XPS study of the band alignment
at ITO/oxide (n-type MoO 3 or p-type NiO) interface
J. C. Berne`de*1, S. Houari2, D. Nguyen3, P. Y. Jouan3, A. Khelil2, A. Mokrani3, L. Cattin3, and P. Predeep4
1MOLTECH Anjou, UMR 6200, Universite´ de Nantes, 2 rue de la Houssinie`re, BP 92208, Nantes 44000, France
2Universite´ d’Oran, LPCM2E, Oran Es-Se´nia, Alge´rie
3Institut J. Rouxel (IMN), UMR 6502, Universite´ de Nantes, 2 rue de la Houssinie`re, BP 92208, Nantes 44000, France
4Labaratory for Unconventional Electronics and Photonics, Department of Physics, National Institute of Technology, Calicut 673 601, Kerala, India
Received 17 June 2011, revised 6 March 2012, accepted 30 March 2012 Published online 2 May 2012
Keywordsband alignment, interfaces, oxides, XPS
*Corresponding author: e-mailjean-christian.bernede@univ-nantes.fr, Phone: 33-2-51 12 55 30, Fax: 33-2-51 12 55 28
While they have different electronic properties n-type MoO3
and p-type NiO are very efficient as buffer layers between the ITO anode and the organic electron donor in organic photo- voltaic cells. While it is admitted that MoO3is n-type, its band structure is still under study. Here, the band alignment at the interface of an ITO/MoO3heterojunction is studied by X-ray photoelectron spectroscopy (XPS). The same study is realized on the structure ITO/NiO, NiO being a p-type semiconductor.
The measurements have been performed on samples obtained under the same experimental conditions as those used to achieve organic photovoltaic cells. The MoO3 (NiO) upper
layer was 3 nm thick. The semidirect XPS technique used to measure the band offsets allows us to estimate the band discontinuities at the interface ITO/MoO3: DEv¼0.50 eV and DEc¼0.90 eV, while at the interface ITO/NiO we have DEv¼ 2.10 eV andDEc¼ 1.90 eV. Therefore, n-type MoO3 and p-type NiO, which are both very efficient anode buffer layers (ABLs), exhibit different band structure at the contact with ITO. However, the measurement, by means of a Kelvin probe, of the work functions of the structures ITO/NiO and ITO/MoO3, shows that they are close and significantly higher than that of ITO alone.
ß2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Because of the recent and impressive progress of the performances of the organic photovoltaic cells (OPVCs) much interest is nowadays awarded to them [1–3].
It is well known that besides good photoactive materials, the properties of the interfaces between the organic layers and the electrodes are crucial for achieving high carrier collection efficiency and high power-conversion efficiency (PCE) [4].
Many papers have shown that the introduction between the anode, which is usually an ITO film, and the organic electron donor, of a buffer layer [anode buffer layer (ABL)] produces significant improvement in the OPVCs performances [5].
For instance, it has been shown that a MoO3[3–10] or NiO [11–15] thin film is very efficient to improve the device performances and stability. However, even though the MoO3 is n-type [16], NiO is p-type [15], nevertheless both buffer layers are efficient. The reason why buffer layers with different majority carrier types are as efficient are not completely understood. It is well known that a good hole
collection efficiency necessitates a good band matching between the anode and the organic material. Usually, it is admitted that the buffer layer allows such band structure alignment to be achieved. However, in the case of MoO3, there is some controversy, initial studies of thermally evaporated MoO3 layers indicated that the valence band (VB) and conduction band (CB) are at 5.3 and 2.3 eV, the CB and VB being given in absolute values, respectively [8, 17]. Recent results report greater values, ranging between 9.37–9.70 and 6.18–6.70 eV, respectively [16, 18, 19]. These discrepancies for MoO3energy level can be due to different deposition techniques, material purity, and post-treatments. For instance, it is known that MoO3films deposited by simple joule effect are not stoichiometric, the deposited films are oxygen deficient. This oxygen deficiency is very important since the work function of MoO3xvaries with thexvalue [19].
Moreover, in some works based on UPS it has been shown that MoO3xis strongly n-doped [16]. In the case of NiO, it is admitted that its bandgap is between 3.5 and 4 eV and its CB is
applications and materials science
at around 1.8 eV [11–15]. It thus seems desirable to check the band structure of MoO3, which is n-type, and to compare its band discontinuities with ITO to those of NiO, which is p-type, to try to understand the role of the ABL. In the present work we proceed to X-ray photoelectron spectroscopy (XPS) measure- ments on real buffer layers, that is to say deposited in the same experimental conditions as those used to achieve OPVCs, to estimate the band discontinuitiesDEvandDEcat the contacts ITO/MoO3and ITO/NIO. It should be noted that, while MoO3
can be easily deposited by sublimation, it is necessary to use a sputtering apparatus to grow NiO films under vacuum.
Immediately after ABL (MoO3or NiO) vacuum deposition the ITO/ABL structures were either introduced in the vacuum apparatus devoted to organic solar cells realization or into the vacuum chamber of the XPS apparatus. For calibration, the work function of the ITO has been measured using a Kelvin probe. It must be noted that the work function is an extremely sensitive indicator of the state of a surface [20], and therefore of the environment surrounding it. Ambient conditions can influence the energy required to remove an electron from the Fermi level and this is especially true for metal oxides like ITO. It has been shown [21] that the work-function values strongly depend on the measurement techniques because of this environment factor. We have used the Kelvin probe technique as it is known to be highly sensitive.
2 Experimental
2.1 OPVCs realization The OPV multilayer cells were based on the couple copper phthalocyanise/fullerene- CuPc/C60. The transparent conductive oxide (TCO) used was ITO, while the cathode was an aluminum film. Two ABLs) have been used, either MoO3or NiO. The NiO film was deposited by DC reactive magnetron sputtering using a pure nickel target (99.99%) and an Ar–O2gas mixture. We have used a target 2 inch (5.08 cm) in diameter. Substrates were placed at approximately 3 cm from the target. The gases argon and oxygen (10 at%) were introduced in the reactor by a mass flow controller, the discharge current during deposition was 110 mA [22]. The MoO3was deposited by evaporation under vacuum, the deposition rate was 0.5 nm/s and the film thickness was 4.5 nm.
The ITO/ABL structure was introduced in TCO/ABL/
CuPc/C60/BCP/Al classical OPV cells. The BCP (bath- ocuproine) layer, called an exciton blocking layer, signifi- cantly improves the OPV cell performances [23]. All the films were deposited under vacuum. Before utilization the TCO-coated substrates were scrubbed with soap and then rinsed in running deionized water. The thin-film deposition rates and thickness were estimated in situ with a quartz monitor. The deposition rate and final thickness were 0.05 nm/s and 35 nm in the case of CuPc, 0.05 nm/s and 40 nm in the case of C60and 0.1 and 9 nm for BCP. These thicknesses were chosen after optimization.
After organic thin-film deposition, without breaking the vacuum, the aluminum top electrodes, were thermally evaporated through a mask with 2 mm8 mm active areas and then an approximately 50 nm encapsulating layer of
amorphous selenium (a-Se) [24]. The vacuum in the apparatus of thin-film depositions was 104Pa.
After OPV deposition, electrical characterizations were performed with an automated I–V tester, in the dark and under sun global AM 1.5 simulated solar illumination.
2.2 XPS measurements Most of the photoemission measurements published are performed on structures realized in experimental conditions that are considerably different from those used for real OPVCs. Indeed, for ultraviolet photoelectron spectroscopy (UPS) studies, most of the layers of the heterojunction, with the exception of the TCO film, are deposited in ultrahigh vacuum, which is very different from the experimental conditions used by the classical OPVCs technology. Such different experimental conditions may induce large spreads in the band-disconti- nuity values measured. Therefore, in the present work, we study the properties of the contacts ITO/MoO3and ITO/NiO that we used in our OPVCs. Indeed, we have already shown that, 3-nm thick MoO3films deposited by simple thermal evaporation under vacuum allow improving significantly the PCE of organic solar cells [7]. In the case of NiO, the film thickness is optimized. The NiO films are deposited by sputtering onto the ITO substrate.
The transport properties of OPVCs strongly depend on the interface characteristics: potential barrier height, inter- face states, and band discontinuities. The change in the forbidden gap across the interface is distributed between a VB discontinuity,DEv, and a CB discontinuity,DEc.
DEv (DEc) is positive (negative) when the valence (conduction) band edge of the wide-gap semiconductor is lower than that of the smaller-gap semiconductor [25]. For measuringDEv(DEc) two methods based on XPS measure- ments can be used [26]. In the first, the VB XPS spectra for the bare underlayer surface and for increasing upper layer coverage of this surface are measured. At intermediate coverage (1.5–3 nm) both VB leading edges are visible and a direct measurement ofDEvis possible by linear extrapolation of the two edges. This method is direct but cannot be applied to all interfaces. The second method, known as the semidirect XPS technique, is less direct but can be applied to all interfaces;
it allows the study of real interfaces obtained with the deposition processes used to obtain OPVCs such as chemical deposition [25]. This technique is used in this work and allows us to estimate DEc and DEv at the interface of the heterojunctions ITO/MoO3and ITO/NiO. Measurements have been repeated on different samples to check their validity.
In order to accurately determine the band offsets, we measured the In4d, the Mo4p the Ni3p core levels as well as the VB maxima (VBM)Ev. The accuracy of this method was about0.05 eV for each measure. The VB discontinuityDEv and the CB discontinuityDEcat the heterojunction interface ITO/MoO3are given by
DEv¼ EMo4pEvMoO3
ðEIn4dEvITOÞ DECL; (1)
DEc¼DEvEgMoO3þEgITO; (2) where EMo4p (EIn4d) is the binding energy in the bulk of MoO3(ITO), EvMoO3 (EvITO) is the VB maximum in the bulk of MoO3 (ITO), and DECL is the energy difference between the In4d and Mo4p levels at the interfaces ITO/MoO3. One can determine EMo4p (EIn4d) and EvMoO3 (EvITO) from the MoO3(ITO) film.DECLis obtained from the XPS spectrum of an ITO/MoO3sample with a very thin MoO3layer.
In the case of the ITO/NiO structure, the same equations are used having replaced EMo4p, EvMoO3, and EgMoO3 by ENi3p,EvNiO, andEgNiO, respectively.
The validity of this indirect analytical method has been checked by Chichibu et al. [26] and Hashimoto et al. [25] for inorganic materials.
2.3 Kelvin probe measurements The work func- tion of the ITO has been measured using a Kelvin probe instrument (KPTechnology Model SKP5050). All the measurements are made under ambient conditions (tempera- ture around 238C). The vibrating probe consists of a stainless steel tip of diameter 10 mm and having a work function of 4.947 eV. For the measurements the tip is calibrated against a gold surface before and after each measurement. This calibration value varied by approximately 20–30 meV before and after each measurement, thus keeping the measurement error at 30 meV. The sampling surface is illuminated by a Hg–Zn–Cd discharge lamp giving a peak wavelength at about 253 nm (4.947 eV), which is short enough to be absorbed in the ITO. The nonscanning mode is used to measure the work function with about 500 repetitions for a single point. The work function of the sample is given by adding the measured work function (WF) to the correction factor (4.947 eV). The Kelvin method measures the contact potential difference (CPD) between the tip and the surface of the sample that are brought into contact, as a result of Fermi- energy equalization The vibrating capacitor consists of the surface of the sample under test, the reference surface of the electrode and the insulating medium (here air) between them. The CPD is evaluated by inducing an AC current flow by the vibration of one of the surfaces with respect to the other one, in the vibrating capacitor. The contact potential difference is then measured by determining the compensat- ing voltage required to null this current [21]. The resolution of the measures is 3 meV.
3 Experimental results
3.1 OPVCs characterization Figure 1 presents typical I–V characteristics of OPVCs with NiO ABL, the results achieved with NiO and MoO3ABL are summarized in Table 1.
It can be seen that, whatever the ABL, NiO, or MoO3, it improves significantly the performances of the cells. With- out a buffer layer, ITO being only scrubbed with soap and then rinsed in distilled water, a kink effect is clearly visible in theJ–Vcharacteristics. From Fig. 1, we can see that, when a
thin NiO layer is introduced at the interface ITO/CuPc, 4 nm at least are necessary to achieve a good diode effect, for thinner NiO layer there is a high leakage current. For MoO3 we have previously shown that 4 nm are sufficient to achieve the optimum performance [7]. In a general way, the buffer layer significantly improves the device efficiency through Jsc,Voc, and fill factor (FF) effects, which will be discussed in the light of the XPS study.
3.2 XPS study Figures 2 and 3 show typical XPS spectra. From Fig. 2, the majority carrier type of NiO, MoO3, and ITO can be checked. The absolute value of the energy had been corrected using the C 1s signal (C 1s¼284.6 eV), NiO is p-type, while MoO3and ITO are n-type. We can determine the energy differenceDECLfrom Fig. 3. The ITO film is an nþ-type TCO and therefore the nþ–ITO/n–MoO3 and nþ–ITO/p–NiO contacts are abrupt heterojunctions. It is known that, in the case of abrupt heterojunctions, i.e., in the case of a contact between a metal or degenerated semiconductor with a semiconductor, the depleted zone is completely situated in the semiconductor [27, 28]. This indicates that the flat-band condition remains in the ITO with MoO3or NiO coverage and the structure becomes dominated by the MoO3or NiO bands. This hypothesis will be discussed more carefully below.
This indicates that the flat-band condition remains in the ITO with MoO3(NiO) coverage and the structure becomes dominated by the MoO3(NiO) bands.
-200 -100 0 100 200 300 400 500 600 700 -10
0 10 20
Current density (mA/cm2 )
Voltage (mV)
ITO in dark ITO in light NiO_2nm/ITO (dark) NiO_2nm/ITO (light) NiO_4nm/ITO (dark) NiO_4nm/ITO (light)
Figure 1 (online color at: www.pss-a.com)J–Vcharacteristics of solar cells, using NiO as ABL, under AM 1.5 illumination.
Table 1 Photovoltaic characteristics under AM1.5 conditions of devices, with NiO or MoO3as ABL.
anode Jsc Voc FF h
ITO 4.00 0.41 38 0.68
ITO/NiO (2 nm) 7.35 0.40 34 1.00
ITO/NiO (4 nm) 7.34 0.50 54 1.95
ITO/MoO3(4 nm) 7.50 0.47 54 1.90
When the dopant density is low, the surface band bending of the upper layer is linear in the XPS probing region [28] and there is no perturbation of the DECL value. In the present work, the films have a carrier concentration of 3–61016 and 131016cm3 for MoO3 and NiO, respectively, which is in good agreement with the results published before [29], which is sufficiently low to neglect the band-curving effect.
The bandgap of the MoO3 is 3.30 eV, that of NiO is 3.5 eV, while that of the ITO used in this work is 3.70 eV. The measured values are reported in Tables 2 and 3 and in Fig. 4.
x 101
70 80 90 100 110 120
CPS
6 4 2 0 - 2
Binding Energy (eV) x 101
40 50 60 70 80 90 100 110 120
CPS
1 0 -1 -2 -3 -4
Binding Energy (eV) x 101
70 80 90 100 110 120 130 140
CPS
5 4 3 2 1 0 -1 -2 -3 -4
Binding Energy (eV)
(a)
(b)
(c)
Figure 2 (online color at: www.pss-a.com) VB edge (Ev): (a) of ITO, (b) of MoO3, and (c) of NiO.
B F B F
B F
x 102
6 8 10 12 14 16 18 20 22 24
CPS
45 40 35 30 25 20 15 10 5 0
Binding Energy (eV)
Mo4p
In4d
∆ECL = 22,15 eV
Figure 3 (online color at: www.pss-a.com) Example of XPS spec- trum of thin ITO/MoO3heterostructures (MoO3thickness¼3 nm).
Table 2 XPS binding energies and bandgap of ITO, MoO3, and NiO.
ITO (eV) In4d 18.50
Ev 3.10
In4d-Ev 15.40
Eg 3.7
MoO3(eV) Mo4p 40.10
Ev 3.05
Mo4p-Ev 37.05
Eg 3.3
NiO (eV) Ni3p 67.00
EV 0.20
Ni3p-Ev 66.40
Eg 3.5
Table 3 XPS binding energies band offsets at the interfaces ITO/
MoO3and ITO/NiO (The values are given in eV).
ITO/MoO3(eV) In4d 18.40
Mo4p 40.55
DECL 22.15
DEv 0.5
DEc 0.9
ITO/NiO (eV) In4d 17.50
Ni3p 66.80
DECL 49.30
DEv 2.10
DEc 1.90
First, the fact that the bandgap of our ITO thin films is 3.7 eV, whileEvITO¼3.10 eV, means that, at the surface of the ITO films, the Fermi level, EF, is below the CB minimum.
Because indeed, the energetic position ofEFwith respect to the band edge can be directly determined from the binding energy of the VB maximum,Ev, as the zero binding energy corresponds to the position ofEF. This result is unexpected since, as shown by the slow increase of its resistance with the temperature, the ITO is degenerated. Indeed, experience has shown that the surface chemistry of TCO is difficult to control [30]. As said above, the ITO used in the present work is only scrubbed with soap and then rinsed in distilled water, and therefore it is not completely degenerated at the surface of the films, while it is in the bulk of the films. Indeed, the surface of ITO is hydrophilic and it has been shown that ITO surfaces are terminated in hydrolyzed oxides, including, In(OH)3, InOOH, which form immediately upon exposure of the as-deposited ITO surface to atmosphere [30]. This surface contamination can explain the fact that at the surface the ITO films are not degenerated, while they are below this first contaminated atomic layer. Such an apparent difference in surface and bulk Fermi level positions has already been shown. This has been attributed to carrier depletion near the surface [31].
Since the ITO surface is not degenerated the ITO flat- band hypothesis could be questioned. As discussed by Gassenbauer et al. [32, 33], as the ITO surface is not degenerate, a thin depleted zone is present at the ITO surface.
The width of the depleted zone depends on the density of carriers present at the surface. In the present work, as ITO film is degenerate the carrier density in their bulk is very high (>1021cm3). Therefore, even if at the surface the ITO films are not degenerate the carrier density stays order of
magnitude higher than that of the molybdenum and nickel oxides (3–61016 and 1–31016cm3) and therefore approximation of an abrupt heterojunction nþ–ITO/n–MoO3
remains valid.
Figure 4 depicts the energy-band diagram of the ITO/
MoO3 and ITO/NiO heterostructures based on the band parameters. This band diagram allows us to estimateDEvto 0.5 eV and DEc to 0.90 eV for MoO3, while they are DEv¼ 2.10 eV andDEc¼ 1.9 eV for NiO. The accuracy of the XPS measurements in the present work is0.05 eV.
The work function of ITO, measured in room air using a Kelvin probe isWFITO¼4.8 eV, that of ITO/MoO3is 5.05 eV and that of ITO/NiO is 5.1 eV.
4 Discussions The results presented above have been obtained with a top MoO3film thickness of 3 nm. However, structures with MoO3films of 5 and 7 nm thickness have also been probed. We have checked by scanning electron microscopy that, at least when they are 7 nm thick, the MoO3 films are continuous. Similar XPS results are achieved. However, the accuracy of the measures decreases because the intensity of the signals issued from the ITO bottom layer decrease when the MoO3thickness decreases that decreases the precision of the measure ofDECL.
The accuracy of the present work is less than that obtained with an in situ grown heterojunction [25, 26].
However, the samples were grown under the same experimental conditions as those used to achieve OPV cells [7]. Therefore, the values obtained here give more practical information for real solar-cell performance.
Above the VB and CB of MoO3, in the initial studies of thermally evaporated MoO3layers it was proposed that, as the VB was 5.3 eV, it allows an improvement of the band matching between the ITO anode, with a work function, WFITO, of 4.8 eV and the HOMO of the electron donor, which is usually between 5 and 5.5 eV. It is clear from the present study that the VB of MoO3 cannot be situated between WFITOof ITO and the HOMO of the electron donor since DEv¼0.5 eV (Fig. 4). It means that the VB of MoO3is closer to the VB of ITO than to the CB and the WFof ITO. Actually, using the work-function value of ITO measured by a Kelvin probe and the different DEv and DEc deduced from XPS measurements we can estimate to 8.4 and 5.8 eV the VB values of MoO3and NiO, respectively, while their CB values are 5.1 and 2.3 eV, respectively. These measures, in the case of MoO3are in compliance with the values measured by other groups after air contamination [34, 35]. However, the semidirect XPS method used in the present work does not allow measuring directly theWFof different materials and therefore the VB and CB values proposed here are only rough estimations, neglecting some possible interface effect.
Therefore, it can be concluded that the high CB, VB values for MoO3deduced by recent UPS measures seem in good agreement with the values estimated in the present work.
In the case of NiO, the band scheme configuration of the interface ITO/NiO is different from that of ITO/MoO3, here
Mo4p In4d
Ni3p ∆EC= -1.9eV
∆EC=0.9eV
∆EV=0.5eV ∆EV= -2.1eV
NiO ITO
MoO3
Eg=3.5 eV
Eg=3.3 eV Eg=3.7 eV
∆ECL=-22.15eV ∆ECL=-49.30eV
EF
Figure 4 (online color at: www.pss-a.com) Determined energy- band diagram at the ITO/MoO3and ITO/NiO interfaces.
the CB of NiO is far above the CB of ITO, while its VB is below the CB of ITO, but closer to it than to the VB of ITO.
Therefore, in the case of NiO it can be said that it allows an improvement of the band matching between the ITO anode (FMaround 4.6 eV) and the HOMO of the electron donor (5–5.5 eV).
Therefore, while with the p-type NiO, its VB is more or less situated at the level of the HOMO of the electron donor, in the case of n-type MoO3, it is its CB that is more or less situated at the level of the HOMO of the electron donor. We are thus at the level of the interface with ITO, in presence for these two ABL of very different band structures and nevertheless so effective as regards the improvement of OPVCs efficiency.
However, it should be noted that if NiO and MoO3 exhibit different band structures at the contact with ITO, the work functions of the structures ITO/MoO3and ITO/NiO are 5.05 and 5.1 eV, respectively, which is very similar, while of that of ITO alone is 4.8 eV, which is far smaller.
Therefore, if the band structures at the interface ABL/
ITO, are very different, they have a very close work-function value, which shows that this value is the fundamental property in obtaining a good hole-collecting contact.
5 Conclusions The semi-direct XPS technique used in the present work allows, for the first time, to estimate the DEvandDEcvalues of ITO/oxide structures grown under the inexpensive deposition conditions needed for photovoltaic applications.
In the case of MoO3, the values measured (DEv¼0.50 eV andDEc¼0.90 eV), linked to the value of the ITO work function show that our results are in good agreement with recent values of CB a VB proposed for MoO3 exposed to room air [34, 35]. We show also that, if the NiO and MoO3ABL present different band-structure alignment with ITO, but the structures ITO/MoO3and ITO/NiO have a quite similar work-function value, which is higher than that of ITO. This shows that the quality of the anode/electron donor interface depends strongly on the value of the work- function value of the buffer layer.
The results have been obtained on samples grown under the same conditions as those used to achieve OPV cells.
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