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Inorganic–organic photodiodes based on polyaniline doped boric acid and polyaniline doped boric acid:nickel(II) phthalocyanine composite
Sensors and Actuators A 153 (2009) 191–196
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Inorganic–organic photodiodes based on polyaniline doped boric acid and polyaniline doped boric acid:nickel(II) phthalocyanine composite Fahrettin Yakuphanoglu a,∗ , Mehmet Kandaz b , B. Filiz Senkal c a b c
Fırat University, Department of Physics, 23119 Elazı˘g, Turkey Sakarya University, Department of Chemistry, 54140 Esentepe, Sakarya, Turkey Istanbul Technical University, Department of Chemistry, 34469, Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 6 December 2008 Received in revised form 15 May 2009 Accepted 15 May 2009 Available online 23 May 2009 Keywords: Photodiode Organic semiconductor Boric acid doped polyaniline
a b s t r a c t The electronic and photovoltaic properties of hybrid organic photodiodes based on n-Si/boric acid doped polyaniline (PANIB) and n-Si/2,3,7,8,12,13,17,18-octakis(2 -aminophenylsulfanyl)-substituted-nickel(II) phthalocyanine:boric acid doped polyaniline (PANIB-PC) composite have been investigated. The current–voltage characteristics of the n-Si/PANIB and n-Si/PANIB-PC diodes were analyzed under dark and illumination conditions. The open circuit voltage, Voc and short circuit current, Isc values for the n-Si/PANIB and n-Si/PANIB-PC diodes under 105 mW/cm2 were respectively found to be 0.280 V, 6.19 nA and 0.304 V, 0.091 nA. The fabricated inorganic/organic devices can be used as an optical sensor for optoelectronic applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic semiconductors have extensively been used in electronic and photovoltaic device applications such as organic light emitting diodes, thin film transistors, and solar cells due to their functional electrical and optical properties. Optoelectronic devices based on organic materials are becoming a mature technology for a wide range of applications [1]. Photodiodes based on semiconductor–semiconductor and metal–semiconductor junctions can be fabricated by using different organic semiconductors. When these junctions with organic semiconductor are illuminated, electrons and holes are produced and in turn, these devices show a photovoltaic effect. Organic photovoltaic devices are promising candidates for renewable sources of electrical energy because of ease in fabrication and low production cost as well as light weight and flexibility [2–5]. Polyaniline is the most interesting material among the other conductive polymers [6] and it has attracted considerable attention due to its unique properties, in particular its reversibly controlled conversions via redox doping and protonation. This polymer can be used for electronic and optoelectronic device applications. In present study, we have synthesized the boric acid doped polyaniline (PANIB) and 2,3,7,8,12,13,17,18-octakis(2 -aminophenylsulfanyl)substituted-nickel(II) phthalocyanine:boric acid doped polyaniline
∗ Corresponding author. E-mail address: [email protected] (F. Yakuphanoglu). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.05.008
organic semiconductors to fabricate organic–inorganic photovoltaic devices.
2. Experimental 2,3,7,8,12,13,17,18-Octakis(2 -aminophenylsulfanyl)substituted-nickel(II) phthalocyanine (PC) has synthesized and described elsewhere [7]. The polyaniline doped boric acid (PANIB) was synthesized as follows: 1 g of aniline and 1 g of boric acid were added into 50 mL of water. To this solution, 30 mL aqueous solution containing 3 g of ammonium persulfate was added dropwise (15–20 min interval) at 0 ◦ C. The reaction mixture was kept for 24 h at room temperature. The reaction mixture was filtered, washed with excess of water, and finally with ethanol. The sample was dried at room temperature under vacuum, until constant weight was reached. The chemical structure of the polyaniline doped boric acid (PANI-B) is characterized as in Ref. [8]. Nickel(II) phthalocyanine bearing 2 -aminophenylsulfanyl moieties is doped into matrix of the boric acid doped polyaniline. For the fabrication of n-Si/PANIB and n-Si/PANIB-PC diodes, firstly, the back ohmic contact was fabricated by evaporating Al on the n-type Si using a PVD-HANDY/2S-TE (Vaksis Company) thermal evaporation system [9]. Then, the ohmic contact was subjected to a heat treatment at 570 ◦ C for 5 min in N2 atmosphere. The composite of PANIB and PC compounds was prepared with 1 wt% of PC. The PANIB and PANIB-PC samples were dissolved and the solutions were homogenized for 15 min by mixing
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Fig. 1. Energy-band diagram and structure of the diodes. Ef is the Fermi level, d is the thickness of organic layer, ˚b is the barrier height, Eg (org) is the band gap of the organic material, Eg (in) is the band gap of inorganic semiconductor, and w is the width of the depletion region.
with rotation before the deposition. Then, the films of PANIB and PANIB-PC samples were separately prepared by deposition of solution on the Si wafer by dip coating technique [9–11]. After the film deposition, the polymer was removed from the Al contact on Si. The Au contacts were prepared on the PANIB and PANIBPC films. The structures of devices prepared are Al/n-Si/PANIB/Au and Al/n-Si/PANIB-PC/Au, in which anode and cathode electrodes are Au and Al contacts. In these devices, the PANIB-PC serves as an activate layer. The energy band diagram and structure of the diodes are shown in Fig. 1a and b. The current–voltage (I–V) characteristics of the device under dark and illumination conditions were measured using a KEITHLEY 2400 sourcemeter. Photovoltaic measurements were employed using a 200 W halogen lamp.
3. Results and discussion 3.1. Current–voltage characteristics of the n-Si/PANI-B and n-Si/PANIB-PC diodes The current–voltage characteristics of the n-Si/PANIB and nSi/PANIB-PC diodes are shown in Fig. 2a and b. The I–V curves of the n-Si/PANIB and n-Si/PANIB-PC diodes under dark and illumination conditions indicate three different regions. In the first region, the current is limited by shunt resistance, Rsh . In the second region, the current of the diode is exponentially increases with applied voltage. At higher voltages (region III), the current of the diode is controlled by space charge. Under illumination, the diodes give an open circuit voltage and short circuit current values. As seen in Fig. 2, when the
Fig. 2. I–V characteristics of the diodes at dark and illumination conditions: (a) n-Si/PANIB and (b) n-Si/PANIB-PC.
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Fig. 3. Rj –V plots of the diodes: (a) n-Si/PANIB and (b) n-Si/PANIB-PC.
diodes are illuminated, the first region disappeared, this suggests that the electrical behavior of the photovoltaic n-Si/PANIB and nSi/PANIB-PC diodes is dominated by second region, in which the current varies exponentially with applied voltage. The current–voltage expression for organic–inorganic diodes including parasitic resistances is expressed by the following relation [12]: I = Io exp
q(V − IR ) s
nkT
+
V − IRs Rsh
Fig. 4. Plots of dV/d ln I–I and H(I)–I of the diodes: (a) n-Si/PANIB and (b) n-Si/PANIBPC.
by Cheung functions [18]
(1)
dV kT =n + IRs q d ln(I)
where n is the ideality factor, Io is the reverse saturation current, Rs is the series resistance, q is the electronic charge and Rsh shunt resistance. As seen in Fig. 1, the I–V behavior of the diodes is affected by series resistance and shunt resistance. Rs resistance is related to the interfaces between two semiconductors, while Rsh resistance is associated with semiconductor–electrode interface properties [13]. The determination of these parameters is important to get the detail information about the charge transport mechanism of the diodes. The junction resistance Rj for the diodes is expressed as Rj =
dV dI
H(I) = V − n
kT ln q
(3)
I o
(4)
AA∗ T 2
and H(I) = IRs + nB
(5)
Below region III, the series resistance is dominant. Thus, we plotted the curves of dV/d ln I–I and H(I)–I, as shown in Fig. 4(a) and (b). The series resistance RS and ideality factor n values were calculated from dV/d ln I–I plots and are given in Table 1. The B and Rs values were determined from the H(I)–I plots and are given in Table 1. The obtained series resistance values from Cheung method are lower than those obtained from Rj –V plots. The discrepancy between the Rs values results from the saturation region regions in Rj –V plots, as shown in Fig. 3. For the ideal Rj –V curves, Rj values at sufficiently high forward bias should be reached to a good saturation region. Thus, we evaluate that the Cheung method is ideal to obtain the series resistance values of the n-Si/PANIB and n-Si/PANIB-PC diodes. The higher ideality factor of the diodes may be due to the organic semiconductor layer on n-Si surface and the presence of the native oxide layer. The interface oxide layer can
(2)
In order to calculate the series and shunt resistances, we plotted the curve of Rj –V for the diodes, as shown in Fig. 3. These resistances were determined by method defined in Refs. [14–17]. The Rs and Rsh values were calculated from the curves of Rj –V and are given in Table 1. The Rs value of the PANIB diode is lower than that of the n-Si/PANIB-PC diode. This suggests that the PANIB diode has the lower ideality factor and barrier height values; because these parameters are affected from the series resistance, interface state density and higher electrical conductivity of PANIB with respect to PANIB-PC. The series resistance of the diodes can be also analyzed Table 1 The electrical parameters of the diodes. Diode
Rs ()a
Rsh ()
n
b (eV)
Rs ()b
Rs ()c
n-Si/PANIB n-Si/PANIB-PC
8.04 × 105 1.69 × 107
4.35 × 107 8.12 × 108
5.93 7.07
0.80 0.87
7.21 × 105 1.37 × 107
7.42 × 105 1.35 × 107
a
Rj –V plot. dV/d ln I–I plot. c H(I)–I plot. b
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Fig. 5. Plots of log I–log V of the diodes: (a) n-Si/PANIB and (b) n-Si/PANIB-PC.
be formed, when the organic layer film is deposited onto the surface of silicon due to the water or vapour adsorbed. The n-Si/PANIB and n-Si/PANIB-PC diodes have the higher series resistance due to the electrical conductivity of the PANIB and PANIB-PC organic semiconductors. At higher voltages, in order to analyze the charge transport mechanism of the diodes, the current–voltage characteristics of the diodes are shown in Fig. 5a and b. The I–V curves in logarithmic scale indicate two current regions. These regions can be analyzed via I ∝ Vm relation. m values for each region were calculated from the slope of Fig. 5a and b. For the first region, m values are higher than 2 (2.95–3.12 for n-Si/PANIB diode and 4.02–3.71 for n-Si/PANIB-PC). This suggests that the current in the first region is controlled by trap-charge-limited conductivity mechanism (TCLC). In this mechanism, the current is limited by trapped charges, when the quasi-Fermi level is located in the trap distribution. In turn, the n-Si/PANIB and n-Si/PANIB-PC diodes show one TCLC mechanism which relates to one kind of traps. In this model, the relation between and voltage is expressed as [19–21] I=
qAN
εε l o
d2l+1
ePo kTt
V l+1
(6)
where ε is the dielectric constant of the semiconductor, e is the electronic charge, is the mobility of carrier charges, N is the effective density of states in electronic band edge, d is the thickness of the sample, l (l = m − 1) is a parameter given by l = Tt /T, Tt is a characteristic temperature of the exponential distribution of the traps. As seen in Fig. 5a and b, the slope of the plots changes with temperature. This indicates that Tt values vary with temperature. It is evaluated that the change in Tt values of the traps indicates to start
Fig. 6. I–V characteristics of the diodes at different illumination intensities: (a) nSi/PANIB and (b) n-Si/PANIB-PC.
the filling of the traps. In region II, the increase rate of current with voltage decreases. This suggests that most of traps are filled and contribution of free carrier to electric field becomes appreciable [22]. 3.2. Photovoltaic properties of the n-Si/PANI-B and n-Si/PANIB-PC diodes The reverse I–V curves of the diodes under various illuminations are measured, as a photodiode operates under reverse bias conditions and I–V curves are shown in Fig. 6a and b. As seen in Fig. 6a and b, the light on reverse I–V characteristics is translated downward moved in the −I direction along the current axis, giving an open circuit voltage (Voc ) along with a short photocurrent (Isc ) [23] due to the generation of the electron–hole pairs, which were effectively generated in the junction by incident photons. Thus, the current in the reverse direction is strongly increased by illumination. The diodes have an open circuit voltage and short-circuit current. The Voc and Isc values for the n-Si/PANIB and n-Si/PANIB-PC diodes under 105 mW/cm2 were respectively found to be 0.280 V, 6.19 nA and 0.304 V, 0.091 nA. In the literature, electrical and solar cell parameters of a Schottky structure fabricated using CuPc and fluorinated tin oxide (FTO) have been investigated and the obtained Isc value is 2.2 mA/m2 under 98 mW/cm2 [24]. In another study, NiPc/p-Si junction has been shown a photovoltaic characteristic
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4. Conclusions The current–voltage characteristics of the n-Si/PANIB and n-Si/PANIB-PC diodes have been investigated under dark and illumination conditions. The I–V characteristics of the diodes are affected by series and shunt resistances. The Voc and Isc values for the n-Si/PANIB and n-Si/PANIB-PC diodes under 105 mW/cm2 were respectively found to be 0.280 V, 6.19 nA and 0.304 V, 0.091 nA. The obtained results indicate that n-Si/PANIB and n-Si/PANIB-PC junctions are inorganic–organic semiconductor photodiodes. Acknowledgments This work was supported by the National Boron Research Institute (BOREN) (Project Number: BOREN-2006-26-C¸25-19). The authors wish to thank BOREN. References
Fig. 7. Plot of Iph –P of n-Si/PANIB and n-Si/PANIB-PC diodes.
with short circuit current (Isc ) of 186 A [25]. In others studies, Isc values of 4-tricyanovinyl-N,N-diethylaniline/p-silicon hybrid organic–inorganic solar cells [26], hybrid organic/inorganic semiconductor photodiode [27] and p-Si/C70 heterojunction [28] devices are 9.15 mA/cm2 , 0.10 A and 0.35 A, respectively. The short circuit current values of n-Si/PANIB and n-Si/PANIB-PC diodes are lower than that of FTO/CuPc/Al and NiPc/p-Si devices. These results suggest that the performance of a organic/inorganic device is strongly impacted by organic materials. The studied diodes can be operated as a heterojunction photodiode. The obtained photovoltaic parameters are typical values for a photodiode, because photodiodes are routinely designed to achieve a spectral response or a rapid time response [23]. In order to analyze photoconductivity mechanism of the diodes, the variation of the photocurrent with light intensity of the PANIB and PANIB-PC diodes is shown in Fig. 7. The photocurrent dependence of light intensity is expressed as [17] Iph = BP ˛
(7)
where Iph is the photocurrent, B is a constant, ˛ is a exponent and P is the intensity of light. The ˛ values for the n-Si/PANIB and nSi/PANIB-PC diodes were determined from the slope of Iph vs P plots and were found to be 0.52 and 0.29, respectively. The ˛ = 0.52 value for the n-Si/PANIB diode indicates a bimolecular recombination process, while the ˛ = 0.29 value for the n-Si/PANIB-PC diode indicates the participation of another recombination path. The recombination mechanism of the n-Si/PANIB diode varies with the doping of PC organic semiconductor. When the diodes are illuminated, photocarriers are generated at organic–inorganic interface and in turn, the reverse current increases the efficient substantially. The increase in charge production is dependent on the difference in the electron affinities between n-Si and PANIB, PANIB-PC semiconductors.
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constants, (ii) refractive index dispersion. (C) liquid crystals and electro-optical properties. (D) nanostructure semiconductor materials and their electronic devices applications.
Biographies
Mehmet Kandaz graduated from the Blacksea Technical University Trabzon, Turkey in 1987, where he received his M.S degree in 1989. He received his Ph.D. degree in inorganic chemistry on “The Synthesis and Properties of Novel New Substituted Multifunctional Star Phthalocyanines” from the I˙ stanbul Technical University under supervisor of Prof. Dr. Ö. Bekaro˘glu in I˙ stanbul. He joined the Department of Chemistry at Sakarya University as an assistant professor in 1998 and, after a postdoctoral stay both in Nortwestern University in Chicago USA (undersupervisor of Brian M. Hoffman) from 1998 until the end of 1999, and again in I˙ stanbul Technical University was promoted to Associate Professor in 2004. Since 2004, he has been a Professor of inorganic chemistry at Sakarya University in Sakarya. His main research interests focus on functional phthalocyanines, porphyrazines, fluoresans probes, selective sensors, electroactive molecules, electron-transfer processes, electrochemistry, spectroelectrochemistry, catalytical effect of phthalocyanines, and liquid crystals.
Fahrettin Yakuphanoglu obtained his master degree from Solid State Physics, Firat University, Elazı˘g, Turkey, 1996–1998, Ph.D. from Solid State Physics, Firat University, Elazı˘g, Turkey, 1998–2002. He was Assoc. Prof. in Solid State Physics, Firat University, Elazı˘g, Turkey, 2004. His main achievements are—(A) organic semiconductors and their electronic devices applications such as Schottky diode, P–N heterojunction diode, metal–insulator–semiconductor junctions, solar cells, thin film transistor, photodiode, optical sensor. (B) optical materials: (i) determination of the optical
B. Filiz Senkal was graduated from Istanbul Technical University, Dept. of Chemical Engineering. She obtained her master degree from Istanbul Technical University, Dept. of Chemistry, 1991, Ph.D. from Istanbul Technical University, Dept. of Chemistry, 1996. She has been Assoc. Prof. in Istanbul Technical University, Dept. of Chemistry since 2002. Her main subjects are polymer modification, functional polymers, chemical preparation of conducting polymers and synthesis of new polymers.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Inorganic–organic photodiodes based on polyaniline doped boric acid and polyaniline doped boric acid:nickel(II) phthalocyanine composite Fahrettin Yakuphanoglu a,∗ , Mehmet Kandaz b , B. Filiz Senkal c a b c
Fırat University, Department of Physics, 23119 Elazı˘g, Turkey Sakarya University, Department of Chemistry, 54140 Esentepe, Sakarya, Turkey Istanbul Technical University, Department of Chemistry, 34469, Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 6 December 2008 Received in revised form 15 May 2009 Accepted 15 May 2009 Available online 23 May 2009 Keywords: Photodiode Organic semiconductor Boric acid doped polyaniline
a b s t r a c t The electronic and photovoltaic properties of hybrid organic photodiodes based on n-Si/boric acid doped polyaniline (PANIB) and n-Si/2,3,7,8,12,13,17,18-octakis(2 -aminophenylsulfanyl)-substituted-nickel(II) phthalocyanine:boric acid doped polyaniline (PANIB-PC) composite have been investigated. The current–voltage characteristics of the n-Si/PANIB and n-Si/PANIB-PC diodes were analyzed under dark and illumination conditions. The open circuit voltage, Voc and short circuit current, Isc values for the n-Si/PANIB and n-Si/PANIB-PC diodes under 105 mW/cm2 were respectively found to be 0.280 V, 6.19 nA and 0.304 V, 0.091 nA. The fabricated inorganic/organic devices can be used as an optical sensor for optoelectronic applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic semiconductors have extensively been used in electronic and photovoltaic device applications such as organic light emitting diodes, thin film transistors, and solar cells due to their functional electrical and optical properties. Optoelectronic devices based on organic materials are becoming a mature technology for a wide range of applications [1]. Photodiodes based on semiconductor–semiconductor and metal–semiconductor junctions can be fabricated by using different organic semiconductors. When these junctions with organic semiconductor are illuminated, electrons and holes are produced and in turn, these devices show a photovoltaic effect. Organic photovoltaic devices are promising candidates for renewable sources of electrical energy because of ease in fabrication and low production cost as well as light weight and flexibility [2–5]. Polyaniline is the most interesting material among the other conductive polymers [6] and it has attracted considerable attention due to its unique properties, in particular its reversibly controlled conversions via redox doping and protonation. This polymer can be used for electronic and optoelectronic device applications. In present study, we have synthesized the boric acid doped polyaniline (PANIB) and 2,3,7,8,12,13,17,18-octakis(2 -aminophenylsulfanyl)substituted-nickel(II) phthalocyanine:boric acid doped polyaniline
∗ Corresponding author. E-mail address: [email protected] (F. Yakuphanoglu). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.05.008
organic semiconductors to fabricate organic–inorganic photovoltaic devices.
2. Experimental 2,3,7,8,12,13,17,18-Octakis(2 -aminophenylsulfanyl)substituted-nickel(II) phthalocyanine (PC) has synthesized and described elsewhere [7]. The polyaniline doped boric acid (PANIB) was synthesized as follows: 1 g of aniline and 1 g of boric acid were added into 50 mL of water. To this solution, 30 mL aqueous solution containing 3 g of ammonium persulfate was added dropwise (15–20 min interval) at 0 ◦ C. The reaction mixture was kept for 24 h at room temperature. The reaction mixture was filtered, washed with excess of water, and finally with ethanol. The sample was dried at room temperature under vacuum, until constant weight was reached. The chemical structure of the polyaniline doped boric acid (PANI-B) is characterized as in Ref. [8]. Nickel(II) phthalocyanine bearing 2 -aminophenylsulfanyl moieties is doped into matrix of the boric acid doped polyaniline. For the fabrication of n-Si/PANIB and n-Si/PANIB-PC diodes, firstly, the back ohmic contact was fabricated by evaporating Al on the n-type Si using a PVD-HANDY/2S-TE (Vaksis Company) thermal evaporation system [9]. Then, the ohmic contact was subjected to a heat treatment at 570 ◦ C for 5 min in N2 atmosphere. The composite of PANIB and PC compounds was prepared with 1 wt% of PC. The PANIB and PANIB-PC samples were dissolved and the solutions were homogenized for 15 min by mixing
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Fig. 1. Energy-band diagram and structure of the diodes. Ef is the Fermi level, d is the thickness of organic layer, ˚b is the barrier height, Eg (org) is the band gap of the organic material, Eg (in) is the band gap of inorganic semiconductor, and w is the width of the depletion region.
with rotation before the deposition. Then, the films of PANIB and PANIB-PC samples were separately prepared by deposition of solution on the Si wafer by dip coating technique [9–11]. After the film deposition, the polymer was removed from the Al contact on Si. The Au contacts were prepared on the PANIB and PANIBPC films. The structures of devices prepared are Al/n-Si/PANIB/Au and Al/n-Si/PANIB-PC/Au, in which anode and cathode electrodes are Au and Al contacts. In these devices, the PANIB-PC serves as an activate layer. The energy band diagram and structure of the diodes are shown in Fig. 1a and b. The current–voltage (I–V) characteristics of the device under dark and illumination conditions were measured using a KEITHLEY 2400 sourcemeter. Photovoltaic measurements were employed using a 200 W halogen lamp.
3. Results and discussion 3.1. Current–voltage characteristics of the n-Si/PANI-B and n-Si/PANIB-PC diodes The current–voltage characteristics of the n-Si/PANIB and nSi/PANIB-PC diodes are shown in Fig. 2a and b. The I–V curves of the n-Si/PANIB and n-Si/PANIB-PC diodes under dark and illumination conditions indicate three different regions. In the first region, the current is limited by shunt resistance, Rsh . In the second region, the current of the diode is exponentially increases with applied voltage. At higher voltages (region III), the current of the diode is controlled by space charge. Under illumination, the diodes give an open circuit voltage and short circuit current values. As seen in Fig. 2, when the
Fig. 2. I–V characteristics of the diodes at dark and illumination conditions: (a) n-Si/PANIB and (b) n-Si/PANIB-PC.
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Fig. 3. Rj –V plots of the diodes: (a) n-Si/PANIB and (b) n-Si/PANIB-PC.
diodes are illuminated, the first region disappeared, this suggests that the electrical behavior of the photovoltaic n-Si/PANIB and nSi/PANIB-PC diodes is dominated by second region, in which the current varies exponentially with applied voltage. The current–voltage expression for organic–inorganic diodes including parasitic resistances is expressed by the following relation [12]: I = Io exp
q(V − IR ) s
nkT
+
V − IRs Rsh
Fig. 4. Plots of dV/d ln I–I and H(I)–I of the diodes: (a) n-Si/PANIB and (b) n-Si/PANIBPC.
by Cheung functions [18]
(1)
dV kT =n + IRs q d ln(I)
where n is the ideality factor, Io is the reverse saturation current, Rs is the series resistance, q is the electronic charge and Rsh shunt resistance. As seen in Fig. 1, the I–V behavior of the diodes is affected by series resistance and shunt resistance. Rs resistance is related to the interfaces between two semiconductors, while Rsh resistance is associated with semiconductor–electrode interface properties [13]. The determination of these parameters is important to get the detail information about the charge transport mechanism of the diodes. The junction resistance Rj for the diodes is expressed as Rj =
dV dI
H(I) = V − n
kT ln q
(3)
I o
(4)
AA∗ T 2
and H(I) = IRs + nB
(5)
Below region III, the series resistance is dominant. Thus, we plotted the curves of dV/d ln I–I and H(I)–I, as shown in Fig. 4(a) and (b). The series resistance RS and ideality factor n values were calculated from dV/d ln I–I plots and are given in Table 1. The B and Rs values were determined from the H(I)–I plots and are given in Table 1. The obtained series resistance values from Cheung method are lower than those obtained from Rj –V plots. The discrepancy between the Rs values results from the saturation region regions in Rj –V plots, as shown in Fig. 3. For the ideal Rj –V curves, Rj values at sufficiently high forward bias should be reached to a good saturation region. Thus, we evaluate that the Cheung method is ideal to obtain the series resistance values of the n-Si/PANIB and n-Si/PANIB-PC diodes. The higher ideality factor of the diodes may be due to the organic semiconductor layer on n-Si surface and the presence of the native oxide layer. The interface oxide layer can
(2)
In order to calculate the series and shunt resistances, we plotted the curve of Rj –V for the diodes, as shown in Fig. 3. These resistances were determined by method defined in Refs. [14–17]. The Rs and Rsh values were calculated from the curves of Rj –V and are given in Table 1. The Rs value of the PANIB diode is lower than that of the n-Si/PANIB-PC diode. This suggests that the PANIB diode has the lower ideality factor and barrier height values; because these parameters are affected from the series resistance, interface state density and higher electrical conductivity of PANIB with respect to PANIB-PC. The series resistance of the diodes can be also analyzed Table 1 The electrical parameters of the diodes. Diode
Rs ()a
Rsh ()
n
b (eV)
Rs ()b
Rs ()c
n-Si/PANIB n-Si/PANIB-PC
8.04 × 105 1.69 × 107
4.35 × 107 8.12 × 108
5.93 7.07
0.80 0.87
7.21 × 105 1.37 × 107
7.42 × 105 1.35 × 107
a
Rj –V plot. dV/d ln I–I plot. c H(I)–I plot. b
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Fig. 5. Plots of log I–log V of the diodes: (a) n-Si/PANIB and (b) n-Si/PANIB-PC.
be formed, when the organic layer film is deposited onto the surface of silicon due to the water or vapour adsorbed. The n-Si/PANIB and n-Si/PANIB-PC diodes have the higher series resistance due to the electrical conductivity of the PANIB and PANIB-PC organic semiconductors. At higher voltages, in order to analyze the charge transport mechanism of the diodes, the current–voltage characteristics of the diodes are shown in Fig. 5a and b. The I–V curves in logarithmic scale indicate two current regions. These regions can be analyzed via I ∝ Vm relation. m values for each region were calculated from the slope of Fig. 5a and b. For the first region, m values are higher than 2 (2.95–3.12 for n-Si/PANIB diode and 4.02–3.71 for n-Si/PANIB-PC). This suggests that the current in the first region is controlled by trap-charge-limited conductivity mechanism (TCLC). In this mechanism, the current is limited by trapped charges, when the quasi-Fermi level is located in the trap distribution. In turn, the n-Si/PANIB and n-Si/PANIB-PC diodes show one TCLC mechanism which relates to one kind of traps. In this model, the relation between and voltage is expressed as [19–21] I=
qAN
εε l o
d2l+1
ePo kTt
V l+1
(6)
where ε is the dielectric constant of the semiconductor, e is the electronic charge, is the mobility of carrier charges, N is the effective density of states in electronic band edge, d is the thickness of the sample, l (l = m − 1) is a parameter given by l = Tt /T, Tt is a characteristic temperature of the exponential distribution of the traps. As seen in Fig. 5a and b, the slope of the plots changes with temperature. This indicates that Tt values vary with temperature. It is evaluated that the change in Tt values of the traps indicates to start
Fig. 6. I–V characteristics of the diodes at different illumination intensities: (a) nSi/PANIB and (b) n-Si/PANIB-PC.
the filling of the traps. In region II, the increase rate of current with voltage decreases. This suggests that most of traps are filled and contribution of free carrier to electric field becomes appreciable [22]. 3.2. Photovoltaic properties of the n-Si/PANI-B and n-Si/PANIB-PC diodes The reverse I–V curves of the diodes under various illuminations are measured, as a photodiode operates under reverse bias conditions and I–V curves are shown in Fig. 6a and b. As seen in Fig. 6a and b, the light on reverse I–V characteristics is translated downward moved in the −I direction along the current axis, giving an open circuit voltage (Voc ) along with a short photocurrent (Isc ) [23] due to the generation of the electron–hole pairs, which were effectively generated in the junction by incident photons. Thus, the current in the reverse direction is strongly increased by illumination. The diodes have an open circuit voltage and short-circuit current. The Voc and Isc values for the n-Si/PANIB and n-Si/PANIB-PC diodes under 105 mW/cm2 were respectively found to be 0.280 V, 6.19 nA and 0.304 V, 0.091 nA. In the literature, electrical and solar cell parameters of a Schottky structure fabricated using CuPc and fluorinated tin oxide (FTO) have been investigated and the obtained Isc value is 2.2 mA/m2 under 98 mW/cm2 [24]. In another study, NiPc/p-Si junction has been shown a photovoltaic characteristic
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4. Conclusions The current–voltage characteristics of the n-Si/PANIB and n-Si/PANIB-PC diodes have been investigated under dark and illumination conditions. The I–V characteristics of the diodes are affected by series and shunt resistances. The Voc and Isc values for the n-Si/PANIB and n-Si/PANIB-PC diodes under 105 mW/cm2 were respectively found to be 0.280 V, 6.19 nA and 0.304 V, 0.091 nA. The obtained results indicate that n-Si/PANIB and n-Si/PANIB-PC junctions are inorganic–organic semiconductor photodiodes. Acknowledgments This work was supported by the National Boron Research Institute (BOREN) (Project Number: BOREN-2006-26-C¸25-19). The authors wish to thank BOREN. References
Fig. 7. Plot of Iph –P of n-Si/PANIB and n-Si/PANIB-PC diodes.
with short circuit current (Isc ) of 186 A [25]. In others studies, Isc values of 4-tricyanovinyl-N,N-diethylaniline/p-silicon hybrid organic–inorganic solar cells [26], hybrid organic/inorganic semiconductor photodiode [27] and p-Si/C70 heterojunction [28] devices are 9.15 mA/cm2 , 0.10 A and 0.35 A, respectively. The short circuit current values of n-Si/PANIB and n-Si/PANIB-PC diodes are lower than that of FTO/CuPc/Al and NiPc/p-Si devices. These results suggest that the performance of a organic/inorganic device is strongly impacted by organic materials. The studied diodes can be operated as a heterojunction photodiode. The obtained photovoltaic parameters are typical values for a photodiode, because photodiodes are routinely designed to achieve a spectral response or a rapid time response [23]. In order to analyze photoconductivity mechanism of the diodes, the variation of the photocurrent with light intensity of the PANIB and PANIB-PC diodes is shown in Fig. 7. The photocurrent dependence of light intensity is expressed as [17] Iph = BP ˛
(7)
where Iph is the photocurrent, B is a constant, ˛ is a exponent and P is the intensity of light. The ˛ values for the n-Si/PANIB and nSi/PANIB-PC diodes were determined from the slope of Iph vs P plots and were found to be 0.52 and 0.29, respectively. The ˛ = 0.52 value for the n-Si/PANIB diode indicates a bimolecular recombination process, while the ˛ = 0.29 value for the n-Si/PANIB-PC diode indicates the participation of another recombination path. The recombination mechanism of the n-Si/PANIB diode varies with the doping of PC organic semiconductor. When the diodes are illuminated, photocarriers are generated at organic–inorganic interface and in turn, the reverse current increases the efficient substantially. The increase in charge production is dependent on the difference in the electron affinities between n-Si and PANIB, PANIB-PC semiconductors.
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constants, (ii) refractive index dispersion. (C) liquid crystals and electro-optical properties. (D) nanostructure semiconductor materials and their electronic devices applications.
Biographies
Mehmet Kandaz graduated from the Blacksea Technical University Trabzon, Turkey in 1987, where he received his M.S degree in 1989. He received his Ph.D. degree in inorganic chemistry on “The Synthesis and Properties of Novel New Substituted Multifunctional Star Phthalocyanines” from the I˙ stanbul Technical University under supervisor of Prof. Dr. Ö. Bekaro˘glu in I˙ stanbul. He joined the Department of Chemistry at Sakarya University as an assistant professor in 1998 and, after a postdoctoral stay both in Nortwestern University in Chicago USA (undersupervisor of Brian M. Hoffman) from 1998 until the end of 1999, and again in I˙ stanbul Technical University was promoted to Associate Professor in 2004. Since 2004, he has been a Professor of inorganic chemistry at Sakarya University in Sakarya. His main research interests focus on functional phthalocyanines, porphyrazines, fluoresans probes, selective sensors, electroactive molecules, electron-transfer processes, electrochemistry, spectroelectrochemistry, catalytical effect of phthalocyanines, and liquid crystals.
Fahrettin Yakuphanoglu obtained his master degree from Solid State Physics, Firat University, Elazı˘g, Turkey, 1996–1998, Ph.D. from Solid State Physics, Firat University, Elazı˘g, Turkey, 1998–2002. He was Assoc. Prof. in Solid State Physics, Firat University, Elazı˘g, Turkey, 2004. His main achievements are—(A) organic semiconductors and their electronic devices applications such as Schottky diode, P–N heterojunction diode, metal–insulator–semiconductor junctions, solar cells, thin film transistor, photodiode, optical sensor. (B) optical materials: (i) determination of the optical
B. Filiz Senkal was graduated from Istanbul Technical University, Dept. of Chemical Engineering. She obtained her master degree from Istanbul Technical University, Dept. of Chemistry, 1991, Ph.D. from Istanbul Technical University, Dept. of Chemistry, 1996. She has been Assoc. Prof. in Istanbul Technical University, Dept. of Chemistry since 2002. Her main subjects are polymer modification, functional polymers, chemical preparation of conducting polymers and synthesis of new polymers.