Dielectric Spectroscopic Studies of Boron ... - Springer Link

Electronic Materials Letters, Vol. 9, No. 1 (2013), pp. 101-106 DOI: 10.1007/s13391-012-2089-8

Dielectric Spectroscopic Studies of Boron Subphthalocyanine Chloride Thin Films Mandeep Singh,1 Aman Mahajan,1,* R. K. Bedi,1 and D. K. Aswal2 1

Material Science Research Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar-143 005, India 2 Thin Films Devices Section, Technical Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India (received date: 25 May 2012 / accepted date: 2 August 2012 / published date: January 2013) Boron subphthalocyanine chloride (Cl-BsubPc) thin film have been deposited by thermal evaporation technique and studied for structural and dielectric properties. AFM study indicates the formation of uniform and cracks free films. To study the dielectric properties as function of both frequency and voltage, Al/ Cl-BsubPc/Al heterostructure has been fabricated. The impedance-frequency study indicates the formation of space charge region in lower frequency range. The conduction mechanism in Cl-BSubPc film, under applied ac field, found to be electronic hopping. The mobility and charge carrier concentration of Cl-BsubPc films have also been measured using capacitance techniques. Keywords: subphthalocyanine, mobility, carrier concentration, conduction mechanism

1. INTRODUCTION Phthalocyanines (Pcs), are the most studied and bestknown class of organic semiconducting molecules, have attracted considerable attention due to their unique biological, electronic, optical and structural properties.[1-3] Among phthalocyanines, boron subphthalocyanines chloride (ClBsubPc, Fig. 1) are the lowest homologues of phthalocyanine have attracted a great deal of interest due to their unique chemistry and photophysical properties.[4,5] ClBsubPc is composed of three diiminoisoindole rings Nfused around a boron core.[4] The 14π-electron aromatic core system of Cl-BsubPc along with their nonplanar coneshaped structure and octupolar character make them poten-

Fig. 1. Schematic diagram of Boron subphthalocyanine chloride (ClBsubPc) molecules. *Corresponding author: [email protected] ©KIM and Springer

tial candidates to design hybrid inorganic-organic materials with outstanding tunable properties for photo-induced electron energy transfer, optical data storage, nonlinear optics and sensing applications.[6-8] Cl-BsubPc has recently emerged as functional materials and found potential applications in photovoltaic and light emitting devices.[9-12] Cl-BsubPc based organic photovoltaic devices are found to be superior than other phthalocyanines based ones due to better energy level alignment and large open circuit voltage (VOC). Kristin et al.[13] have reported that photovoltaic cell based on boron subphthalocyanine chloride/fullerene (SubPc/C60) show increase in VOC by 27% as compared to copper phthalocyanine/fullerene (CuPc/C60). Moreover, the presence of Cl-BsubPc as a hole injecting layer in OLEDs improves both electron and hole injection efficiencies.[12] Considering the potentiality of Cl-BsubPc compound for optoelectronic device applications, the structural and optical properties of Cl-BsubPc thin film have extensively been studied.[14-17] To optimize the devices performance, a detail understanding of the fundamental of charge-carrier transport properties of material is also required, but till now there is almost no report available on the carrier transport properties of this material. Keeping this end in view, an attempt has been made in this paper to study the transport properties of Al/Cl-BsubPc/ Al heterostructures prepared by thermal evaporation technique. Besides these, simultaneous measurements of charge carrier density and mobility in Cl-BsubPc films have also been reported using capacitance techniques.

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Mandeep Singh et al.: Dielectric Spectroscopic Studies of Cl-BsubPc Thin Films

2. THEORETICAL BACKGROUND In organic semiconductor devices, the process of conduction mechanism and carrier transport dominates their performance. The carrier concentration and mobility plays a crucial in determining the performance of the device. Many experimental methods namely time of flight (TOF), Hall effect coupled with conductivity measurements, the magneto-resistance, the equilibrium charge carrier extraction, the conductivity/concentration (σ/n) etc. have been developed to extract these electrical parameters.[18-23] TOF is perhaps the most general technique to extract the carrier mobility of organic materials.[24] However, this method is of limited use due the requirement of thick film of several microns or more to provide a well defined flight distance. Moreover, the presence of intrinsic or extrinsic traps inside the organic film, results in a highly dispersive time of flight transient which is hindering the extraction of relaxation time. Thus, TOF appears not to be a practical means for mobility evaluation in dispersive materials.[25] In these directions, dielectric spectroscopy has been recently emerged as a technique to study conduction mechanism and transport properties of the material in bulk form as well as in thin film form. Dielectric spectroscopy is known to be a powerful technique for the simultaneous evaluation of carrier concentration and mobility along with conduction mechanism.[26,27] Data from dielectric spectroscopy can be analyzed using different complex formalisms, each of which consists of real and imaginary parts, namely:[28] • Complex permittivity, ε = ε' – jε'' • Complex Impedance, Z = R + jX = R + 1/jωC

(1)

The differential susceptance ∆B can be evaluated by using the relation: −∆B = −ω(C − Cgeo)

(2)

where ω, C and Cgeo are the angular frequency, capacitance and geometric capacitance of the film respectively. The ac conductivity σac was determined by using the relation: σac = εo ω εr tan δ

Cl-BsubPc film was monitored by Quartz monitor and found to be 100 nm. The structural analysis of the films under investigation was done using x-ray diffraction (XRD) technique. The Bruker diffractrometer was used for this purpose. X-ray diffraction spectra have been recorded in the range 3° - 70° (2θ) at a scanning speed of 0.25° per minute. The surface topographical studies were carried out using Nanosurf easyscan2 (Switzerland) Atomic Force Microscope (AFM). HIOKI 3532-50 LCR HITESTER have been used to study the dielectric properties of Al/Cl-BsubPc/Al heterostructure. All the measurements were carried out at room temperature.

4. RESULTS AND DISCUSSION 4.1 Structural properties Figure 2 shows the x-ray diffraction spectrum of ClBsubPc film deposited on the glass substrate kept at room temperature. The broad peak centered at 2θ = 22.88° has been observed in the x-ray diffraction spectrum of ClBsubPc film and this peak represented (002) plane.[29] This indicates that Cl-BsubPc molecule exhibit a preferential alignment of the molecular crystals with the (002) orientation perpendicular to the substrate. The AFM images ClBsubPc film deposited on the glass substrate (Fig. 3(a,b)) reveals that the growth of the film is uniform and crack free. The average length and base width of grains are found to be 325 nm and 253 nm respectively. Further, root mean square (RMS) surface roughness is found to be 5.39 nm. 4.2 Dielectric properties 4.2.1 Capacitance frequency characteristics The variation of capacitance of Cl-BsubPc film with frequency ranging from 10 Hz to 1 KHz at constant voltage is shown in Fig. 4. It is found that with increase in frequency the capacitance is found to be decreased and tends to attain a constant value which is equivalent to the geometric capacitance Cgeo = ε0εrA/d of the Cl-BsubPc films (where ε0 is the

(3)

where εo is the permittivity in free space and tan δ is the dissipation factor.

3. EXPERIMENTAL PROCEDURE The Cl-BsubPc powder was procured from sigma Aldrich. The heterostructure was deposited onto chemically and ultrasonically cleaned glass substrate by thermal evaporation technique at a pressure of about 10−5 Torr. A molybdenum boat and tungsten filament were used for depositing the Cl-BsubPc and Al layers respectively. The thickness of

Fig. 2. X-ray diffraction spectrum of Cl-BsubPc film.

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permittivity of free space, εr is the relative permittivity of Cl-BsubPc, A is the area of the Cl-BsubPc thin film and d is the thickness of the Cl-BsubPc film). The geometrical capacitance of Cl-BsubPc thin films is found to be 2.35 nF. This type of behavior of capacitance with frequency can more precisely be explained in terms of Goswami and Goswami equivalent circuit model.[30] In this model the capacitor system is assumed to comprise a frequency independent capacitive element C' in parallel with a discrete temperature resistive element R, both in series with a constant low value resistance r. According to this model, the measured series capacitance Cs is given by: Cs = C' + 1/ω2 R2 C'.

(4)

According to the equation (4) the series capacitance Cs should decreased with increase in frequency and attains a constant value C' at higher frequency. All these effects were clearly observed in Fig. 4. It is seen that the capacitance decreases in the low frequency range and attains a constant value in the high frequency range, which the usual behavior is observed in many dielectric films.[31,32] 4.2.2 Dielectric constant and dissipation factor The dielectric constant (εr) of Cl-BsubPc thin films has been evaluated from the capacitance measurements given by the equation: εr = Cd/ε0A.

(5)

Fig. 3. (a) Two dimensional AFM image of Cl-BsubPc film (b) Three dimensional AFM image of Cl-BsubPc film.

where C is the observed capacitance, d the thickness of the film, A the area of the film and ε0 the permittivity of free space. The εr values are obtained in the frequency range from 10 Hz to 1 KHz at constant bias voltage. Figures 5 and 6 shows the variation of dielectric constant and dissipation factor of Cl-BsubPc films with frequency at constant volt-

Fig. 4. Variation of capacitance with frequency of Cl-BsubPc film at constant bias voltage.

Fig. 5. Variation of dielectric constant with frequency of Cl-BsubPc film at constant bias voltage.


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ages. It is found that the dielectric constant lies between 2.89 and 2.96. Further both dielectric constant and dissipation factor are decreased with increase in frequency and tends to attain the constant values at higher frequency. This behavior may be attributed to the dipole relaxation phenomenon. This phenomenon reflects about the delay time of the dipoles subjected to an electric field in the frequency response. This delay time is occurred due to the inability of dipoles (responsible for polarization) to follow the oscillations of the electric field at particular frequencies, the dipole reorientation and the field reversal become out of phase giving rise to dissipation of energy. 4.2.3 Impedance frequency characteristics Figure 7 shows the variation of impedance with frequency (10 Hz - 1 KHz) at constant bias voltage. It is observed that as the frequency increased the impedance of Cl-BsubPc films found to be decreased and tends to attain a constant value at higher frequency. This behaviour can be explained by using the complex impedance equation (1). At higher frequencies, the second term in equation (1) is negligibly small and the impedance of the Cl-BsubPc films tends to attain a constant value R, resistance of the films. But as the frequency decreases, the capacitor offer additional reactance to the charge carrier. Due to this the charge carriers got trapped in the materials or on Al/(Cl-BsubPc) interface, which results the formation of space charge region. This space charge region significantly increased the capacitance of the Cl-BsubPc films. 4.2.4 Mobility determination In order to calculate charge carrier mobility’s[33,34] in the Cl-BsubPc film, the differential susceptance ∆B at different dc biasing voltage is plotted as a function of frequency

Fig. 7. Variation of impedance with frequency of Cl-BsubPc film at constant bias voltage.

Fig. 8. Variation of ∆B with frequency of Cl-BsubPc film at constant bias voltage.

(Fig. 8). There exists a maximum ∆B value at particular frequency fr = τr−1 (where fr,τr are relaxation frequency and relaxation time respectively). The positions of these maxima found to lie in 140 - 340 Hz for biasing voltage of 1 - 3 V. This can be related to the average transit time by the relation tdc = 0.56τr. The holes mobility can be calculated by the following relation: µdc = (d2/Vdc tdc)

Fig. 6. Variation of dissipation factor (tan δ) with frequency of ClBsubPc film at constant bias voltage.

(6)

where d and Vdc are the film thickness and dc bias voltage, respectively. The calculated values of relaxation time and holes mobility’s at different dc bias voltage are listed in Table 1. The observed decrease in hole mobility is due to shift of maxima of ∆B toward the lower frequency as the dc bias voltage increased. This shift of ∆B maxima also increases the rate of relaxation.



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Table 1. AC electrical parameters of Cl-BsubPc films. Applied Bias voltage

σ (S/m)

1V

6.88 × 10−

0

10

2V

3.83 × 10

3V

2.98 × 10

−10 −10

n

A

0.85

3.94 × 10−

0.57 0.45

12

3.12 × 10 6.37 × 10

−11 −11

µ (cm V− s− )

∆B Maxima (Hz)

τr (s)

340

2.9 × 10−

230 140

2

2

4.3 × 10 7.1 × 10

−2 −2

1

1

6.07 × 10−

9

2.05 × 10

−9

8.33 × 10−

10

Fig. 10. Variation of ac conductivity with frequency of Cl-BsubPc film at constant bias voltage. Fig. 9. Variation of 1/C with voltage of Cl-BsubPc film at constant frequency. 2

4.2.5 Carrier concentration The voltage dependence of capacitance provides an information about the carrier concentration in space charge region (Nsc) using the following equation:[35] 1/C2 = 2(V + Vbi)/qεNscA2

(7)

where C is the capacitance, q is electronic charge of carriers, ε is permittivity of the Cl-BsubPc film, Vbi is the built-in potential, A is the effective area of the device and V is the voltage. Figure 9 shows the variation of 1/C2 with voltage. Charge carriers concentration Nsc has been determined from the slope of these plots at different frequencies and obtained values are lying in the range 3 × 109 to 7 × 109 cm−3. 4.2.6 AC conductivity and conduction mechanism The evaluation of ac conductivity (σac) from the dielectric loss (tan δ), gives valuable information on the nature of conduction occurring in these films. The ac conductivity σac as a function of frequency at constant bias voltage of ClBsubPc thin film is shown in Fig. 10. The variation of ac conductivity shows a frequency independent plateau in the low frequency region and exhibits dispersion at higher frequencies.[36] This behavior obeys the universal power law:[37] σ (w) = σ0 + Aωn

(8)

where ω is the frequency, σ0 is the dc conductivity (fre-

quency independent plateau in the low frequency region), ‘n’ is the power law exponent which generally varies between 0 to 1 and A is the pre-factor which gives the strength of polarizibility. The exponent ‘n’ represents the degree of interaction between mobile ions with lattice around them (n = 1 represents non-interacting Debye system and with decreasing n, interaction between mobile ions and lattice is expected to increase). The conductivity is found to be decreased with decreased in frequency due to the accumulation of charge at the Al/(Cl-BsubPc) interface. The values of parameters σ0, A and n were obtained by fitting equation (8) and are tabulated in Table 1. The values of ‘n’ are in accordance with the theory of hopping conduction in amorphous materials.[38] As the frequency increases, the hopping between the charge centers and the electrodes increase, which results in sharp increase of ac conductivity and the material becomes conducting. Table 1 also indicates that as the bias voltage increased the value of parameter ‘n’ and σ0 decreases. This may be due to the increase of polarizability strength ‘A’ with increase in bias voltage which in turn decreases the conductivity of Cl-BsubPc films. The value of ‘n’ also indicates that the relaxation processes in Cl-BsubPc films are non-Debye type.[37] Hence the interactions between the dipoles cannot be neglected and these interactions increases as the dc biasing voltage increase (value of ‘n’ decreases with increase in dc biasing voltage). Due to these interactions the distribution of relaxation time occurs in the films.


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5. CONCLUSIONS In conclusion we have demonstrated the dielectric spectroscopy technique to determine the charge carrier transport properties of Cl-BsubPc film. The impedance is found to be increased toward the lower frequency, suggesting the blockage of charge carrier at the Al/(Cl-BsubPc) interface. The ac conductivity of Cl-BsubPc film obey the universal power law with index value lie in the range 0.45 - 0.85, indicates the presence of frequency induced electron hopping process. The non-Debye type relaxation process has been observed in these films. The hole mobility and carrier concentration of Cl-BsubPc film are found to be lying in the range 8.33 × 10−10 - 6.07 × 10−9 cm2 V−1 s−1 and 3 × 109 7 × 109 cm−3 respectively.

ACKNOWLEDGMENTS Authors are thankful to DAE-BRNS, Mumbai, India for providing financial assistance to carry out this research work.

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