Roles of Polyethylenimine Ethoxylated in Efficiently Tuning the

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Roles of Polyethylenimine Ethoxylated in Efficiently Tuning Thermoelectric Performance of Poly(3,4-ethylenedioxythiophene)-rich Nanocrystal Film Xuejing Li, Congcong Liu, Weiqiang Zhou, Xuemin Duan, Yukou Du, Jingkun Xu, Changcun Li, Jing Liu, Yanhua Jia, Peipei Liu, Qinglin Jiang, Chan Luo, Cheng Liu, and Fengxing Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00298 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Materials & Interfaces

Roles of Polyethylenimine Ethoxylated in Efficiently Tuning Thermoelectric Performance of Poly(3,4ethylenedioxythiophene)-rich Nanocrystal Film Xuejing Li,† Congcong Liu,† Weiqiang Zhou,† Xuemin Duan,† Yukou Du,§ Jingkun Xu,†,ǁ* Changcun Li,† Jing Liu,† Yanhua Jia,† Peipei Liu,‡* Qinglin Jiang,† Chan Luo,† Cheng Liu,‡ and Fengxing Jiang‡, ǁ* †

College of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330013,

P. R. China. ‡

Department of Physics, Jiangxi Science and Technology Normal University, Nanchang 330013,

P. R. China. §

College of Chemistry, Chemical Engineering and Materials Science, Soochow University,

Suzhou 215123, P.R. China. ǁ

College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology,

Qingdao 266042, P. R. China. KEYWORDS: reductant/oxidant; PEDOT:PSS; PEIE; hole block; thermoelectrics.

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Abstract The regulation of oxidation level is of great importance as an efficient way to optimize the thermoelectric (TE) performance of conducting polymers. Many efforts have been devoted into the acquisition of high TE performance for PEDOT:PSS by oxidation/reduction post-treatment to achieve an effective compromise. However, a strong oxidant/reductant is usually employed to tune TE performance of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with high electrical conductivity ( and Seebeck coefficient (S), and also presents a number of operational challenges depending on fast reaction rate. Herein, a nontoxic polyethylenimine ethoxylated (PEIE) served as a reductant to successfully realize an enhanced S for PEDOT:PSS, followed as another significant anion-blocking role in enabling efficient modulation of oxidation level by sulfuric acid (H2SO4) with a longer operating time. Eventually, a good PEDOT-rich nanocrystal is achieved by sequential dipping process in PEIE/ethylene glycol (PEIE/EG) and H2SO4 solutions. A large TE power factor of 133 μW m-1 K-2 can be ascribed to the good formation of PEDOT-rich nanocrystal and an effective compromise between  and S of PEDOT:PSS films. The mechanism was elucidated for efficient regulation of  and S enabling high performance of organic TE materials.

Introduction Thermoelectric (TE) materials due to the capacity to directly convert heat to electricity provide a promising route to develop electricity production.1,2 The performance evaluation of TE materials is generally based on a dimensionless figure of merit (ZT):

𝑍𝑇 =

𝜎𝑆2 𝜅

𝑇

(1)


2

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where,  S, T, and  are electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity, respectively. As proposed in the definition of ZT, a high power factor (S2) and a low  are required. Conducting polymers possess unique features for TE application because of their low thermal conductivity, low density, easy synthesis and processability compared with inorganic semiconductor materials.3,4 Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), an excellent and commercially available conducting polymer, has made its mark with sparkling accomplishment as an alternative organic electron material in that easily realizable and high electronic transport properties.5,6 Currently, PEDOT:PSS is fully deserved as one of the most promising p-type organic TE materials, owing to its high / ratio.7-9 However, the pristine PEDOT:PSS suffers the disadvantage of low TE performance. Many methods have been developed to improve the TE performance based on the optimization of oxidation level through doping and de-doping processes.10-12 For conducting polymers, their doping level enable the easy and effective modulation to complete a reduction with respect to the concentration of polarons and bipolarons.13,14 Generally, the reducing agents are used to achieve the process of reduction in conducting polymers. Sodium borohydride,15 hydrazine,16 and tetrakis(dimethylamino)ethylene17 as common reductants have been employed to optimize S of PEDOT:PSS with different efficiency. It is crucial through appropriate chemicals to tune its TE performance. Recently, low-cost, nontoxic, and environmentally friendly polyethylenimine ethoxylated (PEIE) containing an aliphatic amine group has been universally used in inorganic and organic optoelectronic devices thereby enabling tunable electrical/optical properties.18-21 Yang et al.21 enhanced the efficiency of polymer light emitting diodes by using PEIE as an efficient electron injector due to the reduced work function and hinder hole injection. Jin et al.19 achieved highly efficient and air-stable inverted


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organic solar cells with PEIE to improve carrier mobility () and prevent leakage. Despite its remarkable advantages in optoelectronic devices, the role of PEIE as an efficient reductant in tuning TE performance of PEDOT:PSS has not yet attracted attention up to now. Although reductant can reduce the oxidation level and improve S of conducting polymers, the

 suffers from a sever slide.14,15 An alternative way is re-oxidation to seek a good compromise. PEDOT:PSS is an individual hybrid composed by conductive PEDOT with superior air-stability and non-conductive PSS as counter-ion and dispersant.22,23 Therefore, the  of PEDOT:PSS does not only depend on the doping level, but also have respect to the formation of conductive PEDOT nanocrystals. As we know, polar organic solvents, ionic liquids, and sulfuric acid have been successfully used to achieve high  of PEDOT:PSS films.5,6 For polar organic solvent and ionic liquids, they enable the conformational change and reorientation of PEDOT:PSS chains attributed to the phase separation between PEDOT and PSS.24,25 A high  (>1000 S cm-1) is generally happened with them when the thickness of PEDOT:PSS film is less than 100 nm. A sophisticated and precise technology is needed to ensure the integrity and uniformity of PEDOT:PSS films. While sulfuric acid treatment is able to realize the increase in doping level and the partial depletion of non-conductive PSS leading to a good contract between PEDOT crystals on the surface of film,26-28 especially for a thicker film. Recently, Fan et al.12 proposed a sequential treatment with acids and bases resulting in an enhanced TE power factor despite the decreased oxidation level occurred with base treatment for PEDOT:PSS. Nevertheless, it is difficult to achieve the effective optimization of doping level through an individual reduction/oxidation process during a sufficient period of operating time. It is of great desires for efficiently tuning TE performance of PEDOT:PSS by improving both

 and S with post-treatment. In this work, PEIE was chosen as nontoxic reductant to gain enhanced


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S for PEDOT:PSS nanofilm. The sulfuric acid was used to not only increase the doping level of PEDOT, but also promote the formation of PEDOT-rich nanocrystal. A novel reduction/oxidation process was proposed by combining PEIE and sulfuric acid post-treatment for PEDOT to complete an effective optimization of TE performance. PEIE played another significant role in blocking further oxidation by sulfuric acid. The impacts of reduction/oxidation on the morphology, structure, and TE performance of PEDOT:PSS nanofilms were systematically investigated by a series of characterizations. A possible mechanism is proposed to tune TE performance of PEDOTrich nanocrystal.

Results and discussion Scheme 1 presents the preparation of PEDOT:PSS nanofilm and the sequential reduction and oxidation processes by PEIE and H2SO4, respectively. It can be observed from the process is very easy to operate without complicated technology. We firstly investigated the microstructure of asprepared PEDOT:PSS nanofilms by atomic force microscopy (AFM), as shown in Figure 1a-f, due to its significant impact on electron transport properties. For the ethanol-dilution filtering PEDOT:PSS nanofilms, an interconnected network is formed resulting in the small and uniform PEDOT grains (Figure 1a) with a root-mean-square (RMS) of 1.67 nm similar to previous report.29 The corresponding image in Figure 1d shows the obvious phase separation between conductive PEDOT and nonconductive PSS chains. After dipping in PEIE/EG solution, one can see that the conductive PEDOT grain aggregated into larger ones in Figure 1b, which is probably attributed to the interactivity with PEIE. Interestingly, the PEDOT:PSS nanofilms become relatively flat with a lower RMS of 1.35 nm, implying a uniform PEIE layer adsorbed on PEDOT:PSS surface due to the charge interaction between PEDOT+ and PEIE. Figure 1e displays the more distinct phase separation. Because PEIE has a strong coupling capacity to sulfur in PEDOT by aliphatic amine


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groups.30,31 Further, the H2SO4 dipping lead to a much rougher surface with RMS of 2.06 nm, as observed in Figure 1c. However, one can find the formation of an extended conductive layer. The phase separation become more distinct further with less nonconductive species on the surface in Figure 1f. This is due to the removal of residual PEIE as well as partial PSS induced further out by H2SO4 resulting in a PEDOT-rich film. It is totally reasonable that the PEIE and H2SO4 dipping would give rise to significant changes in the electron transport properties of PEDOT:PSS. Besides, an optimal molecule stacking of conducting polymers allows the efficient electronic transport that is critical in improving S and .24,32,33 Two-dimensional (2D) Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) was employed to determine the effects of post-treatment with the reduction and oxidation processes on intermolecular stacking and orientation of molecules in PEODT:PSS nanofilms. As shown in Figure 2, 2D GIWAXS data indicate the inner and outer ring corresponding to the scattering from the π-π stacking in horizontal and vertical directions associated with the edge-on and face-on orientated PSS and PEDOT molecules.32 All the PEDOT:PSS display multi-ordered diffractions involving lamellar structure along the qz axis and π-π stacking along the qxy axis structure. Compared to pristine PEDOT:PSS in Figure 2a, the filtering film in Figure 2b shows a stronger diffraction and a less off-axis scattering. While the PEIE/EG dipping in Figure 2c have almost no influence further. Noted that, the further H2SO4 dipping PEDOT:PSS film (Figure 2d) shows afresh a smaller arcing diffractions, indicating consecutive and oriented fractions. The difference in Figure 2 illustrate the dipping processes allow the relative molecular stacking to focus on aggregation and/or crystallization of PEDOT molecules. To further illustrate the orientation of molecules, Figure 3a-d show the linearly offset onedimensional (1D) GIWAXS of horizontal and vertical direction associated with the edge-on and


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face-on orientated PEDOT and molecules. The peaks at 1.26 and ~1.78 Å-1 depend on the stacked PSS and PEDOT molecular, respectively.32 The changes of scatter including intensity and peak width from Figure 3a-d reveal the transform of molecular structure from the predominant PSS stack to the predominant π-π stacking PEDOT due to the preferred edge-on molecular orientation of PEDOT in as-prepared films. It indicates a relatively preferred edge-on molecular orientation with a narrower peak width towards PEDOT crystallites.32,34 This means both the dilution and dipping processes can effectively promote the formation of more oriented PEDOT crystallites/aggregates due to the phase separation of PEDOT and PSS, which can result in a large enhancement in  of PEDOT:PSS.25,35,36 It is worth mentioning that this phase separation was not completed through the treatment of solid PEDOT:PSS film with some low boiling organic solvents in previous report.37 Additionally, the higher PEDOT to PSS ratio in the crystalline fraction indicates the predominant oriented PEDOT crystallites in the PEDOT:PSS films in favor of the improvement of TE performance for PEDOT:PSS.38,39 On the other hand, the interchain coupling has a significant impact on the electronic transport in PEODT:PSS films.40 Figure 3e and 3f show the structural perturbation of PEDOT:PSS by the 1D in-plane (qxy) and out-of-plane (qz) GIWAXS patterns. Initially, the peak of 1.78 Å-1 (Figure 3e) indicates a real d-spacing of 3.65 Å in the π-π stacking direction for the pristine films. By contrast, the peak shifted to 1.83 Å-1 accompanied with the decreased π-π stacking distance of 3.45 Å-1 for the direct filtering PEDOT:PSS, which enables the enhancement of charge transport along the π-π stacking direction.41 After further dipping in PEIE/EG solution, the peak for PEDOT has a slightly shift to 1.81 Å−1 possibly attributed to a small amount of introduction of PEIE molecules, which allows the change of steric configuration42 and the coupling of amine groups with sulfur.31 The peak shifted to 1.86 Å-1 relative to the re-reduced π-π stacking distance of 3.38 Å-1 for the


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H2SO4 dipping PEDOT:PSS implying the re-depletion of partial PEIE in the PEDOT-rich film. In addition, the significant changes can be observed for the lamellar (100) d-spacing (d100) of PEDOT nanocrystals compared to the pristine PEDOT:PSS at 0.30 Å-1 in Figure 3f. H2SO4 dipping induced a larger shift in the qz direction to 0.52 Å-1 corresponding to a small d100 of 12.08 Å-1 compared with the filtering PEDOT:PSS (19.62 Å-1). Noted that PEIE/EG dipping have almost no influence on the lamellar spacing. Based on the preceding discussion, a possible change of steric configuration is proposed in Figure 4 with PEDOT:PSS molecular rearrangement occurred by direct filtering and dipping processes. It refers to the following three aspects: (i) The ethanol-dilution induces the efficient phase separation between PEDOT and PSS as well as the aggregation of PEDOT crystals; (ii) The PEIE/EG dipping results in a slight increase in the π-π stacking direction of PEDOT backbone but no obvious effect on the lamellar distance; (iii) The further H2SO4 dipping significantly decrease in both π-π stacking and lamellar spacing distances leading to a highly ordered PEDOT-rich nanocrystal film. X-ray photoelectron spectroscopy (XPS) analysis were conducted to further study the chemical composition changes in PEDOT:PSS nanofilms, as shown in Figure 5. The S2p peak in Figure 5a between 166 and 172 eV corresponds to the sulfur atoms from the sulphonate group in PSS, while the two peaks from 162 to 166 eV are relative to the sulfur atom from thiophene group in PEDOT.28 43

One can observe that the increased ratio of PEDOT to PSS was 1:1.68 for the filtering

PEDOT:PSS compared with the pristine film (1:2.55),29 indicating a decreased contents of PSS. This is in agreement with the result of 1D GIWAXS in Figure 3. However, the ratio dramatically increased to 1:6.63 after dipping in PEIE/EG solution. This means the PEIE molecules have significant effect on the steric molecular rearrangement referring to PEDOT and PSS due to the


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strong coupling of aliphatic amine group in PEIE with PEDOT. In general, EG dipping is beneficial to the removal of PSS from PEDOT:PSS system according to previous reports.24,35 It also arrows the further aggregation of PSS molecules owing to the interactivity between PEIE and PSS leading to a better orientation for PEDOT crystals, which is associated with the GIWAXS in Figure 3b and 3d. In addition, the peak of PEDOT was slightly shifted to a lower binding energy compared with PEDOT:PSS before PEIE/EG dipping, indicating a reduced oxidation level due to PEIE.16 Further, the H2SO4 dipping increased the ratio of PEDOT to PSS to 1:2.25, which is ascribed to the formation of partial PSSH removed by H2SO4.26-28 Figure 5b further reveals the doping level in filtering PEDOT:PSS films with reduction and oxidation by PEIE/EG and H2SO4 dipping. The N1s peaks at 399.3 eV and 401.9 eV are assigned to the nitrogen atom in neutral amines (N) and protonated amines (N+) from PEIE, respectively.18, 44

The pure PEIE has a small [N+]/[N] ratio based on the reported value by Jin et al,19 indicating

the predominant [N] in PEIE molecule. In the case of PEIE/EG dipping, the [N+]/[N] ratio significantly increases to 0.746, which means the electron transfer occurs from the neutral [N] to protonated [N+]. The PEIE is able to donate its electron to PEDOT2+ due to the presence of an aliphatic amine group, as illustrated in Figure 6, it refers to the route as PEDOT2+:2PSS- + PEIE → PEDOT+:PSS- + PEIE+:PSS-. Therefore, it is reasonable that PEIE with the capacity of donor electron is in favor of the depletion of more PSS from PEDOT aggregation resulting in a better oriented PEDOT crystals. Further, the H2SO4 dipping yielded a much higher [N+]/[N] ratio of 2.016, which is relative to the re-oxidation process as PEDOT+:PSS- + PEIE+:PSS- + 2H2SO4 → PEDOT2+:2HSO4- + PEIEH+:PSS- + H+PSS-. The increased PEIEH+ is due to the presence of uncoupled [N] in PEIE with PEDOT. Different from previous reports,12, 28, 45 the H2SO4 dipping


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process focus on the re-doping for PEDOT with the anionic HSO4-, although partial nonconductive PSS molecules have been removed. To further confirm the validity of tuning the oxidation level by reduction and oxidation, the Raman and Ultraviolet-visible (UV-vis) spectra were performed, as shown in Figure 7. Before PEIE/EG and H2SO4 dipping, Raman spectra show the typical features of pristine PEDOT:PSS for both films.29 Compared to filtering PEDOT:PSS, the peak assigned as C=C vibration in Figure 7a shifts from 1417 cm-1 to 1406 cm-1, which is ascribed to the reduction of PEDOT by PEIE.16 These results are in good agreement with the de-doped PEDOT:PSS films in previous reports.14, 46 This fact is further confirmed by the UV-vis absorption spectra of PEDOT:PSS films in Figure 7b. The filtering PEDOT:PSS nanofilms display a broad infrared absorption over 1200 nm corresponding to the bipolaron of PEDOT2+. The PEIE/EG dipping yield the polaronic PEDOT+ segments from 600 to 1200 nm.16 Interestingly, after H2SO4 dipping, the C = C in-plane vibration re-shifts to 1416 cm-1 due to the increase in doping level of PEDOT caused by anionic HSO4-, but without further blue shift more than 1416 cm-1, indicating the significant hole-blocking role of PEIE.21 The increased absorption for UV-vis over 1100 nm indicate the reappearing bipolaron of PEDOT2+. Noted that a lower bipolaron content after H2SO4 dipping can be observed compared to before. These results are further consistent with the proceeding mechanism. As we known, the electronic transport properties are of great dependence on morphology and molecule orientation as well as the degree of polarization for PEDOT:PSS.17, 47 Therefore, many methods have been developed to complete crystallization of PEDOT:PSS involving commonly available solvent-treatment with acids/alkalis, organic solvents, or ionic liquids,5,6 because of the low electrical conductivity of 0.3 S cm-1 for pristine PEDOT:PSS. For a good TE material, high S and  are simultaneously necessary for PEDOT:PSS considering its inherently low . As shown


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in Figure 8a, the direct filtering PEDOT:PSS nanofilms achieved a large  of 1313 S cm-1. This is ascribed to the efficient phase separation between PEDOT and PSS resulting in a good crystallization of PEDOT. Unfortunately, the low S (14.3 μV K-1) restricts the enhancement of TE power factor. It is desired to further improve S by an effective method. Generally, a de-doping process enables increase in S by addition/treatment with alkalis or reductants.14-16 Meanwhile, it is inevitable that this process will cause a sharp decline in  due to the reduced carrier concentration (n). The PEIE/EG dipping PEDOT:PSS nanofilm gain an enhanced Seebeck coefficient of 24.6 μV K-1 from more imine protonation in PEIE. Importantly, the  is still more than 700 S cm-1 during limited dipping time (180 s) different from previous reports with common reductive treatment in a short time (

Roles of Polyethylenimine Ethoxylated in Efficiently Tuning the

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