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ITO multilayer for optoelectronic applications
Vacuum 86 (2012) 1318e1322
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Influence of Ag thickness on electrical, optical and structural properties of nanocrystalline MoO3/Ag/ITO multilayer for optoelectronic applications Mohsen Ghasemi Varnamkhasti a, *, Hamid Reza Fallah a, b, Mojtaba Mostajaboddavati a, Ali Hassanzadeh c a
Department of Physics, University of Isfahan, Isfahan, Iran Quantum Optics Research Group, University of Isfahan, Isfahan, Iran c Department of Chemistry, University of Urmia, Urmia, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2011 Received in revised form 25 September 2011 Accepted 5 December 2011
In this study, MoO3/Ag/ITO/glass (MAI) nano-multilayer films were deposited by the thermal evaporation technique and then were annealed in air atmosphere at 200 C for 1 h. The effects of Ag layer thickness on electrical, optical and structural properties of the MoO3(45 nm)/Ag(5e20 nm)/ITO(45 nm)/glass nanomultilayer films were investigated. The sheet resistance decreased rapidly with increasing Ag thickness. Above a thickness of 10 nm, the sheet resistances became somewhat saturated to a value of 3(U/,). The highest transparency over the visible wavelength region of spectrum (85%) was obtained for 10 nm Ag layer thickness. Carrier mobility, carrier concentrations, transmittance and reflectance of the layers were measured. The allowed direct band-gap for an Ag thickness range 5e20 nm was estimated to be in the range 3.58e3.71 eV. The XRD pattern showed that the films were polycrystalline. X-ray diffraction has shown that Ag layer has a (111) predominant orientation when deposited. The figure of merit was calculated for MAI multilayer films. It has been found that the Ag layer thickness is a very important factor in controlling the electrical and optical properties of MAI multilayer films. The optimum thickness of the Ag layer for these films was determined. The results exhibit that the MAI transparent electrode is a good structure for use as the anode of optoelectronic devices. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: MoO3/Ag/ITO/glass multilayer films X-ray diffraction Transparent conductive anode Optical properties
1. Introduction Transparent conducting oxide (TCO) films have been widely researched for use in a variety of optoelectronic devices [1,2], energy-efficient windows [3], organic light emitting diodes (OLEDs) [4], flat panel displays (FPD) [5], storage-type cathode ray tubes [6], surface layers in electroluminescent applications [7], gas sensors [8], photocatalysts [9] and organic photovoltaic cells [10]. A transparent conductive oxide must simultaneously have low resistivity and high transparency. When it is used as anode, in organic photovoltaic cells or in organic light emitting diodes, it must permit effective hole exchange between the anode and the organic material. In order to improve the electrical conductivity a very thin metal film can be used and oxide semiconductor layers are deposited on both sides of the metal film to prevent reflection from the metal in the visible region and obtain a selective transparency. Among the
* Corresponding author. Tel.: þ98 311 7932428; fax: þ98 311 7932409. E-mail addresses: [email protected], [email protected] (M. Ghasemi Varnamkhasti), [email protected] (H.R. Fallah), [email protected] (M. Mostajaboddavati), [email protected] (A. Hassanzadeh). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.12.002
existing TCOs, indium tin oxide is one of the most frequently used materials because of its unique characteristics such as high conductance, high optical transmittance over the visible wavelength region, chemical stability and excellent adhesion to substrates [11]. In this work, the proposed structure for the anode is MoO3/Ag/ITO/glass or MAI, since Ag metal films have low resistivity and MoO3 is an oxide buffer layer between the anode and the organic material [12]. To the best of our knowledge according to the literature, less attention has been paid to a study of the electrical, optical and structural properties of MAI multilayer films deposited by the thermal evaporation technique. The effect of this anode on performance of MAI-based organic photovoltaics has not been reported. The optical and electrical properties of Ag thin films will change significantly with different thickness of film, which will greatly affect the performance of the multilayer films. We report the effect of Ag thickness on electrical, optical and structural properties of nanocrystalline MAI multilayer films deposited by thermal evaporation on glass substrates, which were post annealed in the air. Properties of the MAI multilayer were measured as a function of Ag film thickness by analyzing electrical resistivity, Xray diffraction (XRD) spectra, optical transmittance, reflectance, and morphology. After optimization the Ag thickness in the MAI
M. Ghasemi Varnamkhasti et al. / Vacuum 86 (2012) 1318e1322
multilayer films they were used as an effective transparent anode in organic photovoltaic cells. 2. Experiments 2.1. Sample preparation Glass slides were immersed in boiling sulfuric acid and Milli-Q water for 30 min. Afterwards they were ultrasonically cleaned in acetone and absolute ethanol for 10 min. Finally they were rinsed with distilled water and dried in hot air. Ag Thin films and MAI multilayer have been prepared on glass. ITO thin films with composition of 10 wt% SnO2 and 90 wt% In2O3, were deposited on glass substrates using thermal evaporation. High-purity metal Ag powder (99.99% purity) and MoO3 (99.99% purity) were used as source material. Substrate temperature during the deposition process was kept at room temperature (25 C). Deposition of all films was performed at a pressure of 5 105 mbar in the vacuum chamber. Deposition rate and film thickness were measured by a quartz crystal system. During the optimization of the MoO3/Ag/ ITO/glass performance the deposition rates of ITO and MoO3 were 0.05 nm s1and that of silver was 0.1 nm s1. The thicknesses of ITO and MoO3 films were fixed at 45 nm, while the Ag layer thickness was varied from 5 to 20 nm. Optical measurements of the samples were done in the wavelength range from 300 to 800 nm with a double-beam spectrophotometer (Shimadzu UV 3100) for recording the UVevisible optical transmission and reflection spectra of samples. The sheet resistance of films was measured by a four-point probe. Electrical properties of the films were investigated by Hall effect measurements. The crystal structure of MAI thin films deposited at different thicknesses were characterized using the XRD technique with a D8 Advanced Bruker X-ray diffractometer at room temperature, with monochromated CuKa (l ¼ 1.54 A) in the scan range of 2q between 15 and 90 and a step size of 0.01 (2q/s). Measurements were taken under beamacceleration conditions of 40 kV/35 mA. The surface morphology of the Ag layer grown on the ITO film was examined as a function of the Ag thickness by scanning electron microscopy (SEM).
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Table 1 Electrical characteristics of ITO (45 nm)/Ag/MoO3 (45 nm) structures. Ag Thickness (nm)
Resistivity (Ucm)
5 10 15 20
4.75 8 4.2 3.3
104 105 105 105
Carrier concentration (cm3) 7.6 38.4 66.9 75.3
1020 1020 1020 1020
Mobility (cm2 V1 S1) 17.1 20.4 22.3 25.2
was over the thickness of 10 nm, where the sheet resistances were somewhat saturated at 3(U/,). The enhancement of the Ag layer crystallinity with thickness, results in the decrease of sheet resistance. The sheet resistance of the films for 10 nm Ag film thickness was 4(U/,) which was much lower than the value previously reported for ITO/Ag/ITO multilayers [13,14]. This value of sheet resistance is sufficiently low to use as an anode electrode for organic photovoltaic cells. A continuous layer of Ag has very good electrical conductivity. The electrical behaviour of these films is attributed to transition from distinct islands of Ag atoms to the formation of a continuous film. The comparison of the sheet resistance of all MAI multilayer films and the Ag layer leads to the conclusion that the conductivity of the multilayer system is essentially due to the metal Ag film. The sheet resistance is proportional to both the carrier concentration and carrier mobility. When the thickness is above 5 nm, holes among the associated islands are gradually filled and electron mobility increases. When the film thickness is larger than 10 nm, the holes are almost filled, so the sheet resistance plot demonstrates an almost constant level. The carrier concentration and carrier mobility values of MAI multilayer films are shown in Table 1. The results indicated that the carrier mobility and carrier concentration was increased when the Ag thickness increased. Therefore, the resistivity of MAI multilayer films decreased. 3.2. Optical properties
Fig. 1 shows the sheet resistance of MoO3(45 nm)/Ag/ ITO(45 nm)/glass multilayer with different Ag layer thickness. In this figure, the overall variations are divided in two regions. Region I was roughly up to the Ag thickness of 10 nm, where the sheet resistance decreased rapidly with the Ag thickness and region II
The transmittance and reflectance of MAI samples were measured in the wavelength region from 300 to 800 nm. A high transparency for the thin films in the visible wavelength region is required in various applications such as transparent electrodes in optoelectronic devices. Fig. 2 shows the optical transmission of the films in the mentioned region. As can be seen, the optical transmission of MAI multilayer films improves with increase of Ag layer thickness from 5 to 10 nm and it decreases with further increase. Of all the MAI films, the multilayer films with a 10 nm thick Ag indicated the highest optical transmittance in the visible wavelength region which is consistent with the absorption wavelength of the organic active material. The films with 20 nm-Ag thickness showed
Fig. 1. Dependence of sheet resistance of MAI multilayer films as a function of Ag layer thickness.
Fig. 2. Dependence of optical transmission spectra of MAI multilayer films on the Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
3. Results and discussions 3.1. Electrical properties
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Fig. 3. Dependence of reflection spectra of MAI multilayer films on the Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
the lowest optical transmittance. This may be attributed to the reflection of light from a fairly thick Ag film. It seems that the 5 nm thickness of silver film is not yet continuous. This can cause scattering of the incident light by the discontinuous Ag layer [15]. The conductance values of these films will confirm this explanation. When the Ag thickness increases, the absorption edge is shifted to the higher wavelength. The absorption edge shift in this study is consistent with the results of others [14]. As it can be seen from Fig. 3 the reflection of the multilayer system increased with Ag layer thickness. With increasing amount of silver, the reflectance in the near infrared increased. The increase in the reflection in the near infrared region is due to the interaction of free electrons with the incident radiation.
Fig. 5. Variation of band-gap energy of the MAI multilayer films with Ag layer thickness.
layer thickness. The optical band-gap of the MAI multilayer system decreases with increasing Ag layer thickness. This may be attributed to the introduction of some defects which create localized states in the band-gap and therefore decrease the band-gap. Thus, the results showed that a lower optical band-gap can be obtained by increasing the thickness of Ag film. 3.3. Figure of merit The figure of merit (FTC), defined by Haacke [18], is an important index for evaluating the performance of transparent conductive oxide films and is defined as:
FTC ¼ 3.2.1. Calculation of the optical band-gap The optical transmission (T) and reflectance (R) data were used to calculate absorption coefficients of the MAI films at different wavelengths. The absorption coefficient for the direct allowed transition can be described as a function of photon energy [16]:
ðahyÞ2 ¼ A hy Eg
where Eg is energy gap, hy is the photon energy, a is the absorption coefficient and A is a constant. Fig. 4 shows the dependence of plots of (ahy)2 versus hy of MAI multilayer films on the Ag layer thickness. The plots had a linear region, and extrapolation of the straight line to zero absorption gave the effective energy gap for different multilayer system. It should be noticed that such plots are not strictly appropriate for these systems and should be used with caution. Nevertheless, it is conventional to obtain the effective band-gap from this plot [17]. Fig. 5 exhibits the variation of the energy gap, Eg, of the MAI multilayer system as a function of Ag
Fig. 4. Dependence of plots of (ahy)2 versus hy of MAI multilayer films on the Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
10 Tav Rs
where Tav is the transmittance and Rs is the sheet resistance. Fig. 6 shows a plot of FTC as a function of Ag thickness. From the plot, we can see that the best FTC is obtained when the Ag layer is continuous for MAI multilayer films. In this study, since a higher FTC results in better quality transparent conductive oxide films [19], the MAI films with a 10 nm Ag inter-layer have better opto-electrical properties than the other MAI films. However the films with a thicker Ag layer did not show high enough optical transmission to be employed as transparent conductive electrodes, MAI multilayer films still have good potential for use in specialized applications because of their high conductance. The optimum Ag thickness in MAI multilayer films was found to be 10 nm for thermally evaporation films with the evaporation parameters mentioned above. 3.4. Structural properties In order to investigate the effect of thickness Ag layer on the structural properties of MAI multilayer films, XRD and SEM
Fig. 6. Dependence of figure of merit of MAI multilayer films on Ag layer thickness.
M. Ghasemi Varnamkhasti et al. / Vacuum 86 (2012) 1318e1322
Fig. 7. XRD patterns of MAI multilayer films as a function of Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
analyses were performed. The XRD patterns of MAI multilayer films deposited with different thicknesses of Ag layer are shown in Fig. 7. The line at 2q at 31.5 corresponds to the reflection from the (222) plane. This peak is close to the position of the strongest line of the reference indium oxide pattern. The other peaks are due to the reflection from the (211), (400), (431), (440) and (622) planes. An Ag (111) orientation was observed at about 38.2 . None of the patterns indicated any characteristic peaks of MoO3, which means that the MoO3 layer was amorphous in the multilayer system. This may be attributed to the fact that the annealing temperature is not sufficient for crystallisation of MoO3. We have also used the average full width at half maximum (FWHM) of the strongest XRD line to evaluate the grain size of ITO in the MAI multilayer films using the Scherrer equation [20]:
D ¼
0:9l BcosqB
where l ¼ 0.154 nm and B is the measured broadening of the diffraction line peak at an angle of 2q in radians, at FWHM. The ITO
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grain size remains almost the same with increasing Ag layer thickness. The ITO grain size is about 25 nm. Since the silver peak width decreases with increasing Ag thickness the grain size of silver increases. In order to investigate the transition of Ag deposits from islands to a continuous film, SEM analyses were performed. Fig. 8 shows SEM images of free surface of the Ag film on the ITO layer as a function of the Ag thickness. It can be seen that the surface morphology of the Ag films changed significantly with thickness. When the Ag thickness is 5 nm a discontinuous Ag film was formed. Disconnected Ag films lead to an increase in sheet resistance and light absorption. As shown in Fig. 8(b), more agglomeration of Ag islands is observed in a 10 nm thick film. The sheet resistance of this film was significantly decreased. The low sheet resistance of the MAI multilayer films with the thickness of Ag above 10 nm was attributed to the better connection of Ag islands. As mentioned above, the transmittance of MAI multilayer films decreased for the 20 nm thick Ag layer. This decrease could be attributed to the ITO layer being completely coated by the Ag film. 3.5. Organic solar cell In order to examine and demonstrate the effect of the fabricated MAI films, MAI/PEDOT: PSS/CuPc/C60/LiF/Ag organic solar cells were fabricated. A 40 nm-thick layer of poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT-PSS) was deposited by spin coating on the anode coated glass substrates, acting as the hole transport layer. CuPc, C60 and LiF were deposited in a vacuum of 5 105 mbar. The thin film deposition rates and thickness of layers were evaluated in situ with a quartz monitor. The deposition rate and final thickness were 0.05 nm/s and 40 nm in the case of CuPc and C60. A 1 nm-thick LiF buffer layer was deposited using a 0.01 nm/s deposition rate. After deposition of the buffer layer, a 100 nm-thick Ag layer was deposited by thermal evaporation as the cathode. This organic solar cell was evaluated when the anode was a 100 nm-thick ITO. A single layer of ITO has 20(U/,) and 85% transparency in the visible wavelength region.
Fig. 8. SEM images of the free surface of the Ag film on the ITO layer as a function of Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
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typically in the range of 10e20 nm. But the thicker Ag films lead to a rapid decrease in optical transmission. Also, from the results we may conclude that the Ag thickness plays a major role in controlling the optoelectronic properties of the MAI multilayer films. From figure of merit results we can conclude that the MAI films with a 10 nm Ag inter-layer have better optoelectronic properties than other films in this study. We have demonstrated that the MoO3/Ag/ ITO/glass structure can be used as an effective transparent anode in organic photovoltaic cells. The performances of the organic photovoltaic cells with an MAI multilayer anode are better than the cells with a single ITO anode. Thus, the MAI multilayer is a promising anode that can be used instead of a conventional ITO anode.
Acknowledgment Fig. 9. Current densityevoltage characteristics of the organic photovoltaic cells fabricated on the optimized MAI and single ITO anodes. Table 2 Photovoltaic parameters for the devices with different anodes (under an illumination of 100 mW/cm2 with an AM 1.5 G sun simulator). Anode
Voc (V)
Jsc (mA/cm2)
FF (%)
h (%)
ITO ITO/Ag (10 nm)/MoO3
0.45 0.45
3.9 6
38.4 51.3
0.675 1.38
We want to stress here that we do not intend to achieve the best overall cell performance. We show that MAI multilayer films can be used as an effective anode in organic photovoltaic cells. The active area for the device is about 0.2 cm2. Fig. 9 shows the current densityevoltage (JeV) characteristics of the organic solar cell fabricated on MAI and ITO that were measured with a Keithley 2400 source meter under an illumination 100 mW/cm2 irradiation from a solar simulator. For comparison, identical organic films and cathodes were deposited on the MAI and ITO anode electrodes simultaneously under the same deposition conditions. Photovoltaic parameters of both types of organic photovoltaic cell are shown in Table 2. Since the structure of both organic photovoltaic cells is identical, both types exhibited similar open circuit voltage (Voc). The fill factor (FF) and short circuit current (Jsc) values increased for the MAI multilayer anode electrode because the FF and Jsc were critically affected by the sheet resistance of the electrodes. Series resistance of anode material has no effect on Voc, but reduces the Jsc values [21]. Also, the power conversion efficiency increased from 0.675% to 1.38%, due to improved FF and Jsc values. In this study, two factors influence the power conversion efficiency. One is the higher optical transmittance in the wavelength 400e800 nm, which is the absorption region of the active layer of the CuPc-C60 based organic photovoltaic cell and another is the low sheet resistance of MAI multilayer films. Also, in the case of CuPc-C60 cells the maximum transmission of the MAI multilayer anode is matched with the CuPc absorption. 4. Conclusions MoO3/Ag/ITO/glass (MAI) multilayer films with different Ag thickness have been prepared by thermal evaporation. It is found that the electrical, optical and structural properties of these multilayer films clearly depend on the Ag film thickness. The effect of the intermediate Ag thickness on the optoelectronic properties of the films was investigated. The MAI multilayer electrode with 10 nm thick Ag layer exhibited a very low sheet resistance of 4(U/ ,) and optical transmission of 85% in the visible wavelength region. This work shows that one of the key factors in achieving both high transmittance and low sheet resistance in these electrodes is to obtain continuous Ag films, for which their thickness is
The authors would like to thank the Graduate Office of Isfahan University for their support. The authors are greatly indebted to the Iran Nanotechnology initiative council for their financial support.
References [1] Wang CH, Chen SW, Wu JM, Wei CN, Bor HY. Effect of postdeposition oxidation and subsequent reduction annealing on electric and optical properties of amorphous ZnOeSnO2 transparent conducting films. Electrochem Solid-State Lett 2011;14:5e8. [2] Jain VK, Kumar P, Bhandari D, Vijay YK. Growth and characterization of transparent conducting nanostructured zinc indium oxide thin films. Thin Solid Films 2010;519:1082e6. [3] Fallah HR, Varnamkhasti MG, Vahid MJ. Substrate temperature effect on transparent heat reflecting nanocrystalline ITO films prepared by electron beam evaporation. Renew Energ 2010;35:1527e30. [4] Nam E, Kang YH, Jung D, Kim YS. Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes. Thin Solid Films 2010;518:6245e8. [5] Lee BH, Kim IG, Cho SW, Lee SH. Effect of process parameters on the characteristics of indium tin oxide thin film for flat panel display application. Thin Solid Films 1997;302:25e30. [6] Pankove JI. Display devices, topics in applied physics, vol. 40. Berlin: SpringerVerlag; 1980. [7] Meng LJ, Li CH, Zhong GZ. The influence of the concentration of Er3þ ions on the characteristics of AC-electroluminescence in ZnS: ErF3 thin films. J Lumin 1987;39:11e7. [8] Min BK, Choi SD. SnO2 thin film gas sensor fabricated by ion beam deposition. Sensors Actuators B 2004;98:239e46. [9] Poznyak SK, Golubev AN, Kulak AI. Correlation between surface properties and photocatalytic and photoelectrochemical activity of In2O3 nanocrystalline films and powders. Surf Sci 2000;454:396e401. [10] Yu J, Wang N, Zang Y, Jiang Y. Sol Energ Mat Sol Cells 2011;95:664e8. [11] Fallah HR, Ghasemi M, Hassanzadeh A, Steki H. The effect of annealing on structural, electrical and optical properties of nanostructured ITO films prepared by e-beam evaporation. Mater Res Bull 2007;42:487e96. [12] Kanai Y, Matsushima T, Murata H. Improvement of stability for organic solar cells by using molybdenum trioxide buffer layer. Thin Solid Films 2009;518:537e40. [13] Lewis J, Grego S, Chalamala B, Vick E, Temple D. Highly flexible transparent electrodes for organic light-emitting diode-based displays. Appl Phys Lett 2004;85:3450e2. [14] Guillen C, Herrero J. Transparent conductive ITO/Ag/ITO multilayer electrodes deposited by sputtering at room temperature. Opt Commun 2009;282:574e8. [15] Sahu DR, Lin SY, Huang JL. ZnO/Ag/ZnO multilayer films for the application of a very low resistance transparent electrode. Appl Surf Sci 2006;252:7509e14. [16] Mohamed SH. Effects of Ag layer and ZnO top layer thicknesses on the physical properties of ZnO/Ag/Zno multilayer system. J Phys Chem Solids 2008;69:2378e84. [17] Cho JY, Kim IK, Jung IO, Moon JH, Kim JH. Effects of Mg doping concentration on the band gap of ZnO/MgxZn1xO multilayer thin films prepared using pulsed laser deposition method. J Electroceram 2009;23:442e6. [18] Haacke G. New figure of merit for transparent conductors. J Appl Phys 1976; 47:4086e9. [19] Kim D. Properties of ITO/Cu/ITO multilayer films for application as low resistance transparent electrodes. Trans Electr Electron Mater 2009;10:165e8. [20] Kloppel A, Kriegseis W, Meyer BK, Scharmann A, Daube C, Stollenwerk J, et al. Dependence of the electrical and optical behaviour of ITOesilvereITO multilayers on the silver properties. Thin Solid Films 2000;365:139e46. [21] Luque A, Hegedus S. Handbook of photovoltaics science and engineering. New York: John Wiley & Sons Ltd.; 2003.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Influence of Ag thickness on electrical, optical and structural properties of nanocrystalline MoO3/Ag/ITO multilayer for optoelectronic applications Mohsen Ghasemi Varnamkhasti a, *, Hamid Reza Fallah a, b, Mojtaba Mostajaboddavati a, Ali Hassanzadeh c a
Department of Physics, University of Isfahan, Isfahan, Iran Quantum Optics Research Group, University of Isfahan, Isfahan, Iran c Department of Chemistry, University of Urmia, Urmia, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2011 Received in revised form 25 September 2011 Accepted 5 December 2011
In this study, MoO3/Ag/ITO/glass (MAI) nano-multilayer films were deposited by the thermal evaporation technique and then were annealed in air atmosphere at 200 C for 1 h. The effects of Ag layer thickness on electrical, optical and structural properties of the MoO3(45 nm)/Ag(5e20 nm)/ITO(45 nm)/glass nanomultilayer films were investigated. The sheet resistance decreased rapidly with increasing Ag thickness. Above a thickness of 10 nm, the sheet resistances became somewhat saturated to a value of 3(U/,). The highest transparency over the visible wavelength region of spectrum (85%) was obtained for 10 nm Ag layer thickness. Carrier mobility, carrier concentrations, transmittance and reflectance of the layers were measured. The allowed direct band-gap for an Ag thickness range 5e20 nm was estimated to be in the range 3.58e3.71 eV. The XRD pattern showed that the films were polycrystalline. X-ray diffraction has shown that Ag layer has a (111) predominant orientation when deposited. The figure of merit was calculated for MAI multilayer films. It has been found that the Ag layer thickness is a very important factor in controlling the electrical and optical properties of MAI multilayer films. The optimum thickness of the Ag layer for these films was determined. The results exhibit that the MAI transparent electrode is a good structure for use as the anode of optoelectronic devices. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: MoO3/Ag/ITO/glass multilayer films X-ray diffraction Transparent conductive anode Optical properties
1. Introduction Transparent conducting oxide (TCO) films have been widely researched for use in a variety of optoelectronic devices [1,2], energy-efficient windows [3], organic light emitting diodes (OLEDs) [4], flat panel displays (FPD) [5], storage-type cathode ray tubes [6], surface layers in electroluminescent applications [7], gas sensors [8], photocatalysts [9] and organic photovoltaic cells [10]. A transparent conductive oxide must simultaneously have low resistivity and high transparency. When it is used as anode, in organic photovoltaic cells or in organic light emitting diodes, it must permit effective hole exchange between the anode and the organic material. In order to improve the electrical conductivity a very thin metal film can be used and oxide semiconductor layers are deposited on both sides of the metal film to prevent reflection from the metal in the visible region and obtain a selective transparency. Among the
* Corresponding author. Tel.: þ98 311 7932428; fax: þ98 311 7932409. E-mail addresses: [email protected], [email protected] (M. Ghasemi Varnamkhasti), [email protected] (H.R. Fallah), [email protected] (M. Mostajaboddavati), [email protected] (A. Hassanzadeh). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.12.002
existing TCOs, indium tin oxide is one of the most frequently used materials because of its unique characteristics such as high conductance, high optical transmittance over the visible wavelength region, chemical stability and excellent adhesion to substrates [11]. In this work, the proposed structure for the anode is MoO3/Ag/ITO/glass or MAI, since Ag metal films have low resistivity and MoO3 is an oxide buffer layer between the anode and the organic material [12]. To the best of our knowledge according to the literature, less attention has been paid to a study of the electrical, optical and structural properties of MAI multilayer films deposited by the thermal evaporation technique. The effect of this anode on performance of MAI-based organic photovoltaics has not been reported. The optical and electrical properties of Ag thin films will change significantly with different thickness of film, which will greatly affect the performance of the multilayer films. We report the effect of Ag thickness on electrical, optical and structural properties of nanocrystalline MAI multilayer films deposited by thermal evaporation on glass substrates, which were post annealed in the air. Properties of the MAI multilayer were measured as a function of Ag film thickness by analyzing electrical resistivity, Xray diffraction (XRD) spectra, optical transmittance, reflectance, and morphology. After optimization the Ag thickness in the MAI
M. Ghasemi Varnamkhasti et al. / Vacuum 86 (2012) 1318e1322
multilayer films they were used as an effective transparent anode in organic photovoltaic cells. 2. Experiments 2.1. Sample preparation Glass slides were immersed in boiling sulfuric acid and Milli-Q water for 30 min. Afterwards they were ultrasonically cleaned in acetone and absolute ethanol for 10 min. Finally they were rinsed with distilled water and dried in hot air. Ag Thin films and MAI multilayer have been prepared on glass. ITO thin films with composition of 10 wt% SnO2 and 90 wt% In2O3, were deposited on glass substrates using thermal evaporation. High-purity metal Ag powder (99.99% purity) and MoO3 (99.99% purity) were used as source material. Substrate temperature during the deposition process was kept at room temperature (25 C). Deposition of all films was performed at a pressure of 5 105 mbar in the vacuum chamber. Deposition rate and film thickness were measured by a quartz crystal system. During the optimization of the MoO3/Ag/ ITO/glass performance the deposition rates of ITO and MoO3 were 0.05 nm s1and that of silver was 0.1 nm s1. The thicknesses of ITO and MoO3 films were fixed at 45 nm, while the Ag layer thickness was varied from 5 to 20 nm. Optical measurements of the samples were done in the wavelength range from 300 to 800 nm with a double-beam spectrophotometer (Shimadzu UV 3100) for recording the UVevisible optical transmission and reflection spectra of samples. The sheet resistance of films was measured by a four-point probe. Electrical properties of the films were investigated by Hall effect measurements. The crystal structure of MAI thin films deposited at different thicknesses were characterized using the XRD technique with a D8 Advanced Bruker X-ray diffractometer at room temperature, with monochromated CuKa (l ¼ 1.54 A) in the scan range of 2q between 15 and 90 and a step size of 0.01 (2q/s). Measurements were taken under beamacceleration conditions of 40 kV/35 mA. The surface morphology of the Ag layer grown on the ITO film was examined as a function of the Ag thickness by scanning electron microscopy (SEM).
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Table 1 Electrical characteristics of ITO (45 nm)/Ag/MoO3 (45 nm) structures. Ag Thickness (nm)
Resistivity (Ucm)
5 10 15 20
4.75 8 4.2 3.3
104 105 105 105
Carrier concentration (cm3) 7.6 38.4 66.9 75.3
1020 1020 1020 1020
Mobility (cm2 V1 S1) 17.1 20.4 22.3 25.2
was over the thickness of 10 nm, where the sheet resistances were somewhat saturated at 3(U/,). The enhancement of the Ag layer crystallinity with thickness, results in the decrease of sheet resistance. The sheet resistance of the films for 10 nm Ag film thickness was 4(U/,) which was much lower than the value previously reported for ITO/Ag/ITO multilayers [13,14]. This value of sheet resistance is sufficiently low to use as an anode electrode for organic photovoltaic cells. A continuous layer of Ag has very good electrical conductivity. The electrical behaviour of these films is attributed to transition from distinct islands of Ag atoms to the formation of a continuous film. The comparison of the sheet resistance of all MAI multilayer films and the Ag layer leads to the conclusion that the conductivity of the multilayer system is essentially due to the metal Ag film. The sheet resistance is proportional to both the carrier concentration and carrier mobility. When the thickness is above 5 nm, holes among the associated islands are gradually filled and electron mobility increases. When the film thickness is larger than 10 nm, the holes are almost filled, so the sheet resistance plot demonstrates an almost constant level. The carrier concentration and carrier mobility values of MAI multilayer films are shown in Table 1. The results indicated that the carrier mobility and carrier concentration was increased when the Ag thickness increased. Therefore, the resistivity of MAI multilayer films decreased. 3.2. Optical properties
Fig. 1 shows the sheet resistance of MoO3(45 nm)/Ag/ ITO(45 nm)/glass multilayer with different Ag layer thickness. In this figure, the overall variations are divided in two regions. Region I was roughly up to the Ag thickness of 10 nm, where the sheet resistance decreased rapidly with the Ag thickness and region II
The transmittance and reflectance of MAI samples were measured in the wavelength region from 300 to 800 nm. A high transparency for the thin films in the visible wavelength region is required in various applications such as transparent electrodes in optoelectronic devices. Fig. 2 shows the optical transmission of the films in the mentioned region. As can be seen, the optical transmission of MAI multilayer films improves with increase of Ag layer thickness from 5 to 10 nm and it decreases with further increase. Of all the MAI films, the multilayer films with a 10 nm thick Ag indicated the highest optical transmittance in the visible wavelength region which is consistent with the absorption wavelength of the organic active material. The films with 20 nm-Ag thickness showed
Fig. 1. Dependence of sheet resistance of MAI multilayer films as a function of Ag layer thickness.
Fig. 2. Dependence of optical transmission spectra of MAI multilayer films on the Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
3. Results and discussions 3.1. Electrical properties
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Fig. 3. Dependence of reflection spectra of MAI multilayer films on the Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
the lowest optical transmittance. This may be attributed to the reflection of light from a fairly thick Ag film. It seems that the 5 nm thickness of silver film is not yet continuous. This can cause scattering of the incident light by the discontinuous Ag layer [15]. The conductance values of these films will confirm this explanation. When the Ag thickness increases, the absorption edge is shifted to the higher wavelength. The absorption edge shift in this study is consistent with the results of others [14]. As it can be seen from Fig. 3 the reflection of the multilayer system increased with Ag layer thickness. With increasing amount of silver, the reflectance in the near infrared increased. The increase in the reflection in the near infrared region is due to the interaction of free electrons with the incident radiation.
Fig. 5. Variation of band-gap energy of the MAI multilayer films with Ag layer thickness.
layer thickness. The optical band-gap of the MAI multilayer system decreases with increasing Ag layer thickness. This may be attributed to the introduction of some defects which create localized states in the band-gap and therefore decrease the band-gap. Thus, the results showed that a lower optical band-gap can be obtained by increasing the thickness of Ag film. 3.3. Figure of merit The figure of merit (FTC), defined by Haacke [18], is an important index for evaluating the performance of transparent conductive oxide films and is defined as:
FTC ¼ 3.2.1. Calculation of the optical band-gap The optical transmission (T) and reflectance (R) data were used to calculate absorption coefficients of the MAI films at different wavelengths. The absorption coefficient for the direct allowed transition can be described as a function of photon energy [16]:
ðahyÞ2 ¼ A hy Eg
where Eg is energy gap, hy is the photon energy, a is the absorption coefficient and A is a constant. Fig. 4 shows the dependence of plots of (ahy)2 versus hy of MAI multilayer films on the Ag layer thickness. The plots had a linear region, and extrapolation of the straight line to zero absorption gave the effective energy gap for different multilayer system. It should be noticed that such plots are not strictly appropriate for these systems and should be used with caution. Nevertheless, it is conventional to obtain the effective band-gap from this plot [17]. Fig. 5 exhibits the variation of the energy gap, Eg, of the MAI multilayer system as a function of Ag
Fig. 4. Dependence of plots of (ahy)2 versus hy of MAI multilayer films on the Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
10 Tav Rs
where Tav is the transmittance and Rs is the sheet resistance. Fig. 6 shows a plot of FTC as a function of Ag thickness. From the plot, we can see that the best FTC is obtained when the Ag layer is continuous for MAI multilayer films. In this study, since a higher FTC results in better quality transparent conductive oxide films [19], the MAI films with a 10 nm Ag inter-layer have better opto-electrical properties than the other MAI films. However the films with a thicker Ag layer did not show high enough optical transmission to be employed as transparent conductive electrodes, MAI multilayer films still have good potential for use in specialized applications because of their high conductance. The optimum Ag thickness in MAI multilayer films was found to be 10 nm for thermally evaporation films with the evaporation parameters mentioned above. 3.4. Structural properties In order to investigate the effect of thickness Ag layer on the structural properties of MAI multilayer films, XRD and SEM
Fig. 6. Dependence of figure of merit of MAI multilayer films on Ag layer thickness.
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Fig. 7. XRD patterns of MAI multilayer films as a function of Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
analyses were performed. The XRD patterns of MAI multilayer films deposited with different thicknesses of Ag layer are shown in Fig. 7. The line at 2q at 31.5 corresponds to the reflection from the (222) plane. This peak is close to the position of the strongest line of the reference indium oxide pattern. The other peaks are due to the reflection from the (211), (400), (431), (440) and (622) planes. An Ag (111) orientation was observed at about 38.2 . None of the patterns indicated any characteristic peaks of MoO3, which means that the MoO3 layer was amorphous in the multilayer system. This may be attributed to the fact that the annealing temperature is not sufficient for crystallisation of MoO3. We have also used the average full width at half maximum (FWHM) of the strongest XRD line to evaluate the grain size of ITO in the MAI multilayer films using the Scherrer equation [20]:
D ¼
0:9l BcosqB
where l ¼ 0.154 nm and B is the measured broadening of the diffraction line peak at an angle of 2q in radians, at FWHM. The ITO
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grain size remains almost the same with increasing Ag layer thickness. The ITO grain size is about 25 nm. Since the silver peak width decreases with increasing Ag thickness the grain size of silver increases. In order to investigate the transition of Ag deposits from islands to a continuous film, SEM analyses were performed. Fig. 8 shows SEM images of free surface of the Ag film on the ITO layer as a function of the Ag thickness. It can be seen that the surface morphology of the Ag films changed significantly with thickness. When the Ag thickness is 5 nm a discontinuous Ag film was formed. Disconnected Ag films lead to an increase in sheet resistance and light absorption. As shown in Fig. 8(b), more agglomeration of Ag islands is observed in a 10 nm thick film. The sheet resistance of this film was significantly decreased. The low sheet resistance of the MAI multilayer films with the thickness of Ag above 10 nm was attributed to the better connection of Ag islands. As mentioned above, the transmittance of MAI multilayer films decreased for the 20 nm thick Ag layer. This decrease could be attributed to the ITO layer being completely coated by the Ag film. 3.5. Organic solar cell In order to examine and demonstrate the effect of the fabricated MAI films, MAI/PEDOT: PSS/CuPc/C60/LiF/Ag organic solar cells were fabricated. A 40 nm-thick layer of poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT-PSS) was deposited by spin coating on the anode coated glass substrates, acting as the hole transport layer. CuPc, C60 and LiF were deposited in a vacuum of 5 105 mbar. The thin film deposition rates and thickness of layers were evaluated in situ with a quartz monitor. The deposition rate and final thickness were 0.05 nm/s and 40 nm in the case of CuPc and C60. A 1 nm-thick LiF buffer layer was deposited using a 0.01 nm/s deposition rate. After deposition of the buffer layer, a 100 nm-thick Ag layer was deposited by thermal evaporation as the cathode. This organic solar cell was evaluated when the anode was a 100 nm-thick ITO. A single layer of ITO has 20(U/,) and 85% transparency in the visible wavelength region.
Fig. 8. SEM images of the free surface of the Ag film on the ITO layer as a function of Ag layer thickness (a: 5 nm, b: 10 nm, c: 15 nm, d: 20 nm).
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typically in the range of 10e20 nm. But the thicker Ag films lead to a rapid decrease in optical transmission. Also, from the results we may conclude that the Ag thickness plays a major role in controlling the optoelectronic properties of the MAI multilayer films. From figure of merit results we can conclude that the MAI films with a 10 nm Ag inter-layer have better optoelectronic properties than other films in this study. We have demonstrated that the MoO3/Ag/ ITO/glass structure can be used as an effective transparent anode in organic photovoltaic cells. The performances of the organic photovoltaic cells with an MAI multilayer anode are better than the cells with a single ITO anode. Thus, the MAI multilayer is a promising anode that can be used instead of a conventional ITO anode.
Acknowledgment Fig. 9. Current densityevoltage characteristics of the organic photovoltaic cells fabricated on the optimized MAI and single ITO anodes. Table 2 Photovoltaic parameters for the devices with different anodes (under an illumination of 100 mW/cm2 with an AM 1.5 G sun simulator). Anode
Voc (V)
Jsc (mA/cm2)
FF (%)
h (%)
ITO ITO/Ag (10 nm)/MoO3
0.45 0.45
3.9 6
38.4 51.3
0.675 1.38
We want to stress here that we do not intend to achieve the best overall cell performance. We show that MAI multilayer films can be used as an effective anode in organic photovoltaic cells. The active area for the device is about 0.2 cm2. Fig. 9 shows the current densityevoltage (JeV) characteristics of the organic solar cell fabricated on MAI and ITO that were measured with a Keithley 2400 source meter under an illumination 100 mW/cm2 irradiation from a solar simulator. For comparison, identical organic films and cathodes were deposited on the MAI and ITO anode electrodes simultaneously under the same deposition conditions. Photovoltaic parameters of both types of organic photovoltaic cell are shown in Table 2. Since the structure of both organic photovoltaic cells is identical, both types exhibited similar open circuit voltage (Voc). The fill factor (FF) and short circuit current (Jsc) values increased for the MAI multilayer anode electrode because the FF and Jsc were critically affected by the sheet resistance of the electrodes. Series resistance of anode material has no effect on Voc, but reduces the Jsc values [21]. Also, the power conversion efficiency increased from 0.675% to 1.38%, due to improved FF and Jsc values. In this study, two factors influence the power conversion efficiency. One is the higher optical transmittance in the wavelength 400e800 nm, which is the absorption region of the active layer of the CuPc-C60 based organic photovoltaic cell and another is the low sheet resistance of MAI multilayer films. Also, in the case of CuPc-C60 cells the maximum transmission of the MAI multilayer anode is matched with the CuPc absorption. 4. Conclusions MoO3/Ag/ITO/glass (MAI) multilayer films with different Ag thickness have been prepared by thermal evaporation. It is found that the electrical, optical and structural properties of these multilayer films clearly depend on the Ag film thickness. The effect of the intermediate Ag thickness on the optoelectronic properties of the films was investigated. The MAI multilayer electrode with 10 nm thick Ag layer exhibited a very low sheet resistance of 4(U/ ,) and optical transmission of 85% in the visible wavelength region. This work shows that one of the key factors in achieving both high transmittance and low sheet resistance in these electrodes is to obtain continuous Ag films, for which their thickness is
The authors would like to thank the Graduate Office of Isfahan University for their support. The authors are greatly indebted to the Iran Nanotechnology initiative council for their financial support.
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