Research Article | Open Access
Irfan, Franky So, Yongli Gao, "Photoemission Spectroscopy Characterization of Attempts to Deposit MoO2 Thin Film", International Journal of Photoenergy, vol. 2011, Article ID 314702, 6 pages, 2011. https://doi.org/10.1155/2011/314702
Photoemission Spectroscopy Characterization of Attempts to Deposit MoO2 Thin Film
Attempts to deposit molybdenum dioxide (MoO2) thin films have been described. Electronic structure of films, deposited by thermal evaporation of MoO2 powder, had been investigated with ultraviolet photoemission and X-ray photoemission spectroscopy (UPS and XPS). The thermally evaporated films were found to be similar to the thermally evaporated MoO3 films at the early deposition stage. XPS analysis of MoO2 powder reveals presence of +5 and +6 oxidation states in Mo 3d core level along with +4 state. The residue of MoO2 powder indicates substantial reduction in higher oxidation states while keeping +4 oxidation state almost intact. Interface formation between chloroaluminum phthalocyanine (AlPc-Cl) and the thermally evaporated film was also investigated.
Applications of organic semiconductors (OSCs) are increasingly attracting interest of both scientific and industrial communities because of its low synthesis cost and tunable material properties. Potential applications, including organic photovoltaic cells (OPVs) [1–3], organic light emitting diodes (OLEDs) [4–6], and organic thin film transistors (OTFTs) [7, 8], have been successfully demonstrated. Despite the impetus from these early successful applications, tremendous scope of performance enhancement is expected with proper understanding of each interface in the organic device. Therefore, detailed investigations are desired for each interface with the characterization of electronic structure and interface energy level alignments.
A great deal of efforts have been made in order to improve the charge transport and collection at the electrodes. Introduction of a high work function (WF) transition metal oxide insertion layer between conducting indium tin oxide (ITO) and organic semiconductors was an attempt first made by Tokito et al. . While improvement of device performance is well established with the oxide insertion layer [10–13], consensus on the mechanism of this improvement is yet to be achieved. Kroger et al. [14, 15] have reported a possible mechanism for hole injection improvement, originating from electron extraction from the highest occupied molecular orbital (HOMO) of organic semiconductor to the ITO anode through conduction band of MoO3. In our reports [13–16], we have established the efficiency enhancement by a reduced hole injection barrier and a drift field from hole accumulation in the organic material at the ITO/hole injection layer interface. We have also underlined the importance of the high work function (WF) of MoO3 for the performance enhancement of devices [16–19]. Some other groups have emphasized on gap-state-assisted transport . The role of defect states on performance has also been raised by many research groups [21–23]. Greiner et al.  and Vasilopoulou et al.  have recently reported that the reduced molybdenum trioxide film (MoOx and ) outperforms the stoichiometric trioxide film, due to the presence of defect states in the former. These reports invoke interest in the investigation of MoO2 film as an interlayer.
In the present, work we discuss our investigation of MoO2 deposition attempts and the interface formation with chloroaluminum phthalocyanine (AlPc-Cl) using ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). We investigated the electronic properties of films deposited by thermally evaporated MoO2 powder. At the early stage of MoO2 evaporation, the electronic structure of the deposited film resembles to that of thermally evaporated MoO3 thin films [16–19, 24]. We probed molybdenum 3d core level spectra for MoO2 powder and residue powder from MoO2 evaporation boat to compare the difference. We observed strong reduction in +6 oxidation state, while +4 oxidation state was almost intact. We also studied the interface formation of the deposited film (by thermal evaporation of MoO2 powder) with AlPc-Cl, and results are discussed.
2. Experimental Details
UPS measurements were performed using a VG ESCA Lab UHV system equipped with a He discharge lamp. The UHV system consists of three interconnecting chambers, a spectrometer chamber, an in situ oxygen plasma (OP) treatment chamber, and an evaporation chamber. The base pressure of the spectrometer chamber is typically 8 × 10−11 torr. The base pressure of the evaporation chamber is typically 1 × 10−6 torr. The UPS spectra were recorded by using unfiltered He I (21.22 eV) excitation, as the excitation source with the sample biased at −5.00 V to observe the low-energy secondary cutoff. The UV light spot size on the sample is about 1 mm in diameter. The typical instrumental resolution for UPS measurements ranges from 0.03 to 0.1 eV with the photon energy dispersion of less than 20 meV. XPS spectra were measured with a Surface Science Laboratories’ SSX-100 system equipped with a monochromatic Al anode X-ray gun (Kα 1486.6 eV). The base pressure of the system is 9 × 10−11 torr. The spot size of the X-ray was selected to be 1000 micron in diameter. The resolution of this system is 0.5 eV. One of the substrate was a borosilicate glass from Corning, coated with 250 nm thick conducting ITO, with resistivity 15 Ω per square. The ITO substrate was treated in situ in oxygen ambience of 600 milli-torr at bias voltage of −500 V for 30 sec in the VG system’s evaporation chamber. Other substrates were cut from gold-coated Si wafer. All the substrates were roughly about 8 mm × 8 mm in size. All the substrates were ultrasonically cleaned before being loaded in to the UHV chambers. All the measurements were performed at room temperature.
3. Results and Discussion
3.1. Thin Film Deposited on Indium Tin Oxide (ITO) by Evaporation of MoO2 Powder
Thermally deposited thin films from MoO2 powder on indium tin oxide (ITO) were investigated with ultraviolet photoemission spectroscopy (UPS). The current required for the early evaporation was 1.5 times higher than the current required for the thermal evaporation of MoO3 powder. The thermal evaporation of MoO2 powder was performed in the evaporation chamber of the VG ESCA system, as described in the experimental details. The evaporation became harder with increasing deposition, and beyond 64 Å, the current required was more than twice of usual current for the thermal evaporation of MoO3 powder. At such a high current, the pressure in the preparation chamber was ~1 × 10−3 torr from usual 1 × 10−6 torr and eventually the evaporation boat broke/melted down due to excessive heat.
In Figure 1, the UPS data are presented for the cut-off region (a) and the valence band region (b), with increasing thickness of the thermally evaporated films. All the spectra have been normalized to the same height for visual clarity. The data presented in the cut-off region provides a quick visualization of surface work function (WF) changes. The cut-off binding energy (BE) position of oxygen-plasma- (OP-) treated ITO surface is measured to be 15.49 eV from which the WF of 5.73 eV can be measured by subtracting photon energy (21.22 eV). The WF first increased with the evaporated film thickness up to 16 Å and then started to decrease. The WF at 2 Å, 4 Å, 8 Å, and 16 Å thicknesses were measured to be 6.28 eV, 6.24 eV. 6.42 eV, and 6.48 eV, respectively. The WF after the 16 Å thickness started to decrease and was found 5.72 eV and 5.10 eV for 32 Å and 64 Å film thickness, respectively. The initial (upto 16 Å thickness) WF enhancement is associated with +6 and +5 oxidation state of molybdenum oxide rather than +4. The reported work function value of MoO3 thin film is 6.7 ± 0.2 eV [13–19]. The highest occupied molecular orbital (HOMO) region as presented in Figure 1(b) provides a quick visualization of changes in the occupied levels (valence band region for inorganic materials) lying within few eV of binding energy (BE) with respect to the Fermi level of the system. In Figure 1(b), we notice a clear change in the valence region from ITO to 2 Å film thickness. With the 2 Å thickness, we observed the characteristic molybdenum trioxide peak with the peak position at 4.25 eV . Increasing the MoO2 evaporation thickness results in the peak position at 4.17 eV, 4.09, and 4.00 eV for 4 Å, 8 Å, and 16 Å thicknesses, respectively. The shift of ~0.2 eV in the valence band peak towards the lower BE is similar to the shift of WF from 2 Å to 16 Å thickness. These shifts indicate a minor electron trap at the interface of MoOx/ITO. At 8 Å thickness we observed a gap state around ~1 eV and an another gap state start to appear around ~2 eV at 64 Å thickness. The valence band spectra of initial 2 Å and 4 Å thicknesses is exactly similar to the thermally evaporated MoO3 films . The presence of oxychloride formation at the AlPc-Cl/MoOx interface could not be ascertained but cannot be completely ruled out .
3.2. XPS of MoO2 and Residue MoO2 Powder
In order to further confirm the preferential evaporation of molybdenum trioxide and molybdenum suboxide at early stage rather than molybdenum dioxide, we performed X-ray photoemission spectroscopic (XPS) measurements on MoO2 powder and residue MoO2 powder from inside of an evaporation boat after ~20% of the filled MoO2 powder from the boat was evaporated. Both MoO2 and the residue powder were mixed with methanol to make it like a paste. The pastes were dropped on gold substrates and were automatically dried after methanol was quickly evaporated. Both the samples were then transferred in the SSX-100 system for the XPS measurements.
In Figure 2(a), Mo 3d core levels are presented for MoO2 powder and the residue. We observed different oxidation state of molybdenum in MoO2 powder itself. The presence of higher oxidation states was puzzling. The origin of the formation of higher oxidation state may be due to absorption of oxygen from air or by decomposition of unstable MoO2 molecules in to the metallic and higher oxidation states. A quick comparison between MoO2 and the residue vividly illustrates the preferential reduction of higher oxidation states of molybdenum in the residue. In Figure 2(b), peak fitting Mo 3d spectrum of MoO2 powder is performed. The peak fitting was done with fixed separation of 3.2 eV for 3d5/2 and 3d3/2 spin orbit coupling splitted peaks, fixed area ratio of 3 : 2, and the same full width at half the maximum height (FWHM) for both the peaks. The positions of 3d5/2 peaks of +4, +5, and +6 oxidation state of molybdenum were measured to be 228.83 eV, 230.36 eV, and 232.00 eV, respectively. The observed peak positions agree well with earlier reports [21, 25, 26]. The percent intensity of +4, +5, and +6 oxidation states of molybdenum were measured to be 22%, 19%, and 59%, respectively. In Figure 2(c), peak fitting of Mo 3d spectrum of residue MoO2 powder is presented. The positions of 3d5/2 peaks of +4, +5, and +6 states were found to be 228.77 eV, 230.37 eV, and 232.01 eV, respectively. The intensity of +4, +5, and +6 oxidation states was measured to be 37%, 33%, and 30%, respectively. While the intensity reduction of +4 and +5 oxidation states was small in the residue, the +6 oxidation state was reduced to one fourth. The above finding is consistent with our observation in the first section that the evaporation of MoO2 powder at early stage results mostly in MoO3 deposition.
3.3. Interface Formation of MoO2 with Chloroaluminum Phthalocyanine (AlPc-Cl)
After being convinced that the early stage evaporation of MoO2 results in the deposition of MoO3 and after few nanometers a little mixture of oxide of molybdenum with lower oxidation states, we performed interface formation of MoO2 evaporated film with AlPc-Cl. Interface formation is crucial in order to understand possible device performance and possibly comment on the controversy of performance enhancement by molybdenum trioxide versus suboxide interlayer between an organic semiconductor and the electrode. The current required for the early evaporation of MoO2 powder was 1.5 times higher than the current required for the thermal evaporation of MoO3 powder. In the present study, the thermal evaporation of MoO2 powder was performed inside the VG ESCA system, as described in the experimental section. The evaporation became harder with increasing deposition, and, beyond 30 Å thickness, the current, required was more than twice of usual current for the thermal evaporation of MoO3 powder. At such a high current the pressure in the preparation chamber was ~1 × 10−5 torr from usual 8 × 10−11 torr, and we stopped further evaporation of MoO2 powder. AlPc-Cl was deposited in the preparation chamber, on top of the 30 Å MoO2 powder evaporated film.
In Figure 3, the UPS data are plotted for the cut-off region (a) and the highest occupied molecular orbital (HOMO) region (b) with increasing thickness of MoO2 evaporated film and then AlPc-Cl films. All the spectra have been normalized to the same height for visual clarity. In Figure 3(a), the WF of gold substrate was measured to be 5.0 eV. The surface WF first increased with the deposition of 5 Å and 10 Å film and was measured to be 6.51 eV and 6.68 eV, respectively. With further deposition of MoO2 WF was measured to be 6.25 eV and 5.49 eV, respectively. The WF reduction may be originating due to contribution of oxides of molybdenum with lower oxidation state, or exposure of deposited MoO3 film [17–19] to unavoidable high pressure inside the chamber due to excessive heat in the MoO2 evaporation boat, or a combination of both. After the deposition of the 30 Å film, we deposited AlPc-Cl layer by layer on top. With deposition of AlPc-Cl, the WF continuously decreased before reaching a saturation value of 4.82 eV at 32 Å thickness. In Figure 3(b), we observed the Fermi edge from the substrate. The early evaporation of MoO2 again results in the valence region spectra similar to that of MoO3 [16, 17]. With the deposition of AlPc-Cl on 30 Å MoO2 film, the features of AlPc-Cl [16, 17] started to appear. The HOMO features of AlPc-Cl became prominent between 4 Å to 8 Å thickness, and the HOMO onsets were measured to be 0.45 eV and 0.47 eV, respectively. With further increase in the AlPc-Cl thickness, there was no significant change.
The energy level alignment at the AlPc-Cl/MoO3 interface  is dramatically different than at the AlPc-Cl/MoO2 evaporated film interface. The early HOMO onset value of 0.45 eV observed in the present case is more than 1.5 times of 0.28 eV observed with MoO3. Energy levels are more or less flat with the increasing thickness of AlPc-Cl in the present case, while there was strong band-bending-like region in the case of AlPc-Cl/MoO3 interface. In the absence of band-bending-like region, the expected hole extraction should not be efficient. Thus, with MoO2 evaporated film interlayer, at the early stage, the larger value of hole injection barrier and later the absence of drift electric field would make it less efficient. We attribute this change to the low work function of deposited film either due to the contribution of oxides of molybdenum with lower oxidation states than +6 or exposure of early deposited MoO3 film.
In conclusion, we presented investigations on MoO2 evaporated film insertion layer and interface formation with an organic semiconductor. We observed that the early stage MoO2 evaporation results in MoO3 deposition marked with high surface work function. After few nm of evaporation, we observed the deposited film to have some contribution of lower oxidation state of molybdenum, marked with low work function and a prominent defect state at the binding energy around ~2 eV. Our X-ray photoemission study reveals the dramatic reduction in the +6 oxidation state of molybdenum in the residue MoO2 powder. The interface formation of AlPc-Cl/MoO2 evaporated film brings out the higher hole injection barrier and no band bending in AlPc-Cl, which would hamper the device performance. Therefore, reduced MoO3 oxide may be performing nicely the extreme reduction to MoO2 which would have deleterious effects on the device performance.
The authors would like to acknowledge the support of the National Science Foundation Grant no. DMR-1006098.
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