*3.2.4. The tandem OLEDs incorporating the combination of Li2CO3:PTCDA and Li2CO3:BCP*

Tandem OLEDs consisting of two light emitting units stacked vertically in series have been demonstrated [40, 41]. Compared to the single light emitting unit, the two-unit tandem OLED can provide the prolonged working lifetime and higher luminance but nearly the same power efficiency, because the driving voltage of the tandem OLED is twice that of the single light emitting unit at a given current density. Therefore, the tandem OLEDs have been widely recognized as a promising technology for organic flat-panel displays and solidstate lighting sources in the market competition. The future development of the tandem OLEDs is to further reduce their high driving voltage via both the structure optimization of the single light emitting unit and the improvement of the interconnecting structure.

It has been recognized that the voltage drop across the interconnected structure is the key to determining the operation voltage of the tandem OLEDs [40]. In order to minimize the series resistance of the interconnected structure, it is of great necessity to reduce the ohmic loss of the current conduction in this structure provided that the energy losses due to the internal charge generation and injections have already been optimized. Thus, it is meaningful to seek for a higher-conductivity alternative to the conventional n-doped organic electron transporters. Recently, the n-doped PTCDA has been used to reduce the driving voltage of OLEDs due to its higher conductance than the conventional n-doped organic electron conductors (BCP and Alq3) [13, 42], implying that the n-doped PTCDA may act as a potential candidate for the n-type section to reduce the current loss in the tandem structure.

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

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 mechanism of the improved interconnecting structure is also discussed.

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

166 Organic Light Emitting Devices

20).

*Li2CO3:BCP* 

tandem structure.

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

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

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

*3.2.4. The tandem OLEDs incorporating the combination of Li2CO3:PTCDA and* 

the single light emitting unit and the improvement of the interconnecting structure.

It has been recognized that the voltage drop across the interconnected structure is the key to determining the operation voltage of the tandem OLEDs [40]. In order to minimize the series resistance of the interconnected structure, it is of great necessity to reduce the ohmic loss of the current conduction in this structure provided that the energy losses due to the internal charge generation and injections have already been optimized. Thus, it is meaningful to seek for a higher-conductivity alternative to the conventional n-doped organic electron transporters. Recently, the n-doped PTCDA has been used to reduce the driving voltage of OLEDs due to its higher conductance than the conventional n-doped organic electron conductors (BCP and Alq3) [13, 42], implying that the n-doped PTCDA may act as a potential candidate for the n-type section to reduce the current loss in the

Tandem OLEDs consisting of two light emitting units stacked vertically in series have been demonstrated [40, 41]. Compared to the single light emitting unit, the two-unit tandem OLED can provide the prolonged working lifetime and higher luminance but nearly the same power efficiency, because the driving voltage of the tandem OLED is twice that of the single light emitting unit at a given current density. Therefore, the tandem OLEDs have been widely recognized as a promising technology for organic flat-panel displays and solidstate lighting sources in the market competition. The future development of the tandem OLEDs is to further reduce their high driving voltage via both the structure optimization of

decreases and gives rise to the decreased IBOLED current.

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 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.

#### **3.2.4.2. The electrical properties of the IS1 and IS2**

For studying the electrical properties, we have fabricated the following two devices: Device 24: ITO (anode)/ 1:2 Li2CO3:PTCDA 5 nm/ MoO3 5 nm/ NPB 80 nm/ Al (cathode). Device 25: ITO (anode)/ 1:4 Li2CO3:BCP 5 nm/ MoO3 5 nm/ NPB 80 nm/ Al (cathode).

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 169

**3.2.4.3. The performance comparison between the S, T1, and T2 devices** 

**Figure 25.** The schematic diagrams for the structures of the T1 and T2 device.

increased power efficiency at current density 45 mA/cm2.

appreciable microcavity effect.

Fig. 26 shows the electroluminescent spectra of S, T1, and T2 devices. It is obvious that these three devices produce nearly the same Alq3 emission peaking at a wavelength of about 520 nm, demonstrating that IS1 and IS2 behave as good optical spacers without bringing any

The electrical and luminous properties of the three devices are shown in Fig. 27. As expected, the T1 device gives higher current density than T2 device, which may be mostly attributed to the more efficient current conduction in IS1 than in IS2. Accordingly, the device T1 is also brighter than the device T2. At a driving voltage of 19 V, the luminance of T1 is 3656.8 cd/m2, brighterr than 2323.9 cd/m2 of T2 device. Because of some absorption in IS1 over the Alq3 emission, the T1 device shows slightly less current efficiency than T2 device when the current density ranges from 0.1 to 80 mA/cm2. The maximum current efficiency of T2 device (4.86 cd/A) is found to be almost twice that of S device (2.51 cd/A). However, due to the reduced working voltage required for T1 than for T2, these two tandem OLEDs show comparable power efficiencies when the current density ranges from 0.1 to 80 mA/cm2. Both of them have the maximum power efficiency of about 0.84 lm/W, slightly greater than that of the single-unit device S (0.80 lm/W). Note that, compared to both T1 and T2 devices, S device gives much lower power efficiency at a current density ≤ 10 mA/cm2 and slightly

A single OLED S with structure of ITO/ NPB 80 nm/ Alq3 55 nm/ 1:4 Li2CO3:BCP 5 nm/ Al and two tandem OLEDs, T1 and T2, have been fabricated for this study as shown in Fig. 25.

The I-V characteristics of devices 24 and 25 are shown in Fig. 24. Because the ITO anode is unable to inject holes into n-doped PTCDA and BCP, and the Al cathode provides very inefficient electron injection into NPB, the current versus voltage plots of devices 24 and 25 mostly represent the internal charge generation and transport in the devices IS1 and IS2. Because devices 24 and 25 have the same interfaces of 5 nm MoO3 and 80 nm NPB, the internal charge generation and hole conduction for the two devices are identical. Therefore, the difference between the I-V characteristics of devices 24 and 25 exhibit mostly the electron conduction through the IS1 and IS2. The current density of device 24 shown in Fig. 24 is found to be greater than that of device 25, indicating the resistance of IS1 is less than that of the IS2. Hence, the IS1 tandem structure is advantageousthan IS2 because IS1 results in less ohmic loss of electron conduction than IS2.

**Figure 24.** The I-V characteristics of devices 24 (blak curve ) and 25 (red curve).

The LUMO-LUMO offset at the 1:2 Li2CO3:PTCDA/MoO3 interface can be estimated via calculating the interfacial dipole through Eq. (2). The term (1/1+1/2) is assumed to be 0.1, taking 120 and 220 because the low-frequency dielectric constant of the doped PTCDA is larger compared to the undoped one. Provided that the CNL for n-doped PTCDA-4.8 eV [30] and the CNL for MoO3-6.9 eV, Eq. (2) yields an interface dipole =2.0 eV, leading to a 2.0 eV upward shift of the vacuum level on the MoO3 side. Thus, the electron injection barrier at the 1:2 Li2CO3:PTCDA/MoO3 heterojunction is estimated to be roughly 0.1 eV, favoring the very efficient electron injection from MoO3 into 1:2 Li2CO3:PTCDA. Likewise, the electron transport barrier from MoO3 to 1:4 Li2CO3:BCP is estimated to be 0.18 eV [14], meaning that the electron current can flow from MoO3 into 1:4 Li2CO3:BCP very efficiently as well. As a result, the increased electron transport in the IS1 over IS2 may be mostly attributed to the higher electron conductivity of 1:2 Li2CO3:PTCDA than 1:4 Li2CO3:BCP [13].

#### **3.2.4.3. The performance comparison between the S, T1, and T2 devices**

168 Organic Light Emitting Devices

**3.2.4.2. The electrical properties of the IS1 and IS2** 

ohmic loss of electron conduction than IS2.

**Figure 24.** The I-V characteristics of devices 24 (blak curve ) and 25 (red curve).

The LUMO-LUMO offset at the 1:2 Li2CO3:PTCDA/MoO3 interface can be estimated via calculating the interfacial dipole through Eq. (2). The term (1/1+1/2) is assumed to be 0.1, taking 120 and 220 because the low-frequency dielectric constant of the doped PTCDA is larger compared to the undoped one. Provided that the CNL for n-doped PTCDA-4.8 eV [30] and the CNL for MoO3-6.9 eV, Eq. (2) yields an interface dipole =2.0 eV, leading to a 2.0 eV upward shift of the vacuum level on the MoO3 side. Thus, the electron injection barrier at the 1:2 Li2CO3:PTCDA/MoO3 heterojunction is estimated to be roughly 0.1 eV, favoring the very efficient electron injection from MoO3 into 1:2 Li2CO3:PTCDA. Likewise, the electron transport barrier from MoO3 to 1:4 Li2CO3:BCP is estimated to be 0.18 eV [14], meaning that the electron current can flow from MoO3 into 1:4 Li2CO3:BCP very efficiently as well. As a result, the increased electron transport in the IS1 over IS2 may be mostly attributed to the higher electron conductivity of 1:2 Li2CO3:PTCDA than 1:4 Li2CO3:BCP [13].

For studying the electrical properties, we have fabricated the following two devices: Device 24: ITO (anode)/ 1:2 Li2CO3:PTCDA 5 nm/ MoO3 5 nm/ NPB 80 nm/ Al (cathode). Device 25: ITO (anode)/ 1:4 Li2CO3:BCP 5 nm/ MoO3 5 nm/ NPB 80 nm/ Al (cathode).

The I-V characteristics of devices 24 and 25 are shown in Fig. 24. Because the ITO anode is unable to inject holes into n-doped PTCDA and BCP, and the Al cathode provides very inefficient electron injection into NPB, the current versus voltage plots of devices 24 and 25 mostly represent the internal charge generation and transport in the devices IS1 and IS2. Because devices 24 and 25 have the same interfaces of 5 nm MoO3 and 80 nm NPB, the internal charge generation and hole conduction for the two devices are identical. Therefore, the difference between the I-V characteristics of devices 24 and 25 exhibit mostly the electron conduction through the IS1 and IS2. The current density of device 24 shown in Fig. 24 is found to be greater than that of device 25, indicating the resistance of IS1 is less than that of the IS2. Hence, the IS1 tandem structure is advantageousthan IS2 because IS1 results in less A single OLED S with structure of ITO/ NPB 80 nm/ Alq3 55 nm/ 1:4 Li2CO3:BCP 5 nm/ Al and two tandem OLEDs, T1 and T2, have been fabricated for this study as shown in Fig. 25.


**Figure 25.** The schematic diagrams for the structures of the T1 and T2 device.

Fig. 26 shows the electroluminescent spectra of S, T1, and T2 devices. It is obvious that these three devices produce nearly the same Alq3 emission peaking at a wavelength of about 520 nm, demonstrating that IS1 and IS2 behave as good optical spacers without bringing any appreciable microcavity effect.

The electrical and luminous properties of the three devices are shown in Fig. 27. As expected, the T1 device gives higher current density than T2 device, which may be mostly attributed to the more efficient current conduction in IS1 than in IS2. Accordingly, the device T1 is also brighter than the device T2. At a driving voltage of 19 V, the luminance of T1 is 3656.8 cd/m2, brighterr than 2323.9 cd/m2 of T2 device. Because of some absorption in IS1 over the Alq3 emission, the T1 device shows slightly less current efficiency than T2 device when the current density ranges from 0.1 to 80 mA/cm2. The maximum current efficiency of T2 device (4.86 cd/A) is found to be almost twice that of S device (2.51 cd/A). However, due to the reduced working voltage required for T1 than for T2, these two tandem OLEDs show comparable power efficiencies when the current density ranges from 0.1 to 80 mA/cm2. Both of them have the maximum power efficiency of about 0.84 lm/W, slightly greater than that of the single-unit device S (0.80 lm/W). Note that, compared to both T1 and T2 devices, S device gives much lower power efficiency at a current density ≤ 10 mA/cm2 and slightly increased power efficiency at current density 45 mA/cm2.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 171

**Figure 27.** The electroluminescent spectra of the S, T1, and T2 devices.

luminance, current efficiency, and power efficiency, respectively.

**Table 1.** The performance data of the devices S, T1, and T2 . I, L, CF, and PF represent current density,

**Figure 26.** The I-V (a), luminance versus voltage (b), current efficiency versus current density (c), and power efficiency versus current density (d) characteristics of the S, T1, and T2 devices.

In Table I are listed the performance data of S device at a driving voltage of 9 V and those of T1 and T2 devices at a driving voltage of 18 V. The current density of T2 device at 18 V is lower than that of the S device at 9 V, while the luminance and current efficiency of the T2 device are nearly twice of those of the S device, and the power efficiencies of the T2 and S devices were nearly same. This indicates that the IS2 can effectively connect the two units in T2 device but with a marked voltage drop across it. The current density of T1 device at 18 V is 50% higher than that of S device at 9 V, its luminance is about three times that of S device, its current efficiency is twice that of S device, and its power efficiency is equal to that of S device. The current density of T1 device at 18 V is more than that of S device at 9 V. This may be attributed to the following two factors. Firstly, the voltage drop across IS1 is markedly lower compared to IS2 and secondly, the NPB layer in the upper unit is of much less thickness than that in the bottom unit.

**Figure 27.** The electroluminescent spectra of the S, T1, and T2 devices.

**Figure 26.** The I-V (a), luminance versus voltage (b), current efficiency versus current density (c), and

In Table I are listed the performance data of S device at a driving voltage of 9 V and those of T1 and T2 devices at a driving voltage of 18 V. The current density of T2 device at 18 V is lower than that of the S device at 9 V, while the luminance and current efficiency of the T2 device are nearly twice of those of the S device, and the power efficiencies of the T2 and S devices were nearly same. This indicates that the IS2 can effectively connect the two units in T2 device but with a marked voltage drop across it. The current density of T1 device at 18 V is 50% higher than that of S device at 9 V, its luminance is about three times that of S device, its current efficiency is twice that of S device, and its power efficiency is equal to that of S device. The current density of T1 device at 18 V is more than that of S device at 9 V. This may be attributed to the following two factors. Firstly, the voltage drop across IS1 is markedly lower compared to IS2 and secondly, the NPB layer in the upper unit is of much

power efficiency versus current density (d) characteristics of the S, T1, and T2 devices.

less thickness than that in the bottom unit.


**Table 1.** The performance data of the devices S, T1, and T2 . I, L, CF, and PF represent current density, luminance, current efficiency, and power efficiency, respectively.

It should be pointed out that, due to the poor ability of Li2CO3:PTCDA to inject electrons into the traditional electron transport materials (e.g., Alq3, BCP), an n-doped layer with high-lying LUMO level, n-doped BCP must be involved to facilitate the electron injection from n-doped PTCDA into the traditional electron transport materials [13]. Thus, IS1 cannot only reduce the ohmic loss of current conduction, but also can offer comparable electron injection into the bottom unit, relative to IS2.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 173

*State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,* 

The authors are grateful for the financial supports from the National Science Foundation of China (Grant No 50803014) and from Open research fund of state key laboratory of polymer physics and chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of

[1] C.W. Tang and S. A. Vanslyke, Organic electroluminescent diodes. Appl. Phys. Lett. 51:

[2] S. Braun, W. Osikowicz, Y. Wang and W. R. Salaneck W R, Energy level alignment regimes at hybrid organic-organic and inorganic-organic interfaces. Org. Electron. 8: 14

[3] S. Braun S, W. R. Salaneck and M. Fahlman, Energy-level alignment at organic/metal

[4] K. Walzer K, B. Maennig, M. Pfeiffer and K. Leo,Highly Efficient Organic Devices Based

[5] G. H. Cao, D. S. Qin, J. S. Cao, M. Guan, Y. P. Zeng and J. M. Li, Organic light emitting diodes with an organic acceptor/donor interface involved in hole injection. Chin. Phys.

[6] G. H. Cao, D. S. Qin, J. S. Cao, M. Guan, Y. P. Zeng and J. M. Li, Improved performance in organic light emitting diodes with a mixed electron donor-acceptor film involved in

[7] Y. Y. Yuan, S. Han, D. Grozea and Z. H. Lu, Fullerene-organic nanocomposite: a flexible material platform for organic light-emitting diodes. Appl. Phys. Lett. 88:

[8] X. Zhou, M. Pfeiffer, J. S. Huang, J. Blochwitz-Nimoth, D. S. Qin, A. Werner, J. Drechesel, B. Maennig and K. Leo, Low-voltage inverted transparent vacuum deposited organic light-emittingdiodes using electrical doping. Appl. Phys. Lett. 81:

[9] T. Y. Chu, J. F. Chen, S. Y. Chen, C. H. Chen, Comparative study of single and multiemissive layers in inverted white organic light-emitting devices. Appl. Phys. Lett.

[10] N. J. Watkins, L. Yan and Y. Gao, Electronic structure symmetry of interfaces between

and organic/organic interfaces. Adv. Mater. 21: 1450 (2009).

hole injection. J. Appl. Phys. 101: 124507 (2007).

pentacene and metals. Appl. Phys. Lett. 80: 4384 (2002).

on. Electrically Doped Transport Layers. Chem.Rev. 107: 1233 (2007).

*Chinese Academy of Sciences, Changchun, Jilin Province, People's Republic of China* 

Jidong Zhang

Sciences.

**5. References** 

913 (1987).

Lett. 24: 1380 (2007).

093503 (2006).

922 (2002).

89: 113502 (2006).

(2007).

**Acknowledgement** 
