*3.2.3. The inverted OLEDs incorporating the combination of n-NTCDA and Li2CO3:BCP*

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 163

Device 22. ITO/NTCDA 5 nm/ BCP:Li2CO3 (4:1) 5 nm /Alq3 40nm/ CBP 60 nm/

Device 23. ITO/NTCDA 7 nm/ BCP:Li2CO3 (4:1) 3 nm /Alq3 40nm/ CBP 60 nm/

**Figure 18.** The XRD (a) and UV-Vis absorption spectra (b) of the 5, 10, 15 nm NTCDA thin films on the ITO and the 15 nm NTCDA thin film on the glass. Due to the limitation of the instrument, we were unable to detect the sub-gap absorption in the 5 nm NTCDA on the ITO. Note that, the 3 nm NTCDA

Fig. 19(a) shows the I-V characteristics of devices 19 and 20. As expected, device 20 showed higher current density than device 19 at a given voltage between 3 and 10 V. To generate a current density of 100 mA/cm2, device 20 needed a driving voltage of 8.0 V, smaller than that for device 19 (9.0 V). Fig. 19(b) shows device 20 also exhibits enhanced luminance than device 19. At a driving voltage of 10 V, the luminance of device 20 is found to be 11706 cd/m2, in comparison with 4016 cd/m2 for device 19. Fig. 19(c) shows that the maximum current efficiency in device 20 is 2.29 cd/A, 38% higher than that of 1.66 cd/A in device 19, mostly attributed to the following two factors: firstly, the intervention of the NTCDA improved the hole-electron balance in device 20 relative to device 19; secondly, the NTCDA was nearly transparent in the visible-light range. Hence, it may be concluded that the

thin film on the ITO is also confirmed to be crystalline.

MoO3 10 nm/Al;

MoO3 10 nm/Al.

To maximize the performance for the combination of two n-ETLs, the n-ETL contacting the cathode needs to meet the two requirements: high conductivity and good transparency over the visible light range. Thus, the n-NTCDA was chosen to function with Li2CO3:BCP.

### **3.2.3.1. The properties of the NTCDA deposited onto the ITO**

NTCDA is a class of n-type organic semiconductor with a LUMO level at 4.0 eV [36]. It can have the electronic interactions with the active metals like In or Mg at room temperature, which is attributed to the partial charge transfer from the metal to carbonyl oxygen of NTCDA [37]. It is also reported that NTCDA shows strong electronic interaction with metal atoms in an ITO substrate to form charge transfer (CT) complexes or gap states [38]. The formation of a CT complex can not only lower the energy barrier for hole injection from ITO into N,N'-bis-(1-naphthl)-diphenyl-1,1'-biphenyl-4,4'-diamine, but also can offer a conducting path to assist hole transport [38]. Thus, the introduction of NTCDA onto the ITO anode can significantly improve the hole current and thereby the performance of regular OLED. Nevertheless, it needs to be stressed that the electronic interaction between active metal and NTCDA can leave NTCDA n-doped via the spontaneous charge transfer from metal atom to NTCDA [37].

The four NTCDA thin films have been fabricated and characterized in Fig. 18. Fig. 18(a) shows that the NTCDA thin films both deposited on ITO and glass are crystalline, proven by their same diffraction peaks at 2=11.8o. Fig. 18(b) shows the UV-VIS absorption spectra of the four NTCDA thin films. All the NTCDA thin films gave two peaks at the wavelengths of 368 nm and 390 nm assigned to π-π\* . Compared with the 15 nm NTCDA thin film on glass, the 15 nm NTCDA thin film on ITO shows a broad absorption peak from 420 nm to 600 nm, which becomes weaker with the decreasing thickness of the NTCDA on ITO [39]. The emergence of a new sub-gap absorption indicates the formation of the CT complex between NTCDA and ITO as a result of the upward diffusion of In atoms from ITO into NTCDA and the concomitant interaction between In and anhydride groups of NTCDA. Thus, the NTCDA thin films on the ITO are conclusively crystalline and n-doped. The conductivity of the n-doped NTCDA (n-NTCDA) is reported to be as high as 9.29×10-4 S/cm [4], almost two orders of magnitude higher than that of n-BCP [31]. Hence, according to the concept of using two n-ETLs mentioned above, the NTCDA on the ITO may be applied as the first n-ETL to enhance the electron conduction in IBOLEDs.

#### **3.2.3.2. The comparison between the electron injection structures of ITO/ BCP:Li2CO3 (4:1) 10 nm and ITO/NTCDA 3 nm/ BCP:Li2CO3 (4:1) 7 nm**

For this study we fabricated the following IBOLEDs: Device 19. ITO/ BCP:Li2CO3 (4:1) 10 nm/ Alq3 40 nm/ CBP 60 nm/ MoO3 10 nm/Al; Device 20. ITO/NTCDA 3 nm/ BCP:Li2CO3 (4:1) 7 nm /Alq3 40nm/CBP 60 nm/ MoO3 10 nm/Al; Device 21. ITO/NTCDA 3 nm/ BCP 7 nm /Alq3 40nm/ CBP 60 nm/ MoO3 10 nm/Al; Device 22. ITO/NTCDA 5 nm/ BCP:Li2CO3 (4:1) 5 nm /Alq3 40nm/ CBP 60 nm/ MoO3 10 nm/Al; Device 23. ITO/NTCDA 7 nm/ BCP:Li2CO3 (4:1) 3 nm /Alq3 40nm/ CBP 60 nm/ MoO3 10 nm/Al.

162 Organic Light Emitting Devices

metal atom to NTCDA [37].

of 368 nm and 390 nm assigned to π-π\*

the first n-ETL to enhance the electron conduction in IBOLEDs.

**10 nm and ITO/NTCDA 3 nm/ BCP:Li2CO3 (4:1) 7 nm** 

For this study we fabricated the following IBOLEDs:

MoO3 10 nm/Al;

*3.2.3. The inverted OLEDs incorporating the combination of n-NTCDA and Li2CO3:BCP* 

To maximize the performance for the combination of two n-ETLs, the n-ETL contacting the cathode needs to meet the two requirements: high conductivity and good transparency over

NTCDA is a class of n-type organic semiconductor with a LUMO level at 4.0 eV [36]. It can have the electronic interactions with the active metals like In or Mg at room temperature, which is attributed to the partial charge transfer from the metal to carbonyl oxygen of NTCDA [37]. It is also reported that NTCDA shows strong electronic interaction with metal atoms in an ITO substrate to form charge transfer (CT) complexes or gap states [38]. The formation of a CT complex can not only lower the energy barrier for hole injection from ITO into N,N'-bis-(1-naphthl)-diphenyl-1,1'-biphenyl-4,4'-diamine, but also can offer a conducting path to assist hole transport [38]. Thus, the introduction of NTCDA onto the ITO anode can significantly improve the hole current and thereby the performance of regular OLED. Nevertheless, it needs to be stressed that the electronic interaction between active metal and NTCDA can leave NTCDA n-doped via the spontaneous charge transfer from

The four NTCDA thin films have been fabricated and characterized in Fig. 18. Fig. 18(a) shows that the NTCDA thin films both deposited on ITO and glass are crystalline, proven by their same diffraction peaks at 2=11.8o. Fig. 18(b) shows the UV-VIS absorption spectra of the four NTCDA thin films. All the NTCDA thin films gave two peaks at the wavelengths

glass, the 15 nm NTCDA thin film on ITO shows a broad absorption peak from 420 nm to 600 nm, which becomes weaker with the decreasing thickness of the NTCDA on ITO [39]. The emergence of a new sub-gap absorption indicates the formation of the CT complex between NTCDA and ITO as a result of the upward diffusion of In atoms from ITO into NTCDA and the concomitant interaction between In and anhydride groups of NTCDA. Thus, the NTCDA thin films on the ITO are conclusively crystalline and n-doped. The conductivity of the n-doped NTCDA (n-NTCDA) is reported to be as high as 9.29×10-4 S/cm [4], almost two orders of magnitude higher than that of n-BCP [31]. Hence, according to the concept of using two n-ETLs mentioned above, the NTCDA on the ITO may be applied as

**3.2.3.2. The comparison between the electron injection structures of ITO/ BCP:Li2CO3 (4:1)** 

Device 19. ITO/ BCP:Li2CO3 (4:1) 10 nm/ Alq3 40 nm/ CBP 60 nm/ MoO3 10 nm/Al; Device 20. ITO/NTCDA 3 nm/ BCP:Li2CO3 (4:1) 7 nm /Alq3 40nm/CBP 60 nm/

Device 21. ITO/NTCDA 3 nm/ BCP 7 nm /Alq3 40nm/ CBP 60 nm/ MoO3 10 nm/Al;

. Compared with the 15 nm NTCDA thin film on

the visible light range. Thus, the n-NTCDA was chosen to function with Li2CO3:BCP.

**3.2.3.1. The properties of the NTCDA deposited onto the ITO** 

**Figure 18.** The XRD (a) and UV-Vis absorption spectra (b) of the 5, 10, 15 nm NTCDA thin films on the ITO and the 15 nm NTCDA thin film on the glass. Due to the limitation of the instrument, we were unable to detect the sub-gap absorption in the 5 nm NTCDA on the ITO. Note that, the 3 nm NTCDA thin film on the ITO is also confirmed to be crystalline.

Fig. 19(a) shows the I-V characteristics of devices 19 and 20. As expected, device 20 showed higher current density than device 19 at a given voltage between 3 and 10 V. To generate a current density of 100 mA/cm2, device 20 needed a driving voltage of 8.0 V, smaller than that for device 19 (9.0 V). Fig. 19(b) shows device 20 also exhibits enhanced luminance than device 19. At a driving voltage of 10 V, the luminance of device 20 is found to be 11706 cd/m2, in comparison with 4016 cd/m2 for device 19. Fig. 19(c) shows that the maximum current efficiency in device 20 is 2.29 cd/A, 38% higher than that of 1.66 cd/A in device 19, mostly attributed to the following two factors: firstly, the intervention of the NTCDA improved the hole-electron balance in device 20 relative to device 19; secondly, the NTCDA was nearly transparent in the visible-light range. Hence, it may be concluded that the combination of n-NTCDA and 1:4 Li2CO3:BCP excels the single 1:4 Li2CO3:BCP in promoting the current conduction and efficiency of the IBOLED.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 165

**Figure 20.** The I-V characteristics of device 21. Note that, there was no Alq3 emission observed in the

**Figure 21.** Schematic diagrams for depicting the electronic structures at the interfaces of n-NTCDA/BCP (i) and n-NTCDA/1:4 Li2CO3:BCP (ii), both deposited onto the ITO substrates. For simplicity, the gap states are not shown. The EF, HOMO, and horizontal dotted arrow represent the Fermi level, highest occupied molecular orbital, and the electron-injecting process, respectively.

compared to that (1.6) of the undoped NTCDA and taking BCP1.4, (1/1+1/2) is estimated to be 0.7. Provided that the CNL for n-NTCDA is roughly -4.2 eV due to the Fermi level pinning [22] and taking the CNL of BCP-3.8 eV, Eq. (2) yields an interface dipole 1=-0.26 eV, which results in a 0.26 eV downward shift of the vacuum level (VL) on the BCP side as seen in Fig. 21(i). Thus, the energy barrier from the LUMO of n-NTCDA to that of BCP is estimated to be about 0.74 eV, determining that the electron transport through the n-NTCDA/BCP interface is very poor in the case of device 22. For the n-NTCDA/1:4 Li2CO3:BCP heterojunction, (1/1+1/2) is assumed to be 0.2, because the low-frequency dielectric constants of the doped materials are larger compared to the undoped materials. Provided that the CNL for n-NTCDA -4.2 eV and the CNL for n-BCP-3.2 eV, Eq. (2) yields

measurement range.

**Figure 19.** The I-V (a), luminance versus voltage (b), current efficiency versus current density (c) characteristics for devices 19 (squares) and 20 (circles).

#### **3.2.3.3. The mechanism of electron transport from n-NTCDA into BCP:Li2CO3 (4:1)**

In device 20, how the electrons are efficiently transported from n-NTCDA into 1:4 Li2CO3:BCP needs to be addressed, since the LUMO level (4.0 eV) of NTCDA is 1.0 eV lower than that (3.0 eV) of BCP. In quest of the relevant mechanism, device 21 is fabricated and its characteristics are shown in Fig. 20. As seen in Fig. 20, device 21 exhibits poor current- voltage characteristics. At a driving voltage of 10 V, it yields a current of 3.63 mA/cm2 and no light emission, which is clearly contrary to 601 mA/cm2 and 11709 cd/m2 achieved in device 20. The greatly improved performance of device 20 relative to device 21 indicates that the energy barrier for electron injection from n-NTCDA into BCP is significantly reduced by fulfilling the n-doping on the BCP side [13].

The electronic structures of the interfaces of n-NTCDA/BCP and n-NTCDA/1:4 Li2CO3:BCP are schematically shown in Fig. 21. The LUMO-LUMO offset at the organic heterojunction can be varied by the interfacial dipole (∆) expressed as Eq. (2). In the case of n-NTCDA/BCP heterojunction, considering the dielectric constant of n-NTCDA is large

the current conduction and efficiency of the IBOLED.

characteristics for devices 19 (squares) and 20 (circles).

fulfilling the n-doping on the BCP side [13].

combination of n-NTCDA and 1:4 Li2CO3:BCP excels the single 1:4 Li2CO3:BCP in promoting

**Figure 19.** The I-V (a), luminance versus voltage (b), current efficiency versus current density (c)

**3.2.3.3. The mechanism of electron transport from n-NTCDA into BCP:Li2CO3 (4:1)** 

In device 20, how the electrons are efficiently transported from n-NTCDA into 1:4 Li2CO3:BCP needs to be addressed, since the LUMO level (4.0 eV) of NTCDA is 1.0 eV lower than that (3.0 eV) of BCP. In quest of the relevant mechanism, device 21 is fabricated and its characteristics are shown in Fig. 20. As seen in Fig. 20, device 21 exhibits poor current- voltage characteristics. At a driving voltage of 10 V, it yields a current of 3.63 mA/cm2 and no light emission, which is clearly contrary to 601 mA/cm2 and 11709 cd/m2 achieved in device 20. The greatly improved performance of device 20 relative to device 21 indicates that the energy barrier for electron injection from n-NTCDA into BCP is significantly reduced by

The electronic structures of the interfaces of n-NTCDA/BCP and n-NTCDA/1:4 Li2CO3:BCP are schematically shown in Fig. 21. The LUMO-LUMO offset at the organic heterojunction can be varied by the interfacial dipole (∆) expressed as Eq. (2). In the case of n-NTCDA/BCP heterojunction, considering the dielectric constant of n-NTCDA is large

**Figure 20.** The I-V characteristics of device 21. Note that, there was no Alq3 emission observed in the measurement range.

**Figure 21.** Schematic diagrams for depicting the electronic structures at the interfaces of n-NTCDA/BCP (i) and n-NTCDA/1:4 Li2CO3:BCP (ii), both deposited onto the ITO substrates. For simplicity, the gap states are not shown. The EF, HOMO, and horizontal dotted arrow represent the Fermi level, highest occupied molecular orbital, and the electron-injecting process, respectively.

compared to that (1.6) of the undoped NTCDA and taking BCP1.4, (1/1+1/2) is estimated to be 0.7. Provided that the CNL for n-NTCDA is roughly -4.2 eV due to the Fermi level pinning [22] and taking the CNL of BCP-3.8 eV, Eq. (2) yields an interface dipole 1=-0.26 eV, which results in a 0.26 eV downward shift of the vacuum level (VL) on the BCP side as seen in Fig. 21(i). Thus, the energy barrier from the LUMO of n-NTCDA to that of BCP is estimated to be about 0.74 eV, determining that the electron transport through the n-NTCDA/BCP interface is very poor in the case of device 22. For the n-NTCDA/1:4 Li2CO3:BCP heterojunction, (1/1+1/2) is assumed to be 0.2, because the low-frequency dielectric constants of the doped materials are larger compared to the undoped materials. Provided that the CNL for n-NTCDA -4.2 eV and the CNL for n-BCP-3.2 eV, Eq. (2) yields an interface dipole 2=-0.90 eV, leading to a 0.90 eV downward shift of the VL in the n-BCP side, as as shown in Fig. 21(ii). The electron injection barrier from n-NTCDA to 1:4 Li2CO3:BCP is estimated to be roughly 0.10 eV. Thus, the interface becomes ohmic, enabling the efficient electron conduction and hence enhaced device performance (the case of device 20).

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 167

**Figure 22.** The I-V characteristics of devices 20 (circles), 22 (uptriangles), and 23 (downtriangles).

mechanism of the improved interconnecting structure is also discussed.

to 700 nm, due to the narrow optical band gap of PTCDA (1.7 eV).

**Figure 23.** The optical properties of the IS1 and IS2 deposited on the quartz glasses.

**3.2.4.1. The optical properties of the IS1 and IS2** 

Here, a 5 nm lithium carbonate doped PTCDA (1:2 Li2CO3:PTCDA) has been incorporated in the n-type section of the interconnected structure. Compared to the tandem OLED of 5 nm Li2CO3 doped BCP (1:4 Li2CO3:BCP)/ MoO3, the tandem OLED of 5 nm 1:2 Li2CO3:PTCDA/ 5 nm MoO3 showed the enhanced electrical and luminous performance, due to the higher electron conductivity of 1:2 Li2CO3:PTCDA than that of 1:4 Li2CO3:BCP. The working

Fig. 23 shows the optical transmittance of two interconnected structures. It can be seen that the 5 nm 1:2 Li2CO3:PTCDA/ 5 nm MoO3 (IS1) is nearly transparent in the visible-light range, but the 5 nm 1:4 Li2CO3:BCP/ 5 nm MoO3 (IS2) shows slight light absorption from 400

#### **3.2.3.4. The effect of the NTCDA thickness on the current conduction of IBOLED**

It is found that the device performance is strongly influenced by the thickness of the NTCDA on ITO as a hole injection layer [39]. Therefore, the dependence of the IBOLED performance on the NTCDA on ITO as n-ETL has been investigated and the observed characteristics are shown in Fig. 22 for devices 20, 22, and 23. It can be seen that the current density of the IBOLED using the 3 nm NTCDA is found to be higher than that of the IBOLEDs using the 5 and 7 nm NTCDA at a given voltage between 3 and 10 V. It may be explained as follows. When the thickness of the NTCDA exceeds 3 nm, the formation of the CT complex near the NTCDA top surface starts to vanish [38], that is, the surface density of the CT complex near NTCDA top surface starts to decrease, leading to a reduced generation of free electrons therein. Thus, the electron conduction across the whole NTCDA thin film decreases and gives rise to the decreased IBOLED current.
