Session D22: Focus Session: Organic Electronics: Synthesis and Materials

The role of symmetry and charge delocalization in two-dimensional molecules conjugated molecules for optoelectronic applications

Our group is investigating whether star molecules offer any advantage over linear polymers when used as the active layer in light-emitting diodes (LEDs), organic transistors (OFETs) or in photovoltaics (PVs). Specifically, we are investigating the role of architecture, synthesizing some novel molecules that contain a central tetra substituted phenyl ring. These molecules have a tendency to pi-pi stack, further delocalizing the carriers. The synthetic strategy used to prepare these molecules is versatile so that the four arms in the molecules do not have to be identical. The placement of differing arms also affords the opportunity to study the effect of symmetry on the properties of these molecules. The HOMO and LUMO levels can be changed via the type and placement of arms. Recent results obtained with these materials and their applications in photovoltaics and light-emitting diodes will be described.   

Ultra-pure organic semiconductors with improved charge carrier transport properties

The electrical performance of devices based on organic semiconducting films depend critically on the synthesis and processing of the materials. The quality of organic semiconductors depends on synthetic conditions and purification procedures, which have not yet been optimized. Ultra-pure semiconductors permit to establish sound correlations between molecular structure, functional properties of the films and performance of devices based thereon, e.g. field-effect transistors (FETs). Understanding structure-property relationships in films is the crucial condition for predicting chemical structures with superior properties. We report on FETs based on oligothiophene that exhibit dramatically improved charge transport, due to the ultra-purity of the organic semiconductors. The latter was achieved using a synthetic methodology that takes advantage of (i) heterogeneous catalysts, (ii) microwave activation to shorten reaction times and suppress side reactions.   

Side Chain Effects on the Structure and Dynamics of PPEs in different Complex Fluids

Poly (para phenyleneethynylenes) (PPEs) are electro-optically active macromolecules with immense potential in organic electro-optical devices. The polymer backbones are substituted by side chains that affect their conformation, association modes and dynamics. Consequently, they modify the optical response of the polymer. The present work introduces a small angle neutron scattering (SANS) and neutron spin echo (NSE) measurement of PPE substituted with a bulky triisopropylsiloxyl (TIPS) side chain in toluene. The results are compared with those previously obtained results of alkyl substituted PPEs. While alkyl substituted PPEs interacts via pi-pi stacking and forms a fragile gel, TIPS-PPE associates predominantly via the side chains and forms a stable gel over a large temperature range. The dynamics of PPEs in gel includes both fast and slow components and a cooperative dynamics between the solvent and PPE molecules.   

Thermal, structural, and electrical characterization of two high performance semiconducting polymers

Polymer semiconductors are inexpensive solution processable alternatives to amorphous silicon for applications in flexible large area electronics. Recently, thin films of spun-cast poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (pBTTT) have been demonstrated to exhibit exceptional hole mobilities in thin film transistors (TFTs) after heating into a mesophase. Poly(dialkylthieno[3,2-b]thiophene-2,5-bithiophene) (pTTBT) is a newly synthesized isomer of pBTTT with side chains attached to the thienothiophene rather than the bithiophene unit. This subtle structural change results in a very different response to heating. FTIR, spectroscopic ellipsometry (SE), AFM, X-ray diffraction, and NEXAFS were utilized to determine the root of the different thermal behavior. The structural transitions of the isomers are generally similar; however, the side chain melting transition Tm occurs about 50\r{ }C lower in pTTBT than in pBTTT. The significant drop in Tm appears to correlate with a subtle decrease in main chain packing interactions. Both materials exhibit high hole mobility, even in their respective mesophases. The slight overall higher order in pBTTT is reflected in the device performance.   

Synthesis and characterization of conducting polymer inserted carbon nanotubes

The carbon nanotubes filled with the photo-conducting polymer poly($N$-vinyl carbazole) and the conducting polymer polypyrrole were prepared by polymerizing the monomers inside the nanotubes using the supercritical carbon dioxide. The endohedral nanotubes were characterized by HRTEM and $^{1}$H NMR, which confirmed that the inserted material was indeed the conducting polymer [1]. I-V characteristics of the polymer inserted carbon nanotubes are presented. \newline [1] Johannes Steinmetz, Soyoung Kwon, Hyun-Jung Lee, Edy Abou-Hamad, Robert Almairac, Christophe Goze-Bac, Hwayong Kim, Yung-Woo Park,, Chem. Phys. Lett., \textbf{431}, 139 (2006)   

X-ray scattering Study of Ordering in Liquid Crystalline Semiconducting Polymers

The electrical performance of thin-film transistors, TFTs, formed with semiconducting polymers is approaching that of amorphous silicon. The highest performance TFTs are obtained from semiconducting polymers with liquid crystalline (LC) mesophases. Thermal annealing of these materials after deposition can increase the field effect mobility by as much as an order of magnitude. We will report the results of detailed x-ray scattering studies of the impact of thermal annealing on the crystalline ordering in thin films of poly(2,5-bis(3-n-alkyl-2yl)thieno[3,2-b]thiophene), PBTTT. The packing structure of this polymer has a lamellar stacking direction due to the alkyl sidechains and a $\pi $-stacking direction due to the planar backbones. Measurements of the lamellar d-spacing show that the lamellar packing in the as-cast state is distinct from the packing in annealed films. The change in ordering occurs after the film is heated into the LC mesophase. The LC mesophase is highly ordered with maintenance of both the lamellar and $\pi $-stacking of the polymer chains. The largest differences in these phases are found in the ordering of the sidechains with smaller changes in the $\pi $-stacking. The influence of these phases on electrical transport will be discussed.   

Improving the Electrical Conductivity of Polyaniline Through Molecular and Structural Control

We have investigated the electrical conductivity of polyaniline (PANI) that is template synthesized with a polymer acid of poly(2-acrylamino-2-methyl-1-propanesulfonic acid), PAAMPSA, as a function of the polymer acid molecular characteristics, including its molecular weight and molecular weight distribution. The electrical conductivity of PANI-PAAMPSA increases with decreasing PAAMPSA molecular weight. Additionally, PANI that is doped with narrow molecular weight distribution PAAMPSA is twice as conductive as PANI that is doped with PAAMPSA of comparable molecular weight having a broader molecular weight distribution. We can further increase the electrical conductivity of PANI-PAAMPSA post-synthesis by exposing the polymer film to dichloroacetic acid (DCA). In this case, the conductivity of PANI-PAAMPSA improves by more than two orders of magnitude. UV-vis-NIR spectroscopy and X-ray photoelectron spectroscopy suggest that DCA moderates the ionic interactions between PANI and PAAMPSA, allowing the polymer blend to adopt a structurally more favorable extended chain conformation.   

Surface photoisomerization activity vs. functionalization of azobenzene derivatives

Azobenzene and its derivatives can be reversibly photoisomerized between their \textit{cis} and \textit{trans} conformations in solution. The photoisomerization process is wavelength selective and results in a large length change. Hence, it is ideally suited to actuating molecular nanomachines on surfaces. However, it has recently been shown [1] that to recover photoisomerization activity on a metallic surface molecules must be functionalized with bulky spacing groups to decouple the optically active part of the molecule from the surface. This results in various trade-offs between molecular optical activity and overall flexibility/functionality. We have explored the photoisomerization activity of different azobenzene derivatives on metallic surfaces using a scanning tunneling microscope (STM) with optical access to the sample. The effects on molecular photo-activity and self-assembly for different substituent groups has been studied. [1] Matthew J. Comstock, Niv Levy, Armen Kirakosian, Jongweon Cho, Frank Lauterwasser, Jessica H. Harvey, David A. Strubbe, Jean M. J. Fr\'echet, Dirk Trauner, Steven G. Louie, and Michael F. Crommie Phys. Rev. Lett. \textbf{99}, 038301 (2007)   

Electrical and Optical Properties of a Novel Nonconjugated Conductive Polymer, Polynorbornene

We report electrical and optical properties of a novel nonconjugated conductive polymer, polynorbornene which has an isolated double bond in the repeat. Electrical conductivity of this polymer increases by more than ten orders of magnitude to about 0.01 S/cm upon doping with iodine. Optical absorption measurements of the polymer film have been made at different dopant concentrations. For a lightly doped polymer, two absorption peaks: one corresponding to cation radicals and the other corresponding to charge transfer between the double bond and the dopant were observed at 4.20 eV (295nm) and 3.13 eV (396nm) respectively. FTIR spectroscopic measurements have shown a reduction in the intensity of the C=C stretching and =C-H bending vibration bands upon doping indicating formation of radical cations. Photoluminescence studies have shown an emission band with a peak at $\sim $ 425nm when excited at 300nm. Nonlinear optical studies of this novel nonconjugated conductive polymer are in progress.   

Quadratic Electro-optic Measurements in the Nonconjugated Conductive Polymer, Poly($\beta $-pinene) at 800 nm and 1550 nm

Electro-optic effect in the nonconjugated conductive polymer, iodine-doped poly($\beta $-pinene) measured at 633 nm has been recently reported. In this presentation, results of quadratic electro-optic measurements at longer wavelengths will be reported. The electro-optic measurement has been made using the field-induced birefringence technique in the cross-polarized geometry with lock-in detection. Films with a medium doping level of iodine have been used in the measurements. Modulation depths of about 1.1{\%} at 800 nm and 0.06{\%} at 1550 nm were observed for an applied ac field of about 1 Volt/$\mu $m and for a film thickness of about 1 $\mu $m. More detailed measurements are in progress. The results are highly promising for applications of these materials in electro-optic modulators in the channel waveguide configuration. This exceptionally large quadratic electro-optic effect has been attributed to the confinement of this electronic system within a sub-nanometer dimension and the special electronic structure of this doped system.   

Enantiotropic Polymorphism in Di-Indenoperylene

The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP), an organic semiconductor material, with transition temperature of 403 K, has been structurally characterized using single crystal X-ray diffraction. Both the low temperature $\alpha $- and the high temperature $\beta $-phase have a herringbone-type structure, with the $\alpha $-phase being triclinic with doubled unit cell volume compared to the monoclinic $\beta $-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from the $\beta $- to the $\alpha $-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules. The $\beta $-phase of DIP is equivalent to the thin film phase.   

Direct Nanoscale Characterization of Submolecular Mobility in Complex Organic Non-linear Optical Systems

For organic non-linear optical (NLO) materials composed of intricate molecular building blocks, the challenge is to deduce meaningful molecular scale mobility information to understand complex relaxation and phase behavior. This is crucial, as the process of achieving a robust acentric alignment strongly depends on the availability of inter- and intra-molecular mobilities outside the temperature range of the device operation window. Here, we introduce a nanoscale methodology based on scanning probe microscopy that provides direct insight into structural relaxations and shows great potential to direct material design of sophisticated macromolecules. It also offers a means by which mesoscale dynamics and cooperativity involved in relaxation processes can be quantified in terms of dynamic entropy and enthalpy. This study demonstrates this methodology to describe the mesocale dynamics of two systems (1) organic networking dendronized NLO molecular glasses that self-assemble into physically linked polymers due to quadrupolar phenyl-perfluorophenyl interactions and (2) dendronized side-chain electro-optic (EO) polymers. For the self assembling glasses, the degree of intermolecular cooperativity can be deduced using this methodology, while for the dendronized side-chain polymers, specific side chain mobilities are exploited to improve EO properties.