Session Z44: Focus Session: Organic Electronics and Photonics -- New Materials and Applications

Ambipolar Electric Double Layer Transistors Using Organic Single Crystals

Among organic devices, ambipolar transistors are very unique device, in which both electrons and holes are equally mobile and we are able to observe light emission through the recombination of them. Progress in the applications of such light-emitting transistors (LETs) based on organic single crystals has provided possibilities in developing organic laser. However, in these LETs, the current density is still low for lasing, and, therefore, a different device structure is necessary to overcome this issue. Here we show the first demonstration of organic ambipolar electric double layer transistors (EDLTs), in which the gate dielectric is not a conventional insulator but an electrolyte. The peculiar merit of EDLT is extremely high conductivity due to the huge capacitance of the EDL formed at the organic/electrolyte interfaces. Consequently, we can increase current density. In this study, we used rubrene single crystal and ion-gel as the active material and electrolyte, respectively. These present results will provide a prospect for further development in LET operation.   

Refractive Indices of Specific Nonconjugated Conductive Polymers: Organic Nanometallic Systems

Exceptionally large quadratic electro-optic effect and two-photon absorption coefficients have been recently reported for nonconjugated conductive polymers after doping. These polymers include: cis- and trans- polyisoprene, poly($\beta $-pinene) and others. The large optical nonlinearities observed in these materials have been attributed to the nanometallic state with subnanometer dimensions that is formed upon doping and charge-transfer. Measurement and calculation of linear refractive indices of these novel nonlinear optical systems before and after doping are important. Linear absorption coefficients (UV-Visible) of trans-1,4-polyisoprene have been measured for different doping levels of iodine. Refractive indices have been calculated using Kramers-Kronig transformation of absorption data for different doping levels. Numerical integration using MATLAB software was used for these calculations. Refractive indices at specific wavelengths have been determined by measuring reflectivity at normal incidence. The calculated and measured values of refractive indices have been compared. Results on calculations and measurements on these novel systems will be discussed.   

Sub-diffraction limited features in three-dimensional photopatterned, two-photon excimer-forming fluorescent dye-doped films

3D Photopatterning is a key process in optical data storage, photolithography and other applications. Two photon active systems are a popular choice to pattern in 3D. The main challenges pertain to the contrast and density of the patterned features. By making use of a thermal threshold process which induces dye deaggregation, high contrast features are written with diameters smaller than the system's diffraction limit. A polymer film was doped with a two photon active dye that possesses two distinct fluorescence states in its monomer and excimer phases. The film's phases are stable at room temperature, and have a threshold response to heating. By selective exposure to a pulsed 675 nm source, the film is photopatterned in 3D as the focused pulses are absorbed and thus anneal the sample. Because the change is physical rather than chemical, the mechanism is shown to be a threshold process. As a result, sub diffraction limit photopatterns are demonstrated in the medium.   

Low threshold conjugated polymer lasers by intrinsically directed resonator design

Creation of laser cavities requires external imposition of an optical feedback system onto the gain medium. By contrast, we use functional conjugated polymers that can be chemically or physically patterned from within to form diffractive laser resonators. One realization is based on a chemically-modified polyfluorene, which can be patterned into distributed feedback (DFB) resonators of any desired grating period. A different route is by physically pattering conjugated polymers into sub-micrometer colloids via microfluidics. These systems can be tuned with respect to particle size and their arrangement in the resulting laser structure. We show random lasing for a photonic-glass, self-assembled from monodisperse conjugated polymer particles.   

Effects of polar analytes on the transport properties of organic semiconductor field-effect chemical sensors

Chemical recognition or sensing in organic thin film transistors (TFTs) can be achieved by direct analyte interaction or the inclusion of specific receptor molecules added to the semiconducting surface. Overall, sensing is dependent on the interactions occurring between the molecule and the OTFT active region, which includes the semiconductor and semiconductor-insulator interface. The magnitude of the interaction will depend on the molecules polarizability and the partition function of the analyte vapor. We employ a range of analytes (cyclohexane, ethanol, and styrene) with different solvations parameters (polarizability/dipolarity levels) to gain more clarity on their effects of the charge transport properties in OTFTs. Receptors are used to understand in more detail the physical and chemical interactions, which contribute to the sensor response. The receptors themselves have diverse polarizability/dipolarity parameters, which produce varying sensing behaviors dependent on the solvation parameters of the analytes.   

Chemical vapor sensors using a poly(triarylamine) semiconductor modified through incorporation of organic receptors

A poly(triarylamine) (PTAA) semiconducting active layer has been employed in order to produce OFET chemical sensors displaying excellent stability in air, with minimal bias stress effects. We propose a ``chemical fingerprint'' sensing array comprised of a single base polymer (PTAA), with the selectivity of individual devices modified through the incorporation of a variety of small molecule receptors. This allows for consistent device operation and optimization of the array. The effect of various receptors will be discussed, and reported while sensing alcohol vapors. Different methodologies will be proposed for incorporation of receptors into the device, including incorporation into the PTAA film and separate deposition on top of the PTAA film. The relative merits of each approach will be discussed, including the effect on both threshold voltage and carrier mobility.   

Nonadiabatic exciton dynamics in conjugated polymers

The results of mixed quantum/classical simulations of the nonadiabatic excited state dynamics of sexithiophene and the C60-sexithiophene interface are presented. The model is capable of describing the photogeneration and subsequent time-evolution of excitons in conjugated polymers at model bulk heterojunction interfaces. The effect of chain length and electric field on exciton mobility is discussed for both single-chain and $\pi$-stacked sexithiophene oligomer. In addition the dynamics of exciton dissociation at the C60-sexithiophene interface are described.   

Solution Processable Organic Solar Cell Microarrays for Use in MEMS

We have developed an innovative way to fabricate organic solar arrays for application as DC power supplies in electrostatic MEMS devices. The generation 1 microarray consists of 20 small (1 mm\textsuperscript{2}) solar cells connected in series (total device area of 2.2 cm\textsuperscript{2}). The device uses an active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC$_{61}$BM), which are mixed together (1:1 mass ratio) in appropriate solvent. We manipulated active layer nanomorphology by choice of solvents and annealing conditions. The optimized generation 1 device has an open-circuit voltage of 11.5V, short-circuit current density of 1 mA/cm\textsuperscript{2}, and a power conversion efficiency of 2\% under simulated solar AM1.5 illumination. The generation 2 microarray has a new design with reduced series resistance and improved cell occupancy. The generation 2 arrays have demonstrated improved device efficiency and power output density. Detailed analysis of device physics in both generation microarrays will be presented. The procedure described has potential for producing microarrays as small as 0.01 mm\textsuperscript{2}.   

Synthesis and characterization of a new TPA-Thiophene based molecule for potential Organic PV applications

Triphenylamine (TPA) containing molecules possess unique optical and photovoltaic properties. These molecules show very strong visible absorption due to unique electronic structure. Another class of molecules, thiophene derivatives and resulting polymers have higher hole mobility amongst other conjugate co-polymers. In view of these aspects, we have designed a new conjugated thiophene containing molecule with TPA. A solution processable technique is reported to synthesize this newly designed star shaped molecule with TPA derivative as its core and thiophene derivative as branches. The synthesized molecule is systematically characterized by studying Mass spectroscopy, Raman spectroscopy, UV-Vis spectroscopy, and cyclic voltammetry. This molecule shows solubility in various common organic solvents, broad absorption in spectral range of 300- 650 nm, and good thermal stability. An atomistic simulations based on density functional theory was carried out to validate HOMO and LUMO levels. The cyclic voltammetry analysis is consistent with atomistic simulation results. A 3D simulated orbital image reveals that the HOMO level corresponds to oxygen as well as carbon in a benzene ring.   

Quadratic Electro-optic Effect in the Nonconjugated Conductive Polymer Trans-1,4-polyisoprene Measured at 1.55 $\mu $m

Quadratic electro-optic effect in the nonconjugated conductive polymer, iodine-doped trans-1,4-polyisoprene has been measured at a longer wavelength (1.55 $\mu $m) and x-ray diffraction of the polymer film has been studied. Optical absorption spectra of trans-1,4-polyisoprene (TPI) show two peaks after doping, one at 4.2eV (295 nm) due to radical cation and the other at 3.2eV (388 nm) due to charge-transfer from double to dopant. X-ray diffractometer scans show that the trans-1,4-polyisoprene film has $\gamma $-phase crystal structure. Upon doping intensities of two peaks were observed increase without significant changes in overall peak positions. Field induced birefringence technique has been used to measure quadratic electro optic effect at 1.55 $\mu $m (which is away from resonance) and an exceptionally large Kerr coefficient (2.5x10$^{-10}$ m/V$^{2})$ has been obtained. This large nonlinearity has been attributed to the subnanometer-size metallic quantum dot structure produced upon doping of this polymer.   

Photo-active Silicon Containing Polymer Films; An Approach Towards All-Solution Processable Devices

The continuous drive for smaller, cheaper electronic devices elucidates the necessity for the design and incorporation of materials with improved properties along with a greater ease of processability. Polycarbosilanes (PCSs) are a class of organic-inorganic hybrid materials which exhibit increased chemical and thermal stability, appreciable hydrophobicity, and significant mechanical robustness. Recently, we have developed UV-crosslinkable cyclolinear PCSs by the incorporation of aryl substituted disilacyclobutane (DSCB) rings into the backbone. This novel material exhibits a low dielectric constant (k), making it an attractive new material in the development of interlayer dielectrics. Preliminary data shows that there is a maximum UV-crosslinking depth of approximately 30 microns. Additionally we have successfully demonstrated the resist capabilities of this material by patterning micro-scale (about 10 microns) features with a surface roughness variation of approx. 1 percent. Finally, new approaches towards taking advantage of the reactivity of polysilanes and the effect of molecular weight and polydispersity will be discussed.