**2.1 Preparation of SLG source electrodes**

*Preparation of substrates*: The transparent VOLET substrate used here was prepared with a pre-patterned bottom gate electrode, consisting of a transparent ITO layer (80-nm-thick, 30 ohm square<sup>−</sup><sup>1</sup> sheet resistance) on a glass substrate, and a sputterdeposited aluminum oxide (Al2O3) top layer (400 nm) as a gate dielectric over the ITO gate (glass/ITO/Al2O3). The VOLET substrate used was then cleaned with alcohol, followed by a UV treatment (5 min), prior to the fabrication of the devices.

*Synthesis and transfer of SLG*: The procedure used for transferring the chemicalvapor-deposition (CVD)-grown graphene onto a target substrate [40–45], in this case an FET substrate, a VOLET substrate, or a glass substrate, is described below. The first step involves the CVD growth of graphene on a copper (Cu) foil [42–45]. A clean Cu foil was placed in a quartz tube vacuum chamber and then the temperature of the chamber was increased to 1000°C under Ar (10 sccm). For the growth of graphene, a mixed gas of methane (CH4, 30 sccm) and hydrogen (H2, 10 sccm) was used at approximately 2.7 × 10<sup>−</sup><sup>2</sup> Pa. The next step involved spin-coating a solution of polymethyl methacrylate (PMMA) at 3000 rpm for 60 s onto the graphene on the Cu foil [42–45]. The graphene on the back side of the Cu foil was removed by atmosphericpressure oxygen plasma. Next, a PMMA-coated Cu/Gr (Cu/Gr/PMMA) block (width: 4 mm, length: 20 mm) was floated on an aqueous FeCl3 etching solution used to etch the Cu foil entirely, at 50°C for 20 min [40]. Next, the Gr/PMMA block was rinsed with deionized (DI) water two to five times (10 min) and transferred onto a target substrate, after which the graphene-transferred substrate was dried under reduced pressure (~1 Pa) for 1 h and left in air for 24 h. The PMMA layer was then removed by dissolving the PMMA layer in chloroform (~60 min), monochlorobenzene (~30 min), and chloroform again (~30 min), to obtain a SLG source electrode. The optical characteristics of the SLG-transferred VOLET substrate were monitored using a UV–visible spectroscopy system. The average optical transmittance (~92%) of a SLG source on a VOLET substrate in the visible range (400–800 nm) was found to be similar to that (~92%) of a conventional ITO-coated glass substrate for OLEDs.

## **2.2 Characterization of SLGs**

In this study, a transferred SLG was investigated as a source contact, where the FeCl3 doping is processed spontaneously during the graphene transfer process [40]. The basic properties of the three SLG sources are shown in **Figures 3** and **4** and are summarized in **Table 1**.

In order to identify the SLG used, the surface composition of the SLG on the SiO2/Si substrates was analyzed by X-ray photoelectron spectroscopy (XPS). **Figure 3(a)** presents the wide-scan XPS spectra, showing strong photoelectron lines at binding energies of ~104, ~285, and ~531 eV, which are attributed to Si2p, C1s and O1s, respectively. Note that there is no Cu peak in the range of 932–935 eV (Cu2p and Cu2+), implying the complete etching of the Cu foil. In addition, the XPS spectra also revealed small but measurable amounts of Cl and Fe. These are likely residues of the etchant (FeCl3) used during the etching process. When such FeCl3 residues adsorb onto the SLG, the transfer of electrons to the Cl from the SLG (chlorination) [46] induces unintentional p-doping of the SLG.

**Figure 3(b)** shows the surface morphology of the SLG on the VOLET substrate as measured by a noncontact atomic force microscope (AFM). As indicated by the AFM morphology, the SLG samples exhibit a fairly smooth surface; the SLG presented an AFM morphology that was nearly identical at different positions on the investigated SLG samples with low RMS roughness levels of 1.4–2.0 nm.

The surface-contact potential difference (*V*CPD) of the SLG on the VOLET substrate was also monitored using a simultaneous Kelvin prove force microscope (KPFM) by applying AC voltage (1 V) with a frequency of 18 kHz to a Pt/Ir-coated silicon KPFM cantilever. In order to calibrate the work function of the sample, *V*CPD of highly oriented pyrolytic graphite (HOPG) was used as a reference *V*CPD. The work function of the SLG (*W*SLG) was obtained by a comparison of *V*CPDs for the SLG and the HOPG,

$$\mathcal{W}\_{\text{SLG}} = \mathcal{W}\_{\text{HOPG}} + \left[V\_{\text{CPD}}(\text{HOPG}) - V\_{\text{CPD}}(\text{SLG})\right],\tag{1}$$

**147**

**Figure 4.**

*VOLET substrate.*

**Figure 3.**

*Vertical-Type Organic Light-Emitting Transistors with High Effective Aperture Ratios*

substrate is approximately 5.21 ± 0.07 eV, which is higher than the intrinsic work function (4.5–4.8 eV) of undoped monolayer graphenes [47, 48], mainly due to the

*(a) Wide-scan XPS survey spectra of the studied SLG on a SiO2/Si substrate. AFM topographic image (5 μm × 5 μm) (b) and corresponding work function distribution (c) of the SLG on the VOLET substrate as measured by KPFM. (d) Transport characteristics of the SLG from liquid-gated lateral Gr-FETs at*  V*DS = −100 mV. The insets show the structure of the liquid-gated lateral Gr-FET with an Ag/AgCl reference* 

*(a) Raman spectra of the SLG transferred from the Cu foil onto the VOLET substrate. (b) Polarized optical microscope image of a spin-coated layer of commercial nematic liquid crystals on the SLG transferred to the* 

*electrode in a nonaqueous electrolyte containing ACN and 100 mM of TBAPF6.*

*DOI: http://dx.doi.org/10.5772/intechopen.92833*

FeCl3 doping [49].

where *W*HOPG is the work function of the HOPG (~4.6 eV) [47]. **Figure 3(c)** shows the distributions of the work functions of the SLG as measured by the KPFM. The estimated average work function of the FeCl3-doped SLG on the VOLET *Vertical-Type Organic Light-Emitting Transistors with High Effective Aperture Ratios DOI: http://dx.doi.org/10.5772/intechopen.92833*

substrate is approximately 5.21 ± 0.07 eV, which is higher than the intrinsic work function (4.5–4.8 eV) of undoped monolayer graphenes [47, 48], mainly due to the FeCl3 doping [49].

#### **Figure 3.**

*Liquid Crystals and Display Technology*

approximately 2.7 × 10<sup>−</sup><sup>2</sup>

**2.2 Characterization of SLGs**

summarized in **Table 1**.

step involves the CVD growth of graphene on a copper (Cu) foil [42–45]. A clean Cu foil was placed in a quartz tube vacuum chamber and then the temperature of the chamber was increased to 1000°C under Ar (10 sccm). For the growth of graphene, a mixed gas of methane (CH4, 30 sccm) and hydrogen (H2, 10 sccm) was used at

methyl methacrylate (PMMA) at 3000 rpm for 60 s onto the graphene on the Cu foil [42–45]. The graphene on the back side of the Cu foil was removed by atmosphericpressure oxygen plasma. Next, a PMMA-coated Cu/Gr (Cu/Gr/PMMA) block (width: 4 mm, length: 20 mm) was floated on an aqueous FeCl3 etching solution used to etch the Cu foil entirely, at 50°C for 20 min [40]. Next, the Gr/PMMA block was rinsed with deionized (DI) water two to five times (10 min) and transferred onto a target substrate, after which the graphene-transferred substrate was dried under reduced pressure (~1 Pa) for 1 h and left in air for 24 h. The PMMA layer was then removed by dissolving the PMMA layer in chloroform (~60 min), monochlorobenzene (~30 min), and chloroform again (~30 min), to obtain a SLG source electrode. The optical characteristics of the SLG-transferred VOLET substrate were monitored using a UV–visible spectroscopy system. The average optical transmittance (~92%) of a SLG source on a VOLET substrate in the visible range (400–800 nm) was found to be similar to that

In this study, a transferred SLG was investigated as a source contact, where the FeCl3 doping is processed spontaneously during the graphene transfer process [40]. The basic properties of the three SLG sources are shown in **Figures 3** and **4** and are

In order to identify the SLG used, the surface composition of the SLG on the SiO2/Si substrates was analyzed by X-ray photoelectron spectroscopy (XPS). **Figure 3(a)** presents the wide-scan XPS spectra, showing strong photoelectron lines at binding energies of ~104, ~285, and ~531 eV, which are attributed to Si2p, C1s and O1s, respectively. Note that there is no Cu peak in the range of 932–935 eV (Cu2p and Cu2+), implying the complete etching of the Cu foil. In addition, the XPS spectra also revealed small but measurable amounts of Cl and Fe. These are likely residues of the etchant (FeCl3) used during the etching process. When such FeCl3 residues adsorb onto the SLG, the transfer of electrons to the Cl from the SLG

**Figure 3(b)** shows the surface morphology of the SLG on the VOLET substrate as measured by a noncontact atomic force microscope (AFM). As indicated by the AFM morphology, the SLG samples exhibit a fairly smooth surface; the SLG presented an AFM morphology that was nearly identical at different positions on the investigated SLG samples with low RMS roughness levels of 1.4–2.0 nm.

The surface-contact potential difference (*V*CPD) of the SLG on the VOLET substrate

*W*SLG = *W*HOPG + [*V*CPD(HOPG)– *V*CPD(SLG)], (1)

was also monitored using a simultaneous Kelvin prove force microscope (KPFM) by applying AC voltage (1 V) with a frequency of 18 kHz to a Pt/Ir-coated silicon KPFM cantilever. In order to calibrate the work function of the sample, *V*CPD of highly oriented pyrolytic graphite (HOPG) was used as a reference *V*CPD. The work function of the SLG (*W*SLG) was obtained by a comparison of *V*CPDs for the SLG and the HOPG,

where *W*HOPG is the work function of the HOPG (~4.6 eV) [47]. **Figure 3(c)** shows the distributions of the work functions of the SLG as measured by the

KPFM. The estimated average work function of the FeCl3-doped SLG on the VOLET

(~92%) of a conventional ITO-coated glass substrate for OLEDs.

(chlorination) [46] induces unintentional p-doping of the SLG.

Pa. The next step involved spin-coating a solution of poly-

**146**

*(a) Wide-scan XPS survey spectra of the studied SLG on a SiO2/Si substrate. AFM topographic image (5 μm × 5 μm) (b) and corresponding work function distribution (c) of the SLG on the VOLET substrate as measured by KPFM. (d) Transport characteristics of the SLG from liquid-gated lateral Gr-FETs at*  V*DS = −100 mV. The insets show the structure of the liquid-gated lateral Gr-FET with an Ag/AgCl reference electrode in a nonaqueous electrolyte containing ACN and 100 mM of TBAPF6.*

#### **Figure 4.**

*(a) Raman spectra of the SLG transferred from the Cu foil onto the VOLET substrate. (b) Polarized optical microscope image of a spin-coated layer of commercial nematic liquid crystals on the SLG transferred to the VOLET substrate.*


**Table 1.**

*Summary of the electronic properties of the FeCl3-doped SLG used.*

Next, the transport characteristics of the SLG used were observed by assessing a liquid-gated lateral FET with SLG channels, a Gr-FET, as shown in **Figure 3(d)**. The lateral FET substrate was prepared using the VOLET substrate or a heavily doped n-type Si wafer substrate (0.05-ohm cm) with a thermally grown SiO2 layer (300-nm-thick) as the gate dielectric for the OTFT, together with a laterally patterned metal source and drain electrodes consisting of a Cr layer (5.5-nm-thick) and a Au layer (50-nm-thick) formed on the substrate via a vacuum deposition process with a mask. The channel length (*L*) and width (*W*) of the FET were 50 μm and 1600 μm, respectively (see inset in **Figure 3(d)**). Regarding the transport characteristics of the SLG studied, a liquid-gated lateral Gr-FET was prepared using an FET substrate with acetonitrile (ACN) with 100 mM of tetrabutylammonium hexafluorophosphate (TBAPF6). The channel of the studied SLG of the Gr-FET was gated through the ACN electrolyte with an Ag/AgCl (3.5 M KCl) reference electrode by sweeping the gate voltages from −0.8 to 0 V and then to +0.8 V with a sweep rate of 30 mV s<sup>−</sup><sup>1</sup> at *V*DS = 100 mV. In general, the liquid-gate Gr-FET has better transfer characteristics than the conventional back-gate Gr-FET because the liquid gate exhibits higher capacitance than the back gate [49, 50].

For the SLG used here, the Gr-FET showed a clear asymmetrical V-shaped *I*DS-*V*<sup>G</sup> curve with a charge-neutral gate voltage (or Dirac point, *V*Dirac) of ~0.54 V/VAg/AgCl. This large positive value of *V*Dirac clearly indicates that the SLG used is p-type (hole) doped graphene due to the chlorination of graphene by FeCl3 [49]. According to the *V*Dirac value of the SLG, the energy level of the Dirac point *E*D, relative to the vacuum level, can be estimated with regard to the redox potential of a probe material of ferrocene via the following relationship [51]:

$$E\_{\rm D} = \left\lfloor -\left( \mathbf{e} \, V\_{\rm G,Dirac} - E\_{1/2} \{ \mathbf{Fc}/\mathbf{Fc} + \lambda \} \right) - 4.8 \right\rfloor \text{ eV.} \tag{2}$$

Here, 4.8 eV is the absolute energy level of the ferrocene and ferrocenium (Fc/Fc+) redox couple below the vacuum energy level, and *E*1/2(Fc/Fc+) = 0.45 eV [51]. From Eq. (2), the *V*Dirac value of ~0.54 V/VAg/AgCl for the SLG gives a Dirac point energy *E*D of approximately ~4.89 eV. Note that the *E*D value of 4.89 eV is higher than that (~4.49 eV) of epitaxial monolayer graphene [52], also confirming the p-type doping of the SLG. From the transfer characteristics, the carrier (hole) mobility *μ* of the SLG was also estimated using the relationship [53] of

$$\mu = \left\{ L / W C\_{\text{g}} V\_{\text{DS}} \right\} \left( \Delta I\_{\text{DS}} / \Delta V\_{\text{G}} \right), \tag{3}$$

**149**

less than 2.7 × 10<sup>−</sup><sup>4</sup>

electron microscopy (SEM).

*Vertical-Type Organic Light-Emitting Transistors with High Effective Aperture Ratios*

are very small disorder-induced D bands around ~1340–1350 cm<sup>−</sup><sup>1</sup>

scattering of phonons at the zone boundary [54, 55]. It can be observed that there

a comparison of these with other examples in an earlier report of the relationship between the G and 2D peak positions of graphenes [55], it was verified that the SLG

Subsequently, the densities of the defects, the distances between the defects, and the porosities of nano-defects for the SLG were estimated from the ratio of the Raman intensities of the G bands to the D bands, ID/IG, as shown in the Raman spectra above. The density of the defects (*n*D) and the distances between the defects (*L*D) for the SLG, as estimated from the carbon amorphization trajectory (ID/

studied here are nonporous high-quality SLGs with a negligible number of porous

Next, for this SLG, polarized optical microscopy was also carried out using SLG covered with commercial nematic liquid crystals (NLCs, Merck LC ZLI-2293) in a crossed polarization state [58]. As shown in **Figure 4(b)**, the polarized optical microscopic image of a spin-coated NLC layer on the SLG shows large graphene domains (with an average radius of the domains >100 μm) in the form of highly uniform optical retardation, in addition to small domains of several hundreds of nanometers in size [59, 60], clearly indicating that the SLG studied here is high-

**Figure 5** presents a schematic illustration of the structure used and the stages

an epoxy resin in a glove box. The photograph in **Figure 5** shows the microscopic morphology of the device cross section as observed by field emission scanning

Pa (Step 4). Finally, the fabricated device was encapsulated with

of the fabrication of the SLG-based VOLETs (Gr-VOLETs) with an ITO gate separated by an Al2O3 gate dielectric layer, a SLG source, organic channel layers, and an Al drain. The fabrication steps of the Gr-VOLET investigated are described below. To construct the Gr-VOLET, SLG (4 mm by 20 mm) was transferred onto a VOLET substrate, as mentioned above (Steps 1, 2). The source electrode used was FeCl3-doped SLG. Next, organic semiconducting materials were deposited over the SLG source electrode regions; a channel layer of poly(para-phenylene vinylene) copolymer (known as SY, 70-nm-thick) was coated as an emissive channel layer by spin coating (Step 3), after which a 2-nm-thick electron injection layer of CsF and a 80-nm-thick drain electrode of Al were deposited on the top of the SY channel layer

From the Raman peak intensities, it was found that the ratios of the integrated Raman intensities of the G band to the 2D band for the FeCl3-doped SLG were in the approximate range of 1.7–1.8, indicating that the SLGs studied here are high-quality monolayer graphene [55]. Moreover, from the peak positions, it was found that while the G and 2D peaks of the intrinsic undoped SLG are positioned

bonds due to the relatively few defects in the SLGs studied.

, respectively, the G and 2D peak positions of the

and ~2677 cm<sup>−</sup><sup>1</sup>

and *L*D ~ 32.8 nm, respectively, cor-

under a base pressure of

%. This result clearly indicates that the SLGs

, indicating the

. Through

*DOI: http://dx.doi.org/10.5772/intechopen.92833*

and ~2669 cm<sup>−</sup><sup>1</sup>

IG ~ 0.117) [56, 57], were *n*D ~ 3.0 × 1010 cm−<sup>2</sup>

defects introduced during the synthesis and transfer steps.

quality graphene with large-area graphene domains.

**3. VOLETs with a doped CVD graphene source**

in sequence via thermal deposition at a rate of 0.05 nm s<sup>−</sup><sup>1</sup>

**3.1 Fabrication of SLG-based VOLETs**

responding to a porosity of 9.4 × 10<sup>−</sup><sup>2</sup>

used here is p-type doped SLG.

SLG used are correspondingly upshifted to ~1585 cm<sup>−</sup><sup>1</sup>

sparse formation of sp3

at ~1579 cm<sup>−</sup><sup>1</sup>

where *C*g is the top-gate capacitance of graphene (~1.9 μF cm<sup>−</sup><sup>2</sup> ) [50]. The estimated hole mobility for the SLG was approximately ~410cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> .

For the SLG studied here, Raman spectroscopy was also carried out using a confocal Raman system with a laser source operating at 514.5 nm (~1 mW on sample surface). As shown in **Figure 4(a)**, the Raman spectra of the SLGs studied here have two strong characteristic peaks, a G band at around ~1580–1600 cm<sup>−</sup><sup>1</sup> , due to the E2g vibration of sp2 -bonded carbon atoms, and a 2D band at around ~2644–2665 cm<sup>−</sup><sup>1</sup> , which is a second-order type of vibrational mode caused by the

#### *Vertical-Type Organic Light-Emitting Transistors with High Effective Aperture Ratios DOI: http://dx.doi.org/10.5772/intechopen.92833*

scattering of phonons at the zone boundary [54, 55]. It can be observed that there are very small disorder-induced D bands around ~1340–1350 cm<sup>−</sup><sup>1</sup> , indicating the sparse formation of sp3 bonds due to the relatively few defects in the SLGs studied.

From the Raman peak intensities, it was found that the ratios of the integrated Raman intensities of the G band to the 2D band for the FeCl3-doped SLG were in the approximate range of 1.7–1.8, indicating that the SLGs studied here are high-quality monolayer graphene [55]. Moreover, from the peak positions, it was found that while the G and 2D peaks of the intrinsic undoped SLG are positioned at ~1579 cm<sup>−</sup><sup>1</sup> and ~2669 cm<sup>−</sup><sup>1</sup> , respectively, the G and 2D peak positions of the SLG used are correspondingly upshifted to ~1585 cm<sup>−</sup><sup>1</sup> and ~2677 cm<sup>−</sup><sup>1</sup> . Through a comparison of these with other examples in an earlier report of the relationship between the G and 2D peak positions of graphenes [55], it was verified that the SLG used here is p-type doped SLG.

Subsequently, the densities of the defects, the distances between the defects, and the porosities of nano-defects for the SLG were estimated from the ratio of the Raman intensities of the G bands to the D bands, ID/IG, as shown in the Raman spectra above. The density of the defects (*n*D) and the distances between the defects (*L*D) for the SLG, as estimated from the carbon amorphization trajectory (ID/ IG ~ 0.117) [56, 57], were *n*D ~ 3.0 × 1010 cm−<sup>2</sup> and *L*D ~ 32.8 nm, respectively, corresponding to a porosity of 9.4 × 10<sup>−</sup><sup>2</sup> %. This result clearly indicates that the SLGs studied here are nonporous high-quality SLGs with a negligible number of porous defects introduced during the synthesis and transfer steps.

Next, for this SLG, polarized optical microscopy was also carried out using SLG covered with commercial nematic liquid crystals (NLCs, Merck LC ZLI-2293) in a crossed polarization state [58]. As shown in **Figure 4(b)**, the polarized optical microscopic image of a spin-coated NLC layer on the SLG shows large graphene domains (with an average radius of the domains >100 μm) in the form of highly uniform optical retardation, in addition to small domains of several hundreds of nanometers in size [59, 60], clearly indicating that the SLG studied here is highquality graphene with large-area graphene domains.
