One-step post-treatment boosts thermoelectric properties of PEDOT:PSS flexible thin films

doi:10.1016/j.jmst.2022.05.047

Abstract

Abstract:

Developing high-performance poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) significantly widens the practical applications of flexible organic thermoelectric devices, while the water-based co-solvent dopants and/or post-treatments are still rarely studied so far. Here, we develop a one-step post-treatment to improve the power factor of PEDOT:PSS films by using a water-based solution, which is composed of co-solvent (polar solvent dimethylacetamide (DMAC) and deionized water) and organic reducing agent L-ascorbic acid (LAA). The 80 vol.% DMAC solution significantly boosts the room-temperature electrical conductivity of the films from 5 to 964 S cm⁻¹, while the Seebeck coefficient can be further enhanced from 18.7 to 25 µV K⁻¹ by treating with 0.5 mol L⁻¹ LAA, contributing to a significantly improved power factor of 55.3 µW m⁻¹ K⁻². The boosted electrical conductivity is ascribed to the separated PEDOT and PSS phases triggered by the high dielectric constant and polarity of DMAC; while the improved Seebeck coefficient is attributed to the reduced oxidation degree of PEDOT from the reducing agent LAA, both confirmed by the comprehensive structural and morphological characterizations. Furthermore, a maximum power factor of 64.4 µW m⁻¹ K⁻² can be achieved at 360 K and the observed temperature-dependent electrical transport behavior can be well explained by the Mott variable range hopping model. Besides, a flexible thermoelectric device, assembled by the as-fabricated PEDOT:PSS films, exhibits a maximum output power of ∼23 nW at a temperature difference of 25 K, indicating the potential for applying to low-grade wearable electronics.

Key words: Thermoelectric, PEDOT:PSS, Post-treatment, Dimethylacetamide, L-ascorbic acid

Xingyu Liu, Xiao-Lei Shi, Li Zhang, Wei-Di Liu, Yanling Yang, Zhi-Gang Chen. One-step post-treatment boosts thermoelectric properties of PEDOT:PSS flexible thin films[J]. J. Mater. Sci. Technol., 2023, 132: 81-89.

Figures/Tables 7

Fig. 1. (a) Schematic diagram of fabricating PEDOT:PSS film by post-treating with a rationally designed solution composed of polar solvent dimethylacetamide (DMAC), deionized water, and organic reducing agent L-ascorbic acid (LAA). The inset optical image shows that the as-fabricated film exhibits high flexibility. (b) Schematic illustration of the effects on PEDOT:PSS structures by post-treatment with H2O, DMAC, and LAA. The as-achieved power factor hits ～65 μW m?1 K?2 at 360 K post-treated with 80 vol.% DMAC solution and 0.5 mol L?1 LAA.

Fig. 2. (a) Thermoelectric properties including σ, S, and S2σ of PEDOT:PSS films by different volume percentages of DMAC in DMAC/H2O solution. (b) σ, S, and S2σ by different LAA concentrations in DMAC/H2O/LAA solution. (c) Temperature-dependent σ, S, and S2σ by 80 vol.% DMAC solution and 0.5 mol L?1 LAA. (d) Comparison of S2σ between this work and previous literature [26,37,[44], [45], [46]]. (e) ln σ as a function of T?1/4 with a fitting line and (f) S as a function of T1/2 with fitting lines to the Mott variable range hopping (VRH) model of pristine film and the film treated by DMAC/0.5 LAA solutions.

Fig. 3. (a) X-ray photoelectron spectroscopy (XPS) and (b) X-ray diffraction (XRD) results of PEDOT:PSS films without and with post-treating with 80 vol.% DMAC solution and 0.5 mol L?1 LAA. The inset in (a) shows enlarged XPS results near 164 eV. (c) Raman spectrum before and after post-treating with DMAC/H2O solutions with different volume percentages of DMAC. (d) Raman spectrum before and after post-treating with DMAC/H2O/LAA solutions with different concentrations of LAA. The volume percentages of DMAC are all 80%.

Fig. 4. The scanning electron microscopy (SEM) cross-sectional images of (a) pristine PEDOT:PSS film, (b) PEDOT:PSS film post-treated with 100% DMAC, (c) PEDOT:PSS film post-treated with 80 vol.% DMAC solution, and (d) PEDOT:PSS films post-treated with 80 vol.% DMAC solution and 0.5 mol L?1 LAA.

Fig. 5. Atomic force microscopy (AFM) images of PEDOT:PSS thin films before and after post-treatments. Height images: (a) pristine PEDOT:PSS film, (b) PEDOT:PSS film post-treated with 80 vol.% DMAC solution, and (c) PEDOT:PSS film post-treated with 80 vol.% DMAC solution and 0.5 mol L?1 LAA. (d-f) Corresponding phase images and (g-i) three-dimensional (3D) images. The sizes of all images are 1 μm × 1 μm.

Fig. 6. (a) A thermoelectric device composed of as-fabricated PEDOT:PSS films with the polyimide (PI) substrate. (b, c) Wearability of the as-designed device. (d) Open-circuit voltage Voc as a function of temperature difference ΔT. (e) Voc and output power P versus current I at a ΔT of 25 K.

Table 1. Device performance (power generation) of PEDOT:PSS-based films.

Material	Couple number	∆T (K)	Open-circuit voltage, V_oc (mV)	Output power, P (nW)	Output power density (μW cm⁻²)	Refs.
PEDOT:PSS/Te	9	40	13.4	47.7	57.2	[57]
PEDOT:PSS/Te/Cu₇Te₄	8	39.1	31.2	94.7	39.5	[58]
p-type: PEDOT:PSS; n-type: Cu_0.6Ni_0.4	4	30	10.4	121.08	98.115	[36]
p-type: PEDOT:PSS; n-type: nickel	144	65	260	46	-	[59]
PEDOT:PSS	14	12	2.9	-	∼1	[38]
PEDOT:PSS	5	25	2	-	-	[60]
PEDOT:PSS	16	8	4.6	-	-	[55]
PEDOT:PSS		30	20	10.5 μW	1.21	[40]
PEDOT:PSS	8	25	3.4	23	13.31	This work

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