Thermal degradation mechanisms of PEDOT:PSS

Open Journal of Organic Polymer Materials, 2012

PEDOT:PSS buffer layers have been processed with the standard annealing step used for organic solar cells device applications. The d.c. conductivity σ as a function of temperature for two heating rates under He and atmospheric air was studied. Moreover, the stability of the conductivity was investigated at different temperatures and environments vs time.

Organic Electronics, 2014

By simultaneously measuring the Seebeck coefficient and the conductivity in differently processed PEDOT:PSS films, fundamental understanding is gained on how commonly used processing methods improve the conductivity of PEDOT:PSS. Use of a high boiling solvent (HBS) enhances the conductivity by 3 orders of magnitude, as is well-known. Simultaneously, the Seebeck coefficient S remains largely unaffected, which is shown to imply that the conductivity is improved by enhanced connectivity between PEDOT-rich filaments within the film, rather than by improved conductivity of the separate PEDOT filaments. Post-treatment of PEDOT:PSS films by washing with H 2 SO 4 leads to a similarly enhanced conductivity and a significant reduction in the layer thickness. This reduction strikingly corresponds to the initial PSS ratio in the PEDOT:PSS films, which suggests removal and replacement of PSS in PEDOT:PSS by HSO 4 À or SO 4 2À after washing. Like for the HBS treatment, this improves the connectivity between PEDOT filaments. Depending on whether the H 2 SO 4 treatment is or is not preceded by an HBS treatment also the intra-filament transport is affected. We show that by characterization of S and r it is possible to obtain more fundamental understanding of the effects of processing on the (thermo)electrical characteristics of PEDOT:PSS.

The conductivity difference Δσ between two similar PEDOT:PSS films, 120 nm thick, heated by 5 and 15 K/min from 80 to 440 K, the one under inert He and the other under ambient air gives an approximate measure of the influence of oxygen and moisture on the conductivity vs temperature T. The resulting curves Δσ = f(T) exhibit three different regions: at temperatures from 80 to 145 K for heating rate 5 K/min and from 80 to 200 K for 15 K/min, an intense degradation of the sample under ambient atmosphere was revealed by the abrupt increase of the difference of the conductivities between He and atmospheric air. At intermediate temperatures from 145 to 380 K for 5 K/min and 200 to 400 K for 15 K/min, the conductivity difference remains basically constant indicating a stabilization of the damage produced by oxygen and moisture. Finally, for temperatures higher than 380 to 400 K the degradation increases again. An explanation of this behavior is proposed based on the hydrophilic character of the PSS and the destructive role of oxygen at high temperatures. Moreover, the isotherms σ = σ(t), where t is the heating time, at 443 K, under inert He and under atmospheric air for one hour verify the significant role of oxygen and moisture on the electrical conductivity. Under He the conductivity increases monotoni-cally with t, but under ambient air factors the conductivity competes with others decreasing it producing a maximum at about t = 10 min.

Journal of Materials Chemistry A, 2013

In this work, the thermoelectric (TE) properties of poly (3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films at room temperature are studied. Different methods have been applied for tuning the TE properties: 1 st addition of polar solvent, dimethyl sulfoxide (DMSO), into the PEDOT:PSS solution; 2 nd post-treatment of thin films with a mixture of DMSO and ionic liquid, 1-ethyl-3methylimidazolium tetrafluoroborate (EMIMBF 4 ). It is verified that DMSO post-treatment is more efficient than DMSO addition in improving the electrical conductivity with a trivial change in the Seebeck coefficient. The power factor is increased up to 30.1 mW mK À2 for the film with DMSO posttreatment, while the optimized power factor by DMSO addition is 18.2 mW mK À2 . It is shown that both DMSO addition and post-treatment induce morphological changes: an interconnected network of elongated PEDOT grains is generated, leading to higher electrical conductivity. In contrast, for those films post-treated in the presence of EMIMBF 4 , an interconnected network of short and circular PEDOT grains with increased polaron density is created, resulting in the improvement in the Seebeck coefficient and a concomitant compromise in the electrical conductivity. An optimized power factor of 38.46 mW mK À2 is achieved at 50 vol% of EMIMBF 4 , which is the highest reported so far for PEDOT:PSS thin films to our knowledge. Assuming a thermal conductivity of 0.17 W mK À1 , the corresponding ZT is 0.068 at 300 K. These results demonstrate that post-treatment is a promising approach to enhance the TE properties of PEDOT:PSS thin films. Furthermore, ionic liquid, EMIMBF 4 , shows the potential for tuning the TE properties of PEDOT:PSS thin films via a more environmentally benign process.

2007

Employing the unique mechanical, electronic, and optical properties of the conjugated organic and polymer materials several technological and commercial applications have been developed, such as sensors, memories, solar cells and light-emitting diodes (LEDs). In this respect, the central theme of this thesis is the electrical conductivity and mechanisms of charge transport in PEDOT:PSS, a polymer blend that consists of a conducting poly(3,4-ethylenedioxythiophene) polycation (PEDOT) and a poly(styrenesulfonate) polyanion (PSS). PEDOT:PSS is omnipresent as electrode material in ‘plastic electronics’ applications mentioned above. Although the conductivity of PEDOT:PSS can vary by several orders of magnitude, depending on the method by which it is processed into a thin film, the reason for this behavior is essentially unknown. This thesis describes a detailed study of the anisotropic charge transport of PEDOT:PSS and its correlation with the morphology, the shape, and the dimension of ...

2010

A significant increase of the electrical conductivity of PEDOT:PSS films, brought about by the addition of dimethyl sulfate (DMS, (CH3)2SO4), while preserving the films’ excellent flexibility and visible-light transparency, is reported. The electrical and morphological properties of the films were studied as a function of DMS concentration. At an optimal concentration of around 1:25 (DMS to PEDOT:PSS), the conductivity of the films is enhanced by a factor on the order of 1880 times that of pristine PEDOT:PSS films. Extensive spectroscopic measurements using absorbance, Raman, and FTIR techniques, as well as structural characterization by AFM microscopy, were performed. These measurements support the idea that the mechanism responsible for the conductivity enhancement is the partial replacement of the PSS− segments by SO4−2 anionic sulfates when a small amount of DMS is added to a PEDOT:PSS solution. This mechanism is associated with an increase of doping, and this doping can be understood in the following manner: due to that the SO3− ions of the PSS segment only carry one negative charge, it is more probable for them to create polaronic states, whereas the SO4−2 ions are double charged, increasing the possibility of creating bipolaron carriers in the PEDOT backbone. In this way, the partial replacement of the PSS− segments by SO4−2 ions increases the bipolaron population by an ion exchange process, and, as a consequence, the doping level is increased.

Advanced Materials, 2007

Study of the effect of DMSO concentration on the thickness of the PSS insulating barrier in PEDOT: PSS thin films, 2010

One of the most used secondary dopants in thin film processing of PEDOT:PSS is dimethyl sulfoxide (DMSO). In this work, we present results that explain, from the point of view of impedance spectroscopy, the mechanism of the increase in the conductivity observed on films based on PEDOT:PSS. The results obtained with this technique, combined with others such as AFM, and Raman and UV–vis–NIR spectroscopies, clearly show that there is a thinning of the insulating barrier of PSS surrounding conductive grains of PEDOT. It is shown that the thickness of the insulating barrier is related strongly and inversely with the onset frequency of AC conductivity. However, this is not the only existing effect, because for values beyond the optimal concentration of DMSO, we observe a decrease in the conductivity related with an increase of the separation of the PEDOT grains. The AC measurements and the AFM images also show the clear interplay between the increase of the PEDOT average grain size and the separation between them.

Journal of Polymer Science Part B: Polymer Physics, 2012

We have investigated the electrical transport properties of poly(3,4-ethylenedioxythiophen)/poly(4-styrene-sulfonate) (PEDOT:PSS) with PEDOT-to-PSS ratios from 1:1 to 1:30. By combining impedance spectroscopy with thermoelectric measurements, we are able to independently determine the variation of electrical conductivity and charge carrier density with PSS content. We find the charge carrier density to be independent of the PSS content. Using a generalized effective media theory, we show that the electrical conductivity in PEDOT:PSS can be understood as percolation between sites of highly conducting PEDOT:PSS complexes with a conductivity of 2.3 (Xcm) À1 in a matrix of excess PSS with a low conductivity of 10 À3 (X cm) À1 . In addition to the transport properties, the thermoelectric power factors and Seebeck coefficients have been determined. V

Organic Electronics, 2020

Conducting polymers such as poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) have attracted extensive attention for thermoelectric applications due to its solution-processability, mechanical flexibility, low thermal conductivity and tunable electrical conductivity. This work demonstrated a sequential posttreatment method with formamide and sodium formaldehyde sulfoxylate (SFS) to significantly improve thermoelectric properties of PEDOT:PSS film, particularly its Seebeck coefficients. Water-soluble and non-toxic SFS is an ionic-type reducing agent, which is used as an environmentally benign chemical reagent for the first time to treat PEDOT:PSS. First, the PEDOT:PSS film was soaked with formamide (F-PEDOT:PSS), and then was treated with SFS solutions with various molar concentrations (SFS-F-PEDOT). The Seebeck coefficient of F-PEDOT:PSS film treated with 100 mM SFS was steeply increased from 14.8 to 51.8 μV/K primarily due to the proper control of the doping level and the carrier concentration although the corresponding electrical conductivity of the film was reduced from 2,873 to 693 S/cm. The resulting power factor reached its maximum value of 185.8 μW/K 2 m, which was approximately three times that of formamide-treated film (F-PEDOT:PSS: 63.7 μW/K 2 m). The crossplane thermal conductivity of the pristine PEDOT:PSS film was dropped from 0.59 W/mK for the pristine film to 0.29 W/mK for the SFS-F-PEDOT:PSS film, leading to an estimated ZT value of in the range of ~0.07-~0.14 at 300 K. Also, the stability of SFS-F-PEDOT:PSS film was examined under a harsh environment, and results showed that the film retained its electrical conductivity and Seebeck coefficient of more than 85% after continuous exposure under 70 � C and a humidity of 75% RH for 480 h, revealing the excellent long-term environmental stability.