Showcasing research from the Department of Chemical

Engineering, Pohang University of Science and Technology

(POSTECH), Pohang, Korea.

Reduced Pyronin B as a solution-processable and heating-free

n-type dopant for soft electronics

Air-stable and heating-free doping processes in the fabrication

of electronic devices are highly desirable for realizing advanced

flexible electronics on plastic substrates. A synthetic protocol

for an n-type organic dopant using an organic cationic dye,

Pyronin B, via a simple chemical reduction in various organic

non-polar solvents is developed, in which Pyronin B is

transformed into its leuco form. The developed dopant has

been used as a highly efficient solution-processable n-dopant

for graphene field-effect transistors under ambient conditions

at room temperature.

As featured in:

See Joon Hak Oh et al.,J. Mater. Chem. C, 2018, 6, 6672.

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6672 | J. Mater. Chem. C, 2018, 6, 6672--6679 This journal is©The Royal Society of Chemistry 2018

Cite this: J.Mater. Chem. C, 2018,

6, 6672

Reduced Pyronin B as a solution-processable andheating-free n-type dopant for soft electronics†

Eun Kwang Lee, a Yonghee Kim,a Jisu Back,b Eunsung Lee b and Joon Hak Oh *a

Air-stable and heating-free doping processes are highly desirable for realizing advanced flexible

electronics on plastic substrates. A synthetic protocol for a high-performance n-type organic dopant

using an organic cationic dye, Pyronin B, via a simple chemical reduction method with NaBH4 in various

organic non-polar solvents is developed, in which the cationic dye Pyronin B is transformed into its leuco

form. The reduced Pyronin B (rPyB, leuco form) spontaneously interacts with carbon nanomaterials, such

as graphene and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), via hydride-mediated electron

transfer doping, which is confirmed by electron spin resonance (ESR) analysis. After coating rPyB onto

graphene, the non-polar solvents are removed by simple evacuation (B1 � 10�3 Torr) without an

annealing step. The work function of graphene is reduced from 4.52 eV to 3.98 eV after doping. Multiple

coatings of rPyB on graphene field-effect transistors (FETs) result in air stability for 90 days under

ambient conditions. Selective n-type doping using a polydimethylsiloxane (PDMS) stamp and rPyB has

also been performed on a large-area 16 � 16 graphene FET array. Our findings demonstrate a viable

methodology for the cost-effective synthesis of an n-type dopant and its application in soft electronics.

Introduction

Two-dimensional (2D) nanomaterials including graphene andtransition metal dichalcogenides (TMDCs) and organic semi-conductors are highly promising for use in soft electronicsowing to their outstanding mechanical flexibility.1–6 Moreover,due to the simple tunability of their electrical performance bymodifying their surface conditions, surface transfer dopingtechniques have been intensively investigated.7–9 However, incontrast to the rapid advances in hole-transporting (p-type)semiconductors, the relatively low performance and instabilityof n-type semiconductors under ambient conditions havesignificantly impeded their practical applications such as p–njunctions and complementary circuits.10–13 One way to tacklethese problems is to develop highly effective n-type dopingmethods to enhance and stabilize electron transport underambient conditions with simple solution processes. Surfacetransfer doping using organic dopants, which is a nondestructivedoping technique, has attracted great interest compared withinterstitial doping in soft electronics in this regard.14,15

In Si-based electronics, high-temperature doping such asthermal diffusion and ion implantation have been used to

control the conductivity in doping processes of Si-based electronicsover 500 1C.16,17 However, such a high-temperature doping systemis not applicable on flexible or stretchable plastic substrates in softelectronics.

In n-type molecular doping, the energetically high-lyinghighest occupied molecular orbital (HOMO) level of n-typeorganic dopants is necessary for the efficient transfer of electronsto the lowest unoccupied molecular orbital (LUMO) of hostsemiconductors. However, n-type organic dopants with highHOMO levels generally show instability under ambient conditionsdue to oxidative degradation by oxygen and water molecules.18

Solution-processable n-type organic dopants for surface transferdoping have rarely been investigated. A reduced benzyl viologen(BV) was utilized as a molecular n-dopant for a variety ofsemiconductors.18–20 However, its toxicity (BV derivatives aremain compounds in the herbicide industry) required carefulhandling during the device fabrication steps and hamperedpractical applications.

Air-stable n-type organic field-effect transistors (OFETs) dopedby 1H-benzoimidazole derivatives have been reported.21,22 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI) deriva-tives are known to promote hydrogen- and/or electron-transfer viathe formation of radical species. Mixing of (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)-dimethylamine (N-DMBI)and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) improvedthe electron mobility of PCBM-based OFETs. However, the post-thermal treatment at 80 1C overnight was necessary to formN-DMBI neutral radicals. Its energetically high-lying singly

a Department of Chemical Engineering, Pohang University of Science and

Technology (POSTECH), Pohang 37673, Korea. E-mail: joonhoh@postech.ac.krb Department of Chemistry, Pohang University of Science and Technology

(POSTECH), Pohang 37673, Korea

† Electronic supplementary information (ESI) available: Table S1, Fig. S1–S11,and experimental section. See DOI: 10.1039/c8tc02094a

Received 1st May 2018,Accepted 24th May 2018

DOI: 10.1039/c8tc02094a

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occupied molecular orbital (SOMO) level (�2.36 eV) formed byheating allowed the electron transfer from the N-DMBI neutralradical to PCBM. In addition, Bao et al. reported electrontransfer from DMBI to PCBM occurring via hydride-mediatedelectron transfer (HMET).23 The n-doping of single-walledcarbon nanotubes by leuco malachite green via HMET was alsoachieved by post heat-treatment at 80 1C.24 Recently, dimericorganometallic reductants were investigated as n-type organic–inorganic hybrid dopants activated by the cleavage of thedimers via post heat-treatment or photoirradiation.25–30

On the other hand, organic cationic dyes can be utilized asn-type dopants in organic electronics.31–34 Pyronin B (PyB, themolecular structure is shown in Fig. 1), which has beenfrequently studied as an n-type molecular dopant, can be trans-formed into both leuco and neutral radical forms. Previous studiesrevealed that the neutral radical form of PyB acts as an effectiven-type dopant due to its high-lying SOMO level of�3.34 eV.32,33,35,36

However, the n-type doping process required thermal evaporation ata high temperature under high vacuum (B1� 10�6 Torr). Similarly,the post heat-treatment or post-activation of the doped system hasoften been a prerequisite to trigger the n-type doping effect. Thus, aheating-free or room-temperature organic doping methodology maygreatly increase the degree of freedom in the process architecture ofsoft electronics.

Herein, we have developed a solution-processable n-typeorganic dopant using PyB. A positively charged state of PyB is

reduced to a leuco form of PyB in a biphasic system. The reducedPyB (rPyB) shows effective n-type doping behaviors on a graphenelayer via HMET. In addition, the low work function (WF) ofn-doped graphene electrode enhanced the electron transport ofn-type OFETs. By properly choosing non-polar solvents (NPSs),the rPyB solutions do not require any thermal annealing processfor effective n-type doping. Because of its advantages of solutionprocessability and heating-free processes, a simple and selectivedoping using an rPyB solution and a polydimethylsiloxane(PDMS) stamp has been successfully performed on a large areagraphene FET array under ambient conditions without thermalannealing. In addition, our technique has demonstrated a molecularn-type doping on a low glass transition temperature (TG) plasticsubstrate without any morphological damage.

Experimental sectionMaterials and instrumentation

All starting materials were purchased either from Sigma Aldrichor Alfa Aesar and used without further purification. All solventswere ACS grade unless otherwise noted. An Agilent 5500 scanningprobe microscope running with a Nanoscope V controller wasused to obtain atomic force microscopy (AFM) images. AFMimages were recorded in a high-resolution tapping mode underambient conditions. Raman spectroscopy was conducted at awavelength of 532 nm (WITec, Micro Raman). Kelvin probe (KP)mapping was obtained from a Kelvin probe photoemissionsystem (APS02-5050, KP Technology Ltd) under ambient conditions.A direct current was applied to the sample to nullify the contactpotential difference between the Kelvin probe force microscopy(KPFM) tip and the sample surface to estimate the sample WF.The WF of the gold film (5.1 eV) was measured as a reference in KPmeasurements. ESCALAB 250XI of Thermo Fisher Scientific at theUlsan National Institute of Science and Technology (UNIST) wasutilized to measure ultraviolet photoelectron spectroscopy (UPS)data under vacuum, B1 � 10�10 Torr using He I excitation lines(hn = 21.22 eV).

Synthesis of rPyB

Various concentrations (concentration: from 10 mM to 5 mM) ofthe PyB (Pyronin B iron(III) chloride complex, CAS Number:2150-48-3, empirical formula: C42H54Cl8Fe2N4O2) were preparedin 10 mL of deionized water, followed by the addition of 10 mL ofvarious NPSs (cyclopentane, hexane, cyclohexane, benzene andtoluene) to form a biphasic solution. Two milligrams of NaBH4

were added to the biphasic solution. The resulting biphasicsolution was left for 1 day. The dark purple color of PyB indeionized water became transparent after the reaction. The non-polar solvent phase was carefully separated using a pipette.

Fabrication of graphene electrodes

A monolayer graphene film was grown on a copper foil (0.025 mmthickness) by a conventional chemical vapor deposition (CVD)method using H2 and CH4 gases. After the growth of graphene,poly(methyl methacrylate) (PMMA) was coated onto the graphene

Fig. 1 Synthesis of rPyB (a–c): (a) NPS was layered on top of the PyB solutiondissolved in water. (b) NaBH4 was added to the PyB solution to synthesize rPyB.The rPyB is not charged, thus rPyB molecules moved to the NPS spontaneously.(c) After 24 h, rPyB molecules moved to the NPS completely. (d) Chemicalreaction of PyB with NaBH4. The chemical reaction results in the formation ofrPyB (leuco form) in which the conjugation length is shortened. (e) UV-visabsorption spectra of PyB (black) and rPyB (red). (f) X-band (microwavefrequency = 9.4508 GHz) ESR spectra of undoped and rPyB-doped PCBM.

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surface to support the graphene for further processing. A copperfoil was etched by an ammonium persulfate ((NH4)2S2O8) aqueoussolution (0.1 M). After the copper foil was etched away, PMMA/graphene was transferred onto the substrate and annealed at130 1C for 30 min. Then, the PMMA was removed using acetone.To pattern the transferred graphene film, a photolithographytechnique was used. A positive photoresist (PR, DSAM-3037from Dongjin Semichem) was spin-coated on the graphene film(4000 rpm for 30 s) and annealed at 100 1C for 60 s. UV light wasilluminated on the PR with a photomask, and the pattern wasdeveloped using a 2.15% tetramethylammonium hydroxide(TMAH) solution. The undesired graphene area for patterningwas etched away using a reactive ion etching (RIE) plasma(100 W for 5 s). A PR stripper removed the remaining PR. Thechannel width and length were 200 and 50 mm, respectively.

Fabrication of graphene and N2200 FETs

For the fabrication of graphene FETs, conventional CVD-growngraphene was prepared on a SiO2/n++Si wafer. Au electrodeswere thermally deposited via a shadow mask. The channelwidth and length were 200 and 50 mm, respectively. For thefabrication of the N2200 FETs, a commercially available polymersemiconductor, N2200 (Polyera Corp.), was dissolved in chloro-benzene (3 mg mL�1). The solution was spin-coated at 3000 rpm ina N2 atmosphere. The spin-coated N2200 film was placed in avacuum oven at 150 1C for 2 h.

Fabrication of a rPyB-doped graphene FET array on a flexiblepoly(ethylene terephthalate) (PET) film

A freshly washed PET film (thickness: 100 mm) was prepared.Gate electrodes (Cr 4 nm/Au 40 nm) were thermally depositedon the PET film through a shadow mask. A 500 nm-thickparylene layer was deposited on the PET film using a parylenecoater. The measured capacitance for the 500 nm-thick parylenewas 7 nF cm�2. CVD-grown graphene was wet-transferred andpatterned using photolithography and reactive ion etching(RIE). Then, source and drain electrodes (Cr 4 nm/Au 40 nm)were thermally deposited on the samples.

Electrical measurements

The electrical performance of the rPyB-doped graphene FETsand N2200 FETs was measured under ambient conditionsusing a Keithley 4200-SCS semiconductor parametric analyzer.The charge carrier mobility (mFE) was calculated by using thefollowing eqn (1) and (2) for linear and saturation regimes,respectively:

IDS ¼W

LmFECg VGS � VTHð ÞVDS (1)

IDS ¼W

2LmFECg VGS � VTHð Þ2 (2)

where IDS is the drain current, VDS is the source–drain voltage, Lis the channel length, W is the channel width, Cg is thecapacitance per unit area of the total gate dielectric layer, VGS andVTH are the gate voltage and the threshold voltage, respectively.

A typical device performance was measured under the followingdevice parameters: W = L = 50 mm, Cg = 1.08 � 10�8 F cm�2 forSiO2 300 nm. The transistors operated in the linear and saturationregimes for graphene (VDS = 1 V) and N2200 (VDS = 100 V),respectively.

Results and discussionChemical and electronic characterization of rPyB

The reduction of organic cationic dyes is thermodynamicallyfavorable.37 In addition, organic cationic dyes have differentcolors in their oxidized and reduced states, and their majorspectral bands typically do not overlap with each state.37

Photographs of the brief synthesis protocol of rPyB are shownin Fig. 1a–c. First, a PyB solution was prepared in deionizedwater by dissolving a PyB iron(III) chloride complex. Then,10 mL of a non-polar solvent (NPS) was added on top of thePyB solution, resulting in a biphasic solution, as shown in Fig. 1a(top layer: NPS, bottom layer: PyB aqueous solution). The list ofsix NPSs used in these experiments is tabulated in Table S1(ESI†). The standard for choosing the NPS is the dipole momentin this experiment. A large difference of the dipole momentsbetween the NPS and water can induce the spontaneousformation of a biphasic solution when those solvents are placedtogether. NaBH4 as a reducing agent was slowly added into thebiphasic solution (Fig. 1b). NaBH4 is a well-known strongreducing agent used in wide applications in chemistry andthe dyeing industry. After the reduction of PyB for 24 h, rPyBwas moved to the upper NPS medium with H2 bubbles. Thetransparent NPS solution containing rPyB was separated fromthe aqueous solution, as shown in Fig. 1c. After the reduction ofPyB, the conjugation length in its main backbone was shortened,resulting in a dramatic color change from violet to colorless. Thechemical structure of rPyB in the toluene phase was confirmed tobe the leuco form of PyB using 1H nuclear magnetic resonance(NMR) spectroscopy analysis (Fig. S1a, ESI†) and electron spinresonance (ESR) in which no signal of radical formation was found(Fig. S1b, ESI†). The reduction of PyB by NaBH4 occurs by hydridetransfer, which changes PyB to rPyB (leuco form).38 The schematicchemical reaction is depicted in Fig. 1d and can be described bythe following reaction:

2PyBðaqÞðcationic formÞ þ 2BH4ðaqÞ�

������!H2O=hexane2rPyBðleuco formÞ þ 2BH3ðaqÞ þH2ðgÞ

The rPyB solution was kept under ambient conditions for 1 weekto check the stability of the rPyB solution. No color change in therPyB solution was observed, indicating that the rPyB solution wasstable under ambient conditions for 1 week (Fig. S2, ESI†).

To explore the capability of rPyB as an n-type dopant, a dropof the rPyB solution (B5 mL) was placed onto a spinninggraphene at 3000 rpm for 1 min. The NPS was removed byevacuation at 1 � 10�3 Torr for 10 min. Among the NPSs,hexane showed the most reliable uniformity for film formation,as confirmed by optical images (Fig. S3, ESI†). In addition, the

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drying process was completed by only vacuum treatment atroom temperature without any heating process owing to thelow boiling point of hexane (69 1C). Hereafter, hexane waschosen as the solvent to perform the characterization of then-doping effect on semiconductors. The black and red lines inFig. 1e correspond to the normalized UV-vis absorption spectraof PyB in water and rPyB in hexane, respectively. The absorptionbands were significantly changed after reduction for 24 h. Themaximum absorption intensity of PyB appeared at 553 nm. After thereduction of PyB using NaBH4, the peak at 553 nm disappeared, anda new absorption peak was observed at 366 nm. These resultsindicate that a new optical energy state was formed after thereduction.

Hydride-mediated electron transfer of rPyB

Prior to exploring the rPyB doping effect on graphene films,rPyB was mixed with PCBM in hexane. PCBM is a suitablecarbon-based nanomaterial to analyze the n-type doping effectdue to its solution processability. ESR analysis of PCBM dopedwith rPyB revealed the existence of a radical with a g value of2.0075 (Fig. 1f). This result indicates that fullerene radicalanions and stable PyB cations were formed upon doping, whichis similar to the case of N-DMBI doping on PCBM. Thus, themechanism of rPyB doping on PCBM is consistent with that ofN-DMBI doping on PCBM, which is a HMET doping.23,24,34

Fig. 2a depicts the HMET mechanism of rPyB on graphene. Twoelectrons/H+ or hydrides (H�) are transferred to the graphenematrix, resulting in the formation of graphene radical anions

and PyB (the cationic form). It is noteworthy that the hydro-genation of graphene takes place by the doping effect of rPyB.Thus, the thermodynamics and kinetics of the doping processcannot be simply explained by considering only the dopantionization energy and the host semiconductor electron affinityto explain the simple electron transfer between them.23

To identify the concentration of the rPyB solution for theoptimal surface coverage of rPyB on the graphene film, rPyBlayers prepared on graphene using various molar concentrationsof rPyB (10 mM, 20 mM, 0.1 mM, 1 mM, and 5 mM) were examinedusing AFM. The rPyB-coated area gradually increased with themolar concentration (Fig. S4 and S5, ESI†). Five millimolardoping of rPyB on a graphene film resulted in the full coverageof the graphene surface (Fig. S5, ESI†). Therefore, we chose 5 mMrPyB for the characterization of the doping effect on graphene.

The optical transparencies of pure graphene and rPyB-coated graphene were measured as a function of the coatingnumber of 5 mM rPyB (Fig. 2b). The optical transmittanceof CVD-grown graphene was 96.6% at 550 nm, while that ofrPyB-coated graphene was 93.2% at the same wavelength.The transmittance decreased from 93.2% for a one-time rPyBcoating to 79.3% for a 4-time rPyB coating process on graphene.We found that even though rPyB was transparent in thesolution phase right after the reaction, the dried rPyB filmreturned to purple (its original color) upon exposure to ambientconditions due to the strong tendency to return to PyB. In otherwords, the strong tendency allowed the HMET without anypost-heating process.

Fig. 2 (a) Schematic illustration of HMET using rPyB solution on a graphene film. The blue-shaded area indicates that hydride is transferred from rPyB tographene through the HMET process. (b) Transmittance of graphene and graphene after various rPyB coatings. (c) Raman spectra of graphene before andafter coating of rPyB. UPS spectra at (d) the secondary electron cutoff region and (e) the low binding energy region near the Fermi level for the grapheneelectrode (black) and the rPyB-doped graphene electrode (red).

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Analysis of the electronic properties of rPyB-doped graphene

Raman spectroscopy data (Fig. 2c) revealed that the pristinegraphene film exhibited a 2D peak (2683 cm�1) and a G peak(1592 cm�1). After the 1-time coating process of rPyB ongraphene, the 2D peak was downshifted to 2677 cm�1. On theother hand, the G peak was upshifted to 1602 cm�1. TheseRaman shifts are consistent with the previous n-type dopingresults on graphene.39–41 A p–p interaction between rPyB andgraphene contributed to the graphene-enhanced Raman scattering(GERS) enhancement as shown in Fig. S6 in the ESI.† 42–48 Inaddition, the D peak (B1340 cm�1) intensity was barely changed,indicating that defects were not formed on the graphene after thedoping.

The electrostatic properties of graphene doped with rPyBwere investigated using KPFM under ambient conditions.Fig. S7a and b in the ESI† illustrate the WF mapping resultsscanned by KPFM (scan size = 0.40 mm2, scan points = 100)before and after doping of rPyB on the graphene film underambient conditions, respectively. Average WFs of 4.62 eV(�0.02 eV) and 4.48 eV (�0.02 eV) were obtained on the pristinegraphene and the 1-time coated graphene, respectively. Thisresult indicates that a uniform n-type doping effect of rPyB wasclearly observed over the large-area graphene film. In addition,the WF of graphene was gradually reduced up to 4.39 eV after a4-time coating of rPyB under ambient conditions as shown inFig. S8 in the ESI.†

UPS spectra at the secondary electron cutoff region shown inFig. 2d and the low binding energy region near the Fermi level(EF) shown in Fig. 2e were also obtained for a closer examinationof the interfacial contact between rPyB and the graphene film.The WFs of the pristine graphene and rPyB-doped graphenefilms were estimated to be 4.52 eV and 3.98 eV, respectively. Thepotential difference (0.54 eV) at the interface is caused by theHMET from the rPyB to the graphene film. The discrepancies inthe estimated WFs between the KP and UPS results might arisefrom the different atmospheric and experimental conditions.

rPyB-Doped graphene as an active layer in an FET andelectrodes in an OFET

Fig. 3a and b exhibit the transfer characteristics (IDS–VGS) of thepristine graphene film and rPyB-doped graphene FETs, respectively.The Dirac point (VDirac), where charges are neutralized, was shiftedfrom +39 V to over �60 V with a 1-time rPyB coated graphene,indicating that strong n-type doping occurred. The calculated holeand electron mobilities were 810 and 264 cm2 V�1 s�1 for pristinegraphene under ambient conditions, respectively (Fig. 3a). After thecoating of rPyB on the graphene FETs, electron transport was onlyobserved with a mobility of 1703 cm2 V�1 s�1 (Fig. 3b). To controlthe rPyB doping efficacy on graphene at lower concentrations, thedopant concentration varied from 10 mM to 1 mM. The VDirac wascontrollably placed at +9 and �32 V with 10 mM and 1 mM dopingof rPyB, respectively, as shown in Fig. 3c. Particularly, 20 mM dopingof rPyB on a graphene FET showed a VDirac close to 0 V. This resultagreed with the dramatically negative shift of VDirac in accordancewith the concentration trends, as shown in Fig. 3d. A relatively large

hysteresis behaviour in the transfer curve was observed after thedoping of rPyB on graphene FETs (Fig. S9, ESI†). This phenomenoncan be explained by the transformation of the rPyB into PyB whichis a positively charged form (cationic) on the graphene surfaceacting as a scattering center.49,50

In addition, we fabricated bottom-gate bottom-contact OFETsusing pristine graphene (Fig. 3e) and rPyB-doped graphene (Fig. 3f)as electrodes with a commercially available n-type polymer semi-conductor, poly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)} (N2200,Polyera Corporation, the chemical structure is shown in theinset of Fig. 3e). The electron mobility was 6.2 � 10�3 and1.3� 10�2 cm2 V�1 s�1 for the graphene electrodes and rPyB-dopedgraphene electrodes, respectively. The notable differences betweenthese two devices before and after rPyB doping are a huge increasein both the on/off ratio and transconductance. In addition, fastsaturation was observed in the case of the rPyB-doped grapheneelectrodes. This is attributed to the lowering of the WF of thegraphene electrodes using rPyB, which is better matched with theLUMO (�3.84 eV) of the polymer semiconductor.

Fig. 3 Transfer curves of graphene (a) before and (b) after rPyB coating.(c) Typical transfer curves of rPyB-doped graphene FETs with a controlledconcentration of rPyB. (d) Dirac point change of rPyB-doped grapheneFETs with respect to the doping concentration of rPyB. The Dirac pointsgradually decreased with increasing the doping concentration. Transfercurves of N2200 FETs with (e) pure graphene electrodes and (f) grapheneelectrode coated rPyB in the configuration of the bottom gate bottomcontact (BGBC). Insets in (a), (b), (e) and (f) are schematic illustrations ofeach device structure.

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The HOMO and LUMO of N2200 are �5.36 and �3.91 eV,respectively.51,52 Electrons are transported from the source todrain electrodes via the LUMO of N2200. The injection barrierbetween the LUMO level and the WF of the electrodes needs tobe reduced for efficient electron transport. According to theUPS results, the injection barrier was dramatically reducedfrom 0.61 to 0.07 eV when the graphene electrode was replacedby rPyB-doped graphene. Thus, the reduced injection barrierresulted in the fast saturation and enhanced electrical propertiesof N2200 FETs (Fig. S10a, ESI†). The electron mobilities ofgraphene FETs with multiple rPyB-coatings from one to fourtimes were monitored for 90 days in order to investigate the airstability (Fig. S10b, ESI†). In the case of the 1-time coating ofrPyB, the electron mobility was maintained up to 75% after90 days. The electron mobility was maintained up to 97% after90 days in the case of the 4-time coating process of rPyB. Thethickness of the dopant on graphene was increased with multiplecoatings. The multiple coatings of rPyB enhanced the stability ofthe electron mobility under ambient conditions due to theincreased amount of the dopant on the graphene surface, whichcould increase the n-type doping effects and prevent the activechannel from further oxidation.

On-demand contact doping using rPyB and a PDMS stamp

To demonstrate simple and effective graphene doping on alarge-area device array system using the developed n-typedopant, stamp doping using polydimethylsiloxane (PDMS,Dow Chemical) was applied to a 16 � 16 graphene FET arrayon a 300 nm SiO2/n++Si wafer. The prepared PDMS stamp

(the fabrication of the PDMS stamp and schematic illustrationof the PDMS stamp are described in the Appended experimentalsection in the ESI†) was soaked in the rPyB dopant solution for1 min. After being vacuumized to remove the hexane solvent onthe surface of the PDMS stamp at a pressure of 1 � 10�3 Torr for10 min, the stamp was placed on the top of the graphene FETarray (Fig. S11a and b, ESI†) to induce a physical contactbetween the PDMS stamps and active channels of the grapheneFETs for 5 min. After detaching the PDMS stamp from the16 � 16 graphene FET array, the transfer curves of the grapheneFET were obtained, as shown in Fig. S11 in the ESI.† The strongp-type doped behavior (Fig. S11c, ESI†) of the 16 � 16 grapheneFETs originated from the chemicals (ca. PR, developer, stripper,etc.) involved in the photolithographic fabrication process. AfterrPyB doping using the PDMS stamp, the typical transfer curveexhibited a VDirac of approximately +9 V (Fig. S11d, ESI†),demonstrating successful n-type doping. To exploit the advan-tages of stamp doping, spare PDMS pieces were placed at thecorners of the array to provide sufficient space between thePDMS stamp and the 16 � 16 graphene FET array. The centerpart of the PDMS stamp was touched with a fingertip to induce adirect contact between the stamp and the 16 � 16 graphene FETarray, as shown in Fig. 4a. Before stamp doping, VDirac was notobserved with the gate sweep range of �60 to +60 V (Fig. 4b).After stamp doping, VDirac was placed at an approximately +9 Vgate voltage on the touched area (Fig. 4c). The response timeupon rPyB-stamp doping was calculated from the IDS vs. timecurve (Fig. S12, ESI†). The rPyB-coated PDMS stamp was pressedwith a fingertip for 30 s. The calculated response time was 14 s.

Fig. 4 Solution-processable, selective on-demand contact doping using a PDMS stamp and a graphene FET array on a PET film. (a) A rPyB-coated PDMSstamp was placed on top of the graphene FET array. Fingertip touched the center of PDMS for the on-demand contact doping. Fingertip was detached.VDirac mapping (b) before and (c) after selective stamping doping at the center of the PDMS stamp. (d) Photograph of graphene FET array on a PET film(top). Optical microscopy image of flexible graphene FET (bottom); the inset is a schematic illustration of the graphene FET structure.

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The long response time revealed that a sufficiently long contactis required for the effective doping.

Flexible n-doped graphene FET array on a low TG substrate

In addition, the rPyB doping system was applied to a grapheneFET array fabricated on a flexible PET film (Fig. S13a, ESI†) ofwhich the TG is 76 1C. After coating of the rPyB solution onthe prepared graphene FET array on the PET film, evacuation(B1 � 10�3 Torr) was performed without annealing. Thefabricated devices were flexible (Fig. 4d) and morphologicaldamages on the flexible devices were not observed after thecomplete doping process as shown in Fig. S13a in the ESI.† Theeffect of rPyB as a solution-processable n-type dopant onflexible devices was also clearly confirmed by the comparisonof the transfer curves before (Fig. S13b, ESI†) and after (Fig. S13c,ESI†) the doping.

Conclusions

In conclusion, we demonstrated a solution-processable andheating-free n-type molecular doping for the first time, usinga reduced organic dopant, rPyB, which was prepared via asimple phase-transfer chemical reduction of PyB in anNaBH4/NPS biphasic solution. The doping mechanism of theprepared n-type dopant was characterized using ESR analysison the rPyB/PCBM mixed system, which revealed that it isrelated to the HMET process. The n-type rPyB dopant washighly effective for tuning the WF of graphene without anyadditional heating process. Multiple coatings of rPyB were veryeffective for improving the air stability of the n-type dopedgraphene FETs. Finally, we demonstrated that a large-area16 � 16 graphene FET array could be selectively doped inn-type using a PDMS stamp and rPyB solution with a fingertip.The rPyB dopant solution could be applied to a commerciallyavailable low TG plastic substrate for advanced flexible devices.The results describe a simple and yet very powerful syntheticapproach for the facile fabrication of low-cost n-type dopantsfor high-performance graphene and organic electronics. Ourdeveloped doping technique can also be applied in a variety ofnon-soft electronics areas, for example, energy level engineeringon electrodes in energy conversion devices and bioelectronics,and as catalysts for organic reactions and gas storage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation ofKorea (NRF) grant (No. 2017R1E1A1A01074090), the Nano Mate-rial Technology Development Program (No. 2017M3A7B8063825),and the Center for Advanced Soft Electronics under the GlobalFrontier Research Program (No. 2013M3A6A5073175) fundedby the Ministry of Science and ICT (MSIT), Korea. We thank

Prof. T.-W. Lee and S. H. Jung for their help in Kelvin probemeasurements.

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