Doping Versatile n-Type Organic Semiconductors via Room

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Doping Versatile n-Type Organic Semiconductors via Room-Temperature Solution-Processable Anionic Dopants Chu-Chen Chueh, Chang-Zhi Li, Feizhi Ding, Zhong'an Li, Nathan Cernetic, Xiaosong Li, and Alex K. -Y. Jen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14375 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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ACS Applied Materials & Interfaces

Doping Versatile n-Type Organic Semiconductors via

Room-Temperature

Solution-Processable

Anionic Dopants Chu-Chen Chueha+, Chang-Zhi Lia,c+, Feizhi Dingb, Zhong’an Lia, Nathan Cernetica, Xiaosong Lib, and Alex K.-Y. Jena,b,c* Dr. C.-C. Chueh, Dr. C.-Z. Li, Dr. Z. Li, Dr. N. Cernetic, Prof. A. K.-Y. Jen Department of Materials Science & Engineering, University of Washington, Seattle, WA, 98195, USA, E-mail: [email protected]

a

Dr. F. Ding, Prof. X. Li, Prof. A. K.-Y. Jen Department of Chemistry, University of Washington, Seattle, WA, 98195, USA

b

Dr. C.-Z. Li, Prof. A. K.-Y. Jen c MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, P. R. China +

These authors are equally contributed

KEYWORDS: n-doping, anion-induced electron transfer, conductivity, organic electronics, anionic dopant

ABSTRACT. In this study, we describe a facile solution processing method to effectively dope versatile n-type organic semiconductors, including fullerene, n-type small molecule, and graphene by commercially available ammonium and phosphonium salts via in-situ anion-induced electron transfer (AIET). In addition to the Lewis basicity of anions, we unveiled that the ionic 1

ACS Paragon Plus Environment


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binding strength between the cation and anion of the salts is also crucial in modulating the electron transfer strength of the dopants to affect the resulting doping efficiency. Furthermore, combining with the rational design of n-type molecules, an n-doped organic semiconductor is demonstrated to be thermally and environmentally stable. This finding provides a simple and generally applicable method to make highly efficient n-doped conductors, which complements the well-established p-doped organics such as PEDOT:PSS for organic electronic applications.

1. Introduction Organic semiconductors are commonly known to possess lower charge carrier density and mobility compared to their inorganic counterparts.1-3 This is because charge transport in organic semiconductors is mainly based on charge hopping within the molecular manifolds that is strongly dependent on the degree of π-orbital overlap of molecules. Hence, charge carriers might be localized at the trapping sites, deteriorating the overall charge mobility. In this regard, significant efforts have been devoted to improve the charge-transporting properties of organic semiconductors to enhance their performance in optoelectronic applications.1-12 Among them, the molecular doping approaches have attracted significant interests since it can simultaneously improve the charge-transporting and electrical properties of organic semiconductors.6-17 In principle, through the electron transfer from dopant to an organic molecule, it can increase the charge carrier concentration of organic semiconductor as a result of the extra free carriers (hole or electron) generated in their corresponding valence or conduction bands.6-17 Although considerable progress has been made for developing efficient p-doped organic semiconductors, the progress of n-doped organic semiconductors is still lagged behind due to the scarcity of stable and solution processable n-dopants.4-12 To achieve efficient n-doping, the 2


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molecular dopant requires an up-shifted highest occupied molecular orbital (HOMO) or singly occupied molecular orbital (SOMO) than the lowest unoccupied molecular orbital (LUMO) level of the n-type organic molecule. This high-lying HOMO/SOMO level will impose the vulnerability of n-dopant to ambient environment (oxygen and moisture), thereby hampering the doping process. Most of the reported n-dopants thus far, such as reactive alkali metals, cationic dyes,4-5,12,18-19 strong electron donors,20-21 or organometallic complexes,8-9,22-24 suffer from this problem, and thus either possess marginal ambient stability or require strict vacuum processing. Hence, it is very challenging to develop facile, solution-processable n-dopants, not to mention achieving a stable n-doped system that can be processed at low temperatures resemble the renowned PEDOT:PSS conducting polymer. -

-

-

-

Recently, it has been found the anions of ammonium salts (such as anions: I , Br , Cl , AcO , -

-

F , and OH ) can serve as a Lewis base to dope π-acidic [6,6]-phenyl-C61-butyric methyl ester (PC61BM) in solid-state to significantly improve its conductivity. The effective doping of PC61BM originates from the in-situ anion-induced electron transfer (AIET).25-27 The closepacked charge transfer (CT) complexes comprising anions and fullerenes could form as the anions approach the fullerene cores within few angstrom distance through anion-π interaction. Such re-hybridization of π-wave-functions in solid-state eventually enables AIET due to the disassociation of CT complex into charged species.28-29 Therefore, in addition to the frontier energy levels, the interactions between the anion and cation of the dopant and between the targeted π-systems will also affect such re-hybridization process to modulate the doping efficiency.

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In this study, we first compare the n-doping efficiency of ammonium and phosphonium salts and manifest that the phosphonium salt doped PC61BM exhibits ~5 times higher conductivity than that of the ammonium doped system (1.73 × 10−2 vs 0.38 × 10−2 S/cm). It also reveals that the competitions between the anion and cation (of the dopant) as well as the molecular π-system play a pivotal role in affecting the resultant doping efficiency as illustrated in Figure 1. This affirms that the AIET doping efficiency can be improved by rational selection of the anionic dopants. Finally, we extend this n-doping method to other organic semiconductors to demonstrate its general applicability. 2. Results and Discussion 2.1. n-Doping of Fullerenes by Ammonium/Phosphonium Salts Based on our previous work,25-27 we investigate the correlation between the dopant compositions and the resultant doping effects. It was reported in the literature that Columbic interaction between the cation and anion in phosphonium-based ionic liquid is weaker than that of the ammonium-based counterpart,30-31 which might be attributed to the intrinsically larger ionic radius of phosphor (P) atom relative to that of nitrogen (N) atom. Therefore, we chose tetrabutyl-ammonium bromide (TBABr) and tetrabutylphosphonium bromide (TBPBr) as a group to compare their n-doping efficiency. It is believed that the reduced cation-anion attraction of n-dopant is conducive to facilitate the accessibility of the anion to the semiconductor πsystems

due

to

anion-π interaction

(Figure

1).

In

addition

to

the

above

pair,

methyltriphenylphosphonium bromide (mTPPBr) was also investigated to understand the possible roles of the n-dopant side-chain might play in affecting doping. Note that all these bromide salts could be easily dissolved in common organic solvents with good ambient stability 4


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for solution processing.32 To test this hypothesis, the charge-transporting properties of the n-doped PC61BM films by the studied salts were examined using the bottom gate/top contact field-effect transistors (FETs) as reported in our previous works.6-7,25-27 As clearly shown in Figure 2a-b, all these salts efficiently dope PC61BM. The original semiconducting property of PC61BM was converted into metallic-like conduction after doping, wherein the drain current (Id) at zero gate voltage (Vg) arose 7 orders from 10-11 to 10-4 A. Besides, the field-dependent behavior vanished and became field-independent, showing linear Id-drain voltage (Vd) dependence. Figure 2c and Table S1 summarized the conductivity of these n-doped PC61BM films, in which the conductivity was derived from each of the Id-Vd characteristics (at Vg = 0 V) in films (Figure S1). A clear trend of increased conductivity was shown while the dopants were changed from the ammonium to phosphonium salts. The PC61BM film doped by TBPBr showed 2-3 times higher value than that doped by TBABr, in which the highest conductivity achieved is around 9.4 × 10−3 S/cm with a doping concentration of 20 mol%. We noted that the morphology of the doped films starts to degrade if doping concentration exceeds 20 mol%, resulting in the decreased conductivity. This can be ascribed to poor miscibility between the alkyl salts and PC61BM.25-27 The higher conductivity observed for TBPBr-doped PC61BM than TBABr-doped one plausibly verified our hypothesis of modulating doping efficiency by altering the ionic binding force of the dopants. Then, mTPPBr salt with a phenyl side-chain was used to dope PC61BM to study the effect of side-chain interaction of n-dopant on doping efficiency since the phenyl group on mTPPBr can potentially enhance its compatibility with PC61BM due to π-π interactions. As can be seen in Figure 2a, the overall Id of mTPPBr-doped PC61BM (1.6 - 2.0 × 10-4 A) is slightly higher than 5



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that of the TBPBr-doped film (7.8 × 10-5 - 1.2 × 10-4 A). The corresponding conductivity of mTPPBr-doped PC61BM is calculated to be ~1.42 × 10−2 and ~1.73 × 10−2 S/cm at 10 mol% and 20 mol% doping concentration, which are higher than the values of TBPBr-doped systems (Figure 2c). This enhancement could be in part attributed to the enhanced anion-π interaction as a result of improved compatibility between mTPPBr and PC61BM. To better elucidate the side-chain effects on doping, we have investigated thermal stability of these n-doped systems. It is important to mention that both phosphonium salts (TBPBr and mTPPBr) have high decomposition temperatures (Td) above 300 °C while the alkyl ammonium salt has a lower Td of ~150 °C (from the study using differential scanning calorimeter). Therefore, the phosphonium salts are expected to afford better thermal stability of the doped systems. Both of the phosphonium salts doped PC61BM showed decent conductivity of ~3 × 10−3 S/cm even after being annealed at 100°C for 20 mins (Figure 2d) while the TBABr-doped PC61BM film showed dramatically diminished conductivity after annealing. Impressively, the mTPPBrdoped PC61BM can retain a high conductivity of ~3 × 10−4 S/cm even after being annealed at 200°C for 20 mins (melting point of mTPPBr is ~234°C). The low melting point (~105 °C) of TBPBr impeded the derived doped film to be thermally stable because the homogeneity of the blend film was significantly destroyed. Besides, PC61BM may also aggregate to contribute to the decreased conductivity since heating at high temperatures not only deteriorates the miscibility with dopant but also hampers the AIET efficiency and charge transport.33 2.2. AIET Doped n-Type Small Molecules

6


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Besides three-dimension (3D) fullerenes, we have further examined the efficacy of in-situ AIET doping on other organic π-systems. A small molecule, dicyanomethylene-substituted quinoidalthienoisoindigo (DCN), was designed and synthesized as the target n-type organic semiconductor. The synthetic route was shown in Figure 3a and more details were described in the Supporting Information. By altering the conjugation of the thienoisoindigo core into quinoid system,34-35 the DCN adopts a nice planar conformation to possess a deep-lying LUMO level. The cyclic voltammetry (CV) measurement (Figure S2) shows two reversible reductive peaks and the corresponding LUMO is estimated to be ~ -4.2 eV, which is about 0.5 eV lower than that of PC61BM (-3.7 eV). This relatively low-lying LUMO level of DCN than PC61BM endows its better stability against ambient oxygen and moisture.16-17,25-27 The transfer and output characteristics of pristine DCN were presented in Figure 3b-c. It showed a typical n-channel semiconducting characteristics with an estimated electron mobility of 0.070-0.085 cm2 V-1 s-1, comparable to that of PC61BM (~0.087 cm2 V-1 s-1) (Table S2). This high mobility can be attributed to its good lamella-like packing as portrayed in Figure 4 (and Table S3), wherein a planar configuration and tightly packed pattern with a stacking distance of 0.334 nm can be clearly observed. To investigate AIET doping on DCN, the ET between the phosphonium salts and DCN was first evaluated by electron paramagnetic resonance (EPR) spectroscopy, as shown in Figure S3. It clearly suggested that electron transfer (ET) indeed occurred in solid film but remained dormant in solution, being consistent with our previous finding in the doped PC61BM.25-27 Based on this evidence, the TBPBr/mTPPBr-doped DCN FET devices were fabricated. As illustrated in Figure 5a-b, all the doped DCN devices exhibited decent conductivities. The conductivity for the TBPBr-doped DCN films is extracted to be 2.42 - 4.65 × 10−2 S/cm while the value for the 7



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mTPPBr-doped ones is around 0.74 - 1.28 × 10−2 S/cm, as summarized in Table S4. The resultant conductivity rose as the doping concentration gradually increased from 10 to 30 mol% for both salts. However, it starts to drop when the doping concentration exceeds 30 mol% due to the deteriorated morphology of the doped films, which is similar to the doped PC61BM case. The higher conductivity of TBPBr/DCN over mTTPBr/DCN can also be attributed to the better miscibility of TBPBr with DCN due to its two relatively flexible alkyl side chains. These results affirm the efficacy of in-situ AIET doping for regular organic π-systems. Owing to its relatively high-lying LUMO level (3.8 - 4.3 eV), the n-doped PC61BM has been proven to show poor ambient stability.16-17,25-27 Providing the deeper-lying LUMO level of DCN, we speculated the n-doped DCN should possess better ambient stability. To verify this, the ambient stability of pristine DCN was examined. As shown in Table S5, the mobility of DCN only slightly decreased from 0.07 to 0.05 cm2 V-1 s-1 after being exposed to air for 72 h. However, we noticed that the on/off ratio increases after air exposure (Figure S4). It indicates that the intrinsically trapped charges in DCN will be gradually quenched by oxygen and water after air exposure and then drive the off current back to the neutral state while these charges are stably delocalized at the LUMO level of DCN under inert condition. The ambient stability of n-doped DCN was traced and presented in Figure 5c and S5 and summarized in Table S6. As shown, the n-doped DCN is stable under inert conditions, similar to the self-doped fulleropyrrolidinium ions (FPI) system.25-27 The slightly increase in conductivity under inert storage stems from the dynamic feature of in-situ AIET. After being exposed to air for 72h, the conductivity of doped DCN films remained reasonably stable in contrast to the doped PC61BM (Figure 5d and Figure S6) manifesting its significantly improved ambient stability due to our rational molecular design. 8


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2.3. AIET n-Doped Graphene Graphene is an ideal system to understand the doping efficiency of in-situ AIET due to its unique semimetal nature, for which its valence and conduction band touch at the Brillouin zone corners, known as the Dirac point.36 In principle, the relative position between the Fermi level and the Dirac point will influence the charge transport of graphene and it can be detected from the shifts of Dirac voltage (VDirac). Generally, the VDirac for neutral (undoped) graphene is around 0 V; however, it will shift away while the graphene is doped because the dopants alter its intrinsic charge density and shift the Fermi level away from the Dirac point and the shift magnitude correlates with the doping level. The work function (WF) of graphene is around 4.1 4.6 eV,37-38 which is suitable for in-situ AIET. Therefore, the AIET-doped graphene can provide both the direct evidence of electron injection from anions to graphene and probe its doping efficiency by tracking the VDirac (Fermi level) shift. Thus, FET devices of AIET-doped graphene were fabricated in two different device configurations as shown in Figure 6. For the first structure (Figure 6a), the phosphonium dopants (TBPBr/mTPPBr) were spin-coated onto the top of the completed graphene FET devices. As shown, the pristine graphene device showed the typical ambipolar behavior with a VDirac around 0 ± 10 V. Whereas, after sequentially depositing a layer of TBPBr/mTPPBr, the VDirac was significantly shifted to around -90 ± 10 V, indicating that considerable amount of electrons are transferred from dopants to graphene. We suspect that dopant anions have good interfacial contact with graphene resulted from the strong anion-π interaction to facilitate the formation of associated anion-π complexes and electron transfer to greatly promote the Fermi level of graphene, leading to the converted n-type characteristic.

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Figure 6b presents the other FET configuration, in which dopants were sandwiched between graphene and source/drain (S/D) electrodes. This architecture has more extended interfaces between dopants and graphene than the previous device structure (Figure 6a). As show, the doped-graphene was similarly converted to n-type behavior; however, the p-type characteristics vanished completely, indicating that VDirac was significantly shifted to beyond -100 V. This profoundly shifted VDirac arose from not only the extended interface between anions and graphene but also possibly the WF tuning of the S/D electrodes induced by the dopant layer.14,1617,39-40

The extended anion/graphene interface reinforces the in-situ AIET doping while the

dopants might tune the electrode WF through interfacial dipole to facilitate electron injection.4142

Interestingly, Figure S7 presents the respectable ambient stability of those n-doped graphene

devices after being exposed to air for 24h. It reveals the dopant layer serves as a protection layer for underlying graphene to retard the infiltration of oxygen or moisture to increase its ambient stability.41-42 2.4. Quantum-Chemical Modeling To better understand the mechanism of in-situ AIET doping, we resorted to quantumchemical modeling using density functional theory (DFT). While it is known that most of standard DFT are not able to give a quantitatively accurate description of charge transfer excited sates, incorporation of certain amount of exact exchange (e.g., in B3LYP) can give qualitatively reasonable results, especially when the charge transfer state is the ground state.43 The energetics and frontier molecular orbitals of C60 and bromide as representatives were specifically analyzed herein through static fixed-point calculations and dynamic simulation. As shown in Figure 7, the -

CT characteristics of C60/Br could be clearly observed from both static and dynamic approaches. 10


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-

The CT characteristics were enhanced when Br is getting closer to the fullerene core from 0.6 nm to 0.2 nm (the C-Br covalent bond length is ~0.19 nm and the usual π-π stacking distance is ~0.35 nm) (Figure 7a). The calculated potential curve resembles a typical Lennard-Jones -

potential, which indicates that the C60/Br complex possesses the lowest energy state of the CT characteristic while its distance is around 0.3 nm. The CT species can then dissociate through -

-

either electron transfer from Br to fullerene or restore to the initial Br and neutral fullerene, while the two resulting species have comparable energy state (∆EET = -7 kcal/mol). Such process is similar to the CT and ET conditions in the donor/acceptor system, where the transition belongs -

to the same energy continuum. Also, the dynamic simulation showed that the charge on Br

would be significantly reduced and transferred to C60 when they are in close distance (Figure 7b). These results explain the reason why ET is less efficient in solution because anions and fullerene are separated apart by surrounding solvent cage, while they are closely packed in solid-state. 3. Conclusion In summary, we describe a facile solution-processing method to effectively dope versatile ntype organic semiconductors under mild conditions by ammonium/phosphornium salts. We manifested that the doping efficiency of in-situ AIET is correlated to the associated Columbic interaction between the cation and anion of dopant. Furthermore, combined with the rational design of n-type molecules, an ambient- and thermal-stable n-doped organic semiconductor could be accomplished. The efficacy of AIET was also demonstrated in doping graphene. The VDirac of the doped graphene showed a quite significant negative shift of 100 V, revealing the effective doping efficiency by in-situ AIET approach. This study offers new insights for

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developing solution-processable n-doped organics with desired mild processability and environmental stability.

Experiment Section Material preparation and Characterization: TBABr, TBPBr, and mTPPBr were purchased from Sigma-Aldrich and PC61BM was purchased from the American Dye Source. The matrix for MALDI-TOF-MS used 2:1 mixture of alpha-cyano-4-hydroxycinnamic acid (CHCA)/2,5dihydroxybenzoic acid (DHB) in acetonitrile. Unless specifically noted, materials and HPLC grade solvents used for experiments were purchased from Sigma-Aldrich, and used after appropriate purification. Divinyltetramethyldisiloxane bis(benzocyclobutene) (BCB) was purchased from the Dow Chemicals Company. All reactions dealing with air- or moisturesensitive compounds were carried out using standard Schlenk technique. The synthesis of dicyanomethylene-substituted quinoidalthienoisoindigo (DCN) is described later. CCDC 958395 contains the supplementary crystallographic data for this paper. These data can be obtained free of

charge

from

The

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/data_request/cif. Graphene was prepared on SiO2 substrates according to the procedure reported in the literature.44 All 1H (500 MHz) and

13

C (125 MHz) spectra were

recorded on a Bruker AV500 spectrometer. Spectra were reported in parts per million from internal tetramethylsilane (δ 0.00 ppm) or residual protons of the deuterated solvent for 1H NMR and from solvent carbon (e.g. δ 77.00 ppm for chloroform) for 13C NMR. CV measurement was carried out in a one-compartment cell under N2, equipped with a glassy-carbon working electrode, a platinum wire counter electrode, and an Ag/Ag+ reference electrode. Measurement was

conducted

in

dichloromethane

(0.5

mM)

containing

12


tetrabutylammonium

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hexafluorophosphate (0.1 M) as a supporting electrolyte with a scan rate of 100 mV/s. All potentials were corrected against ferrocene/ferrocenium (Fc/Fc+) redox couple. Fabrication and Characterization of FETs: The studied FETs were fabricated in a topcontact/bottom-gate device configuration, in which heavily doped p-type silicon substrates with a 300 nm thermal oxide layer (purchased from the Montco Silicon Technologies Inc.) were used. After cleaning the substrate by sequential ultrasonication in acetone and isopropyl alcohol (15 min for each step), air plasma treatment was applied, followed by the passivation of a thin BCB buffer layer. The BCB precursor solution (1 wt% in toluene) was spun onto the silicon oxide at 4000 rpm and then annealed at 250 °C overnight. The measured total capacitance density by parallel-plate capacitors was 10.6 nF/cm2. The active layers were spincoated from chloroform (5 mg/ml) at 1000 rpm. For the DCN device, the films were annealed at 80 and 150 °C for 10 min. The thickness of PC61BM and DCN layer is 30 and 20 nm, respectively (measured by AFM). Interdigitated source and drain electrodes (W=1000 µm, L=20 µm) were defined by evaporating Ag (50 nm, for all n-doped PC61BM) or Au (40 nm, for all ndoped DCN) through a shadow mask at 10-7 Torr. OFET characterization was carried out in a N2–filled glovebox using an Agilent 4155B semiconductor parameter S6 analyzer. The field-effect mobility was estimated from a linear fit of (Ids)1/2 vs Vgs in the saturation regime. The threshold voltage (Vt) was estimated as the x intercept of the linear section of the plot of (Ids)1/2 vs Vgs. Conductivity was extracted from gated twoterminal measurements at zero gate voltage according to the equation of σ = (L/A)(Id/Vd), where L (m) and A (m2) represent the channel length and cross-sectional area of devices, respectively.4,6-7,10,25-27 For the graphene FET, the dopant layers were spin-coated from the

13



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solutions of phosphonium salts (5 mg/ml) before or after the Au evaporation based on the targeted device configuration, in which the thicknesses are 10 nm and 20 nm, respectively

.

Synthesis of dicyanomethylene-substituted quinoidalthienoisoindigo (DCN): Malononitrile (40 mg, 0.61 mmol) was added to a suspension of sodium hydride (30 mg, 1.22 mmol) in 10 mL of dry THF at 0°C under Argon. The mixture was then stirred at room temperature for 30 min. To this mixture, Br-Thin (100 mg, 0.15 mmol) (the synthesis of Br-Thin, (E)-2,2'-dibromo-4,4'bis(2-Ethylhexyl)-[6,6'-bithieno[3,2-b]pyrrolylidene]-5,5'(4H,4'H)-dione,

is

following

the

reported procedure34) and tetrakis(triphenylphosphine) -palladium (0) (35 mg, 0.031 mmol) was added. The system was then heated to reflux for overnight. The mixture was cooled to 0°C and diluted hydrochloric acid (2 M, 5 mL) was added, following by 5 mL of saturated Br2/H2O solution was added. This suspension was stirred at 0°C for 30 min and then the resulting mixture was extracted with chloromethane. The collected extracts were then washed with brine and dried MgSO4. After evaporating the solvent, the residue was purified by column chromatography on silica-gel eluted with n-hexane/dichloromethane (2/1), then dichloromethane to give the final product (45%). DCN 1H NMR (300 MHz, CDCl3): δ 0.95 (m, 12H, CH3), 1.33 (m, 16H, CH2), 1.79 (m, 2H, CH), 3.63 (d, 4H, NCH2), 6.50 (s, 2H, thisophene-H); 13C NMR (125 MHz, CDCl3): δ 10.39, 14.01, 23.00, 23.72, 28.34, 29.71, 30.28, 38.33, 46.35, 75.13, 104.00, 112.08, 112.25, 114.19; MALDI -TOF-MS: calcd. for [C34H38N6O2S2], 626.65, found [M-H]- , 625.27. Computational Modeling: The static approach was carried out by fixed-point ground state energy calculations at various intermolecular distances between Br-/C60, and the dynamic simulation was performed within the Ab initio Born-Oppenheimer molecular dynamics (BOMD) framework. The dynamics was initiated with the bromo atom 0.23 nm away from the C60 surface. 14


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The initial velocities of all the atoms were taken from the thermal sampling of the system at 298 K. A step size of 0.5 fs and a total number of 1000 steps were used in the simulation. The total energy was conserved to within 0.08 kcal/mol. All calculations were carried out using the GAUSSIAN 09 program package.45

ASSOCIATED CONTENT Supporting Information Detailed FET characteristics and derived conductivity of the studied ndoped PC61BM and DCN via in-situ AIET measured at varied dopant concentrations, annealing temperature, and environment, CV and crystal data of DCN, and air-stability of MLG FETs were supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected] (A. K.-Y. Jen); Author Contributions +

These authors are equally contributed

ACKNOWLEDGMENT The authors thank the support from the Asian Office of Aerospace R&D (FA2386-15-1-4106), and the Office of Naval Research (N00014-14-1-0170). Alex K.-Y. Jen thanks the Boeing Foundation for support. X. Li and F. Ding acknowledge US NSF CHE-CAREER 0844999 for

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support. The authors thank Dr. Werner Kaminsky for single crystal analysis of DCN, and Prof. Xiaodong Xu for providing the facilities in his laboratory for the CVD-grown MLG.

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Figure 1. The chemical structures of studied dopants and n-type organic materials for in-situ AIET n-doping in this study.

(b)

-3

10

Drain current, -Id (A) (x10-6)

(a)

-4

Drain Current (A)

10

-5

10

-6

10

-7

10

Pristine PCBM 10 mol%: 20 mol% Vg = TBABr 0 TBABr TBPBr TBPBr mTPBr mTPBr

-8

10

-9

10

-10

10

-11

10

Gave Voltage (V)

-2

TBABr

5 mol%

100

TBPBr

10 mol%

Vg= 0 ~ 80 V

50

0 0

10

20

30

40

50

60

Drain voltage (V)

(d)

mTPPBr

1

0.1

150

20 mol%

o

R.T.

-2

(c)

PCBM (20 mol% mTPPBr)

200

10 20 30 40 50 60 70 80

Conductivity (S/cm) (x 10 )

-20 -10 0

Conductivity (S/cm) (x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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@100 C

1

0.1

N/A 0.01

TBPBr

Doping Concentration

o

@200 C

mTPPBr

Figure 2. N-channel FET (a) Transfer and (b) Output characteristics of the studied n-doped PC61BM via in-situ AIET; Conductivities of the n-doped PC61BM films at (c) Varied dopant concentrations and (d) Different annealing temperatures. 20


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Figure 3. (a) Synthetic route of DCN; (b) N-channel FET transfer; and (c) Output characteristics of DCN.

Figure 4. Single-crystal X-ray structure of DCN, which adopts a planar configuration and short π-π distance of 0.334 nm in Triclinic unit cell. A slice hkl(321) representing a lamella-like packing.

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(b)

-3

Drain Current (A)

-2

10

Conductivity (S/cm) (x10 )

(a)

-4

10

-5

10

-6

10

Pristine DCN TBPBr: mTPPBr 10 mol% 10 mol% 20 mol% 20 mol% 30 mol% 30 mol%

10

10 mol%

30 mol%

0.1

0.01 TBPBr

mTPPBr

Doping Concentration

Gave Voltage (V)

(c) 10

20 mol %

1

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

(d)

-3

100

-4

-5

10

-6

10

-7

10

-8

10

-9

10

20 mol% TBPBr-doped DCN Fresh InAir: 0.5h 3.0h 48h

1W in N2 1.0h 6.0h 72h

1.5h 24h

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Gave Voltage (V)

Percentage (%)

10

Drain Current (A)

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75 FPI FPI-doped PCBM (20 mol%) FPI-doped PCBM (50 mol%) TBPBr-doped DCN (20 mol%) mTPPBr-doped DCN (20 mol%)

50

25

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Exposure Time in Air (h)

Figure 5. (a) FET transfer characteristics and (b) Conductivity of the n-doped DCN via in-situ AIET; (c) FET transfer characteristics and (d) Remaining percentage of the conductivities of the n-doped DCN after exposure to air. The data of FPI and FPI-doped PCBM were reprinted with permission from ref. 25. Copyright @ 2013 John Wiley and Sons.

Figure 6. The FET transfer curves of graphene with the dopants (a) Atop the S/D electrodes and (b) Sandwiched between itself and S/D electrodes. 22


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Figure 7. (a) Calculated potential-energy surfaces for charge transfer from bromide to [60]fullerene under varied distance; (b) Dynamic Simulation for charge transfer form bromide to fullerene.

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Table of Contents Graphic

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Doping Versatile n-Type Organic Semiconductors via Room

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