The rapid development of novel organic technologies has led to significant applications of the organic electronic devices such as light-emitting diodes, solar cells, and field-effect transistors. There is a great need for conducting polymers with high conductivity and transparency to act as the charge transport layer or electrical interconnect in organic devices. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS), well-known as the most remarkable conducting polymer, has this role owing to its good film-forming properties, high transparency, tunable conductivity, and excellent thermal stability. In this Review, various of interesting physical and chemical approaches that can effectively improve the electrical conductivity of PEDOT:PSS are summarized, focusing especially on the mechanism of the conductivity enhancement as well as applications of PEDOT:PSS films. Prospects for future research efforts are also provided. It is expected that PEDOT:PSS films with high conductivity and transparency could be the focus of future organic electronic materials breakthroughs.

Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most successful conducting polymer in terms of practical application. It possesses many unique properties, such as good film forming ability by versatile fabrication techniques, superior optical transparency in visible light range, high electrical conductivity, intrinsically high work function and good physical and chemical stability in air. PEDOT:PSS has wide applications in energy conversion and storage devices. This review summarizes its applications in organic solar cells, dye-sensitized solar cells, supercapacitors, fuel cells, thermoelectric devices and stretchable devices. Approaches to enhance the material/device performances are highlighted.

Review on application of PEDOTs and PEDOT:PSS in energy conversion and storage devices

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations.

Flexible thermoelectric materials and devices

A biomaterial scaffold is one of the key factors for successful tissue engineering. In recent years, an increasing tendency has been observed toward the combination of scaffolds and biomolecules, e.g. growth factors and therapeutic genes, to achieve bioactive scaffolds, which not only provide physical support but also express biological signals to modulate tissue regeneration. Huge efforts have been made on the exploration of strategies to prepare bioactive scaffolds. Within the past five years, electrospun scaffolds have gained an exponentially increasing popularity in this area because of their ultrathin fiber diameter and large surface-volume ratio, which is favored for biomolecule delivery. This paper reviews current techniques that can be used to prepare bioactive electrospun scaffolds, including physical adsorption, blend electrospinning, coaxial electrospinning, and covalent immobilization. In addition, this paper also analyzes the existing challenges (i.e., protein instability, low gene transfection efficiency, and difficulties in accurate kinetics prediction) to achieve biomolecule release from electrospun scaffolds, which necessitate further research to fully exploit the biomedical applications of these bioactive scaffolds.

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Bioactive electrospun scaffolds delivering growth factors and genes for tissue engineering applications.

We report that a simple chemical dedoping treatment of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanofilms enhances the thermoelectric properties of the polymer nanofilms. The dedoping process was done by over-coating a mixture of dimethyl sulfoxide (DMSO) and hydrazine (HZ), a strong chemical reducing agent, onto the PEDOT:PSS nanofilms. This additional step led to the removal of excess PSS chains and the formation of neutral states of PEDOT chains, resulting in an improvement in the Seebeck coefficient, from 30 μV K−1 to 142 μV K−1, and a decrease in the electrical conductivity from 726 S cm−1 to 2 S cm−1. By controlling the concentration of HZ, we obtained an optimized power factor of 112 μW m−1 K−2 at 0.0175 wt% of HZ in DMSO at room temperature. The corresponding electrical conductivity and Seebeck coefficient under optimized conditions were 578 S cm−1 and 67 μV K−1, respectively. We expect that this simple dedoping process can be applied to general thermoelectric nanofilms based on chemically doped polymers in order to enhance the power factor.

Enhanced thermoelectric properties of PEDOT:PSS nanofilms by a chemical dedoping process

Sequential doping and dedoping increased the conductivity and optimized the oxidation level of transparent and flexible poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS) films, resulting in an improvement in the thermoelectric figure of merit ZT. The electrical conductivity (σ) increased from 970 to 1260 S cm−1 and the power factor from 66.5 to 70.7 μW mK−2 at the optimum concentration of the chemical dopant p-toluenesulfonic acid monohydrate (TSA). Then, the doped PEDOT:PSS films were treated with hydrazine/DMSO solutions with different hydrazine concentrations to precisely control the oxidation level. During the hydrazine/DMSO treatment (dedoping), σ of the films continuously decreased from 1647 to 783 S cm−1 due to a decrease in the carrier concentration, whereas the Seebeck coefficient (S) steeply increased from 28 to 49.3 μV K−1 at the optimum oxidation level. A power factor of 318.4 μW mK−2 (σ = 1310 S cm−1, S = 49.3 μV K−1), the highest among all existing thermoelectric nanofilms, was achieved while maintaining polymer film flexibility and transparency (88.3% of optical transmittance). In addition, the thermal conductivity (κ) of the PEDOT:PSS films decreased from 0.38 to 0.30 W mK−1 upon removal of PSS. At the lowest κ value, a high ZT value of 0.31 was achieved at room temperature.

Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency

The thermoelectric performance of poly(3,4-ethylenedioxythiophene) complexed with a poly(4-styrenesulfonic acid) (PEDOT:PSS) film was enhanced by a three-step process. First, ultrafiltration was applied to remove non-complexed PSS from PEDOT:PSS grains. The ultrafiltration treatment leads to an improvement in the power factor (S2σ) from 54.9 to 83.2 μW m−1 K−2 and a decrease in the thermal conductivity (κ) from 0.330 to 0.170 W m−1 K−1. Second, the fully de-doped PEDOT:PSS solution was prepared by addition of hydrazine, which acted as a reducing agent. Third, the two PEDOT:PSS solutions (ultrafiltered and hydrazine-treated) were mixed in different ratios by simple ultrasonication blending without any post-treatment. The optimal S2σ value of 115.5 μW m−1 K−2 (∼10 times higher than that of the pristine PEDOT:PSS film) and ZT value of 0.2 were recorded at 33 wt% of the hydrazine-treated, ultrafiltered PEDOT:PSS in the blend.

Novel solution-processable, dedoped semiconductors for application in thermoelectric devices

Ionically crosslinked Ad/chitosan nanocomplexes processed by electrospinning for targeted cancer gene therapy.

Tae-Joon Park, Yeon Jae Jung, Sung-Wook Choi, Hongkwan Park, Hyungsup Kim, Eunkyoung Kim, Sang Hyun Lee*,4, and Jung Hyun Kim*,1 Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea Department of Biotechnology, The Catholic University of Korea, Gyeonggi-do 420-743, Korea Department of Textile Engineering, Konkuk University, Seoul 143-701, Korea Department of Microbial Engineering, Konkuk University, Seoul 143-701, Korea

Hongkwan Park

Papers

Enhanced thermoelectric properties of PEDOT:PSS nanofilms by a chemical dedoping process

Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency

Novel solution-processable, dedoped semiconductors for application in thermoelectric devices

Ionically crosslinked Ad/chitosan nanocomplexes processed by electrospinning for targeted cancer gene therapy.

Native Chitosan/Cellulose Composite Fibers from an Ionic Liquid via Electrospinning