G. Srdanov

Large area, full-color, digital image sensors made with semiconducting polymers

Abstract The high performance polymer photovoltaic cells and photodiodes demonstrated recently are of great interests not only for traditional applications but also for large area, digital image sensing. In this presentation, we demonstrate large sensing area, photodiode arrays and matrices made with semiconducting polymers and polymer blends. These photosensors are of high photosensitivity, low dark current, large dynamic range and fast response time. For full-color image sensing applications, large area photosensor arrays were developed to have true-color (24-bits) resolution. The red, green, and blue color primaries were achieved by coupling a set of color filters with the polymer sensor pixels with response to visible light radiation. Image recovery process from the pixel photocurrent data was developed, which is suitable generally to image arrays with power law light intensity dependence and with finite pixel dark current. Large sensing length photodiode arrays (2.5–5 in.) were fabricated in 40–100 dot-per-inch (dpi). They were used as the sensing unit in page-size document scanners. Voltage-switchable polymer photodetectors were developed. Their photosensitivity can be switched on and off with external bias. These devices can be used as the sensing elements in x–y addressable sensor matrices. The high on/off switching ratio and high photocurrent/darkcurrent ratio allow such two-dimensional, passive photosensor matrices to be used for image sensing applications.

On the complexities of short range ferromagnetic exchange in a nitronyl nitroxide

Abstract The molecular solid state structure and properties of 2-(4-nitrophenyl)-4,4,5,5-tetramethy-4,5-dihydro-1H-imidazolyl-1-oxyl 3-oxide (1) were examined. The solid state structure of two ( α and β ) out of three phases, together with relevant short intermolecular contacts (to 3.60A) are presented. Solid state phase transformations, as detected by differential scanning calorimetry and electron spin resonance are described. As reported previously by Awaga, one phase of 1 exhibits short range intermolecular ferromagnetic interaction of the unpaired electrons. Explanation for the structural origin of this electron-electron interaction is hampered by the fact that 1 forms structurally completely different solid state phases, yet each phases shows qualitatively the same temperature dependance of the magnetic susceptibility but different electron spin resonance linewidths and linewidth behavior as a function of temperature.

Polymer-polymer rectifying heterojunction based on poly(3,4-dicyanothiophene) and MEH-PPV

Thin, homogeneous films of a high electron affinity (n-dopable) polymer, poly(3,4-dicyanothiophene) (PDCTh), were prepared by in-situ thermal polymerization of 2,5-diiodo-3,4-dicyanothiophene on substrates. As a result of the presence of two cyano substituents on the thiophene backbone, PDCTh is an electronegative polymer, with the LUMO at ca. 3.6 eV and the HOMO at ca. 6.7 eV. We demonstrate the fabrication of polymer-polymer rectifying heterojunctions using PDCTh and poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) polymer layers on ITO-glass. Rectification ratios in the current voltage characteristics of these devices exceed 10.3 This device exhibits a photovoltaic effect with a dc sensitivity at -3 V reverse bias of 4 × 10-4 A/W or quantum yield of 0.1% electrons/photon.

Observation of a photoinduced electron transfer from a conducting polymer (MEHPPV) onto C60

Abstract Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto Buckminsterfullerene, C 60 , is reported. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C 60 anion. Since the photoluminescence in the conducting polymer is quenched by interaction with C 60 , the data imply that charge transfer from the excited state occurs on a picosecond time scale.

Fibres of poly(methoxy-2-ethyl-hexyloxy) phenylenevinylene prepared from the soluble, fully conjugated polymer

Abstract Electrically conductive fibres were prepared of poly (methoxy-2-ethyl-hexyloxyphenylenevinylene), which is soluble in its conjugated form, employing a simple wet-spinning process, without the need of a precursor-conversion reaction. The enhanced processibility of the polymer, however, comes at the expense of a reduction in both mechanical and electrical properties in comparison with the unsubstituted parent polymer.

Short range ferromagnetic exchange in 1,5,6-triphenyl-3-(4-nitrophenyl)-verdazyl

Abstract 1,5,6-Triphenyl-3-(4-nitrophenyl)-verdazyl has been synthesized and characterized by X-ray diffraction, magnetic susceptibility, ESR, and other spectroscopic methods, as well as elemental analysis. The product χ spin T was found to increase below 100 K to reach a maximum (0.529 emu.K/mole) at 3.9 K. From 100 to 30 K the spin susceptibility follows the Curie-Weiss law with C = 0.382 and Θ = +1.6 K, indicating ferromagnetic interactions between S = 1 2 species. A strong deviation from the Curie-Weiss law was observed below 30 K where the susceptibility reaches a maximum χ spin = 0.241 emu/mole at 2.15 K. Using the spin density rule proposed by McConnell, an attempt was made to correlate the unusual magnetic properties of this solid with its structure.

High-performance polymer photovoltaic cells and photodetectors

Polymer photovoltaic cells and photodetectors have passed their infancy and become mature technologies. The energy conversion efficiency of polymer photovoltaic cells have been improved to over 4.1% (500 nm, 10 mW/cm2). Such high efficiency polymer photovoltaic cells are promising for many applications including e-papers, e-books and smart- windows. The development of polymer photodetectors is even faster. The performance parameters have been improved to the level meeting all specifications for practical applications. The polymer photodetectors are of high photosensitivity (approximately 0.2 - 0.3 A/Watt in visible and UV), low dark current (0.1 - 1 nA/cm2), large dynamic range (> 8 orders of magnitude), linear intensity dependence, low noise level and fast response time (to nanosecond time domain). These devices show long shelf and operation lives. The advantages of low manufacturing cost, large detection area, and easy hybridization and integration with other electronic or optical components make the polymer photodetectors promising for a variety of applications including chemical/biomedical analysis, full-color digital image sensing and high energy radiation detection.

Pleiadene Dimerization and Its Application to the Construction of Novel Carbon Networks

The synthesis of the anti- and syn-pleiadene dimers 2 has been carried out in solution and in the solid state from (6b-10b-dihydrobenzo(j)cyclobut(a)acenaph- thylene) (3). The structures of the anti- and syn-2 dimers were determined by X-ray crystallography and by NMR spectroscopy and mass spectrometry. An exhaustive thermal analysis of the two dimers has been carried out, establishing that the equilibrium for their interconversion occurs via pleiadene as an intermediate species. Vinyl-6b-10b-dihydrobenzo(j)cyclobut(a)acenaphthylene (5) has been synthesized and used as a pre-monomer for the preparation of poly(pleiadene) dimer through either thermal or photochemical polymerization.

A stable free radical as donor : A layer-structure organic pressure sensor

Abstract A novel charge transfer salt was prepared by the addition of TCNQ to the stable free radical, 1,3-diphenyl-1,2,4-benzotriazin-4-yl ( R •), resulting in (TCNQ) 5 ( R •) 2 . Analysis by x-ray revealed the crystal structure to be triclinic, P 1 , with a = 7.696(4) A, b=16.428(3) A, c=16.706(2) A, α=91.96(1)°, β=103.16(2)°, and γ=97.36(3)°, V=2035(1) A 3 , Z=2, D x =1.297 gcm −3 . Both variable temperature ESR and magnetic susceptibility measurements indicated the material behaved as a Curie paramagnet down to 20 K, where a transition to an antiferromagnetically ordered state occurred. The pressed pellet room temperature conductivity was 10 −4 S/cm. Single crystal conductivity measurements of the current parallel to the TCNQ layer indicated a strong pressure dependence of the conductivity; where ‘sigma’ increased two orders of magnitude at 2 kbar.

DECACYANOOCTATETRAENE DIANION : A MODEL SYSTEM FOR CYANOCARBON-BASED CONJUGATED POLYMERS

Abstract Decacyanooctatetraene (DCOT) has been prepared as its dianion and studied as a model compound for the prototypical n-dopable polymer poly(dicyanoacetylene) (PDCA).