- Email: [email protected]
ITO Schottky diodes
Physica B 406 (2011) 533–536
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Direct current and impedance spectroscopic studies on MoO3 modified ZnPc/ITO Schottky diodes Imran Murtaza a,n, Ibrahim Qazi a,b, Khasan S. Karimov a,c, Muhammad H. Sayyad a a
Faculty of Engineering Sciences, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, Pakistan Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan c Physical Technical Institute of Academy of Sciences, Dushanbe 734025, Tajikistan b
a r t i c l e in f o
abstract
Article history: Received 7 September 2010 Received in revised form 8 November 2010 Accepted 9 November 2010
We use experimental results of direct current and low signal impedance spectroscopy to investigate the conduction mechanism in organic semiconductor ZnPc. The experimental results demonstrate an increase in current and holes mobility by the introduction of a thin MoO3 film at the ITO/ZnPc interface. This significantly improves the device performance. The improvement is explained in terms of the reduction in the effective barrier for charge transfer from ZnPc to ITO. & 2010 Elsevier B.V. All rights reserved.
Keywords: Organic semiconductor Zinc phthalocyanine Molybdenum oxide Impedance spectroscopy Schottky effect Dielectric constant Holes mobility
1. Introduction Thin films of organic semiconductors have been the focus of numerous investigations over the past few years in view of their promising applications in optoelectronic devices [1,2]. Organic materials are now being used in flat-panel displays and the endless potential for organic material synthesis and their structural flexibility have made them promising candidate, for their use as active layer, in organic field effect transistors and organic photovoltaic cells [3–5]. Due to the relatively low cost and relatively good electronic properties, metal phthalocyanines (MPc) are the most promising materials for these organic devices. In order to optimize the device performance it is very important to have knowledge about intrinsic properties, particularly the charge transport and charge injection properties. However, the charge transport mechanism in organic semiconductors is not as simple, since there may be many interpretations of the observed characteristics like Poole–Frenkel, Schottky or space charge limited effects, etc., therefore cross checks between different electrical characterizations techniques are needed to correctly interpret the experimental results. There are different techniques to investigate the conduction mechanism in organic semiconductors. For phthalocyanines, in particular and
n
Corresponding author. E-mail address: [email protected] (I. Murtaza).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.11.029
for organic semiconductors in general, impedance spectroscopy (IS) has been proposed to characterize the carrier dynamics in organic semiconductors [6–8]. In order to improve the hole injection efficiency at the anode/organic interface, for better device performance, thin interlayers of molybdenum oxide (MoO3) have been recently used in single carrier devices [9–11]. In this study, therefore, both the dc and IS characteristics of ITO/ZnPc/Al and ITO/MoO3/ZnPc/Al Schottky type diodes are investigated and conduction mechanisms and dielectric properties are discussed.
2. Experimental details ITO coated glass substrates (Applied Films Corp.), with a sheet resistance of about 20 O/&, were cleaned by sonication in detergent, de-ionized water and methanol. ZnPc and MoO3, obtained in powder form, from Sigma-Aldrich, were thermally deposited on ITO sub˚ strates at a deposition rate of 2 and 0.5 A/s, respectively. During the deposition the chamber pressure was 5 10 6 Torr. The ITO substrates were maintained at room temperature and the thickness of the films was measured by quartz vibrating monitor. Finally Al electrode was deposited on ZnPc layer through a shadow mask at a deposition ˚ rate of 2 A/s. The active area of the investigated samples was 0.785 mm2 and the thicknesses of MoO3, ZnPc and Al thin films were 3, 150 and 100 nm, respectively. The schematic view of devices is shown in Fig. 1. The direct current (dc)–voltage characteristics of the
534
I. Murtaza et al. / Physica B 406 (2011) 533–536
devices were measured by a Keithley 236 source meter and impedance spectroscopic (IS) characterization was done on a frequency response analyzer (Solatron 1255HF) connected to a potentiostat/ galvanostat (EG & G Princeton Applied Research Model 273). The IS data of the samples were obtained in the frequency range between 100 Hz and 100 MHz, keeping the ac amplitude fixed at 50 mV and for dc biased values of 0, 71, 72 and 73 V. All measurements were carried out in air at room temperature.
3. Results and discussion 3.1. Dc results and discussion Fig. 2 shows the semi-log J–V characteristics of the investigated devices under dark condition. In the forward bias, for both ITO/ ZnPc/Al (device A) and ITO/MoO3/ZnPc/Al (device B) devices. The positive bias was applied to the ITO contact. The J–V characteristics of both the Schottky barrier diodes showed rectification behavior and the rectification ratio, defined as the ratio of forward to reverse current at a particular voltage are found to be 2950 and 4220 for devices A and B at 74 V, respectively. It is well known that barrier is present at the ITO/organic material interface. The introduction of an interfacial layer of MoO3 between the ITO and ZnPc allows to decrease the barrier height at the interface and facilitates the holes injection. The energy levels of HOMO and LUMO for ZnPc are 5.17 and 3.78 eV, while the work functions of ITO, MoO3 and Al are 4.3, 5.4 and 4.2 eV, respectively [9,10,12]. It would be barriers of 0.87 and 0.64 eV for devices A and B, respectively, at the ITO/organic semiconductor interface. It is supposed that the carriers are transported through the MoO3 layer by tunneling effect. This tunneling effect decreases with increase in thickness of the MoO3 layer. It was admitted that up to 3 nm, the 5
4
6
3
2
1 Glass Fig. 1. Cross sectional view of ITO/MoO3/ZnPc/Al device: 1-ITO, 2-MoO3 (absent in device A), 3-ZnPc, 4-Al, 5 and 6-terminals.
efficiency of the tunneling effect is maximum [9]. Furthermore, it has been reported [9] that MoO3 films are strongly oxygen deficient and when some oxygen is removed from the MoO3 crystal, holes can cross the insulating film by multiple tunneling steps through these gap states introduced by the oxygen vacancies. As a result, the probability of tunneling through the barrier increases causing higher rectification ratio and larger current in device B. To understand the conduction mechanism in both the devices, logarithmic scale of current density is plotted against V1/2 in Fig. 3 showing a linear variation, represented by straight solid lines, at the bias values greater than 1 V. This type of behavior is generally attributed to either a Poole–Frenkel or Schottky effect [13]. Current density expressions for the Schottky and Poole–Frenkel effects are [14] j b V 1=2 J ¼ AT 2 exp s exp s 1=2 ð1Þ KT KT d
J ¼ J0 exp
bpf ¼ 2bs ¼
Current density (A/cm2)
10-4 10-5 10-6
1=2
e
ð3Þ
pe0 er
Device A Device B
10-1
10-3
where, A is the Richardson constant, js is the Schottky barrier height at the anode/organic semiconductor interface, J0 is the low field current density, e is the electron charge, er is the relative dielectric permittivity, e0 is the vacuum permeability and bs and bpf are the Schottky and Poole–Frenkel field lowering coefficients, respectively. The theoretical values of bs and bpf are 2.00 10 5 and 4.00 10 5 eV m1/2 V 1/2, respectively (for er ¼3.6, obtained from IS data in the next sub-section). The values of b calculated from the slopes of linear regions in Fig. 3, are 2.57 10 5 and 2.71 10 5 eV m1/2 V 1/2 for devices A and B, respectively. These values are very close to the theoretical values of Schottky and Poole–Frenkel field lowering coefficients. Generally, it is difficult to distinguish between the two effects [13], nevertheless our results show that at bias values above 1 V, Schottky effect is more probable as the values of b are more close to Schottky coefficient, which is in agreement with our interpretation of the increase in rectification ratio and current density in
Device A Device B
10-2
ð2Þ
KT d1=2
100
10-1
!
where,
100
Current density (A/cm2)
bpf V 1=2
10-2 10-3 10-4 10-5
10-7 10-6
10-8
10-7
10-9 -10
-8
-6
-4
-2
0 2 Bias (V)
4
6
8
10
Fig. 2. Current density vs. voltage (J–V) characteristics of ITO/ZnPc/Al (device A) and ITO/MoO3/ZnPc/Al (device B).
1.0
1.5
2.0
2.5
Bias (V1/2) Fig. 3. Forward bias semi-log plot of current density as a function of V1/2 for ITO/ ZnPc/Al (device A) and ITO/MoO3/ZnPc/Al (device).
I. Murtaza et al. / Physica B 406 (2011) 533–536
device B. Below 1 V, the bias dependence in both the devices is Ohmic and space charge limited conduction, which is clear from their J–V characteristics and which are the two conduction mechanisms generally found in MPc thin films.
2.0
3.2. Impedance results and discussion
1.0
0.50
Device A
0.45
Applied D.C. Bias (V) 3 2 1 0 -1 -2 -3
0.40 0.35 0.30
Capacitance (nF)
0.25
Device A
1.5
0.5
ΔB (μS)
Fig. 4 shows the frequency dependent capacitance (C) of devices A and B at different dc bias voltages. Clearly the capacitance increases with applied dc bias due to increase in amount of injected holes and thus increasing amount of stored charge. As expected, from the dc analysis in the previous section, it is clear from Fig. 4 that capacitance is higher in device B than device A. Particularly at higher dc forward bias the increase in capacitance in device B is more pronounced than device A, which shows better injection and charge storing property of device B due to the surface passivation of ITO by MoO3 thin film. It is also seen in Fig. 4 that, at a fixed dc bias, capacitance decreases with increase in frequency and reaches a steady value of 0.167 nF for device A. This should be the same as the geometric capacitance Cgeo ¼ e0erA/d of the device (where e0 is the permittivity of free space, er is the relative permittivity of ZnPc, A is the area of the device¼7.85 10 7 m2 and d is the thickness of the ZnPc thin film¼150 nm). Using this relation for Cgeo, the value of er for ZnPc is found to be equal to 3.6, which is the same as that given in literature for CuPc (13). The value of Cgeo for device B is found 0.186 nF from Fig. 4. IS results can be used to evaluate the hole mobility in organic transport layer. In order to calculate charge carrier mobilities in the both devices, the differential susceptance DB¼ o(C Cgeo), at the dc biasing of 1 V is plotted as a function of frequency (See Fig. 5). The differential susceptance DB is obtained from IS data. There exists a maximum DB value at particular frequency fr ¼ tr 1. The positions of
535
0.0 Device B 2.0 1.5 1.0 0.5 0.0 102
103 104 Frequency (Hz)
105
Fig. 5. Differential susceptance (DB) as a function of frequency.
this maxima are at 15,800 and 31,600 Hz for devices A and B, respectively. This can be related to the average transit time by the relation tdc ¼0.56tr [7]. Hence the holes mobility mdc ¼d2/tdcVdc can be calculated (where d and Vdc are the film thickness and dc bias voltage, respectively). The calculated values for holes mobilities are 6.35 10 6 and 1.27 10 5 cm2 V 1 s 1 for devices A and B, respectively. We propose that the smaller mobility in device A is because of interfacial trap states caused by surface interaction between ZnPc and ITO, while in device B the effect of interfacial trap states becomes weaker by introducing MoO3 surface passivation of ITO.
0.20 4. Conclusions
0.15
The analyses of the direct current and impedance spectroscopic data have shown improvement in the device performance by introduction of MoO3 layer. The current, rectification ratio and holes mobility increase by the introduction of MoO3 thin film at ITO/ZnPc interface. J–V characteristics of both the devices have shown that at positive bias values below 1 V, the conduction is Ohmic and space charge limited while at values above 1 V the conduction process can be best explained by Schottky effect. Using IS results dielectric constant and hole mobilities were found which are important factors in the electrical characterization of any device.
Device B
0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10
Acknowledgement
102
103 104 Frequency (Hz)
105
Fig. 4. Capacitance as a function of frequency for ITO/ZnPc/Al (device A) and ITO/ MoO3/ZnPc/Al (device B).
This work is supported by Higher Education Commission Pakistan through the ‘‘International Research Support Initiative program’’ awarded to Mr. Imran Murtaza. The authors also acknowledge the technical and scholastic support provided by Dr. Ian G. Hill, Department of Physics, Dalhousie University, Halifax, Canada.
536
I. Murtaza et al. / Physica B 406 (2011) 533–536
References [1] N.C. Greenham, R.H. Friend, Solid State Physics, Series s.l, in: H. Ehrenreich, F. Saepen (Eds.), Academic Press, New York, 1995, p. 1 vol. 48. [2] S.R. Forrest, Chem. Rev. 97 (1997) 1793. [3] Xiujin Haibo Wang, Bo Wang, Yu yanhou Geng, Yan Donghang, Appl. Phys. Lett. 93 (2008) 113303. [4] Nana Wang, yu Junsheng, Yue Zang, Jiang Huang, Yadong Jiang, Sol. Energ. Mater. Sol. C. 94 (2010) 263. [5] L.L. Chen, W.L. Li, H.Z. Wei, B. Chu, B. Li, Sol. Energ. Mater. Sol. C. 90 (2006) 1788. [6] F.T. Reisa, D. Mencaraglia, S. Oould Saad, I. Seguy, M. Oukachmih, P. Jolinat, P. Destruel, Synthetic Met. 138 (2003) 33.
[7] S.W. Tsang, S.K. So, J. Appl. Phys. 99 (2006) 013706-1. [8] G. Jarosz, J. Non-Cryst. Solids 352 (2006) 4264. [9] L. Cattin, F. Dahou, Y. Lare, M. Morsli, R. Tricot, S. Houari, A. Mokrani, K. Jondo, J. Appl. Phys. 105 (2009) 034507. [10] Lu Zhaoyue, Deng Zhenbo, Zheng Jianjie, Ye Zou, Zheng Chen, Xu Denghui, Wang Yongsheng, Physica E 41 (2009) 1806. [11] Tetsuro Hori, Takeshi Shibata, Varutt Kittichungchit, Hiroki Moritou, Jun Sakai, Hitoshi Kubo, Thin Solid Films 518 (2009) 522. [12] D.J. Milliron, I.G. Hill, A. Kahn, J. Schwarz, J. Appl. Phys. 87 (2000) 572. [13] R.D. Gould, Coordin. Chem. Rev. 156 (1996) 237. [14] T.S. Shafai, Thin Solid Films 517 (2008) 1200.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Direct current and impedance spectroscopic studies on MoO3 modified ZnPc/ITO Schottky diodes Imran Murtaza a,n, Ibrahim Qazi a,b, Khasan S. Karimov a,c, Muhammad H. Sayyad a a
Faculty of Engineering Sciences, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, Pakistan Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan c Physical Technical Institute of Academy of Sciences, Dushanbe 734025, Tajikistan b
a r t i c l e in f o
abstract
Article history: Received 7 September 2010 Received in revised form 8 November 2010 Accepted 9 November 2010
We use experimental results of direct current and low signal impedance spectroscopy to investigate the conduction mechanism in organic semiconductor ZnPc. The experimental results demonstrate an increase in current and holes mobility by the introduction of a thin MoO3 film at the ITO/ZnPc interface. This significantly improves the device performance. The improvement is explained in terms of the reduction in the effective barrier for charge transfer from ZnPc to ITO. & 2010 Elsevier B.V. All rights reserved.
Keywords: Organic semiconductor Zinc phthalocyanine Molybdenum oxide Impedance spectroscopy Schottky effect Dielectric constant Holes mobility
1. Introduction Thin films of organic semiconductors have been the focus of numerous investigations over the past few years in view of their promising applications in optoelectronic devices [1,2]. Organic materials are now being used in flat-panel displays and the endless potential for organic material synthesis and their structural flexibility have made them promising candidate, for their use as active layer, in organic field effect transistors and organic photovoltaic cells [3–5]. Due to the relatively low cost and relatively good electronic properties, metal phthalocyanines (MPc) are the most promising materials for these organic devices. In order to optimize the device performance it is very important to have knowledge about intrinsic properties, particularly the charge transport and charge injection properties. However, the charge transport mechanism in organic semiconductors is not as simple, since there may be many interpretations of the observed characteristics like Poole–Frenkel, Schottky or space charge limited effects, etc., therefore cross checks between different electrical characterizations techniques are needed to correctly interpret the experimental results. There are different techniques to investigate the conduction mechanism in organic semiconductors. For phthalocyanines, in particular and
n
Corresponding author. E-mail address: [email protected] (I. Murtaza).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.11.029
for organic semiconductors in general, impedance spectroscopy (IS) has been proposed to characterize the carrier dynamics in organic semiconductors [6–8]. In order to improve the hole injection efficiency at the anode/organic interface, for better device performance, thin interlayers of molybdenum oxide (MoO3) have been recently used in single carrier devices [9–11]. In this study, therefore, both the dc and IS characteristics of ITO/ZnPc/Al and ITO/MoO3/ZnPc/Al Schottky type diodes are investigated and conduction mechanisms and dielectric properties are discussed.
2. Experimental details ITO coated glass substrates (Applied Films Corp.), with a sheet resistance of about 20 O/&, were cleaned by sonication in detergent, de-ionized water and methanol. ZnPc and MoO3, obtained in powder form, from Sigma-Aldrich, were thermally deposited on ITO sub˚ strates at a deposition rate of 2 and 0.5 A/s, respectively. During the deposition the chamber pressure was 5 10 6 Torr. The ITO substrates were maintained at room temperature and the thickness of the films was measured by quartz vibrating monitor. Finally Al electrode was deposited on ZnPc layer through a shadow mask at a deposition ˚ rate of 2 A/s. The active area of the investigated samples was 0.785 mm2 and the thicknesses of MoO3, ZnPc and Al thin films were 3, 150 and 100 nm, respectively. The schematic view of devices is shown in Fig. 1. The direct current (dc)–voltage characteristics of the
534
I. Murtaza et al. / Physica B 406 (2011) 533–536
devices were measured by a Keithley 236 source meter and impedance spectroscopic (IS) characterization was done on a frequency response analyzer (Solatron 1255HF) connected to a potentiostat/ galvanostat (EG & G Princeton Applied Research Model 273). The IS data of the samples were obtained in the frequency range between 100 Hz and 100 MHz, keeping the ac amplitude fixed at 50 mV and for dc biased values of 0, 71, 72 and 73 V. All measurements were carried out in air at room temperature.
3. Results and discussion 3.1. Dc results and discussion Fig. 2 shows the semi-log J–V characteristics of the investigated devices under dark condition. In the forward bias, for both ITO/ ZnPc/Al (device A) and ITO/MoO3/ZnPc/Al (device B) devices. The positive bias was applied to the ITO contact. The J–V characteristics of both the Schottky barrier diodes showed rectification behavior and the rectification ratio, defined as the ratio of forward to reverse current at a particular voltage are found to be 2950 and 4220 for devices A and B at 74 V, respectively. It is well known that barrier is present at the ITO/organic material interface. The introduction of an interfacial layer of MoO3 between the ITO and ZnPc allows to decrease the barrier height at the interface and facilitates the holes injection. The energy levels of HOMO and LUMO for ZnPc are 5.17 and 3.78 eV, while the work functions of ITO, MoO3 and Al are 4.3, 5.4 and 4.2 eV, respectively [9,10,12]. It would be barriers of 0.87 and 0.64 eV for devices A and B, respectively, at the ITO/organic semiconductor interface. It is supposed that the carriers are transported through the MoO3 layer by tunneling effect. This tunneling effect decreases with increase in thickness of the MoO3 layer. It was admitted that up to 3 nm, the 5
4
6
3
2
1 Glass Fig. 1. Cross sectional view of ITO/MoO3/ZnPc/Al device: 1-ITO, 2-MoO3 (absent in device A), 3-ZnPc, 4-Al, 5 and 6-terminals.
efficiency of the tunneling effect is maximum [9]. Furthermore, it has been reported [9] that MoO3 films are strongly oxygen deficient and when some oxygen is removed from the MoO3 crystal, holes can cross the insulating film by multiple tunneling steps through these gap states introduced by the oxygen vacancies. As a result, the probability of tunneling through the barrier increases causing higher rectification ratio and larger current in device B. To understand the conduction mechanism in both the devices, logarithmic scale of current density is plotted against V1/2 in Fig. 3 showing a linear variation, represented by straight solid lines, at the bias values greater than 1 V. This type of behavior is generally attributed to either a Poole–Frenkel or Schottky effect [13]. Current density expressions for the Schottky and Poole–Frenkel effects are [14] j b V 1=2 J ¼ AT 2 exp s exp s 1=2 ð1Þ KT KT d
J ¼ J0 exp
bpf ¼ 2bs ¼
Current density (A/cm2)
10-4 10-5 10-6
1=2
e
ð3Þ
pe0 er
Device A Device B
10-1
10-3
where, A is the Richardson constant, js is the Schottky barrier height at the anode/organic semiconductor interface, J0 is the low field current density, e is the electron charge, er is the relative dielectric permittivity, e0 is the vacuum permeability and bs and bpf are the Schottky and Poole–Frenkel field lowering coefficients, respectively. The theoretical values of bs and bpf are 2.00 10 5 and 4.00 10 5 eV m1/2 V 1/2, respectively (for er ¼3.6, obtained from IS data in the next sub-section). The values of b calculated from the slopes of linear regions in Fig. 3, are 2.57 10 5 and 2.71 10 5 eV m1/2 V 1/2 for devices A and B, respectively. These values are very close to the theoretical values of Schottky and Poole–Frenkel field lowering coefficients. Generally, it is difficult to distinguish between the two effects [13], nevertheless our results show that at bias values above 1 V, Schottky effect is more probable as the values of b are more close to Schottky coefficient, which is in agreement with our interpretation of the increase in rectification ratio and current density in
Device A Device B
10-2
ð2Þ
KT d1=2
100
10-1
!
where,
100
Current density (A/cm2)
bpf V 1=2
10-2 10-3 10-4 10-5
10-7 10-6
10-8
10-7
10-9 -10
-8
-6
-4
-2
0 2 Bias (V)
4
6
8
10
Fig. 2. Current density vs. voltage (J–V) characteristics of ITO/ZnPc/Al (device A) and ITO/MoO3/ZnPc/Al (device B).
1.0
1.5
2.0
2.5
Bias (V1/2) Fig. 3. Forward bias semi-log plot of current density as a function of V1/2 for ITO/ ZnPc/Al (device A) and ITO/MoO3/ZnPc/Al (device).
I. Murtaza et al. / Physica B 406 (2011) 533–536
device B. Below 1 V, the bias dependence in both the devices is Ohmic and space charge limited conduction, which is clear from their J–V characteristics and which are the two conduction mechanisms generally found in MPc thin films.
2.0
3.2. Impedance results and discussion
1.0
0.50
Device A
0.45
Applied D.C. Bias (V) 3 2 1 0 -1 -2 -3
0.40 0.35 0.30
Capacitance (nF)
0.25
Device A
1.5
0.5
ΔB (μS)
Fig. 4 shows the frequency dependent capacitance (C) of devices A and B at different dc bias voltages. Clearly the capacitance increases with applied dc bias due to increase in amount of injected holes and thus increasing amount of stored charge. As expected, from the dc analysis in the previous section, it is clear from Fig. 4 that capacitance is higher in device B than device A. Particularly at higher dc forward bias the increase in capacitance in device B is more pronounced than device A, which shows better injection and charge storing property of device B due to the surface passivation of ITO by MoO3 thin film. It is also seen in Fig. 4 that, at a fixed dc bias, capacitance decreases with increase in frequency and reaches a steady value of 0.167 nF for device A. This should be the same as the geometric capacitance Cgeo ¼ e0erA/d of the device (where e0 is the permittivity of free space, er is the relative permittivity of ZnPc, A is the area of the device¼7.85 10 7 m2 and d is the thickness of the ZnPc thin film¼150 nm). Using this relation for Cgeo, the value of er for ZnPc is found to be equal to 3.6, which is the same as that given in literature for CuPc (13). The value of Cgeo for device B is found 0.186 nF from Fig. 4. IS results can be used to evaluate the hole mobility in organic transport layer. In order to calculate charge carrier mobilities in the both devices, the differential susceptance DB¼ o(C Cgeo), at the dc biasing of 1 V is plotted as a function of frequency (See Fig. 5). The differential susceptance DB is obtained from IS data. There exists a maximum DB value at particular frequency fr ¼ tr 1. The positions of
535
0.0 Device B 2.0 1.5 1.0 0.5 0.0 102
103 104 Frequency (Hz)
105
Fig. 5. Differential susceptance (DB) as a function of frequency.
this maxima are at 15,800 and 31,600 Hz for devices A and B, respectively. This can be related to the average transit time by the relation tdc ¼0.56tr [7]. Hence the holes mobility mdc ¼d2/tdcVdc can be calculated (where d and Vdc are the film thickness and dc bias voltage, respectively). The calculated values for holes mobilities are 6.35 10 6 and 1.27 10 5 cm2 V 1 s 1 for devices A and B, respectively. We propose that the smaller mobility in device A is because of interfacial trap states caused by surface interaction between ZnPc and ITO, while in device B the effect of interfacial trap states becomes weaker by introducing MoO3 surface passivation of ITO.
0.20 4. Conclusions
0.15
The analyses of the direct current and impedance spectroscopic data have shown improvement in the device performance by introduction of MoO3 layer. The current, rectification ratio and holes mobility increase by the introduction of MoO3 thin film at ITO/ZnPc interface. J–V characteristics of both the devices have shown that at positive bias values below 1 V, the conduction is Ohmic and space charge limited while at values above 1 V the conduction process can be best explained by Schottky effect. Using IS results dielectric constant and hole mobilities were found which are important factors in the electrical characterization of any device.
Device B
0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10
Acknowledgement
102
103 104 Frequency (Hz)
105
Fig. 4. Capacitance as a function of frequency for ITO/ZnPc/Al (device A) and ITO/ MoO3/ZnPc/Al (device B).
This work is supported by Higher Education Commission Pakistan through the ‘‘International Research Support Initiative program’’ awarded to Mr. Imran Murtaza. The authors also acknowledge the technical and scholastic support provided by Dr. Ian G. Hill, Department of Physics, Dalhousie University, Halifax, Canada.
536
I. Murtaza et al. / Physica B 406 (2011) 533–536
References [1] N.C. Greenham, R.H. Friend, Solid State Physics, Series s.l, in: H. Ehrenreich, F. Saepen (Eds.), Academic Press, New York, 1995, p. 1 vol. 48. [2] S.R. Forrest, Chem. Rev. 97 (1997) 1793. [3] Xiujin Haibo Wang, Bo Wang, Yu yanhou Geng, Yan Donghang, Appl. Phys. Lett. 93 (2008) 113303. [4] Nana Wang, yu Junsheng, Yue Zang, Jiang Huang, Yadong Jiang, Sol. Energ. Mater. Sol. C. 94 (2010) 263. [5] L.L. Chen, W.L. Li, H.Z. Wei, B. Chu, B. Li, Sol. Energ. Mater. Sol. C. 90 (2006) 1788. [6] F.T. Reisa, D. Mencaraglia, S. Oould Saad, I. Seguy, M. Oukachmih, P. Jolinat, P. Destruel, Synthetic Met. 138 (2003) 33.
[7] S.W. Tsang, S.K. So, J. Appl. Phys. 99 (2006) 013706-1. [8] G. Jarosz, J. Non-Cryst. Solids 352 (2006) 4264. [9] L. Cattin, F. Dahou, Y. Lare, M. Morsli, R. Tricot, S. Houari, A. Mokrani, K. Jondo, J. Appl. Phys. 105 (2009) 034507. [10] Lu Zhaoyue, Deng Zhenbo, Zheng Jianjie, Ye Zou, Zheng Chen, Xu Denghui, Wang Yongsheng, Physica E 41 (2009) 1806. [11] Tetsuro Hori, Takeshi Shibata, Varutt Kittichungchit, Hiroki Moritou, Jun Sakai, Hitoshi Kubo, Thin Solid Films 518 (2009) 522. [12] D.J. Milliron, I.G. Hill, A. Kahn, J. Schwarz, J. Appl. Phys. 87 (2000) 572. [13] R.D. Gould, Coordin. Chem. Rev. 156 (1996) 237. [14] T.S. Shafai, Thin Solid Films 517 (2008) 1200.