*3.1.1. The hole generation at the planar CuPc/PTCDA interface*

We firstly fabricated the following light emitting devices: Device 1: ITO/ CuPc 5 nm/ NPB 75 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 2: ITO/ PTCDA 10 nm/ NPB 70 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 3: ITO/ PTCDA 20 nm/ NPB 60 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 4: ITO/ PTCDA 10 nm/ CuPc 5 nm/ NPB 65 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 5: ITO/ PTCDA 20 nm/ CuPc 5 nm/ NPB 55 nm/ Alq3 60 nm/ Mg:Ag/ Ag;

Note that, the devices 4 and 5 are fabricated with planar PTCDA/ CuPc interfaces for hole injection.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 147

Fig. 1 shows the properties of devices 1, 2, and 4. As shown in Fig. 1(a), device 4 reaches a current density of 130 mA/cm2 at a voltage of 10 V, which is 4.6 times higher than that of 23.2 mA/cm2 for device 1 and 12.3 times higher than that of 9.8 mA/cm2 for device 2. There is a decrease in the operating voltage at a given current density in the order of device 2 > device 1 > device 4, clearly demonstrating the structure of ITO/ PTCDA 10 nm/ CuPc 5 nm can generate much enhanced hole current than the structures of ITO/ CuPc 5 nm and ITO/ PTCDA 10 nm. Likewise, Fig. 1(b) shows that device 4 also exhibits much higher luminance than devices 1 and 2. As seen in Fig. 1(c), there is a decrease in the external quantum efficiency (EQE) in the order of device 1 > device 2 > device 4, mostly resulting from some absorption in CuPc and PTCDA over the Alq3 emission; nevertheless, the power efficiency at a certain current density decreases in an order of device 4 > device 1 > device 2. It may thus,be concluded that the structure of ITO/ PTCDA 10 nm/ CuPc 5 nm outperforms the

In order to figure out the hole generation in OLEDs with the ITO/ PTCDA (10 and 20) nm/ CuPc structure, the influence of the PTCDA layer thickness on the device current has been studied. We have shown the observed I-V characteristics of devices 2, 3, 4 and 5 in Fig. 2. It shows that the device 2 (10 nm) has got nearly the same I-V characteristics as device 3 (20 nm), indicating that the hole generation in the ITO/ PTCDA (10 and 20) nm structure is weakly dependent on the PTCDA thickness [20], while device 4 gives much increased current density than device 5, implying that the hole generation in the ITO/ PTCDA (10 and 20) nm/ CuPc structure is strongly subject to the PTCDA layer thickness. It may therefore be concluded that besides a minor process of ITO injecting holes directly into the PTCDA layer [20, 21], devices 4 (10 nm) and 5 (20 nm) possess a major hole-generating process which

**Figure 2.** The I-V characteristics of devices 2 (10 nm PTCDA), 3 (20 nm PTCDA), 4 (10nm PTCDA/

Fig. 3 depicts the major hole generation process in the ITO/ PTCDA (20) nm/ CuPc structure. Under the forward bias, due to the influence of holes accumulated at the ITO/ PTCDA interface [22], the PTCDA/ CuPc interface may become polarized, that is, half electrons from

structures of ITO/ CuPc 5 nm and ITO/ PTCDA 10 nm in OLEDs.

devices 2 and 3 do not have.

CuPc) and 5 (20 nm PTCDA/ CuPc).

**Figure 1.** Characteristics of (a) I-V, (b) luminance-voltage (L-V), and (c) efficiency-current density of devices 1, 2, and 4. The external quantum efficiencies are calculated, provided that all of the devices are assumed to function as Lambertian sources.

Fig. 1 shows the properties of devices 1, 2, and 4. As shown in Fig. 1(a), device 4 reaches a current density of 130 mA/cm2 at a voltage of 10 V, which is 4.6 times higher than that of 23.2 mA/cm2 for device 1 and 12.3 times higher than that of 9.8 mA/cm2 for device 2. There is a decrease in the operating voltage at a given current density in the order of device 2 > device 1 > device 4, clearly demonstrating the structure of ITO/ PTCDA 10 nm/ CuPc 5 nm can generate much enhanced hole current than the structures of ITO/ CuPc 5 nm and ITO/ PTCDA 10 nm. Likewise, Fig. 1(b) shows that device 4 also exhibits much higher luminance than devices 1 and 2. As seen in Fig. 1(c), there is a decrease in the external quantum efficiency (EQE) in the order of device 1 > device 2 > device 4, mostly resulting from some absorption in CuPc and PTCDA over the Alq3 emission; nevertheless, the power efficiency at a certain current density decreases in an order of device 4 > device 1 > device 2. It may thus,be concluded that the structure of ITO/ PTCDA 10 nm/ CuPc 5 nm outperforms the structures of ITO/ CuPc 5 nm and ITO/ PTCDA 10 nm in OLEDs.

146 Organic Light Emitting Devices

**Figure 1.** Characteristics of (a) I-V, (b) luminance-voltage (L-V), and (c) efficiency-current density of devices 1, 2, and 4. The external quantum efficiencies are calculated, provided that all of the devices are

assumed to function as Lambertian sources.

In order to figure out the hole generation in OLEDs with the ITO/ PTCDA (10 and 20) nm/ CuPc structure, the influence of the PTCDA layer thickness on the device current has been studied. We have shown the observed I-V characteristics of devices 2, 3, 4 and 5 in Fig. 2. It shows that the device 2 (10 nm) has got nearly the same I-V characteristics as device 3 (20 nm), indicating that the hole generation in the ITO/ PTCDA (10 and 20) nm structure is weakly dependent on the PTCDA thickness [20], while device 4 gives much increased current density than device 5, implying that the hole generation in the ITO/ PTCDA (10 and 20) nm/ CuPc structure is strongly subject to the PTCDA layer thickness. It may therefore be concluded that besides a minor process of ITO injecting holes directly into the PTCDA layer [20, 21], devices 4 (10 nm) and 5 (20 nm) possess a major hole-generating process which devices 2 and 3 do not have.

**Figure 2.** The I-V characteristics of devices 2 (10 nm PTCDA), 3 (20 nm PTCDA), 4 (10nm PTCDA/ CuPc) and 5 (20 nm PTCDA/ CuPc).

Fig. 3 depicts the major hole generation process in the ITO/ PTCDA (20) nm/ CuPc structure. Under the forward bias, due to the influence of holes accumulated at the ITO/ PTCDA interface [22], the PTCDA/ CuPc interface may become polarized, that is, half electrons from the highest occupied molecular orbital (HOMO) of CuPc are transferred into the lowest unoccupied molecular orbital (LUMO) of PTCDA, which is possibly favored by the following three reasons: Firstly, the PTCDA/ CuPc interface can effectively dissociate the photo-generated excitons into hole and electrons and then prevent them from recombining [23]; secondly, there is a strong Coulomb force between PTCDA and CuPc [7], thereby making the overlap between the HOMO of CuPc and LUMO of PTCDA significant, and furthermore the electric field can assist electrons in the HOMO of CuPc overcoming 0.6 eV energy barrier to get into the LUMO of PTCDA; thirdly, the interfacial dipole at the PTCDA/ CuPc interface can facilitate the generation of hole-electron pairs [19] instead of excitons. After electrons get transferred into the LUMO of PTCDA, the vacancies (holes) are generated in the HOMO of CuPc, and those transferred into the PTCDA layer are rapidly transported into the ITO anode, generating very efficient hole current. For the ITO/ PTCDA (10 and 20) nm/ CuPc structure, when the thickness of the PTCDA layer increases, the electrostatic inducement effect of holes accumulated at the ITO/ PTCDA interface on the PTCDA/ CuPc interface, presumably considered proportional to the inverse of the square of the PTCDA layer thickness, decreases, and accordingly the charge generation at the PTCDA/ CuPc interface becomes less efficient, accounting for the decreased current density in device 3 as compared to that in device 4.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 149

efficiency of 2.7 cd/A and a power efficiency of 1 lm/W, whereas the device with the ITO/CuPc anode has 2.3 cd/A and 0.7 lm/W, respectively. Therefore, the ITO/PTCDA:CuPc

**Figure 4.** (a) I-V-L and (b) efficiency-luminance characteristics of two OLEDs with structures of ITO/CuPc 5 nm/NPB 75 nm/Alq3 60 nm/Mg:Ag (filled squares) and ITO/PTCDA:CuPc (1:2 in mass

structure is better when the thickness of PTCDA:CuPc is changed from 10 to 20 nm.

**Figure 5.** (a) Maximum current efficiency and driving voltage vs PTCDA:CuPc thickness and (b) plots of power efficiency as a function of current density for OLEDs with structure of ITO/1:2 PTCDA:CuPc x nm/NPB 80-x nm/Alq3 60 nm/Mg:Ag (open circles), where the PTCDA:CuPc thicknesses are 5, 10, 20,

and 30 nm, respectively.

The influence of PTCDA:CuPc thickness on the device performance was studied. Fig. 5(a) shows that the maximum current efficiencies of the devices with 10 and 20 nm PTCDA:CuPc are slightly different and higher than those of the devices with 5 and 30 nm PTCDA:CuPc. The driving voltage for the devices with 10, 20, and 30 nm PTCDA:CuPc, such as at I=50 mA/cm2 or I=150 mA/cm2, is very close and all are lower than that for the device with 5 nm PTCDA:CuPc. Fig. 5(b) shows that the power efficiency of the devices with 10 and 20 nm PTCDA:CuPc is nearly the same and both are higher than those of the devices with 5 and 30 nm PTCDA:CuPc. It may therefore be concluded that the ITO/ PTCDA:CuPc anode

ratio) 10 nm/NPB 70 nm/Alq3 60 nm/Mg:Ag (open circles), respectively.

anode structure is more efficient than the ITO/CuPc anode structure.

**Figure 3.** Schematic flat-band energy level diagram for description of the major hole-generating process in devices 4 and 5. The energy level alignments at the PTCDA/ CuPc interface have been ignored for simplicity.

#### *3.1.2. Characterization of OLEDs with ITO/ CuPc and ITO/PTCDA:CuPc anodes*

We compare the performance of two devices with structures of ITO/CuPc 5 nm/NPB 75 nm/Alq3 60 nm/Mg:Ag and ITO/PTCDA:CuPc (1:2 in mass ratio) 10 nm/NPB 70 nm/Alq3 60 nm/Mg:Ag, respectively. As shown in Fig. 4(a), the device with the ITO/ PTCDA:CuPc anode has higher current density and luminance at the same driving voltage compared to the device with the ITO/CuPc anode. In addition, as shown in Fig. 4(b), the latter has better current and power efficiencies than the device with the ITO/CuPc anode. For example, at a brightness of 2000 cd/m2, the device with the ITO/PTCDA:CuPc anode has a current efficiency of 2.7 cd/A and a power efficiency of 1 lm/W, whereas the device with the ITO/CuPc anode has 2.3 cd/A and 0.7 lm/W, respectively. Therefore, the ITO/PTCDA:CuPc anode structure is more efficient than the ITO/CuPc anode structure.

148 Organic Light Emitting Devices

3 as compared to that in device 4.

simplicity.

the highest occupied molecular orbital (HOMO) of CuPc are transferred into the lowest unoccupied molecular orbital (LUMO) of PTCDA, which is possibly favored by the following three reasons: Firstly, the PTCDA/ CuPc interface can effectively dissociate the photo-generated excitons into hole and electrons and then prevent them from recombining [23]; secondly, there is a strong Coulomb force between PTCDA and CuPc [7], thereby making the overlap between the HOMO of CuPc and LUMO of PTCDA significant, and furthermore the electric field can assist electrons in the HOMO of CuPc overcoming 0.6 eV energy barrier to get into the LUMO of PTCDA; thirdly, the interfacial dipole at the PTCDA/ CuPc interface can facilitate the generation of hole-electron pairs [19] instead of excitons. After electrons get transferred into the LUMO of PTCDA, the vacancies (holes) are generated in the HOMO of CuPc, and those transferred into the PTCDA layer are rapidly transported into the ITO anode, generating very efficient hole current. For the ITO/ PTCDA (10 and 20) nm/ CuPc structure, when the thickness of the PTCDA layer increases, the electrostatic inducement effect of holes accumulated at the ITO/ PTCDA interface on the PTCDA/ CuPc interface, presumably considered proportional to the inverse of the square of the PTCDA layer thickness, decreases, and accordingly the charge generation at the PTCDA/ CuPc interface becomes less efficient, accounting for the decreased current density in device

**Figure 3.** Schematic flat-band energy level diagram for description of the major hole-generating process in devices 4 and 5. The energy level alignments at the PTCDA/ CuPc interface have been ignored for

We compare the performance of two devices with structures of ITO/CuPc 5 nm/NPB 75 nm/Alq3 60 nm/Mg:Ag and ITO/PTCDA:CuPc (1:2 in mass ratio) 10 nm/NPB 70 nm/Alq3 60 nm/Mg:Ag, respectively. As shown in Fig. 4(a), the device with the ITO/ PTCDA:CuPc anode has higher current density and luminance at the same driving voltage compared to the device with the ITO/CuPc anode. In addition, as shown in Fig. 4(b), the latter has better current and power efficiencies than the device with the ITO/CuPc anode. For example, at a brightness of 2000 cd/m2, the device with the ITO/PTCDA:CuPc anode has a current

*3.1.2. Characterization of OLEDs with ITO/ CuPc and ITO/PTCDA:CuPc anodes* 

**Figure 4.** (a) I-V-L and (b) efficiency-luminance characteristics of two OLEDs with structures of ITO/CuPc 5 nm/NPB 75 nm/Alq3 60 nm/Mg:Ag (filled squares) and ITO/PTCDA:CuPc (1:2 in mass ratio) 10 nm/NPB 70 nm/Alq3 60 nm/Mg:Ag (open circles), respectively.

The influence of PTCDA:CuPc thickness on the device performance was studied. Fig. 5(a) shows that the maximum current efficiencies of the devices with 10 and 20 nm PTCDA:CuPc are slightly different and higher than those of the devices with 5 and 30 nm PTCDA:CuPc. The driving voltage for the devices with 10, 20, and 30 nm PTCDA:CuPc, such as at I=50 mA/cm2 or I=150 mA/cm2, is very close and all are lower than that for the device with 5 nm PTCDA:CuPc. Fig. 5(b) shows that the power efficiency of the devices with 10 and 20 nm PTCDA:CuPc is nearly the same and both are higher than those of the devices with 5 and 30 nm PTCDA:CuPc. It may therefore be concluded that the ITO/ PTCDA:CuPc anode structure is better when the thickness of PTCDA:CuPc is changed from 10 to 20 nm.

**Figure 5.** (a) Maximum current efficiency and driving voltage vs PTCDA:CuPc thickness and (b) plots of power efficiency as a function of current density for OLEDs with structure of ITO/1:2 PTCDA:CuPc x nm/NPB 80-x nm/Alq3 60 nm/Mg:Ag (open circles), where the PTCDA:CuPc thicknesses are 5, 10, 20, and 30 nm, respectively.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 151

absorption feature at λ=539 nm for neat PTCDA film [26] is strongly suppressed in

Hole generation in the ITO/PTCDA:CuPc anode cannot be simply described as holes being directly injected from ITO to PTCDA:CuPc, as the hole injection barrier from ITO to PTCDA:CuPc is not reduced, as compared to that from ITO to CuPc [5]. In order to figure out the working mechanism of the ITO/PTCDA:CuPc hole injection structure, the dependence of the device performance on NPB spacer thickness was studied. As shown in Fig. 7(a), with increasing NPB spacer thickness, the device current decreases significantly; but when the NPB spacer thickness is greater than or equal to 20 nm, the devices exhibit nearly identical I−V characteristics. Fig. 7(b) shows that the device efficiency decreases with

**Figure 7.** (a) I-V and (b) efficiency-current density curves for OLEDs with structure of ITO/ NPB spacer *x* nm/ PTCDA:CuPc 10 nm/ NPB 70-*x* nm/ Alq3 60 nm/ Mg:Ag, where NPB spacer thicknesses are 0 nm (open circles), 10 nm (filled circles), 20 nm (upward triangles), and 30 nm (downward triangles),

The variation of device current with NPB spacer thickness indicates that static-induced holeelectron pairs generation is likely to take place in PTCDA:CuPc composites, expressed as

SI

where SI stands for static inducement. One paired electron in the HOMO of one CuPc molecule can be transferred to the LUMO of one PTCDA molecule in proximity, favored by (i) the PTCDA/CuPc interface can effectively dissociate the photogenerated excitons into holes and electrons and prevent them from recombining [23] and (ii) there is intermolecular hydrogen bonding between PTCDA and CuPc, and consequently the overlap between the HOMO of CuPc and the LUMO of PTCDA becomes significant. Thus, via process (1) holes and electrons can be efficiently generated in the HOMO of CuPc and in the LUMO of PTCDA, respectively, in PTCDA:CuPc composites. Note that the static inducement effect decreases rapidly with increasing NPB spacer thickness and almost vanishes when the NPB spacer thickness is greater than or equal to about 20 nm, as shown in Fig. 7(a). The working

CuPc PTCDA CuPc PTCDA , (1)

PTCDA:CuPc film .

respectively.

increasing NPB spacer thickness.

**Figure 6.** (a) XRD patterns and (b) absorption spectra for 100 nm CuPc, PTCDA, and 1:2 PTCDA:CuPc thin films deposited on quartz glasses.

The structural and optical properties of 1:2 PTCDA:CuPc composite have also been studied. As shown in Fig. 6(a), for the mixed PTCDA:CuPc thin film, there is a prominent peak present at 2 =6.8°, corresponding to diffraction from the (200) plane of the α-CuPc phase [18], and a broad peak present at 2 =21.0°, assigned to the reflection from the quartz substrate. This clearly demonstrates that some CuPc aggregates are crystalline, while almost all PTCDA aggregates are amorphous in the 1:2 PTCDA:CuPc film. Intermolecular hydrogen bonding is possibly formed between the outer ring of CuPc and carboxylic dianhydride of PTCDA [25]. Thus, during the codeposition, CuPc could be bonded to PTCDA and accordingly destroy PTCDA (102) aggregates, but PTCDA does not affect all of the CuPc (200) stacks, presumably ascribed to the smaller amount of PTCDA than CuPc. Fig. 6(b) displays the electronic absorption spectra of CuPc, PTCDA, and PTCDA:CuPc films. In the low-energy Q band of CuPc, as compared to CuPc film, PTCDA:CuPc film shows a decreased relative intensity of the CuPc dimeric peak at λ=629 nm to the CuPc monomeric peak at λ=693 nm, consistent with the earlier analyses of XRD data [25]. In the high-energy zone, there is a prominent absorption peak at λ=536 nm for PTCDA:CuPc film; the broad absorption feature at λ=539 nm for neat PTCDA film [26] is strongly suppressed in PTCDA:CuPc film .

150 Organic Light Emitting Devices

thin films deposited on quartz glasses.

**Figure 6.** (a) XRD patterns and (b) absorption spectra for 100 nm CuPc, PTCDA, and 1:2 PTCDA:CuPc

The structural and optical properties of 1:2 PTCDA:CuPc composite have also been studied. As shown in Fig. 6(a), for the mixed PTCDA:CuPc thin film, there is a prominent peak present at 2 =6.8°, corresponding to diffraction from the (200) plane of the α-CuPc phase [18], and a broad peak present at 2 =21.0°, assigned to the reflection from the quartz substrate. This clearly demonstrates that some CuPc aggregates are crystalline, while almost all PTCDA aggregates are amorphous in the 1:2 PTCDA:CuPc film. Intermolecular hydrogen bonding is possibly formed between the outer ring of CuPc and carboxylic dianhydride of PTCDA [25]. Thus, during the codeposition, CuPc could be bonded to PTCDA and accordingly destroy PTCDA (102) aggregates, but PTCDA does not affect all of the CuPc (200) stacks, presumably ascribed to the smaller amount of PTCDA than CuPc. Fig. 6(b) displays the electronic absorption spectra of CuPc, PTCDA, and PTCDA:CuPc films. In the low-energy Q band of CuPc, as compared to CuPc film, PTCDA:CuPc film shows a decreased relative intensity of the CuPc dimeric peak at λ=629 nm to the CuPc monomeric peak at λ=693 nm, consistent with the earlier analyses of XRD data [25]. In the high-energy zone, there is a prominent absorption peak at λ=536 nm for PTCDA:CuPc film; the broad Hole generation in the ITO/PTCDA:CuPc anode cannot be simply described as holes being directly injected from ITO to PTCDA:CuPc, as the hole injection barrier from ITO to PTCDA:CuPc is not reduced, as compared to that from ITO to CuPc [5]. In order to figure out the working mechanism of the ITO/PTCDA:CuPc hole injection structure, the dependence of the device performance on NPB spacer thickness was studied. As shown in Fig. 7(a), with increasing NPB spacer thickness, the device current decreases significantly; but when the NPB spacer thickness is greater than or equal to 20 nm, the devices exhibit nearly identical I−V characteristics. Fig. 7(b) shows that the device efficiency decreases with increasing NPB spacer thickness.

**Figure 7.** (a) I-V and (b) efficiency-current density curves for OLEDs with structure of ITO/ NPB spacer *x* nm/ PTCDA:CuPc 10 nm/ NPB 70-*x* nm/ Alq3 60 nm/ Mg:Ag, where NPB spacer thicknesses are 0 nm (open circles), 10 nm (filled circles), 20 nm (upward triangles), and 30 nm (downward triangles), respectively.

The variation of device current with NPB spacer thickness indicates that static-induced holeelectron pairs generation is likely to take place in PTCDA:CuPc composites, expressed as

$$\text{CuPcc} + \text{PTCDA} \xrightarrow{\text{Sl}} \text{CuPc}^+ + \text{PTCDA}^- \tag{1}$$

where SI stands for static inducement. One paired electron in the HOMO of one CuPc molecule can be transferred to the LUMO of one PTCDA molecule in proximity, favored by (i) the PTCDA/CuPc interface can effectively dissociate the photogenerated excitons into holes and electrons and prevent them from recombining [23] and (ii) there is intermolecular hydrogen bonding between PTCDA and CuPc, and consequently the overlap between the HOMO of CuPc and the LUMO of PTCDA becomes significant. Thus, via process (1) holes and electrons can be efficiently generated in the HOMO of CuPc and in the LUMO of PTCDA, respectively, in PTCDA:CuPc composites. Note that the static inducement effect decreases rapidly with increasing NPB spacer thickness and almost vanishes when the NPB spacer thickness is greater than or equal to about 20 nm, as shown in Fig. 7(a). The working

mechanism of the ITO/PTCDA:CuPc anode is described as follows: when the ITO is biased positive, holes confined at the ITO/PTCDA:CuPc interface may effectively polarize PTCDA:CuPc pairs, efficiently generating holes in CuPc and electrons in PTCDA, and then holes are transferred into the NPB layer by CuPc aggregates and electrons into the ITO by PTCDA aggregates, thus generating a very efficient hole current. The static-inducement for very efficient hole-electron pair generation in PTCDA:CuPc is the reason why the ITO/PTCDA:CuPc anode structure is superior to the ITO/CuPc anode structure. For the ITO/ PTCDA:CuPc anode structure, if the thickness of the PTCDA: CuPc composite (such as 5 nm) is smaller than the optimal thickness of 10–20 nm, the number of PTCDA:CuPc pairs involved in hole-electron pair generation is smaller, hence generating less efficient hole current and resulting in reduced device performance. If the thickness of the PTCDA:CuPc composite is larger (e.g. 30 nm) than the optimal thickness of 10–20 nm, there is no more enhancement of hole-electron pairs generation, but the absorption of PTCDA:CuPc composite over Alq3 emission becomes remarkable, causing the degradation in device performance.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 153

**3.2.1.1. Optical and structural properties of Li2CO3:PTCDA and Li2CO3:BCP** 

Li2CO3:BCP composite [29].

Al (cathode);

Al (cathode).

The optical properties of 1:2 Li2CO3:PTCDA and 1:4 Li2CO3:BCP are studied here and presented in Fig. 8. As seen in Fig. 8(a), compared to the intrinsic PTCDA, Li2CO3:PTCDA exhibits much lower intermolecular optical absorption at 558 nm [26], indicating severe distortion of the ordered - stack of PTCDA molecules caused by the doping with Li2CO3. In addition, a new sub-gap absorption centered at a wavelength of 686 nm appears, indicating the formation of the charge transfer state between the matrix and dopant in Li2CO3:PTCDA composite [28]. Fig. 8(b) shows that the edge of the optical absorption for 1:4 Li2CO3:BCP was slightly blue-shifted relative to that of intrinsic BCP, which implies the occurrence of electron transfer from the O2- bonded to Li to BCP 1:4

**Figure 8.** UV-vis absorption spectra for the 20 nm PTCDA and 1:2 Li2CO3:PTCDA thin films (a), and the 20 nm BCP and 1:2 Li2CO3:PTCDA thin films (b). The inset in (b) is the molecular structure of PTCDA.

For studying the electrical properties we have fabricated the following two devices: Device 6: ITO (anode)/ 1:4 Li2CO3:BCP 10 nm/ 1:2 Li2CO3:PTCDA 5 nm/ Alq3 65 nm/

Device 7: ITO (anode)/ 1:4 Li2CO3:BCP 5 nm/ 1:2 Li2CO3:PTCDA 10 nm/ Alq3 65 nm/

In device 6 with the structure of ITO (anode)/1:4 Li2CO3:BCP *x* nm/1:2 Li2CO3:PTCDA 15-*x* nm/Alq3 65 nm/Al (cathode), the electron current passes through Alq3 to 1:2 Li2CO3:PTCDA and to 1:4 Li2CO3:BCP connected in series. Therefore, the relative conductivity of 1:2 Li2CO3:PTCDA to 1:4 Li2CO3:BCP can be detected by observing the influence of 1:4 Li2CO3:BCP thickness on the device current. Fig. 9 exhibits that the current of device 6 is much lower than that of device 7 at a given driving voltage, demonstrating that 1:2 Li2CO3:PTCDA is much more conductive than 1:4 Li2CO3:BCP. This can be due to the following two factors: firstly, the charge carriers are more efficiently generated in 1:2 Li2CO3:PTCDA than in 1:4 Li2CO3:BCP due to the lower LUMO level of PTCDA than that of BCP. Secondly, the electron mobility of intrinsic PTCDA (310-6 cm2·V-1·s-1) is much higher

**3.2.1.2. Electrical properties of Li2CO3:PTCDA and Li2CO3:BCP** 
