Great Enhancement in the Seebeck Coefficient and Thermoelectric Properties of Solid PEDOT:PSS Films Through Molecular Energy Filtering by Zwitterions

Organic thermoelectric (TE) materials are considered as the next‐generation TE materials owing to their merits including high mechanical flexibility, low cost, abundant elements, and nontoxicity. However, their Seebeck coefficient is lower than that of the inorganic counterparts by around one order of magnitude, and thus they have a lower dimensionless figure of merit (ZT) value. Herein, the significant enhancement in the Seebeck coefficient and thus the overall TE properties of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) that is the most popular TE polymer by adding a zwitterion like rhodamine 101 (R101), N‐dodecyl‐N,N‐dimethyl‐3‐ammonio‐1‐propane‐sulfonate (DDMAP), or 1‐(N,N‐dimethylcarbamoyl)‐4‐(2‐sulfoethyl) pyridinium hydroxide (DMCSP) are reported. In particular, R101 can enhance the Seebeck coefficient of the acid‐then‐base‐treated PEDOT:PSS from 21.2 to 61.6 μV K−1. The PEDOT:PSS/R101 film can exhibit a power factor of 546 μW m−1 K−2 and a ZT of 0.46 that is the highest for pure organic solid films. The enhancement in the Seebeck coefficient is ascribed to the energy filtering induced by the dipole moment of zwitterion and the π–π overlapping between conjugated rhodamine 101 and PEDOT:PSS. To distinguish it from the conventional methods, this method is named as the molecular energy filtering.


Introduction
[3][4] The TE performance of a material is evaluated by the dimensionless figure of merit (ZT), ZT = S 2 σT/κ, where S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ the thermal conductivity.S 2 σ is called the PF. [5,6]The conventional TE materials are inorganic semiconductors or semimetals like bismuth-based alloys. [7,8][11][12][13][14] However, conducting polymers usually exhibit a Seebeck coefficient lower than the inorganic counterpart by about one order in magnitude.As a result, their overall TE properties are notably inferior to the latter. [15,16][19][20][21][22][23][24] It can be dispersed in water or some polar organic solvents, and high-quality PEDOT:PSS films can be readily prepared by solution processing techniques such as coating and printing. [25]ut an as-prepared PEDOT:PSS film obtained from its aqueous solution has a Seebeck coefficient of only 15-18 μV K À1 , a conductivity of less than 1 S cm À1 , and thus a very low ZT value. [26]Because PEDOT:PSS has a low intrinsic thermal conductivity, the methods to increase its TE properties are usually to increase the electrical conductivity and/or the Seebeck coefficient.[42][43][44][45][46] For example, a triple treatment with H 2 SO 4 can enhance the conductivity of PEDOT:PSS films to >3000 S cm À1 , while the Seebeck coefficient is only 14-18 μV K À1 . [47][50] For instance, chemical dedoping of PEDOT: tosylate (PEDOT:Tos) can notably increase the Seebeck coefficient but simultaneously lower the electrical conductivity.At the optimal doping level, the PF is 324 μW m À1 K À2 , and the corresponding Seebeck coefficient, conductivity, and ZT are 210 μV K À1 , 73 S cm À1 , and 0.25, respectively.[53][54][55] For example, the sequential treatments of PEDOT:PSS with an acid and base can enhance the PF to 334 μW m À1 K À2 with the Seebeck coefficient of 39.2 μV K À1 and the electrical conductivity of 2170 S cm À1 . [47]he Seebeck coefficient improvement by dedoping is achieved at the sacrifice of the electrical conductivity.[58] A foreign material with a different Fermi level is usually mixed into a TE material for the energy filtering.The energy barrier at the interface of the two materials can selectively scatter the charge carriers of low energy and thus increase the mean energy of charge carriers, thereby increasing the Seebeck coefficient.[61] For instance, Bi 2 Te 3 that has a different Fermi level from PEDOT:PSS can induce the energy filtering and increase the Seebeck coefficient to 45.0 μV K À1 and the ZT value to 0.2. [59]Reduced graphene oxide (rGO) and tellurium nanowires (Te NWs) can induce the energy filtering to PEDOT:PSS as well, and they can enhance the Seebeck coefficient to 202 μV K À1 and the ZT to 0.21. [56]In addition to inorganic conductors, inorganic ferroelectric materials like barium titanate (BaTiO 3 ) nanoparticles can also induce the energy filtering to PEDOT:PSS and enhance the Seebeck coefficient to 40.7 μV K À1 and the ZT value to 0.15. [62][65] For instance, nanowires with PEDOT as the core and polypyrrole as the shell were introduced into PEDOT:PSS, and this can enhance the Seebeck coefficient to 31.9 μV K À1 and ZT to 0.1. [64]he energy filtering reported in literature is usually carried out by mixing nanometer-sized fillers into a TE polymer matrix.However, due to the limited interfacial area between the polymer matrix and the fillers, the energy filtering efficiency may be compromised.In addition, the inorganic materials usually have a thermal conductivity much higher than the TE polymers.Guan et al. reported surface energy filtering of rhodamine 101 on PEDOT:PSS films. [63]Although this can enhance the Seebeck coefficient and PF to 47.2 μV K À1 and 401.2 μW m À1 K À2 , respectively, it can have the effect only in a short depth.
Here, we report that a zwitterion-like rhodamine 101 (R101, chemical structure shown in Figure 1) can induce the energy filtering to PEDOT:PSS and thus greatly enhance the Seebeck coefficient and the ZT value.R101 can form contact with PEDOT:PSS at the molecular level, thereby giving rise to much higher interfacial area than that with nanometer or interfacial fillers.It can enhance the Seebeck coefficient from 21.2 to 61.6 μV K À1 .The optimal PF is 546 μW m À1 K À2 , and the ZT is 0.46.This ZT value is the highest for pure organic solid films.The energy filtering is ascribed to the high intrinsic dipole of R101 and π-π overlapping between conjugated rhodamine 101 and PEDOT:PSS.conductivity of less than 1 S cm À1 , and a PF of 0.008 μW m À1 K À2 . [66,67]The addition of 5 vol% DMSO into the PEDOT:PSS aqueous solution can enhance the electrical conductivity, Seebeck coefficient, and PF to 982 S cm À1 , 21.2 μV K À1 , and 44 μW m À1 K À2 , respectively.These values are consistent with that reported in literature. [6,63]70] A methanol solution of rhodamine 101 (R101) was mixed into the PEDOT:PSS aqueous solution added with 5 vol% DMSO.No precipitation was observed in the mixture.The solid films of PEDOT:PSS/R101 (PR) were prepared by spin coating the mixture (Figure 2a).As displayed in Figure 2b, R101 can greatly enhance the Seebeck coefficient of PEDOT:PSS.The Seebeck coefficient first increases with increasing R101 loading, and it reaches the maximum value of 43.3 μV K À1 at the R101 loading of 1.6 wt%.It then slightly decreases with the further increase in R101 loading.Simultaneously, the electrical conductivity decreases with increasing R101 loading (Figure 2c).The optimal PF is 132 μW m À1 K À2 at the R101 loading of 1.6 wt%, and the corresponding electrical conductivity is 657 S cm À1 (Figure 2d).

Results and Discussion
To understand the relationship between the structure of the zwitterions and the TE properties of PEDOT:PSS, the other two zwitterions including DMCSP and DDMAP were investigated here as the additives for PEDOT:PSS.DMCSP has a saturated structure, while DMCSP has a conjugated pyridine structure.The corresponding polymer films are denoted as PDm and PDd, respectively.These two zwitterions can also enhance the Seebeck coefficient of PEDOT:PSS (Figure 2b).The presence of DMCSP and DDMAP also lowers the electrical conductivity of PEDOT:PSS (Figure 2c).At the same zwitterion concentration, PDd shows the highest conductivity, while PR exhibits the lowest conductivity.The decrease in the conductivity in PEDOT:PSS films by the zwitterions can be ascribed to the insulating nature of the zwitterions and the filtration of the low-energy charge carriers.The optimal DMCSP and DDMAP loadings are 4 and 3 wt% in terms of the PF, respectively (Figure 2d).The optimal PFs are 93 and 80 μW m À1 K À2 for PDm and PDd, and the corresponding Seebeck coefficients are 40.1 and 34.9 μV K À1 , respectively.These Seebeck coefficients and PFs are notably lower than that of PR.
Presumably, the different enhancements in the Seebeck coefficient by the three zwitterions are related to the conjugated structure of the zwitterions.R101 that has the largest conjugated structure gives rise to the highest Seebeck coefficient, DMSCP that has a conjugated pyridine ring can produce the modest Seebeck coefficient, while DDMAP that has a saturated structure gives rise to the lowest enhancement of Seebeck coefficient.In addition, there is an optimal zwitterion loading in terms of the Seebeck coefficient, particularly for R101.When the zwitterion loading is higher than the optimal loading, the Seebeck coefficient slightly decreases with the increasing zwitterion loading.This may be related to the interaction between PEDOT:PSS and the zwtterions, because an interaction like the π-π overlapping between them can affect the electronic structure of PEDOT.

Enhancement in the TE Properties of Acid-Treated PEDOT: PSS by Zwitterions
To attain high TE properties, the PEDOT:PSS and PEDOT:PSS/ zwitterion films were treated with 8 M of methanesulfonic acid (MSA) aqueous solution.This acid treatment can induce secondary doping and greatly enhance electrical conductivity. [47,53,55]As shown in the Figure 3, the acid-treated PEDOT:PSS films (PA) exhibit an electrical conductivity of 2998 S cm À1 , a Seebeck coefficient of 15.8 μV K À1 , and a PF of 75 μW m À1 K À2 .These values are consistent with the ones reported in literature. [19,47]he acid treatment strikingly affects the TE properties of the PEDOT:PSS/zwitterion films as well.PRA, PDmA, and PDdA are used to represent the acid-treated PR, PDm, and PDd, respectively.The acid treatment lowers the Seebeck coefficient of the polymer films, and the maximum Seebeck coefficients become 30.4,29.2, and 26.8 μV K À1 for PRA, PDmA, and PDdA, respectively.Among them, the acid treatment causes the biggest drop in the Seebeck coefficient of PR.Probably, this is related to the protonation of the zwitterions (Figure 3a).R101 has a carboxylic group, which can be protonated much easier than DMCSP and DDMAP, which have a sulfonate group.The protonation can lower the dipole moment of the zwitterions.Similar to the acid treatment of PEDOT:PSS, the acid treatment can greatly enhance the electrical conductivity of PEDOT:PSS/zwitterion films.The electrical conductivity after the acid treatment exhibits similar dependence on the zwitterion loading to that before the acid treatment.The optimal zwitterion loadings in terms of the PF are not affected by the acid treatment.At the optimal zwitterion loading, the PFs of PRA, PDmA, and PDdA at the optimal zwitterion loading are enhanced from 132, 93, and 80 μW m À1 K À2 to 209, 175, and 158 μW m À1 K À2 , respectively, by the acid treatment.Among them, PRA exhibits the highest PF because it has the highest Seebeck coefficient.

Enhancement in the TE Properties of Acid-Then-Base-Treated PEDOT:PSS by Zwitterions
The acid-treated PEDOT:PSS films were sequentially treated with 1 M of NaOH aqueous solution.This base treatment can enhance the PF from 75 to 258 μW m À1 K À2 , and the corresponding Seebeck coefficient and conductivity of the acid-then-base-treated PEDOT:PSS (PAB) films are 37.5 μV K À1 and 1835 S cm À1 , respectively.The enhancement in the Seebeck coefficient and the PF can be attributed to the partial dedoping of PEDOT by NaOH.
The NaOH treatment can remarkably enhance the Seebeck coefficient of the acid-treated PEDOT:PSS/zwitterion films as well.Figure 4 presents the variation of the Seebeck coefficient, conductivity, and PF of the acid-then-base-treated polymer/ zwitterion films with the zwitterion loading.The Seebeck coefficient of the acid-then-base-treated PR (PRAB) films increases with the increasing R101 loading and reaches the maximum value of 61.6 μV K À1 at the R101 loading of 2 wt% (Figure 4a).It then slightly decreases with the further increase in the R101 loading.The base treatment also enhances the Seebeck coefficients of PDmA and PDdA.The maximum Seebeck coefficients of PDmAB and PDdAB are 55.5 and 46.8 μV K À1 , respectively.Nevertheless, the NaOH treatment lowers the electrical conductivity of the PEDOT:PSS/zwitterion films, and the conductivity decreases with the increasing zwitterion concentration (Figure 4b). Figure 4c shows the variations of the PFs of the acid-then-base-treated PEDOT:PSS and PEDOT:PSS/zwitterion films with zwitterion loading.Among the three zwitterions, R101 always gives rise to the highest PF.The optimal PF of PRAB is 546 μW m À1 K À2 at the R101 loading of 2 wt%.The optimal PFs of PDmAB and PDdAB are only 385 and 323 μW m À1 K À2 , respectively.
The thermal conductivity of PRAB was measured by the laser flash method.It is 0.234 W m À1 K À1 for the PRAB films with the optimal PF and 0.204 W m À1 K À1 for the untreated PEDOT:PSS films along the out-of-plane direction.Because the in-plane thermal conductivity of PEDOT:PSS is about 1.4 times of the out-of-plane one, [16] the in-plane thermal conductivity of PRAB and untreated PEDOT:PSS films are 0.328 and 0.286 W m À1 K À1 , respectively.These values are comparable to those (0.2-0.34 W m À1 K À1 ) reported in literature. [48,52]Thus, the optimal ZT value of PRAB is 0.46.[73] Although PAB film coated with an ionic liquid can exhibit a PF of 754 μW m À1 K À2 and ZT of 0.75, [74] it is not a monolithic solid film owing to the presence of the ionic liquid layer.

Characterizations
The zwitterions and PEDOT:PSS can form uniform polymer films.As revealed by the scanning electron microscopic (SEM) images, a PR film has a very smooth surface (Figure 5).No noticeable change can be observed on the surface after sequential treatments with the acid and base.The energy-dispersive X-ray spectroscopy (EDX) images of N and S indicate that R101 can be uniformly dispersed in PEDOT:PSS.
The interaction between the zwitterions and PEDOT:PSS is studied by fluorescence spectroscopy.Figure 6 shows the fluorescence spectra of an aqueous solution of R101 or DMSCP in the presence or absence of PEDOT:PSS.Under the excitation light of 585 nm, the R101 aqueous solution without PEDOT:PSS shows strong fluorescence emission in the wavelength range of 600-800 nm, while PEDOT:PSS is not fluorescent.The presence of PEDOT:PSS quenches the fluorescence of R101.This cannot    The interaction between the zwitterions and PEDOT:PSS is further studied by Raman spectroscopy.To avoid the fluorescence of the zwitterions, an infrared laser with the wavelength of 785 nm was used as the excitation light.As shown in Figure 6c, an intense Raman band appears between 1350 and 1500 cm À1 , which originates from the C α = C β stretching vibration of PEDOT. [75,76]This band appears at 1430 cm À1 for pristine PEDOT:PSS.It shifts to 1427, 1424, or 1423 cm À1 for PDd, PDm, or PR, respectively, and it becomes narrower.The redshift is not due to the possible Raman signal of the zwitterions, since their loading is very low.Because the zwitterions cannot oxidize or reduce PEDOT:PSS, the change in this Raman band suggests the conformational change of PEDOT from the benzoid structure to the quinoid structure. [75]Xia et al. also reported that zwitterions can cause the conformational change of PEDOT. [77]A PEDOT chain with the quinoid structure is more planar, and it can have more π-π overlapping with the conjugated structure of a zwitterion.
The conformational change can be related to the π-π overlapping between the zwitterions and PEDOT.The nonconjugated DDMAP does not have π-π overlapping with PEDOT, while the conjugated R101 can have great π-π overlapping with PEDOT.These are consistent with the redshifts of the Raman band of PDd (3 cm À1 ) and PR (7 cm À1 ).After the acid treatment, the presence of R101 can also induce the redshift of the Raman band.It appears at 1421 cm À1 for PA, and it shifts to 1416 cm À1 for PRA (Figure 6d).R101 can also induce the redshift of the Raman band of the acid-then-base-treated PEDOT:PSS films (Figure S2, Supporting Information).
Figure 7a shows the X-ray photoelectron spectra (XPS) of PEDOT:PSS, PR, PDm, and PDd.The XPS doublet between 162 and 166 eV is due to the S atom of PEDOT, and the doublet between 167 and 172 eV originates from the S atom of PSS. [63]he zwitterions particularly R101 can induce notable redshift of the S2p XPS band of PEDOT.The S 2p binding energies are 164.10 and 165.00 eV for PEDOT:PSS, and they shift to 163.85 and 164.75 eV for PR and 163.95 and 164.9 eV PDm.Instead, PDd does not induce the redshift of the S2p binding energies.The redshifts of PR and PDm indicate the electron transfer from R101 or DMCSP to PEDOT.This is also consistent with their π-π overlapping with PEDOT.
The presence of R101 can cause the redshift of the S2p XPS bands of PEDOT after the acid treatment.As shown in Figure 7b, the intensity of the S2p XPS bands of PEDOT relative to that of PSS À significantly increases due to the removal of PSS after the acid treatment, while the binding energies of PA are almost the same as that of PEDOT:PSS.cause the redshift of the S2p XPS bands of PEDOT after the sequential treatments with the acid and base.As shown in the Figure 7c, the bind energies are 163.85 and 164.90 eV for PAB, and they shift to 163.8 and 164.85 eV for PRAB. Figure 7d presents the N1s XPS spectra of R101, PR, PRA, and PRAB.The N1s band of R101 can be deconvoluted into a major band at 399.66 eV and a minor shoulder at 401.54 eV.The former is due to the neutral N atom, while the latter arises from NH þ as a result of the proton transfer from the carboxylic group to a nitrogen atom. [63]The minor N1s XPS band due to NH þ becomes remarkable for PR and PRA, but it becomes inconspicuous for PRAB.These results indicate that the protonation of the N atoms of R101 occurs in the PEDOT:PSS aqueous solution that has a pH value of %2 and deprotonation during the base treatment.In addition, the major N1s binding energy shifts to blue for PR, PRA, and PRAB in comparison to neat R101.The blueshift of this N1s binding energy suggests the electron transfer from R101 to PEDOT, which is consistent with the redshift of S2p binding energies of PEDOT.
Remarkable blueshift of the major N1s binding energy can be observed for DMCSP and PDm as well (Figure S3a, Supporting Information).It appears at 401.38 eV for DMCSP, and it shifts to 403.32 eV for PDm.This also suggests the electron transfer from DMCSP to PEDOT.Instead, no notable shift can be observed for the N1s XPS band from neat DDMAP to PDd (Figure S3b, Supporting Information).Hence, electron transfer does not take place between PEDOT and saturated DDMAP.
The UV-vis absorption spectra also evidence the electron transfer from R101 to PEDOT:PSS (Figure 8a).The absorption band near 900 nm is related to the polaron, and the absorption near 600 nm is due to the neutral state of PEDOT. [78]he polaron band becomes more remarkable for PRA than PA, which is not due to the absorption of R101, as shown in Figure S4a, Supporting Information.A similar effect by R101 on the polaron band can be observed on the polymer films after sequential acid and base treatments (Figure S4b, Supporting Information).
Presumably, the electron transfer from R101 to PEDOT should affect the work function of PEDOT:PSS.This is supported by the ultraviolet electron spectroscopy (UPS) (Figure 8b and Figure S5, Supporting Information).The presence of R101 shifts the cut-off edge to higher binding energy for PA or PAB.The work function of PRA is 4.58 eV, that is lower than that (5.07 eV) of PA by 0.49 eV, and it is 4.73 eV for PRAB, that is lower than that (4.88 eV) for PAB by 0.15 eV.The surface potentials of PA, PRA, PAB, and PRAB were also studied by Kelvin probe force microscopy (KPFM).As shown in the Figure S6, Supporting Information, the presence of R101 significantly changes the surface potentials.The surface potential of PRA is 755 mV, higher than that (493 mV) of PA.The surface potential of PRAB is 670 mV, higher than that (483 mV) of PAB.The work function (Φ s ) of a sample is related to the surface potential (V sp ) by Φ s ¼ Φ tip À eV sp , where Φ tip is the work function of the KPFM tip. [79]Thus, the effects of R101 on the surface  Because the presence of zwitterions can decrease the conductivity of PEDOT:PSS, they can affect the conduction mechanism.This was studied by measuring the resistances of the polymer films from 110 up to 350 K (Figure 8c and S7, Supporting Information).The resistance of all samples decreases with the elevating temperature at the temperature below 290 K.It then increases with the increasing temperature at the temperature above 290 K.Although the resistances of all the samples show similar trend to the temperature, the presence of a zwitterion increases the temperature dependence of the resistance.
The temperature-dependent resistances were analyzed by the 1D variable range hopping (VRH) model.
where T 0 = 16/k B N(E F )L // L ⊥ 2 is the characteristic temperature associated with activation energy required for hopping which represents the energy barrier between localized states, N(E F ) is the density of states at the Fermi level, and L // and L ⊥ are the localization length in the parallel and perpendicular directions, respectively.The T 0 values were obtained by the best fitting of the lnR-(T À1/2 ) relationship of the samples at the temperature below 290 K (Figure S8, Supporting Information).As shown in Figure 8d, the T 0 value increases with the increasing R101 loading.The presence of DMCSP or DDMAP also increases the T 0 value, but the T 0 value of PDmAB or PDdAB is lower than that of PRAB at the same zwitterion loading.The effects of the zwitterions on the T 0 value are consistent to their effects on the conductivity of PAB.
The presence of a zwitterion can affect the surface morphology of the polymer films as revealed by the atomic force microscopy (AFM) (Figure S9, Supporting Information).The presence of R101 can increase the domain size of PEDOT:PSS or PA films.This can be related to their effect on the PEDOT conformation.More linear PEDOT chains can lead to more aggregation and thus larger domain size.

Mechanism of the Zwitterion Effects
Although the zwitterions with conjugated structure including R101 an DMCSP can enhance the Seebeck coefficient of PEDOT:PSS, DDMAP that has a saturated structure can also enhance the Seebeck coefficient.Presumably, the dipole moment of a zwitterion can induce the energy filtering of the charge carriers by scattering the charge carriers.Zwitterions can have a high dipole moment.For example, R101 has a dipole moment 6.13D, as calculated by the density functional theory (DFT) method using the Gaussian16.A.03 software with the M06-2X function and 6-31þþg(d) basis (Figure 9a).The dipole moment is quite high, because that of a free water monomer is only 1.86D and that of ethanol is 1.66D.This energy filtering can increase the mean free energy (E J ) of the accumulated holes in PEDOT:PSS under temperature gradient and thus increase the Seebeck coefficient, because the Seebeck coefficient (S) is proportional to the difference between E J and the Fermi energy (E F ), S ∝ |E J -E F |.
In terms of this mechanism, the dipole moment should be very close to the PEDOT chains for energy filtering.It thus depends on the interaction between a zwitterion and PEDOT.When a zwitterion like R101 has a conjugated structure, it can form the π-π overlapping with PEDOT and thus induce much more significant energy filtering than a saturated zwitterion-like DDMAP.The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of R101 were calculated using the Gaussian16.A.03 software with the M06-2X function and 6-31þþg(d) basis. [80]As shown in Figure 9b, the LUMO of R101 is formed mainly by the benzene ring bonded with the carboxylic group, while the HOMO involves the fused ring structure.The LUMO and HOMO of R101 are À0.9 and À4.8 eV, respectively.The HOMO level is higher than the Fermi level (À5.2 eV) of PEDOT.Thus, the electrons on the HOMO level of R101 can transfer to PEDOT þ . [81,82]Therefore, R101 can greatly enhance the Seebeck coefficient of PEDOT:PSS.

Conclusion
In this article, we demonstrate that the Seebeck coefficient and PF of PEDOT:PSS film can be further enhanced by rhodamine 101, DDMAP, and DMCSP zwitterions.The Seebeck coefficient and PF of the as-prepared PEDOT:PSS films from aqueous solution are only 21.2 μV K À1 and 44.0 μW m À1 K À2 , respectively.The Seebeck coefficient can be enhanced to 61.6, 55.5, and 46.8 μV K À1 for PRAB, PDmAB, and PDdAB films, respectively.For PRAB, the PF value of 546 μW m À1 K À2 and the ZT value of this polymer film are calculated as 0.46, which is the highest value for pure organic solid PEDOT:PSS films.The different enhancement in Seebeck coefficient is attributed to different molecular energy filtering effects arising from intrinsic dipole moment of different zwitterions, interaction, and charge transfer-induced intermolecular dipole moment between PEDOT:PSS and different zwitterions.Interaction between PEDOT:PSS and zwitterions can facilitate the energy filtering effect by dipole moment of zwitterions.The molecular energy filtering can increase the mean free energy of the transporting charge carriers by filtering the low-energy charge carriers and thus increase the Seebeck coefficient.

Figure 2 .
Figure 2. a) Schematic illustration of the sample preparation.b) The Seebeck coefficient, c) electrical conductivity, and d) PF of PEDOT:PSS films added with zwitterions.

Figure 3 .
Figure 3. a) Schematic illustration of the protonation of the carboxylic group of R101.b) The Seebeck coefficient, c) electrical conductivity, and d) PF of the PEDOT:PSS/zwitterion films after the acid treatment.

Figure 4 .
Figure 4. a) The Seebeck coefficient, b) electrical conductivity, and c) PF of acid-then-base treated PEDOT:PSS and PEDOT:PSS/zwitterion films.d) The PFs and ZTs of the state-of-the-art pure organic solid films and this work at room temperature.

Figure 5 .Figure 6 .
Figure 5. a) Surface SEM, b) nitrogen EDX, and c) sulfur EDX images of a PR film.d) Surface SEM, e) nitrogen EDX, and f ) sulfur EDX images of PRAB films.The R101 loading was 2 wt% in the PR and PRAB films.

Figure 7 .
Figure 7. a) S2p XPS spectra of PEDOT:PSS and PEDOT:PSS/zwitterion films.b) S2p XPS spectra of PA and PRA.c) S2p XPS spectra of PAB and PRAB.d) N1s XPS spectra of R101, PR, PRA, and PRAB.The zwitterions were at the optimal loading in the polymer films in terms of the PF.

Figure 8 .
Figure 8. a) UV-vis absorbance spectra of PA and PAR films.The R101 loading is 10 wt% in PAR.The small band at around 600 nm marked with * of the PRA film is the absorption of R101.b) UPS spectra of PA, PRA, PAB, and PRAB films with the R101 loading of 2 wt%.c) Variations of the normalized resistances of PAB and PRAB films with the temperature.The R101 loadings in the PRAB films are 1 and 4 wt%.d) Variation of the T 0 value with the R101 loading.The T 0 values of PDmAB and PDdAB with the optimal zwitterion loadings are presented in (d) as well.
potential of PA and PAB are consistent with their effect on the work function.

Figure 9 .
Figure 9. a) Mechanism of molecular energy filtering effect in zwitteriondoped PEDOT:PSS films.b) The molecular orbitals of R101 calculated by the DFT method.