We report the electrical transport properties of a hybrid organic/inorganic diode device consisting of a layer of
an organic ferrimagnetic semiconductor V[TCNE]x (x~2, TCNE: tetracyanoethylene; TC ~ 400 K, EG ~ 0.5 eV, σ~ 10-2
S/cm) and a GaAs/AlGaAs p-i-n diode. Comparison with a control excluding the V[TCNE]x~2 reveals that the addition of
the V[TCNE]x~2 layer shifts the turn-on voltage and ideality of the diode in accordance with bulk V[TCNE]x~2 properties.
This result has implications for the use of inorganic systems as probes of spin physics in organic and molecular systems.
The field effect devices prepared using active channels fabricated from doped conducting polymers, such as PEDOT/PSS (poly-(3,4-ethyldioxythiophene/poly(styrene sulfonic acid)), polypyrrole/Cl-, and polyaniline/Cl- with various dopants are reported. Normally in the "on" state, the devices have a sharp switch off at a small gate voltage. The current ratio Ion/Ioff can exceed 104 at room temperature. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage. The activation energy, T0, increases even for on/off ratios as small as 1.07 demonstrating that the electric field effect has changed the bulk charge transport in the active channel despite the expected screening due to mobile charge carriers. Based on these transport and optical studies we propose that the conducting polymer is near the metal-insulator transition and that the field effect in the transistor is related with electric field modulating this transition in the region underneath of gate through field induced ion motion. The time dynamics of current with the gate modulation strongly supports our proposal. Application of the Doped Polymer Field Effect Devices (DPFEDs) to form circuit components has been demonstrated.
We present photo- and electro-luminescence, and hole mobility measurements of carbazole (Cz) substituted polyacetylene (PA-Cz) and poly(diphenylacetylene) (PDPA-Cz). The photoluminescence (PL) of the interband transition in PA-Cz thin film is quenched. PDPA-Cz shows a green-yellow emission with a PL efficiency about 30 percent of the interband transition. The hole mobility of PDPA-Cz is determined to be approximately 10 7 cm2/Vs and the ionization energy is 5.3 eV. PDPA-Cz forms robust thin films and is thermally stable up to 470 degrees C. For a structure of ITO/PDPA-Cz/Alq(tris(8-quinolinolato) aluminum)/MgAg EL quantum efficiency over 1 percent is achieved.
We present the absorption and emission properties of the phenylene-based copolymers, poly(pyridyl vinylene-phenylene vinylene) (PPyVPV), poly(thienylene p-phenylene) (PTpP) and poly(dithienylene p-phenylene) (PDTpP), which incorporate `straps' to bridge the 2 and 5 positions of the phenyl rings. The absorption and luminescence properties of the PPyVPV polymers are morphology dependent. The absorption maxima are at approximately 3.0 eV, with films having absorption edges that tail into the IR. The photoluminescence (PL) spectra are red shifted for films compared to solutions and powders, with the presence of smaller red shifted photoluminescence for the PPyVPV polymers with straps. A low lying absorption maximum occurs at 2.7 eV for both solution and film forms of PTpP. The (PL) spectra also are similar with features at 2.1 eV and 2.3 eV assigned to the 0-1 and 0-0 vibronic transitions, respectively. Solution and film photoluminescence excitation (PLE) profiles qualitatively follow the absorption spectra. Two low energy absorption maxima are seen for PDTpP at 2.8 eV and 3.2 eV with a single feature observed at 2.3 eV for the PL. The PLE spectra in PDTpP also follow the corresponding absorption features in both solution and film samples. Time resolved PL studies (ps to ns) show that there is smaller spectral shift for the films of polymers with straps, showing the importance of reduced aggregation. Steady state photoinduced absorption studies (ms) indicate the roles of both polarons and triplet excitons in these systems.
Recently there have been reports on color variable light emitting devices. Here we present a new approach to such devices based on conjugated polymers. the device consists of a blend of pyridine-phenylene and thiophene-phenylene based copolymers sandwiched between two redox polymers: emeraldine base form of polyaniline and sulfonated polyaniline (SPAN). ITO and Al are used as electrodes. The devices work under either polarity of driving voltage with different colors of light being emitted from different locations, red from emitting polymer/SPAN interface under forward bias and green from bulk of the emitting polymers under reverse bias. Electroluminescence of the devices peak at 550 nm with a shoulder at 585 nm under reverse bias while they show a single peak at 665 nm under forward bias. The relative fast time response allows the rapid switch of colors and AC operation.
Recent x-band optically detected magnetic resonance studies of spin 1/2 polaron and spin 1 triplet exciton dynamics in ladder-type poly(p-phenylene) and poly(p-phenylene ethynylene) (PPE) films and solutions are described. The results suggest that the polaron resonance is due to interchain polaron pairs with lifetimes 10 microsecond(s) <EQ (tau) <EQ 4 ms range. The absence of delayed fluorescence at such times as well as other observations lead to the conclusion that the mechanism responsible for this resonance is the nonradiative quenching of singlet excitons by the polarons, whose population decreases at the field-for- resonance. The full-field triplet pattern is 700 - 1200 G wide, consistent with a triplet localized on a phenylene ring. In frozen toluene solutions of PPE, the amplitudes (Delta) I/I of both the polaron and half-field triplet resonances decrease similarly with concentration, and increase similarly with laser power P as P(alpha ), where 1/3 <EQ (alpha) <EQ 1/2. Hence, the triplet resonance is believed to be due to triplet intrinsically trapped and stabilized at similar interchain sites. Possible scenarios which link the processes yielding the resonances are also discussed.
We report the fabrication and properties of three-, four-, and five-layer electroluminescent devices fabricated from light emissive N-based heterocyclic novel polymeric derivatives of PPP and PPV with which they are isoelectronic. They include poly(pyridine vinylene), (PPyV), and poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene), (PPV.PPyV). Some of the devices operate in both forward and reverse bias modes thus enabling operation in an ac mode. One type of device has the general construction: M/I/polymer/I/ITO where M equals Cu or Al, I equals polyaniline (emeraldine base, EB) or poly(3-hexylthiophene), (P3HT), and polymer equals PPV.PPyV. Under low frequency ac (sinusoidal) driving, light pulses with twice the driving frequency were observed in a device where M equals Al or Cu, I equals EB and polymer equals PPV.PPyV; and in a device where M equals Al, I equals P3HT and polymer equals PPV.PPyV. In the latter device the electroluminescence spectrum in the reverse bias mode differed from that in the forward bias mode. It was also shown that blends of PPyV in Nylon 6,6 exhibit a lower operating voltage than the pure polymer.
Symmetrically configured ac light-emitting (SCALE) devices based on conjugated polymers utilizing indium-tin oxide (ITO) and aluminum as electrodes have been demonstrated recently. Here we report the fabrication of SCALE devices using a more stable high workfunction metal, such as gold, as a charge (both electron and hole) injection electrode. Also, a variation of such devices in which the electroluminescent polymer, instead of being separated from the insulating polymer, is dispersed in the insulating polymer to form a unified emitter-insulator is reported. These devices emit light in both forward and reverse dc bias with symmetric current- voltage characteristics. Under low frequency ac (sinusoidal) driving voltage, light pulses with double the driving frequency are observed. A model is proposed to account for the device operation.
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