University of Groningen
Structure-property and film formation mechanism in PEDOT:PSS based and perovskites systems
Dong, Jingjin
DOI:
10.33612/diss.166892884
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Publication date: 2021
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Dong, J. (2021). Structure-property and film formation mechanism in PEDOT:PSS based and perovskites systems. University of Groningen. https://doi.org/10.33612/diss.166892884
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Chapter 3 Engineering the Thermoelectrical Properties of
PEDOT:PSS by Alkali Metal Ion Effect
Jingjin Dong, Jian Liu, Xinkai Qiu, Ryan Chiechi, L. Jan Anton Koster, Giuseppe Portale Engineering, accepted.
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3.1 Introduction
Organic electronic (OE) materials have recently attracted increasing interest from the scientific community.[1–3] Because of the flexibility and lightweight properties, they hold great potential in a broad range of applications such as wearable sensors and artificial skins.[4– 6] However, it is important to tune and control the properties of these materials in order to prepare ideal devices for different applications.[7,8] Poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonate) (PEDOT:PSS, structure shown in Scheme 3.1) has been extensively studied amongst various other OE materials as it exhibits excellent processability, mechanical and electrical properties.[9,10] One of the hottest application fields is thermoelectrics (TE) due to the potential for energy harvesting in daily life. The efficiency of the TE devices is determined by the so-called figure of merit ZT=𝜎S2T
κ ,
[11,12] where is the electrical conductivity, S is the Seebeck coefficient, is the thermal conductivity and T is the absolute temperature. For polymers, the co-called power factor 𝑃𝐹 = σ𝑆2 is the main aspect to improve due to the intrinsically low thermal conductivity of polymeric TE materials.[13,14]
Polar solvents with high boiling point were first found to be able to enhance the electrical properties of PEDOT:PSS when used as additives or as post-treatment solvents.[2,15,16] For example, the addition of dimethyl sulfoxide (DMSO) into PEDOT:PSS films enhances phase separation and triggers the formation of interconnected networks of crystalline, elongated PEDOT domains. This leads to a great enhancement of σ (from originally ~100 to optimized ~103 S cm-1) without compromising S.[17] On the base of DMSO addition, Lee et al. applied a dedoping process by over-coating a strong reducing agent such as hydrazine onto the PEDOT:PSS nanofilms,[18] which led to an enhanced Seebeck coefficient and PF ( = 578 S cm-1, S = 67 μV K-1, PF = 259 μW m-1 K-2). Fan et al. introduced sequential post-treatments with H2SO4 and NaOH on PEDOT:PSS thin film.[19] After the acid treatment, σ increased dramatically to 2000-3000 S cm-1 due to the sufficient removal of PSS and subsequently formed nanosized fibrous structures which significantly prompt the charge mobility in the film. It was shown that the base treatment induces a low oxidation level of PEDOT and thus improves the Seebeck coefficient. Although was found to decrease during the base treatment, an optimized power factor 334 μW m-1 K-2 was obtained. These results showed that larger power factors can be achieved by first improving the electrical conductivity by morphology and structure modification and then increasing the Seebeck coefficient by lowering the oxidation state of PEDOT. Moreover, the results proved the dependence of the post-treatment agents’ pH on the TE performance. While acid treatment has been well studied due to its positive effect on the electrical conductivity,[20–23] a detailed study on the base post-treatment is still missing, especially using different cations. Unanswered questions are: do certain cations have an effect on the performances, even when samples are exposed to solution with the same pH? If so, what is the mechanism behind the performance change? In this work, a very easy and environmentally-friendly process is applied to tune the thermoelectric properties of PEDOT:PSS. Three different alkali base solutions (LiOH, NaOH,
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and KOH dissolved in DI water) are used as post-treatment agents to the pristine PEDOT:PSS:DMSO thin films deposited by spin-coating (Scheme 3.1). To allow comparison, the same concentration of 1M is used at first, which means the anions and cations can fully dissociate and the pH of the solution is the same.[24–26] Electrical conductivity and Seebeck coefficient are probed to determine the film performances and a series of characterization methods including atomic force microscope (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), UV-vis-NIR spectra and attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) are used to study the effect of the alkali solution treatment on the PEDOT:PSS structure. Besides, different solution concentrations are also used to get a database for tuning the TE properties and other potential applications.
Scheme 3.1 Materials, fabrication and the post-treatment process. 3.2 Results and Discussions
TE properties
In Figure 3.1, the TE properties measured for the pristine film and the films post-treated with LiOH, NaOH and KOH solutions are reported (films referred to as LiOH_PT, NaOH_PT and KOH_PT in the following). As expected, the electrical conductivity decreases upon treatment with alkaline solutions, while the Seebeck coefficient S increases compared to the pristine film. However, a clear trend for both and S of the post-treated films with the kind of cation used (i.e. cations of different size) is observed. The larger the cation size, the lower
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and the higher S. drops significantly from 600 S cm-1 in the pristine film to 244 S cm-1, 201 S cm-1 and 184 S cm-1 in the LiOH, NaOH and KOH post-treated films, respectively. At the same time, S undergoes a dramatic increase from initial 15.0 V K-1 to 20.9 V K-1, 37.8 V K-1 and finally 51.9 V K-1. Considering that the pH of the different solution is identical, we speculate that the main impact on the thermoelectric properties is the cation nature, as it will be discussed below. As a result of the dramatic increase of S, the calculated PF also shows an increasing trend from LiOH_PT to KOH_PT (see Figure 3.1b). Notably, the KOH_PT film exhibits an optimized value of about 50.0 W m-1 K-2 which is almost two times higher than the value for the pristine PEDOT:PSS:DMSO film and is competitive to many other complex engineering methods.[14]
Figure 3.1 Thermoelectric properties of the different PEDOT:PSS:DMSO thin films
post-treated using different basic solutions: (a) conductivity and Seebeck coefficient S together with (b) the calculated power factor PF. The concentration is kept as 1 M.
Surface morphology and crystalline structure
We further investigated the relationship between the properties and structure of the treated films using microscopy and grazing-incidence wide-angle X-ray scattering (GIWAXS). At first, the large-scale film homogeneity of the films was tested using optical microscopy (OM). All the films show homogeneous structure without any significant large-scale inhomogeneity. As an example we report the OM images for the KOH_PT sample (Figure S3.1).
The surface structure was then measured by atomic force microscopy (AFM). The height and phase images of the studied films are captured using tapping mode AFM and summarized in
Figure 3.2. We observed interconnected networks of PEDOT crystals (they appeared as
elongated grains in Figure 3.2a) in the pristine PEDOT:PSS:DMSO film in both height and phase images, which is in good agreement with literature.[17] These interconnected networks are barely visible on the films post-treated with LiOH (labeled as LiOH_PT in Figure 3.2b), instead we observed circular grains (highlighted in Figure 3.2b) in both height and phase images. This morphological transition is persistent also in the films post-treated by NaOH
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(labeled as NaOH_PT in Figure 3.2c) and KOH (labeled as KOH_PT in Figure 3.2d). The surface roughness values for all the studied films is very similar. The formation of a morphology composed by circular grains in the post-treated films can be rationalized by spherical crystalline PEDOT islands being spatially distributed into the matrix of PSS. The transition from an interconnected PEDOT crystallite network to isolated crystalline islands observed here, corroborates reported observations in localized charge carriers and low carrier mobility due to shortened mean free path.[27] On the other hand, potential barriers generated from the grain boundaries in post-treated films only allows the transport of high energy charge carriers to the cold side,[28] thus explaining the increase in the Seebeck coefficient reported in Figure 3.1.
Figure 3.2 AFM height (top) and phase (bottom) images of pristine PEDOT:PSS:DMSO
films (a) and PEDOT:PSS:DMSO films post-treated with LiOH (b), NaOH (c) and KOH (d).
The semi-crystalline structure of the pristine and post-treated films was investigated by GIWAXS as shown in Figure 3.3a-d. Similar to the surface morphology, the bulk film crystalline structure undergoes a great change after the post-treatment with base solutions. To better visualize the structural changes, full integration line profiles from the images are also reported in Figure 3.3e. The low angle region features two peaks, the 100 reflection located at the lowest q-values and a second peak located around 0.48 Å-1. Note that the dip caused by the gaps in the Pilatus detector sits in between these two peaks in the full integration profile as shown in Figure 3.3e. However these two peaks are clearly visible in the intensity linecuts along the vertical out-of-plane (qz) direction as is shown in Figure S3.2.
The first thing to note is that the 100 peak located around q = 0.24 Å-1 for the pristine film decreases in intensity after post-treatment (decrease of 4 %, 20 % and 37 % for LiOH, NaOH
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and KOH, respectively) and shifts to 0.28 Å-1, 0.32 Å-1 and 0.36 Å-1 for the LiOH_PT, NaOH_PT and KOH_PT films, respectively. This means that the d-spacing of the 100 planes becomes smaller and smaller going from the pristine to KOH_PT film (from 26.2 Å to 22.4, 19.6 and 17.4 Å, respectively). This more compact packing along the 100 direction is expected to help the charge carrier transport inside the crystal.[29] The 100 d-spacing for the spin coated PEDOT:PSS:DMSO films reported here is somehow bigger than what we have previously reported for the drop-casting films, indicating differences in the packing as obtained from these two film preparation methods.[30] Remarkably, the intensity of peak located at q = 0.48 Å-1 along the qz direction exhibits a great increase going from the pristine to the KOH_PT film. While the 100 peak position clearly shifts with the alkali metal atom used, the position of the peak at q = 0.48 Å-1 is almost unchanged. Moreover, the trend in intensity of these two peaks is opposite. These observations suggest that the peak at q = 0.48 Å-1 cannot be associated to the second order of the 100 peak, as usually reported for pristine PEDOT:PSS:DMSO. Indeed, two different types of packing were recently reported by Biemann et al., each one characterized by a different 100 peak position.[31] Type I PEDOT crystals show sufficiently doped PEDOT chains closely surrounded by PSS and exhibit a larger d-spacing along the 100 direction (100 peak located towards smaller q-values). On the contrary, Type II PEDOT crystals show little or even no PSS inside the PEDOT crystals, making the 100 d-spacing smaller (100 peak located towards higher q-values). In our case, the PEDOT packing varies from mainly Type I to a mixture of Type I and Type II, depending on the nature of the alkaline solution used (the calculated ratio of Type I to Type II changed from 1.80 to 1.31, 1.10 and 1.05 going from pristine to LiOH_PT, NaOH_PT and KOH_PT, respectively). This indicates the progressive removal of doping PSS from the PEDOT crystallites and thus, the occurrence of a base-induced dedoping process. Besides a change on the PEDOT packing structure, also the free PSS chains not directly associated to PEDOT show changes following the base treatment. The free PSS peak exhibits a clear shift from around 1.32 Å-1 for pristine film to 1.24 Å-1, 1.28 Å-1 and 1.28 Å-1 for LiOH_PT, NaOH_PT and KOH_PT, respectively. This observation indicates that the alkaline solutions have an interaction with the free PSS inside the films, making the average distance among PSS chains larger (from 4.8 Å for pristine to 5.1 Å, 4.9 Å and 4.9 Å for LiOH_PT, NaOH_PT and KOH_PT, respectively). In contrast, the 010 peak which is associated to the ordering along the PEDOT − stacking direction remains at the same peak position around 1.83 Å-1, suggesting that the post-treatment agents do not affect the inner molecular packing distance of PEDOT chains along the − stacking direction. However, changes are observed in the 010 peak width after post-treatment (see Figure 3.3e). The observed 010 peak broadening is minor for LiOH_PT, but not negligible for the other two samples. The 010 peak width significantly increases for the NaOH_PT and KOH_PT films, which means that the crystal coherence length along the 010 direction (S010) becomes smaller. S010 changes from 18.5 Å to 15.7 Å and thus the average number of − stacking layers decreases from 5.4 to 4.5 in average. This crystal quality loss well explains the observed decrease in film conductivity.[30] Moreover, the 010 peak shows a great intensity enhancement along the in-plane horizontal qy direction (indicating edge-on orientation) and a decrease along the out-of-plane vertical qz direction (indicating face-on orientation) upon film post-treatment (see azimuthal intensity
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profiles in Figure 3.3f). The estimated fractions of face-on and edge-on crystals for pristine PEDOT:PSS:DMSO are 53.3 % and 12.3 %, respectively, while these fractions change to 48.0 % and 18.1 % for the post-treated samples (values calculated using the method presented in our previous work [30]). This indicates that base post-treatment promotes a more edge-on orientation of the PEDOT crystallites. This crystal orientation change could partially compensate for the conductivity loss, limiting the drop in . Together with the AFM results, the GIWAXS observations clearly reveal that the elongated grains present in the pristine film are formed by heavily doped PEDOT crystals with preferential face-on orientation, while the smaller globular domains appearing in the post-treated films are formed by less doped (even neutral) smaller PEDOT crystals with a less pronounces orientation, but still mainly with face-on orientation.
Figure 3.3 (a-d) GIWAXS images, (e) full integration line profiles and (f) peak intensity
around q = 1.83 Å-1 (PEDOT - stacking) against the azimuthal angle for different base solutions post-treated PEDOT:PSS:DMSO thin films. The dips in the intensity profiles of Figure (e) and (f) are due to the gaps in the Pilatus detector.
Electronic structure
UV-vis-NIR absorption spectrum was applied to study the oxidation state of the thin films post-treated with the different alkali base solutions. As shown in Figure 3.4a, compared to the pristine PEDOT:PSS:DMSO film, the post-treated films feature a remarkable increase in the signal intensity in the wavelength range 400-700 nm and 700-1200 nm, which represents the neutral and polaron states according to the conventional explanation.[32,33] Concomitantly, the spectra of the post-treated films show a clear intensity decrease in the 1200 nm - 1600 nm wavelength region which represents the bipolaron state. Among the three bases,
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LiOH_PT and NaOH_PT exhibit quite close behavior, while KOH_PT shows a much stronger change. These variations are quite similar to the ones observed by Stepien et al. for PEDOT:PSS films prepared from KOH added solution,[34] and constitute a direct evidence of the dedoping process caused by the base solutions. The shrink of bipolaron band and bipolarons dissociation into polarons and even neutral species observed here is associates to an upward shift of the Fermi level, far away from the valence band, which makes the material a non-degenerate semi-conductor and well explains the high Seebeck coefficient measured for the post-treated films (see Figure 3.1a).[35,36] We also note that recently the interpretation of the PEDOT:PSS UV-vis-NIR spectra has been revised. Zozoulenko et al. reported on the basis of DFT calculations that the peak at 700-1000 nm could be attributed to both polarons and bipolarons, and the peak at NIR range could be attributed to polaronic and bipolaronic states coming from PEDOT with high oxidation level.[37] However, the conclusions drawn here still stand also in the view of this new interpretation, as the increase/decrease trend at the vis/NIR range supports the variation from high oxidation state to low oxidation state after base solution post-treatment.
To further study the effect of the base post-treatment on the chemical structures, attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) was employed. As shown in Figure
3.4b, several changes can be observed when comparing the ATR-FTIR of the post-treated
films with the pristine film. The peak at wavenumber 1155 cm-1 decreases dramatically upon base treatment. This signal is associated to the asymmetric S=O stretching of PSS in the proton form, indicating that –SO3H changed to –SO3-.[38] The increase of the peak at 1524 cm-1 which belongs to symmetric C=C stretching of the thiophene ring and the decrease of the peak at 1557 cm-1 (shifted to 1547 cm-1) which belongs to the asymmetric C=C stretching together suggest a change in the structure from a more quinoid to a more benzoid structure.[39,40] A red shift from 1263 cm-1 to 1249 cm-1 which represents the C−C’ inter-ring stretching also gives the idea that the C−C’ varied from a more double bond structure (quinoid) to single bond structure (benzoid). All these changes further support the base-induced dedoping process discussed above. Importantly, LiOH_PT exhibited a clear shoulder at the wavenumber 1220 cm-1. This peak could be attributed to S=O stretching from -SO3 -Li+, indicating a stronger interaction between PSS- and Li+. This observation is very important and will help understanding the working mechanism discussed below.
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Figure 3.4 (a) UV-vis-NIR and (b) ATR-FTIR spectra for different bases post-treated
PEDOT:PSS:DMSO thin films.
Working principle
According to all the findings discussed above, a possible mechanism can be summarized. As shown in Scheme 3.2, exposure to a base solution causes a neutralization reaction taking place with PSSH changing to PSS-. At the same time, part of the highly doped PEDOT chains (bipolarons) get reduced (into polaron state). Due to the different ion sizes, the three alkali ions have different affinity to PSS-. Li+ as the smallest would have the highest affinity, while K+ as the biggest would have the lowest affinity. In this situation, we can consider PSS- as an ion exchange resin. In the next step, washing with DI water removes all the free ions. For KOH_PT, free ions means all the K+ applied, while for LiOH_PT, some Li+ still remain inside the PSS matrix. This is evidenced by the PSS peak shift (larger d-spacing) shown in GIWAXS and the appearance of the salt form sulfonate peak in ATR-FTIR. To balance the negative charges (dissociated sulfonate groups in the PSS matrix) caused by the removal of positive ions, the p-type doped PEDOT will accept the electrons from PSS- and get dedoped. As KOH_PT has the most dissociated sulfonate groups, it is supposed to have the lowest doping state. The difference in ion affinity for the PSS will be particularly true when the interaction is not in the hydrated state,[41] as is in our case.
In Figure S3.3a, time evolution of the integrated GIWAXS profiles for 1 M LiOH solution post-treatment process before washing is shown (similar trends are also observed for NaOH and KOH). After alkali solution exposure, water evaporates within the first 5-7 minutes. Right after the excess liquid droplet is evaporated from the surface of the film (time point t ~ 420 s), the free PSS-/Li+ peak is located at q ~ 1.25 Å-1. Its position does not shift anymore in time until the end of the drying process (t ~ 1800 s), but the free PSS peak only becomes sharper during drying. Moreover, the sample peak position is recorded after the washing and temperature annealing steps (see Figure 3.3e). The average distance among the PSS chains is around 5.0 Å for LiOH_PT and around 4.9 Å for NaOH_PT and KOH_PT, suggesting that within the free PSS domains tight nanochannels are present, allowing only the diffusion of "dehydrated" cations.[41] The mechanism unveiled here well explains the counter-intuitive TE
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performance difference among PEDOT:PSS:DMSO films post-treated by bases with the same reducing equivalent but different alkali cations. Our results are in agreement and complement previously published works on acid-base treatment of PEDOT:PSS.[42]
Scheme 3.2 Proposed mechanism of the alkali base solution post-treatment process to the
PEDOT:PSS based thin films.
To further confirm this hypothetical mechanism, another three post-treatment agents were also studied. As shown in Figure 3.5, PEDOT:PSS:DMSO films post-treated with aqueous LiCl, NaCl, and KCl solutions show a lower electrical conductivity and higher Seebeck coefficient performance than the pristine PEDOT:PSS:DMSO film. However, there is no obvious trend among themselves. stays around 400-500 S cm-1 and S around 22.2 V K-1 (Figure 3.5a). Also, no clear trend is observed for the calculated PF that oscillates between 23 and 27 W m-1 K-2 (Figure 3.5b). Moreover, these values are quite close to the value obtained when PEDOT:PSS:DMSO is treated with simple water washing. This observation matches well with our hypothesis of the ion exchange resin effect, as it strongly depends on the pH.[43,44] While free PSSH can be successfully transformed into PSS- by the OH- when exposed to the alkali base solutions, the Cl- ions have much lower interaction with PSSH, so the ion exchange resin mechanism does not apply to the salt solution post-treatment. In addition, it excludes the energy filtering effect principle. Assuming that the energy filtering effect dominates the Seebeck coefficient difference, the trend among chloride salts treated samples is supposed to be the same with the bases. However it is not the case here.[45,46]
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Figure 3.5 TE properties of PEDOT:PSS:DMSO thin films post-treated with different salt
solutions: (a) conductivity and Seebeck coefficient and (b) Power Factor calculated.
Concentration dependence
In Figure 3.6(a-c), three TE property maps against both different alkali metal ions and solution concentrations are shown. With the increase in concentration, drops from 600 S cm-1 to 60 S cm-1, and S increases from 16 V K-1 to 56 V K-1. Notably, when the highest concentration of 5 M was used for KOH, the Seebeck coefficient shows a decrease compared to lower concentrations. This could be explained by the fact that the high concentration of the base destroys not only the elongated chain structure but also the crystallinity, so that the thermal driven carriers would be blocked when moving to the cold side.[39,47]
Figure S3.4 shows the GIWAXS results of NaOH_PT films post-treated with different
concentrations, and the structure revealed matches well with the TE properties shown here. The data presented in Figure 3.6 highlight the sensitivity of PEDOT:PSS towards the concentration and the nature of the alkali solution.
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Figure 3.6 (a) Electrical conductivity, (b) Seebeck coefficient and (c) Power Factor of
LiOH_PT, NaOH_PT and KOH_PT films with different post-treatment agent concentrations.
3.3 Conclusions
In summary, in this paper we show that the thermoelectric performance of PEDOT:PSS thin films can be finely tuned using exposure to different alkali metal ions basic solutions. The post-treatment method explored here is simple and green. Various characterization techniques including AFM, GIWAXS, UV-vis-NIR, and ATR-FTIR are employed to reveal the possible working principle. A series of different post-treatment concentrations was applied, allowing to reach an optimal PF of 56 W m-1 K-2 when using a 2M KOH solution. Based on the measured TE properties, a database for the electrical conductivity and Seebeck coefficient as a function of the post-processing conditions is presented here.
The changes in the material thermoelectric properties are explained using an ‘ion exchange resin effect’, based on the different affinity between the alkali cations and the PSS- chains. The alkali metal ion effect reported here could be further explored for potential applications in different fields such as hole transport layer for solar cells, organic electrochemical transistors (OECTs) and memristors for neuromorphic devices.
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Experimental Section
Materials: PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus.
DMSO(99.8%), LiOH, NaOH and KOH were purchased from Sigma Aldrich.
Materials: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
aqueous solution (Clevios PH1000) was purchased from Heraeus, Germany. Dimethyl sulfoxide (DMSO) (99.8%), LiOH, NaOH, and KOH were purchased from Sigma Aldrich, Germany.
Film preparation: The borosilicate glass substrates (17.8 mm × 13.8 mm × 0.7 mm) were
sequentially washed using detergent, acetone, and isopropanol. Next, the substrates were dried using a nitrogen gun and treated by ultraviolet (UV)–ozone for 10 min. The thin films were prepared by spin-coating PH1000-5% DMSO solution on the borosilicate glass. The speed was kept at 2000 rpm (revolutions per minute) and the thin film was subsequently placed on a 130 °C hot plate for 10 min and then left to cool at room temperature. The film thickness (d) was about 60–65 nm. For the post-treated films, a total of 200 L base solutions were injected onto the pristine film for 1 min. After removing the solvents, deionized water was used to wash the film three times. The film was subsequently placed on a 50 °C hot plate for 1 min to gently remove the remaining water; then the temperature was increased to 130 °C for 10 min. After cooling at room temperature, the final thin-film samples, which were extremely homogeneous, were successfully prepared. For the ultraviolet-visible-near-infrared (UV-vis-NIR)absorption spectrum, the films were cast on quartz substrates using the same process. All the processes were performed in our lab under stable humidity of around 30%–35%.
Characterization of thin films: The electrical conductivity was measured by an Ossila
four-point probe station. For the Seebeck coefficient measurements, we kept the setup the same as reported before.[48] The UV-vis-NIR absorption spectrum of the solid films was measured using a Shimadzu UV-3600. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) measurement was carried out with a PIKE MIRacle ATR accessory with a diamond prism in a Vertex 70 spectrometer (Bruker, USA), and the system was continuously purged with nitrogen gas (N2). Infrared (IR) spectra were acquired at a resolution of 4 cm−1 and 32 scans were taken between 1800 and 800 cm−1. The presented spectra were baseline-corrected using the same rubber band correction for all spectra. The measurements were performed on the diEnviroScope (Veeco, USA) with TESP probes (Bruker, USA) . Microscope measurement was performed on a Nikon E200 at ten-fold magnification. For surface morphology measurement, tapping mode AFM measurements were performed on a Bruker AFM multimode MMAFM-2 equipped with a RTESPA-300 probe (resonant frequency 300 kHz, spring constant 40 N·m−1, Burker, USA). The height images and phase images were captured at a scan rate of 0.8 Hz and 640 points per line. The data were analyzed with Nanoscope Analysis 1.5 (provided by Bruker, USA). To determine the thickness, small scratches were made in the film using a very fine needle. The scan direction was set perpendicular to the scratch direction, allowing the determination of the height of the scratch (and therefore the film thickness). Grazing-incidence wide-angle X-ray scattering (GIWAXS)
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measurements were performed at the Dutch–Belgian beamline (DUBBLE) BM26B at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. An X-ray beam with an energy of 12 keV ( = 1.033 Å, where represents the beam wavelength) was used with a sample-to-detector distance of 407.4 mm. GIWAXS frames were collected using a Pilatus 1M camera and using an exposure time of 30 s per frame. All the necessary corrections for GIWAXS data have been taken into account (detector efficiency, flat field, solid angle, and polarization). The beam center was estimated using the known position of diffracted rings from standard Silver Behenate and -Al2O3 powders. The scattering vector q was defined with respect to the center of the incident beam and has a magnitude of q = (4/)sin(2), where 2 is the scattering angle of the X-ray beam. Herein, we opted to present the wedge-shaped corrected images, where qxy and qz are the in-plane and near-out-of-plane scattering vectors, respectively. The scattering vectors are defined as follows:
𝑞 = {
𝑞𝑥= 2𝜋𝜆 (cos (2𝜃𝑓) cos(𝛼𝑓) − cos (𝛼𝑖)) 𝑞𝑦 = 2𝜋 𝜆 (sin(2𝜃𝑓) cos(𝛼𝑓)) 𝑞𝑧 = 2𝜋 𝜆 (sin(𝛼𝑖) + sin (𝛼𝑓)) ,
where f is the exit angle in the vertical direction, i is the incident angle and 2f is the in-plane scattering angle, in agreement with standard GIWAXS notation. The parallel component of the scattering vector is calculated as:
𝑞𝑟 = √𝑞𝑥2+ 𝑞𝑦2
An incident angle i = 0.25° was used for all the samples. The crystal coherence length along the 010 direction (CCL010) was calculated by the following equation: CCL010= 2/FWHM, where FWHM represents the full width of the half maximum of the peak. The
in situ GIWAXS setup was the same as we reported previously.[49] A total of 200 L base solutions were remotely controlled to drop onto the pristine films, and were left to dry at room temperature. The GIWAXS images were acquired after the dropping of the bases.
Acknowledgements
The ESRF and NWO are acknowledged for allocating the beam-time at the Dutch-Belgian beamline (DUBBLE, ESRF, Grenoble) for the GIWAXS experiments. The authors are grateful to the DUBBLE team for their help during the beam time. G.P. acknowledges the Zernike Institute for Advanced Materials for the startup funds. J.D. and G.P. are grateful to the China Scholarship Council (CSC No. 201606340158).
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Supplementary Figures
Figure S3.1 Optical microscope of the KOH_PT sample (a) with and (b) without a scratch
showing great large-scale homogeneity. To note: this scratch is made with a fine needle on purpose for better contrast. Note that the black spots are caused by some dust in our optics.
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Figure S3.2 GIWAXS integration linecuts in the out-of-plane direction (qz) for pristine
PEDOT:PSS:DMSO films and PEDOT:PSS:DMSO films post-treated with 1 M LiOH, NaOH and KOH solutions. The dips are due to the gaps in the Pilatus detector.
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Figure S3.3 Time evolution of the integrated GIWAXS profiles for 1 M LiOH solution
post-treatment process (a) and the profiles at the time point T = 420 s and T = 1800 s (b). The sharp peaks are due to the LiOH crystallization on the film surface.
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Figure S3.4 GIWAXS images (a-e) and full integration linecuts of NaOH_PT films with
different concentrations of solution. The dip in the intensity profiles in Figure (f) are due to the gaps in the Pilatus detector.
