Ultrafast Laser-induced excellent thermoelectric performance of PEDOT:PSS Films

Due to poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is water-processable, thermally stable and highly conductive, PEDOT:PSS and its composites have been considered to be one of the most promising ﬂexible thermoelectric materials. However, the PEDOT:PSS ﬁlm prepared from its commercial aqueous dispersion usually has quite low conductivity, thus cannot be directly utilized for thermoelectric applications. Here, a simple environmentally friendly strategy via femtosecond laser irradiation without any chemical dopants and treatments was demonstrated. Under optimal conditions, the electrical conductivity of the treated ﬁlm is increased to 803.1 S/cm from 1.2 S/cm around three order of magnitude higher, and the power factor is improved to 19.0 μ W · m -1 · K -2 , which is enhanced more than 200 times. The mechanism for such remarkable enhancement was attributed to the transition of the PEDOT chains from a coil to a linear or expanded coil conformation, reduction of the interplanar stacking distance, and the removal of insulating PSS with increasing the oxidation level of PEDOT, facilitating the charge transportation. This work presents an eﬀective route for fabricating high-performance ﬂexible conductive polymer ﬁlms and wearable thermoelectric devices


Introduction
Conductive polymers show promising applications in thermoelectric devices, owing to low thermal conductivity, mechanical flexibility, light weight, low cost, easy solution processing, and low toxicity. [1,2]The performance of thermoelectric materials is described by a dimensionless quantity, the thermoelectric figure of merit ZT: [3] ZT ¼ where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute operating temperature of the device.Considering the significant effect of the thermal conductivity term on the overall ZT, the inherently low thermal conductivity of polymer materials is attractive for their thermoelectric applications.6][7] PEDOT:PSS consists of a positively charged conjugated PEDOT chain and a negatively charged PSS chain, where PEDOT is a derivative of polythiophene.PSS is a polymeric surfactant that not only acts as a chargebalancing counterion but also allows the dispersion of PEDOT in aqueous environments.0] To improve PEDOT:PSS thermoelectric performance, adding organic solvents, post-treatment, and sequential treatment are common methods. [8,11,12]In these methods, hazardous chemicals are inevitably used or produced.[15] The electron beam irradiation has been reported to enhance the PEDOT:PSS electrical conductivity, benefitting from the loss of PSS units and the increase of oxidation level in PEDOT. [16]Photonirradiation techniques have also been demonstrated to be an effective way to tune the electrical conductivity of PEDOT:PSS films. [13]Compared with conventional laser irradiation technique, femtosecond (fs) laser pulse irradiation can provide extremely high peak power and localize the thermal effect on the thin film, which has been demonstrated as a versatile micro-nano fabrication technique. [17]n this work, a simple environmental friendly method via femtosecond laser irradiation without any chemical dopants and treatments was proposed.Under optimal laser fluence, the electrical conductivity of the treated film is increased to 803.1 S cm À1 from 1.2 S cm À1 around three order of magnitude higher than that of the pristine ones, and the power factor is improved to 19.0 μW m À1 K À2 , which is more than 200 times of the original value.We provide the mechanic insights for such remarkable enhancement of conductivity after ultrashort pulse irradiation, and it was attributed to the intensive photo-chemistry effect, driving the PEDOT chains transiting from a coil to a linear or expanded coil The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/eem2.12650.DOI: 10.1002/eem2.12650Because poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is water processable, thermally stable, and highly conductive, PEDOT:PSS and its composites have been considered to be one of the most promising flexible thermoelectric materials.However, the PEDOT:PSS film prepared from its commercial aqueous dispersion usually has very low conductivity, thus cannot be directly utilized for TE applications.Here, a simple environmental friendly strategy via femtosecond laser irradiation without any chemical dopants and treatments was demonstrated.Under optimal conditions, the electrical conductivity of the treated film is increased to 803.1 S cm À1 from 1.2 S cm À1 around three order of magnitude higher, and the power factor is improved to 19.0 μW m À1 K À2 , which is enhanced more than 200 times.The mechanism for such remarkable enhancement was attributed to the transition of the PEDOT chains from a coil to a linear or expanded coil conformation, reduction of the interplanar stacking distance, and the removal of insulating PSS with increasing the oxidation level of PEDOT, facilitating the charge transportation.This work presents an effective route for fabricating high-performance flexible conductive polymer films and wearable thermoelectric devices.
conformation.Besides, the following localized shock pressure and heat after electron-phonon interaction, reducing the interplanar stacking distance and inducing the removal of insulating PSS thus facilitating the charge transportation.This work presents an effective route to fabricate high-performance flexible conductive polymer films and provides a wide range of applications in wearable thermoelectric devices, optoelectronics and other flexible electronics.

Results and Discussion
The mechanism of laser enhancement of PEDOT:PSS thermoelectric properties is shown in Figure 1.When the ultrashort pulse is irradiated on PEDOT, the carrier excitation and delocalization of the conjugate backbone in PEDOT chains will occur during the ultrafast electron-photon interaction.The conformations of PEDOT chains tend to shift from irregular clusters to regular linear ones, which is often beneficial to carrier mobility.At the following short period, the electron transfers the excess energy to the molecules via electron-phonon scattering process, and the ultra-high shock pressure will be generated in the confined focus zone.The π-π stack spacing of PEDOT is effectively reduced.At the longer time period, a huge amount of heat accumulated overcomes the Coulombic interaction between PEDOT and PSS, causing the partial removal of PSS and promoting the separation of PEDOT and PSS.The comprehensive result is that charge transfer within and between PEDOT chains becomes much easier.
In order to confirm the suitable laser annealing power range, we first performed single-point exposure of 2500 pulses with different laser fluences on the PEDOT:PSS films and obtained the ablation threshold of the film.Figure S2, Supporting Information, reveals the initial stage (craters) of foaming of PEDOT:PSS films from PET substrates under laser pulse irradiation.As the laser fluence increases, the lateral and vertical dimensions of the craters increase until they become bumps. [18]A further increase in laser fluence stimulates the growth of the central bumps, and the micro-scale holes emerge finally.The thresholds for the above three stages can be estimated by the following formula: [19] where D is the structure diameter, F th is the threshold fluence for forming the corresponding structures.It is revealed in Figure S3, Supporting Information, that the modification threshold of PEDOT: PSS films is below the damage threshold.Therefore, we selected the laser fluence between the modification and damage thresholds to treat films.The SEM in Figure S4a, Supporting Information, shows that the pristine film drying at 60 °C has a dense and uniform morphology, which is normal for the drop-coating PEDOT: PSS.At 1.14 mJ cm À2 fluence, the morphology of the film does not show obvious change (as shown in Figure S4b, Supporting Information).When laser fluence increases to 3.42 mJ cm À2 , an obvious stripe-like morphology with 10 μm spacing can be observed, which is consistent with the experimentally setting line spacing (as shown in Figure S4c, Supporting Information).However, the film is obviously damaged when the laser fluence reaches 4.56 mJ cm À2 .A large number of ripple-like black ablated regions appear along the laser scanning path (as shown in Figure S4d, Supporting Information).The optical microscope images reflect the same phenomena (as shown in Figure S5, Supporting Information).
Figure 2a shows that the conductivity of PEDOT:PSS films was gradually enhanced with increasing laser fluence.The average conductivity can be reached 803.1 S cm À1 under the optimal laser fluence, which is around three orders of magnitude higher than the initial value.Above 3.42 mJ cm À2 , the electrical properties of the films rapidly decline.As shown in Figure 2b, the Seebeck coefficient drops with increasing laser fluence.The highest power factor of 19.0 μW m À1 K À2 can be obtained owing to the greatly improved conductivity at room temperature (as shown in Figure 2c).Additionally, the temperature-varying thermoelectric property tests were performed.The laser-treated thermoelectric properties of the flexible films do not change significantly from 20 °C to 120 °C (as shown in Figure 2d-f).For intrinsic or lightly doped conducting polymers, conductivity in amorphous domains is temperature dependent and occurs through polaron or bipolaron hopping between adjacent charge sites.Specifically, the conduction mechanism can be described by Mott's variable range hopping (VRH) model: [20,21] where σ 0 is the conductivity prefactor, T 0 is related to the activation energy (which describes the charge transport distance and potential barrier), and d is the dimension of the conduction.By fitting the slope of ln σ ð Þ ∼ T À 1 d curves, the activation energy information required for charge jumping transport before and after laser treatment can be obtained (as shown in Figure S6, Supporting Information).For example, in the 1D-VRH model (d = 1), the absolute value of T 0 decreases from 6192.11 to 39.43 K after 3.42 mJ cm À2 femtosecond laser treatment, and the corresponding activation energy decreases from 533.57 to 4.25 meV.The decrease of the jump barrier indicates the delocalization of the carriers during femtosecond laser irradiation, which makes carrier transport more efficient.
In order to explore the effect on the oxidation level, which has a significant impact on PEDOT:PSS on carrier concentration, UV-Vis absorption tests were conducted on PEDOT films before and after laser treatments (as shown in Figure 3a,b).All samples exhibit broad absorption in the visible and near-infrared wavelengths.Since the absorption of PSS above 300 nm is much lower than that of PEDOT, it can be considered that all the absorption in the spectra comes from PEDOT. [22]Specifically, the PEDOT chain in the neutral state shows absorption around 600 nm and shows absorption around 900 nm for polaron state, while the absorption in the near-infrared band is mainly from the PEDOT chain in the bipolar state. [23,24]An increase in the polaron and bipolaron state content of the annealed films is observed at 3.42 mJ cm À2 laser fluence.Therefore, the ultrashort pulse irradiation process effectively increases the oxidation level of PEDOT:PSS.And unlike the traditional method to increase the film conductivity by adding bipolarons, the higher carrier concentration in our work is mainly due to the polaron delocalization effect on the PEDOT-conjugated backbone.The increase in the charge carrier concentration negatively affects the Seebeck coefficient, which is also in accordance with the experimentally measured TE performance.
The Raman spectra are shown in Figure 3c.The strongest peaks located around 1420 cm À1 correspond to the symmetrical stretching vibrations of C α =C β in PEDOT. [23]The peak near 1245 cm À1 is attributed to the inter-ring stretching vibration between C α -C α 0 bonds, while Energy Environ.Mater.2024, 7, e12650 the peak around 1357 cm À1 is corresponding to intra-ring stretching vibration between C β -C β bond. [16,25]When the laser fluence is gradually increased from 0 to 3.42 mJ cm À2 , the C α =C β vibrational peak shifts to lower wavenumbers (as shown in Figure 3d).This change implies the change of the PEDOT conjugated skeleton from benzene to quinone structure.The broadening of FWHM can be attributed to the increase of the oxidized states (at the higher wavenumber in Raman spectrum) caused by irradiation.Likewise, the vibrational peaks located between 1510 and 1570 cm À1 represent the PEDOT asymmetric stretching mode. [26]It also has a tendency to shift toward lower wavenumbers after laser irradiation, which shows the partial enhancement of the low-wavenumber oxidation state PEDOT.Besides, the damage threshold of PEDOT:PSS film is also reflected in Raman spectra.For the maximum laser fluence of 4.56 mJ cm À2 , the Raman spectra do not show any characteristic peaks of PEDOT or PSS, but exhibit a broad Dband at 1360 cm À1 and a G-band at 1584 cm À1 like laser-induced graphene (corresponding to sp2 lattice carbon atoms).However, this does not mean that the films at this laser fluence have excellent conductivity.Generally, the I D /I G (the intensity ratio of D-band to G-band) value of carbon materials is less than 0.8 when graphitization occurs, but the calculation value is 1.99.Excessive intensity of D-band indicates that there are lots of defects inside the film, which is unfavorable for carrier transport.
The FTIR spectra of pristine and laser-irradiated PEDOT:PSS films are obtained in Figure S7, Supporting Information.The peak at 1270 cm À1 represents the C α -C α 0 interring stretching, and blue-shifts to 1273 cm À1 at 3.42 mJ cm À2 laser fluence, indicating a transition from more singlebond structure (benzoid) to double-bond structure (quinoid).At the excessive laser fluence of 4.56 mJ cm À2 , the intensity of the C-O-C vibrational peaks (at 1058 and 1240 cm À1 ) is almost absent, indicating that the part connected to the thiophene ring in PEDOT was destroyed at this time. [27,28]And excessive laser fluence makes the EDOT units unable to connect in the form of double bonds (the peak at 1270 cm À1 disappears), and the C-C and C=C stretching vibration peaks on the thiophene ring between 1300 and 1500 cm À1 tend to degenerate.These are the reasons why carriers cannot be transported smoothly inside the film after high laser fluence treatment.
Usually, the impact pressure of ultrafast laser on material surface cannot be ignored.Figure 3e,f shows the PEDOT:PSS molecular structural formula and XRD data of the PEDOT:PSS films as pristine and after annealing with different laser fluence.The characteristic peaks around 2θ = 26°correspond to π-π stacking distance d (010) of PEDOT rings. [29]With the laser fluence increasing, the characteristic peaks d (010) shift to a higher angle and center at 26.52°for the 1.14 mJ cm À2 laser-treated film and at 26.78°for the 3.42 mJ cm À2 laser-treated film.The specific (010) interplanar distance (d (010) ) can be deduced according to the Bragg diffraction equation.The calculated d (010) decreases from the original 0.3494 to 0.3414 nm for the 3.42 mJ cm À2 laser-treated film.The decrease in π-π stacking distance lowers the barrier for carrier hopping transport between PEDOT chains and enhances the electrical conductivity (as shown in Figure 3g). [30,31]It has been confirmed that a similar phenomenon can be observed by the hot pressing method, and effectively improving the conductivity of PEDOT:PSS films. [32]Here, under femtosecond laser pulse irradiation, shock pressure followed by the localizing heat can be effectively induced and plays a similar role to reduce the interplanar distance of PEDOT:PSS films (as shown in Figure S8, Supporting Information).In addition, the 3.42 mJ cm À2 laser-treated film exhibited narrower and higher relative intensity in the main diffraction peak compared to the pristine film, which implies better crystallinity of PEDOT:PSS after irradiated.As for the maximum laser fluence, a diffraction peak at 10.8°was found, which corresponds to the increase of graphite oxide under intense laser irradiation.The ultrafast laser irradiation technique is a novel nonequilibrium process.Specifically, during the ultrashort pulse irradiation, ultrafast photon-electron and electron-phonon energy processes will be initiated, resulting in an ultrafast temperature field with a Gaussian distribution in the focal region, which can be estimated by the following equation: [33] ΔT t where erfc(x) is the complementary error function, ω is the laser beam radius, d is the absorption depth, C is the heat capacity, and ρ is the mass density.Q ¼ P=ν ¼ Fπω 2 is pulse energy with P, ν, and F being the average laser power, repetition frequency, and laser fluence, respectively.t 0 ¼ πω 2 Cρ=k is the heat diffusion time with k being thermal conductivity (detailed information in Table S1 and Figure S9, Supporting Information).As shown in Figure S10, Supporting Information, each pulse causes about 31 K temperature rise to the sample within a picosecond timescale, followed by quenching in microseconds.Due to the low thermal conductivity of the PEDOT:PSS films, the cumulative heat effect is very obvious (Figure S11, Supporting Information and Figure 4a).Figure 4b shows the results of the original PEDOT:PSS thermogravimetric test.The polymer mass loss is mainly concentrated in the range of 300 °C-500 °C.Specifically, the TGA curve has two peaks around 340 °C and 380 °C, respectively corresponding to the removal of PSS chain and the degradation of PEDOT chain.PEDOT cores have strong absorption at 1030 nm wavelength, generating a large amount of heat during the process of (bi)polaron excitation.
The film heats up to around 338 °C under 3.42 mJ cm À2 laser fluence.During this process, the PSS chains wrapping on the PEDOT chains are broken and partially removed.Figure 4c-f and Figure S12, Supporting Information, are X-ray photoelectron spectroscopy (XPS) measurements on pristine and laser-treated PEDOT: PSS films.36][37] Each S(2p) peak can be resolved in two components 2p 3/2 and 2p 1/2 .The composition ratio of PSS and PEDOT on the surface can be simply estimated by the S(2p) peak area ratio.As it is shown in Figure 4g, the calculated ratio of PSS to PEDOT on sample surface continuously drops from the initial value of 2.14-1.69when the laser fluence increases.The removal of a large amount of insulating PSS is a key reason for the substantial increase in conductivity.This also results in a weakened coulomb interaction between PEDOT chains and PSS chains and increased PEDOT crystallinity on the surface of the films, as the XRD patterns showed before.The change of PSS content was also supported by the change of film thickness (Figure S13, Supporting Information).In addition, each 2p sub-peak of the S element shifts toward the lower binding energy direction after the laser fluence reaches 3.42 mJ cm À2 as shown in Figure 4h. [38]This is mainly due to the decrease in PSS content.
The high-performance thermoelectric devices on the market are usually integrated with many micro-generating units.The high-density array brings challenges to the manufacturing process.Here, a 10 × 1 number of integrated thermoelectric device on a 30 mm × 30 mm flexible substrate is presented (Figure S14, Supporting Information).The ten PEDOT:PSS thermoelectric bars are fabricated by selective femtosecond laser irradiation, followed by sputtering gold wires to connect each functional bars in series.Because the electrical conductivity of the proregion is much higher than that of the unprocessed region, the series superposition of thermovoltage of each function bar can be realized.An output voltage of 0.7 mV is obtained with an average temperature difference of 11.1 K (Figure S15, Supporting Information).This approach avoids the step of removing nonfunctional region of PEDOT: PSS films and shows the potential of femtosecond laser in flexible highpower micro-thermoelectric devices.

Conclusions
In conclusion, we developed a new strategy to achieve high conductivity in drop-coating PEDOT:PSS films on PET substrates by fs laser irradiation.Ultrafast laser treatment of PEDOT:PSS films can significantly improve the electrical conductivity, reaching the highest value of 803.1 S cm À1 under 3.42 mJ cm À2 laser fluence, the power factor can be improved to a maximum of 19.0 μW m À1 K À2 .Detailed tests and characterization explained such remarkable enhancement of electronic transport performance after ultrashort pulse irradiation.We believe that laser treatment is a facile and efficient alternative to traditional chemical processing of polymer thermoelectric materials.And it is expected to be used in the preparation of large-area flexible polymer conductive films and micro-nano thermoelectric devices.
Fabrication of PEDOT:PSS films: The PEDOT:PSS film was prepared by drop coating.Typically, 40 μL of PEDOT:PSS aqueous solution (filtered with 220 nm filter head) was drop-coated on a 14 mm × 4 mm PET substrate.Before deposition, the PET substrate was cleaned with dish soap solution followed by ultrasonic cleaning in DI water and ethanol for 15 min.Then the substrates were dried in oven for 30 min at 60 °C.After that, they were irradiated in UV cleaner for 15 min.After deposition, the films were annealed at 60 °C for 40 min.Finally, a film with a thickness of about 5 μm can be obtained (the cross-section image of the prepared film is shown in Figure S1, Supporting Information).
Femtosecond laser irradiation of PEDOT:PSS thin films: The film was placed on a three-dimensional mechanical stage, and a galvo scanning system was used to irradiate the film with a 1030 nm femtosecond laser source (Pharos, Light Conversion Inc.).Specifically, the laser pulse width is 260 fs, and the pulse repetition frequency (ν) is 1 MHz.The scanning interval for each sample is 10 μm, and the specified laser scanning speed is 10 mm s À1 .In the experiment, the focused laser Energy Environ.Mater.2024, 7, e12650 spot radius (ω 0 ) is 12.5 μm.Laser beams with a certain range of power (P) were selected, which were 5.6, 8.4, 11.2, 14, 16.8, 19.6, and 22.4 mW.The irradiation laser fluence F ¼ P= ν Á πω 2 0 À Á can be calculated.Characterization techniques: Surface morphology was investigated by field emission scanning electron microscope (FESEM, TESCAN MIRA LMS, Czech).The film thickness was measured by Bruker DektakXT.X-ray diffraction (XRD) tests were conducted on a Bruker D8 Discover X-ray diffractometer (Cu Kα X-ray source, λ = 1.5406Å).Absorption spectra were acquired by using a UV-VIS-NIR spectrophotometer (PerkinElmer Lamda 750).Raman spectra were measured by Thermo Fischer DXR.X-ray photoemission spectra were measured by Thermo Fischer, ESCALAB Xi+ (the peak calibration was done according to the C1s peak at 284.8 eV).ATR-FTIR spectra were measured by Nicolet 6700.For all samples, the electrical conductivity and Seebeck coefficient were measured by CTA-3/500 thermoelectric performance test system.The surface temperature of PEDOT:PSS films during laser irradiation was monitored with a UTi320E infrared thermal imager.The thermogravimetric analysis (TGA) was measured by NETZSCH STA 2500.

Figure 1 .
Figure 1.Schematic of the mechanism for conductivity-enhanced PEDOT:PSS by femtosecond laser pulse treatment.

Figure 2 .
Figure 2. Thermoelectric properties of PEDOT:PSS films after annealing with different laser fluence: a) electrical conductivity, b) Seebeck coefficient, c) power factor, d-f) thermoelectric properties of PEDOT:PSS film vary with temperature after annealing by laser compared with pristine film.

Figure 3 .
Figure 3. a) UV-Vis absorption spectra of pristine and laser treated PEDOT:PSS films, b) electronic structure evolution for different status of PEDOT molecular.c) Raman spectra of pristine and laser-treated PEDOT:PSS films, d) Raman shift and FWHM of the C α =C β symmetric stretching peaks of PEDOT: PSS films with or without laser treatments, e) molecular structure of PEDOT:PSS, f) XRD spectra of pristine and laser treated PEDOT:PSS films, g) schematic for interlayer stacking changes of PEDOT.

Figure 4 .
Figure 4. a) Center surface temperature of PEDOT:PSS films measured by infrared thermal imager at different laser fluences.(Insert: infrared thermal images, scale bar: 200 μm) b) TG and DTG curves of PEDOT:PSS.c-f) Convoluted XPS for S2p of PEDOT:PSS films, g) surface composition of PSS with respect to PEDOT in pristine and laser-irradiated PEDOT:PSS films, h) changes in binding energy of S element.