Low work function Ca doped graphene as a transparent cathode for organic opto-electronics and OLEDs

Journal Pre-proof Low work function Ca doped graphene as a transparent cathode for organic optoelectronics and OLEDs

Chen Klein, Sivan Linde, Rafi Shikler, Gabby Sarusi PII:

S0008-6223(19)31037-1

DOI:

https://doi.org/10.1016/j.carbon.2019.10.028

Reference:

CARBON 14690

To appear in:

Carbon

Received Date:

08 August 2019

Accepted Date:

12 October 2019

Please cite this article as: Chen Klein, Sivan Linde, Rafi Shikler, Gabby Sarusi, Low work function Ca doped graphene as a transparent cathode for organic opto-electronics and OLEDs, Carbon (2019), https://doi.org/10.1016/j.carbon.2019.10.028

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof

Low work function Ca doped graphene as a transparent cathode for organic opto-electronics and OLEDs Chen Klein1, 3, Sivan Linde2, 3, Rafi Shikler2, 3 and *Gabby Sarusi1, 3 Electro-Optics and Photonics Engineering Dept.1, Electrical and Computer Engineering Dept.2, School of Electrical and Computers Engineering1,2 and Ilsa Katz Center for Nano-Science and Technology3, Ben-Gurion University of the Negev, Beer-Sheva, Israel *Corresponding author’s e-mail: [email protected]

Abstract Graphene is an excellent candidate for a transparent electrode since it has high lateral conductivity and optical transparency, but due to its high work function, between 4.5 - 5eV, it can serve as a good transparent anode but poor transparent cathode due to a barrier for electrons injection. Reducing graphene’s work function is thus essential to make it an ideal transparent cathode as well. Naturally, n-type doping of graphene increases its Fermi level and thus reduces the work function. Among many choices of n-type dopants alkaline element can be good candidate, here we present a study of calcium (Ca) n-doped graphene to be serve as a transparent cathode. Several measurements were carried out on Ca doped graphene and compared them with pristine graphene as a reference, such as: Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) Field effect transistor (FET), transparent electron microscopy (TEM), and photoemission spectroscopy. Our results show that doping graphene with 1nm of evaporated Ca on the surface reduces its work function by nearly 1eV. We further performed current-voltage characteristics of graphene/Alq3/Ag structures having undoped and doped graphene cathodes. The structures with doped graphene showed an increase of two orders of magnitude in current under the same applied bias due to the contacts barrier reduction. The results confirm the effectiveness of Ca-doped graphene for reducing the barrier for electron injection from graphene to next adjacent organic layer. It demonstrates the feasibility of Ca doped graphene to serve as a transparent cathode in organic devices in general and in OLEDs in particular. 1

Journal Pre-proof

1. Introduction During the last few years, graphene has drawn attention among others, as the next-generation for flexible transparent electrode due to its high electrical conductivity and optical transparency[1], which can make it an ideal transparent anode [2]. These characteristics could make graphene also a good candidate for Transparent Conductive Cathode (TCC), for various applications such as photo-transistors, photovoltaic cells and organic light emitting diodes (OLED), provided that it would be possible to reduce its high work-function that is around 4.5-5eV [3]. Such reduction of the work function will lower the energy barrier for electrons injection from the graphene and the next adjacent organic or semiconductor layer. Three main characteristics define TCC: low electrical resistivity, high optical transmittance, and low work function [4]. Out of these three main characteristics, lowering the work function is the most challenging since this is not an inherent characteristic of a pristine graphene. Numerous methods have been reported in the literature to reduce the work function including external electric field [5], chemical doping [6], organic-molecule induced dipoles [7-8] and self-assembled methods [9]. Intercalation of different atoms such as Br2 [10], FeCl3 [11] and Li [12] has also been investigated in order to modify the work function of graphene. These methods usually create p-type graphene, while ntype doped graphene seems to be more difficult to obtain. The challenge of getting n-type graphene is attributed to several factors: i) n-dopants must have ionization energy that is lower than the electron affinity of the host material. These dopants having low electron affinity are very reactive mainly to oxygen. ii) Alignment of the energy levels with the adjacent electron-transport layers (particularly in OLEDs), which generally have an electron affinity smaller than 3eV, calls for dopants having low ionization energy [13]. Few studies have been conducted on n-doping of graphene in order to reduce its work function. Wei et. Al. showed that the molecular reductant 2(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) can lead to a reduction of 0.7 eV in the work function of graphene [14]. Recent study showed a new group of materials, high-molecular-weight ethylene amines, for strong n-type doping which reduces the WF of graphene by 0.4 eV and with a high stability, such as amino group that can be utilized as an electrons donor for many chemical reactions [15]. Triethylenetetramine (TETA) is an example for a short ethylene amine, with four amino groups that has the ability to form a high electrons injection per unit area, thereby improving electrical conductivity. Chang et. Al. showed 2

Journal Pre-proof

a large deviation in the work function by using cesium fluoride (CsF) in an aqueous solution. They managed to decrease the WF of graphene by 1.25 eV [16]. They also succeeded to reduce the sheet resistance of graphene to 300 Ω/□ after doping with CsF. Other graphene dopant such as , polyethylenimine ethoxylated (PEIE) can also reduce the WF by 0.3eV [16]. Alkali metal carbonates are also suitable, since metal carbonates have good solubility in water and metal ions with poor electron properties are spontaneously combined with carbon atoms of the graphene sheet due to the negative Gibbs free energy [17]. Yuan et al. [18] showed that a combination of n-doped graphene with alkaline metals and electrostatic gating can reduce the WF of graphene by 1eV[18]. This low WF was achieved by first decreasing the WF by 0.7 eV via an electrostatic gating and after that reducing it to 1 eV by doping graphene with Cs/O in an ultrahigh vacuum (UHV). In this process, first the Fermi level was increased by the electrostatic gating, following by doping graphene with Cs/O. Another example of alkaline metal that was reported to n-dope graphene is Cesium Carbonate (Cs2CO3) [17]. Researchers showed that Cs2CO3 can also lower graphene’s work function [17]. Sanders et al. showed that Cs2CO3 doped graphene can be used as a cathode for OLEDs, since it has the ability to modify graphene’s work function as well as other electrical properties [19]. A summary table on n-type dopants that were used to reduce the work function of graphene is shown in table 1. Table 1: List of common n-type dopants in the literature and work function reduction achieved

Dopant

Type

Δ WF(eV)

Reference

Meo-DMBI n -0.7 14 CsF n -1.2 16 Teta n -0.4 15 PEIE n -0.3 16 Cs2CO3 n -1 17 Another alkaline candidate that can be used is pure calcium (Ca), it is one of the most commonly used electron injectors in organic based devices electrodes [20]. Brown et al. showed that electron injection from Al to organic layer could be enhanced by introducing an ultra-thin Ca layer between the electron-transport layer (ETL) and the Al cathode [21]. This suggests that using a thin layer of Ca as a graphene dopant can significantly reduce the required applied voltage and thus can aid in an efficient electron injection [21]. Owing to its low work function (ϕ = 2.9 eV), low electron affinity, Ca can be used as a metallic cathode to facilitate an efficient 3

Journal Pre-proof

electron injection [22]. Calcium capped with a protective layer of aluminum is a common choice for cathodes as it has been shown to lead to devices with satisfactory luminescent characteristics if it is encapsulated properly [23]. On the other hand, for transparent cathodes, such a combination of Ca-Al, although very thin, has a very low optical transparency of about 50%. Doping of graphene with Ca was not investigated in depth, especially as cathodes to OLED devices where it is expected that the optical transparency would be above 90%. Based on Density Functional Simulations, Legesse et. al. showed that adsorption of Ca atoms on graphene results in a shift of the Fermi level due to transfer of electrons from metal dopants to the pristine graphene [24]. Lee et al. used DFT calculations to simulate the adsorption of Ca atoms on graphene. They found that Ca atoms prefer to be individually adsorbed on the zigzag edge of graphene and thus, the amount Ca atoms clustering is reduced [25]. Han et al. studied the doping effect of Ca in gapped bi-layer graphene (BLG) by low temperature transport measurements. They found that the impurities introduce localized in-gap states, even when the impurity level is in the conduction band [26]. No work was done so far to dope graphene with Ca for the purpose of having an efficient transparent cathode. Here we report of a study on Ca doped graphene where we optimized the doping conditions to enable an efficient transparent cathode. Our research affords a better understanding of the doping mechanism of alkaline metals such as Ca of graphene and emphasizes the potential of graphene as a transparent cathode for organic devices and light emitting and light detection semiconductor devices.

2. Experimental and methods 2.1 Preparation of Graphene Mono-layers of graphene were grown by chemical vapor deposition (CVD) on a copper catalyst (area of 6×4cm2). The sheet-resistance of the specimens was measured using a four-points probe and were around

350Ω/□. Graphene was then transferred to a Si/SiO2 (2.5×2.5cm2)

substrate using wet transfer process. We used a carrier layer of poly (methyl methacrylate PMMA 495K diluted to 2% in Anisole) that was spin-coated on the graphene/Cu foil for 50sec at a speed of 4500 rpm. The PMMA-coated graphene/Cu foil was floated on the ferric chloride (FeCl3) surface to etch the Cu substrate and then to be transferred onto Si/SiO2 substrates. The PMMA 4

Journal Pre-proof

was then removed using acetone for 30 minutes. Ca vapor deposition technique was used to evaporate 1nm thick of Ca and thus dope the graphene (after it was transferred to the Si/SiO2 substrate) using VST TFDS-462 thermal evaporator, where the samples were placed in the evaporator’s vacuum chamber face down. At a vacuum of 10-6 Torr, a thin layer (1nm) of Ca was deposited on the graphene surface. Without exposure to oxygen, the samples were sealed with quartz cap and were placed inside evacuated plastic bags in order to further prevent oxidation when moving them to the characterization systems. Electrical contacts of 80 nm silver (Ag) for conductivity measurements were deposited using a thermally evaporation, at 10-6 Torr at a rate of 0.3 A˚/s through a shadow mask. 2.2 Preparation of Field effect transistor (FET) and four points probe 80 nm of Ag contacts were evaporated on top of the graphene surface using a shadow mask in order to define the source and drain, where the Si substrate serves as a common gate to all devices. The area of the graphene FET channel was around 1mm2. The back side of the Si substrate was scratched with a diamond pen to remove the native oxide and to provide a common gate to all transistors on the wafer. Undoped and doped graphene FET were fabricated-and characterized in the N2 glove box having O2 concentration of less than 0.1 ppm. Sheet resistance (Rsh) of the doped graphene was measured in a four points probe configuration over different periods of time after the evaporation. 2.3 Characterization of pristine and doped Graphene by Raman, TEM, XPS and UPS

The quality of the graphene was evaluated by Raman spectroscopy using a confocal micro-Raman spectrometer (NT-MDT, NTEGRA SPECTRA) having a 514nm laser excitation source (the laser power at sample’s surface was below 0.1mW), the system uses a X100 objective and a CCD camera for aiming and alignment. The focused laser spot has a diameter around 2µm, with a spectral resolution of about 3cm-1. The wavenumber calibration was achieved based on the standard values for crystalline silicon band checked at 520cm-1 and the vibrational stretching mode of atmospheric nitrogen at 2332cm-1. All spectra were recorded at room temperature. Transmission electron microscope (TEM) micrographs were acquired using a JEOL JEM-2100F system with accelerating voltage of 200 kV. For the TEM measurements a CVD-grown single-layer graphene was prepared, via transferring the graphene sheet on Au foil 5

Journal Pre-proof

onto a TEM lacey carbon-coated grid. Topographic images were made with a MFP-3D-Bio AFM (Asylum Research/Oxford Instruments) in AC-mode ("tapping mode"), using standard cantilevers for imaging in air (AC160TS, HQ300Au, Asylum Research). X-ray photoelectron spectroscopy (XPS) spectra were carried out with a UHV ESCALAB 290Xi spectrometer, equipped with a hemispherical electron energy analyzer. A Kratos AXIS ULTRA system using a concentric hemispherical analyzer for photo-excited electron detection was used. XPS measurements were performed using a monochromatic Al K X-ray source (h = 1486.6eV) at 75W and detection pass energies ranged between 20 and 80eV. UPS (Ultra-violet Photoemission Spectroscopy) measurements were carried out using a Kratos AXIS ULTRA system and a concentric hemispherical analyzer for photo-excited electron detection. UPS was measured with a helium discharge lamp of He I (21.22eV) and He II (40.8eV) emission lines. Energy level with respect to the Fermi level was measured on a bare Au substrate. The vacuum level was obtained from the secondary-electron cutoff (photoemission onset) measured in the low kinetic energy region of the He (I) spectra. Total energy resolution was less than 100meV, as was determined from the Fermi edge of Au reference sample.

3. Results and Discussion Raman spectra of undoped and doped graphene films on SiO2 (285 nm thick)/Si are shown in Figure 1. Three main Raman bands were used to characterize graphene: The G band (1580 cm-1), 2D band (2700 cm-1) and the D band (1360cm-1) [27]. The G band arises from the stretching of the of sp2 carbon atoms [28]. The 2D band arises from the two phonon double resonance Raman process [29], and the D band arises from breathing modes of sp2 atoms in the ring [27]. The 2D band is mainly used to distinguish between single-layer graphene from multilayer graphene films [30], while the D-band is used to quantify structural defects [27]. After doping, the G band was red shifted by 12 cm-1, indicating that the graphene becomes n-type. On the other hand, the shift of the 2D band by 5 cm-1, is observed; these changes in the 2D band is attributed to phonons at the K+Δk points of the Brillouin zone [30]. After doping, a shift of the Fermi level may cause a change in the equilibrium carbon lattice which is attributed to phonon dispersions near Kohn anomalies [31]. The theoretical dependence of the G-band shift as a

6

Journal Pre-proof

function of doping in Raman measurement was also calculated [32-33]. The doping concentration (Nd) can be quantified from the frequency shift of the G peak by using the relation [32]: (1)

ℏ∆𝜔𝐺 = 𝛼 ′ |∆𝐸𝐹 |

ΔωG is the G peak position variation of the doped and undoped graphene, where 𝛼 ′ = 4.72*10-3. From this equation, we can calculate the shift of Fermi level and relate it to the carrier density using the following relation [32]: (2)

𝑛 = 𝜋 −1 (∆𝐸𝐹/ ℏ𝜐𝐹) 2

From the obtained parameters and equations 1 and 2, we evaluated Fermi level shifts ΔEF and the electron carrier density n. The Fermi energy shift of ΔEF = 0.5 eV corresponds to the electron carrier density of n = 1.2 × 1013 cm−2 for doped graphene. Another important parameter is the ratio between I2D/IG bands [31]. Studies have shown that the decrease of I2D/IG ratio indicates charge transfer between the dopant to the graphene [15]. As shown in Fig 1A, after doping, the 2D mode of doped graphene becomes more pronounced with respect to the undoped graphene. This may indicate a successful process of modifying p-type graphene into n-type [14]. The dynamic nature (over several periods of time after the Ca evaporation) of the Raman modes after doping is also analyzed, as shown in Fig. 1(B, C) in more detail, by using a Lorentzian fitting method. It can be seen that the 2D modes are well fitted (Fig 1 B, C) by the single Lorentzian with a full-width half-maximum (FWHM) of 41 and 48 cm-1, respectively. This is in agreement with the assumption that the samples are made of monolayer graphene. As for the D band, it is used to indicate disordered structure of the graphene. The Raman spectrum of pristine graphene (Fig 1A) shows a very small D-peak (black) which is positioned at 1350 cm-1, indicating the presence of low number of defects and grain boundaries in the graphene plane due to small number of structural and physical defects created during growth and the transfer of the graphene. It should be also noted that after Ca doping, no significance changes in the D peak occurred for monolayer Ca doped graphene. This may indicate that a rather low defect density of the basal plane reactions and formation of substitution impurities do not take place by the Ca dopants. The intensity of the D band peak amplitude looks smaller due to that the

7

Journal Pre-proof

spectrum of the doped graphene was measured through quartz cap that prevent oxidization of the Ca compare to the pristine graphene that was measured with no Quartz capping.

Fig.1. A) Raman spectra of Si/SiO2/G (black) undoped graphene and doped graphene Si/SiO2/G/Ca (red). The inset in (A) is a zoom-in into the region around the G peak variation before(black) and after doping(red) B) 2D Lorentzian fits of the pristine graphene and C) Doped graphene by Ca. Both Si/SiO2/G (black) and Si/SiO2/G/Ca (red) substrates were measured through sealed transparent quartz substrate

In order to further confirm the conversion of the graphene to be n- type, graphene field-effect transistor (GFET) was fabricated in a structure shown in figure 2A and the current−voltage characteristics of both undoped and doped graphene were measured as shown in Fig 2B.

8

Journal Pre-proof

Figure 2. (A) A schematic structure of back gated graphene field effect transistor with Ca doping on the surface (top); (B) Transfer curve of undoped and doped graphene field effect transistor

Figure 2B shows the drain current (IDS) measured for a constant drain-source voltage VDS = 0.1V as a function of the gate voltage applied on the backside of the Si/SiO2 substrate. The GFETs of undoped graphene shows a small positive Dirac Point (DP) voltage at 9 V, suggesting a p-type character of the graphene that was exposed to air. This shift can be associated with the poly (methyl methacrylate) (PMMA) residue on the surface of the CVD graphene introduced during transfer [34], as well as defects when graphene is exposed to moisture and oxygen and the mounting on SiO2 surface [35]. In contrast, graphene doped with 1 nm of Ca shows a pronounced DP voltage shifting to

−8 V indicating a clear n-type behavior of the doped

graphene. We did XPS analysis to characterize the core levels spectra of the dopants at the Graphene/Ca interface. Figure 3 shows XPS spectra of undoped and doped graphene layers. The quantitative chemical analyses of each graphene film, obtained from these spectra, are summarized in Table 2 where the Ca atomic percent values are presented in Table 2 with respect to the carbon content. Table 2: Elemental composition of pristine and Ca doped graphene

Sample Pristine Graphene Graphene Doped Ca

O 34.5 41.6 9

C 48.6 37.3

Si 16.6 10.6

Ca 9.1

Journal Pre-proof

As seen in table 2, carbon signals are well observed indicating that graphene layer was not removed or damaged during doping process. The presence of Ca signal presented in Fig.3B of the doped graphene reveals that Ca atoms/clusters are covering the surface and that it is a ndoped graphene. The fitting of the C1s spectra measured for pristine and doped graphene is also presented in Fig. 3A and 3B. Five main components in the XPS spectra can be observed in Fig 3A and 3B, one at 284.3eV related to the C=C sp2 bonds, second one at 285.8 eV that is associated with C–C sp3 hybridization. Third component is at 286.3 eV which is associated with C–O bonds, the fourth is a carbonyl group (C=O) near 287.6 eV, and the fifth is hydroxyl group (C=OH) group near 290.1 eV. The existence of the oxidized carbon signals can be attributed to the transfer of CVD graphene, which uses organic materials such as PMMA and acetone. The C 1s peak shift takes place after the Ca doping, with a shift of all the peak positions to higher binding energies. Furthermore, the effect of Ca doped graphene is indicated in Fig 3A, B through the variation in the IC=C/IC–C intensity ratio. We can observe from our quantitative analysis that IC=C/IC–C ratio slightly decreases from 5.1 for pristine graphene to 3.7 in doped graphene. These findings may suggest in a strong chemical bonding between graphene and the adsorbed Ca. They are also in accordance with Kwon et. al., Larrude et. al. and Zhou et. al. which showed that the decreased in the IC=C/IC–C ratio is associated with n doping [17,36-37]. Fig. 3C shows the Ca 3d core-level spectra of the Ca doped graphene. The Ca 3d core-level spectrum shows a high intensity of the Ca 3d peak. The high signal of the Ca 3d spectra verifies the adsorption of Ca on graphene surface after doping.

10

Journal Pre-proof

Fig.3. High resolution C1s XPS spectra of Si/SiO2/G (A) and Si/SiO2/G/Ca (B) films. The C 1s peak was fitted by four components: sp2 carbon (C=C), sp3 carbon (C–C), C–O bond, C=O bond, and the C-OH bond. A small peak of C-Ca is evident in the doped graphene; (c) Ca 3d spectra

In order to examine the surface morphology of pristine and Ca doped graphene we used an atomic force microscope (AFM) technique. Fig. 4 show the AFM topographic images of pristine graphene (Fig. 4A) and Ca doped graphene (Fig. 4B). The surface morphology of pristine graphene is smooth (roughness =1.2nm RMS). After Ca doping, the graphene sample was covered with Ca clusters of the average size of 30nm and most probably Ca atoms in between that can’t be resolved by the AFM. Figure 4B is an example of cluster that is indicated by a red arrow. The measured RMS roughness of these clusters was 6.2nm, where the number density of Ca particles is 912 not including the non-observed Ca atoms. The images correspond well with the XPS measurements (Table 2) and Ca clusters coverage of about 9%. Fig. 4c shows highmagnification Transmission Electron Microscope - TEM images of the doped graphene. After Ca doping, sub-micrometer-size bright particles of Ca indicated by the red arrow, can be spotted on the graphene surface.

11

Journal Pre-proof

Fig.4. AFM images of (a) undoped, and (b) doped graphene by calcium (1nm), respectively. The red arrow denotes the clustering of Ca on the doped graphene (c) High-magnification TEM image of the doped graphene, the red arrow indicates Ca clusture with the size of ~30nm

UPS measurements were carried out to investigate the direct effect of n-type doping on the work-function of the doped graphene. Fig. 5 shows the results of pristine graphene and of Ca doped graphene. The cut-off binding energy of single layer graphene is 16.65 eV, and the work function is 4.57 eV. After Ca doping, the cut-off binding energy is increased to be 17.62eV which corresponds to a lowering of the work-function by nearly 1 eV.

12

Journal Pre-proof

Fig. 5: UPS spectra covering the secondary electron threshold Region for pristine graphene (black) and Ca doped graphene (red). In the inset, a schematic of graphene Fermi–Dirac cones showing the Fermi level shift due to Ca doped graphene

This behavior is typical for n-doped graphene, where the decrease in the work function can be attributed to the shift of the Fermi level from the Dirac point to the conduction band. Our results are consistent with the doping mechanism for alkaline metals reported in the literature [38]. Two main doping mechanisms can explain our results in converting pristine graphene into n-type graphene and reducing the work function: i) Since graphene has low density of states near the Fermi level, charge transfer of electrons from the Ca to the graphene fills un occupied conduction band states. Because of the sharp conical shape of the band structure, this contributes to a significant increases of the Fermi level and thus a reduction of the work function [24]. ii) second mechanism that can explain the reduction in the work function is attributed to the dipole potential, which shifts the graphene states with respect to the vacuum level. This dipole can also cause a shift of the Fermi level [38]. In any case or a combination of the two mechanisms it results with a reduction of the work function as desired. 13

Journal Pre-proof

Next, we investigated the effect of the doping on the sheet resistance of pristine and doped graphene. The sheet resistance was measured as a function of the elapsed time after ending of the doping process i.e., 2 minutes, 1 hour, 24 hours. The measurements were done in a N2 ventilated glove box that is attached to the evaporator chamber, so no oxidation of the Ca could take place. It should be noted that in this measurement, the contacts were evaporated before the Ca doping, so the contacts were attached to the pristine graphene having high work function and thus high barrier for electron injection for pristine and doped graphene. The measurements reveal that no significance changes in the sheet resistances occurred for monolayer Ca doped graphene. The fact that in Ca doped graphene the lateral conductivity did not decrease is in agreement the Raman results stating that due to the doping process the sp2 carbon lattice was not damaged and thus, there was no change in the sheet resistance. Next, in order to show that the doping affect only the electron injection from the graphene cathode to the Alq3 Electron Transport Layer (ETL) due to the reduction of the barrier, we compared the pristine graphene having a structure of Ag/Alq3/Graphene/SiO2/Si with Ca doped graphene of similar structure. But in the latter case the doping was done before the evaporation of the metal contacts, so the doping was underneath the contact and thus the barrier height was decreased. We characterized the current density vs. voltage (J−V) of the two diodes structures: (1) Pristine graphene/Alq3/Ag; and (2) Ca (1nm) doped graphene /Alq3/Ag. The two device structures and J−V characteristics are presented in Figure 6A, B, C, respectively. As can be seen, diode 1 shows nearly symmetric J−V characteristics, with a low current density in the order of 10−5 mA/cm2 under a bias of +2V. Low current injections from both Ag and graphene are observed which is consistent with high injection barriers due to the high work function of the graphene. In diode 2 on the other hand, a symmetric J−V characteristic is observed; having a significantly higher current density under the same bias (positive bias on top Ag electrode). This is attributed to an efficient electron injection from the doped graphene having reduced work function due to Ca n-type doping. The current under bias of +2V is more than two orders of magnitude larger in diode 2 compared to diode 1, consistent with the injection barrier lowering of 1V due to the Ca doping.

14

Journal Pre-proof

B

A

Fig.6. Schematics of two diode structures: (A) diode 1 - Pristine graphene/Alq3/Ag, and (B) diode - 2 doped graphene: Ca (1nm)/Alq3/Ag. (C) J−V characteristics of two diodes, the inset shows optical microscope image of the device.

4. Conclusion In this work, we showed an effective way to reduce the work function of graphene by 1eV and thus reduce the electron-injection barrier between a Ca doped single-layer graphene and an organic electron transport material, Alq3. This makes the Ca doped graphene an efficient 15

Journal Pre-proof

transparent cathode. The graphene’s work-function was reduced by ~1eV after a deposition of 1 nm of Ca, which enhances the electron-injection to the organic ETL. This reduction of the workfunction is due to the transfer of electrons from the Ca dopant to the graphene, which increases the Fermi level above the Dirac point to the conduction band. Raman spectroscopy, XPS, TEM and photoemission spectroscopy measurements confirmed the dopant-induced changes in the electronic properties of the graphene layer without any damage to the graphene. The effectiveness of electron injection from the low work-function graphene was demonstrated with series of Graphene/Alq3/Ag diodes comprising undoped graphene and doped graphene. The latter show an order of magnitude increase in electron current under the same bias only due to the reduction of the barrier to electron injection from the cathode. Our results pave the way for use of Ca doped Graphene as a transparent cathode for organic optoelectronic devices. Acknowledgement: The authors would like to acknowledge Dr. Tatyana Bendikov from Weizmann Institute of Science for the UPS measurements and the fruitful discussions. This work was partially funded by INNI Israel National Nanotechnology Initiative under the FTA programs

References [1].

Hideo Aoki , Mildred S.Dresselhaus (2 ed.), Physics of Graphene (Springer 2013). doi:10.1007/978-3-319-02633-610.

[2].

K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V. V. Khotkevich, S. V. Morozov, A.K. Geim, Two-dimensional atomic crystals, Proc. Natl. Acad. Sci. 102 (2005),pp. 10451– 10453. doi:10.1073/pnas.0502848102.

[3].

Zheng Qingbin, Kim Jang-Kyo. (1 ed). Graphene for Transparent Conductors Synthesis, Properties and Applications, (Springer; 2015) doi: 10.1007/978-1-4939-2769-2

[4].

A.E. Mansour, M.M. Said, S. Dey, H. Hu, S. Zhang, R. Munir, Y. Zhang, K. Moudgil, S. Barlow, S.R. Marder, Facile Doping and Work-Function Modification of Few-Layer Graphene Using Molecular Oxidants and Reductants,Advanced Functional materials, 27 (2017),pp.1602004. doi:10.1002/adfm.201602004. 16

Journal Pre-proof

[5].

Y.J. Yu, Y. Zhao, S. Ryu, L.E. Brus, K.S. Kim, P. Kim, Tuning the Graphene Work Function by Electric Field Effect,Nano Lett,9 (2009), 3430-3434.doi: 10.1021/nl901572a.

[6].

H. Liu, Y. Liua and D. Zhua, Chemical doping of graphene, Journal of Materials Chemistry ,21 (2011),pp. 3335–3345. doi:10.1039/c0jm02922j.

[7]

A. Kasry,M. A. Kuroda,G. J. Martyna,G. S. Tulevski,and A. A. Bol, Chemical Doping of Large-Area Stacked Graphene Films for Use as Transparent , Conducting Electrodes,ACS,Nano, 4 (2010),pp. 3839–3844.doi: doi.org/10.1021/nn100508g.

[8]

T.O. Wehling, K.S. Novoselov, S. V Morozov, E.E. Vdovin, Molecular Doping of Graphene,Nano Lett.8 (2008),pp.173-177. doi:10.1021/nl072364w.

[9]

R. Phillipson, C.J. Lockhart, D. Rosa, J. Teyssandier, P. Walke, D. Waghray, Y. Fujita, J. Adisoejoso, I. Asselberghs, C. Huyghebaert, H. Uji-i, S. De Gendt, S. De Feyter, Tunable doping of graphene by using physisorbed self-assembled networks,Nano scale,48

[10]

A.E. Mansour, S. Dey, A. Amassian, M.H. Tanielian, Bromination of Graphene: A New Route to Making High Performance Transparent Conducting Electrodes with Low Optical Losses,Applied materials and interfaces,7 (2015),pp. 17692-17699. doi:10.1021/acsami.5b03274.

[11]

I. Khrapach, F. Withers, T.H. Bointon, D.K. Polyushkin, W.L. Barnes, S. Russo, M.F. Craciun, Novel Highly Conductive and Transparent Graphene-Based Conductors,Advanced materials,24 (2012),pp. 2844–2849. doi:10.1002/adma.201200489.

[12]

W. Bao, J. Wan, X. Han, X. Cai, H. Zhu, D. Kim, D. Ma, Y. Xu, J.N. Munday, H.D. Drew, M.S. Fuhrer, L. Hu, lithium intercalation, Nat. Commun. 5(2014),pp. 1–9. doi:10.1038/ncomms5224.

[13]

A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Electron Transport Materials for Organic Light-Emitting Diodes,Chem.Mat,16 (2004),pp. 4556–4573. doi:10.1021/cm049473l. 17

Journal Pre-proof

[14]

P. Wei,N. Liu, H. R. Lee, E. Adijanto, L. Ci, B. D. Naab,J. Q. Zhong, J. Park, W. Chen, Y. Cui, and Z. Bao,Nano Lett,13 (2013),pp.189-1897. doi:10.1021/nl303410g.

[15]

Y. Kim, J.Ryu, M. Park, E. S. Kim, J. M. Yoo, J. Park, J. H. Kang, and B. H. Hong, Vapor-Phase Molecular Doping of Transparent Electrodes,ACS Nano, (2014),pp. 868– 874. doi:10.1021/nn405596j.

[16]

J.Chang, W. Lin, J. Taur, T. Chen, G. Liao, T. Pi,M. Chen,and C. Wu, Graphene Anodes and Cathodes: Tuning the Work Function of Graphene by Nearly 2 eV with an Aqueous Intercalation Process, 7(2015) pp.17155–17161. doi:10.1021/acsami.5b03934.

[17]

K.C. Kwon, K.S. Choi, B.J. Kim, J. Lee, S.Y. Kim, Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates, J. Phys. Chem.C, 116 (2012),pp. 26586-26591. doi:10.1021/jp3069927.

[18]

H. Yuan, S. Chang, I. Bargatin, N.C. Wang, D.C. Riley, H. Wang, J.W. Schwede, J. Provine, E. Pop, Z. Shen, P.A. Pianetta, N.A. Melosh, R.T. Howe, Engineering Ultra-Low Work Function of Graphene, Nano Lett,15(2015),pp.6475-6480. doi:10.1021/acs.nanolett.5b01916.

[19]

S. Sanders, A. Cabrero‐Vilatela, P. R. Kidambi, J. A. Alexander‐Webber, C. Weijtens, P. Braeuninger‐Weimer, A. I. Aria, M. M. Qasim, T. D. Wilkinson, J. Robertson, S. Hofmann, J. Meyer, Nanoscale, 7(2015),pp. 13135–13142.doi: 10.1039/c5nr03246f.

[20]

S. L. Lai, M. Y. Chan, C. S. Lee, and S. T. Lee, J.App. Phys. 94 (2003),pp.7297-7299. doi.org/10.1063/1.1623326.

[21] T.M. Brown, R.H. Friend, Efficient electron injection in blue-emitting polymer lightemitting diodes with LiF / Ca / Al cathodes Efficient electron injection in blue-emitting polymer light-emitting diodes, J.App. Phys,Lett, 174 (2001),pp. 77–80. doi:10.1063/1.1383800. [22]

D. Braun and A. J. Heeger, Visible light emission from semiconducting polymer diodes, J.App. Phys,Lett ,58 (1998),pp.1982-1984. doi.org/10.1063/1.105039. 18

Journal Pre-proof

[23]

T.M. Brown, R.H. Friend, Efficient electron injection in blue-emitting polymer lightemitting diodes with LiF / Ca / Al cathodes Efficient electron injection in blue-emitting polymer light-emitting diodes, J.App. Phys,Lett, 174 (2001),pp. 77–80. doi:10.1063/1.1383800.

[24]

M.Legessea,F.ElMellouhia,E.T.Bentriaa,M.E.Madjeta,T.S.Fisherb,S.Kaisa,F.H.Alharbi Reduced work function of graphene by metal adatoms,App.Surface science,394(2017),pp.98-107.doi: org/10.1016/j.apsusc.2016.10.097.

[25]

H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Calcium-Decorated Graphene-Based Nanostructures for Hydrogen Storage, Nanostructures for Hydrogen Storage, 10(2010) pp.793–798. doi:10.1021/nl902822s.

[26]

Qi Han, Baoming Yan, Zhenzhao Jia, Jingjing Niu, Dapeng Yu, and Xiaosong Wu, Effect of impurity doping in gapped bilayer graphene,App.Physics.Lett, 107 (2016) pp.163104. doi:10.1063/1.4934489.

[27] A.C. Ferrari, Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (2007),pp. 47–57. doi:10.1016/j.ssc.2007.03.052. [28]

S. J. Kim, S. J. Park, H. Y. Kim, G. S. Jang, D. J. Park, Ji-Yong Park, Soonil Lee, and Y. H. Ahn, Characterization of chemical doping of graphene by in-situ Raman spectroscopy spectroscopy, 108 (2016),pp. 203111. doi:10.1063/1.4950969.

[29]

R. Beams, L.G. Canc, L. Novotny, Raman characterization of defects and dopants in graphene, Journal of Physics: Condensed Matter,27(2015),pp. 083002. doi:10.1088/09538984/27/8/083002.

[30]

A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Raman Spectrum of Graphene and Graphene Layers, Physical review Lett. ,97 (2006) ,pp. 187401. doi:10.1103/PhysRevLett.97.187401.

[31]

A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U. V Waghmare, K.S. 19

Journal Pre-proof

Novoselov, H.R. Krishnamurthy, A.K. Geim, A.C. Ferrari, A.K. Sood, Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor,Nature Nanotech, 3 (2008),pp. 1–6. doi:10.1038/nnano.2008.67. [32]

S. Pisana, M. Lazzeri, C. Casiraghi, K.S. Novoselov, A.K. Geim, A.C. Ferrari, F. Mauri, Breakdown of the adiabatic Born – Oppenheimer approximation in graphene,Nature.Mat, 6 (2007) ,pp.3–6. doi:10.1038/nmat1846.

[33]

K. Yokota,K. Takai and T. Enoki, Carrier Control of Graphene Driven by the Proximity Effect of Functionalized Self-assembled Monolayers ,Nano Lett,11 (2011),pp. 3669–3675. doi:10.1021/nl201607t.

[34] Pirkle, A.; Chan, J.; Venugopal, A.; Hinojos, D.; Magnuson, C.W.; McDonnell, S.; Colombo, L.; Vogel, E. M.; Ruoff, R. S.; Wallace,R. M. The Effect of Chemical Residues on the Physical and Electrical Properties of Chemical Vapor Deposited Graphene Transferred to SiO2. Appl. Phys. Lett. 99(2011),pp.122108.doi: doi.org/10.1063/1.3643444 [35]

Ryu, S.; Liu, L.; Berciaud, S.; Yu, Y. J.; Liu, H.; Kim, P.; Flynn G. W.; Brus, L. E. Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene on a SiO2 Substrate. Nano Lett. 10(2010),pp. 4944−4951.doi: 10.1021/nl1029607.

[36] D. G. Larrude, Y. G.Basabe, F. L. Freire' J.be and M. Luiza M. RoccoElectronic structure and ultrafast charge transfer dynamics of phosphorous doped graphene layers on a copper substrate: a combined spectroscopic study, RSC Adv,5(2015),pp 74189–74197.doi: 10.1039/c5ra12799h. [37]

G. Zhou, G. Pan, L. Wei, T. Li and F. Zhang Heavily N-doped monolayer graphene electrodes used for high-performance N-channel polymeric thin film transistors,RSC Adv,6(2016),pp 93855–93862.doi: 10.1039/c6ra20496a.

[38]

K.T. Chan, J.B. Neaton, M.L. Cohen, First-principles study of metal adatom adsorption on graphene, Phys. Rev. B,77 (2008),pp. 1–12. doi:10.1103/PhysRevB.77.235430.

20

Journal Pre-proof

Declaration of interests I declare That I and my co-authors do not have any conflict of interests submitting this paper Prof. Gabby Sarusi