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We report electroabsorption measurements of the built-in electrostatic potential in metal/C60-doped polymer/metal structures to investigate chemical potential pinning due to equilibrium electron transfer from a metal contact to the electron acceptor energy level of C60 molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and C60-doped poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were measured. For undoped MEH-PPV, which has an energy gap of about 2.4 eV, the maximum built-in potential is about 2.1 eV, whereas for C60-doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the C60 molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of C60 and decreases the built-in potential of the structure. From the systematic dependence of the built-in potential on the metal work function we find that the electron acceptor energy level of C60 in MEH-PPV is about 1.7 eV above the hole polaron energy level of MEH-PPV.

We present measurements of the modulation of the near absorption edge optical properties of single carrier diodes made from a soluble poly(p-phenylene vinylene) derivative. Pulsed electrical excitation was used to acquire the optical characteristics for average injected charge densities up to 1018 cm−3. For injected carrier densities below 5×1017 cm−3, the modulation of the optical properties are dominated by electroabsorption effects; at higher densities, the effects of the injected charge predominate. We find that there is no significant absorption by the injected charge below the optical absorption threshold and that there is substantial bleaching of the lowest energy optical transitions at high carrier density. These results imply that charge carrier absorption in this polymer does not limit the performance of organic light-emitting diodes and diode lasers.

We present measured and calculated current–voltage (I–V) characteristics of diodes fabricated using a soluble poly(p-phenylene vinylene) derivative. Steady-state and pulsed electrical excitation were used to acquire the I–V characteristics for current densities from 1×10−3 to 1.3×103 A/cm2. Hole current is predominant in the diode. The I–V characteristics were fit using a device model that assumes an electric field-dependent hole mobility of the Poole–Frenkel form that is independent of the charge carrier density. The measured and calculated I–V characteristics are in good agreement over the full range of current density. The maximum electric field and carrier density is about 4×106 V/cm and 1×1018 cm−3, respectively. These results demonstrate that an electric field-dependent mobility, without carrier density dependence, provides an accurate description of hole transport in this polymer over this range of field and carrier density.

We present capacitance–voltage and current–voltage measurements of polymer light-emitting electrochemical cells and compare these results with steady state device model calculations. The capacitance–voltage characteristic is used to assess the formation and structure of the electrochemical junction in the device. The cell capacitance and current both increase sharply above a threshold voltage as the bias is increased. The threshold voltage for the rapid increase in capacitance is lower than that for the increase in current, indicating that the electrochemical junction begins to form prior to significant current flow. The electrochemical junction width, estimated from the capacitance measurements, is about 15 nm at a current density of 0.1 A/cm2. The steady state device model calculations are in reasonable agreement with these observations.

We present device model calculations of the current–voltage (I–V) characteristics of organic diodes and compare them with measurements of structures fabricated using MEH-PPV. The structures are designed so that all of the current is injected from one contact. The I–V characteristics are considered as a function of the Schottky energy barrier to charge injection from the contact. Experimentally, the Schottky barrier is varied from essentially zero to more than 1 eV by using different metal contacts. A consistent description of the device I–V characteristics is obtained as the Schottky barrier is varied from small values, less than about 0.4 eV, where the current flow is space-charge limited to larger values where it is contact limited.

We report electrical impedance measurements of polymer light‐emitting diodes employing the soluble, conjugated polymer poly[2‐methoxy, 5‐(2’‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene] (MEH‐PPV) as the light‐emitting layer. The diode structures were metal‐polymer‐metal structures utilizing thin gold as the transparent, positive contact, and calcium as the negative contact. The devices were fabricated using undoped, polymer active layers ∼40 nm thick. The polymer light‐emitting diodes are accurately modeled as a resistor and capacitor in parallel for frequencies from 100 Hz to 1 MHz and for bias conditions from reverse bias to forward current densities of 0.1 A/cm2. The diode capacitance as a function of bias voltage is qualitatively different from conventional Schottky or p‐n junction diodes; in reverse bias, the polymer layer is fully depleted and the capacitance is independent of bias; at small forward bias, traps are charged near the metallic contacts and the capacitance increases; under large forward bias, with significant electron and hole injection, the traps are neutralized and the capacitance decreases. From the magnitude of the initial increase in capacitance with forward bias the trap density is estimated to be only a few times 1016 cm−3.

We report polarized optical transmission and reflection measurements which are used to determine the orientation of polymer chains in spin‐cast thin films of soluble, electroluminescent polymers. We find that the polymer chains lie primarily in the plane of the film. This result has three important implications for polymer light emitting diodes: the electroluminescence is preferentially emitted propagating perpendicular to the polymer film; the relevant dielectric and electrical transport properties are those perpendicular to the polymer chains; and large area, uniform devices can be produced by spin casting.

The authors report femotosecond transient-absorption studies of a five-ring oligomer of polyphenylenevinylene (PPV) prepared in two different forms: as solid-state films and dilute solutions. Both types of samples exhibit a photoinduced absorption (PA) band with dynamics which closely match those of the stimulated emission (SE), demonstrating unambiguously that these features originate from the same species, namely from intrachain singlet excitons. Photo-chemical degradation of the solid-state samples is demonstrated to dramatically shorten the SE dynamics above a moderate incident pump fluence, whereupon the dynamics of the SE and the long-wavelength PA no longer coincide. In contrast to solutions, solid-state films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the efficient formation of non-emissive inter-chain excitons. Correlations in the subpicosecond dynamics of the two PA features, as well as the pump intensity-dependence provide strong evidence that the formation of inter-chain excitons is mediated by intrachain two-exciton states. At high pump levels, the authors see a clear indication of interaction between excited states also in dilute solutions. This is manifested as a superlinear pump-dependence and shortening of the decay dynamics of the SE. They attribute this behavior to the formation of biexcitons resulting from coherent interaction between two excitons on a single chain.

An electroactive polymer with a bandgap of 1.7 eV has been obtained from the monomer 1,3-di(2-thienyl)benzo[c]thiophene.

New measurements of electrical conductivity along the $b$ axis of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ) are combined with published results to provide a comprehensive summary including approximately 600 samples studied at 18 different laboratories. The magnitudes of these measured conductivities do not necessitate the assumption of superconducting fluctuations or any other collective state in which the conductivity exceeds the limitations of single-particle scattering. Since an adequate theory of the limitations of single-particle scattering for TTF-TCNQ does not exist at present, experiment alone does not rule out the possibility that collective effects may somewhat enhance or suppress the conductivity.