**3.2.2.1. Conductivity of MoO3 and Li2CO3:BCP**

For comparing the conductivity due to the incorporation of MoO3 and Li2CO3:BCP in OLEDs, We have fabricated the following two devices:

Device 11: ITO (cathode)/ MoO3 10 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 80 nm/ Al (anode); Device 12: ITO (cathode)/ MoO3 5 nm/ 1:4 Li2CO3:BCP 10 nm/ Alq3 80 nm/ Al (anode).

The conductivity of an organic thin film can be measured via a vertical sandwiched structure. However, the diffusion of the top metal electrode into organic thin film enables the chemical reaction between metal atoms and organic molecules, leading to the unavoidable alteration for the intrinsic property of organic thin film. Thus, devices 11, 12 are fabricated to qualitatively compare the conductivities of MoO3 and 1:4 Li2CO3:BCP. In the electron-only device with structure of ITO (cathode)/ MoO3 *x* nm/ 1:4 Li2CO3:BCP 15-*x* nm/ Alq3 80 nm/ Al (anode), the electrons are firstly injected into MoO3, then transported through MoO3, 1:4 Li2CO3:BCP, and Alq3 in sequence, and finally transfered into Al. Thus, the study of the I-V characteristics with the varying thickness of MoO3 can be studied from devices 11 and 12 as shown in Fig. 13. As seen in Fig. 13, the current density of device 11 is greater than that of device 12 in the whole applied voltage range. This clearly indicates that MoO3 (10 nm thick) is more conducting than 1:4 Li2CO3:BCP (10 nm thick) composite. Thus, MoO3 can be expected to act as a better electron transport layer in the IBOLED

**Figure 13.** The I-V characteristics of devices 11 (open circles) and 12 (fill squares). No Alq3 electroluminescence is detected in the measurement range for these two devices.

#### **3.2.2.2. Inverted OLEDs using the combination of MoO3 and Li2CO3:BCP**

We have fabricated and studied the characteristics of the following IBOLEDs: Device 13: ITO/ MoO3 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 40 nm/ NPB 60 nm/ MoO3 10 nm/Al; Device 14: ITO/ 1:4 Li2CO3:BCP 10 nm/ Alq3 40 nm/ NPB 60 nm/ MoO3 10 nm/Al; Device 15: ITO/ MoO3 5 nm/ BCP 5 nm/ Alq3 40 nm/ NPB 60 nm/ MoO3 10 nm/Al;

Device 16: ITO/ MoO3 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 50 nm/ NPB 60 nm/ MoO3 10 nm/Al; Device 17: ITO/ MoO3 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 60 nm/ NPB 60 nm/ MoO3 10 nm/Al; Device 18: ITO/ MoO3 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 70 nm/ NPB 60 nm/ MoO3 10 nm/Al;

158 Organic Light Emitting Devices

**3.2.2.1. Conductivity of MoO3 and Li2CO3:BCP** 

OLEDs, We have fabricated the following two devices:

injection.

that MoO3 in association with an n-doped layer of BCP can enable the efficient electron

For comparing the conductivity due to the incorporation of MoO3 and Li2CO3:BCP in

The conductivity of an organic thin film can be measured via a vertical sandwiched structure. However, the diffusion of the top metal electrode into organic thin film enables the chemical reaction between metal atoms and organic molecules, leading to the unavoidable alteration for the intrinsic property of organic thin film. Thus, devices 11, 12 are fabricated to qualitatively compare the conductivities of MoO3 and 1:4 Li2CO3:BCP. In the electron-only device with structure of ITO (cathode)/ MoO3 *x* nm/ 1:4 Li2CO3:BCP 15-*x* nm/ Alq3 80 nm/ Al (anode), the electrons are firstly injected into MoO3, then transported through MoO3, 1:4 Li2CO3:BCP, and Alq3 in sequence, and finally transfered into Al. Thus, the study of the I-V characteristics with the varying thickness of MoO3 can be studied from devices 11 and 12 as shown in Fig. 13. As seen in Fig. 13, the current density of device 11 is greater than that of device 12 in the whole applied voltage range. This clearly indicates that MoO3 (10 nm thick) is more conducting than 1:4 Li2CO3:BCP (10 nm thick) composite. Thus,

Device 11: ITO (cathode)/ MoO3 10 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 80 nm/ Al (anode); Device 12: ITO (cathode)/ MoO3 5 nm/ 1:4 Li2CO3:BCP 10 nm/ Alq3 80 nm/ Al (anode).

MoO3 can be expected to act as a better electron transport layer in the IBOLED

**Figure 13.** The I-V characteristics of devices 11 (open circles) and 12 (fill squares). No Alq3

electroluminescence is detected in the measurement range for these two devices.

**3.2.2.2. Inverted OLEDs using the combination of MoO3 and Li2CO3:BCP** 

We have fabricated and studied the characteristics of the following IBOLEDs:

Device 14: ITO/ 1:4 Li2CO3:BCP 10 nm/ Alq3 40 nm/ NPB 60 nm/ MoO3 10 nm/Al; Device 15: ITO/ MoO3 5 nm/ BCP 5 nm/ Alq3 40 nm/ NPB 60 nm/ MoO3 10 nm/Al;

Device 13: ITO/ MoO3 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 40 nm/ NPB 60 nm/ MoO3 10 nm/Al;

Fig. 14 shows the performances of devices 13 and 14. Device 13 shows nearly the same I-V characteristics as device 14, but the luminance and current efficiency of device 13 are higher than those of device 14. At a luminance of 100 cd/m2, device 13 achieves a driving voltage of 7.51 V and a current efficiency of 1.39 cd/A, in comparison with 7.72 V and 1.08 cd/A, respectively, reached in device 14. At a luminance of 1000 cd/m2, device 13 exhibits a driving voltage of 8.86 V and a current efficiency of 2.0 cd/A, in comparison with 9.04 V and 1.62 cd/A obtained in device 14. The improved performance of device 13 over device 14 may be attributed to the reduced dopant diffusion from Li2CO3:BCP into Alq3 as a result of the thinner Li2CO3:BCP in device 13.

**Figure 14.** (a) I-V and L-V characteristics and (b) current efficiency versus current density characteristics for devices 13 (circles) and 14 (squares).

#### **3.2.2.3. Mechanism of operation in the combination of MoO3 and Li2CO3:BCP**

Despite that MoO3 exhibits better electron conduction than 1:4 Li2CO3:BCP composite, device 13 produces nearly the identical electron current as device 14, demonstrating that there must be some voltage drop across the MoO3/1:4 Li2CO3:BCP interface in device 13. In an effort to investigate the mechanism of the electron transfer from MoO3 to 1:4 Li2CO3:BCP, the characteristics of device 15 is studied as presented in Fig. 15, which shows that device 15 exhibits poor I-V characteristics with no Alq3 electroluminescence observed, suggesting there is a big Schottky barrier for electron injection from MoO3 to BCP. The marked performance difference between devices 13 and 15 indicates that the LUMO-LUMO offset at the MoO3/BCP interface can be remarkably lowered due to doping Li2CO3 into BCP, which can be explained by the interfacial dipole calculated by Eq. (2). In the case of the MoO3/BCP heterojunction, taking MoO320 and BCP1.4 [30], the item of (1/1+1/2) is estimated to be 0.76. Considering that the CNL (work function) of MoO3 is -6.9 eV [33] and that of BCP-3.8 eV, Eq. (2) yields an interface dipole 1=-1.92 eV. This results in a 1.92 eV downward shift of the vacuum level on the BCP side. Thus, the Schottky barrier for the electron injection from the LUMO of MoO3 to that of BCP is roughly 1.78 eV, determining that the electron transport through the MoO3/BCP interface is very inefficient in the case of device 15. For the MoO3/1:4 Li2CO3:BCP heterojunction, the item of (1/1+1/2) is assumed to be 0.1, because the low-frequency dielectric constant of the doped BCP is larger compared to the undoped one [30]. Provided that the CNL of MoO3-6.9 eV and that of n-doped BCP-3.2 eV [20], Eq. (2) yields an interface dipole 2=-3.52 eV, leading to a 3.52 eV downwards shift of the vacuum level on the doped BCP side. Thus, the electron injection barrier at the MoO3/1:4 Li2CO3:BCP heterojunction is estimated to be roughly 0.18 eV, favoring the efficient electron injection in MoO3 and hence better device performance (the case of device 13). In addition, the intrinsic Schottky barrier for electron injection from Li2CO3:BCP to Alq3 is estimated to be 0.4 eV according to Eq. (2).

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 161

calculated to be 28.1 (MV/cm)-1/2, approximately one and half times larger than the value of RS=11.6 (MV/cm) -1/2, implied by the RS formula of RS = (e3/4π0r)1/2/kBT (taking <sup>r</sup> 1.6). This discrepancy is consistent with the other reported observations [34]. It should be stressed here that the electric field at the Li2CO3:BCP/Alq3 interface, responsible for electron injection, may be different to the average electric field in the Alq3 layer. Interestingly, when I exceeds 30 mA/cm2, the total device current versus the average electric field in Alq3 diverts from the RS behavior due to the deteriorated linear fit (R2=0.989) to the log I versus F1/2av, Alq3 plot. This diversion is likely to be attributed to the fact that the conclusive electron injection process shifts from the Li2CO3:BCP/Alq3 interface to the MoO3/1:4 Li2CO3:BCP heterojunction as a

**Figure 16.** (a) The I-V characteristics of devices 13 (solid squares), 16 (solid circles), 17 (solid triangles), and 18 (open squares). (b) The total voltage drop across device versus the Alq3 thickness at various current densities (I in mA/cm2). The dashed lines represent the linear fits to the plots, and their slopes denote the the average internal electric fields in the Alq3 layer (Fav, Alq3 in MV/cm). R2 represents the linear-fit correlation coefficient. The inset (a) shows the L-V characteristics for devices 13, 16, 17 and 18.

**Figure 17.** The average internal electric fields in the Alq3 layer (circles, bottom X + left Y) and the dependence of of the total current density on the average internal electric field (squares, top X + right Y)

in device 13. The two straight lines with correlation coefficients (R2) mean the linear fits.

result of the efficient Li2CO3 diffusion towards the anode in device 13.

**Figure 15.** The I-V characteristics of device 15. There was no Aq3 electroluminescence detected in the measurement range, suggesting that the electron current was very inefficient in device 15.

#### **3.2.2.4. The dopant diffusion in the Li2CO3:BCP**

Fig. 16(a) shows the I-V characteristics of devices 13, 16, 17 and18 and Fig. 16(b) presents the plot of the voltage drop as a function of the thickness of Alq3 film. The inset of Fig. 16(a) displays the luminance properties of these devices. Fig. 16(b) indicates that the driving voltage across the device varies linearly with the Alq3 thickness when the device current (I) increases from 2 to 100 mA/cm2. Thus, the average internal electric field in the Alq3 layer (Fav, Alq3) of device 13 is calculated [34, 35] as shown in Fig. 17. It can be seen in Fig. 17 that Fav, Alq3 increases monotonously as I increases from 2 to 30 mA/cm2, and becomes gradually saturated when I 30 mA/cm2. The turning point at I = 30 mA/cm2 of the Fav, Alq3 versus I characteristics can be due to the efficient diffusion of Li2CO3 from Li2CO3:BCP into Alq3, which can not only lead to some exciton-quenching effect in the emissive layer, coincident with the observations in Fig. 14(b), but also it can leave the emissive layer n-doped, thereby reducing the interfacial energy barrier [13] and making Alq3 more conductive. Also seen in Fig. 17 is that when I varies between 2 and 30 mA/cm2, the total device current versus the average electric field in Alq3 appears to follow a Richardson–Schottky (RS) behavior, I exp [(Fav,Alq3)1/2], due to the good linear fit (R2=0.997) to the log I versus F1/2av, Alq3 plot. Accordingly, the coefficient is calculated to be 28.1 (MV/cm)-1/2, approximately one and half times larger than the value of RS=11.6 (MV/cm) -1/2, implied by the RS formula of RS = (e3/4π0r)1/2/kBT (taking <sup>r</sup> 1.6). This discrepancy is consistent with the other reported observations [34]. It should be stressed here that the electric field at the Li2CO3:BCP/Alq3 interface, responsible for electron injection, may be different to the average electric field in the Alq3 layer. Interestingly, when I exceeds 30 mA/cm2, the total device current versus the average electric field in Alq3 diverts from the RS behavior due to the deteriorated linear fit (R2=0.989) to the log I versus F1/2av, Alq3 plot. This diversion is likely to be attributed to the fact that the conclusive electron injection process shifts from the Li2CO3:BCP/Alq3 interface to the MoO3/1:4 Li2CO3:BCP heterojunction as a result of the efficient Li2CO3 diffusion towards the anode in device 13.

160 Organic Light Emitting Devices

that of BCP is roughly 1.78 eV, determining that the electron transport through the MoO3/BCP interface is very inefficient in the case of device 15. For the MoO3/1:4 Li2CO3:BCP heterojunction, the item of (1/1+1/2) is assumed to be 0.1, because the low-frequency dielectric constant of the doped BCP is larger compared to the undoped one [30]. Provided that the CNL of MoO3-6.9 eV and that of n-doped BCP-3.2 eV [20], Eq. (2) yields an interface dipole 2=-3.52 eV, leading to a 3.52 eV downwards shift of the vacuum level on the doped BCP side. Thus, the electron injection barrier at the MoO3/1:4 Li2CO3:BCP heterojunction is estimated to be roughly 0.18 eV, favoring the efficient electron injection in MoO3 and hence better device performance (the case of device 13). In addition, the intrinsic Schottky barrier for electron injection from Li2CO3:BCP to Alq3 is estimated to be 0.4 eV according to Eq. (2).

**Figure 15.** The I-V characteristics of device 15. There was no Aq3 electroluminescence detected in the

Fig. 16(a) shows the I-V characteristics of devices 13, 16, 17 and18 and Fig. 16(b) presents the plot of the voltage drop as a function of the thickness of Alq3 film. The inset of Fig. 16(a) displays the luminance properties of these devices. Fig. 16(b) indicates that the driving voltage across the device varies linearly with the Alq3 thickness when the device current (I) increases from 2 to 100 mA/cm2. Thus, the average internal electric field in the Alq3 layer (Fav, Alq3) of device 13 is calculated [34, 35] as shown in Fig. 17. It can be seen in Fig. 17 that Fav, Alq3 increases monotonously as I increases from 2 to 30 mA/cm2, and becomes gradually saturated when I 30 mA/cm2. The turning point at I = 30 mA/cm2 of the Fav, Alq3 versus I characteristics can be due to the efficient diffusion of Li2CO3 from Li2CO3:BCP into Alq3, which can not only lead to some exciton-quenching effect in the emissive layer, coincident with the observations in Fig. 14(b), but also it can leave the emissive layer n-doped, thereby reducing the interfacial energy barrier [13] and making Alq3 more conductive. Also seen in Fig. 17 is that when I varies between 2 and 30 mA/cm2, the total device current versus the average electric field in Alq3 appears to follow a Richardson–Schottky (RS) behavior, I exp [(Fav,Alq3)1/2], due to the good linear fit (R2=0.997) to the log I versus F1/2av, Alq3 plot. Accordingly, the coefficient is

measurement range, suggesting that the electron current was very inefficient in device 15.

**3.2.2.4. The dopant diffusion in the Li2CO3:BCP** 

**Figure 16.** (a) The I-V characteristics of devices 13 (solid squares), 16 (solid circles), 17 (solid triangles), and 18 (open squares). (b) The total voltage drop across device versus the Alq3 thickness at various current densities (I in mA/cm2). The dashed lines represent the linear fits to the plots, and their slopes denote the the average internal electric fields in the Alq3 layer (Fav, Alq3 in MV/cm). R2 represents the linear-fit correlation coefficient. The inset (a) shows the L-V characteristics for devices 13, 16, 17 and 18.

**Figure 17.** The average internal electric fields in the Alq3 layer (circles, bottom X + left Y) and the dependence of of the total current density on the average internal electric field (squares, top X + right Y) in device 13. The two straight lines with correlation coefficients (R2) mean the linear fits.
