**4.6. A FEOLED with strip electron multiplier**

As shown in Fig. 10, the typical structure of FEOLEDs in this work comprises a CNTs template cathode and the strip electron multiplier formed on an OLED as a part of anode. It can be assembled in a vacuum chamber.

#### *4.6.1. Experimental*

32 Organic Light Emitting Devices

evenly in the EML.

*4.5.1. Original type FEOLEDS* 

emit light through the ITO substrate..

*4.5.3. Luminescence mechanisms in FEOLEDs* 

operate on similar mechanism of emission.

As shown in Fig. 7 (a), FEOLEDs (original type) are similar to FEDs, but they use organic light emitting material instead of a phosphor. In these FEOLEDs an ITO film is used as anode and CNTs template as cathode. By applying the driving voltage to both the electrodes (ITO and CNTs template), then electrons and holes move toward the interface between the two transport layers (HTL, and ETL) and recombine to form excitons. Finally, these excitons

As shown in Fig, 7 (b), FEOLEDs with a dynode structure are classified as triode devices. It comprises the dynode and an organic EL light emitting layer and CNTs template. The

As shown in Fig. 8 (a), there is another kind of triode FEOLED, which is a FEOLED with a strip electron multiplier. In this case a strip electron multiplier is used instead of a dynode, but it is attached directly to the organic EL light emitting layer for protecting the electron injection layer from high-energy electron bombardment and allows the electrons to disperse

To further confirm that the mechanism of operation of FEOLEDs is the same as that of OLEDs, the following experiment was conducted. A hole blocking layer (BCP) is inserted between the hole transport layer (NPB) and the emission layer (Alq3) of the organic formation, as shown in Fig. 9(a). If an OLED is applied a voltage, hole carriers injected from the ITO anode electrode to the hole transport layer would be blocked at the interface of the NPB layer and the BCP layer. The electrons (emitted from the cathode and passing through Alq3) would then recombine with the holes accumulated in the NPB layer. Refer to Fig. 3, this NPB excitation generated by the recombination, according to the active energy levels of OLEDs would give rise to blue light. The Alq3 layer in such case would generate no light. If a cathode luminescence mechanism device is applied with the BCP layer, on the other hand, electron bombardment on the organic material would generate light in the emission layer Alq3, which should have green color. As such, when a BCP layer is inserted in FEOLEDs, if blue light is observed, the luminescent mechanism of the FEOLED must be similar to that of the conventional OLED: if green light is observed, the luminescent mechanism must be similar to that of cathode luminescence. The experiment results showed that blue light was observed, as shown in Fig. 9(b), which clearly illustrates that both FEOLEDs and OLEDs

Thus, the light emission in FEOLEDs also occurs via the following five processes as in OLEDs: 1) both electrons and holes injected from anode and cathode into organic layers, 2) these injected charge carriers are transported towards each other across the organic layer, 3) formation of singlet excitons due to the Coulomb interaction between the injected electrons

*4.5.2. Triode FEOLEDs with a dynode structure (strip electron multiplier)* 

dynode is formed between the cathode and the organic EL lighting layers.

In this section, the current density (J)-applied voltage (V) characteristics and the optical performances of a FEOLED with the strip electron multiplier are studied experimentally.

Fig.10 shows the configuration of FEOLED in this work, where the upper portion is an OLED lower part is the external electron source of CNTs cathode. The Soled switch is use to control OLED and Sext switch is used to control the external electron source of CNTs template.

**Figure 10.** Schematic presentation of an apparatus for characterizing a FEOLED with strip electron multiplier

The structure of an OLED is ITO glass/ m-MTDATA:V**2**O**5**(10 nm, 10 %)/ NPB (30 nm)/ Alq3:C545T (30nm, 3 %)/ Alq3 (10 nm)/ Cs**2**CO**3**(1 nm)/ Al, where, the V**2**O**5** doped m-MTDATA was chosen as the hole transport layer (HTL) and it has a high conductance, intrinsically leading to the formation of many intrinsic carriers between ITO and the organic surface significantly enhancing the hole injection and transport [46]. Additionally, the emission layer (EML) of OLED uses Alq3 as host and C545T dye as green fluorescent material to trap the electrons to build up the space charge and decrease the free electron distribution in the host Alq3 [47], subsequently reducing the current density of the device. Moreover, Cs**2**CO**3** and LiF were chosen as electron injection buffer layer (EIL). And the strip electron multiplier was fabricated from the strip of Al (80 nm) and strip of CsI (110 nm) and integrated with the organic material of FEOLEDs. The entire organic material layer was prepared by using a high vacuum thermal evaporation system.

Field Emission Organic Light Emitting Diode 35

an EL efficiency of 10.2 cd/A at 10 V. This demonstrates very clearly that the OLEDs, in which Cs**2**CO**3** is used as an electron injection layer, show an excellent performance. It indicates that electrons are effectively injected from the cathode to the organic layer due to the lower electron injection barrier, which improves the charge carrier balance and

**Figure 11.** (a) Current density (J)-Applied voltage (V) at S**OLED** closes and S**ext** is open and (b)

**Figure 12.** The field emission current versus electric field (J-E) characteristics of a CNTs template when

Fig. 12 shows the field emission current versus electric field (J–E) characteristics of a CNTs template. The CNTs template is made as an external electron source for FEOLED [51]. A field emission current density of approximately 127mA/cm**2** is produced at an electric field of 1.86 V/μm. The enhanced current density can be attributed to the satisfactory adhesion

Luminance (L)-Applied voltage (V ) at S**oled**=closes and S**ext** is open(Fig.10)

Soled is open and Sext is closed, (Eext=Vext/d) (Fig. 10)

subsequently increases the device efficiency.

Such a structure (Fig.10) is used to examine the current-voltage (J-V) characteristics of OLEDs with a different electron injection layer to prove the electron injection capability of Cs**2**CO**3**. Additionally, the J-V characteristics and luminance of the FEOLED are measured by Keithly-2400, Keithly-237 and TOPCON PR-650, respectively. All the measurements are performed in a high vacuum ambient of 6×10**-6** torr at room temperature.

### *4.6.2. Results and discussion*

In this section, the current density (J)-applied voltage (V) characteristics and the optical performances of a FEOLED with Electron Multiplier are studied experimentally.

Fig.11 (a) shows the J-V characteristics of two OLEDs, one with LiF and the other with CS**2**CO**3** as the electron injection layer when S**oled** is closed and S**ext** is open ( Fig10). To enhance the OLED performance, the CS**2**CO**<sup>3</sup>** is used instead of LiF as the electron injection material. The optimal thickness of CS**2**CO**3** has been characterized and found to be 1 nm. Under the same driving voltage of 10 V, the OLED with the electron injection layer of CS**2**CO**3** (1.0 nm) can get the current density of 93 mA/cm**2** which was higher than that of OLED with the electron injection layer of LiF (0.7 nm). The better performance of OLEDs with Cs**2**CO**3** can be attributed to Cs having a low-work function of 2.14 eV relative to Li (2.9 eV) [50]. The electron injection layer of Cs**2**CO**3** in OLEDs seems to have induced strong ndoping effects in Alq3 and ultimately increases the electron concentrations in the electronstransport layer of Alq3. Moreover, OLED with an increased thickness of Cs**2**CO**3** to 2 nm have shift the J-V curve to a lower current density. Notably, the OLEDs performance depends on the thickness of the Cs**2**CO**3** layer.

Fig.11 (b) shows the luminance (L)-applied voltage (V) characteristics of the same two OLEDs with LiF and CS**2**CO**3** as the electron injection layer when S**oled** is closed and S**ext** is open ( Fig10). According to Figure (b), the OLED with Cs**2**CO**3** (1 nm) as the electron injection layer can achieve a high luminance of 10,820 cd/m2 and a high EL efficiency of 12 cd/A at 10 V. In contrast, OLED with a LiF layer can achieve a luminance of 5,821 cd/m**<sup>2</sup>** and an EL efficiency of 10.2 cd/A at 10 V. This demonstrates very clearly that the OLEDs, in which Cs**2**CO**3** is used as an electron injection layer, show an excellent performance. It indicates that electrons are effectively injected from the cathode to the organic layer due to the lower electron injection barrier, which improves the charge carrier balance and subsequently increases the device efficiency.

34 Organic Light Emitting Devices

system.

*4.6.2. Results and discussion* 

depends on the thickness of the Cs**2**CO**3** layer.

The structure of an OLED is ITO glass/ m-MTDATA:V**2**O**5**(10 nm, 10 %)/ NPB (30 nm)/ Alq3:C545T (30nm, 3 %)/ Alq3 (10 nm)/ Cs**2**CO**3**(1 nm)/ Al, where, the V**2**O**5** doped m-MTDATA was chosen as the hole transport layer (HTL) and it has a high conductance, intrinsically leading to the formation of many intrinsic carriers between ITO and the organic surface significantly enhancing the hole injection and transport [46]. Additionally, the emission layer (EML) of OLED uses Alq3 as host and C545T dye as green fluorescent material to trap the electrons to build up the space charge and decrease the free electron distribution in the host Alq3 [47], subsequently reducing the current density of the device. Moreover, Cs**2**CO**3** and LiF were chosen as electron injection buffer layer (EIL). And the strip electron multiplier was fabricated from the strip of Al (80 nm) and strip of CsI (110 nm) and integrated with the organic material of FEOLEDs. The entire organic material layer was prepared by using a high vacuum thermal evaporation

Such a structure (Fig.10) is used to examine the current-voltage (J-V) characteristics of OLEDs with a different electron injection layer to prove the electron injection capability of Cs**2**CO**3**. Additionally, the J-V characteristics and luminance of the FEOLED are measured by Keithly-2400, Keithly-237 and TOPCON PR-650, respectively. All the measurements are

In this section, the current density (J)-applied voltage (V) characteristics and the optical

Fig.11 (a) shows the J-V characteristics of two OLEDs, one with LiF and the other with CS**2**CO**3** as the electron injection layer when S**oled** is closed and S**ext** is open ( Fig10). To enhance the OLED performance, the CS**2**CO**<sup>3</sup>** is used instead of LiF as the electron injection material. The optimal thickness of CS**2**CO**3** has been characterized and found to be 1 nm. Under the same driving voltage of 10 V, the OLED with the electron injection layer of CS**2**CO**3** (1.0 nm) can get the current density of 93 mA/cm**2** which was higher than that of OLED with the electron injection layer of LiF (0.7 nm). The better performance of OLEDs with Cs**2**CO**3** can be attributed to Cs having a low-work function of 2.14 eV relative to Li (2.9 eV) [50]. The electron injection layer of Cs**2**CO**3** in OLEDs seems to have induced strong ndoping effects in Alq3 and ultimately increases the electron concentrations in the electronstransport layer of Alq3. Moreover, OLED with an increased thickness of Cs**2**CO**3** to 2 nm have shift the J-V curve to a lower current density. Notably, the OLEDs performance

Fig.11 (b) shows the luminance (L)-applied voltage (V) characteristics of the same two OLEDs with LiF and CS**2**CO**3** as the electron injection layer when S**oled** is closed and S**ext** is open ( Fig10). According to Figure (b), the OLED with Cs**2**CO**3** (1 nm) as the electron injection layer can achieve a high luminance of 10,820 cd/m2 and a high EL efficiency of 12 cd/A at 10 V. In contrast, OLED with a LiF layer can achieve a luminance of 5,821 cd/m**<sup>2</sup>** and

performances of a FEOLED with Electron Multiplier are studied experimentally.

performed in a high vacuum ambient of 6×10**-6** torr at room temperature.

**Figure 11.** (a) Current density (J)-Applied voltage (V) at S**OLED** closes and S**ext** is open and (b) Luminance (L)-Applied voltage (V ) at S**oled**=closes and S**ext** is open(Fig.10)

**Figure 12.** The field emission current versus electric field (J-E) characteristics of a CNTs template when Soled is open and Sext is closed, (Eext=Vext/d) (Fig. 10)

Fig. 12 shows the field emission current versus electric field (J–E) characteristics of a CNTs template. The CNTs template is made as an external electron source for FEOLED [51]. A field emission current density of approximately 127mA/cm**2** is produced at an electric field of 1.86 V/μm. The enhanced current density can be attributed to the satisfactory adhesion

between CNTs and the ITO glass substrate. The current density increases with the electric field. Based on the above results, the amount of electrons injected into the Al electrode can be determined by adjusting the electrical field (Fig.10, V**ext**), which is applied to the CNTs template. Detailed operations of FEOLEDs can be described as follows. Initially, S**oled** is turned on to drive the OLED shown in Fig.10. The OLED emits a luminance of 10,820 cd/m**<sup>2</sup>** as the driving voltage reaches 10 V and, simultaneously, S**ext** is switched on to attract the electrons emitted from the CNT emitters.

Field Emission Organic Light Emitting Diode 37

**Figure 14.** The Luminous efficiency (η)- Current density (J)-Electrical field (E) of the OLED and

In a FEOLED, the electrical field under vacuum condition, accelerates the electrons emitted from the CNTs cathode to affect the secondary electron material of CsI; they then pass through the Al and transport are transported through the organic EL light emitting layer. Field emission electrons with a sufficiently large electron energy supplement into OLED to increase the current density. Notably, increasing the number of the electrons that reach the organic EL light emitting layer also increases the luminous efficiency of the OLED. Therefore, the ways in which the OLED and FEOLED differ can be easily observed under the same current density. The luminance of FEOLED exceeds that of conventional OLED, as shown in Fig.13. Our results further demonstrate that the curve of the FEOLED becomes gradually saturated, especially for section C. Notably, injecting external electrons into the OLED continuously does not allow the luminance of the FEOLED to increase linearly with the current density since the quantity of electrons is larger than in the hole in section C. The carrier has become imbalanced again, subsequently decreasing the luminance. Furthermore, the electronic behavior shown in the FEOLED, it can be further demonstrates the amount of

FEOLED devices at both Soled and Sext close (Eext=Vext/d)

electrons is less than holes.

Fig.13 shows the luminance-current density characteristics of OLEDs and FEOLEDs. The curve in section A displays the characteristics of a conventional OLED (S**0led**=close, S**ext**=open, as shown in Fig.10), where sections B and C show the FEOLED (S**oled**=close, S**ext**=close, as shown in Fig.10). At a driving voltage of 10 V on an OLED, the luminance is enhanced from 10,820 cd/m**2** to 27,393 cd/m**<sup>2</sup>** while S**ext** is turned on. Obviously, applying an electrical field (E**ext**) to the CNTs template can enhance the generation of the field emission electrons into the OLED. Additionally, the current density of OLED is increased by the supplementary electrons into the multilayer of the organic light emitting layer with the external electron source (S**oled**=close, S**ext**=close, as shown in Fig.10). Moreover, the current density of the OLED (V**oled**=10 V) with the external electron source increases from 93 mA/cm**<sup>2</sup>** (E**ext**=0.8 V/μm) to 184.5 mA/cm**2** (E**ext**=1.7 V/μm), and the luminance also increases from 10,820 cd/m**<sup>2</sup>** to 27,393 cd/m**2** simultaneously, as shown in Fig.13 (hole block line)

According to the above characteristics of FEOLED in comparison with the OLED under the same operating current density (120mA/cm2), the FEOLED exhibits a higher luminous efficiency of 18.6 cd/A than the luminous efficiency of 11.42 cd/A for OLED, as shown in Fig.14. The FEOLED results can be attributed to the external electron injection into the multilayer organic layer of OLED, thus balancing the hole and electron. Furthermore, increasing the quantity of electrons by using an external electron source significantly increases the current density of OLED and makes the luminance efficiency higher than that of conventional OLED.

**Figure 13.** The Luminance (L)-Current density (J)-Electric field (E) of the OLED and FEOLED devices at both S**oled** and S**ext** close (Eext=Vext/d) (Fig. 10)

of conventional OLED.

both S**oled** and S**ext** close (Eext=Vext/d) (Fig. 10)

electrons emitted from the CNT emitters.

between CNTs and the ITO glass substrate. The current density increases with the electric field. Based on the above results, the amount of electrons injected into the Al electrode can be determined by adjusting the electrical field (Fig.10, V**ext**), which is applied to the CNTs template. Detailed operations of FEOLEDs can be described as follows. Initially, S**oled** is turned on to drive the OLED shown in Fig.10. The OLED emits a luminance of 10,820 cd/m**<sup>2</sup>** as the driving voltage reaches 10 V and, simultaneously, S**ext** is switched on to attract the

Fig.13 shows the luminance-current density characteristics of OLEDs and FEOLEDs. The curve in section A displays the characteristics of a conventional OLED (S**0led**=close, S**ext**=open, as shown in Fig.10), where sections B and C show the FEOLED (S**oled**=close, S**ext**=close, as shown in Fig.10). At a driving voltage of 10 V on an OLED, the luminance is enhanced from 10,820 cd/m**2** to 27,393 cd/m**<sup>2</sup>** while S**ext** is turned on. Obviously, applying an electrical field (E**ext**) to the CNTs template can enhance the generation of the field emission electrons into the OLED. Additionally, the current density of OLED is increased by the supplementary electrons into the multilayer of the organic light emitting layer with the external electron source (S**oled**=close, S**ext**=close, as shown in Fig.10). Moreover, the current density of the OLED (V**oled**=10 V) with the external electron source increases from 93 mA/cm**<sup>2</sup>** (E**ext**=0.8 V/μm) to 184.5 mA/cm**2** (E**ext**=1.7 V/μm), and the luminance also increases from 10,820 cd/m**<sup>2</sup>**

According to the above characteristics of FEOLED in comparison with the OLED under the same operating current density (120mA/cm2), the FEOLED exhibits a higher luminous efficiency of 18.6 cd/A than the luminous efficiency of 11.42 cd/A for OLED, as shown in Fig.14. The FEOLED results can be attributed to the external electron injection into the multilayer organic layer of OLED, thus balancing the hole and electron. Furthermore, increasing the quantity of electrons by using an external electron source significantly increases the current density of OLED and makes the luminance efficiency higher than that

**Figure 13.** The Luminance (L)-Current density (J)-Electric field (E) of the OLED and FEOLED devices at

to 27,393 cd/m**2** simultaneously, as shown in Fig.13 (hole block line)

**Figure 14.** The Luminous efficiency (η)- Current density (J)-Electrical field (E) of the OLED and FEOLED devices at both Soled and Sext close (Eext=Vext/d)

In a FEOLED, the electrical field under vacuum condition, accelerates the electrons emitted from the CNTs cathode to affect the secondary electron material of CsI; they then pass through the Al and transport are transported through the organic EL light emitting layer. Field emission electrons with a sufficiently large electron energy supplement into OLED to increase the current density. Notably, increasing the number of the electrons that reach the organic EL light emitting layer also increases the luminous efficiency of the OLED. Therefore, the ways in which the OLED and FEOLED differ can be easily observed under the same current density. The luminance of FEOLED exceeds that of conventional OLED, as shown in Fig.13. Our results further demonstrate that the curve of the FEOLED becomes gradually saturated, especially for section C. Notably, injecting external electrons into the OLED continuously does not allow the luminance of the FEOLED to increase linearly with the current density since the quantity of electrons is larger than in the hole in section C. The carrier has become imbalanced again, subsequently decreasing the luminance. Furthermore, the electronic behavior shown in the FEOLED, it can be further demonstrates the amount of electrons is less than holes.

As describe above, we can see that the characteristics of the OLED and the FEOLED are listed by Table 1, respectively.

Field Emission Organic Light Emitting Diode 39

**Author details** 

Meiso Yokoyama

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*Department of Electronic Engineering, I-Shou University, Kaohsiung City, Taiwan* 

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**Table 1.** (1) The characteristics of the OLED and (2) The characteristics of the FEOLED
