Integrated manufacturing of REciclable multi-material COmposites for the TRANSport sector

Abstract

The thermoelectric properties of fully sprayed thin films of poly(3-hexylthiophen-2,5-diyl) (P3HT) doped with chloroauric acid are investigated for different film thicknesses. The film thickness increases logarithmically with increasing amount of deposited material on the surfaces. Both the electrical conductivity and measured Seebeck coefficients of the doped thin films show an optimal polymer layer thickness between 275 and 310 nm and yield a maximum power factor of ((1.77,pm ,0.22) frac{mu text {W}}{text {m}cdot text {K}^2}). The optimum layer thickness results from the optimal amount of dopant molecules per monomer between 1.1 and 1.3 at these ratios of P3HT and HAuCl(_4) for the thin film fabrication.

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Introduction

Classical thermoelectric generators1,2 are usually composed of inorganic n- and p-type semiconductors. Instead of bulk materials, recent approaches use, for example, nanostructured Bi networks3,4 or thin SrTiO(_3) films2 to form devices with high electrical conductivity and Seebeck coefficients. With the increasing demand for more sustainable alternatives, organic semiconductors have become a promising alternative for thermoelectric devices.5,6 In the case of organic electronics, both poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and poly(3-hexylthiophen-2,5-diyl) (P3HT) emerged as the two primary model systems in the field of organic photovoltaics,7 sensors,8 and wearables9,10,11 or for optical,12,13,14 electronic,15,16 and energy harvesting applications.17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37 In the case of flexible photovoltaics and thermoelectrics, P3HT is one of the most prominent conjugated semiconducting polymers due to its favorable energy band gap, general availability, environmental stability, and potential scalability.7 The polymer-gold interface in the solar cell stack is of utmost importance for competitive efficiency compared to traditional silicon-based solar cells.38 In addition to its high electric conductivity, it shows thermoelectric properties when doped with metal chlorides or nanoparticles, for example, gold (Au).39,40 To optimize the charge transport and hence the thermoelectric Seebeck coefficients, it is advantageous to form small gold domains inside the polymer films. This is usually achieved by low concentrations of dopants on the surface of the film.41,42 Although there is almost always an intermixing layer of gold and P3HT near the surface,43 higher thermoelectric coefficients can be achieved by implanting the dopant deep within the organic films.42,44 Raman scattering is also used for structural characterization of thin-film thermoelectrics.16,45,46,47,48,49,50,51,52 For future industrial applications, large-scale fabrication techniques, like slot-die coating or wet spray deposition, are preferred, as they are also roll-to-roll compatible.53,54 Recent studies have achieved air-stable prototypes of flexible thermoelectric generators using wire-coated P3HT films, which were doped with ferric or chloroauric acid.42 Taking inspiration from these results, the present study aims to produce fully sprayed thin films using the same sample system (P3HT and HAuCl(_4)). Wet spray deposition can be applied in industrially relevant roll-to-roll processes and can help reduce production costs due to fewer material waste.55 Here, the thermoelectric performance as a function of polymer film thickness and degree of doping is investigated with the aim to find the optimal fabrication parameters for HAuCl(_4)-doped P3HT thin-film thermoelectric functional layers.

MethodsMaterials

For this study, a poly (3-hexylthiophene-2,5-diyl) regioregular variant (P3HT) with molar masses between (M_n=10,000--13,000, frac{textrm{g}}{textrm{mol}}) and a polydispersity of (frac{M_{w}}{M_{n}}=1.3) was used (Polymer Source, Canada). Appropriate amounts of polymer were weighed and dissolved in a 1:1 mixture of chlorobenzene and 1,4-dichlorobenzene (Sigma-Aldrich, Germany) to obtain solutions with a final concentration of (5, frac{textrm{mg}}{textrm{ml}}). This concentration was chosen as an ideal compromise between the high amounts of polymer in the solutions and the low viscosity, which is necessary for spraying. As a dopant material, chloroauric acid (HAuCl(_4) (cdot 4)H(_2)O) (Sigma-Aldrich, Germany) was used. For subsequent spray deposition of the dopant, the appropriate amounts were weighed and dissolved in milliq water (specific resistance (rho =18.3, text {M}Omega cdot text {cm})) for a final concentration of (8, frac{textrm{mg}}{textrm{ml}}). All chemicals were used as received without further purification.

Thin-film fabrication

As the first step in thin-film preparation, silicon wafers (p-type /boron-doped, orientation (), (rho =1-30, Omega cdot text {cm}), Si-Mat Silicon Materials, Germany) and glass (ISO 8037-1 microscope slides, DWK Life Sciences, Germany) were cleaned and activated using the following procedure: 1) ultrasonification in acetone (Sigma-Aldrich, Germany) for 20 min, 2) subsequent washing and cleaning in toluene, isopropanol, and milliq water, and 3) surface activation with a UV ozone cleaner (Ossila, United Kingdom) for 20 min. All substrates had the same dimensions of (25times 25, text {mm}^2=625, text {mm}^2). After UV treatment, thin-film spray deposition was performed using a spray setup (see Fig. 1a) consisting of a heating plate and a spray nozzle (Compact JAUCO, Spraying Systems Co, Wheaton, Illinois, USA) as described in an earlier study.13 The spray nozzle was placed (20, text {cm}) above the substrates, connected to pressurized air (6 bar) and nitrogen gas (1 bar) and controlled using electronic pressure valves and customized Python scripts. Thin films were produced by layer-by-layer deposition on heated substrates ((120^circ text {C}) for P3HT solutions and (55^circ text {C}) for the aqueous HAuCl(_4) solutions). Here, for depositing one layer, a (100, text {ms}) long spray pulse with a subsequent drying time of (10, text {s}) was used. Up to 40 layers of polymer or dopant were deposited in this manner, while the dopant layers were always placed on top of the final polymer film. Using these spray parameters, the amount of molecules, which are deposited on the substrate area per spray pulse, was estimated for the polymer and dopant solution (see Figs. 1b and 1c). A detailed description of the calculation can be found in the supporting information.

Fig. 1

(a) Schematic sketch of the two-step film fabrication process of thin-film thermoelectrics using only spray deposition. Using the setup parameters, the amount of (b) P3HT and (c) HAuCl(_4) per spray pulse and per unit area is estimated. A detailed description of the calculation can be found in the supporting information

Full size imageOptical microscopy

All microscopy images were obtained using the VHX-7000N (Keyence, Japan) and its predefined imaging routines. For the images, magnifications of (100, text {x}), (400, text {x}), or (1500, text {x}) were used.

Spectroscopic ellipsometry

Spectroscopic ellipsometry is one of the primary techniques for determining the optical and structural properties of polymer thin films.56,57,58,59 The measurements were performed with a J.A. Woollam M-2000 instrument.60 Fitting of the optical constants was performed with the J.A. Woollam software (CompleteEASE 6.54). The spectral range was 380 to (1000, text {nm}), and focus lenses were used. The samples were measured at different angles between 50 and (65^circ) with a sampling time of 10 s. A build-in optical microscope was used to choose the region of interest. More details on the data analysis and fitting procedure to obtain the film thickness can be found in the supplementary information.

Conductivity measurements

Conductivity measurements were performed using a four-point probe device (Ossila, United Kingdom). Measurements were made in the middle of the samples and repeated 50 times each. The resulting sheet resistivity was recalculated as the thin-film conductivity using the devices build-in software and the film thicknesses obtained by spectroscopic ellipsometry and X-ray reflectivity.

Seebeck coefficient measurements

To measure the Seebeck coefficient, a custom-built measurement setup was used.61 The temperature gradient was achieved using an electrically heated copper stage on the one side and a water-cooled copper stage on the other side. A JULABO F12 unit (JULABO, Germany) was used to control the water temperature. The measurement chamber was closed to minimize external influences. The temperature on the hot side was set at (40,, ^{circ }hbox {C}) and the cold side at (16,, ^{circ }hbox {C}). The resulting voltage was measured using a Keithley SourceMeter 2400 (Keithley, USA). The thin films were contacted using conductive silver paste and Pt100 thermal sensors. After the copper blocks reached the desired temperatures, the thermovoltage was measured 300 times during 23 min.

Results and discussion

The surface morphology of both, pristine and doped thin films, was investigated using optical microscopy (see Fig. 2). As expected, the surfaces show high roughness and morphological differences for increasing amounts of spray pulses or deposited P3HT on the substrates. Using the prior calibration of deposited material per spray pulse (see Fig. 1b), every number of spray pulses can be recalculated in the actual amount of P3HT on the substrate. For small amounts of P3HT on the surface, the undoped (pristine) P3HT films show the distinct coffee-ring patterns associated with the sprayed films.62 Already for ((0.0363pm 0.0047)) mmol of P3HT corresponding to 10 spray pulses, the entire substrate is covered. The thickness of the film varies locally, which is visible in spots of different colors (see Figs. 2a and 2b). The more purple regions arise from lower film thicknesses, whereas the thicker edges of the coffee rings appear to be yellowish. With increasing amounts of P3HT on the substrates, the polymer films become thicker and more uniform (see Fig. 2d), although the coffee-ring patterns on the film surface never fully disappear (see Figs. 2e, 2f, and 2g). Upon further addition of more polymer material, the overall film color changes again to a dark metallic-shiny yellow (see Figs. 2d, 2e, 2f, and 2g), which is also typical for bulk P3HT. For doping, all polymer films were sprayed with a fixed amount of ((6.93pm 0.87) text {mmol}) of an aqueous solution of (text {HAuCl}_4). When chloroauric acid is used as the dopant, the P3HT films tend to change to a more bluish color.40,63 Since P3HT is very hydrophobic, the distribution of the dopant on the surface strongly depends on the amount of polymer. For fewer polymer material and lower polymer film thicknesses, the bluish coloration of the films is evenly spread across the surface (see Figs. 2h and 2i). With an increasing number of spray pulses or more deposited polymer material, the dopant begins to form clusters that can reach sizes of several hundred micrometers (see Figs. 2j, 2k, and 2l). This is due to the increasing hydrophobicity of the P3HT films and, therefore, a larger contact angle for aqueous solutions.64,65,66,67 This prevents the dopant solutions from spreading across the film surface upon deposition and increases the probability of forming clusters or aggregates. When the films with the highest amount of deposited polymer are doped, the formation of a dopant network with an almost closed layer on the surface is apparent (see Figs. 2m and 2n). Here, the high hydrophobicity of the polymer films hinders the penetration of the dopant into the films, causing the formation of an almost closed dopant layer at the polymer film surface.

The relation between the number of spray pulses and the resulting polymer film thickness was studied using spectroscopic ellipsometry (see Fig. 3). An overview of the data acquisition and fitting procedures can be found in the supporting information.

Fig. 2

Optical microscopy images of (a–g) pristine and (h–n) doped P3HT films with varying polymer amount: (a+h) (n_text {P3HT}=(0.0363pm 0.0047)), (b+i) (n_text {P3HT}=(0.0545pm 0.0070)), (c+j) (n_text {P3HT}=(0.0727pm 0.0094)), (d+k) (n_text {P3HT}=(0.091pm 0.012)), (e+l) (n_text {P3HT}=(0.109pm 0.014)), (f+m) (n_text {P3HT}=(0.127pm 0.016)), and (g+n) (n_text {P3HT}=(0.145pm 0.019) , text {mmol}). The amount of deposited dopant was (n_{text {HAuCl}_4}=(6.93pm 0.87) , text {mmol}) and was kept constant for the images in the bottom row (h–n)

Full size imageFig. 3

Obtained thickness results for increasing amount of P3HT, equivalent to the number of spray pulses, using the spectroscopic ellipsometry measurements. The solid line is a logarithmic fit to the data

Full size image

The ellipsometry data shows a clear trend: the resulting P3HT films increase in thickness with increasing number of spray pulses, which are deposited on the substrate. This general trend has been observed before for sprayed films and different materials.68,69 It also further highlights the excellent sustainability and scalability of spray deposition, due to virtually no material loss (despite solvent evaporation) during wet thin-film fabrication compared to other standard techniques, e.g., doctor blading70 or spin casting.71 Although earlier reports hinted at a linear dependency between film thickness and increasing spray pulses,68 the ellipsometry data, which is shown here, follows a logarithmic trend (see Fig. 3). Earlier theoretical works attributed these logarithmic surface growths to the so-called Mullins diffusion model,72 where the trenches in the film surface are filled by diffusion of newly deposited material into them.73,74 Hong and coworkers reported a similar logarithmic / logistic trend for film thickness with increasing material concentration for wire-bladed P3HT films doped with ferric chloride.75 The experimentally obtained results for the film thickness were modeled with a simple scaled logarithmic equation:

$$begin{aligned} d = C_1 cdot text {ln}left( C_2 x+1 right) , end{aligned}$$ (1)

where d denotes the film thickness in nm, (text {ln}) the natural logarithm and x the number of spray pulses. In the fits, (C_1) and (C_2) are optimized using a least-squares fitting approach. The X-ray reflectivity data follow a steeper increasing logarithmic law compared to the data obtained by spectroscopic ellipsometry. This is also represented for the model parameters (C_1) and (C_2) for both data sets (see Table 1).

Table 1 Results of the least-square fits of equation (1) to the thickness values shown in Fig. 3Full size table

The thermoelectric performance of the thin films was determined using their electrical conductivity and Seebeck coefficients (see Figs. 4a and 4b). Since P3HT is an organic semiconductor, polymer films of varying thickness need to be doped in order to show electrical conductivity. For this study, all polymer films were doped with the same amount of (n_{text {HAuCl}_4}=(6.9pm 0.9), text {mmol}) HAuCl(_4). Only P3HT films with at least 10 spray pulses or (n_text {P3HT}=(0.15pm 0.02), text {mmol}) show nonvanishing conductivity upon doping. All films with less than this amount of deposited P3HT were excluded from further thermoelectric characterization. For increasing P3HT-film thicknesses, the electrical conductivity shows a clear peak around 275–310 nm (see Fig. 4a). In classical theory, the conductivity of a thin semiconductor film increases linearly with increasing film thickness.76,77,78,79 However, sprayed, doped P3HT films show a more complex behavior compared to homogeneous bulk-semiconductor materials. Kang and coworkers found a similar trend for the electrical conductivity of slot-die coated P3HT films doped with various amounts of chloroauric acid.42 They attributed an observed decrease in conductivity at high concentrations of dopants with reduced charge carrier mobility due to overdoping.42 In addition, the open dopant layer structure due to the increased hydrophobicity at higher P3HT layer thicknesses will certainly have an effect on the reduced electrical conductivity (see Fig. 2n). Because the same amount of dopant was deposited on each of the polymer films, the varying polymer thickness can be recalculated as the amount of dopant molecule per monomer. Here, it must be assumed that the dopant material can penetrate the polymer layers. A detailed description of the assumptions and calculation steps can be found in the supporting information. Now, it is apparent that the films become more conductive with increasing dopant amounts per monomer (see Fig. 4a). The peak in conductivity is reached for one dopant molecule per monomer. The measured Seebeck coefficients are always negative and show a similar peak with increasing film thickness (see Fig. 4b). Negative Seebeck coefficients show the n-type character of the resulting doped thin films, which might be caused by the incomplete reduction of AuCl(_4^-) to AuCl(_3) and hence the presence of additional mobile Cl(^-) ions (negative charge carriers). Consequently, the Seebeck coefficients first decrease up to film thicknesses of around 275–310 nm, before rising again for thicker films. For doped P3HT films, Seebeck coefficients have been reported to decrease with increasing film thickness due to increased charge carrier path lengths.75 As expected, the overall trend of the Seebeck coefficient is opposite to the conductivity following Mott’s law.80,81 Thermoelectric efficiency can be measured using the so-called power factor

$$begin{aligned} PF = sigma , S^{2} , end{aligned}$$ (2)

where (sigma) denotes the electrical conductivity and S the Seebeck coefficient. By combining the results of both previous characterization techniques, the obtained doped thermoelectric thin films with thicknesses of around 275 and 310 nm show the highest power factors (see Fig. 4c). These two films also have roughly 1.4 and 1.1 dopant molecules per monomer. The relationship between thermoelectric performance and dopant levels, as well as their localizations, is still under discussion.82,83,84,85,86,87 Molecular dynamics simulations of undoped and doped P3HT show a dense packing of the thiophene rings with distances between 0.4 and 1 nm.82,83,84 This available space between the lamellar stacking of delocalized (pi)-electrons along the rings is the ideal anchoring point for small dopant molecules.83 At high doping levels, dopant molecules contribute to delocalization of charge carriers85 for improved thermoelectric performance.86,87 Previous studies have shown the important role of chloroauric acid as a dual dopant, which can act as an electron donor (AuCl(_4^{-}), n-type) or an acceptor (AuCl(_3), p-type).42 In both cases, one dopant molecules is ideally located close to one thiophene ring.42 This is in line with the observed peak power factor at 1.1 dopant molecules per monomer (see Fig. 4c). The maximum power factor of ((1.77pm 0.22), frac{mu text {W}}{text {m}cdot text {K}^2}) is two orders of magnitude lower than the highest reported in the literature for a P3HT-HAuCl(_4) system.42 Here, a different processing technique (wire coating) was used to produce micrometer-thick films. In the present study, the entire thermoelectric system was manufactured using very dilute solutions and spray deposition, allowing for large-scale and cost-efficient production routes. In another study, which used P3HT films sprayed with a different dopant to form thermoelectric thin-film devices, power factors above (300, frac{mu text {W}}{text {m}cdot text {K}^{2}}) were achieved.55 This further corroborates the excellent application potential of spray deposition for the fabrication of thermoelectric thin films with further efforts in designing the ideal polymer-dopant system.

Fig. 4

Thermoelectric efficiency characterization using (a) the electrical conductivity, (b) the Seebeck coefficients, and (c) resulting power factor measurements of the doped thin-film thermoelectrics with varying polymer thickness. A detailed description of the formulas and assumptions for the calculation of the dopant molecules per monomer can be found in the supporting information

Full size imageConclusions

In this study, the thermoelectric efficiency of fully sprayed thin films of P3HT doped with chloroauric acid is studied for different film thicknesses. Using spray deposition, it is found that the film thickness increases logarithmically with increasing amounts of deposited material on the surfaces. This logarithmic increase is attributed to the Mullins diffusion model, where uneven parts on the film surface are filled by diffusion of newly deposited material into them. For varying the thickness of P3HT and keeping the amount of dopant constant, the electrical conductivity and Seebeck coefficients of the doped thin films show an optimal polymer layer thickness between 275 and 310 nm and yield a maximum power factor of ((1.8pm 0.2), frac{mu text {W}}{text {m}cdot text {K}^2}). The optimum layer thickness stems from the optimal amount of dopant molecules per monomer of approximately 1.1 - 1.3 in these ratios of P3HT and chloroauric acid for the thin-film fabrication. For future commercialization, the combined effect of total film thickness and tailored doping will be essential, which is a route for future studies. These results show the excellent potential of sprayed, functional, doped semiconducting polymer thin films for large-scale, cost-efficient thermoelectric coating applications.

Data availability

All data (raw and analyzed) and samples are available upon reasonable request from the authors.

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