to dc transport measurements.   

An adapted Su-Schrieffer-Heeger Hamiltonian for conducting polymers

We present an adapted Su-Schrieffer-Heeger (SSH) model Hamiltonian to naturally incorporate the materials-specific properties of various conducting polymers, such as poly-(p-phenylene-vinylene) (PPV), polypyrrole (PPy), polyaniline (PANI), and their functional derivatives. Taking PPy as an illustrating example, we treat heterogeneous NH sites in pyrrole as positive ions, or attractive potential wells for $\pi $ electrons. All parameter values are determined by numeric fitting to \textit{ab-initio} calculations and experimental data. The model has excellent agreements with \textit{ab-initio} calculations and experimental data in both the equilibrium configurations and $\pi $ electron structures for the cases of a pyrrole ring, a pyrrole dimer, and a long chain of 20 pyrrole rings. Polarons and bipolarons were found upon charge dopings. Results on the porphyrin model Hamiltonian are discussed.   

Exciton transfer rate between two neutral \textit{trans}-polyacetylene chains

We use the perturbation theory to compute the exciton transfer rate between two neutral \textit{trans}-polyacetylene chains described by the Su-Schrieffer-Heeger (SSH) Hamiltonian. Considering the highly polarizable nature of \textit{trans}-polyacetylene, we compute Coulomb interactions directly without the point dipole approximation as introduced in the conventional Forster approach. We find a much softer chain separation dependence of the energy transfer rate between two \textit{trans}-polyacetylene chains, scaling as 1/r$^{n}$ where n $\sim $ 4 which is different from the well-known n= 6 in the Forster formula or n= 5 in the Hamaker formula for one-dimensional pairwise interactions. At a typical separating distance of 3.5 {\AA}, the $\pi -\pi $ interaction density is 0.21 eV/ {\AA} and the exciton transfer time is 72.6 ps. We are extending our calculations for poly-($p$-phenylenevinylene) (PPV) and the complex porphyrin assembly using our recently developed adapted SSH Hamiltonian.   

Electric and Magnetic Performances of Self-Doped Sulfonated Polyanilines (SPAN) and Their Copolymers with Polycaprolactone (PCL)

A group of ``self-doped'' conducting polymers such as sulfonated polyaniline (SPAN) and their copolymers with biodegradable polymers such as poly(caprolactone) (PCL) have been successfully synthesized with varying degrees of sulfonation. By adding terminators, a low molecular weigh (LMw) conducting biocompatible SPAN was obtained. Conducting biodegradable copolymers (CBCP) of SPAN with PCL were also synthesized by reacting LMw SPAN with caprolactone through a Ring Opening Polymerisation (ROP). These copolymers are not only conducting, but also biodegradable, and can be processed into various conducting biodegradable scaffolds (CBS) for tissue engineering applications through electro-spinning, multilayer self- assembling, soft-lithography. The structure and properties of these materials have being characterized with UV-Vis and FT-IR, NMR, GPC, elemental analysis, CV, TGA, DSC, ESR, and four-probe conductivity measurements. We have discovered that pH has effects on the conductivity of polymers, which can be adjusted by varying the degree of sulfonation. A high conducting S-50 sample was measured with a 0.1 S/cm at pH=7.4, which are very interested in the biomedical application. A comprehensive materials evaluation is on the way and the selected results will be presented.   

Ferroelectricity of carbon based materials

We discuss the possibility of ferroelectric carbon-based materials: organic crystals, conducting polymers and graphene ribbons. Ferroelectricity related to the charge ordering was discovered in quasi-1D organic conductors (TMTTF)$_{2}$X, and in layered compounds like $\delta $-(EDT-TTFCONMe$_{2})_{2}$Br. A microscopic picture is based on two coexisting symmetry lowering effects: the dimerization of bonds (which is build-in) and the dimerization of sites (which is a spontaneous symmetry breaking). Today we can look for a possibility of a ferroelectric polymer. The difference with respect to (TMTTF)$_{2}$X is the reverse of the build-in (sites) and spontaneous (bonds) effects of dimerizations. Such an (AB)$_{x}$ polymer has already appeared, studied for nonlinear optical properties, but not tested for the the ferroelectricity. The theory predicted an existence of solitons with non-integer variable charges, both with and without spin i.e. walls separating domains with opposite electric polarisation. Their physics will serve to relate transient ferroelectric processes and the visible range optics. For the case of zigzag edges of graphene ribbons the build-in dimerization is originated by different chemical surrounding of carbon atoms and the spontaneous one can be the result of Peierls transition along the edges.   

Density fluctuations and phase transitions of ferroelectric polymer nanorods

Permittivity measurements in the high frequency limit (i.e. in spectral regions not affected by dielectric dispersions) represent an effective mean to asses density fluctuations in dependence on temperature. This is used in the current study to investigate the phase transitions of ferroelectric polymer nanorods (crystallization, melting, Curie-transitions) down to diameters as small as 15 nm. As fabrication templates -- and in the same time as measurement platforms in combination with Broadband Dielectric Spectroscopy -- ordered arrays of parallel cylindrical nanopores are employed.