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Temperature-dependent current–voltage characteristics of poly-N-epoxypropylcarbazole complex
Thin Solid Films 516 (2007) 72 – 77 www.elsevier.com/locate/tsf
Temperature-dependent current–voltage characteristics of poly-N-epoxypropylcarbazole complex S.A. Moiz a , M.M. Ahmed b,⁎, K.h.S. Karimov c , M. Mehmood a a
National Center for Nanotechnology, Pakistan Institute of Engg. and App. Sciences, Nilore, Islamabad, Pakistan b Department of Electronic Engineering, M.A. Jinnah University, Jinnah Avenue, 44000, Islamabad, Pakistan c Physical Technical Institute, Aini St. 299/1, Dushanbe, 734063, Tajikistan Received 11 April 2006; received in revised form 7 April 2007; accepted 24 April 2007 Available online 5 May 2007
Abstract In this study, current–voltage (I–V) characteristics of thin films of poly-N-epoxypropylcarbazole (PEPC) doped with anthracene have been investigated. The PEPC films were grown on nickel substrates, at room temperature, by using a centrifugal machine operated at 277 g. I–V characteristics were then evaluated as a function of temperature ranging from 30 to 60 °C. Reversible rectifying characteristics were exhibited by the devices in which current magnitude increases with increasing values of temperature. This has been explained with temperature assisted hopping process of free carriers in the organic film having positional as well as energetic disorders whilst the nonlinear I–V characteristics follow space charge limited current (SCLC) model. By applying the correlated Gaussian disorder mobility model to the experimental SCLC, the energetic disorder parameter and average intersite spacing between hopping locations have been calculated. It has been observed that energetic disorderness and average intersite distance in PEPC complex are relatively higher which could be a cause of low hole mobility in PEPC organic semiconductor. © 2007 Elsevier B.V. All rights reserved. Keywords: Correlated Gaussian disorder model; Organic semiconductors; Temperature-dependent I–V characteristics; Hole mobility; Positional disorder; Energetic disorder
1. Introduction The structural flexibility and good optical properties offered by thin films of organic semiconductors have led to promising electronic and optoelectronic applications within the last few decades [1]. Recently, organic photovoltaic materials have attracted solar cell industry because of the prospects of high throughput using low cost thin film deposition techniques. The efficiency of organic solar cells is generally limited due to the low conductivity of photosensitive materials [2,3]. However, researchers are experimenting different photosensitive organic materials and their doping to enhance the electrical properties of organic solar cells and devices. Amongst those poly-Nepoxypropylcarbazole (PEPC) is considered a good candidate for organic semiconductor devices such as photoelectric converters and solar cells. PEPC forms with number of low
⁎ Corresponding author. E-mail address: [email protected] (M.M. Ahmed). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.161
molecular organic materials charge transfer complexes that have high photosensitivity or sufficiently high electric conductivity. It is known that the structures and the properties of organic semiconductors are highly dependent upon the fabrication technology. To optimize the structural properties which define the electrical behavior of a device centrifugal processing techniques are commonly employed with variable processing parameters. It is thus assumed that a suitable processing technology may result in a better electrical performance of a finished device for a given organic material [4]. Generally, organic semiconductors have large molecular weight and strong intramolecular bonding in the form of specific combination of single and double bonds. However, they offer weak van der Waal's intermolecular bonding. This special arrangement causes the delocalization of charges along the molecules or chain. Under the influence of external applied voltage, the free charges move from one molecule or chain to another molecule or chain by hopping phenomenon. Whereas the rate of hopping is severely affected both by the degree of
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
73
Nomenclature List of variables J Current density q Electronic charge p0 Free hole density pt Trapped hole density E Electric filed μp Field dependent hole mobility μ0 Hole mobility at E = 0 μ∞ Hole mobility at T → ∞ μGDM Gaussian disorder mobility μCGDM Correlated Gaussian disorder mobility εs Permittivity of organic film ε0 Permittivity of free space V Applied voltage L Length of organic film KB Boltzmann constant σ Energetic disorderness γ Temperature-dependent activation co-efficient a Intersite spacing Σ Positional disorder energetic disorderness (σ) and positional disorderness (Σ) offered by an organic semiconductor. Their effects on the charge transport mechanism can be estimated by evaluating the mobility of charge carriers in a given semiconductor. Time of flight (TOF) measurements are commonly used to study the mobility in disordered materials including organic semiconductors. However, to evaluate the carriers' mobility in the absence of space charge low carriers densities are used in TOF experiments. Another method to evaluate the field dependent mobility is based on the device current–voltage (I– V) characteristics. Electrical parameters extracted by employing this technique explain fairly well the observed response of a device and, in some cases, are a preferred approach for devices operated at relatively high density of injected carriers [5]. In this article, thin films of PEPC doped with anthracene (An) have been fabricated by using centrifugal deposition technique on nickel (Ni) substrates. The electrical properties of the fabricated devices were evaluated as a function of temperature ranging from 30 to 60 °C. The charge transport parameters of PEPC complex, exhibiting space charge limited current, were discussed by applying Pool–Frenkel field dependent mobility concept and correlated Gaussian disorder model.
Assuming that at J = 0, the value of p0 = 0 and all the carriers are trapped in their respective trapped centers. When temperature increases the trapped carriers are released and a finite value of p0 is available in the film. These free carriers generate a field gradient in the film which in turn controls the current flow called space charge limited current (SCLC). According to the Gauss's law ∇d E ¼
q qp0 ¼ e0 es e0 es
ð3Þ
From Eqs. (1) and (3) SCLC can be expressed as [7] 9 V2 J ¼ e0 es μp 3 8 L
ð4Þ
A most accurate form of Eq. (4), to represent the current density in organic films by adding trap factor, is given as [8] 2 9 p0 V ð5Þ J ¼ e0 es μp 8 p0 þ pt L 3 3. Device fabrication
2. Space charge limited current The current density of an organic material, under the applied electric field, may be defined as J ¼ qp0 μp E
ð1Þ
where it has been assumed that μp follows the Pool–Frenkel expression, i.e., [6] pffiffiffiffi μp ðE; T Þ ¼ μ0 ðT Þexp γðT Þ E ð2Þ
The molecular structure of the PEPC is shown in Fig. 1a whereas the synthesis of the PEPC is described elsewhere [9]. In Fig. 1a the value of n is 4–6 and the molecular weight of PEPC is approximately 1000 atomic weight units [9]. The dopant was An having molecular structure shown in Fig. 1b. The PEPC forms charge transfer complex with An, because PEPC is an electron donor in many lower molecular weight organic materials [10,11]. This increases the room temperature conductivity of the PEPC many folds [3] and enables the formation of
74
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
Fig. 3. A cross-sectional view of an organic-inorganic device fabricated by using p-type PEPC and Ni substrate.
Fig. 1. Molecular structure of poly-N-epoxypropylcarbazole (PEPC) and anthracene (An).
ohmic contacts despite a significant difference between the work function of ohmic metal and the PEPC [12,13]. The films were deposited by using a centrifugal machine, as shown in Fig. 2 on Ni substrates at room temperature from 5 wt.% solution of PEPC in benzene doped with 10 wt.% An at gravity conditions of 277 g, where g is acceleration due to gravity. It has been demonstrated in another work by the same group that the films grown at 277 g offer better electrical properties as compared to 1 g, 123 g, and 1107 g and the technique is economical relative to its counterparts [3]. The substrate was placed inside an aluminum vessel which was mounted on the centrifugal machine. The machine had 11 cm arm and provided acceleration of 277 g at 2000 rpm. For each film growth two symmetrically installed aluminum vessels filled with solution of equal volume (0.6–0.9 ml) were used. At the acceleration of 277 g, a 30–40 min processing time was required to evaporate the solvent completely and to get the films deposited on the substrate. Optical examination showed that the grown films were homogeneous and of uniform morphology. The nominal thicknesses of the polymer films, estimated by
Fig. 2. Schematic diagram of a table-top centrifuge apparatus used for the growth of organic films.
optical examination and also by scanning electron microscopy, were found to be 11 μm. By using hot-probe method it was confirmed that the grown films were p-type semiconductors. Droplets of liquid gallium (Ga), at room temperature, were deposited on the polymer films to provide soft contacts without damaging the films. A non-rectifying contact is formed by allowing van der Waal's interaction between PEPC and Ga metal [14,15]. The liquid Ga was also used to fabricate ohmic contacts with Ni substrate. Fig. 3 shows a cross-sectional view of a finished device. I–V characteristics were then evaluated by using a direct current (dc) measurement station with temperature adjusting facility. The measurements were carried out in the temperature range of 30 to 60 °C with an experimental temperature error of ± 0.5 °C. 4. Results and discussion I–V characteristics of a fabricated device are shown in Fig. 4. Examination of the figure showed that the observed
Fig. 4. Temperature-dependent I–V characteristics of PEPC organic thin film doped with An.
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
nonlinear I–V characteristics are temperature-dependent. In other words, the device offers higher current at higher temperatures both in forward as well as in reverse bias when observed at the same voltage. A relatively high current flow at elevated temperatures may be associated with either increased number density of free carriers or with high hopping rate. At elevated temperature there may be more detrapping of the carriers from their respective trap centers. This increases the number density of free carriers that eventually defines the magnitude of current flow from the device as given by Eq. (5). In addition to the temperature-dependent increase in the carrier density it is assumed that the carriers hopping process, both intramolecule and intermolecules, also increases due to high thermal excitations and thus improving the overall conductivity of the film as a function of temperature. Whereas the field dependent mobility of free carriers in the SCLC region is assumed to be responsible for nonlinear response of the device. In Fig. 5 ln–ln plot of the observed I–V characteristics of a device at different temperatures is presented. From the slope of the plot it is evident that the current increases with a slope of approximately 2 by increasing the voltages which is a typical behavior of a device that offers SCLC. Assuming εs = 3.5, L = 11 μm and the cross-sectional area offered to the flow of charge is ∼ 160 μm2, the value of J is evaluated from Eq. (5) and found to be in good agreement with experimental data. This clearly demonstrates that the hole current in PEPC is bulk defined SCLC as that of the other organic semiconductors [16,17]. By applying SCLC model to the I–V characteristics of the device, it is possible to estimate μp both as a function of temperature as well as electric field [8]. Fig. 6 represents a plot between ln(μp) vs E1/2 at different temperatures. Examination of the characteristics, shown in Fig. 6, reveals that the hole mobility in PEPC complex follows the mobility model
Fig. 5. Shows logarithmic variation of current flowing from PEPC thin film at various temperatures.
75
Fig. 6. Relationship between mobility and applied electric field at various temperatures for PEPC organic semiconductor.
described in Eq. (2). Although this empirical mobility model was first reported by Pai [18] from TOF measurements at relatively high values of E to explain photo injected holes in poly (N-vinylcarbazole) but the model appeared to be generic for a large class of disordered materials including PEPC complex [19]. This demonstrates that charge transport phenomenon in PEPC like other disordered organic materials could be explained with the field dependent mobility of carriers. Thus, one can determine μ0(T) when E = 0 and also γ(T) by using field and temperature-dependent characteristics of Fig. 6 respectively. The observed value of μp for PEPC complex is lower than the values reported in the literature for many other polymers [20].
Fig. 7. Variation in temperature-dependent zero-field mobility (μ0) of a PEPC thin film doped with An.
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S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
In general one can conclude that a higher positional disorderness causes a higher energetic disorderness and both of these factors then eventually reduce the hopping probability of the carriers as well as their mobility. To synthesize high mobility organic semiconductor it would be, therefore, imperative that the organic molecular structure should offer hopping distance as small as possible with minimum positional as well as energetic disorderness. 5. Conclusions
Fig. 8. Shows field activation coefficient (γ) vs temperature for PEPC thin film.
Bässler Gaussian disorder model (GDM) predicts the high field mobility as [21] " ( ) # 2r 2 r 2 2 pffiffiffiffi E μGDM ¼ μl exp þC R ð6Þ 3KB T KB T where the value of Σ varies from 1 to 4 [22] and C is an empirical constant which describes the width of density of states relative to KBT. It is worth mentioning that GDM neglects spatial correlation due to charge dipole interactions even though such correlations have recently been shown to explain the universal electric field dependence mobility observed in disordered organic films. Taking into account the correlated random potential Novikov et al. [23] have modified Eq. (6) and proposed the following empirical relationship " rffiffiffiffiffiffiffiffi# 3r 2 r 32 qaE μCGDM ¼ μl exp þ0:78 2 5KB T KB T r ð7Þ where C = 0.78 and Σ = 2. When E = 0 Eq. (7) is reduced to 2
lnμ0 ¼ lnμ∞
3σ 5K B
2
1 T2
ð8Þ
By applying the SCLC model on the observed temperaturedependent characteristics μ0 and γ were evaluated and shown in Figs. 7 and 8 respectively. For σ = 0.09 eV the calculated value of μ0 as given in Eq. (8), is in good agreement with the experimental value. Using the value of σ obtained from the fitting of μ0(T ), the value of a can then be evaluated from the slope of γ ∼ T − 1.5 as shown in Eq. (7). The value of a thus determined was ∼ 3.13 nm. A usual reported value of a for polymers which offer better mobility is 1−2 nm [24]. A relatively high value of a could be associated with low value of μp for PEPC complex as observed in Fig. 6.
Thin films of poly-N-epoxypropylcarbazole (PEPC) doped with anthracene (An) have been fabricated. Films were grown on Ni substrates by employing centrifugal machine at 277 g. I– V characteristics of the fabricated films were evaluated as function of temperature ranging from 30 to 60 °C. Reversible rectifying characteristics have been observed in which the magnitude of current increases with increasing values of temperature. This can be associated with the generation of free carriers at elevated temperatures together with enhanced temperature assisted hopping in the organic film. It is further observed that I–V characteristics of the device follow space charge limited conduction model. The field and temperaturedependent hole mobility in PEPC complex is described by the 1=2
empirical relation μp a exp c E . By employing correlated Gaussian disorder mobility model on the observed characteristics, energetic disorderness, σ and intersite distance, a has been determined and found to be 0.09 eV and 3.13 nm respectively. These evaluated values are relatively higher and could be the main cause of low carriers mobility in PEPC complex which in return controls the I–V response of the device. References [1] Y. Shirota, J. Mater. Chem. 10 (2000) 1. [2] Kh.S. Karimov, M.M. Ahmed, S.A. Moiz, M.I. Fedorov, Sol. Energy Mater. Sol. Cells 87 (2005) 61. [3] M.M. Ahmed, Kh.S. Karimov, S.A. Moiz, IEEE Trans. Electron. Dev. 51 (2004) 121. [4] S.A. Moiz, M.M. Ahmed, Kh.S. Karimov, Jpn. J. Appl. Phys. 44 (2005) 1199. [5] J. Reynaert, V.I. Arkhipov, G. Borghs, P. Heremans, Appl. Phys. Lett. 8 (2004) 603. [6] D. Braun, J. Polym. Sci., Part B, Polym. Phys. 41 (2003) 2622. [7] P. Mark, H.V. Seggern, J. Appl. Phys. 33 (1962) 205. [8] S.A. Moiz, M.M. Ahmed, Kh.S. Karimov, Electron. Telecommun. Res. Inst. (ETRI) J. 27 (2005) 319. [9] Kh.M. Akhmedov, Doctor of Science Thesis, Donish, Dushanbe, Tajikistan, 1998. [10] L.L. Regel, W.R. Wilcox, Processing by Centrifugation, Kluwer Academic/Plenum Publisher, New York, NY, 2001. [11] M.C. Petty, M.R. Bryce, D. Bloor, An Introduction to Molecular Electronics, Edward Arnold, London, 1995. [12] Y. Wu, Y. Li, B.S. Ong, J. Am. Chem. Soc. 128 (2006) 4202. [13] M. Pfeiffer, K. Leo, X. Zhoua, J.S. Huanga, M. Hofmanna, A. Wernera, J. Blochwitz-Nimothb, Org. Electron. 4 (2003) 89. [14] D.A. Bernards, T. Biegala, Z.A. Samuels, J.D. Slinker, G.G. Malliaras, S. Flores-Torres, H.D. Abruna, J.A. Rogers, Appl. Phys. Lett. 84 (2004) 3675.
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77 [15] Y. Hirose, A. Kahn, V. Aristov, P. Soukiassian, V. Bulovic, S.R. Forrest, Phys. Rev., B 54 (1996) 13748. [16] K.P. Nazeer, S.C. Jain, A.K. Kapoor, M.T. Thamilselvan, D. Mangalaraj, S.K. Narayandass, J. Yi, Polym. Int. 53 (2004) 898. [17] M. Kiy, P. Losio, I. Biaggio, M. Koehler, A. Tapponnier, P. Guunter, Appl. Phys. Lett. 80 (2002) 1198. [18] D.M. Pai, J. Chem. Phys. 52 (1971) 2285. [19] A.B. Walker, A. Kambili, S.J. Martin, J. Phys., Condens. Matter 14 (2002) 9825.
[20] [21] [22] [23]
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P.W.M. Blom, M.C.J.M. Vissenberg, Mater. Sci. Eng., R 27 (2000) 53. H. Bassler, Phys. Status Solidi, B 175 (1993) 15. P.M. Borsenberger, J.J. Fitzgerald, J. Phys. Chem. 97 (1993) 4815. S.V. Novikov, D.H. Dunlap, V.M. Kenkre, P.E. Parris, A.V. Vannikov, Phys. Rev. Lett. 81 (1998) 4472. [24] V.D. Mihailechi, J.K.J. Van Duren, P.W. Blom, J.C. Hummelen, R.A.A. Janssen, J.M. Karoon, M.T. Rispens, W.J.H. Verhess, M.M. Wienk, Adv. Funct. Mater. 13 (2003) 43.
Temperature-dependent current–voltage characteristics of poly-N-epoxypropylcarbazole complex S.A. Moiz a , M.M. Ahmed b,⁎, K.h.S. Karimov c , M. Mehmood a a
National Center for Nanotechnology, Pakistan Institute of Engg. and App. Sciences, Nilore, Islamabad, Pakistan b Department of Electronic Engineering, M.A. Jinnah University, Jinnah Avenue, 44000, Islamabad, Pakistan c Physical Technical Institute, Aini St. 299/1, Dushanbe, 734063, Tajikistan Received 11 April 2006; received in revised form 7 April 2007; accepted 24 April 2007 Available online 5 May 2007
Abstract In this study, current–voltage (I–V) characteristics of thin films of poly-N-epoxypropylcarbazole (PEPC) doped with anthracene have been investigated. The PEPC films were grown on nickel substrates, at room temperature, by using a centrifugal machine operated at 277 g. I–V characteristics were then evaluated as a function of temperature ranging from 30 to 60 °C. Reversible rectifying characteristics were exhibited by the devices in which current magnitude increases with increasing values of temperature. This has been explained with temperature assisted hopping process of free carriers in the organic film having positional as well as energetic disorders whilst the nonlinear I–V characteristics follow space charge limited current (SCLC) model. By applying the correlated Gaussian disorder mobility model to the experimental SCLC, the energetic disorder parameter and average intersite spacing between hopping locations have been calculated. It has been observed that energetic disorderness and average intersite distance in PEPC complex are relatively higher which could be a cause of low hole mobility in PEPC organic semiconductor. © 2007 Elsevier B.V. All rights reserved. Keywords: Correlated Gaussian disorder model; Organic semiconductors; Temperature-dependent I–V characteristics; Hole mobility; Positional disorder; Energetic disorder
1. Introduction The structural flexibility and good optical properties offered by thin films of organic semiconductors have led to promising electronic and optoelectronic applications within the last few decades [1]. Recently, organic photovoltaic materials have attracted solar cell industry because of the prospects of high throughput using low cost thin film deposition techniques. The efficiency of organic solar cells is generally limited due to the low conductivity of photosensitive materials [2,3]. However, researchers are experimenting different photosensitive organic materials and their doping to enhance the electrical properties of organic solar cells and devices. Amongst those poly-Nepoxypropylcarbazole (PEPC) is considered a good candidate for organic semiconductor devices such as photoelectric converters and solar cells. PEPC forms with number of low
⁎ Corresponding author. E-mail address: [email protected] (M.M. Ahmed). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.161
molecular organic materials charge transfer complexes that have high photosensitivity or sufficiently high electric conductivity. It is known that the structures and the properties of organic semiconductors are highly dependent upon the fabrication technology. To optimize the structural properties which define the electrical behavior of a device centrifugal processing techniques are commonly employed with variable processing parameters. It is thus assumed that a suitable processing technology may result in a better electrical performance of a finished device for a given organic material [4]. Generally, organic semiconductors have large molecular weight and strong intramolecular bonding in the form of specific combination of single and double bonds. However, they offer weak van der Waal's intermolecular bonding. This special arrangement causes the delocalization of charges along the molecules or chain. Under the influence of external applied voltage, the free charges move from one molecule or chain to another molecule or chain by hopping phenomenon. Whereas the rate of hopping is severely affected both by the degree of
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
73
Nomenclature List of variables J Current density q Electronic charge p0 Free hole density pt Trapped hole density E Electric filed μp Field dependent hole mobility μ0 Hole mobility at E = 0 μ∞ Hole mobility at T → ∞ μGDM Gaussian disorder mobility μCGDM Correlated Gaussian disorder mobility εs Permittivity of organic film ε0 Permittivity of free space V Applied voltage L Length of organic film KB Boltzmann constant σ Energetic disorderness γ Temperature-dependent activation co-efficient a Intersite spacing Σ Positional disorder energetic disorderness (σ) and positional disorderness (Σ) offered by an organic semiconductor. Their effects on the charge transport mechanism can be estimated by evaluating the mobility of charge carriers in a given semiconductor. Time of flight (TOF) measurements are commonly used to study the mobility in disordered materials including organic semiconductors. However, to evaluate the carriers' mobility in the absence of space charge low carriers densities are used in TOF experiments. Another method to evaluate the field dependent mobility is based on the device current–voltage (I– V) characteristics. Electrical parameters extracted by employing this technique explain fairly well the observed response of a device and, in some cases, are a preferred approach for devices operated at relatively high density of injected carriers [5]. In this article, thin films of PEPC doped with anthracene (An) have been fabricated by using centrifugal deposition technique on nickel (Ni) substrates. The electrical properties of the fabricated devices were evaluated as a function of temperature ranging from 30 to 60 °C. The charge transport parameters of PEPC complex, exhibiting space charge limited current, were discussed by applying Pool–Frenkel field dependent mobility concept and correlated Gaussian disorder model.
Assuming that at J = 0, the value of p0 = 0 and all the carriers are trapped in their respective trapped centers. When temperature increases the trapped carriers are released and a finite value of p0 is available in the film. These free carriers generate a field gradient in the film which in turn controls the current flow called space charge limited current (SCLC). According to the Gauss's law ∇d E ¼
q qp0 ¼ e0 es e0 es
ð3Þ
From Eqs. (1) and (3) SCLC can be expressed as [7] 9 V2 J ¼ e0 es μp 3 8 L
ð4Þ
A most accurate form of Eq. (4), to represent the current density in organic films by adding trap factor, is given as [8] 2 9 p0 V ð5Þ J ¼ e0 es μp 8 p0 þ pt L 3 3. Device fabrication
2. Space charge limited current The current density of an organic material, under the applied electric field, may be defined as J ¼ qp0 μp E
ð1Þ
where it has been assumed that μp follows the Pool–Frenkel expression, i.e., [6] pffiffiffiffi μp ðE; T Þ ¼ μ0 ðT Þexp γðT Þ E ð2Þ
The molecular structure of the PEPC is shown in Fig. 1a whereas the synthesis of the PEPC is described elsewhere [9]. In Fig. 1a the value of n is 4–6 and the molecular weight of PEPC is approximately 1000 atomic weight units [9]. The dopant was An having molecular structure shown in Fig. 1b. The PEPC forms charge transfer complex with An, because PEPC is an electron donor in many lower molecular weight organic materials [10,11]. This increases the room temperature conductivity of the PEPC many folds [3] and enables the formation of
74
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
Fig. 3. A cross-sectional view of an organic-inorganic device fabricated by using p-type PEPC and Ni substrate.
Fig. 1. Molecular structure of poly-N-epoxypropylcarbazole (PEPC) and anthracene (An).
ohmic contacts despite a significant difference between the work function of ohmic metal and the PEPC [12,13]. The films were deposited by using a centrifugal machine, as shown in Fig. 2 on Ni substrates at room temperature from 5 wt.% solution of PEPC in benzene doped with 10 wt.% An at gravity conditions of 277 g, where g is acceleration due to gravity. It has been demonstrated in another work by the same group that the films grown at 277 g offer better electrical properties as compared to 1 g, 123 g, and 1107 g and the technique is economical relative to its counterparts [3]. The substrate was placed inside an aluminum vessel which was mounted on the centrifugal machine. The machine had 11 cm arm and provided acceleration of 277 g at 2000 rpm. For each film growth two symmetrically installed aluminum vessels filled with solution of equal volume (0.6–0.9 ml) were used. At the acceleration of 277 g, a 30–40 min processing time was required to evaporate the solvent completely and to get the films deposited on the substrate. Optical examination showed that the grown films were homogeneous and of uniform morphology. The nominal thicknesses of the polymer films, estimated by
Fig. 2. Schematic diagram of a table-top centrifuge apparatus used for the growth of organic films.
optical examination and also by scanning electron microscopy, were found to be 11 μm. By using hot-probe method it was confirmed that the grown films were p-type semiconductors. Droplets of liquid gallium (Ga), at room temperature, were deposited on the polymer films to provide soft contacts without damaging the films. A non-rectifying contact is formed by allowing van der Waal's interaction between PEPC and Ga metal [14,15]. The liquid Ga was also used to fabricate ohmic contacts with Ni substrate. Fig. 3 shows a cross-sectional view of a finished device. I–V characteristics were then evaluated by using a direct current (dc) measurement station with temperature adjusting facility. The measurements were carried out in the temperature range of 30 to 60 °C with an experimental temperature error of ± 0.5 °C. 4. Results and discussion I–V characteristics of a fabricated device are shown in Fig. 4. Examination of the figure showed that the observed
Fig. 4. Temperature-dependent I–V characteristics of PEPC organic thin film doped with An.
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
nonlinear I–V characteristics are temperature-dependent. In other words, the device offers higher current at higher temperatures both in forward as well as in reverse bias when observed at the same voltage. A relatively high current flow at elevated temperatures may be associated with either increased number density of free carriers or with high hopping rate. At elevated temperature there may be more detrapping of the carriers from their respective trap centers. This increases the number density of free carriers that eventually defines the magnitude of current flow from the device as given by Eq. (5). In addition to the temperature-dependent increase in the carrier density it is assumed that the carriers hopping process, both intramolecule and intermolecules, also increases due to high thermal excitations and thus improving the overall conductivity of the film as a function of temperature. Whereas the field dependent mobility of free carriers in the SCLC region is assumed to be responsible for nonlinear response of the device. In Fig. 5 ln–ln plot of the observed I–V characteristics of a device at different temperatures is presented. From the slope of the plot it is evident that the current increases with a slope of approximately 2 by increasing the voltages which is a typical behavior of a device that offers SCLC. Assuming εs = 3.5, L = 11 μm and the cross-sectional area offered to the flow of charge is ∼ 160 μm2, the value of J is evaluated from Eq. (5) and found to be in good agreement with experimental data. This clearly demonstrates that the hole current in PEPC is bulk defined SCLC as that of the other organic semiconductors [16,17]. By applying SCLC model to the I–V characteristics of the device, it is possible to estimate μp both as a function of temperature as well as electric field [8]. Fig. 6 represents a plot between ln(μp) vs E1/2 at different temperatures. Examination of the characteristics, shown in Fig. 6, reveals that the hole mobility in PEPC complex follows the mobility model
Fig. 5. Shows logarithmic variation of current flowing from PEPC thin film at various temperatures.
75
Fig. 6. Relationship between mobility and applied electric field at various temperatures for PEPC organic semiconductor.
described in Eq. (2). Although this empirical mobility model was first reported by Pai [18] from TOF measurements at relatively high values of E to explain photo injected holes in poly (N-vinylcarbazole) but the model appeared to be generic for a large class of disordered materials including PEPC complex [19]. This demonstrates that charge transport phenomenon in PEPC like other disordered organic materials could be explained with the field dependent mobility of carriers. Thus, one can determine μ0(T) when E = 0 and also γ(T) by using field and temperature-dependent characteristics of Fig. 6 respectively. The observed value of μp for PEPC complex is lower than the values reported in the literature for many other polymers [20].
Fig. 7. Variation in temperature-dependent zero-field mobility (μ0) of a PEPC thin film doped with An.
76
S.A. Moiz et al. / Thin Solid Films 516 (2007) 72–77
In general one can conclude that a higher positional disorderness causes a higher energetic disorderness and both of these factors then eventually reduce the hopping probability of the carriers as well as their mobility. To synthesize high mobility organic semiconductor it would be, therefore, imperative that the organic molecular structure should offer hopping distance as small as possible with minimum positional as well as energetic disorderness. 5. Conclusions
Fig. 8. Shows field activation coefficient (γ) vs temperature for PEPC thin film.
Bässler Gaussian disorder model (GDM) predicts the high field mobility as [21] " ( ) # 2r 2 r 2 2 pffiffiffiffi E μGDM ¼ μl exp þC R ð6Þ 3KB T KB T where the value of Σ varies from 1 to 4 [22] and C is an empirical constant which describes the width of density of states relative to KBT. It is worth mentioning that GDM neglects spatial correlation due to charge dipole interactions even though such correlations have recently been shown to explain the universal electric field dependence mobility observed in disordered organic films. Taking into account the correlated random potential Novikov et al. [23] have modified Eq. (6) and proposed the following empirical relationship " rffiffiffiffiffiffiffiffi# 3r 2 r 32 qaE μCGDM ¼ μl exp þ0:78 2 5KB T KB T r ð7Þ where C = 0.78 and Σ = 2. When E = 0 Eq. (7) is reduced to 2
lnμ0 ¼ lnμ∞
3σ 5K B
2
1 T2
ð8Þ
By applying the SCLC model on the observed temperaturedependent characteristics μ0 and γ were evaluated and shown in Figs. 7 and 8 respectively. For σ = 0.09 eV the calculated value of μ0 as given in Eq. (8), is in good agreement with the experimental value. Using the value of σ obtained from the fitting of μ0(T ), the value of a can then be evaluated from the slope of γ ∼ T − 1.5 as shown in Eq. (7). The value of a thus determined was ∼ 3.13 nm. A usual reported value of a for polymers which offer better mobility is 1−2 nm [24]. A relatively high value of a could be associated with low value of μp for PEPC complex as observed in Fig. 6.
Thin films of poly-N-epoxypropylcarbazole (PEPC) doped with anthracene (An) have been fabricated. Films were grown on Ni substrates by employing centrifugal machine at 277 g. I– V characteristics of the fabricated films were evaluated as function of temperature ranging from 30 to 60 °C. Reversible rectifying characteristics have been observed in which the magnitude of current increases with increasing values of temperature. This can be associated with the generation of free carriers at elevated temperatures together with enhanced temperature assisted hopping in the organic film. It is further observed that I–V characteristics of the device follow space charge limited conduction model. The field and temperaturedependent hole mobility in PEPC complex is described by the 1=2
empirical relation μp a exp c E . By employing correlated Gaussian disorder mobility model on the observed characteristics, energetic disorderness, σ and intersite distance, a has been determined and found to be 0.09 eV and 3.13 nm respectively. These evaluated values are relatively higher and could be the main cause of low carriers mobility in PEPC complex which in return controls the I–V response of the device. References [1] Y. Shirota, J. Mater. Chem. 10 (2000) 1. [2] Kh.S. Karimov, M.M. Ahmed, S.A. Moiz, M.I. Fedorov, Sol. Energy Mater. Sol. Cells 87 (2005) 61. [3] M.M. Ahmed, Kh.S. Karimov, S.A. Moiz, IEEE Trans. Electron. Dev. 51 (2004) 121. [4] S.A. Moiz, M.M. Ahmed, Kh.S. Karimov, Jpn. J. Appl. Phys. 44 (2005) 1199. [5] J. Reynaert, V.I. Arkhipov, G. Borghs, P. Heremans, Appl. Phys. Lett. 8 (2004) 603. [6] D. Braun, J. Polym. Sci., Part B, Polym. Phys. 41 (2003) 2622. [7] P. Mark, H.V. Seggern, J. Appl. Phys. 33 (1962) 205. [8] S.A. Moiz, M.M. Ahmed, Kh.S. Karimov, Electron. Telecommun. Res. Inst. (ETRI) J. 27 (2005) 319. [9] Kh.M. Akhmedov, Doctor of Science Thesis, Donish, Dushanbe, Tajikistan, 1998. [10] L.L. Regel, W.R. Wilcox, Processing by Centrifugation, Kluwer Academic/Plenum Publisher, New York, NY, 2001. [11] M.C. Petty, M.R. Bryce, D. Bloor, An Introduction to Molecular Electronics, Edward Arnold, London, 1995. [12] Y. Wu, Y. Li, B.S. Ong, J. Am. Chem. Soc. 128 (2006) 4202. [13] M. Pfeiffer, K. Leo, X. Zhoua, J.S. Huanga, M. Hofmanna, A. Wernera, J. Blochwitz-Nimothb, Org. Electron. 4 (2003) 89. [14] D.A. Bernards, T. Biegala, Z.A. Samuels, J.D. Slinker, G.G. Malliaras, S. Flores-Torres, H.D. Abruna, J.A. Rogers, Appl. Phys. Lett. 84 (2004) 3675.
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