Leif A. A. Pettersson

Modeling photocurrent action spectra of photovoltaic devices based on organic thin films

We have modeled experimental short-circuit photocurrent action spectra of poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophene) (PEOPT)/fullerene (C60) thin film heterojunction photovoltaic devices. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the PEOPT/C60 interface. The internal optical electric field distribution inside the devices was calculated with the use of complex indices of refraction and layer thickness of the materials as determined by spectroscopic ellipsometry. Contributions to the photocurrent from optical absorption in polymer and fullerene layers were both necessary to model the experimental photocurrent action spectra. We obtained values for the exciton diffusion range of 4.7 and 7.7 nm for PEOPT and C60, respectively. The calculated internal optical electric field distribution and resulting photocurrent action spectra were used in or...

Optical anisotropy in thin films of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)

Abstract Anisotropic optical constants spectrum of spin-coated thin films of poly(3,4-ethylenedioxythiophene)–poly(4-styrenesulfonate) (PEDOT–PSS) from 200 to 1700 nm were determined using variable-angle spectroscopic ellipsometry and polarized intensity transmission data together with absorption spectroscopy. PEDOT–PSS was found to be very anisotropic, uniaxial with the optic axis parallel to the surface normal. The result is in good agreement with results obtained of chemically polymerized PEDOT layers doped with toluenesulfonate. By adding sorbitol to the PEDOT–PSS dispersion before spin-coating, layers with a higher conductivity were obtained. A detailed study was made of the optical response of these layers in comparison to the PEDOT–PSS prepared from dispersions without sorbitol. The optical anisotropy is important to consider when using PEDOT–PSS in optoelectronic devices, such as polymer light-emitting diodes and photovoltaic devices.

Structure of thin films of poly(3,4-ethylenedioxythiophene)

Abstract Thin films (50–400 nm) of tosylate-doped poly(3,4-ethylene-dioxy-thiophene) (PEDOT) on various substrates have been studied by grazing incidence X-ray diffraction, using synchrotron radiation. The material was found to be highly anisotropic. This is correlated with a strong anisotropy observed in its optical and electronic properties. The crystalline order is limited, and evidence is found for a paracrystalline structure. A structural model is presented, which corroborates with the optical anisotropy. The effect of heating the material to 200 °C was studied. Unlike the situation for other substituted polythiophenes, heating PEDOT increases the crystalline order.

Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids

Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the char...

Spectroscopic ellipsometry studies of the optical properties of doped poly(3,4-ethylenedioxythiophene): an anisotropic metal

Abstract Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer with high electrical conductivity in the doped state, good thermal and chemical stability and fast electrochemical switching. The material is used as an antistatic layer, and has potential for use as a (transparent) electrode material and for electrochromic applications. Thin films ( μ m) of doped PEDOT have been studied with variable angle spectroscopic ellipsometry. Reflectance and transmission measurements have also been performed. From ellipsometry and transmission data, the optical properties and thickness of the PEDOT layers were extracted. By simultaneously fitting optical functions for multiple samples using a recently discussed 4×4 matrix formalism, the analysis reveals that the material is optically anisotropic and of uniaxial character with the optic axis normal to the film surface. The uniaxial anisotropy of PEDOT films reveals an entirely different appearance for the ordinary and extraordinary indices of refraction with the former showing metallic character. The metallic state behavior agrees well with the high electrical conductivity measured for this material at different doping levels. From the measured optical properties and from supplementary optical and structural analysis, a preferential structural orientation of the polymer chains can be deduced.

Interference phenomenon determines the color in an organic light emitting diode

We report on electroluminescence from two-layer organic diodes made of poly(3-methyl-4-octylthiophene) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,2,4-oxadiazole films between electrodes of indium tin oxide and Ca/Al. The diodes emitted light in the green-blue range; the electroluminescence spectra varied between diodes with different thicknesses of the polymer and molecular layers. The optical phenomena were simulated with a model accounting for interference effects; simulated results showed that the electroluminescence from the organic diode can be due neither to luminescence of the polymer nor of the molecular layer. These model simulations, together with electrochemical measurements, can be interpreted as evidence for an indirect optical transition at the polymer/molecule interface that only occurs in a strong electric field. We label this transition an electroplex.

Determination of the emission zone in a single-layer polymer light-emitting diode through optical measurements

We study the emission zone in a single-layer polymer light-emitting diode. The emission zone is found by studying the angular distribution of the electroluminescence. The emission is modeled by acc ...

Quantum efficiency of exciton-to-charge generation in organic photovoltaic devices

We present an analysis of the internal monochromatic quantum efficiency of photovoltaic devices based on polymer and polymer/fullerene thin films. A quantum efficiency of exciton-to-charge generation is defined as the external monochromatic quantum efficiency normalized to the absorption in the active materials of the device. An upper limit of the efficiency can be determined, and results show that much of the light is absorbed in photoactive layers of the device, whereas only a fraction of the generated excitons is converted to charge carriers and can be collected as photocurrent.

Conductivity of de-doped poly(3,4-ethylenedioxythiophene)

Abstract The conductivity of chemically and electrochemically de-doped poly(3,4-ethylenedioxythiophene) (PEDOT) has been investigated in situ. We observe a decrease in the conductivity by 4–5 orders of magnitude. The change of conductivity is correlated to the change of electronic structure. We obtain the dielectric function of the polymer by spectroscopic ellipsometry and note that anisotropy is observed in both doped and neutral states.

Anisotropic optical properties of doped poly(3,4-ethylenedioxythiophene)

Anisotropic optical properties of doped poly(3,4-ethylenedioxythiophene) (PEDOT) thin films have been determined by using variable-angle spectroscopic ellipsometry together with intensity reflectance and transmission spectrophotometry. The optical anisotropy of the PEDOT films are of uniaxial character with the optic axis normal to the film surface. The (ordinary) index of refraction in the plane shows a metallic state behavior while the out of plane (extraordinary) index of refraction shows a near dielectric character. The metallic state behavior in the surface plane agrees well with the high electrical conductivity measured for this material. The anisotropic optical properties of doped PEDOT thin films in terms of the complex indices of refraction together with results from grazing incidence X-ray diffraction studies allow us to deduce a preferential orientation of the polymer chains.