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organic contact
Organic Electronics 11 (2010) 89–94
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Role of air exposure in the improvement of injection efficiency of transition metal oxide/organic contact C.H. Cheung a, W.J. Song b, S.K. So a,* a
Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China Devision of Functional Materials and Nano Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 16 June 2009 Received in revised form 10 September 2009 Accepted 6 October 2009 Available online 13 October 2009 Keywords: Transition metal oxides, Charge injection Organic semiconductor Injection efficiency
a b s t r a c t Oxygen or air exposure to transition metal oxides (TMOs) was demonstrated to be essential in improving the hole injection (HI) efficiency at the contact formed by TMOs and small organic hole transporter. Current–voltage (J–V) and dark-injection space-charge-limited current (DI-SCLC) techniques were used to cross-examine the TMO/organic contacts. The hole transporter under investigation was N,N0 -diphenyl-N,N0 -bis(1-naphthyl)(1,10 biphenyl)-4,40 diamine (NPB). The improvement was attributed to the reduction in the energy barrier at TMO/NPB interface, which was a consequence of the work function enhancement of TMO by the oxidation of oxygen in air. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic semiconductors have received tremendous attention for optoelectronic applications in recent decades with various advantages such as low cost processing and structural flexibility [1,2]. Owing to the low intrinsic carrier concentration in organic semiconductors, external charge injection from a contact electrode is necessary for making the semiconductors electrically conducting. As a matter of fact, efficient carrier injection at electrode/organic contact is critically important in ‘turning on’ organic optoelectronic devices and thus realizing devices with high efficiency [3]. For hole injection, ideally, if interface dipole is negligible, the hole injection efficiency is mainly determined by the energy barrier between the work function of anode and the highest occupied molecular orbital (HOMO) level of organic hole transporting layer (HTL) [4]. A smaller barrier should result in a better efficiency. * Corresponding author. E-mail address: [email protected] (S.K. So). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.10.003
The most efficient injection contact, i.e. an Ohmic contact, appears in the absence of an injection barrier. Indium tin oxide (ITO) is the preferred anode for organic optoelectronic devices due to its optical transparency in visible spectrum. However, with a work function of 4.8 eV, a finite energy barrier always exists at ITO/HTL interface as most HT materials possess high-lying HOMO levels (>5.4 eV). To date, the most commonly used method to minimize the ITO/organic barrier is to insert a hole injection layer (HIL) between ITO and HTL [3,5]. Different HI materials were realized, such as copper phthalocyanine (CuPc) [3] and polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS) [5], but an Ohmic contact has not been achieved yet. Recently, transition metal oxides (TMOs) with high work functions have been shown to be promising candidates to facilitate hole injection and improve the performance of organic optoelectronic devices [6–10]. Popular oxides include molybdenum trioxide (MoO3), vanadium pentoxide (V2O5) and tungsten trioxide (WO3). While most reports aim at showing the improved performance of devices by TMOs, a critical factor, the
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preparation condition (substrate temperature and oxygen partial pressure, etc.) of TMOs, which strongly affects the stoichiometry of TMOs and their work functions, was neglected. In this study, we address the injection problems encountered in using TMOs as HIL. We will show that the post-treatment of TMO films (MoO3, V2O5, and WO3) at different oxygen partial pressures before HTL deposition can affect the HI efficiency into an organic HTL, N,N0 -diphenylN,N0 -bis(1-naphthyl)(1,10 biphenyl)-4,40 diamine (NPB). NPB is a widely used amorphous HT material in OLEDs [11]. It has excellent film forming ability and air stability; it also exhibits trap-free HT property. Besides, similar to many common small-molecule HT materials, NPB has a highlying HOMO level of 5.4 eV. Therefore, NPB is an ideal candidate for examining charge transport and injection mechanism in amorphous organic semiconductors [12,13]. Two electrical techniques, current–voltage (J–V) and dark-injection space-charge-limited-current (DI-SCLC) techniques, are employed to perform the examination. These techniques have been demonstrated to be valuable tools for injection contact examination [12,14,15]. Besides, carrier mobility of NPB can also be evaluated by DI-SCLC techniques [16]. The carrier mobility deduced from DISCLC technique will be compared to those obtained from independent time-of-flight (TOF) technique. In addition, X-ray photoemission spectroscopy (XPS) was employed to study the surface composition and the stoichiometry of TMO films.
2. Experimental TMOs and NPB were purchased from Strem Chemicals and E-ray, respectively. Details of TOF experiments have been reported elsewhere [17]. For J–V and DI-SCLC experiments, samples have the general structures of ITO/ TMO(20 nm)/NPB(4 lm)/Ag. Prior to film deposition, ITO-coated glass slides were cleaned by using sequential baths of deconex, de-ionized water, and acetone. Subsequently, the ITO substrates were exposed to UV–ozone [18]. TMOs and NPB were thermally evaporated onto the substrates at rates of 0.05 and 1 nm/s, respectively. Since only hole injection was considered, 100 nm of Ag was used to be the hole collecting and electron blocking cathode. All films were deposited inside a high vacuum evaporator with a base pressure of about 106 Torr. In the following, instead of MoO3, V2O5 and WO3, we will use the notation of MoOx, V2Ox, and WOx, to represent the stoichiometry of thermally evaporated TMOs film. Non-stoichiometric TMO films are usually obtained from thermal evaporation [19]. There are three kinds of devices used in this work. Their differentiation is based on how the TMO layer was modified prior to the deposition of the NPB layer. They are distinguished as follows: Method A – the TMO film was not modified, i.e., TMO and NPB were deposited sequentially under the same vacuum; Methods B and C – the TMO films were exposed to air and oxygen for 10 min, respectively, prior to the deposition of NPB. All film thicknesses were measured in situ by a quartz crystal sensor and ex situ by a step profilometer (Tencor Alpha-Step
500). After fabrication, the samples were immediately loaded inside a vacuum cryostat with a pressure of less than 103 Torr for measurements. All measurements were performed at room temperature. The J–V experiments were carried out with a computer-controlled Keithley SourceMeasure Unit (model 236). For DI-SCLC experiments, the samples were subjected to a rectangular voltage pulse by using a pulse generator (HP model 241B) for injecting holes into the samples. A digital oscilloscope was used to capture the voltage across a current sensing resistor, which was connected in series to the samples. The current can be computed afterwards. For XPS, MoOx was taken for surface composition study. Two samples were fabricated: Sample 1 – 20 nm MoOx covered with 100 nm NPB, by which the two films were deposited sequentially on top of ITO in the same vacuum; Sample 2 – bare MoOx film on top of ITO. The buried MoOx film in sample 1 represents the MoOx film without gas exposure, while the film in sample 2 corresponds to air-exposed oxide. The XPS experiment was performed inside Kratos AXIS ULTRADLD system using monochromic Al Ka radiation as the X-ray source and the surface composition can be determined. The surface composition of MoOx in sample 1 can be characterized only after the removal of the NPB layer by Ar+ ion sputtering. 3. Results and discussion Fig. 1a shows the J–V characteristics for devices having the structure of ITO/HIL/NPB/Ag, where the HIL = PEDOT:PSS, TMOs with and without air exposure. Ideally, if an organic material is trap-free and under the condition of Ohmic injection contact, the steady-state current should follow the space-charge-limited current (SCLC) (JSCL) [20]:
J SCL ¼
pffiffiffi F 2 9 l0 e0 er expð0:89b F Þ 8 d
ð1Þ
where e0 is the permittivity in free space, er is the dielectric constant (3 for organic materials), F is the applied electric field strength and d is the thickness of organic layer respectively. The factor b is the Poole–Frenkel slope, which indicates the sensitivity of carrier mobility of material with respect to applied electric field. Carrier mobility of materials with smaller b shows less field-dependence. On the other hand, l0 is zero-field mobility, i.e., the mobility when F = 0. Both b and l0 can be obtained from independent time-of-flight experiments, and were found to be 1.3 103(cm/V)1/2 and 2.7 104cm2/Vs, respectively [21]. In addition, the HI efficiency (gINJ), can be obtained from the J–V characteristics and is given by [22]:
gINJ ¼ JINJ =JSCL
ð2Þ
where JINJ and JSCL are, respectively, the measured and computed hole current from Eq. (1). The solid line in Fig. 1a is the computed theoretical JSCL which serves as the upper bound for the experimental current density. PEDOT:PSS has been demonstrated to be an promising HIL for OLEDs. The J–V characteristic of device with PEDOT:PSS here is used as a reference showing how
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improvements clearly reveal that the hole injection at the TMO/NPB interface is enhanced upon exposing the oxides to air before the deposition of NPB, which is solely originated from the reduced injection barrier at TMO/NPB interface. The observations are, perhaps, not surprising. TMOs have been demonstrated to be the materials for gas sensors [23,24]. Their work functions change upon exposing to different gases. Exposing the oxides to oxidizing/reducing gases will result in an increase/decrease of their work functions. In air, the oxidizing constituent is oxygen. In order to verify if the improvement is solely originated from the oxygen exposure, samples with oxygen-exposed TMOs were fabricated. Fig. 2a shows the J–V characteristics for NPB, with ITO/ MoOx as hole injecting anodes. The solid lines are the computed theoretical JSCL using Eq. (1). In Fig. 2a, solid triangles (N), open square (h) and open circle (s) represent the samples with no gas-exposed MoOx (device A), air-exposed
Fig. 1. (a) Current–voltage (J–V) characteristics and (b) injection efficiency plot of ITO/HIL/NPB/Ag devices using different HIL materials. The HILs are PEDOT:PSS, MoOx, V2Ox, and WOx. For the TMOs, closed and open symbols correspond to the TMOs with and without air exposure, respectively.
an efficient HIL behaves. Clearly, at high field, the experimental J–V curves nearly overlap with the theoretical one for the PEDOT:PSS device, which indicates the HIL can form nearly Ohmic contact with NPB. On the other hand, those devices with HIL = air-exposed TMOs show larger deviations between experimental and theoretical J–V curves when compared to PEDOT:PSS device, indicating poorer injection condition. Furthermore, for those devices with HIL = TMOs without air exposure (except for WOx), the measured J–V curves deviate substantially from the theoretical curve. This indicates the currents flowing through the devices are completely injection limited. Substantial improvement in current density was observed from the devices with HIL = TMOs after air exposure. Fig. 1(b) shows the HI efficiency (gINJ) plot of all the anodes. Same as J–V characteristics, when the TMOs are exposed to air, the HI efficiency has a marked improvement when compared to those without air exposure. About fivefold increase in gINJ can be found when F > 50 kV/cm. The
Fig. 2. (a) J–V characteristics and (b) injection efficiency plot of ITO/ MoOx/NPB/Ag devices. The MoOx were treated differently in these devices: device A (N) – no gaseous exposure; device B (h) – exposed to air for 10 min; device C (s) – exposed to oxygen for 10 min. In (a), the solid line is the simulated J–V curve using SCLC theory.
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MoOx (device B) and oxygen-exposed MoOx (device C), respectively. Same as device B, substantial improvement in current density was observed from device C, when compared to that of device A. In addition, deviation between the theoretical fit and the experimental J–V curves from device C is smaller when compared to device B, in the entire range of electric field. Fig. 2b shows the HI efficiency plots for the corresponding TMOs containing devices. The results are consistent with the observations from the J–V plots, showing improved gINJ from gas-exposed TMOs. The contacts of device C have the best gINJ among all the devices, especially when F > 50 kV/cm. Analogous experiments were performed on V2Ox, and the results are summarized in Table 1. The table shows gINJ at two different applied electric field strengths, F = 100 kV/cm and 150 kV/cm. Same observation of gINJ improvement can be found from V2Ox containing devices. Since WOx showed no effect upon air exposure, only MoOx and V2Ox were taken to be further investigated here. The discrepancy between gINJ of oxygen-exposed and air-exposed TMOs is likely due to the presence of moisture in air, which acts as a reducing agent for TMOs [23]. Moisture counteracts the effect of oxygen and will reduce the effect of work function enhancement of oxides. In addition, the gINJ of PEDOT:PSS/NPB contact were found to be 0.64 and 0.74 at F = 100 kV/cm and 150 kV/cm from Fig. 1b, respectively. Oxygen-exposed TMOs show comparable injection performance to PEDOT:PSS, which suggests that TMOs can be an alternative choice for HIL in OLEDs to replace PEDOT:PSS, with an additional advantage of being the top electrodes of organic thin film transistors (OTFTs). Besides J–V characteristics, the injection behavior of TMO/NPB contacts was further examined by DI-SCLC experiment. Fig. 3a shows the DI signals at 100 V (F = 250 kV/cm) from all the devices (A, B and C). The signals from device B and C clearly exhibit the characteristics which resemble the ideal DI transient as shown in Fig. 3b, while the signal from device A is featureless. The ideal current transient reaches a maximum at a well-defined flight time sDI and decays to a saturated value after a long time. This transient can only be observed if the following conditions are satisfied: (i) anode/organic contact is Ohmic and (ii) organic/cathode contact is electron blocking. The transient current density J(t) can be derived from Poisson’s equation for a trap-free material under the condition of unipolar Ohmic injection. The carrier mobility lDI can be calculated as follows [25]: 2
lDI ¼
0:787d sDI V
ð3Þ
where sDI is the arrival time of the fastest carriers at the non-injecting electrode, and is related to the space-charge
Fig. 3. (a) DI-SCLC transients from device A, B and C at 100 V (F = 250 kV/ cm). (b) An ideal DI-SCLC transient.
free carrier transit time s by sDI = 0.787s; d and V are the organic layer thickness and voltage applied across the sample. In Fig 3a, we can clearly extract the sDI from devices B and C while no distinct sDI appeared from device A. Moreover, the DI transients under different applied voltages were measured. From Eq. (3), we can extract the hole transit time and hence the hole mobilities of NPB from the DI transients. The results from MoOx containing devices are summarized in Fig. 4. The values of hole mobilities of NPB extracted from all the devices are consistent to the reported values [12,21]. In Fig. 4, device C (s) shows slightly higher hole mobilities than device B (j). Fig. 4 also
Table 1 Summary of HI efficiency (gINJ) of different oxide contacts. MoOx
w/o gas exposure Air exposure O2 exposure
V2Ox
F = 100 kV/cm
F = 150 kV/cm
F = 100 kV/cm
F = 150 kV/cm
0.08 0.43 0.58
0.11 0.51 0.72
0.06 0.37 0.6
0.08 0.43 0.75
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lar to the case of doping donor atoms into an intrinsic inorganic semiconductor, which will shift the Fermi level of semiconductor towards the conduction band. Under this circumstance, non-stoichiometric TMOs are produced. While at high pO2, oxygen is incorporated at an oxygen vacancy and takes two electrons from the valence band of TMOs, leaving two holes to contribute to p-type conductivity. This will shift the Fermi level to the valence band [26,27]. As the work function equals to the difference between the vacuum level and Fermi level, the shift of Fermi level results in the variation of work function. In order to
Fig. 4. The hole mobilities of NPB against the square root of the electric field at 290 K derived from DI-SCLC and TOF techniques for ITO/MoOx/ NPB/Ag devices.
shows the hole mobilities of NPB obtained from independent TOF experiment. In TOF, as the charge carriers inside NPB are optically generated by pulse laser, problems related to the injection contact can be neglected. Therefore, carrier mobility from TOF reveals the actual mobility of materials. The mobilities obtained from DI experiments are in good agreement to those from TOF. Same observation can also be found from V2Ox containing devices. The small discrepancy can be originated from the imperfectly Ohmic injection from the oxides contact. Larger discrepancy can be found in the range of small electric field, as the injection barrier height is lowered upon increasing the applied voltages. The observation is consistent to that of J–V experiment showing incomplete overlap between the measured J–V curves and the theoretical fits, especially when F is small. Although significant difference cannot be found from the DI mobilities evaluated from the devices B and C, the observation of DI transients from those devices with gas-exposed TMOs indicate the contacts are nearly Ohmic. On the other hand, absence of DI transients and the large deviation between measured and theoretical J–V curve indicate the contacts formed by TMOs with no gas exposure and NPB (device A) are significantly injection limited. The contact transition from non-Ohmic to nearly Ohmic further verifies the air/oxygen exposure is essential for reducing the energy barrier at TMO/NPB interface. Energy barrier reduction at the TMO/NPB interface after air/oxygen exposure is solely originated from the gas sensing properties of TMOs. As mentioned above, exposing the oxides to oxidizing gases will result in an increase of their work functions. The mechanism of work function enhancement in our case can be understood as follows. Under an oxygen deficient environment, i.e. low oxygen partial pressure (pO2), TMOs undergo reduction by losing oxygen, which creates an oxygen vacancy and generates electrons contributing to n-type conductivity. This situation is simi-
Fig. 5. Bond representation of (a) stoichiometric MoO3 prior to evaporation; (b) thermally evaporated MoOx film under vacuum; (c) MoOx after exposing to air. The dots (d) denote the electrons left behind after the loss of O-atoms in the lattice.
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verify if the air-exposed TMO film has different oxygen content to that without gas exposure, complementary X-ray photoemission spectroscopy (XPS) experiment was performed on MoOx films. By monitoring the ratio of the peak areas of Mo 3d and O 1s, the values of x in the MoOx films with and without air exposure were found to be 2.6 and 1.4 respectively, which suggests the oxygen content increases upon air exposure. The value of x 2.6 from air-exposed MoOx film is also consistent to the value from Kanno et. al. (x 2.7) [19]. Fig. 5 shows a schematic diagram of the redox reactions of MoO3 during our fabrication. In our case, the deposition of TMO film was carried out inside a vacuum evaporator, which provides an oxygen deficient environment. The outcome is the creation of oxygen vacancies inside the film, which shifts the Fermi level towards the conduction band and thus reduces the work function of the oxide as outlined above (Fig. 5b). However, upon air/oxygen exposure, oxygen will be incorporated back to the oxide films at the oxygen vacancies. For every oxygen vacancies, the process removes up to two electrons. The overall result is a net increase in the work function due to the lowering of the Fermi level of the oxide (Fig. 5c). As a result, the work function of oxide is enhanced, which reduces the energy barrier at TMO/NPB, and improves the HI efficiency. In contrast to MoOx and V2Ox, WOx behaves differently for the work function enhancement in this study. In this case, the J–V characteristics in Fig. 1 showed that only a small improvement can be found when WOx is exposed to air. This is an indication that thermally evaporated WOx film remains highly stoichiometric, with relatively few oxygen vacancies. According to the work of Molenda et al., a slightly non-stoichiometric species, WO2.7, can only be created from WO3 in ultra-high vacuum environment [28]. However, in our case, the film deposition was carried out under high vacuum (106 Torr). So, it is unlikely that the non-stoichiometric oxide can be formed, and thus, the reduction of the oxide by oxygen does not occur. As a result, variation of the work function of WOx cannot be observed and thus, the HI efficiency improvement cannot be found in WOx. On the other hand, even in low vacuum environment, non-stoichiometric MoO3 and V2O5 can be easily obtained [29], which are consistent to our observation as discussed above.
ment is attributed to the oxidation of TMOs by oxygen upon air exposure. Finally, we emphasize that this improvement is not only valuable for enhancing the performance of organic optoelectronic devices, but also providing an insight for the fabrication of transition metal oxide films. Acknowledgements Support of this research by the Research Grant Council of Hong Kong and the Research Committee of Hong Kong Baptist University under Grant #HKBU211107E, #HKBU211209E and FRG/07-08/II-66 is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
4. Conclusion In conclusion, we demonstrate that exposing a TMO film to air can effectively reduce the energy barrier at TMO/NPB interface, and improve the HI efficiency. J–V and DI-SCLC techniques were used to cross-examine the injection behavior of the contacts. The origin of improve-
[26] [27]
[28] [29]
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Role of air exposure in the improvement of injection efficiency of transition metal oxide/organic contact C.H. Cheung a, W.J. Song b, S.K. So a,* a
Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China Devision of Functional Materials and Nano Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 16 June 2009 Received in revised form 10 September 2009 Accepted 6 October 2009 Available online 13 October 2009 Keywords: Transition metal oxides, Charge injection Organic semiconductor Injection efficiency
a b s t r a c t Oxygen or air exposure to transition metal oxides (TMOs) was demonstrated to be essential in improving the hole injection (HI) efficiency at the contact formed by TMOs and small organic hole transporter. Current–voltage (J–V) and dark-injection space-charge-limited current (DI-SCLC) techniques were used to cross-examine the TMO/organic contacts. The hole transporter under investigation was N,N0 -diphenyl-N,N0 -bis(1-naphthyl)(1,10 biphenyl)-4,40 diamine (NPB). The improvement was attributed to the reduction in the energy barrier at TMO/NPB interface, which was a consequence of the work function enhancement of TMO by the oxidation of oxygen in air. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic semiconductors have received tremendous attention for optoelectronic applications in recent decades with various advantages such as low cost processing and structural flexibility [1,2]. Owing to the low intrinsic carrier concentration in organic semiconductors, external charge injection from a contact electrode is necessary for making the semiconductors electrically conducting. As a matter of fact, efficient carrier injection at electrode/organic contact is critically important in ‘turning on’ organic optoelectronic devices and thus realizing devices with high efficiency [3]. For hole injection, ideally, if interface dipole is negligible, the hole injection efficiency is mainly determined by the energy barrier between the work function of anode and the highest occupied molecular orbital (HOMO) level of organic hole transporting layer (HTL) [4]. A smaller barrier should result in a better efficiency. * Corresponding author. E-mail address: [email protected] (S.K. So). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.10.003
The most efficient injection contact, i.e. an Ohmic contact, appears in the absence of an injection barrier. Indium tin oxide (ITO) is the preferred anode for organic optoelectronic devices due to its optical transparency in visible spectrum. However, with a work function of 4.8 eV, a finite energy barrier always exists at ITO/HTL interface as most HT materials possess high-lying HOMO levels (>5.4 eV). To date, the most commonly used method to minimize the ITO/organic barrier is to insert a hole injection layer (HIL) between ITO and HTL [3,5]. Different HI materials were realized, such as copper phthalocyanine (CuPc) [3] and polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS) [5], but an Ohmic contact has not been achieved yet. Recently, transition metal oxides (TMOs) with high work functions have been shown to be promising candidates to facilitate hole injection and improve the performance of organic optoelectronic devices [6–10]. Popular oxides include molybdenum trioxide (MoO3), vanadium pentoxide (V2O5) and tungsten trioxide (WO3). While most reports aim at showing the improved performance of devices by TMOs, a critical factor, the
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preparation condition (substrate temperature and oxygen partial pressure, etc.) of TMOs, which strongly affects the stoichiometry of TMOs and their work functions, was neglected. In this study, we address the injection problems encountered in using TMOs as HIL. We will show that the post-treatment of TMO films (MoO3, V2O5, and WO3) at different oxygen partial pressures before HTL deposition can affect the HI efficiency into an organic HTL, N,N0 -diphenylN,N0 -bis(1-naphthyl)(1,10 biphenyl)-4,40 diamine (NPB). NPB is a widely used amorphous HT material in OLEDs [11]. It has excellent film forming ability and air stability; it also exhibits trap-free HT property. Besides, similar to many common small-molecule HT materials, NPB has a highlying HOMO level of 5.4 eV. Therefore, NPB is an ideal candidate for examining charge transport and injection mechanism in amorphous organic semiconductors [12,13]. Two electrical techniques, current–voltage (J–V) and dark-injection space-charge-limited-current (DI-SCLC) techniques, are employed to perform the examination. These techniques have been demonstrated to be valuable tools for injection contact examination [12,14,15]. Besides, carrier mobility of NPB can also be evaluated by DI-SCLC techniques [16]. The carrier mobility deduced from DISCLC technique will be compared to those obtained from independent time-of-flight (TOF) technique. In addition, X-ray photoemission spectroscopy (XPS) was employed to study the surface composition and the stoichiometry of TMO films.
2. Experimental TMOs and NPB were purchased from Strem Chemicals and E-ray, respectively. Details of TOF experiments have been reported elsewhere [17]. For J–V and DI-SCLC experiments, samples have the general structures of ITO/ TMO(20 nm)/NPB(4 lm)/Ag. Prior to film deposition, ITO-coated glass slides were cleaned by using sequential baths of deconex, de-ionized water, and acetone. Subsequently, the ITO substrates were exposed to UV–ozone [18]. TMOs and NPB were thermally evaporated onto the substrates at rates of 0.05 and 1 nm/s, respectively. Since only hole injection was considered, 100 nm of Ag was used to be the hole collecting and electron blocking cathode. All films were deposited inside a high vacuum evaporator with a base pressure of about 106 Torr. In the following, instead of MoO3, V2O5 and WO3, we will use the notation of MoOx, V2Ox, and WOx, to represent the stoichiometry of thermally evaporated TMOs film. Non-stoichiometric TMO films are usually obtained from thermal evaporation [19]. There are three kinds of devices used in this work. Their differentiation is based on how the TMO layer was modified prior to the deposition of the NPB layer. They are distinguished as follows: Method A – the TMO film was not modified, i.e., TMO and NPB were deposited sequentially under the same vacuum; Methods B and C – the TMO films were exposed to air and oxygen for 10 min, respectively, prior to the deposition of NPB. All film thicknesses were measured in situ by a quartz crystal sensor and ex situ by a step profilometer (Tencor Alpha-Step
500). After fabrication, the samples were immediately loaded inside a vacuum cryostat with a pressure of less than 103 Torr for measurements. All measurements were performed at room temperature. The J–V experiments were carried out with a computer-controlled Keithley SourceMeasure Unit (model 236). For DI-SCLC experiments, the samples were subjected to a rectangular voltage pulse by using a pulse generator (HP model 241B) for injecting holes into the samples. A digital oscilloscope was used to capture the voltage across a current sensing resistor, which was connected in series to the samples. The current can be computed afterwards. For XPS, MoOx was taken for surface composition study. Two samples were fabricated: Sample 1 – 20 nm MoOx covered with 100 nm NPB, by which the two films were deposited sequentially on top of ITO in the same vacuum; Sample 2 – bare MoOx film on top of ITO. The buried MoOx film in sample 1 represents the MoOx film without gas exposure, while the film in sample 2 corresponds to air-exposed oxide. The XPS experiment was performed inside Kratos AXIS ULTRADLD system using monochromic Al Ka radiation as the X-ray source and the surface composition can be determined. The surface composition of MoOx in sample 1 can be characterized only after the removal of the NPB layer by Ar+ ion sputtering. 3. Results and discussion Fig. 1a shows the J–V characteristics for devices having the structure of ITO/HIL/NPB/Ag, where the HIL = PEDOT:PSS, TMOs with and without air exposure. Ideally, if an organic material is trap-free and under the condition of Ohmic injection contact, the steady-state current should follow the space-charge-limited current (SCLC) (JSCL) [20]:
J SCL ¼
pffiffiffi F 2 9 l0 e0 er expð0:89b F Þ 8 d
ð1Þ
where e0 is the permittivity in free space, er is the dielectric constant (3 for organic materials), F is the applied electric field strength and d is the thickness of organic layer respectively. The factor b is the Poole–Frenkel slope, which indicates the sensitivity of carrier mobility of material with respect to applied electric field. Carrier mobility of materials with smaller b shows less field-dependence. On the other hand, l0 is zero-field mobility, i.e., the mobility when F = 0. Both b and l0 can be obtained from independent time-of-flight experiments, and were found to be 1.3 103(cm/V)1/2 and 2.7 104cm2/Vs, respectively [21]. In addition, the HI efficiency (gINJ), can be obtained from the J–V characteristics and is given by [22]:
gINJ ¼ JINJ =JSCL
ð2Þ
where JINJ and JSCL are, respectively, the measured and computed hole current from Eq. (1). The solid line in Fig. 1a is the computed theoretical JSCL which serves as the upper bound for the experimental current density. PEDOT:PSS has been demonstrated to be an promising HIL for OLEDs. The J–V characteristic of device with PEDOT:PSS here is used as a reference showing how
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improvements clearly reveal that the hole injection at the TMO/NPB interface is enhanced upon exposing the oxides to air before the deposition of NPB, which is solely originated from the reduced injection barrier at TMO/NPB interface. The observations are, perhaps, not surprising. TMOs have been demonstrated to be the materials for gas sensors [23,24]. Their work functions change upon exposing to different gases. Exposing the oxides to oxidizing/reducing gases will result in an increase/decrease of their work functions. In air, the oxidizing constituent is oxygen. In order to verify if the improvement is solely originated from the oxygen exposure, samples with oxygen-exposed TMOs were fabricated. Fig. 2a shows the J–V characteristics for NPB, with ITO/ MoOx as hole injecting anodes. The solid lines are the computed theoretical JSCL using Eq. (1). In Fig. 2a, solid triangles (N), open square (h) and open circle (s) represent the samples with no gas-exposed MoOx (device A), air-exposed
Fig. 1. (a) Current–voltage (J–V) characteristics and (b) injection efficiency plot of ITO/HIL/NPB/Ag devices using different HIL materials. The HILs are PEDOT:PSS, MoOx, V2Ox, and WOx. For the TMOs, closed and open symbols correspond to the TMOs with and without air exposure, respectively.
an efficient HIL behaves. Clearly, at high field, the experimental J–V curves nearly overlap with the theoretical one for the PEDOT:PSS device, which indicates the HIL can form nearly Ohmic contact with NPB. On the other hand, those devices with HIL = air-exposed TMOs show larger deviations between experimental and theoretical J–V curves when compared to PEDOT:PSS device, indicating poorer injection condition. Furthermore, for those devices with HIL = TMOs without air exposure (except for WOx), the measured J–V curves deviate substantially from the theoretical curve. This indicates the currents flowing through the devices are completely injection limited. Substantial improvement in current density was observed from the devices with HIL = TMOs after air exposure. Fig. 1(b) shows the HI efficiency (gINJ) plot of all the anodes. Same as J–V characteristics, when the TMOs are exposed to air, the HI efficiency has a marked improvement when compared to those without air exposure. About fivefold increase in gINJ can be found when F > 50 kV/cm. The
Fig. 2. (a) J–V characteristics and (b) injection efficiency plot of ITO/ MoOx/NPB/Ag devices. The MoOx were treated differently in these devices: device A (N) – no gaseous exposure; device B (h) – exposed to air for 10 min; device C (s) – exposed to oxygen for 10 min. In (a), the solid line is the simulated J–V curve using SCLC theory.
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MoOx (device B) and oxygen-exposed MoOx (device C), respectively. Same as device B, substantial improvement in current density was observed from device C, when compared to that of device A. In addition, deviation between the theoretical fit and the experimental J–V curves from device C is smaller when compared to device B, in the entire range of electric field. Fig. 2b shows the HI efficiency plots for the corresponding TMOs containing devices. The results are consistent with the observations from the J–V plots, showing improved gINJ from gas-exposed TMOs. The contacts of device C have the best gINJ among all the devices, especially when F > 50 kV/cm. Analogous experiments were performed on V2Ox, and the results are summarized in Table 1. The table shows gINJ at two different applied electric field strengths, F = 100 kV/cm and 150 kV/cm. Same observation of gINJ improvement can be found from V2Ox containing devices. Since WOx showed no effect upon air exposure, only MoOx and V2Ox were taken to be further investigated here. The discrepancy between gINJ of oxygen-exposed and air-exposed TMOs is likely due to the presence of moisture in air, which acts as a reducing agent for TMOs [23]. Moisture counteracts the effect of oxygen and will reduce the effect of work function enhancement of oxides. In addition, the gINJ of PEDOT:PSS/NPB contact were found to be 0.64 and 0.74 at F = 100 kV/cm and 150 kV/cm from Fig. 1b, respectively. Oxygen-exposed TMOs show comparable injection performance to PEDOT:PSS, which suggests that TMOs can be an alternative choice for HIL in OLEDs to replace PEDOT:PSS, with an additional advantage of being the top electrodes of organic thin film transistors (OTFTs). Besides J–V characteristics, the injection behavior of TMO/NPB contacts was further examined by DI-SCLC experiment. Fig. 3a shows the DI signals at 100 V (F = 250 kV/cm) from all the devices (A, B and C). The signals from device B and C clearly exhibit the characteristics which resemble the ideal DI transient as shown in Fig. 3b, while the signal from device A is featureless. The ideal current transient reaches a maximum at a well-defined flight time sDI and decays to a saturated value after a long time. This transient can only be observed if the following conditions are satisfied: (i) anode/organic contact is Ohmic and (ii) organic/cathode contact is electron blocking. The transient current density J(t) can be derived from Poisson’s equation for a trap-free material under the condition of unipolar Ohmic injection. The carrier mobility lDI can be calculated as follows [25]: 2
lDI ¼
0:787d sDI V
ð3Þ
where sDI is the arrival time of the fastest carriers at the non-injecting electrode, and is related to the space-charge
Fig. 3. (a) DI-SCLC transients from device A, B and C at 100 V (F = 250 kV/ cm). (b) An ideal DI-SCLC transient.
free carrier transit time s by sDI = 0.787s; d and V are the organic layer thickness and voltage applied across the sample. In Fig 3a, we can clearly extract the sDI from devices B and C while no distinct sDI appeared from device A. Moreover, the DI transients under different applied voltages were measured. From Eq. (3), we can extract the hole transit time and hence the hole mobilities of NPB from the DI transients. The results from MoOx containing devices are summarized in Fig. 4. The values of hole mobilities of NPB extracted from all the devices are consistent to the reported values [12,21]. In Fig. 4, device C (s) shows slightly higher hole mobilities than device B (j). Fig. 4 also
Table 1 Summary of HI efficiency (gINJ) of different oxide contacts. MoOx
w/o gas exposure Air exposure O2 exposure
V2Ox
F = 100 kV/cm
F = 150 kV/cm
F = 100 kV/cm
F = 150 kV/cm
0.08 0.43 0.58
0.11 0.51 0.72
0.06 0.37 0.6
0.08 0.43 0.75
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lar to the case of doping donor atoms into an intrinsic inorganic semiconductor, which will shift the Fermi level of semiconductor towards the conduction band. Under this circumstance, non-stoichiometric TMOs are produced. While at high pO2, oxygen is incorporated at an oxygen vacancy and takes two electrons from the valence band of TMOs, leaving two holes to contribute to p-type conductivity. This will shift the Fermi level to the valence band [26,27]. As the work function equals to the difference between the vacuum level and Fermi level, the shift of Fermi level results in the variation of work function. In order to
Fig. 4. The hole mobilities of NPB against the square root of the electric field at 290 K derived from DI-SCLC and TOF techniques for ITO/MoOx/ NPB/Ag devices.
shows the hole mobilities of NPB obtained from independent TOF experiment. In TOF, as the charge carriers inside NPB are optically generated by pulse laser, problems related to the injection contact can be neglected. Therefore, carrier mobility from TOF reveals the actual mobility of materials. The mobilities obtained from DI experiments are in good agreement to those from TOF. Same observation can also be found from V2Ox containing devices. The small discrepancy can be originated from the imperfectly Ohmic injection from the oxides contact. Larger discrepancy can be found in the range of small electric field, as the injection barrier height is lowered upon increasing the applied voltages. The observation is consistent to that of J–V experiment showing incomplete overlap between the measured J–V curves and the theoretical fits, especially when F is small. Although significant difference cannot be found from the DI mobilities evaluated from the devices B and C, the observation of DI transients from those devices with gas-exposed TMOs indicate the contacts are nearly Ohmic. On the other hand, absence of DI transients and the large deviation between measured and theoretical J–V curve indicate the contacts formed by TMOs with no gas exposure and NPB (device A) are significantly injection limited. The contact transition from non-Ohmic to nearly Ohmic further verifies the air/oxygen exposure is essential for reducing the energy barrier at TMO/NPB interface. Energy barrier reduction at the TMO/NPB interface after air/oxygen exposure is solely originated from the gas sensing properties of TMOs. As mentioned above, exposing the oxides to oxidizing gases will result in an increase of their work functions. The mechanism of work function enhancement in our case can be understood as follows. Under an oxygen deficient environment, i.e. low oxygen partial pressure (pO2), TMOs undergo reduction by losing oxygen, which creates an oxygen vacancy and generates electrons contributing to n-type conductivity. This situation is simi-
Fig. 5. Bond representation of (a) stoichiometric MoO3 prior to evaporation; (b) thermally evaporated MoOx film under vacuum; (c) MoOx after exposing to air. The dots (d) denote the electrons left behind after the loss of O-atoms in the lattice.
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verify if the air-exposed TMO film has different oxygen content to that without gas exposure, complementary X-ray photoemission spectroscopy (XPS) experiment was performed on MoOx films. By monitoring the ratio of the peak areas of Mo 3d and O 1s, the values of x in the MoOx films with and without air exposure were found to be 2.6 and 1.4 respectively, which suggests the oxygen content increases upon air exposure. The value of x 2.6 from air-exposed MoOx film is also consistent to the value from Kanno et. al. (x 2.7) [19]. Fig. 5 shows a schematic diagram of the redox reactions of MoO3 during our fabrication. In our case, the deposition of TMO film was carried out inside a vacuum evaporator, which provides an oxygen deficient environment. The outcome is the creation of oxygen vacancies inside the film, which shifts the Fermi level towards the conduction band and thus reduces the work function of the oxide as outlined above (Fig. 5b). However, upon air/oxygen exposure, oxygen will be incorporated back to the oxide films at the oxygen vacancies. For every oxygen vacancies, the process removes up to two electrons. The overall result is a net increase in the work function due to the lowering of the Fermi level of the oxide (Fig. 5c). As a result, the work function of oxide is enhanced, which reduces the energy barrier at TMO/NPB, and improves the HI efficiency. In contrast to MoOx and V2Ox, WOx behaves differently for the work function enhancement in this study. In this case, the J–V characteristics in Fig. 1 showed that only a small improvement can be found when WOx is exposed to air. This is an indication that thermally evaporated WOx film remains highly stoichiometric, with relatively few oxygen vacancies. According to the work of Molenda et al., a slightly non-stoichiometric species, WO2.7, can only be created from WO3 in ultra-high vacuum environment [28]. However, in our case, the film deposition was carried out under high vacuum (106 Torr). So, it is unlikely that the non-stoichiometric oxide can be formed, and thus, the reduction of the oxide by oxygen does not occur. As a result, variation of the work function of WOx cannot be observed and thus, the HI efficiency improvement cannot be found in WOx. On the other hand, even in low vacuum environment, non-stoichiometric MoO3 and V2O5 can be easily obtained [29], which are consistent to our observation as discussed above.
ment is attributed to the oxidation of TMOs by oxygen upon air exposure. Finally, we emphasize that this improvement is not only valuable for enhancing the performance of organic optoelectronic devices, but also providing an insight for the fabrication of transition metal oxide films. Acknowledgements Support of this research by the Research Grant Council of Hong Kong and the Research Committee of Hong Kong Baptist University under Grant #HKBU211107E, #HKBU211209E and FRG/07-08/II-66 is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
4. Conclusion In conclusion, we demonstrate that exposing a TMO film to air can effectively reduce the energy barrier at TMO/NPB interface, and improve the HI efficiency. J–V and DI-SCLC techniques were used to cross-examine the injection behavior of the contacts. The origin of improve-
[26] [27]
[28] [29]
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