PSS Dispersion Research Articles

Poly(thieno[3,4-b]thiophene)−Poly(styrene sulfonate): A Low Band Gap, Water Dispersible Conjugated Polymer

Herein we report the oxidative chemical polymerization of thieno[3,4-b]thiophene (T34bT) using several different oxidants including ferric sulfate, ammonium persulfate, and hydrogen peroxide in the presence of poly(styrenesulfonic acid) in water and properties of the resulting poly(thieno[3,4-b]thiophene)-poly(styrenesulfonic acid) (PT34bT-PSS) dispersion. The PT34bT-PSS is rendered a colloidal dispersion in water with a particle size diameter ranging between 180 and 220 nm depending on the oxidant used for polymerization. PT34bT-PSS films have band gaps of ca. 1 eV (1260 nm) as determined by the onset of the pi to pi transition from the vis-NIR spectrum with absorption maxima ranging from 1.4 eV (912 nm) to 1.7 eV (724 nm). The neutral and oxidized forms of PT34bT-PSS prepared from ferric sulfate dispersed in water were blue and lime green, respectively, whereas the neutral and oxidized forms of PT34bT-PSS prepared from ammonium persulfate and hydrogen peroxide were blue and blue-green, respectively. Spectral properties of the PT34bT-PSS dispersion can be tuned by the combination of oxidants. PT34bT-PSS films showed ca. 100% cation dominant ion transport behavior as determined by electrochemical gravimetry with each charge-discharge cycle and the doping level of the polymer was calculated to be 26%. Electrical conductivities for these polymers were found to be dependent on chemical oxidants used and varied from 10(-2) to 10(-4) S/cm.

Effect of the alkali metal content on the electronic properties of PEDOT:PSS

The effect of the sodium and cesium ion surface concentration on the electronic properties of spin-coated poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) films, known as PEDOT:PSS, has been studied by means of ultraviolet and X-ray photoelectron spectroscopy. The sodium and cesium concentration in the film has been varied by the addition of NaOH or CsOH to the PEDOT:PSS dispersion. Hydrogen ions of the acid PSSH are exchanged for sodium or cesium ions, resulting in the salt PSSNa or PSSCs without changing the oxidation state of PEDOT, i.e., without doping/dedoping the material. The work function changes from 5.1 to 4.0 eV with increasing alkali surface concentration. The ionization potential remains constant at 5.0 eV above 1 at% alkali metal content and coincides with the work function below 1 at%. Thus, the material changes from a semiconductor-like to a metal-like state.

Modification of PEDOT:PSS as hole injection layer in polymer LEDs

Poly(3,4-ethylenedioxythiophene):poly(styrenesulphonic acid) (PEDOT:PSS) is commonly used as an anode in polymer light-emitting diodes (PLED). We have studied the effect of the pH and Na+ ion concentration of the aqueous PEDOT:PSS dispersion on the bulk and surface properties of spincoated films by various techniques, including UV-vis-NIR optical absorbance spectrometry, Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoemission Spectroscopy (UPS). A pH increase by addition of NaOH modifies the PEDOT:PSS properties in a similar way as electrochemical dedoping: the IR absorbance decreases, the Raman peaks shift, sharpen and increase in intensity, and the work function decreases. Consequently, a barrier for hole injection is introduced for several classes of light-emitting polymers. We argue that the mechanism of the pH-effect is different from electrochemical dedoping, and originates from a change in the relative stability of polarons and bipolarons on the doped thiophene. The changes in the electronic properties of PEDOT:PSS point to the determining role of the counter-ion in the stabilisation of oxidised thiophene units. Polymer LEDs comprising Na+-rich, proton poor PEDOT:PSS can show lower lifetime and efficiency than the corresponding Na+-free, proton-rich devices. For light emitting polymers which suffer from the addition of sodium to the hole injecting PEDOT:PSS, the decreased lifetime hints at hole injection as limiting factor in the degradation of these PLEDs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

Films of poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS), prepared by coating the aqueous PEDOT–PSS dispersion and by coating a mixture of the PEDOT–PSS dispersion and different solvents, have been studied using four-point probe conductivity measurements, atomic force microscopy and photoelectron spectroscopy. The electrical conductivity of thin films of the second type (further on called PEDOT–PSS–solvents) was increased by a factor of about 600 as compared to films of the first type (further on called PEDOT–PSS–pristine). Morphological and physical changes occur in the polymer film due to the presence of the solvent mixture, the most striking being that the ratio of PEDOT-to-PSS in the surface region of the films is increased by a factor of ∼2–3. This increase of PEDOT at the surface indicates that the thickness of the insulating PSS ‘shell’ that surrounds the conducting PEDOT–PSS grains is reduced. The (partial) reduction of the excess PSS layer that surrounds the conducting PEDOT–PSS grains is proposed to lead to an increased and improved connectivity between such grains in the film and hence a higher conductivity. When PEDOT–PSS–solvents receives a post-coating heat treatment, the increased PEDOT-to-PSS ratio at the surface is maintained or even slightly improved, as is the increase in electrical conductivity, even though spectroscopy show that the solvent molecules are removed. This suggests that screening or doping by the solvents throughout the film are not likely to be the key mechanisms for the improved conductivity and supports our proposed mechanism of improved conductivity through improved connectivity between the conducting grains.

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.