**3.2 Operating characteristics of Gr-VOLETs**

The operating characteristics of the Gr-VOLET were observed using a luminance meter in conjunction with two source meters. To operate the Gr-VOLETs, sourcedrain voltage *V*DS (= −*V*SD) on the Al drain and gate voltage, *V*GS (or *V*G), were applied with respect to the SLG source contact, held at the ground potential. During the operation of the Gr-VOLET, an electron injection occurs from the Al drain into the SY channel layer, and the hole injection from the SLG source to the SY channel layer can be modulated by controlling the gate voltage *V*GS (or *V*G), as discussed below.

**Figure 6** shows the EL light emissions of a Gr-VOLET operating under different *V*G values with a fixed *V*SD of 3.8 V. As shown in **Figure 6(b)**, the EL light emission is uniform and bright (in the on-state) and evenly dark (off-state) over the entire surface of the active area for negative and positive *V*G values, respectively. Hence, *V*G essentially influences the radiative recombination process in the emissive channel layers.

#### **Figure 6.**

*EL light emission from a Gr-VOLET for different gate voltages, VGs, for a given source-drain voltage, VSD, of 3.8 V in bright (a) and dark (b) conditions (active area: 4 mm × 2 mm, white squares).*

**151**

**Figure 7.**

*curves, OLED operations).*

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

The output current and luminance characteristics of the Gr-VOLET were investigated as described below. For comparative purposes, the diode characteristics of the Gr-VOLET were also observed with the gate electrodes isolated from the external circuits (Gr-OLED). As shown in **Figure 7**, the current density-voltage (*J*SD*-V*SD) characteristics and luminance-voltage (*L-V*SD) characteristics of the Gr-VOLET present the following key features: (i) the *J*SD*-V*SD characteristics are similar to the diode characteristics, as is generally observed in vertical organic FETs due to the short channel lengths [61]. (ii) Similar behaviors were observed in the *L-V*SD characteristics; (iii) for a given *V*SD, both *J*SD and *L* increase with a decrease in *V*G, even at low *V*SD values, indicating that current modulation by *V*G can change the EL emission brightness. Thus, the *V*G dependent turn-on voltage, *V*onset, can be reduced to well below *V*onset of the Gr-OLED, and (iv) both *J*SD and *L* also depend on the direction of change of *V*G, that is, upward or downward, implying hysteretic

Interestingly, as shown in **Figure 7**, at *V*G = −40 V, the Gr-VOLET with the doped

, respectively, at *V*G = ±40 V. Thus, the gate-bias-induced modulation effect of

Next, the device performance, *AR*eff, and *PPC*, of the Gr-VOLET were estimated in the on-state in comparison with a control OLED fabricated using an identical process on an ITO anode (ITO-OLED) (**Figure 8**). As shown in **Figure 8**, the Gr-VOLET exhibits EL luminance higher than that of the control OLED (ITO/SY/CsF/Al) in the *V*SD < 4.0 V region. Moreover, the Gr-VOLET was shown to be more efficient than the control OLED (**Figure 8(c)**). For instance, at an EL luminance level of

ITO-OLED. Thus, the *AR*eff value of the Gr-VOLET can be estimated to be 154% due to its full surface emission, just like the OLED. Thus, it is clear that the Gr-VOLET has a greatly enhanced *AR*eff compared to other reported devices (**Table 2**). This result offers another substantial advantage: given the level of *AR*eff, the brightness of the device can be maintained under a lower *J*SD, providing a longer device lifetime

*Gate-voltage (VG)-dependent current density-voltage (JSD*-*VSD) (a) and luminance-voltage (L-VSD) (b) characteristics of a Gr-VOLET with a FeCl3-doped SLG source for upward and downward changes in VG. For comparison, the characteristics of a Gr-OLED (i.e., gate-disconnected Gr-VOLET) are also shown (dotted* 

, which is approximately 154% of the *η*C result (4.64 cd A<sup>−</sup><sup>1</sup>

, the Gr-VOLET emitted EL light with a current efficiency, *η*C, of 7.13 cd

and

) for the control

SLG source exhibits high device performance, superior to that of the Gr-OLED, indicating an improved and balanced charge injection from the SLG source for negative *V*G values. Conversely, at *V*G = +40 V, *J*SD and *L* of the Gr-VOLET are much lower, due to the switching off effect of the hole injections from the SLG source. The highest values of the peak on/off ratios for *J*SD and *L* were ~102

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

Gr-VOLET is shown to be quite efficient.

behavior.

~104

500 cd m<sup>−</sup><sup>2</sup>

A<sup>−</sup><sup>1</sup>

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

The output current and luminance characteristics of the Gr-VOLET were investigated as described below. For comparative purposes, the diode characteristics of the Gr-VOLET were also observed with the gate electrodes isolated from the external circuits (Gr-OLED). As shown in **Figure 7**, the current density-voltage (*J*SD*-V*SD) characteristics and luminance-voltage (*L-V*SD) characteristics of the Gr-VOLET present the following key features: (i) the *J*SD*-V*SD characteristics are similar to the diode characteristics, as is generally observed in vertical organic FETs due to the short channel lengths [61]. (ii) Similar behaviors were observed in the *L-V*SD characteristics; (iii) for a given *V*SD, both *J*SD and *L* increase with a decrease in *V*G, even at low *V*SD values, indicating that current modulation by *V*G can change the EL emission brightness. Thus, the *V*G dependent turn-on voltage, *V*onset, can be reduced to well below *V*onset of the Gr-OLED, and (iv) both *J*SD and *L* also depend on the direction of change of *V*G, that is, upward or downward, implying hysteretic behavior.

Interestingly, as shown in **Figure 7**, at *V*G = −40 V, the Gr-VOLET with the doped SLG source exhibits high device performance, superior to that of the Gr-OLED, indicating an improved and balanced charge injection from the SLG source for negative *V*G values. Conversely, at *V*G = +40 V, *J*SD and *L* of the Gr-VOLET are much lower, due to the switching off effect of the hole injections from the SLG source. The highest values of the peak on/off ratios for *J*SD and *L* were ~102 and ~104 , respectively, at *V*G = ±40 V. Thus, the gate-bias-induced modulation effect of Gr-VOLET is shown to be quite efficient.

Next, the device performance, *AR*eff, and *PPC*, of the Gr-VOLET were estimated in the on-state in comparison with a control OLED fabricated using an identical process on an ITO anode (ITO-OLED) (**Figure 8**). As shown in **Figure 8**, the Gr-VOLET exhibits EL luminance higher than that of the control OLED (ITO/SY/CsF/Al) in the *V*SD < 4.0 V region. Moreover, the Gr-VOLET was shown to be more efficient than the control OLED (**Figure 8(c)**). For instance, at an EL luminance level of 500 cd m<sup>−</sup><sup>2</sup> , the Gr-VOLET emitted EL light with a current efficiency, *η*C, of 7.13 cd A<sup>−</sup><sup>1</sup> , which is approximately 154% of the *η*C result (4.64 cd A<sup>−</sup><sup>1</sup> ) for the control ITO-OLED. Thus, the *AR*eff value of the Gr-VOLET can be estimated to be 154% due to its full surface emission, just like the OLED. Thus, it is clear that the Gr-VOLET has a greatly enhanced *AR*eff compared to other reported devices (**Table 2**). This result offers another substantial advantage: given the level of *AR*eff, the brightness of the device can be maintained under a lower *J*SD, providing a longer device lifetime

#### **Figure 7.**

*Liquid Crystals and Display Technology*

**3.2 Operating characteristics of Gr-VOLETs**

*dielectric, a SLG source, organic channel layers, and an Al drain.*

The operating characteristics of the Gr-VOLET were observed using a luminance meter in conjunction with two source meters. To operate the Gr-VOLETs, sourcedrain voltage *V*DS (= −*V*SD) on the Al drain and gate voltage, *V*GS (or *V*G), were applied with respect to the SLG source contact, held at the ground potential. During the operation of the Gr-VOLET, an electron injection occurs from the Al drain into the SY channel layer, and the hole injection from the SLG source to the SY channel layer can be modulated by controlling the gate voltage *V*GS (or *V*G), as discussed

*Schematic illustration of the fabrication steps of a Gr-VOLET and a cross-sectional scanning electron microscopy (SEM) image of the Gr-VOLET with stacked layers of an ITO gate separated with an Al2O3 gate* 

**Figure 6** shows the EL light emissions of a Gr-VOLET operating under different *V*G values with a fixed *V*SD of 3.8 V. As shown in **Figure 6(b)**, the EL light emission is uniform and bright (in the on-state) and evenly dark (off-state) over the entire surface of the active area for negative and positive *V*G values, respectively. Hence, *V*G essentially influences the radiative recombination process in the

*EL light emission from a Gr-VOLET for different gate voltages, VGs, for a given source-drain voltage, VSD, of* 

*3.8 V in bright (a) and dark (b) conditions (active area: 4 mm × 2 mm, white squares).*

**150**

**Figure 6.**

below.

**Figure 5.**

emissive channel layers.

*Gate-voltage (VG)-dependent current density-voltage (JSD*-*VSD) (a) and luminance-voltage (L-VSD) (b) characteristics of a Gr-VOLET with a FeCl3-doped SLG source for upward and downward changes in VG. For comparison, the characteristics of a Gr-OLED (i.e., gate-disconnected Gr-VOLET) are also shown (dotted curves, OLED operations).*

#### **Figure 8.**

*J-V (a), L-V (b), and ηC-L (c) comparisons of the Gr-VOLET in the bright on-state (VG = −40 V) with its ITO-based control OLED (ITO-OLED). Note that ITO-OLED = (ITO/SY/CsF/Al).*


#### **Table 2.**

*Comparison of the effective aperture ratio, AReff, and parasitic power consumption, PPC, for various devices at a luminance level of 500 cd m<sup>−</sup><sup>2</sup> .*

[62]. It is noteworthy that *η*C (7.13 cd A<sup>−</sup><sup>1</sup> ) for the Gr-VOLET was approximately 1.4 times higher than *η*C (5.17 cd A<sup>−</sup><sup>1</sup> ) of the ITO-OLED, possessing the optimized HIL of PEDOT:PSS. Thus, it is clear that the SLG source in the Gr-VOLET provides amplification of the emission and current efficiency, although further optimization of the electrodes is still possible.

Next, the *PPC* of the Gr-VOLET was deduced, which achieved luminance of 500 cd m<sup>−</sup><sup>2</sup> at *V*SD = 3.82 V with an *AR*eff rate of 154%, as discussed above. For the control ITO-OLED to emit a luminous flux through an aperture while transmitting 154% of its light, thereby matching the Gr-VOLET, it must emit 324 cd m<sup>−</sup><sup>2</sup> , requiring an applied voltage of 3.62 V. This indicates that 0.2 V of the *V*SD (3.82 V) for the Gr-VOLET was dropped through its embedded transistor element, leading to a considerably reduced *PPC* of only 5.2%. This is much lower than that (6.2%) of the previous CNT-VOLET and the levels (>50%) for a TFT-OLED and a MIS-OLET (**Table 2**) [19, 32].

#### **3.3 Charge injection process at SLG sources**

At this point, our investigation turns to the hole injection mechanism at the interface between the SY channel layer and the SLG source. To be injected across the interface (SLG/SY), the holes must overcome the barrier height at the interface either via thermionic emission or tunneling processes [63–68]. **Figure 9(a)** shows examples of Fowler-Nordheim (F-N) curves [63–67] for the Gr-VOLET at various *V*Gs during upward changes in *V*G. All of the curves show two different hole injection processes with transition voltages (*V*Ts), at which the injection mechanism changes to tunneling from Schottky thermionic emission [63–68]. In the figure, it is interesting to note that *V*G affects both the Schottky thermionic emission and tunneling, indicating that *V*T strongly depends on *V*G. It is also noteworthy that because EL emission of the Gr-VOLETs occurs when *V*SD > *V*onset (*V*onset > *V*T), the main

**153**

injection [69, 70].

**Figure 9.**

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

hole injection process for EL emission is the tunneling injection in the Gr-VOLETs. According to the modified tunneling current model [70], the tunneling current density (*J*) of a single charge carrier through a triangular barrier at a polymer/metal junction is related to the potential barrier height (*Φ*) and the temperature (*T*),

*(a) Fowler-Nordheim plots of the Gr-VOLET with the SLG source with various VG values for upward VG changes. (b) Gate-bias-modulated hole tunneling barrier height, Φ, extracted from the curve fittings in the hole-dominant regimes. The inset shows a schematic energy band diagram for the thermionic emission and tunneling at the interface between the SLG and the SY layer along the normal direction of the interface. Φ*

*denotes the interface potential barrier height between the SLG and the SY channel layer.*

where *k*B is the Boltzmann constant and *P*i are parameters related to *Φ*. This relationship allows the F-N plots to be analyzed, and the potential barrier heights *Φ* at the SLG/SY interface between the Fermi level of SLG and the highest occupied molecular orbital (HOMO) level of the SY channel layer (~5.3 eV) [22] to be obtained (**Figure 9(b)**). As shown in the figure, the SLG/SY interface exhibited strong gate-bias-induced *Φ* modulation (*ΔΦ*) along the direction normal to the SLG/SY interface; that is, *ΔΦ* at *V*G = ±40 V was approximately 0.11 eV, leading to the effective modulation of the device performance of the Gr-VOLET tested here. Note that tunneling at the SLG/SY interface is the major process of hole injection, being responsible for the radiative recombinations of electron-hole pairs. Thus, the analysis of the tunneling process described above provides clear evidence that the Gr-VOLET operates based on vertical barrier height modulation along the direction normal to the SLG source surface (i.e., parallel to the direction of gate field), in contrast to the CNT-VOLET devices based on lateral modulation of Schottky barrier height along the horizontal direction on the CNT source surface (i.e., perpendicular to the direction of gate field) [31, 32], and differently from conventional graphene-based barristors that operate via modulation of the Schottky thermionic

The observations above show the working principle of the Gr-VOLET, as illustrated in the energy-level diagrams in **Figure 10**. At a given *V*SD, a positive gate voltage induces an upward shift of the SLG Fermi level in a direction that increases the *Φ*, resulting in reduced hole tunneling injections into the HOMO level of the channel layer (SY). In contrast, a negative gate voltage induces a downward shift of the SLG Fermi level, decreasing *Φ* significantly (enhancing tunneling) and hence allowing increased hole injections and improved EL performance. Thus, together with the band-bending effect [31], the main operating mechanism of the Gr-VOLET

) = − *P*1/*V* + ln (*P*2/*V*) − ln [sin (*P*3/*V*)], withΦ = (2/3) π *k*B*T* (*P*1/ *P*3), (4)

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

ln (*J*/ *V*<sup>2</sup>

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

**Figure 9.**

*Liquid Crystals and Display Technology*

[62]. It is noteworthy that *η*C (7.13 cd A<sup>−</sup><sup>1</sup>

*Reference devices used the green phosphorescent emitter Ir(ppy)3.*

*.*

**3.3 Charge injection process at SLG sources**

1.4 times higher than *η*C (5.17 cd A<sup>−</sup><sup>1</sup>

of the electrodes is still possible.

*at a luminance level of 500 cd m<sup>−</sup><sup>2</sup>*

500 cd m<sup>−</sup><sup>2</sup>

*a*

**Table 2.**

**Figure 8.**

(**Table 2**) [19, 32].

) for the Gr-VOLET was approximately

,

) of the ITO-OLED, possessing the optimized

HIL of PEDOT:PSS. Thus, it is clear that the SLG source in the Gr-VOLET provides amplification of the emission and current efficiency, although further optimization

*Comparison of the effective aperture ratio, AReff, and parasitic power consumption, PPC, for various devices* 

**Devices Reference Source type** *AR***eff [%]** *PPC* **[%]** TFT + OLED [15] 53 MIS-OLET [19] 51 CNT-VOLETa [32] Porous CNT networks 98 6.2 Gr-VOLET This work [49] FeCl3-doped SLG 154 5.2

*J-V (a), L-V (b), and ηC-L (c) comparisons of the Gr-VOLET in the bright on-state (VG = −40 V) with its* 

*ITO-based control OLED (ITO-OLED). Note that ITO-OLED = (ITO/SY/CsF/Al).*

Next, the *PPC* of the Gr-VOLET was deduced, which achieved luminance of

requiring an applied voltage of 3.62 V. This indicates that 0.2 V of the *V*SD (3.82 V) for the Gr-VOLET was dropped through its embedded transistor element, leading to a considerably reduced *PPC* of only 5.2%. This is much lower than that (6.2%) of the previous CNT-VOLET and the levels (>50%) for a TFT-OLED and a MIS-OLET

At this point, our investigation turns to the hole injection mechanism at the interface between the SY channel layer and the SLG source. To be injected across the interface (SLG/SY), the holes must overcome the barrier height at the interface either via thermionic emission or tunneling processes [63–68]. **Figure 9(a)** shows examples of Fowler-Nordheim (F-N) curves [63–67] for the Gr-VOLET at various *V*Gs during upward changes in *V*G. All of the curves show two different hole injection processes with transition voltages (*V*Ts), at which the injection mechanism changes to tunneling from Schottky thermionic emission [63–68]. In the figure, it is interesting to note that *V*G affects both the Schottky thermionic emission and tunneling, indicating that *V*T strongly depends on *V*G. It is also noteworthy that because EL emission of the Gr-VOLETs occurs when *V*SD > *V*onset (*V*onset > *V*T), the main

control ITO-OLED to emit a luminous flux through an aperture while transmitting 154% of its light, thereby matching the Gr-VOLET, it must emit 324 cd m<sup>−</sup><sup>2</sup>

at *V*SD = 3.82 V with an *AR*eff rate of 154%, as discussed above. For the

**152**

*(a) Fowler-Nordheim plots of the Gr-VOLET with the SLG source with various VG values for upward VG changes. (b) Gate-bias-modulated hole tunneling barrier height, Φ, extracted from the curve fittings in the hole-dominant regimes. The inset shows a schematic energy band diagram for the thermionic emission and tunneling at the interface between the SLG and the SY layer along the normal direction of the interface. Φ denotes the interface potential barrier height between the SLG and the SY channel layer.*

hole injection process for EL emission is the tunneling injection in the Gr-VOLETs. According to the modified tunneling current model [70], the tunneling current density (*J*) of a single charge carrier through a triangular barrier at a polymer/metal junction is related to the potential barrier height (*Φ*) and the temperature (*T*),

$$\begin{array}{l} \ln\left(f/V^{2}\right) = -P\_{1}/V + \ln\left(P\_{2}/V\right) - \ln\left[\sin\left(P\_{3}/V\right)\right],\\ \text{with} \Phi = \text{(2/3)} \text{ } \pi \, k\_{\text{B}}T \text{ (}P\_{1}/P\_{3}\text{)},\end{array} \tag{4}$$

where *k*B is the Boltzmann constant and *P*i are parameters related to *Φ*. This relationship allows the F-N plots to be analyzed, and the potential barrier heights *Φ* at the SLG/SY interface between the Fermi level of SLG and the highest occupied molecular orbital (HOMO) level of the SY channel layer (~5.3 eV) [22] to be obtained (**Figure 9(b)**). As shown in the figure, the SLG/SY interface exhibited strong gate-bias-induced *Φ* modulation (*ΔΦ*) along the direction normal to the SLG/SY interface; that is, *ΔΦ* at *V*G = ±40 V was approximately 0.11 eV, leading to the effective modulation of the device performance of the Gr-VOLET tested here. Note that tunneling at the SLG/SY interface is the major process of hole injection, being responsible for the radiative recombinations of electron-hole pairs. Thus, the analysis of the tunneling process described above provides clear evidence that the Gr-VOLET operates based on vertical barrier height modulation along the direction normal to the SLG source surface (i.e., parallel to the direction of gate field), in contrast to the CNT-VOLET devices based on lateral modulation of Schottky barrier height along the horizontal direction on the CNT source surface (i.e., perpendicular to the direction of gate field) [31, 32], and differently from conventional graphene-based barristors that operate via modulation of the Schottky thermionic injection [69, 70].

The observations above show the working principle of the Gr-VOLET, as illustrated in the energy-level diagrams in **Figure 10**. At a given *V*SD, a positive gate voltage induces an upward shift of the SLG Fermi level in a direction that increases the *Φ*, resulting in reduced hole tunneling injections into the HOMO level of the channel layer (SY). In contrast, a negative gate voltage induces a downward shift of the SLG Fermi level, decreasing *Φ* significantly (enhancing tunneling) and hence allowing increased hole injections and improved EL performance. Thus, together with the band-bending effect [31], the main operating mechanism of the Gr-VOLET

**Figure 10.**

*Energy-level diagrams of the Gr-VOLET for high (a) and low Φs (b) at two distinct values of VG and a given VSD. Φ depicts the tunneling barrier height for the hole injection. EF: Fermi energy level of the SLG source used.*

is energy band matching, with charge balance achieved even without a HIL through gate voltage-induced modulation of the hole carrier tunneling injection at the p-type doped SLG source with FeCl3.

In addition, notable instances of hysteresis were clearly observed, as shown above. Thus, bistable-like switching operations of a Gr-VOLET can allow novel applications for simple and inexpensive driving schemes together with low power consumption. However, this hysteresis effect may become an issue when attempting to realize high-quality grayscale outcomes and should be carefully, therefore, controlled when preparing the dielectric layer.

#### **3.4 Inkjet-printing arrays of Gr-VOLET micro-pixels**

Next, we turn our attention to a micro-pixel fabrication process for the Gr-VOLET using the inkjet-printing technique, as commonly used in solutionprocessable OLEDs [39, 71, 72]. Here, the inkjet technique used is based on the deposition of a small solvent drop onto an insulator layer, which can be easily redissolved and preferentially redeposited at the edge of the sessile drop (the contact line of the solvent drop), resulting in the formation of a via-hole with the shape of a crater, that is inkjet-etching [39].

To investigate the in situ formation of micro Gr-VOLET pixels created by means of inkjet-etching, an insulating polymer of poly(4-vinylpyridine) (P4VP) was introduced as a via-forming material, as P4VP is a hydrophilic polymer that dissolves in dimethyl formamide (DMF), toluene, chloroform, in lower alcohols, and in aqueous mineralic acids [71]. To fabricate a via-hole forming layer, a solution of P4VP with isopropanol (IPA) was spin-coated on top of the light-emitting channel layer of SY pre-coated onto a Gr-VOLET substrate (VOLET substrate/SLG/ SY/P4VP). For micro-patterning, an etching solvent of chloroform for P4VP was inkjet-printed on top of the SY/P4VP layers (**Figure 11(a)**). This inkjet-printed solvent drop of chloroform can dissolve the P4VP layer, and the capillary flow of the solvent pushes the dissolved P4VP from the center to the contact line of droplet due to the coffee ring effect [39, 72–74], resulting the formation of the via-hole through the P4VP layer. Thus, after the deposition of even a single solvent droplet (~150 pL per droplet) on a 30-nm-thick P4VP film, the P4VP polymers are removed from the printed position and completely etched, forming via-holes through the P4VP layer, of which the inner and outer diameters are ~90 μm and ~120 μm, respectively, and finally uncovering the surface of the underlying SY layer. These P4VP via-holes on the light-emitting SY layer act as micro-patterned pixel openings

**155**

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

for the light-emitting active areas of the Gr-VOLETs. Then, to complete the device of an array of micro Gr-VOLETs, the CsF/Al/Ag cathode is deposited following the

*) vs. 1/VSD (left) and ln(L/VSD2*

*) vs. 1/VSD (right) for the* 

*(a) Left: A photographic image of a single drop of solvent ejected from an inkjet nozzle for drop formation. Right: Light emission from inkjet-printed Gr-VOLET pixels (white squares) for two different gate voltages VGs, at a fixed VSD = 4.0 V. (b) Gate-voltage (VG)-dependent current density-voltage (JSD*-*VSD) (left) and luminance-voltage (L-VSD) (right) characteristics of the inkjet-printed Gr-VOLET pixels for various upward* 

**Figure 11(a)** also presents the switching behavior of EL light emissions from the array of micro Gr-VOLET pixels for two different gate voltages, *V*Gs, at a fixed *V*SD of 4.0 V. The photographic images show that only the isolated micro-patterned micro-pixel areas in contact with the SY layer of the Gr-VOLET pixels can emit EL light with a width of 85 μm, similar to the dimensions of the via-hole opening (90 μm). Note that the different solubility characteristics between the P4VP and SY polymers prevents any solvent erosion of the SY layer, as expected. As also clearly shown in the figure, the EL light emission from the micro Gr-VOLET pixels with a fixed droplet spacing of ~140 μm (~180 dpi) is highly bright (on-state) and dark (off-state) over the entire active area of the pixels for negative (−40 V) and positive values of *V*G (+40 V), respectively. Hence, *V*G clearly controls the radiative recombi-

Next, the output characteristics of the inkjet-printed Gr-VOLET pixels were investigated. As shown in **Figure 11(b)**, the *J*SD*-V*SD and *L-V*SD characteristics of the inkjet-printed Gr-VOLET pixels present features similar to those of the spin-coated Gr-VOLET (**Figure 7**). For example, at *V*G = −40 V, the luminance

(*V*onset = 3.0 V). In contrast, at *V*G = +40 V, *J*SD and *L* are greatly suppressed due to the switching off effect of the injection of holes from the SLG source, result-

presents F-N plots of the micro Gr-VOLET pixels for various *V*Gs. As also shown

with a current efficiency of ~5.0 cd/A at *V*SD = 6.0 V

at *V*G = ±40 V. **Figure 11(c)**

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

procedure described in Section 3.1.

*VG changes. (c) Fowler-Nordheim plots, ln(JSD/VSD2*

*inkjet-printed Gr-VOLET pixels at various VG values.*

**Figure 11.**

nation process in the emissive pixel areas.

ing in a peak on/off ratio of *L* of approximately 103

reaches *L* ~ 1200 cd m<sup>−</sup><sup>2</sup>

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

#### **Figure 11.**

*Liquid Crystals and Display Technology*

p-type doped SLG source with FeCl3.

**Figure 10.**

controlled when preparing the dielectric layer.

shape of a crater, that is inkjet-etching [39].

**3.4 Inkjet-printing arrays of Gr-VOLET micro-pixels**

is energy band matching, with charge balance achieved even without a HIL through gate voltage-induced modulation of the hole carrier tunneling injection at the

*Energy-level diagrams of the Gr-VOLET for high (a) and low Φs (b) at two distinct values of VG and a given VSD. Φ depicts the tunneling barrier height for the hole injection. EF: Fermi energy level of the SLG source used.*

In addition, notable instances of hysteresis were clearly observed, as shown above. Thus, bistable-like switching operations of a Gr-VOLET can allow novel applications for simple and inexpensive driving schemes together with low power consumption. However, this hysteresis effect may become an issue when attempting to realize high-quality grayscale outcomes and should be carefully, therefore,

Next, we turn our attention to a micro-pixel fabrication process for the Gr-VOLET using the inkjet-printing technique, as commonly used in solutionprocessable OLEDs [39, 71, 72]. Here, the inkjet technique used is based on the deposition of a small solvent drop onto an insulator layer, which can be easily redissolved and preferentially redeposited at the edge of the sessile drop (the contact line of the solvent drop), resulting in the formation of a via-hole with the

of inkjet-etching, an insulating polymer of poly(4-vinylpyridine) (P4VP) was introduced as a via-forming material, as P4VP is a hydrophilic polymer that dissolves in dimethyl formamide (DMF), toluene, chloroform, in lower alcohols, and in aqueous mineralic acids [71]. To fabricate a via-hole forming layer, a solution of P4VP with isopropanol (IPA) was spin-coated on top of the light-emitting channel layer of SY pre-coated onto a Gr-VOLET substrate (VOLET substrate/SLG/ SY/P4VP). For micro-patterning, an etching solvent of chloroform for P4VP was inkjet-printed on top of the SY/P4VP layers (**Figure 11(a)**). This inkjet-printed solvent drop of chloroform can dissolve the P4VP layer, and the capillary flow of the solvent pushes the dissolved P4VP from the center to the contact line of droplet due to the coffee ring effect [39, 72–74], resulting the formation of the via-hole through the P4VP layer. Thus, after the deposition of even a single solvent droplet (~150 pL per droplet) on a 30-nm-thick P4VP film, the P4VP polymers are removed from the printed position and completely etched, forming via-holes through the P4VP layer, of which the inner and outer diameters are ~90 μm and ~120 μm, respectively, and finally uncovering the surface of the underlying SY layer. These P4VP via-holes on the light-emitting SY layer act as micro-patterned pixel openings

To investigate the in situ formation of micro Gr-VOLET pixels created by means

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*(a) Left: A photographic image of a single drop of solvent ejected from an inkjet nozzle for drop formation. Right: Light emission from inkjet-printed Gr-VOLET pixels (white squares) for two different gate voltages VGs, at a fixed VSD = 4.0 V. (b) Gate-voltage (VG)-dependent current density-voltage (JSD*-*VSD) (left) and luminance-voltage (L-VSD) (right) characteristics of the inkjet-printed Gr-VOLET pixels for various upward VG changes. (c) Fowler-Nordheim plots, ln(JSD/VSD<sup>2</sup> ) vs. 1/VSD (left) and ln(L/VSD2 ) vs. 1/VSD (right) for the inkjet-printed Gr-VOLET pixels at various VG values.*

for the light-emitting active areas of the Gr-VOLETs. Then, to complete the device of an array of micro Gr-VOLETs, the CsF/Al/Ag cathode is deposited following the procedure described in Section 3.1.

**Figure 11(a)** also presents the switching behavior of EL light emissions from the array of micro Gr-VOLET pixels for two different gate voltages, *V*Gs, at a fixed *V*SD of 4.0 V. The photographic images show that only the isolated micro-patterned micro-pixel areas in contact with the SY layer of the Gr-VOLET pixels can emit EL light with a width of 85 μm, similar to the dimensions of the via-hole opening (90 μm). Note that the different solubility characteristics between the P4VP and SY polymers prevents any solvent erosion of the SY layer, as expected. As also clearly shown in the figure, the EL light emission from the micro Gr-VOLET pixels with a fixed droplet spacing of ~140 μm (~180 dpi) is highly bright (on-state) and dark (off-state) over the entire active area of the pixels for negative (−40 V) and positive values of *V*G (+40 V), respectively. Hence, *V*G clearly controls the radiative recombination process in the emissive pixel areas.

Next, the output characteristics of the inkjet-printed Gr-VOLET pixels were investigated. As shown in **Figure 11(b)**, the *J*SD*-V*SD and *L-V*SD characteristics of the inkjet-printed Gr-VOLET pixels present features similar to those of the spin-coated Gr-VOLET (**Figure 7**). For example, at *V*G = −40 V, the luminance reaches *L* ~ 1200 cd m<sup>−</sup><sup>2</sup> with a current efficiency of ~5.0 cd/A at *V*SD = 6.0 V (*V*onset = 3.0 V). In contrast, at *V*G = +40 V, *J*SD and *L* are greatly suppressed due to the switching off effect of the injection of holes from the SLG source, resulting in a peak on/off ratio of *L* of approximately 103 at *V*G = ±40 V. **Figure 11(c)** presents F-N plots of the micro Gr-VOLET pixels for various *V*Gs. As also shown in the figure, the negative slopes of the F-N plots clearly confirm that tunneling injection process is the major charge injection process for the light emission, being responsible for radiative recombinations of electron–hole pairs in the inkjet-printed Gr-VOLET pixels.
