Saeed Masoumia,
Kuanysh Zhussupbekov
b,
Nadezda Prochukhan
c,
Michael A. Morris
c and
Amir Pakdel
*a
aDepartment of Mechanical, Manufacturing, and Biomedical Engineering, Trinity College Dublin, The University of Dublin, D02PN40 Dublin, Ireland. E-mail: pakdela@tcd.ie
bSchool of Physics, CRANN and AMBER Research Centres, Trinity College Dublin, The University of Dublin, D02PN40 Dublin, Ireland
cSchool of Chemistry, CRANN and AMBER Research Centres, Trinity College Dublin, The University of Dublin, D02PN40 Dublin, Ireland
First published on 30th July 2024
The demand for sustainable power sources for wearable electronic devices has spurred exploration into novel energy harvesting technologies. Among these, flexible thermoelectric (TE) materials and generators (TEGs) show promise for transforming body heat into electrical power. For this purpose, organic and polymeric semiconducting materials, such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), have been at the forefront of research recently due to their flexibility, nontoxicity, easy processability, high electrical conductivity, and low thermal conductivity. However, hybridization with inorganic materials, optimization of doping levels, and structural modifications to enhance their TE conversion efficiency are still ongoing areas of investigation, and a comprehensive study encompassing all these approaches is lacking. This research presents the first comprehensive and encyclopedic study of the TE properties of PEDOT:PSS thin films hybridized with Bi0.5Sb1.5Te3 particles to explore the relationship between particle size/concentration, chemical post-treatment strategies, and the resulting microstructural, morphological, and TE properties. Initially, PEDOT:PSS was hybridized with Bi0.5Sb1.5Te3 particles with a wide range of size (0.3–1.4 μm) and concentration (5–60 wt%), and the microstructure and TE properties of the resulting composite films were systematically studied. The composite with 40 wt% Bi0.5Sb1.5Te3 particles exhibited the highest power factor at room temperature, was chosen as the optimal sample, and underwent various single and sequential chemical post-treatments using secondary dopants (DMSO, EG, H2SO4) and a dedoping agent (NaOH). While single post-treatment with 0.5 M H2SO4 could increase the power factor up to 240 times compared to the pristine composite, a sequential pos-treatment with 0.5 M H2SO4 and 0.1 M NaOH resulted in an 864-fold increase in the power factor at room temperature. Furthermore, the temperature-dependent electrical conductivity and Seebeck coefficient of the composite thin films were explored in each case (between 300 K and 375 K) to determine the dominant charge transport mechanism. It was revealed that the charge carrier transport mechanism was three-dimensional variable range hopping in pristine composite samples; however, metallic conduction was observed in the samples after sequential chemical post-treatment. The findings of this paper make it a thorough reference for understanding the TE properties of inorganic/polymeric composites, and offer a detailed perspective on strategies to optimize their power factor for applications in flexible TEGs.
Over the past two decades, significant advancements have been made in the performance of inorganic TE materials, including chalcogenides, skutterudite, half-Heusler, Zintl phases, and oxides. Recently, state-of-the-art TE materials such as GeTe,8 SnSe,9 and Cu2Se10 have demonstrated promising results, with ZT values exceeding 2.4. However, ensuring consistent reproducibility of these outcomes remains a challenge, and integrating these new materials effectively into practical devices is still difficult.11 For near-room-temperature applications, Bi2Te3 alloys have been the most promising materials, offering high ZT values for both n- and p-type TE materials. By alloying Bi2Te3 with Sb2Te3 and Bi2Se3, optimal electrical conductivity and Seebeck coefficient levels can be achieved, effectively reducing lattice thermal conductivity.12 The most frequently used p- and n-type compositions are Bi0.5Sb1.5Te313 and Bi2Te2.7Se0.3,14 respectively. However, the scarcity of tellurium (Te) element has emerged as a challenge; thus, alternative alloys and compounds such as Mg3Sb2/MgAgSb15 and Mg3Bi216 have gained much attention as new Te-free high-ZT materials. Nevertheless, the application of such inorganic TE materials in flexible TEGs is hindered by high costs, complex fabrication processes, and mechanical rigidity.17–19
An alternate strategy is to use organic semiconducting materials in TEGs, which offer flexibility, intrinsically low thermal conductivity, but a moderate electrical conductivity and Seebeck coefficient.20 These organic TE materials include conducting polymers, coordination polymers, carbon-based nanomaterials, small molecules, and single molecules.21 Among them, conducting polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have demonstrated superior TE performance at room temperature, especially when hybridized with inorganic TE materials.22,23 Hybridization enhances the electrical conductivity and Seebeck coefficient of PEDOT:PSS by increasing the charge carrier concentration and filtering low-energy charge carriers at the interfaces, respectively. Examples include incorporation of various concentrations of Bi2Te324–26 and Bi0.4Te3Sb1.627 microparticles, Bi0.5Sb1.5Te3 nanosheets,28,29 and Bi2Te3 nanowires30 into the PEDOT:PSS matrix. However, there is a lack of a comprehensive study on the size effect of these inorganic particles on the TE properties of the composites.
Another strategy to enhance the electrical conductivity of PEDOT:PSS is by secondary doping through a post-treatment with chemical agents. These include (a) organic polar solvents, e.g. dimethyl sulfoxide (DMSO), dimethylacetamide (DMAC), and ethylene glycol (EG), (b) acids, e.g. sulfuric acid (H2SO4),31 and (c) small molecules, e.g. 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO–OH).32 Such post-treatment processes increase the electrical conductivity due to segregation and removal of excess PSS particles, as well as conformational changes in PEDOT.31 Alternatively, enhancement of the Seebeck coefficient in PEDOT:PSS films can be achieved by chemical dedoping and lowering the charge carrier concentration through the transition of PEDOT chains from bipolaron to polaron and polaron to neutral states.21 The common reducing agents for this purpose are NaOH, hydrazine, sodium borohydride (NaBH4), L-ascorbic acid (LAA), and ammonium formate (AF).33
Wu et al.34 reported an increase in the electrical conductivity of PEDOT:PSS fibres to 4464 S cm−1 via post-treating with concentrated H2SO4, which partially removed excessive PSS, leading to a PF of 80.8 μW mK−2. Zhang et al.35 prepared PEDOT:PSS/single-walled carbon nanotube (SWCNT) composites and optimized the TE properties through pre-treatment with DMSO and post-treatment with NaBH4. As a result, a high σ of 1718 S cm−1 and a large S of 49 μV K−1 were obtained. Lim et al.36 developed a PEDOT:PSS/Bi0.5Sb1.5Te3 nanorod composite through a two-step reduction process, followed by a DMSO post-treatment, and achieved a PF of 308 μW mK−2 at room temperature. Kim et al.37 prepared a PEDOT:PSS/Bi2Te3 nanowire composite and after modifying the interfacial energy barrier through polar solvent vapor annealing (PSVA) achieved a PF of 223 μW mK−2 at room temperature. Wang et al.38 optimized interfacial carrier transports in a PEDOT:PSS/Bi0.5Sb1.5Te3 composite by coating the Bi0.5Sb1.5Te3 fillers with a highly conductive CuTe layer, followed by DMSO and H2SO4 post-treatments, resulting in a PF of 312 μW mK−2 at room temperature. However, in each of these studies a composite with a single concentration of inorganic particles was prepared, and the effects of filler concentration and particle size were not investigated.
The most effective approach to improve both the electrical conductivity and the Seebeck coefficient in PEDOT:PSS is sequential post-treatment, which involves applying both secondary doping and chemical dedoping treatments sequentially to achieve a combined effect.39 Luo et al.40 reported a two-step post-treatment, in which a PEDOT:PSS film was first treated by a polar solvent such as DMSO, and then reduced by a reducing agent such as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). Similarly, Fan et al.39 improved the TE properties of PEDOT:PSS thin films through sequential post-treatments with H2SO4 and NaOH. In some cases, sequential post-treatment involving both secondary doping and chemical dedoping agents can be conducted in a one-step process by mixing the chemical solutions. For instance, Liu et al.41 proposed a single-step post-treatment method to enhance the PF of PEDOT:PSS films. They utilized a water-based solution comprising DMAC, deionized water, and LAA. Employing an 80 vol% DMAC solution notably increased the room-temperature electrical conductivity of the films from 5 to 964 S cm−1. Additionally, treatment with 0.5 mol L−1 LAA further elevated the Seebeck coefficient from 18.7 to 25 μV K−1, resulting in a substantially improved PF of 55.3 μW mK−2. However, these studies used PEDOT:PSS thin films without any inorganic fillers. Consequently, there is a lack of comprehensive research on investigating the effect of inorganic filler concentration and particle size followed by sequential post-treatment on the TE properties of composite thin films.
Generally, organic TE materials have inherently low thermal conductivity, typically falling between 0.1 W m−1 K−1 and 1 W m−1 K−1.42 Similarly, the thermal conductivity of conducting polymers ranges between 0.03 W m−1 K−1 and 0.6 W m−1 K−1.43,44 In particular, PEDOT:PSS thin films exhibit thermal conductivity in the range of 0.2–0.4 W m−1 K−1 due to the presence of defects such as polymer chain ends, entanglement, different orientations, voids, and impurities.40,45–49 The thermal conductivity of conducting polymers is predominantly governed by phonon transport, and these defects act as phonon scattering sites for heat transfer.40,45–49 Consequently, the thermal conductivity of PEDOT:PSS is inherently low and much less affected by doping/dedoping processes in comparison with electrical conductivity. Therefore, the power factor has become the dominant measure for TE performance of such materials in the literature.50 In this regard, the main research focus in the TE field has been the enhancing of both σ and S to maximize the power factor.
In this work, a thorough study on the microstructural and TE properties of PEDOT:PSS/Bi0.5Sb1.5Te3 thin film composites was performed, where the thin films had a wide range of filler size and concentration, and underwent various single and sequential post-treatment processes. Bi0.5Sb1.5Te3 (hereon referred to as BST) particles of nano to micro size were prepared by ball milling and sequential centrifugation. Then, the effect of BST particle concentration and size on the surface morphology and TE properties of the pristine thin film composites were investigated. The maximum PF for pristine PEDOT:PSS/BST thin films was obtained in samples with an average BST particle size of 1.389 μm and a concentration of 40 wt%. Given that the Seebeck coefficient and electrical conductivity in a matrix/filler system can be explained using either series or parallel connected models, both models were applied to the pristine thin film composites in this study. The results demonstrated that the electrical properties of the samples closely align with the series connected model. The temperature dependency of the electrical conductivity and Seebeck coefficient in the pristine thin film composites suggested that their dominant charge charrier transport mechanism was 3D variable range hopping (VRH). Later, the composite films with the optimum particle size and concentration were subjected to single and sequential post-treatments with secondary dopants such as dimethyl sulfoxide (DMSO), ethylene glycol (EG), and sulfuric acid (H2SO4) and chemical dedoping agents such as sodium hydroxide (NaOH). Single post-treatment with 0.5 M H2SO4 boosted the power factor 240 times and the sequential post-treatment with 0.5 M H2SO4 and 0.1 M NaOH improved the PF 864 times as compared to the pristine thin film. The temperature dependency of electrical conductivity and Seebeck coefficient in post-treated samples showed a metallic transport in the charge carriers, in contrast with the 3D VRH mechanism in pristine samples.
Besides the single post-treatment procedures, sequential post-treatments were performed on the composite thin films by repeating the post-treatment with DMSO several times, as shown in the ESI,† Fig. S5a. This was repeated 1 to 4 times, using the optimum concentration of DMSO. The subsequent steps in the sequential post-treatment followed a procedure similar to the single post-treatment.
Then, another type of sequential post-treatment was performed on the thin film composites through post-treating with secondary dopants (DMSO, EG, and H2SO4) at an optimum concentration and number of post-treatments followed by post-treating with chemical dedoping agent (NaOH) at different concentrations from 0 M to 2 M, as described in the ESI,† Fig. S5b. The optimum concentration of each chemical agent achieved during single post-treatment was used for maximizing the power factor in sequential post-treatments as well. Also, the number of post-treatments with secondary dopants matched those with DMSO.
The absorption spectra of the films on a quartz substrate were obtained using a PerkinElmer Lambda 1050 spectrometer, ranging from 190 to 1600 nm, with a step size of 3 nm. A purpose-built automated system was used for measuring the Seebeck coefficient and electrical conductivity of the samples.51–53 Two independently controlled microheaters were used for providing the desired temperature difference along the samples. Two K-type thermocouples were placed on top of the sample at the cold and hot ends for measuring the temperatures. The electrical resistance and Seebeck voltage were measured by obtaining the current–voltage characteristic of the samples. An interface circuit and graphical user interface were developed to obtain the current–voltage characteristic, measure temperature differences along the sample, measure the Seebeck voltage and electrical resistance, and control the entire measurement process. Morphological and thermoelectric characterization was performed on all samples to enhance the validity of the experimental data in this study. The error bars in all graphs represent the range between minimum and maximum values of the data relative to the average value.
It should also be noted that many powdered materials, including the present BST powder, can form oxide layers during ball milling. Rinsing BST powders with an HCl solution can effectively remove such oxide layers;54 thus enhancing the power factor of PEDOT:PSS/BST composites. Zhang et al.54 performed X-ray photon spectroscopy (XPS) on BST surfaces before and after HCl rinsing. They concluded that the characteristic peaks of Bi–O and Te–O chemical states are completely removed after HCl rinsing and the peaks associated with BST remain at the same position. Consequently, the HCl rinsing process is very important before mixing BST powders with PEDOT:PSS. To investigate the effect of HCl rinsing in this work, two sets of BST powders (with an average particle size of 1.389 μm) were prepared – one rinsed with HCl and the other without any rinsing. Then, PEDOT:PSS/40 wt% BST thin films were fabricated from both sets of powders and their TE properties were investigated (Fig. S10k, l and S11a–c, ESI†).
It was confirmed that using HCl-rinsed BST in the composite decreased the electrical conductivity while increasing the Seebeck coefficient and power factor. This is important not only for the present research but also for other TE nanocomposite studies, as many other semiconducting or metallic nanomaterials often face surface oxidation during processing. In this study, the BST powders were rinsed with HCl before being added to the PEDOT:PSS matrix.
Fig. 2a–c display typical SEM micrographs of PEDOT:PSS thin films containing 0, 20, and 40 wt% BST particles, respectively. It is evident that particle distribution is homogenous in the composite films. Fig. 2d–f show the AFM surface topology of the films. More SEM and AFM images of PEDOT:PSS films containing 0–60 wt% BST are provided in the ESI,† Fig. S12 and S13. The variation of root mean square (RMS) roughness of the films with increasing BST particle concentration and size is shown in Fig. 2g and h. The PEDOT:PSS thin film exhibited a relatively smooth and uniform surface with no obvious phase separation in the structure and an RMS of 1.69 nm, consistent with values reported in the literature.55 The RMS roughness of the films increased gradually by adding 5, 10, and 20 wt% BST, but a sharp increase was observed thereafter. The increase in RMS roughness with BST concentration across the scanning area can be extrapolated to the entire thin film. Similarly, when the BST concentration was held constant, an increase in BST particle size resulted in higher RMS roughness. AFM step-height profiles were used to calculate the thickness of all the composite films, as shown in the ESI,† Fig. S14 and S15. There was not a considerable change in the average thickness of the composite films containing 5–60 wt% BST (i.e. 105–125 nm).
To investigate the effect of filler concentration on TE properties, PEDOT:PSS/BST composite thin films containing 0–60 wt% BST with an average particle size of 1.389 μm were fabricated via the procedure outlined in Section 2.3 (Fig. S10a–f, ESI†). The variation in electrical conductivity, Seebeck coefficient, and power factor of the films with BST concentration is illustrated in Fig. 3a. The electrical conductivity of the films increased with BST concentration from 0 wt% to 40 wt%, but it decreased afterwards. The Seebeck coefficient however decreased with BST concentration from 0 wt% to 10 wt%, and gradually increased afterwards. The maximum power factor of 0.05 μW m K−2 at 300 K was achieved for the PEDOT:PSS/40 wt% BST composite film.
To explore the effect of filler particle size on TE properties, PEDOT:PSS/40 wt% BST composite films containing BST powders with different average particle sizes were fabricated (Fig. S10g–j, ESI†). The variation in electrical conductivity, Seebeck coefficient, and power factor in the samples with BST particle size is illustrated in Fig. 3b. The electrical conductivity of the composite thin films decreased with BST particle size, while the Seebeck coefficient increased. The maximum power factor of 0.05 μW mK−2 at 300 K was achieved for the composite with an average BST particle size of 1.389 μm.
Using a simple modelling approach to evaluate the electrical properties of polymer nanocomposite, the Seebeck coefficient and electrical conductivity can be described as either series- or parallel-connected models as a basic guideline, supposing that interfacial interactions are negligible.38,54,56–58 The equations for series- and parallel-connected models for PEDOT:PSS (matrix: M) and BST (filler: F) can be written as:
σparallel = σM(1 − x) + σFx | (1) |
![]() | (2) |
![]() | (3) |
![]() | (4) |
The lower values of the Seebeck coefficient in experiment compared to the predicted line in Fig. 3d could be due to the numerous nanointerfaces that exist between the different materials.38,56 In fact, a potential barrier can exist between these materials, attributed to energetic mismatches at the ceramic/polymer interfaces, and leading to preferential filtering of low-energy charge carriers. Consequently, only high-energy carriers can effectively pass through the interface, resulting in an increase in the Seebeck coefficient. The band diagrams of BST and PEDOT:PSS before and after connection are illustrated in Fig. 3e and f. One of the most common ways to study the charge carrier transport characteristics of organic conjugated systems is to analyse the temperature-dependency of their electrical conductivity and Seebeck coefficient.59 In organic and polymeric materials, such as PEDOT:PSS, these properties can be typically described in the framework of Mott's variable range hopping (VRH) theory (instead of band transport used for ordered solids) as:60
![]() | (5) |
S ∝ Tn−1/n+1 | (6) |
The mechanism is further elucidated by T0, which characterizes the energy barrier for effective carrier hopping. The T0 values were derived from the linear fitting of lnσ versus T−1/4 for pristine PEDOT:PSS/BST thin film composites with various BST particle concentrations and sizes (Fig. 4e and f). The T0 value consistently increased with BST particle size but decreased with BST particle concentration until 10 wt%, after which it began to increase. The decrease in T0 correlates with enhanced electrical conductivity and reduced Seebeck coefficient values, indicating a lower energy barrier for charge carrier transport and an enlarged localization length in the films. On the other hand, the increase in T0 is associated with reduced electrical conductivity and enhanced Seebeck coefficient values, suggesting an elevated energy barrier for charge carrier transport. The presented results in Fig. 4e and f substantiate the electrical conductivity and Seebeck coefficient dependency with BST particle concentration and size shown in Fig. 3a and b.
In contrast to the above, post-treatment with NaOH (as a dedoping agent) led to a decrease in the electrical conductivity and an increase in the Seebeck coefficient in PEDOT:PSS films, as shown in Fig. 5d. For instance, post-treatment with a 5 M NaOH solution led to a four-fold decrease in the electrical conductivity, a nearly four-fold increase in the Seebeck coefficient, and achieving a power factor of 0.2 μW mK−2 at 300 K. Chemical dedoping has been widely proven to be an effective approach for Seebeck coefficient enhancement in PEDOT:PSS films by modifying the oxidation level of PEDOT and tuning the carrier concentration.69 During the dedoping process, PEDOT chains undergo transition through three states, as depicted in Fig. 6c. Highly oxidized PEDOT chains in the bipolaron state (PEDOT2+) are gradually dedoped to a less oxidized polaron state (PEDOT+), and eventually to a neutral state (PEDOT0) where PEDOT chains have no net charge.40 The overall impact of the dedoping process on PEDOT chains involved a trade-off between electrical conductivity and the Seebeck coefficient. This dynamic behavior is crucial for tailoring the thermoelectric properties of PEDOT:PSS films.
To analyse the effect of BST particles on the overall structure of PEDOT:PSS thin films, XRD characterization was performed on the BST powder, pristine PEDOT:PSS and PEDOT:PSS/40 wt% BST films, and 0.5 M H2SO4–0.1 M NaOH post-treated PEDOT:PSS and PEDOT:PSS/40 wt% BST films. As shown in Fig. S22a–d (ESI†), the diffraction peaks of the powder can be indexed as rhombohedral structured BST with a space group of Rm and the main diffraction peak at 2θ = 28.3° corresponding to the (015) lattice plane (JCPDS no. 49-1713). The pristine films however showed a broad peak at 2θ of ∼20°–32°, attributed to interchain planar stacking of PEDOT planes.38,68,70 The weak intensity and broad diffraction peak is due to the small thickness of the films and also moderate crystallinity, consistent with the literature.24,29,38 The XRD patterns did not show a considerable change after the addition of BST particles due to their low volume percentage as compared to the matrix material. However, literature suggests that micrometre-thick and free-standing composites of PEDOT:PSS/BST can show sharper diffraction peaks, indicating a higher crystallinity in composite samples due to the presence of BST particles.24,29,36,38 Moreover, to see the effect of sequential post-treatment with H2SO4 and NaOH on improved crystallinity of the films, analysis of weak peaks at low 2θ of ∼3° and 7° is required.38,68 For that purpose, SAXS and WAXS measurements were undertaken, however the results did not indicate a considerable change after the addition of BST particles and post-treatment (Fig. S22, ESI†), which could be due to insufficient resolution of the utilized equipment.
Thus far, the optimum concentration of every doping/dedoping chemical agent was determined to maximize the power factor of PEDOT:PSS/40 wt% BST composite thin films after a single post-treatment process. These concentrations were 40 vol%, 40 vol%, 0.5 M, and 0.5 M for DMSO, EG, H2SO4, and NaOH, respectively. Subsequently, the thin films underwent a sequential post-treatment process with 40 vol% DMSO, 1 to 4 rounds, as described in the ESI,† Fig. S5a. The variation in electrical conductivity, Seebeck coefficient, and power factor of these thin films at room temperature with the number of post-treatment process repetitions is presented in Fig. 7a. The electrical conductivity improved with increasing the number of treatment repetitions, while the Seebeck coefficient decreased slightly. The electrical conductivity improved nearly 40% after post-treatment with DMSO four times, whereas the Seebeck coefficient deceased by 18%. The maximum power factor of 18.5 μW mK−2 at 300 K was achieved after PEDOT:PSS/40 wt% BST thin films were sequentially post-treated three times with a 40 vol% solution of DMSO.
Likewise, three rounds of sequential post-treatment with other secondary dopants (i.e. EG and H2SO4) were concluded to yield the maximum power factor in the composite films, given the similar performance of all those secondary dopants.39 In this regard, a different set of sequential post-treatments was developed, including three rounds of treatment with secondary dopants (DMSO, EG, and H2SO4), followed by a final round of treatment with the chemical dedoping agent NaOH (Fig. S5b, ESI†). The effect of NaOH concentration on electrical conductivity, Seebeck coefficient, and power factor of composite thin films sequentially treated with 40 vol% DMSO–NaOH, 40 vol% EG–NaOH, and 0.5 M H2SO4–NaOH at room temperature is shown in Fig. 7b–d. In the case of DMSO–NaOH treatment, the electrical conductivity decreased with NaOH concentration while the Seebeck coefficient increased, leading to a maximum power factor of 31.1 μW mK−2 at 300 K after sequential treatment with 40 vol% DMSO and 0.1 M NaOH. This indicates a 1.7 times improvement in the power factor after sequential post-treatment with DMSO–NaOH, as compared to single post-treatment with DMSO. Similarly, the electrical conductivity in EG–NaOH and H2SO4–NaOH treated samples reduced with NaOH concentration while the Seebeck coefficient improved. Maximum power factors of 13.1 μW mK−2 and 43.2 μW mK−2 at 300 K were achieved after sequential post-treatment with EG–0.1 M NaOH and H2SO4–0.1 M NaOH. These values are 1.5 times and 2.5 times higher than those achieved after single post-treatment with EG and H2SO4, respectively. The combination of enhanced electrical conductivity arising from secondary dopants (DMSO, EG, and H2SO4) and improved Seebeck coefficient facilitated by NaOH dedoping led to the huge boost in the power factor in sequentially post-treated PEDOT:PSS/40 wt% BST thin films. The best TE performance was observed after three rounds of secondary doping with 0.5 M H2SO4 and one round of dedoping with 0.1 M NaOH, showcasing how the TE behavior in PEDOT:PSS/BST thin films can be the fine-tuned through strategic manipulation of chemical processes and careful choices of doping/dedoping agents.
The temperature dependence of electrical conductivity and Seebeck coefficient was measured for both single and sequentially post-treated PEDOT:PSS/40 wt% BST thin films in the temperature range of 300 K to 375 K to explore charge transport mechanisms in these thin films. The results, presented in Fig. 8 and Fig. S17 (ESI†), show that electrical conductivities of the thin films continuously decreased with temperature, while the Seebeck coefficients steadily increased. This trend was observed in all single and sequentially post-treated thin films, suggesting a metallic transport mechanism, characterized by a decrease in electrical conductivity with increasing temperature. This is contrary to the expected behavior in thermally assisted hopping mechanisms, where a simultaneous increase in both temperature and electrical conductivity is observed in the material. In degenerate semiconductors exhibiting metallic charge transport behavior, the relationship between the Seebeck coefficient and electrical conductivity (based on a parabolic band theory and an energy-independent scattering approximation71) is expressed as:72
![]() | (7) |
σ = neμ | (8) |
![]() | (9) |
![]() | ||
Fig. 8 The variation in electrical conductivity and Seebeck coefficient with temperature for PEDOT:PSS/40 wt% BST thin films after: single post-treatment with DMSO (a) and (e), H2SO4 (b) and (f), sequential post-treatment with DMSO–NaOH (c) and (g), and H2SO4–NaOH (d) and (h). The dashed lines in (e)–(h) represent the linear fitting of eqn (7) to the experimental data. The error bars on the experimental data were calculated based on three repetitions of measurements on the samples. |
In general, for lattice scattering, α is often taken as 1.5. As the temperature increases, the mobility μ decreases, which leads to a decrease in electrical conductivity. Eqn (7)–(9) characterize the metallic transport behaviour in samples with an increase in the Seebeck coefficient with temperature, accompanied by a decrease in electrical conductivity, which was observed in the single and sequentially post-treated thin films. Also, the linear fittings of Seebeck coefficient versus temperature, illustrated in Fig. 8e–h and Fig. S17d–f (ESI†), further confirm the metallic transport in these films. These results demonstrate that post-treatment of PEDOT:PSS/40 wt% BST changes the charge transport mechanism from 3D VRH to metallic transport.
Post-treatment of PEDOT:PSS thin films with secondary dopants, e.g. DMSO, EG, and H2SO4, can lead to phase separation between PEDOT and PSS. This process can induce partial removal of the insulating PSS shell and extend the PEDOT chains from the coiled benzenoid structure to a linear quinonoid structure, creating more ordered PEDOT-rich chains. This results in higher RMS roughness, demonstrating that excess PSS is effectively removed during the post-treatment process.35,40,46,47,49 The oriented PEDOT domains and PSS removal facilitate the transport of charge carriers due to reduced energy barriers, thereby boosting the electrical conductivity of PEDOT:PSS after secondary doping.35,40,46,47,49 It should be noted that secondary doping generally has a small effect on the doping level of PEDOT:PSS, hence the Seebeck coefficient. The characterization of the surface topology of post-treated thin films showed a direct correlation among secondary dopants, RMS roughness, and electrical conductivity. In this regard, AFM surface topology images of the composite thin films after single and sequential post-treatments with DMSO, EG, H2SO4, and NaOH are illustrated in the ESI,† Fig. S18 and S19, respectively. The corresponding surface roughness graphs are presented in Fig. 9, revealing the following points: (1) RMS roughness in single-treated samples increased by raising DMSO, EG, and H2SO4 concentrations (Fig. 9a–c), but remained nearly unchanged with NaOH concentration alterations (Fig. 9d), which is in good agreement with previous findings.39 (2) Repeating the sequential treatment with 40 vol% DMSO increased the RMS roughness of the films (Fig. 9e), providing evidence of successful secondary doping in the films. The secondary dopants remove the excessive PSS, causing phase separation in the conductive network of PEDOT, and resulting in a roughened surface. (3) The increase in NaOH concentration raised the roughness of 40 vol% DMSO–NaOH, 40 vol% EG–NaOH, and 0.5 M H2SO4–NaOH thin films (Fig. 9f–h). This could be due to a slight collision-induced agglomeration of the PEDOT chains stemming from the decreased repulsion force between their neighbouring neutralized chains.73 Further investigation into the morphological features of the post-treated films is detailed in the ESI,† Fig. S20 and S21. Moreover, a decrease in the thickness of the films was observed after post-treatment with secondary dopants and rinsing with DI water, as shown in the ESI,† Fig. S20 and S21. As an example, the thickness of post-treated films with DMSO decreased from 116 nm to 56 nm after post-treatment with 20 vol% DMSO. This reduction in thickness can be attributed to the depletion and removal of PSS.36–38,74 Additionally, the SEM micrographs of the thin films before and after rinsing in DI water (Fig. S4b and c, ESI†) did not indicate noticeable changes in the particle concentration and distribution after rinsing.
Ultraviolet-visible-near-infrared (UV-Vis-NIR) absorption spectra of PEDOT:PSS thin films can be used to characterize chemical changes induced by secondary doping and chemical dedoping.33 There are typically two absorption bands between wavelengths of 190 nm and 250 nm, originating from the aromatic rings of PSS.75,76 The reduction in PSS content through the secondary doping process can be indicated by a decreased intensity in these two bands.75,76 Glass substrates (microscope slides) have a cut-off transmission wavelength of 350 nm; thus, to analyse the UV spectra of the PEDOT:PSS-based composites, new samples were fabricated on quartz substrates that have high transmission even at 190 nm. The UV-Vis-NIR absorption spectra of the PEDOT:PSS/40 wt% BST thin films after single post-treatment with different concentrations of DMSO, EG, and H2SO4, and after sequential post-treatment with 40 vol% DMSO are presented in Fig. 10a–d, respectively. It is evident that the two absorption bands between wavelengths of 190 nm and 250 nm decreased after post-treatment with all the secondary dopants. Polar solvents such as H2SO4, DMSO, and EG (with dielectric constants of 84, 46.6, and 37.7 at 20 °C, respectively) induce a strong screening effect in the composite, diminishing Coulomb interactions between positively charged PEDOT and negatively charged PSS dopants. Among them, H2SO4 has a higher dielectric constant that can lead to a stronger screening effect, facilitating easier depletion of PSS after post-treatment compared to DMSO and EG. That is why Fig. 10c exhibits lower absorption intensities in thin films treated with H2SO4, as compared to those treated with DMSO and EG (Fig. 10a and b). No significant change was observed in the visible region of the absorption spectra of PEDOT:PSS thin films after secondary doping.
In the case of dedoping, the three states of PEDOT chains (i.e. bipolaron, polaron, and neutral states) exhibit absorption at different wavelengths. The neutral polymer chains show absorption around 600 nm, chains in the polaron state show absorption around 900 nm, and chains in the bipolaron state display broad absorption in the near infrared region.40,46,49 The UV-Vis-NIR absorption spectra of the PEDOT:PSS/40 wt% BST thin films after single post-treatment with NaOH and sequential post-treatment with 40 vol% DMSO–NaOH, 40 vol% EG–NaOH, and 0.5 M H2SO4–NaOH are presented in Fig. 10e–h, respectively. As the NaOH concentration increased, a broad polaronic transition peak was intensified at 900 nm in all samples, indicating the dedoping of PEDOT chains. Simultaneously, a new transition evolved at around 600 nm, originating from the absorption of neutral PEDOT segments. This peak became more pronounced at higher NaOH concentrations. The dedoping process by tuning the charge carrier concentration resulted in enlargement of the Seebeck coefficient, a decrease in the electrical conductivity, and maximizing the power factor (Fig. 5d and 7b–d).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc00881b |
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