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[Tomohiro Murata](https://orcid.org/0009-0003-9793-8854), Yoshihisa Usami, Akiko Tajima, Tatsuyuki Makita, [Yu Yamashita](https://orcid.org/0000-0001-7966-3197), Jun Takeya

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[Highly Stable Ion‐Exchange Doping of Organic Semiconductor Single Crystals for Reliable Flexible Sensors](https://mdr.nims.go.jp/datasets/6b19bf49-1b3a-4ff1-a56f-cd972c0e1520)

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Highly Stable Ion‐Exchange Doping of Organic Semiconductor Single Crystals for Reliable Flexible SensorsRESEARCH ARTICLEwww.afm-journal.deHighly Stable Ion-Exchange Doping of OrganicSemiconductor Single Crystals for Reliable Flexible SensorsTomohiro Murata,* Yoshihisa Usami, Akiko Tajima, Tatsuyuki Makita, Yu Yamashita,*and Jun Takeya*Thin-film single crystals of organic semiconductors represent a emergingclass of materials for flexible printed electronic devices, including organicfield-effect transistors and strain sensors. Their potential arises from thedemonstration of high-mobility band transport, which significantly enhancesboth device performance and operational reliability. However, achieving stableand precise control over carrier concentration through chemical dopingremains a fundamental challenge, restricting device architecture and broaderapplication. In this study, the challenge is addressed by improving theenvironmental and thermal stability of chemically doped organic singlecrystals through the strategic selection of dopant anions. Specifically,ion-exchange doping using the bulky tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (BArF) anion resulted in high electrical conductivity andexceptional stability under ambient conditions at 80 °C. The doped singlecrystals further exhibited excellent strain sensitivity, maintaining consistentstrain sensing performance over 100 000 bending cycles, with conductivitydrift suppressed to the order of ppm per hour under ambient conditions.These results highlight the importance of dopant ion design for stabilizing thedoped state of organic semiconductors under environmental stress, withoutcompromising mechanical flexibility or electronic functionality. This approachpaves the way for robust and reliable flexible organic electronic devices.T. Murata, Y. Usami, A. Tajima, Y. Yamashita, J. TakeyaDepartment of Advanced Materials ScienceGraduate School of Frontier SciencesThe University of Tokyo5-1-5 Kashiwanoha, Kashiwa, Chiba 277-0882, JapanE-mail: tmurata@edu.k.u-tokyo.ac.jp; YAMASHITA.Yu@nims.go.jp;takeya@k.u-tokyo.ac.jpT.Makita, J. TakeyaPi-Crystal Inc273-1Kashiwa, Kashiwa, Chiba 277-0005, JapanY. Yamashita, J. TakeyaResearchCenter forMaterialsNanoarchitectonicsNational Institute forMaterial Science (NIMS)1-1Namiki, Tsukuba, Ibaraki 305-0044, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adfm.202518055© 2025 The Author(s). Advanced Functional Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distributionand reproduction in any medium, provided the original work is properlycited.DOI: 10.1002/adfm.2025180551. IntroductionThin-film single crystals of organic semi-conductors (OSCs) offer unique opportu-nities for the development of advancedsolution-processed electronic devices, in-cluding field-effect transistors,[1,2] light-emitting diodes,[3,4] and sensors.[5–7] Car-rier transport in OSCs is governed by their𝜋-conjugated system, where 2D molecu-lar packing of the conjugated cores en-ables high-mobility (>10 cm2 V−1 s−1) bandtransport.[8–10] Recent advances in molec-ular design and solution-based processinghave enabled the fabrication of large-areaorganic single crystals with improved solu-bility and high carrier mobility,[2,11] pavingthe way for integrated circuits capable ofoperating in the tens of megahertz rangeand beyond.[12] To fully exploit the potentialof these materials, doping processes thatadjust conductivity after crystal growth areof critical importance.[13–15] Doping in or-ganic semiconductors (OSCs) is achievedby introducing impurity molecules that ex-change electrons with the 𝜋-conjugatedsystem.[16–18] To avoid disrupting the highlyordered structure of OSC single crystals, carrier concentrationcan be modulated by forming a dopant or ionic layer exclusivelyon the crystal surface.[19,20] The resulting doped single crystalsretain coherent charge transport, which is essential for achievingreliable and reproducible responses in sensing applications.[21]Band transport in the soft lattices of OSC single crystals of-fers a foundation for a variety of applications, among which thedevelopment of reliable mechanical sensors is a notable exam-ple. The charge-carrier mobility in OSCs is strongly influencedbymolecular vibrationswithin their soft crystal lattices, which areformed by weak van der Waals interactions.[22,23] Owing to theseweak intermolecular interactions, the mobility is highly sensitiveto mechanical stress, which modulates the amplitude of molec-ular vibrations.[24] These characteristics make OSC single crys-tals attractive for use in strain sensors, which detect mechanicaldeformation and convert it into electrical signals. Strain sensorsare critical components in a wide range of applications, includ-ing structural monitoring and biomedical systems. Their inte-gration into wearable electronics has attracted significant atten-tion, particularly for applications in health monitoring and hu-man motion analysis.[25–27] Furthermore, the sensitive detectionAdv. Funct. Mater. 2026, 36, e18055 e18055 (1 of 8) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbHhttp://www.afm-journal.demailto:tmurata@edu.k.u-tokyo.ac.jpmailto:YAMASHITA.Yu@nims.go.jpmailto:takeya@k.u-tokyo.ac.jphttps://doi.org/10.1002/adfm.202518055http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadfm.202518055&domain=pdf&date_stamp=2025-09-18www.advancedsciencenews.com www.afm-journal.deof mechanical strain is fundamental to the development of label-free chemomechanical sensors that detect molecular adsorptionand related phenomena, including biosensing, gas detection, andchemical reactions.[28–30]The ability to control carrier concentration in OSC single crys-tals is a key requirement for unlocking their potential across awide spectrum of advanced organic electronic devices. For thefabrication of readily measurable two-terminal resistive sensors,chemical doping via redox reactions has been employed to in-troduce carriers at the surface of OSC single crystals.[19,31] Main-taining a high doping level is essential for achieving band trans-port through effective trap filling, which can contribute to the re-duction of electronic noise.[32] However, achieving stable dopinglevels remains a critical challenge, since doped OSCs graduallydegrade through interactions with atmospheric species such aswater and oxygen. To improve the stability of the doped state inOSCs, the choice of dopant ions plays a key role.[33,34] In partic-ular, reduction of p-type doped OSCs by atmospheric water canlead to decreased doping levels and a drop in conductivity.[35,36]Differences in anion hygroscopicity alter the interactions withwater, thereby influencing doping stability.[37,38] These findingssuggest that the proper design of dopant anions may offer aroute to enhance the ambient stability of doped OSCs. However,most prior studies have focused on polymeric semiconductors,in which both holes and anions are distributed throughout thebulk. The specific impact of dopant anions on doping stability insurface-doped OSC single crystals remains unclear.In this study, we demonstrated that the degradation of thedoped state in organic single crystals was significantly sup-pressed through our selection of dopant molecules and the de-sign of the device structure. This approach enhances the long-term performance of strain sensors and facilitates the develop-ment of more reliable and practical organic electronic devices.2. Results and DiscussionThe schematic structure of our device is shown in Figure 1a.Our benchmarked organic single crystal, 3,11-dinonyldinapht-ho[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (C9-DNBDT-NW,Figure 1b),[11,39] was deposited using a meniscus-guided solutionprocess. Details of the process are described elsewhere.[2,11,40]The lower parylene layer is used to improve wettability formaking the OSC film on a polyimide flexible substrate. Theformation and quality of the OSC film were confirmed by cross-polarized optical microscopy, as shown in Figure 1c. Variationsof brightness were observed between the gold electrodes, whichare attributed to differences in film thickness and indicate theformation of molecular step terraces of the single crystal.[2] Thedopant layer consists of an oxidant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ), a salt for ion exchange,and polymethyl methacrylate (PMMA). Two types of salts,cesium(I) bis(trifluoromethanesulfonyl)imide (CsTFSI) andsodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF),were used for the ion exchange (Figure 1b). The oxidant extractselectrons from the OSC single crystal, generating an ion pair[OSC• +F4TCNQ• −]. This anion is then exchanged with otheranions such as TFSI− or BArF−, which are considered to bindto the surface of the single crystal, as illustrated in Figure 1a.The formation of [OSC+TFSI−] or [OSC+BArF−] leads to holeinjection into OSC and contributes to the electrical conductivity.PMMA enables the formation of a uniform dopant layer, therebyserving to retain the dopant molecules, suppress their diffusion,and improve the reproducibility of the doping process.[31] ACYTOP coating is applied on top of the dopant layer to enhancebarrier properties, followed by parylene and aluminum layersthat serve as encapsulation layers.The type of anion significantly influences the doping level.Figure 1d shows the variation in two-terminal sheet conductiv-ity depending on the anion used for ion exchange. The two ter-minal sheet conductivity obtained using BArF− anions was ap-proximately twice that achieved with TFSI− anions. This differ-ence can be attributed to the ion affinities involved in the dopingprocess.[33] To rule out the possibility that differences in chan-nel quality, such as crystal orientation, affect the observed con-ductivity, we performed statistical measurements on doped sam-ples. The average sheet conductivity of 20 independent deviceswas 5.7 ± 0.4 μS for BArF−-doped samples and 2.1 ± 0.2 μS forTFSI−-doped samples. This demonstrates that the higher con-ductivity of the BArF−-doped crystals is attributed to the anion-dependent doping effect rather than variations in pristine crys-tal quality.The difference of conductivity also influences the tempera-ture dependence of conductivity, as shown in Figure S1 (Sup-porting Information). The BArF−-doped sample exhibits a neg-ative temperature coefficient, indicating a carrier density highenough to support band-like rather than hopping transport. Al-though negative temperature dependence of conductivity hasbeen reported for TFSI−-doped samples,[19] our TFSI−-dopedsample shows a positive dependence. A likely cause of this dis-crepancy is the doping environment: unlike previous studies con-ducted under nitrogen, our doping was performed under am-bient air, where water molecules may have interacted with anddegraded the doped state, possibly introduced carrier traps thatresulted in hopping-dominated transport behavior. Indeed, theresulting conductivity was approximately half of the value re-ported in previous studies. In contrast, BArF− doping yieldedband-like temperature dependence even under ambient condi-tions, demonstrating that stable band transport can be achievedwithout the need for a strictly inert atmosphere. This robust-ness offers a practical advantage for scalable manufacturingprocesses.To investigate the stability of the doping state, we applied ther-mal stress and monitored the changes of sheet conductivity overtime. Samples were heated at various temperatures on a hot plate,and their time-dependent conductivities were measured. Nor-malized changes in conductivities are shown in Figure 2. In thecase of doping with the TFSI anion, a progressive decrease inconductivity was observed with increasing temperature. On theother hand, for doping with the BArF anion, almost no degra-dation of conductivity was observed up to 80°C. A comparisonof conductivity changes after 600 hours reveals that TFSI dop-ing results in variations of −4.7,− 23,− 49,− 62% at 40, 60, 80,and 100 °C, respectively, whereas BArF doping shows changes of−1.3, 2.5,− 2.3,− 40%, indicating that conductivity degradationis significantly suppressed at all temperatures except at 100 °C.The degradation of the doped state is attributed to factors suchas dopant diffusion and the influence of water molecules.[35,41] At100 °C, significant degradation was observed in both samples.Adv. Funct. Mater. 2026, 36, e18055 e18055 (2 of 8) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2026, 13, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518055 by National Institute For, Wiley Online Library on [12/02/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 1. Structural and electrical characterization of the organic strain sensor. a) Cross-sectional schematic structure of the fabricated strain sensorwith the crystal structure of the organic semiconductor C9-DNBDT-NW. Approximate thicknesses of each layer are as follows: 100 and 500 nm for lowerand upper parylene layers, 10 nm for OSC layer, 300 nm for the dopant layer, 100 nm for CYTOP layer, and 100 nm for aluminum layer. b) Chemicalstructures of the dopant, anions, and the organic semiconductor. c) Cross-polarized optical microscopy images of the coated organic semiconductorfilm. d) Comparison of the I–V characteristics after doping with BArF− and TFSI− anions.This degradation may be attributed to the glass transition ofPMMA, accelerating the transport of water and/or dopants.The thermal stability difference between the TFSI− anion andthe BArF− anion can be ascribed to differences in hygroscopic-ity. In p-type doping, one of the primary causes of doped statedegradation is the presence of water.[41] The BArF− molecule is amuch bulkier and more hydrophobic anion than TFSI−, whicheffectively mitigates degradation mechanisms such as dopantdiffusion and moisture-induced reactions. To verify the differ-ence in hygroscopicity between the two dopant layers, surfacecontact angles were measured (Figure 3a). The sample coatedwith F4TCNQ/NaBArF/PMMA exhibited a significantly highercontact angle than that with F4TCNQ/CsTFSI/PMMA. The con-tact angle of F4TCNQ/PMMA was nearly identical to that of theF4TCNQ/CsTFSI/PMMA layer (Figure S2, Supporting Informa-tion). Although the present device structure includes multiplesealing layers to prevent moisture ingress, additional sealing isoften impractical in flexible devices due to limitations inmechan-ical compliance. Therefore, the intrinsic hydrophobicity of thedopant layer becomes a key factor in ensuring environmental sta-bility.To further evaluate the effects of anion type and sealing onmoisture-induced degradation, thermal cycling tests were con-ducted under controlled humidity conditions. Since moleculardiffusion accelerates with temperature, cycling the devices be-tween 25 °C and 80 °C at a constant 50% relative humidity (RH)provides a stringent test for dopant stability. We monitored con-ductivity over time to assess the environmental robustness of thedoped state.As shown in Figure 3b, sealing plays a critical role in suppress-ing conductivity degradation during thermal cycling. UnsealedBArF−-doped samples, despite exhibiting surface hydrophobic-ity, showed a monotonic decline in conductivity under repeatedstress. In contrast, sealed BArF−-doped devices exhibited only atransient increase in conductivity during the initial cycles andremained stable under harsh conditions of 80 °C and 50% RH.Adv. Funct. Mater. 2026, 36, e18055 e18055 (3 of 8) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2026, 13, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518055 by National Institute For, Wiley Online Library on [12/02/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 2. Long-term conductivity stability of BArF− and TFSI− doped samples during air exposure at elevated temperatures: a) 40 °C, b) 60 °C, c) 80 °C,and d) 100 °C.For TFSI−-doped samples, even sealed devices exhibited a con-ductivity decrease of approximately 15% within 24 hours overfive thermal cycles, with further degradation observed upon con-tinued cycling. Unsealed TFSI− samples degraded even morerapidly. These results indicate that sealing is essential to sup-press environmental degradation, but the choice of anion is alsocrucial. The BArF− anion renders the surface hydrophobic, andin combination with sealing, effectively prevents water ingress.This dual protection helps suppress conductivity loss under high-temperature and high-humidity conditions, highlighting the im-portance of moisture resistance in achieving long-term doped-state stability.Interestingly, in Figure 3b, TFSI−-doped samples exhibit con-ductivity decreases upon heating, which contrasts with thepositive temperature dependence of conductivity observed inFigure S1 (Supporting Information) attributed to hopping trans-port. This discrepancy is likely due to swelling of the polyimidesubstrate caused by water uptake under 50%RH, inducing strainin the OSC layer. The direction of strain–tensile or compressive–may vary depending on surface morphology of the substrate. Theconsistent monotonic degradation observed in both unsealedBArF−- and all TFSI−-doped devices over repeated thermal stresssuggests that the long-term degradation is caused by doped-state instability.The response to the strain of the device, which achieved a highstable doped state using the BArF− anion, was tested. The largestrain response observed in OSC single crystals is attributed tocarrier transport influenced by molecular vibrations,[24] which isFigure 3. Stability under repeated thermal stress and contact angle measurements of BArF− and TFSI− doped samples. a) Comparison of contact anglesfor the surface of BArF− and TFSI− doped samples (n=3). b) Thermal cycling test of samples doped with BArF− and TFSI− anions under controlled 50%relative humidity. Solid lines represent sealed samples (CYTOP, parylene, and Al), while dotted plots indicate unsealed samples.Adv. Funct. Mater. 2026, 36, e18055 e18055 (4 of 8) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2026, 13, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518055 by National Institute For, Wiley Online Library on [12/02/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 4. Performance of the BArF-doped device as the strain sensor. a) Schematic illustration for the strain test setup. b) Sensor responses to theelevated mechanical strain applied in both positive and negative directions. Each strain level was applied three times. c) Plot of applied mechanicalstrain versus resistance change, used to evaluate the gauge factor of the BArF− doped device.unique to the crystal structure. Previous studies on chemicallydoped organic single crystals strongly suggest that the dopantspecies remain at the crystal surface rather than being incorpo-rated into the lattice,[19] but it is not obvious whether the pres-ence of bulky anions influences the strain response originatingfrom molecular vibrations. To evaluate this, we prepared a setupas shown in Figure 4a, where controlled strain was applied tothe doped sensor device, and its resistance response to strainwas measured.Figure 4b shows the resistance change when strain was ap-plied, with three cycles of strain application overlaid in the graph.Examining the relationship between strain and resistance changerate, a clear linear response was observed for both positive andnegative strain, as shown in Figure 4c. The gauge factor, de-fined as the ratio of resistance change to applied strain, wasfound to be as high as 24 (Figure 4c). Previous studies on pris-tine OSC single crystals, where charge carriers were inducedelectrostatically via gate electrodes, have demonstrated compa-rable strain sensitivities.[24] In this work, we show that chemi-cal doping enables a similar level of strain sensitivity without theneed for external gating, thereby simplifying the device structureand facilitating practical applications. These results indicate thateven with the introduction of bulky anions via surface chemi-cal doping, the strain response was not disturbed, and a linearstrain response wasmaintained. The value of the gauge factor ob-served in the OSC is approximately 10 times higher than that oftypical nichrome strain gauges.[42,43] Higher gauge factors havebeen reported for crack-assisted architectures, such as alignedCNT/elastomer composites (GF≈200[44]) and MXene-basedsensors (GF≈500[45]). Such values, however, typically rely ondomain-separationmechanisms that lead to nonlinear and asym-metric responses for positive and negative strain. By contrast, ourchemically doped organic single-crystal devices exhibit GF≈20,while maintaining linear and symmetric responses to both ten-sile and compressive deformation. These features not only high-light their advantage for sensitive detection of ppm-level defor-mations but also position our devices as a unique and practi-cal platform for stable, reproducible, and low-cost strain sens-ing enabled by scalable solution-coating processes. A summaryof representative resistive strain-sensor performances, includinggauge factors in the small-strain regime (<1%), is provided inTable S1 (Supporting Information).Next, we repeatedly applied a constant strain of approximately300 ppm under ambient conditions for a total of 100 000 cycles toinvestigate whether the resistance change rate varied over time.Figure 5a displays the resistance changes during the first 20 cy-cles, while Figure 5b presents the overlaid responses from all100 000 strain cycles. As shown in Figure 5c, a stable and repro-ducible resistance change was observed in response to constantstrain, confirming that the strain response remained stable de-spite repeated strain application. These findings suggest that thedeposition of bulky anions on the single-crystal surface does notimpair carrier conduction and is highly effective in achieving astable doped state.The baseline resistance drift observed in Figure 5b was ap-proximately 20 ppm/h over a 50-hour measurement period un-der ambient conditions. This level of temporal stability is consid-ered sufficient for reliable detection of ppm-scale strain signals.Adv. Funct. Mater. 2026, 36, e18055 e18055 (5 of 8) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2026, 13, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518055 by National Institute For, Wiley Online Library on [12/02/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 5. Stability of the BArF-doped strain sensor under repeated strain cycles, tested under ambient conditions. a) Sensor output during the first 20cycles of applied strain. b) Overlay of all output signals over the 100 000 strain cycles. c) Stability of sensor signals versus strain cycles.Despite the limited number of studies on the long-term stabilityof surface-doped organic single crystals, C9-DNBDT-NW dopedwith TFSI− has been reported to drift at a rate of 50 ppm/h underambient conditions. The present strategy suppresses this drift towell below half that level. Moreover, it offers a pronounced advan-tage over TFSI-doping in mitigating conductivity loss at elevatedtemperatures (Figure 2). Further decrease in drift and detectionof smaller strain may be attainable by the implementation of ap-propriate drift compensation techniques.[46–48]3. ConclusionThis study highlights the critical role of anion selection in gov-erning the stability and performance of doped organic singlecrystals under mechanical and thermal stress. Compared to theconventional use of TFSI− anion, employing BArF− anion re-sulted in improved stability under ambient conditions and at el-evated temperatures. Our results indicate that moisture acceler-ates conductivity degradation, highlighting the role of low hygro-scopicity of BArF molecule in enhancing durability. Contact an-gle measurements further confirmed the low hygroscopicity ofthe BArF-containing dopant layers. The single crystal doped withBArF− anion retained its intrinsic strain responsiveness even un-der repeated strain cycles, demonstrating mechanical durabil-ity. These results demonstrate that DNBDT single crystals dopedwith BArF− anion are well-suited for detecting minute strain atthe ppm level. This study offers valuable insights into overcom-ing challenges related to doped-state degradation and structuralinstability, paving the way for the development of robust organicelectronic deviceswith enhanced functionality and long-termper-formance.4. Experimental SectionFabrication of Device: A 16 μm thick polyimide film (Toray-Dupont)was used as the substrate. A 100 nm thick parylene layer was depositedonto the substrate to enhance the coating properties of the OSC sin-gle crystal. The benchmark material, 3,11-dinonyl-dinaphtho[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (C9-DNBDT-NW), was coated onto theparylene layer using a continuous edge-casting method. The film was sub-sequently annealed at 80 °C under vacuum overnight. Photolithographywas used to pattern both the OSC film and the gold electrodes in order toprevent unintended electrical connections. The dopant solution, preparedunder a nitrogen atmosphere, consisted of 10 mM CsTFSI or NaBArF, 10mM F4TCNQ as the oxidant, and 2 wt% PMMA in 2,2,2-trifluoroethanol.The solution was spin-coated on the OSC film at 2000 rpm for 60 sec-onds under ambient conditions to form the dopant layer. This layer wassubsequently heated at 120 °C for 30 min to remove the solvent. A protec-tive layer consisting of CYTOP, parylene, and aluminum layers was subse-quently applied. The CYTOP layer was formed by spin-coating a fivefold-diluted CYTOP 809 M solution at 2000 rpm, followed by heating at 120 °Cfor 30 min to improve adhesion. An approximately 500 nm thick paryleneAdv. Funct. Mater. 2026, 36, e18055 e18055 (6 of 8) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2026, 13, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518055 by National Institute For, Wiley Online Library on [12/02/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.delayer was introduced to mitigate damage from the subsequent aluminumdeposition. The aluminum layer, with a thickness of 100 nm,was depositedby thermal evaporation using a shadow mask.Electrical Measurements: Silver paste (DOTITE D-550, FUJIKURA KA-SEI) was used for electrical contact with the gold electrodes. Fortemperature-dependent timemeasurements, a hot plate and a data logger(DAQ970A, KEYSIGHT) were employed. An environmental test chamber(SH-222, ESPEC) was utilized for temperature tests under controlled hu-midity.To apply strain, the device on the polyimide film was affixed to 100 μmof SUS430 substrate and secured in an arched configuration. Strain wasinduced by bending this arched structure through themovement of amov-ing stage (OSMS26-200, OptoSigma). Themoving stage was controlled bya stage controller (SHOT-302GS, OptoSigma). The relationship betweenstage displacement and strain was calibrated using a commercially avail-able strain sensor with a known gauge factor (KFGS-1-120-C1-11 L1M2R,Kyowa). A 1.5 V bias from a DC power supply (PMX18-2A, KIKUSUI) wasapplied to the OSC single crystal, with an 80 kΩ resistor connected inseries, and the voltage across the OSC single-crystal device was moni-tored. Voltage measurements were conducted using an analog-to-digitalconverter (AD7148-8, Analog Devices) and its evaluation kit. All strain ap-plication experiments were performed at room temperature.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was supported in part by JSPS KAKENHI grant (JP22H04959),JST CREST (JPMJCR21O3), and JST FOREST (JPMJFR236R).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Keywordsmolecular doping, organic semiconductor, single crystals, strain sensorReceived: July 14, 2025Revised: August 25, 2025Published online: September 19, 2025[1] K.Myny, S. Steudel, S. Smout, P. Vicca, F. Furthner, B. VanDer Putten,A. Tripathi, G. Gelinck, J. Genoe, W. Dehaene,Org. Electron. 2010, 11,1176.[2] A. Yamamura, S. Watanabe, M. Uno, M. Mitani, C. Mitsui, J. Tsurumi,N. Isahaya, Y. Kanaoka, T. Okamoto, J. Takeya, Sci. Adv. 2018, 4,eaao5758.[3] J. Liu, H. 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Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2026, 13, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518055 by National Institute For, Wiley Online Library on [12/02/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.de Highly Stable Ion-Exchange Doping of Organic Semiconductor Single Crystals for Reliable Flexible Sensors 1. Introduction 2. Results and Discussion 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords