In-Plane Thermoelectric Characterisation of PEDOT:PSS Films with Inkjet-Printed Test Structures ^†

Abstract

A rapid screening method to identify suitable candidate inks for printed electronics applications is necessary. Herein, we investigate the in-plane thermoelectric properties of PEDOT:PSS for energy harvesting applications on human skin using silver nanoparticle inkjet-printed test structures. The in-plane electrical and thermal conductivity are measured. The Seebeck coefficient, ZT figure of merit, and power factor are consequently determined. PEDOT:PSS films resulted in low-efficiency thermoelectric properties at 293 K to 313 K and demonstrated a correlation between film thickness and in-plane thermoelectric properties. This study demonstrates that the test structures enable generalisable characterisation of thin-film inkjet-printable materials for thermoelectric purposes.

1. Introduction

Ultra-low-grade waste heat and low-grade waste heat sources have gained much interest because of their potential in the heating sector, forming approximately 60% of the lost primary energy [1]. Waste heat recovery (WHR) technologies promise major prospects and opportunities in improving heat energy utilisation [2]. Solid-state devices like thermoelectric generators (TEGs) can enhance energy recovery methods by harvesting energy from heat sources with low temperature gradients relative to the environmental temperature [3]. TEGs are advantageous in that they have no moving parts, produce no vibrations and are relatively environmentally friendly [4], and they can be used on human skin for energy harvesting applications. The efficiency of thermoelectric (TE) energy conversion in TEGs is governed by the dimensionless figure of merit [5],

$$ZT={\displaystyle \frac{S^2\sigma T}{k}}$$

(1)

where $\sigma$ is the electrical conductivity, S is the Seebeck coefficient, k is thermal conductivity and T is the average temperature. Highly efficient TEGs comprise materials with high S and $\sigma$, and low k values. The TE performance of the TEG is characterised by the material power factor,

$$PF=S^2\sigma$$

(2)

and the TE improvement thereof leads to a high ZT value. Test structures were developed for the in-plane TE characterisation of PEDOT:PSS thin films for research and development of flexible TEG applications.

2. Materials and Methods

Sample Preparation

The test structures were formed with silver nanoparticle (AgNP) ink (Harima NPS-JL). The AgNP ink was initially prepared by mixing a 3 mL sample with the Corning variable-speed LSE vortex mixer for 30 min. A Fujifilm Dimatix 10 pL drop volume cartridge was filled with 1.5 mL of the prepared ink and thereafter inkjet-printed on a flexible polyimide (PI) substrate (Polyphonic XF-613). The PI was pre-treated with isopropanol alcohol and air-dried before inkjet printing. The inkjet printer settings used for the Dimatix DMP-2850 are indicated in

Table 1. Dimatix DMP-2850 single-nozzle printing parameters.

Printer Parameter	Setting
Waveform	Dimatix model fluid waveform
Jetting voltage	28 V
Cartridge temperature	28 °C
Platen temperature	50 °C
Meniscus setpoint	4 (relative to water)
Cartridge height	1 mm
Drop spacing	20 µm

. The chosen waveform, jetting voltage settings and drop spacing provided effective drops on the Dimatix drop watcher. The inkjet printer platen temperature was set to 50 °C to control ink spreading on the PI substrate. The resultant test structures were cured at 353 K for 12 h and thereafter tested for electrical continuity. PEDOT:PSS ink (1.3 wt % dispersion in water, conductive grade) was purchased from Sigma Aldrich and prepared similar to the AgNP ink, with a vortex mixer for 30 min. The PEDOT:PSS thin films were formed by direct ink deposition onto the flexible test structure samples and further cured at 353 K for 30 min. The various thin-film sample thicknesses ranged from 10 µm to 25 µm.

TE characterisation of AgNP test structures (grey) in Figure 1 was used for in-plane characterisation of the PEDOT:PSS thin films (blue). The probe pads for all test structures were kept to 2.5 mm². Figure 1a shows the van der Pauw resistivity four-point probe measurements [6]. Figure 1b shows a resistive heater–sensor structure for measuring the thermal conductivity and Figure 1c is used to determine the Seebeck coefficient of the thin film.

3. Results

3.1. Sample Profile Characterisation

The samples were analysed to determine the film’s dimensional profile with the Olympus DSX1000 microscope (Olympus Corporation, Tokyo, Japan) including the DSX10-XLOB40X objective (Olympus Corporation, Tokyo, Japan) in dark field (DF) mode. Figure 1d shows a microscopic image of the four-point probe test structure with the directly deposited PEDOT:PPS thin film. The mean surface area measured from the 2D profiles of five PEDOT:PSS samples is 130 µm² and the mean height is 20 µm.

3.2. Measurement Results

The measured results are summarised in

Table 2. Mean thermoelectric characteristics.

Thermoelectric Parameters	Results
Electrical conductivity ($\sigma$)	$4.95\mathrm{S}/\mathrm{cm}$
Thermal conductivity (k)	$0.136\mathrm{W}/\mathrm{mK}$
Seebeck coefficient (S)	$27/\mathrm{K}$
Power factor (PF)	$0.36/{\mathrm{mK}}^2$
Figure of merit (ZT)	0.08

. The four-point probe test structure in Figure 1a was used to measure the PEDOT:PSS film surface resistance and thereafter, the dimensional characteristics and surface resistance results were used to determine the mean sheet resistance and electrical conductivity ($\sigma$). The test structure in Figure 1b was used to measure thermal conductivity by applying 1 VDC across one end of the AgNP heating electrodes, and the current and temperature gradients between the heater and sensor electrodes were monitored over time. By using the Joules heating effect [7], the mean in-plane thermal conductivity was determined. The Seebeck coefficient was determined by subjecting the opposing AgNP electrodes in Figure 1c to a temperature gradient of 22 K and measuring the subsequent thermal voltage across the PEDOT:PSS film [8]. The ZT value and PF were thus calculated from the measured TE results.

4. Discussion

The results of this study demonstrate that the PEDOT:PSS thin films can be rapidly characterised for research and development of flexible TEG applications on human skin. The PEDOT:PSS ink characterised here resulted in low efficiency as the in-plane electrical conductivity and Seebeck coefficient are low compared to enhanced PEDOT:PSS inks [9]. This led to a low power factor performance and subsequently a low ZT figure of merit. Thermal conductivity remained intrinsically low as only the in-plane thermal conductivity was considered in this study. Electrical conductivity and thermal conductivity were inversely proportional to film thickness while the Seebeck coefficient remained relatively constant with the increase in film thickness. The electrical conductivity can be further improved with the addition of a suitable solvent to the PEDOT:PSS ink [10], and thereafter characterised using the proposed method. The measurement accuracy can be improved by inkjet printing the PEDOT:PSS thin films with nanometer thicknesses. This is better suited for monitoring thin-film thermal energy transfer, charge carrier transport and thus thermal properties [11]. Further investigation is required to produce methods for the inclusion of cross-plane characterisation of thin films. The proposed test structures are valid for lab environment production and demonstrate manufacturing readiness level 4 (MRL-4) compliance. Full-rate production implementation requires large-scale printing methods [12] for a production-relevant environment and may require additional measurement technique research. To reach MRL-5, print quality and management strategies of the test structures [13] must be considered. This study demonstrates that conductive inkjet-printable polymers like PEDOT:PSS can be deposited on flexible substrates such as thin films and characterised using the proposed AgNP test structures for rapid research and development ink screening for low-temperature flexible printed electronic applications.