1. Abstract
Although research on organic polymer thermoelectric materials has made significant progress over the past decade, the in-depth mechanism at the molecular level for nanoscale-thickness films remains unclear. This study proposes a strategy to significantly enhance the electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) ultrathin films by reducing their thickness:
- PEDOT:PSS nanofilms were prepared via the spin coating method, followed by subsequent HNO₃ post-treatment;
- It was found that reducing the film thickness significantly improved electrical conductivity: when the thickness decreased from 23 nm to 15 nm, the electrical conductivity increased sharply from 1772 S cm⁻¹ to 3059 S cm⁻¹;
- The core mechanism was revealed: reduced thickness led to a significant decrease in the content of insulating PSSH (polystyrenesulfonic acid), an increase in the oxidation level of PEDOT chains, the formation of a fibrous morphology with large conductive microdomains, and enhanced crystallinity of PEDOT.
This study provides insights for the preparation of high-performance polymer thermoelectric materials and clarifies the carrier transport mechanism of conductive polymer nanofilms.
2. Introduction
Globally, more than half of the primary energy from fossil fuels is unfortunately lost as waste heat. Effectively recycling this waste heat is crucial for addressing the global energy crisis. Thermoelectric technology enables direct interconversion between heat and electricity, offering a promising approach to alleviate the global energy crisis.
The performance of thermoelectric materials is evaluated by the dimensionless figure of merit ZT, defined by the formula: ZT = S²σT/κ, where:
- σ: Electrical Conductivity
- S: Seebeck Coefficient
- T: Absolute Temperature
- κ: Thermal Conductivity
High-performance thermoelectric materials require high σ, high S, and low κ. Since most organic polymer materials exhibit low κ, and in-plane κ of thin films is difficult to measure accurately, the term S²σ (known as Power Factor, PF) is usually used instead of ZT for performance evaluation.
As a widely studied conjugated polymer, PEDOT:PSS possesses advantages such as low resistance, high transparency in the visible region, excellent air stability, and good solution processability (aided by PSS). It has been applied in energy conversion and storage devices, including flexible thermoelectric devices, solar cells, and capacitors. However, pristine PEDOT:PSS films exhibit extremely low electrical conductivity (<1 S cm⁻¹), requiring modification via methods such as solvent treatment, nanoparticle doping, and acid post-treatment. Among these, strong acid treatment (e.g., H₂SO₄, HNO₃) is a relatively simple and efficient strategy; previous studies have reported that HNO₃ treatment can increase the electrical conductivity of PEDOT:PSS to a maximum of 4100 S cm⁻¹.
Existing studies have contradictory conclusions regarding the correlation between PEDOT:PSS film thickness and electrical conductivity (some suggest increased thickness enhances conductivity, while others report the opposite). Additionally, there is a lack of mechanistic research on ultrathin films thinner than 30 nm. This study fills this gap by systematically investigating the effect of thickness on the electrical conductivity of HNO₃-treated PEDOT:PSS ultrathin films and the underlying molecular mechanism.
3. Materials and Preparation
- Substrate Treatment: Commercial glass microscope slides were used as substrates, which were ultrasonically cleaned for 30 minutes in soap solution, acetone, isopropanol, and sodium hydroxide solution sequentially;
- PEDOT:PSS Solution: Heraeus Clevios PH1000 aqueous solution (1.0-1.3 wt% dispersion in water) was used, filtered through a syringe filter with a 0.45 μm pore-size PVDF membrane to remove large particles;
- Film Preparation: The PEDOT:PSS solution was deposited onto glass substrates via spin coating, followed by HNO₃ post-treatment, ultimately obtaining ultrathin films with thickness less than 30 nm.
4. Results and Discussion
All HNO₃-treated PEDOT:PSS samples were prepared under identical conditions, with a slight thickness variation of ±8 nm (caused by minor differences in droplet deposition position before spin coating). However, this variation did not affect the overall trend analysis. The key results are as follows:
- Correlation between Conductivity and Thickness: A slight reduction in ultrathin film thickness led to a significant increase in electrical conductivity. When the thickness decreased from 23 nm to 15 nm, the electrical conductivity increased by approximately 66.9% (from 1772 S cm⁻¹ to 3059 S cm⁻¹);
- Structural and Compositional Changes: Reduced thickness was accompanied by a decrease in the content of insulating PSSH, an increase in the oxidation degree of PEDOT chains, and the formation of fibrous conductive microdomains that facilitate carrier transport;
- Effect of Crystallinity: X-ray Diffraction (XRD) and Raman spectroscopy analyses showed that reduced thickness improved the crystallinity of PEDOT, further promoting carrier migration.
5. Conclusion
- HNO₃ treatment decreased the Seebeck coefficient (S) of PEDOT:PSS from 17.6 μV K⁻¹ (pristine) to 13-15 μV K⁻¹;
- The electrical conductivity increased from <1 S cm⁻¹ (pristine) to over 1700 S cm⁻¹ after HNO₃ treatment, and could be further significantly enhanced by reducing the ultrathin film thickness (up to 3059 S cm⁻¹);
- This study confirms that reducing the thickness of HNO₃-treated PEDOT:PSS ultrathin films is a simple and efficient strategy for conductivity regulation, providing key technical support for the development of high-performance flexible thermoelectric devices.
6. Acknowledgement
This study was supported by the following funds:
- National Natural Science Foundation of China (NSFC) (Grant No. 11934007);
- Science and Technology Innovation Committee Foundation of Shenzhen (Grant No. JCYJ20200109141205978);
- Outstanding Talents Training Fund of Shenzhen (Grant No. 202108).
The authors acknowledge the technical support provided by SUSTech Core Research Facilities.