*3.2.1. Inverted OLEDs incorporated with the combination of Li2CO3:PTCDA and Li2CO3:BCP*

Here first we will describe the optical and structural properties of n-doped materials and then the prperties of OLEDs fabricated from these materials.

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

152 Organic Light Emitting Devices

performance.

of acceptor.

*Li2CO3:BCP* 

IBOLEDs as described below.

**layers** 

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

*3.1.3. The significance and future development of the donor/ acceptor interface in OLEDs* 

Very recently, the planar and mixed donor/acceptor interfaces have also been used as very efficient interconnecting structures for tandem OLEDs [27], showing the versatility of the donor/acceptor pairs. However, it should be noted that all the donors and acceptors currently used to form the hole-generating interfaces in OLEDs exhibit significant absorption in the visible-light range, thus reducing the light out-coupling from the devices. Therefore, it is of great importance to develop wide-bandgap donors and acceptors. In addition, in order to increase the amplitude of hole current at the donor/acceptor interfaces, it is of much necessity to narrow down the offset between the HOMO of donor and LUMO

**3.2. Increasing electron current in IBOLEDs via the combination of two n-doped** 

The n-doped organic electron acceptors, e.g., n-NTCDA, n-PTCDA, n-C60, possess markedly higher conductivities but markedly lower capabilities of injecting electrons into electron transport layer (such as BCP, Alq3, etc.), as compared to the frequently used n-doped materials (such as n-BCP, n-Alq3, etc.) in OLEDs. However, recent work [13-16] shows that the combination of the above two classes of n-doped materals may increase current in

Here first we will describe the optical and structural properties of n-doped materials and

*3.2.1. Inverted OLEDs incorporated with the combination of Li2CO3:PTCDA and* 

then the prperties of OLEDs fabricated from these materials.

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 Li2CO3:BCP composite [29].

**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.

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

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

Al (cathode).

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 than that of BCP (610-7 cm2·V-1·s-1). Therefore, it might be feasible to improve the performance of IBOLED via the introduction of the Li2CO3:PTCDA composite onto the ITO cathode for enhancing current conduction.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 155

**Figure 10.** (a) I-V characteristics, (b) L-V characteristics, (c) current and power efficiencies versus current density of devices 8 (filled squares) and 9 (filled triangles). Both the devices give nearly the

Figs. 10 (a) and (b) show that device 8 performs better that device 9, which may be attributed to the enhanced electron current due to the following : (i) In device 8, the electron injection is realized not only via tunneling, but also via thermionic emission due to relatively smaller energy barrier from ITO into the LUMO of PTCDA. (ii) The conductivity of 1:2 Li2CO3:PTCDA is higher than that of 1:4 Li2CO3:BCP, leading to reduced ohmic loss of

same Alq3 emission.

**Figure 9.** I-V characteristics for devices 6 and 7. As there is no Alq3 emission observed from the two devices in the measurement range, the current in both the two devices is considered to be comprised of electrons only.

#### **3.2.1.3. Inverted OLED using the combination of Li2CO3:PTCDA and Li2CO3:BCP**

We have fabricated the following three inverted devices and studied their characteristics.

Device 8: ITO (cathode)/ 1:2 Li2CO3:PTCDA 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 40 nm/ NPB 40 nm/ MoO3 10 nm/Al (anode);

Device 9: ITO (cathode)/ 1:4 Li2CO3:BCP 10 nm/ Alq3 40 nm/

NPB 40 nm/ MoO3 10 nm/Al (anode);

Device 10: ITO (cathode)/ 1:2 Li2CO3:PTCDA 5 nm/ BCP 5 nm/ Alq3 40 nm/ NPB 40 nm/ MoO3 10 nm/Al (anode).

Fig.10 shows the performance of devices 8 and 9. It can be seen in Fig. 10(a) that the driving voltage of device 8 for achieving a given current was reduced than that of device 9. To generate a current density of 100 mA/cm2, device 8 needs a driving voltage of 7.1 V, smaller than that needed by device 9 (8.3 V). Device 8 also gives higher luminance than device 9 as shown in Fig. 10(b). At a driving voltage of 8 V, the luminance of device 8 is 6983 cd/m2, in contrast to that of 1354 cd/m2 for device 9. Fig. 10(c) shows that the current efficiency of device 8 is slightly lower than that of device 9, mostly because 1:2 Li2CO3:PTCDA exhibits some absorption over Alq3 emission, while 1:4 Li2CO3:BCP is nearly transparent in the visible-light range. Nevertheless, due to the marked reduction of driving voltage, device 8 provides higher power efficiency than device 9. It can be concluded that the two-layer combination of 1:2 Li2CO3:PTCDA and 1:4 Li2CO3:BCP outperforms the single 1:4 Li2CO3:BCP with regard to promoting the current conduction for the IBOLED.

electrons only.

characteristics.

for the IBOLED.

NPB 40 nm/ MoO3 10 nm/Al (anode);

NPB 40 nm/ MoO3 10 nm/Al (anode);

NPB 40 nm/ MoO3 10 nm/Al (anode).

Device 9: ITO (cathode)/ 1:4 Li2CO3:BCP 10 nm/ Alq3 40 nm/

cathode for enhancing current conduction.

than that of BCP (610-7 cm2·V-1·s-1). Therefore, it might be feasible to improve the performance of IBOLED via the introduction of the Li2CO3:PTCDA composite onto the ITO

**Figure 9.** I-V characteristics for devices 6 and 7. As there is no Alq3 emission observed from the two devices in the measurement range, the current in both the two devices is considered to be comprised of

We have fabricated the following three inverted devices and studied their

Fig.10 shows the performance of devices 8 and 9. It can be seen in Fig. 10(a) that the driving voltage of device 8 for achieving a given current was reduced than that of device 9. To generate a current density of 100 mA/cm2, device 8 needs a driving voltage of 7.1 V, smaller than that needed by device 9 (8.3 V). Device 8 also gives higher luminance than device 9 as shown in Fig. 10(b). At a driving voltage of 8 V, the luminance of device 8 is 6983 cd/m2, in contrast to that of 1354 cd/m2 for device 9. Fig. 10(c) shows that the current efficiency of device 8 is slightly lower than that of device 9, mostly because 1:2 Li2CO3:PTCDA exhibits some absorption over Alq3 emission, while 1:4 Li2CO3:BCP is nearly transparent in the visible-light range. Nevertheless, due to the marked reduction of driving voltage, device 8 provides higher power efficiency than device 9. It can be concluded that the two-layer combination of 1:2 Li2CO3:PTCDA and 1:4 Li2CO3:BCP outperforms the single 1:4 Li2CO3:BCP with regard to promoting the current conduction

**3.2.1.3. Inverted OLED using the combination of Li2CO3:PTCDA and Li2CO3:BCP** 

Device 8: ITO (cathode)/ 1:2 Li2CO3:PTCDA 5 nm/ 1:4 Li2CO3:BCP 5 nm/ Alq3 40 nm/

Device 10: ITO (cathode)/ 1:2 Li2CO3:PTCDA 5 nm/ BCP 5 nm/ Alq3 40 nm/

**Figure 10.** (a) I-V characteristics, (b) L-V characteristics, (c) current and power efficiencies versus current density of devices 8 (filled squares) and 9 (filled triangles). Both the devices give nearly the same Alq3 emission.

Figs. 10 (a) and (b) show that device 8 performs better that device 9, which may be attributed to the enhanced electron current due to the following : (i) In device 8, the electron injection is realized not only via tunneling, but also via thermionic emission due to relatively smaller energy barrier from ITO into the LUMO of PTCDA. (ii) The conductivity of 1:2 Li2CO3:PTCDA is higher than that of 1:4 Li2CO3:BCP, leading to reduced ohmic loss of current conduction. Fig. 10(c) shows that the current efficiency of device 8 is slightly lower than that of device 9, mostly because 1:2 Li2CO3:PTCDA exhibits some absorption over Alq3 emission, while 1:4 Li2CO3:BCP is nearly transparent in the visible-light range. Nevertheless, due to the marked reduction of driving voltage, device 8 provides higher power efficiency than device 9. In terms of device 8, the issue of the efficient electron transfer from 1:2 Li2CO3:PTCDA into 1:4 Li2CO3:BCP needs to be addressed, since the LUMO level (4.6 eV) of PTCDA is 1.6 eV lower than that (3.0 eV) of BCP. In quest of the mechanism of electron transport over such a big Schottky barrier, device 10 is fabricated and characterized. As seen in Fig. 11, device 10 shows poor current-voltage and luminancevoltage characteristics. At a driving voltage of 10 V, it exhibits a current of 2.3 mA/cm2 and a luminance of 4.1 cd/m2, in clear contrary to 531 mA/cm2 and 10492 cd/m2 achieved at a driving voltage of 8.4 V in device 8. This greatly improved performance of device 8 relative to device 10 implies that the energy barrier for electron injection from PTCDA into BCP can be significantly reduced by fulfilling the double-sided n-doping to the Schottky interface.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 157

dipole 1=-0.65 eV, resulting a 0.65 eV downwards shift of the vacuum level (VL) on the BCP side LUMO as shown in Fig. 12(i). Thus, the electron injection barrier from the LUMO of 1:2 Li2CO3:PTCDA to hat of BCP reduces roughly to 0.95 eV. This shows that the electron transport through the 1:2 Li2CO3:PTCDA/BCP interface can be small in device 10. For the 1:2 Li2CO3:PTCDA/1:4 Li2CO3:BCP heterojunction, the term (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 the CNL for n-doped PTCDA-4.8 eV and the CNL for ndoped BCP3.2 eV [31], Eq. (2) yields an interface dipole 2= -1.44 eV, leading to a 1.44 eV downwards shift of the vacuum level (VL) in the doped BCP side, as shown in Fig. 12(ii). Accordingly, the electron injection barrier at the 1:2 Li2CO3:PTCDA/1:4 Li2CO3:BCP heterojunction is estimated to be 0.16 eV, suggesting that the double n-doping can induce significant realignment of molecular levels of organic-organic heterojunction. Thus, the contact becomes ohmic, resulting in efficient electron current and increasing device

**Figure 12.** Schematic diagram for depicting the electronic structures of the interfaces of 1:2

*3.2.2. Inverted OLEDs incorporating the combination of MoO3 and Li2CO3:BCP* 

Li2CO3:PTCDA/BCP (i) and 1:2 Li2CO3:PTCDA/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.

There has been some controversy about the intrinsic property of MoO3, which has been intensively investigated in OLEDs for improving the hole injection. It is considered by some groups that MoO3 should act as a hole conductor with highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of 5.4 eV and 2.3 eV, respectively [32]. Recently, however, it is suggested [33] that due to being doped by oxygen vacancy defects MoO3 intrinsically offers n-typed conduction with deep-lying HOMO and LUMO levels of 9.7 eV and 6.7 eV, respectively. The enhanced hole injection by the MoO3 intervention may be attributed to the process that one electron in the HOMO level of organic hole-transporting material can be easily transferred into the LUMO level of MoO3 because the Fermi level of MoO3 is necessarily pinned close to the HOMO level of p-typed organic conductor [33]. Here, we investigate MoO3 as electron injection layer in IBOLEDs and show

performance (the case of device 8).

**Figure 11.** The I-V and L-V characteristics for device 10.

#### **3.2.1.4. The working mechanism of the combination of Li2CO3:PTCDA and Li2CO3:BCP**

The electronic structures of the interfaces of 1:2 Li2CO3:PTCDA/BCP and 1:2 Li2CO3:PTCDA/1:4 Li2CO3:BCP are schematically shown in Fig. 5. The LUMO-LUMO offset at organic heterojunction can be varied by the interfacial dipole given by [30]:

$$
\Delta = \left(1 - \frac{1}{2}(\frac{1}{\varepsilon\_1} + \frac{1}{\varepsilon\_2})\right) \text{(CNL}\_1 - \text{CNL}\_2\text{)}\_{initial} \text{ \textdegree \tag{2}
$$

where 1 and 2 are the low frequency dielectric constants for the two organic materials, CNL1 and CNL2 represent their charge neutrality levels. In the case of 1:2 Li2CO3:PTCDA/BCP heterojunction, considering that the dielectric constant of n-doped PTCDA is larger compared to that of the undoped PTCDA (1.9) and taking BCP1.4, (1/1+1/2) is estimated to be 0.7. Provided that the CNL for n-doped PTCDA is roughly equal to that of undoped PTCDA(-4.8 eV) due to the Fermi level pinning [22] and CNL of BCP-3.8 eV, Eq. (2) yields an interface dipole 1=-0.65 eV, resulting a 0.65 eV downwards shift of the vacuum level (VL) on the BCP side LUMO as shown in Fig. 12(i). Thus, the electron injection barrier from the LUMO of 1:2 Li2CO3:PTCDA to hat of BCP reduces roughly to 0.95 eV. This shows that the electron transport through the 1:2 Li2CO3:PTCDA/BCP interface can be small in device 10. For the 1:2 Li2CO3:PTCDA/1:4 Li2CO3:BCP heterojunction, the term (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 the CNL for n-doped PTCDA-4.8 eV and the CNL for ndoped BCP3.2 eV [31], Eq. (2) yields an interface dipole 2= -1.44 eV, leading to a 1.44 eV downwards shift of the vacuum level (VL) in the doped BCP side, as shown in Fig. 12(ii). Accordingly, the electron injection barrier at the 1:2 Li2CO3:PTCDA/1:4 Li2CO3:BCP heterojunction is estimated to be 0.16 eV, suggesting that the double n-doping can induce significant realignment of molecular levels of organic-organic heterojunction. Thus, the contact becomes ohmic, resulting in efficient electron current and increasing device performance (the case of device 8).

156 Organic Light Emitting Devices

**Figure 11.** The I-V and L-V characteristics for device 10.

current conduction. Fig. 10(c) shows that the current efficiency of device 8 is slightly lower than that of device 9, mostly because 1:2 Li2CO3:PTCDA exhibits some absorption over Alq3 emission, while 1:4 Li2CO3:BCP is nearly transparent in the visible-light range. Nevertheless, due to the marked reduction of driving voltage, device 8 provides higher power efficiency than device 9. In terms of device 8, the issue of the efficient electron transfer from 1:2 Li2CO3:PTCDA into 1:4 Li2CO3:BCP needs to be addressed, since the LUMO level (4.6 eV) of PTCDA is 1.6 eV lower than that (3.0 eV) of BCP. In quest of the mechanism of electron transport over such a big Schottky barrier, device 10 is fabricated and characterized. As seen in Fig. 11, device 10 shows poor current-voltage and luminancevoltage characteristics. At a driving voltage of 10 V, it exhibits a current of 2.3 mA/cm2 and a luminance of 4.1 cd/m2, in clear contrary to 531 mA/cm2 and 10492 cd/m2 achieved at a driving voltage of 8.4 V in device 8. This greatly improved performance of device 8 relative to device 10 implies that the energy barrier for electron injection from PTCDA into BCP can be significantly reduced by fulfilling the double-sided n-doping to the Schottky interface.

**3.2.1.4. The working mechanism of the combination of Li2CO3:PTCDA and Li2CO3:BCP** 

at organic heterojunction can be varied by the interfacial dipole given by [30]:

1 2

 

 

The electronic structures of the interfaces of 1:2 Li2CO3:PTCDA/BCP and 1:2 Li2CO3:PTCDA/1:4 Li2CO3:BCP are schematically shown in Fig. 5. The LUMO-LUMO offset

11 1 1 ( )( ) <sup>2</sup>

where 1 and 2 are the low frequency dielectric constants for the two organic materials, CNL1 and CNL2 represent their charge neutrality levels. In the case of 1:2 Li2CO3:PTCDA/BCP heterojunction, considering that the dielectric constant of n-doped PTCDA is larger compared to that of the undoped PTCDA (1.9) and taking BCP1.4, (1/1+1/2) is estimated to be 0.7. Provided that the CNL for n-doped PTCDA is roughly equal to that of undoped PTCDA(-4.8 eV) due to the Fermi level pinning [22] and CNL of BCP-3.8 eV, Eq. (2) yields an interface

1 2

*CNL CNL initial*

, (2)

**Figure 12.** Schematic diagram for depicting the electronic structures of the interfaces of 1:2 Li2CO3:PTCDA/BCP (i) and 1:2 Li2CO3:PTCDA/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.

#### *3.2.2. Inverted OLEDs incorporating the combination of MoO3 and Li2CO3:BCP*

There has been some controversy about the intrinsic property of MoO3, which has been intensively investigated in OLEDs for improving the hole injection. It is considered by some groups that MoO3 should act as a hole conductor with highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of 5.4 eV and 2.3 eV, respectively [32]. Recently, however, it is suggested [33] that due to being doped by oxygen vacancy defects MoO3 intrinsically offers n-typed conduction with deep-lying HOMO and LUMO levels of 9.7 eV and 6.7 eV, respectively. The enhanced hole injection by the MoO3 intervention may be attributed to the process that one electron in the HOMO level of organic hole-transporting material can be easily transferred into the LUMO level of MoO3 because the Fermi level of MoO3 is necessarily pinned close to the HOMO level of p-typed organic conductor [33]. Here, we investigate MoO3 as electron injection layer in IBOLEDs and show that MoO3 in association with an n-doped layer of BCP can enable the efficient electron injection.

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 159

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; 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

**Figure 14.** (a) I-V and L-V characteristics and (b) current efficiency versus current density

**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

characteristics for devices 13 (circles) and 14 (squares).

thinner Li2CO3:BCP in device 13.
