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[Zhenyun Xiao](https://orcid.org/0000-0001-9892-1753), [Masaki Ishii](https://orcid.org/0009-0008-6846-9610), [Jun Takeya](https://orcid.org/0000-0002-7003-1350), [Katsuhiko Ariga](https://orcid.org/0000-0002-2445-2955), [Yu Yamashita](https://orcid.org/0000-0001-7966-3197)

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[Chemical doping of a semicrystalline polymeric semiconductor realizing high stability and work function](https://mdr.nims.go.jp/datasets/4be080c3-c941-4d58-8fb4-c24bfe511c31)

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Journal NameChemical doping of a semicrystalline polymeric semicon-ductor realizing high stability and work function†Zhenyun Xiao,a,b Masaki Ishii,a,c Jun Takeya,a,b Katsuhiko Ariga,a,b,c and Yu Yamashita∗a,bRecently, doped organic semiconductors with high work func-tions have been studied for opto-electronic device applica-tions. However, a higher work function commonly results inpoorer stability due to redox reactions with water in air, lim-iting device processing and applications. In this study, combi-nations of bulky anions and a semicrystalline polythiophenederivative were explored for chemical doping under ambi-ent conditions. The use of tetrakis(pentafluorophenyl)borate(TFPB) anion resulted in a high work function over 5.5 eVwith remarkably improved stability, with 90% of the conduc-tivity retained after storage in air for 20 days. The stabilityat elevated temperatures of 100 ◦C and 125 ◦C was also dra-matically improved compared with the use of other dopantions. X-ray diffraction measurements suggest that intercala-tion of the TFPB anion occurs in the lamellar structure of thepolymer, while in-plane π-stacking structures are present. De-doping reactions including self degradation at high tempera-tures seemed to be suppressed for this inert dopant anion,which contributed to the observed exceptional stability. Ourfindings provide promising insights into the design of com-binations of polymers and dopants to achieve highly stabledoped organic semiconductors.IntroductionTo improve device performance, doping control of organic semi-conductors has been studied extensively1 through chemical andelectrochemical processes for applications including transistors,Schottky diodes2, light-emitting diodes3,4, and solar cells5–7.aResearch Center for Materials Nanoarchitectonics (MANA), National Institute for Ma-terials Science (NIMS), Namiki 1-1, Tsukuba, Japan.bMaterial Innovation Research Center (MIRC) and Department of Advanced MaterialsScience, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha5-1-5, Kashiwa, JapancGraduate School of Science and Technology, Tokyo University of Science, Yamazaki2641, Noda, Japan† Electronic Supplementary Information (ESI) available: [details of any supplemen-tary information available should be included here]. See DOI: 00.0000/00000000.This approach may lead to printed high-performance opto-electronic devices based on organic semiconductors with process-ability and tunable electronic properties8,9. The importance ofdoping may be obvious considering that the silicon-based elec-tronics industry is established on the foundation of semiconduc-tor doping technology, where the introduction of impurity atoms(dopants) enables precise control of carrier density and electricalproperties10. For organic semiconductors, chemical doping us-ing redox agents has been employed, which introduces carriersand molecular ions into the organic semiconductors. This pro-cess controls carrier concentration, conductivity, work function,and other opt-electronic properties of organic semiconductors in-cluding thermoelecric performance11–14. One of the remainingchallenges in chemical doping is limited stability of dopants anddoped organic semiconductors13,15–17, where dopants and dopedorganic semiconductors are generally prone to react with water oroxygen in environments.Efforts have been made to obtain stable doped organic semi-conductors based on material engineering in semiconductorsand dopants. Tuning the energy levels of the highest oc-cupied molecular orbital (HOMO) for p-type semiconductorsand the lowest unoccupied molecular orbital (LUMO) for n-type semiconductors has been demonstrated to be an impor-tant factor. For p-type doped semiconductors, one of a ma-jor degradation mechanisms is reduction of the doped semicon-ductors by water or other impurities18. Thus, design of sta-ble materials would be different from the nearly intrinsic p-type semiconductors such as ones employed in thin-film tran-sistors19,20, whose degradation may involve oxidation reactionsby oxygen in air. For instance, compared with polythiophenederivatives with alkyl chains, such as poly(3-hexylthiophene-2,5-diyl) (P3HT) or poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), those with alkoxy or ethylene glycol sidechains show shallow ionization potentials (IPs) and improved sta-bility in the doped state21,22. Reactions with water or oxygen of-ten include electron transfer with doped organic semiconductors,where the suppression of such reactions is possible by tuning theJournal Name, [year], [vol.],1–7 | 1PQT-C12 filmaqueous solutionFe3+holeacNFSI-PQT-C12edbFe2+Conductivity (S cm-1)Photon energy (eV)BFFFFFFFFF FFFFFFFFF FF−(i) (ii)Absorbance (a.u.)anionanionTFSI-TFPB-FeCl3-LiTFSIFeCl3-LiTFPBFeCl3-LiNFSI100806040200Fig. 1 Doping of PQT thin films. (a) A model of possible chemical doping mechanism with the illustrations of (i) initial and (ii) final state of dopingsystem. (b) The molecular structures of employed materials. (c) UV-Vis spectra of the treated and untreated PQT thin films. The gap at 800 nm isdue to the change of detector. (d) Conductivity of PQT thin films after the chemical doping process. FeCl3 and the denoted salts were dissolved indoping solutions. The two-terminal geometry with channel length of 400 µm and width of 2 mm was employed. Three samples were measured foreach condition, where the error bar shows one standard deviation. (e) PYS spectra of the treated and untreated PQT thin films.energy levels of carriers in semiconductors. This may be a rea-sonable approach considering that perfectly eliminating water un-der the device operating environment is rather challenging23,24.However, the employment of shallow HOMO p-type semiconduc-tors is not advantageous for achieving a high work function thatis necessary to form ohmic contacts at the interface with other p-type materials6,18. This highlights the necessity to achieve stabil-ity without sacrificing the work function, that is , without relyingon p-type semiconductors with very shallow HOMO levels.As another approach to improve stability, the tuning of dopantions using ion-exchange doping25 has been demonstrated. In thismethod, electron transfer reactions occur between an oxidant of aredox agent and a semiconductor, which leaves the cationic semi-conductor and the anionic reductant. Then, the anionic reductantis replaced by another anionic molecule introduced into the sys-tem, thereby enabling the introduction of various anionic dopantmolecules into the p-doped semiconductors. This method realizesincorporation of various ions in a controlled manner26–28. Im-provements in environmental stability have been demonstratedby introducing bulky hydrophobic anions29,30. It has been stillunclear how high the work function and stability can be simul-taneously achieved by tuning the molecular dopant ions. Here,in addition to the redox reactions with water, various degrada-tion and side reactions need to be considered depending on theemployed materials15,31.In this study, we focused on a polymeric semiconductor that canaccommodate relatively bulky anions to develop ambient-stabledoped polymer films. Specifically, we employed the semi-crystalline polymer poly(3,3’-didodecylquaterthiophene) (PQT).PQT thin films were immersed in aqueous solutions containingferric chloride (FeCl3) as a oxidising agent and hydropho-bic molecular ions, namely bis(trifluoromethylsulfonyl)imide(TFSI), bis(nonafluorobutanesulfonyl)imide (NFSI), andtetrakis(pentafluorophenyl)borate (TFPB). Efficient p-typedoping of the PQT thin films was demonstrated by UV-Visabsorption, conductivity, and photoelectron yield spectroscopy(PYS) measurements. The TFPB-doped PQT films exhibitedboth a high work function over 5.5 eV and robust ambientand thermal stability. X-ray diffraction (XRD) measurementsindicated successful accommodation of the bulky TFPB− in thelamella of PQT. Our study sheds light on the possibility of tuningenergy levels and molecular and thin-film structures to achieve ahigh work function yet stable doped polymeric semiconductorsthrough an ambient solution process.Results and discussionPQT thin films were fabricated by spin-coating a polymer so-lution onto glass substrates. The deposited films underwent athermal annealing treatment followed by slow cooling to roomtemperature. The thin films were chemically doped in differentdopant solutions for 60 min and then dried by blowing Ar. Thedopant solutions contained FeCl3 or a combination of FeCl3 anda salt of Li-TFSI, Li-NFSI, or Li-TFPB (Fig. 1a,b). This composi-tion is designed such that electron transfer reactions between PQTand FeCl3 can occur while intercalation of the additional anions(TFSI−, NFSI−, or TFPB−) into PQT thin films occurs to compen-sate for the positive charges of the holes. Our chemical dopingwas conducted under ambient conditions. To control the degra-dation of dopants through redox reactions with water, we em-ployed aqueous solutions with a controlled pH32. Deactivation2 | 1–7Journal Name, [year], [vol.],a b c de f g hPristineFeCl3FeCl3-LiTFSIFeCl3-LiTFPBFeCl3-LiNFSIFig. 2 XPS spectra of doped PQT thin films. (a) C 1s, (b) S 2p, (c) B 1s, (d) Fe 2p, (e) Cl 2p, (d) O 1s, (e) N 1s, and (f) F 1s regions.of Fe3+ oxidising agents by redox reactions with water is not ex-pected under acidic conditions33. In this study, a mixture of pH 2phosphate buffer solution and 20 wt% acetonitrile was employedas a solvent to suppress the oxidation of water while acceleratingthe doping reactions by swelling the polymer.The doping effect of the PQT thin films was characterisedby UV-vis and conductivity measurements. The pristine PQT filmshowed a peak originating from the HOMO-LUMO transition at527 nm34,35 (Fig. 1c). When the thin film was immersed in theFeCl3 solution without additional salts, no clear changes in thespectrum could be observed. In contrast, addition of Li-TFSI, Li-NFSI, or Li-TFPB resulted in decreased intensity of the peak at527 nm and new features at 770 nm and above 1100 nm. Thesechanges are consistent with p-type doping of the polythiophenederivatives36. The electrical conductivity of the PQT thin filmswas measured by fabricating doped films on glass substrates withprepatterned Cr/Au electrodes. While the conductivity of pristineand FeCl3-treated thin films were 2×10−7 and 7×10−7 S cm−1,ones for anion-exchanged doped thin films were over 50 S cm−1(Fig. 1d), which supports the successful p-type doping of PQT thinfilms. Representative current-voltage plots are shown in Fig. S1.In an efficient doping process, the Fermi energy and IP of thesemiconductor would show a large shift, which was confirmedby PYS measurements (Fig. 1e). The IPs of the thin films weredetermined based on the energy threshold of the third root ofphotoelectron yield (γ)37. Fitting of the PYS spectra is shown inFig. S2. The IP of the pristine PQT thin film sample was estimatedto be 4.78 eV. The PQT thin film immersed in the FeCl3 solutionexhibited a slightly increased IP of 4.87 eV. In contrast, IPs for PQTthin films doped with a combination of FeCl3 and additional saltswere 5.25 eV for Li-TFSI, 5.47 eV for Li-NFSI, and 5.56 eV for Li-TFPB. These large shifts in IPs are consistent with the efficient p-type doping of PQT, where electrons are removed from the HOMOdensity of states. Importantly, these IPs of a highly conductingpolymer are equal to the work functions, where the Fermi energyis inside the HOMO density of the state. The difference in workfunctions depending on the employed salts may not be explainedby the difference in doping level, considering the almost identicalresults in the UV-Vis and conductivity measurements. This pointis discussed after confirming the composition of dopants insidethe thin films.X-ray photoelectron spectroscopy (XPS) measurements wereperformed to determine the composition of the doped PQT thinfilms (Fig. 2). The PQT films were prepared in the same manneras in the above experiments on glass substrates with pre-coatedCr/Au. All samples showed peaks in the C 1s and S 2p regions,which is consistent with the molecular structure of PQT. WhenFeCl3 was employed in the doping solution, almost no changeswere observed in the C 1s and S 2p peaks. Fe 2p and Cl 2p peakswere not observed, which is consistent with the low doping levelfor this condition. When combinations of FeCl3 and additionalsalts were employed, the Fe 2p and Cl 2p peaks were not ob-served. Instead, when Li-TFSI and Li-NFSI were employed, theintroduction of TFSI− and NFSI− was suggested based on theJournal Name, [year], [vol.],1–7 | 3observations of N 1s, O 1s, and F 1s peaks. In addition, in theS 2p region (Fig. 2b), the use of Li-TFSI and Li-NFSI resulted inadditional peaks at 167.5 and 168 eV. These positions are consis-tent with the reported values for sulfur atoms in the sulfonylim-ide structure38,39. When the combination of FeCl3 and Li-TFPBwas employed, peaks were observed in the B 1s and F 1s re-gions, in addition to the C 1s and S 2p regions. In the B 1s re-gion (Fig. 2c), there are peaks that may originate from impuritiespresent in the pristine sample, whereas only the sample using Li-TFPB showed an additional peak at 188.3 eV. The above resultsverify that our chemical doping process introduces TFSI−, NFSI−,or TFPB− when salts of these anions and FeCl3 are dissolved inthe doping solutions.Table 1 Atomic compositions (%) estimated from XPS measurements.Sample C S B O N FPristine 92.4 7.6 - - - -FeCl3 92.6 7.4 - - - -FeCl3 + Li-TFSI 78.2 8.7 - 4.4 0.9 7.9FeCl3 + Li-NFSI 71.4 7.1 - 4.4 1.5 15.6FeCl3 + Li-TFPB 78.9 5.0 0.4 - - 15.7Atomic compositions were calculated based on the XPS spec-tra, and summarized in Table 1. Base on the carbon and fluorineatomic compositions, the ratios of dopant/monomer around thesurface were calculated to be 0.70, 0.54, and 0.52 for TFSI−-, NFSI−-, and TFPB−-doped films respectively, which agrees thehigh doping levels achieved in our process.Considering that bulky anions were introduced to the PQT thinfilms, the anion-dependent work functions observed in PYS mea-surements may be attributed to the effects of vacuum level shiftsowing to dipoles between holes and counter anions18,40,41. Inthis case, the size of the dipole depends on the distance betweenthe positive and negative charges, which should be affected bythe size of the counter anion. Thus, the large sizes of NFSI−and TFPB− may have contributed to the large work functions ob-served when these anions were employed compared to the casewith TFSI−.The ambient and thermal stability of the doped PQT thin filmswere evaluated using UV-Vis spectroscopy and conductivity mea-surements. In UV-Vis measurements, one-hour heating of dopedfilms under air at 125 ◦C resulted in decreased doping levelsfor TFSI−- and NFSI−-doped films, which is evidenced by theincreased intensity of the peak around 540 nm (Fig. 3a-b). Incontrast, this heating process did not alter the spectrum of theTFPB−-doped film, indicating the high thermal stability of thesample (Fig. 3c). When the conductivity was measured underair at room temperature, the TFPB−-doped film exhibited a highretention ratio of approximately 90.1% after 20 days (Fig. 3d),which was superior to that of TFSI−- and NFSI−-doped films. Thestability of the conductivity was also tested at 100 ◦C and 125 ◦Cunder air (Figs. 3e-f), wherein the PQT films doped with TFPB−demonstrated the highest stability among the tested samples.There are various mechanisms that lead to decrease in dopinglevels of p-type doped polymers under ambient conditions andhigh temperatures. For high work function materials, redox reac-tions with water are anticipated, where use of lowly hygroscopicmaterials are important to improve stability18,42,43. In additionto this, degradation through other mechanisms need to be con-sidered. When polymers are doped with FeCl3 or anion-exchangedoped using FeCl3, side reactions with FeCl3 or chemicals origi-nating from it including FeCl−4 and Cl− possibly occur15. Otherpossibilities include deprotonation of doped polymers that leavesthe polymer neutral state31. To understand the major cause ofthe dedoping in our system, stability tests were also conducted ina nitrogen purged glove box. Here, we observed severe dedopingfor TFSI−- and NFSI−-doped films while TFPB−-doped film didnot show dedoping features (Supplementary Information Fig. S4and S5). This manifests that dedoping of TFSI−- and NFSI−-doped films mainly occurs not by redox reactions with water butby self degradation of the employed materials at high tempera-tures. In our case, the degradation by side reactions with FeCl3or chemicals originating from it may not be the main cause forthe observed dedoping considering that Fe and Cl atoms were notobserved in XPS measurements of the thin films after the dop-ing and surface washing procedures. Other degradation reactionssuch as deprotonation of the positively charged polymers31 mightbe the main cause of the dedoping, where the proton affinity ofdopant ions and the relative position between the semiconductorand dopant ion would be important.To understand structural changes upon heating for the dopedPQT thin films, XRD measurements were performed (Fig. 4).When a proper choice of dopant molecule is employed to apolythiophene derivative, highly crystalline doped state has beendemonstrated44–46. In the out-of-plane direction, the pristine filmshowed the (100) peak at the 0.29 Å−1 corresponding to the d-spacing of 21.4 Å , consistent with the literature47. While thethin film treated with the FeCl3 solution showed almost identi-cal peak position of the (100) diffraction, TFSI−-, NFSI−- andTFPB−-doped PQT thin films showed increase in d-spacing. Theestimated values of d100 are summarized in Table 2. The d100of TFPB−-doped PQT was greater than other samples, indicat-ing that the larger molecular size of TFPB− resulted in the largerlamellar spacing compared to the case with NFSI− and TFSI−similarly to the literature48. After one-hour heating at 125 ◦C,TFSI−- and NFSI−-doped PQT thin films showed decrease in d100,where the shrink of lamellar spacing occurred probably throughremoval of dopant anions from the thin films. On the other hand,the TFPB−-doped film showed the almost unchanged lamellarspacing after the thermal treatment, which manifests structuralstability of the combination of PQT and TFPB−.Evaluation in the in-plane direction was conducted based onwide angle X-ray scattering images (Fig. S3), which was con-verted to the one-dimensional plots (Fig. 4e-h). The (010) peaksthat correspond to π-stacking periodicity49 were observed forTFSI− and NFSI−-doped films with d-spacing of 3.5 Å. For theTFPB−-doped film, this peak was clearly observed after the ther-mal treatment, which indicates that this thermal treatment wasnecessary to obtain stable crystalline structure for this sample.The d-spacing of the TFPB− after the thermal treatment was 3.5 Å,where this small value is advantageous for the two-dimensionalcarrier transport44.4 | 1–7Journal Name, [year], [vol.],cfadFeCl3-LiTFSIas preparedafter heatingRatio of conductivity (%)Ratio of conductivity (%)Wavelength (nm)FeCl3-LiTFPBas preparedafter heatingWavelength (nm)Time (h) Time (h)beFeCl3-LiNFSIas preparedafter heatingRatio of conductivity (%)Wavelength (nm)Time (h)FeCl3-LiTFSIFeCl3-LiTFPBFeCl3-LiNFSIFeCl3-LiTFSIFeCl3-LiTFPBFeCl3-LiNFSIFeCl3-LiTFSIFeCl3-LiTFPBFeCl3-LiNFSIFig. 3 Ambient and thermal stability of the doped PQT thin films. UV-Vis spectra of (a) TFPB−-, (b) NFSI−-, and (c) TFSI−doped thin films beforeand after heating at 125 ◦C under air. The gap at 800 nm is due to the change of detector. (d) Ambient stability of the conductivity of PQT thinfilms. (e) Thermal stability of the conductivity of doped PQT thin films at 100 ◦C and (f) 125 ◦C under air. After storing the thin films at elevatedtemperatures, conductivity measurements were performed at room temperature. The two-terminal geometry with channel length of 400 µm and widthof 2 mm was employed.a b c de f g hpristineFeCl3after heatingas prepared0.2 0.30.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0(010)(100)after heatingas preparedIntensity (a.u.)pristineFeCl3Intensity (a.u.)Intensity (a.u.)qz (Å-1)0.2 0.3(100)Intensity (a.u.)qz (Å-1)0.2 0.3(100)Intensity (a.u.)qz (Å-1)0.2 0.3(100)Intensity (a.u.)qz (Å-1)qxy (Å-1)0.5 1.0 1.5 2.0(010)Intensity (a.u.)qxy (Å-1)qxy (Å-1)0.5 1.0 1.5 2.0(010)Intensity (a.u.)qxy (Å-1)FeCl3-LiTFSI FeCl3-LiTFPBFeCl3-LiNFSIFig. 4 XRD measurements of the doped PQT thin films. Intensities for the (a-d) out-of-plane and (e-h) in-plane directions. Orange plots wereobtained from thin films after treatments with FeCl3 or combinations of FeCl3 and additional salts. Yellow plots were obtained after heating thetreated samples at 125 ◦C under air.Journal Name, [year], [vol.],1–7 | 5Table 2 D-spacing and FWHM of the (100) peak in doped PQT thinfilms, before and after heating.Sample d100 (Å) FWHM (Å−1)pristine 21.35 ± 0.01 0.0647 ± 0.0007FeCl3 as prepared 21.37 ± 0.01 0.0521 ± 0.0006FeCl3 after heating 20.30 ± 0.07 0.0406 ± 0.0028TFSI− as prepared 23.92 ± 0.03 0.0381 ± 0.0010TFSI−after heating 21.73 ± 0.01 0.0276 ± 0.0005NFSI− as prepared 26.17 ± 0.03 0.0308 ± 0.0010NFSI−after heating 22.76 ± 0.01 0.0204 ± 0.0003TFPB− as prepared 27.87 ± 0.06 0.0355 ± 0.0015TFPB− after heating 27.77 ± 0.02 0.0209 ± 0.0004The above XRD measurements suggest that intercalation of em-ployed dopant anions into the PQT lamella occurs in our method.Considering that the combination of PBTTT and TFPB− was re-ported to result in disordered structure50, the longer separationbetween alkyl side chains of PQT compared to P3HT and PBTTTmay have contributed to formation of large void space to accom-modate TFPB−. When intercalation of dopant anions occurs, theanions and π-stacked semiconductor backbones are in close con-tact. The degradation of polymers through the deprotonationprocess31 might be accelerated by oxygen atoms in TFSI− andNFSI−. After this process, the protonated dopant anions may beremoved from the PQT lamella, which is supported by the shrinkof lamellar spacing after heating observed in our XRD measure-ments. Suppression of such self degradation reactions for the casewith TFPB−doped film may owe to the inert molecular structureof the TFPB− without oxygen atoms. Also, the lowly hygroscopicnature of TFPB− would be advantageous to suppress possible de-doping by redox reactions with water. Further chemical analysisincluding mass spectroscopy15, nuclear spin resonance51,53 or vi-brational spectroscopy31 of the degraded polymers would clarifythe dedoping mechanisms which can vary depending on the em-ployed materials and environments.ConclusionIn this study, the chemical doping of PQT thin films was per-formed under ambient conditions using aqueous solutions con-taining FeCl3 and additional salts. The successful doping of thePQT thin films was corroborated by UV-Vis, conductivity, PYS, andXPS measurements, where holes and TFSI−, NFSI−, or TFPB−were introduced into the thin films. When the bulkest TFPB−served as the dopant anion, enhanced stability of the doped statewas demonstrated in the aging test at room temperature and hightemperatures of 100 ◦C and 125 ◦C. The stability performancewas markedly superior to those of other dopant candidates andpreviously reported literature. Through XRD measurements, thethin-film structures of the doped PQT were evaluated, and PQTwas found to accommodate the bulky anions while maintainingthe lamellar and π-stacking structures. For the TFPB−doped thinfilm, the UV-Vis, conductivity, and d-spacings remained almostunaltered upon heating at 125 ◦C for 1 h. Compared to otheremployed dopant ions, suppression of the self degradation reac-tions at high temperatures seemed to be the key in addition to thesuppression of redox reactions with water in air. The high workfunction and stability observed in the TFPB−-doped PQT thin filmhighlights the possibility of achieving a high work function andstable doped organic semiconductors by designing the host-guestcombinations.ExperimentalFabricationFor the fabrication of PQT thin films, EAGLE XG glass substrateswere employed for UV-Vis, conductivity, PYS, and XPS measure-ments. Cr/Au were pre-deposited for conductivity, PYS, and XPSmeasurements. Si wafers with naturally oxidized layers wereemployed as the substrate for XRD measurements. The thinfilm deposition process involved spin coating from an 1 wt% o-dichlorobenzene solution at a spinning speed of 2,000 rpm for1 minute. Subsequently, the deposited films underwent annealingin a vacuum oven, maintained at 100 ◦C for 1 hour, followed by aslow cooling process. The thickness of the pristine PQT film wasdetermined to be ca. 25 nm based on the AFM images in Fig. S6.Thickness of doped films were assumed to be increased by thesame factor as the increase in lamellar spacings. The PQT filmswere doped by simply immersing them in aqueous dopant solu-tions at room temperature. The doping solutions were preparedby dissolving dopant salts (100 mM for LiTFSI and 1 mM for LiN-FSI and LiTFPB) and FeCl3(10 mM) in a binary solvent mixtureconsisting of an aqueous sulfuric acid solution (80 wt%) at pH 2and acetonitrile (20 wt%). The immersion process was carriedout for 60 minutes, after which the residual doping solution wasremoved by blowing Ar gas. The surface of samples were washedwith pure water after this process.EvaluationElectrical conductivities were measured in air using a Keithley2612B source meter with the two-terminal geometry with channellength of 400 µm and width of 2 mm. UV-vis absorption spectrawere obtained using a V670 (JASCO) spectrometer. XRD datawere acquired using a RIGAKU SmartLab with a MicroMax-007HFX-ray generator using Cu Kα radiation (λ = 0.15418 nm). XPSwas performed using the KRATOS ULTRA 2 instrument with themonochromatic Al Kα X-ray.Author ContributionsZ. X. and Y. Y. designed the experiments. Z. X. and M. I. per-formed experiments. J. T. and K. A. supervised the work. Z. X.and Y. Y. analyzed the data and wrote the manuscript.AcknowledgmentsThis work was supported in part by JSPS KAKENHI grant (no.JP20H00392, JP22H02160). 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