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Controlling of silicon–insulator–metal junction by organic semiconductor polymer thin film
Synthetic Metals 160 (2010) 1551–1555
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Controlling of silicon–insulator–metal junction by organic semiconductor polymer thin film Fahrettin Yakuphanoglu ∗ Firat University, Faculty of Science, Department of Physics, 23169 Elazig, Turkey
a r t i c l e
i n f o
Article history: Received 2 April 2010 Received in revised form 17 May 2010 Accepted 19 May 2010 Available online 15 June 2010 Keywords: Metal–oxide–semiconductor contact Light sensitive capacitor Organic semiconductor
a b s t r a c t Electrical and photovoltaic properties of a metal–semiconductor–insulator–polymer–metal diode were investigated. The n-Si/SiO2 /MEH-PPV/Al diode shows a rectifying behavior with the rectification ratio of 2.22 × 105 at ±5 V and exhibits a non-ideal behavior due to the series resistance and oxide-organic layers. The organic semiconductor makes a contribution to the I–V characteristics of the diode and the trap-charge limited space charge and space charge limited current mechanisms were observed for the diode. The current–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode under different illumination intensities give an open circuit voltage (Voc ) along with a short circuit current (Isc ). This suggests that the n-Si/SiO2 /MEH-PPV/Al diode is a photovoltaic device with Voc = 0.456 V and Jsc = 7.89 × 10−8 A/cm2 values under 100 mW/cm2 illumination intensity. The photoconductivity mechanism of the diode is controlled by monomolecular recombination. The interface state density Dit values with time constant it of the diode under dark and illumination conditions were found to be 2.53 × 1010 eV−1 cm−2 with 5.09 × 10−5 s and 2.50 × 1010 eV−1 cm−2 with 8.27 × 10−5 s, respectively. The obtained results indicate that the n-Si/SiO2 /MEH-PPV/Al diode is a photo-sensitive diode. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The metal–insulator–semiconductor (MIS) devices which play a crucial role in constructing some useful devices in electronic technology have been still investigated and have attracted much attention during recent years [1–3]. A MIS structure is the also most useful device in the study of semiconductor surfaces. Since the reliability and stability of all semiconductor devices are intimately related to their surface conditions, an understanding of the surface physics with the help of MIS diodes is of great importance to device operations [4–7]. The existence of organic layer can modify the electrical properties of MIS structure. Such a layer between metal and semiconductor with interface state, series resistance parameters causes the electrical characteristics of MIS Schottky diodes to be non-ideal [8–12]. Poly[2-methoxy-5-(2-ethyl)hexoxy-1,4-phenylenevinylene] (MEH-PPV) has been considered as one of the most potential conducting polymers for various optoelectronic applications such as organic light emitting diodes (OLED), sensors and organic solar cells because of its good environmental stability, easy conductivity control and cheap production in large quantities [13].
∗ Tel.: +90 424 237 0000 3621; fax: +90 424 233 0062. E-mail addresses: fyhanoglu@firat.edu.tr, [email protected]. 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.05.024
Apart from our work, and to best of our knowledge, no other information on silicon/silicon oxide/MEH-PPV organic semiconductor has been reported in any study in the literature. The aim of this study is to fabricate a photo-sensitive diode based on metal–semiconductor–insulator–polymer–metal junction for photovoltaic applications and to investigate whether or not this device shows a photovoltaic behavior.
2. Experimental For the diode fabrication, n-type single crystal silicon 1 1 1 with a thickness of 600 m and poly[2-methoxy-5-(2-ethyl)hexoxy-1,4phenylenevinylene] were used. In order to remove the native oxide on surface on n-Si, the substrate was etched by HF and then was rinsed in deionised water using an ultrasonic bath for 10–15 min and finally was chemically cleaned according to method based on successive baths of methanol and acetone. For formation of ohmic contact, high purity (99.999%) Au with a thickness of 100 nm was thermally evaporated onto the whole backside of the substrate at pressure of 4.5 × 10−5 Torr in vacuum pump system. Then it was rinsed in deionised water and finally was again chemically cleaned. The oxide layer to grown on Si surface without metal, Si wafer was replaced into a furnace and oxide layer was thermally formed on Si surface at 1000 ◦ C for 30 min. The oxide layer thickness was found to be 15 nm. Film of the poly[2-methoxy-5-(2ethyl)hexoxy-1,4-phenylenevinylene] (MEH-PPV) was prepared
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Fig. 2. Plots of dV/dln(I) vs. I and H(I) vs. I of the n-Si/SiO2 /MEH-PPV/Al diode.
linear behavior, whereas at higher voltages, the I–V characteristics deviate from the linear behavior due to the series resistance and interface state density. In such as case, in order to determine the diode parameters, Cheung’s method was used to determine diode parameters and Cheung’s functions are defined as [17–18]: dV kT =n + IRs q dln(I)
Fig. 1. Current–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode.
by evaporating the solvent from a solution of the sample with subsequent drying of the film deposited on SiO2 /Si/Au structure [14]. After depositing by drop coating, the film was dried at 50 ◦ C for 10 min on a hot plate to evaporate the solvent. After film deposition process, the substrate was immediately placed in vacuum system for the processes. Al metal contacts were formed on MEH-PPV organic layer by vacuum thermal evaporation of Al at pressure of approximately 3.0 × 10−5 Torr using a VAKSIS thermal evaporator system. The current–voltage characteristics of the diode were preformed using a KEITHLEY 2400 sourcemeter. The capacitance–conductance–voltage and capacitance–conductance–frequency characteristics were measured using a HIOKI 3532 LCR meter. The photoelectrical measurements were performed using a white light source of 200 W. The intensity of illumination was measured using a solar power meter. 3. Results and discussion 3.1. Dark current–voltage characteristics of Au/n-Si/SiO2 /MEH-PPV/Al diode Fig. 1 shows the current–voltage characteristics of the nSi/SiO2 /MEH-PPV/Al diode. The diode shows a rectifying behavior and the rectification ratio which is the ratio of forward current to reverse current at certain voltage was found to be 2.22 × 105 at ±5 V. The current–voltage characteristics of the diode can be expressed by the following relation [15–16]: I = Io exp
q(V − IR ) s
nkT
q(V − IR ) s
1 − exp −
kT
(1)
where Rs is the series resistance, V is the applied voltage, n is the ideality factor, k is the Boltzmann constant, T is the temperature and Io is the reverse saturation current given by
q B
Io = AA∗ T 2 exp −
kT
(2)
where B is the barrier height and A is the contact area, A* is the Richardson constant (112 A cm−2 K−2 for n-type Si) [15]. The diode indicates a non-ideal behavior due to the series resistance and oxide-organics layers. Under low voltages, the diode exhibits a
and H(I) = V − n
kT ln q
(3)
I o AA∗ T 2
= IRs + nB
(4)
The plots of dV/dln(I) vs. I and H(I) vs. I are shown in Fig. 2 and give a straight line in series resistance region. The Rs and n values were calculated from the slope and intercept of dV/dln(I) vs. I plot and were found to be 5.76 × 105 and 1.70, respectively. The Rs and B values were calculated from the slope and intercept of H(I) vs. I plot and were found to be 5.77 × 105 and 0.74 eV, respectively. The obtained series resistance is higher due the oxide layer and the resistance of the MEH-PPV organic semiconductor which has the low mobility and electrical conductivity with respect to an inorganic semiconductor. In order to analyze the contribution of higher voltages to I–V characteristics of the diode, I–V characteristic of the diode was plotted in log scale and was shown in Fig. 3. I–V curve of the diode were analyzed via I = kVm relation. Here I is the current, V is the voltage and m is a constant. m values were determined from slopes of regions I and II of Fig. 3 and were found to vary from 10.82 to 2.58. This curve shows that in the first region, trap-charge limited space charge mechanism (TCLC) is dominant, whereas in the second region, the current is controlled by space charge limited current mechanism (SCLC) [18]. It is evaluated that the organic semiconductor makes a contribution to the I–V characteristics of the diode. 3.2. Illuminated current–voltage characteristics of Au/n-Si/SiO2 /MEH-PPV/Al diode The current–voltage characteristics of the n-Si/SiO2 /MEHPPV/Al diode under various illumination intensities with step of 20 mW/cm2 are shown in Fig. 4 As seen in Fig. 4, the I–V curves give an open circuit voltage (Voc ) along with a short photocurrent (Isc ). The illumination increases strongly the reverse current due to the generation of carrier charges. The open circuit voltage Voc and short-circuit current density Jsc values of the device under 100 mW/cm2 illumination were found to be 0.456 V and 7.89 × 10−8 A/cm2 , respectively. This suggests that the n-Si/SiO2 /MEH-PPV/Al diode can be operated as a photodiode. The
F. Yakuphanoglu / Synthetic Metals 160 (2010) 1551–1555
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Fig. 3. Plot of log I–log V of the n-Si/SiO2 /MEH-PPV/Al diode.
Fig. 5. Phototransient current plot of the n-Si/SiO2 /MEH-PPV/Al diode.
values of Voc and Jsc for the photodiode is considerable lower and these values satisfy a photodiode, because, the photodiodes are routinely designed to achieve a spectral response or a rapid time response [18]. In order to analyze photoconductivity mechanism of the diode, the variation of the photocurrent with illumination intensity was plotted and the photocurrent increases with increasing illumination intensity. This indicates that the light illumination increases production of electron–hole pairs. The photocurrent dependence of light intensity is expressed as [19]:
and decay times for the diode indicate that the photoconductivity rise and decay of the diode increase and decrease rapidly, when the light is turned on and off, respectively. This confirms that the number of the generated free charge carriers increases with time. Fig. 6 shows the transient photocurrent plot for the reverse and forward characteristics of the diode. The reverse current of the diode increases strongly, when the light is switched on the diode, whereas the forward current does not change significantly. This result suggests that the n-Si/SiO2 /MEH-PPV/Al diode can be used for the optical sensor applications.
Iph = BF ˛
(5)
where Iph is the photocurrent, B is a constant, ˛ is a exponent and F is the intensity of light. The ˛ value was determined from the slope of curve of Iph vs. F plotted and was found to be 1.08. The ˛ values for 0.5 and 1.0 correspond to bimolecular recombination and monomolecular recombination mechanism, respectively [20]. The obtained ˛ value for the diode suggests that the photoconductivity mechanism is controlled by monomolecular recombination. Fig. 5 shows the transient photoconductivity of the diode. After reaching a steady-state condition, the light is switched on and off and the rise and decay currents are recorded with time. The rise
Fig. 4. The current–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode under different illumination intensities with step of 20 mW/cm2 .
3.3. Effect of illumination on capacitance–voltage and interface state density properties of the Au/n-Si/SiO2 /MEH-PPV/Al diode The plots of the capacitance–frequency at 0.0 V under dark and illumination conditions are shown in Fig. 7. The capacitance of the diode increases with decreasing frequency, because the interface states can follow the alternating current (AC) signal. The capacitance of the diode increases with illumination intensity. This suggests that the illumination increases the time constant of interface charges following the AC signal. At higher frequencies,
Fig. 6. Phototransient current plots of forward and reverse characteristics of the n-Si/SiO2 /MEH-PPV/Al diode.
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Fig. 7. Plots of C–f of the n-Si/SiO2 /MEH-PPV/Al diode at V = 0 V under dark and illumination conditions. Fig. 9. Plots of G/ω–f of the n-Si/SiO2 /MEH-PPV/Al diode at V = 0 V under dark and illumination conditions.
the interface states cannot follow the alternating current signal due to non-dispersive capacitance and thus, the high frequency capacitance does not make an important contribution to the total capacitance. The interface state density gives important information about the quality of the metal–insulator layer–semiconductor junctions. In order to analyze interface state properties of n-Si/SiO2 /MEHPPV/Al diode, we have used admittance spectroscopy. For this, the equivalent circuit of metal–insulator–semiconductor capacitor is shown in Fig. 8a. Here, Cd is the depletion layer capacitance, Cit is the interface trap capacitance, Cox is the oxide capacitance and Rit is the resistance of the interfacial layer. The simplified equivalent circuit of Fig. 8 is shown in Fig. 8b [21]. In the circuit, Cp and Gp values are expressed as [21]: Cp = Cd +
Cit tan−1 (ω) 2ω
(6)
and Gp qωit Dit = ω 1 + (ωit )2
(7)
where Dit is density of the interface states, q is the charge of the electron, ω is the angular frequency, it is the time constant of the interface states. The admittance of the metal–oxide–semiconductor devices can be expressed as follows: Y = Gm + iωCm
where Cox is the capacitance measured in strong accumulation. The plots of (G/ω) vs. log f of the diode at 0.0 V under dark and illuminations are shown in Fig. 9. The plots show a peak due to the presence of interface states and the peak position shifts to lower frequencies with illumination. The interface state density of the diode can be obtained by the following relation [21–22]: Dit =
(G/ω)max 0.402qA
(10)
The Dit values with it of the diode under dark and illumination conditions were found to be 2.50 × 1010 eV−1 cm−2 with 8.27 × 10−5 s and 2.53 × 1010 eV−1 cm−2 with 5.09 × 10−5 s, respectively. The Dit value does not almost change with the illumination, whereas, the illumination decreases time constant of the interface states of the diode. Fig. 10 shows the capacitance–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode under dark and illumination conditions. Under dark condition, the capacitance increases and reaches a saturation when the voltage is scanned from −5 V to 5 V. Whereas, under illumination condition, illumination intensity increases capacitance and saturates to the value which is the different as in the accumulation region. The increase in capacitance of the diode with increasing light power intensity is due to the shrinking of the depletion region width under illumination.
(8)
where Gm and Cm are respectively measured conductance and capacitances and the parallel conductance Gp can be expressed by the following relation [21–22]: Gp ωGm Cox 2 = 2 ω Gm + ω2 (Cox − Cm )2
(9)
Fig. 8. Schematic diagram for interface states [21] (a) MIS capacitor with interface trap time constant, (b) simplified equivalent circuit and (c) the measured circuit.
Fig. 10. Plots of C–V of the n-Si/SiO2 /MEH-PPV/Al diode under dark and illumination conditions.
F. Yakuphanoglu / Synthetic Metals 160 (2010) 1551–1555
4. Conclusions The electronic properties of the n-Si/SiO2 /MEH-PPV/Al diode was fabricated were investigated. The n-Si/SiO2 /MEH-PPV/Al diode shows a photovoltaic device with the rectification ratio of 2.22 × 105 at ±5 V, open circuit voltage Voc = 0.456 V and short-circuit current density Jsc = 7.89 × 10−8 A/cm2 values under 100 mW/cm2 illumination intensity. The photoconductivity mechanism of the diode is controlled by monomolecular recombination. The interface state density Dit values with time constant it of the diode under dark and illumination conditions were found to be 2.53 × 1010 eV−1 cm−2 with 5.09 × 10−5 s and 2.50 × 1010 eV−1 cm−2 with 8.27 × 10−5 s, respectively. The illuminated current–voltage characteristics suggest that the nSi/SiO2 /MEH-PPV/Al diode is considered as a photosensitive diode. Acknowledgements This work was supported by Feyzi AKKAYA Scientific Activates Supporting Fund (FABED). Author wishes to thank FABED for financial support. References [1] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981.
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[2] E.H. Rhoderick, R.H. Williams, Metal–Semiconductor Contacts, second ed., Oxford University Press, Oxford, 1988. [3] J.H. Warner, U. Rau, in: J.F. Luy, et al. (Eds.), Springer Series in Electronics and Photonics, vol. 32, 1994. [4] M. Özer, D.E. Yıldız, S¸. Altındal, M.M. Bülbül, Solid-State Electron. 51 (2007) 941–949. [5] H.C. Card, E.H. Rhoderick, J. Phys. D 4 (1971) 1589. [6] H.A. C¸etinkara, et al., Eur. Phys. J. Appl. Phys. 6 (1999) 89. [7] C¸. Nuho˘glu, E. Ayyıldız, M. Sa˘glam, A. Türüt, Appl. Surf. Sci. 135 (1998) 350– 356. [8] R. Castagne, A. Vapaille, Surf. Sci. 28 (1) (1971) 157. [9] A. Tataro˘glu, S¸. Altındal, Microelectron. Eng. 85 (2008) 233–237. [10] H. Deuling, E. Klausmann, A. Goetzberger, Solid State Electron. 15 (5) (1972) 559. [11] U. Kelberlau, R. Kassing, Solid State Electron. 22 (1) (1979) 37. [12] M.O. Aboelfotoh, J. Appl. Phys. 69 (5) (1991) 3351. [13] A. Ltaief, A. Bouazizi, J. Davenas, R. Ben Chaabane, H. Ben Ouada, Synth. Met. 147 (2004) 261. [14] F. Yakuphanoglu, J. Phys. Chem. Solids 69 (2008) 949–954. [15] S. Okur, F. Yakuphanoglu, M. Ozsoz, P.K. Kadayifcilar, Microelectron. Eng. 86 (2009) 2305–2311. [16] F. Yakuphanoglu, S. Okur, Microelectron. Eng. 87 (2010) 30–34. [17] S.K. Cheung, N.W. Cheung, Appl. Phys. Lett. 49 (1986) 85. [18] R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Company, New York, 1996. [19] F. Yakuphanoglu, Sens. Actuator A: Phys. 141 (2008) 383. [20] R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. [21] E.H. Nicollian, J.R. Brews, MOS (Metal Oxide Semiconductor) Physics and Technology, Wiley, New York, 1982. [22] F. Yakuphanoglu, Sens. Actuators A 147 (2008) 104–109.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Controlling of silicon–insulator–metal junction by organic semiconductor polymer thin film Fahrettin Yakuphanoglu ∗ Firat University, Faculty of Science, Department of Physics, 23169 Elazig, Turkey
a r t i c l e
i n f o
Article history: Received 2 April 2010 Received in revised form 17 May 2010 Accepted 19 May 2010 Available online 15 June 2010 Keywords: Metal–oxide–semiconductor contact Light sensitive capacitor Organic semiconductor
a b s t r a c t Electrical and photovoltaic properties of a metal–semiconductor–insulator–polymer–metal diode were investigated. The n-Si/SiO2 /MEH-PPV/Al diode shows a rectifying behavior with the rectification ratio of 2.22 × 105 at ±5 V and exhibits a non-ideal behavior due to the series resistance and oxide-organic layers. The organic semiconductor makes a contribution to the I–V characteristics of the diode and the trap-charge limited space charge and space charge limited current mechanisms were observed for the diode. The current–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode under different illumination intensities give an open circuit voltage (Voc ) along with a short circuit current (Isc ). This suggests that the n-Si/SiO2 /MEH-PPV/Al diode is a photovoltaic device with Voc = 0.456 V and Jsc = 7.89 × 10−8 A/cm2 values under 100 mW/cm2 illumination intensity. The photoconductivity mechanism of the diode is controlled by monomolecular recombination. The interface state density Dit values with time constant it of the diode under dark and illumination conditions were found to be 2.53 × 1010 eV−1 cm−2 with 5.09 × 10−5 s and 2.50 × 1010 eV−1 cm−2 with 8.27 × 10−5 s, respectively. The obtained results indicate that the n-Si/SiO2 /MEH-PPV/Al diode is a photo-sensitive diode. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The metal–insulator–semiconductor (MIS) devices which play a crucial role in constructing some useful devices in electronic technology have been still investigated and have attracted much attention during recent years [1–3]. A MIS structure is the also most useful device in the study of semiconductor surfaces. Since the reliability and stability of all semiconductor devices are intimately related to their surface conditions, an understanding of the surface physics with the help of MIS diodes is of great importance to device operations [4–7]. The existence of organic layer can modify the electrical properties of MIS structure. Such a layer between metal and semiconductor with interface state, series resistance parameters causes the electrical characteristics of MIS Schottky diodes to be non-ideal [8–12]. Poly[2-methoxy-5-(2-ethyl)hexoxy-1,4-phenylenevinylene] (MEH-PPV) has been considered as one of the most potential conducting polymers for various optoelectronic applications such as organic light emitting diodes (OLED), sensors and organic solar cells because of its good environmental stability, easy conductivity control and cheap production in large quantities [13].
∗ Tel.: +90 424 237 0000 3621; fax: +90 424 233 0062. E-mail addresses: fyhanoglu@firat.edu.tr, [email protected]. 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.05.024
Apart from our work, and to best of our knowledge, no other information on silicon/silicon oxide/MEH-PPV organic semiconductor has been reported in any study in the literature. The aim of this study is to fabricate a photo-sensitive diode based on metal–semiconductor–insulator–polymer–metal junction for photovoltaic applications and to investigate whether or not this device shows a photovoltaic behavior.
2. Experimental For the diode fabrication, n-type single crystal silicon 1 1 1 with a thickness of 600 m and poly[2-methoxy-5-(2-ethyl)hexoxy-1,4phenylenevinylene] were used. In order to remove the native oxide on surface on n-Si, the substrate was etched by HF and then was rinsed in deionised water using an ultrasonic bath for 10–15 min and finally was chemically cleaned according to method based on successive baths of methanol and acetone. For formation of ohmic contact, high purity (99.999%) Au with a thickness of 100 nm was thermally evaporated onto the whole backside of the substrate at pressure of 4.5 × 10−5 Torr in vacuum pump system. Then it was rinsed in deionised water and finally was again chemically cleaned. The oxide layer to grown on Si surface without metal, Si wafer was replaced into a furnace and oxide layer was thermally formed on Si surface at 1000 ◦ C for 30 min. The oxide layer thickness was found to be 15 nm. Film of the poly[2-methoxy-5-(2ethyl)hexoxy-1,4-phenylenevinylene] (MEH-PPV) was prepared
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Fig. 2. Plots of dV/dln(I) vs. I and H(I) vs. I of the n-Si/SiO2 /MEH-PPV/Al diode.
linear behavior, whereas at higher voltages, the I–V characteristics deviate from the linear behavior due to the series resistance and interface state density. In such as case, in order to determine the diode parameters, Cheung’s method was used to determine diode parameters and Cheung’s functions are defined as [17–18]: dV kT =n + IRs q dln(I)
Fig. 1. Current–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode.
by evaporating the solvent from a solution of the sample with subsequent drying of the film deposited on SiO2 /Si/Au structure [14]. After depositing by drop coating, the film was dried at 50 ◦ C for 10 min on a hot plate to evaporate the solvent. After film deposition process, the substrate was immediately placed in vacuum system for the processes. Al metal contacts were formed on MEH-PPV organic layer by vacuum thermal evaporation of Al at pressure of approximately 3.0 × 10−5 Torr using a VAKSIS thermal evaporator system. The current–voltage characteristics of the diode were preformed using a KEITHLEY 2400 sourcemeter. The capacitance–conductance–voltage and capacitance–conductance–frequency characteristics were measured using a HIOKI 3532 LCR meter. The photoelectrical measurements were performed using a white light source of 200 W. The intensity of illumination was measured using a solar power meter. 3. Results and discussion 3.1. Dark current–voltage characteristics of Au/n-Si/SiO2 /MEH-PPV/Al diode Fig. 1 shows the current–voltage characteristics of the nSi/SiO2 /MEH-PPV/Al diode. The diode shows a rectifying behavior and the rectification ratio which is the ratio of forward current to reverse current at certain voltage was found to be 2.22 × 105 at ±5 V. The current–voltage characteristics of the diode can be expressed by the following relation [15–16]: I = Io exp
q(V − IR ) s
nkT
q(V − IR ) s
1 − exp −
kT
(1)
where Rs is the series resistance, V is the applied voltage, n is the ideality factor, k is the Boltzmann constant, T is the temperature and Io is the reverse saturation current given by
q B
Io = AA∗ T 2 exp −
kT
(2)
where B is the barrier height and A is the contact area, A* is the Richardson constant (112 A cm−2 K−2 for n-type Si) [15]. The diode indicates a non-ideal behavior due to the series resistance and oxide-organics layers. Under low voltages, the diode exhibits a
and H(I) = V − n
kT ln q
(3)
I o AA∗ T 2
= IRs + nB
(4)
The plots of dV/dln(I) vs. I and H(I) vs. I are shown in Fig. 2 and give a straight line in series resistance region. The Rs and n values were calculated from the slope and intercept of dV/dln(I) vs. I plot and were found to be 5.76 × 105 and 1.70, respectively. The Rs and B values were calculated from the slope and intercept of H(I) vs. I plot and were found to be 5.77 × 105 and 0.74 eV, respectively. The obtained series resistance is higher due the oxide layer and the resistance of the MEH-PPV organic semiconductor which has the low mobility and electrical conductivity with respect to an inorganic semiconductor. In order to analyze the contribution of higher voltages to I–V characteristics of the diode, I–V characteristic of the diode was plotted in log scale and was shown in Fig. 3. I–V curve of the diode were analyzed via I = kVm relation. Here I is the current, V is the voltage and m is a constant. m values were determined from slopes of regions I and II of Fig. 3 and were found to vary from 10.82 to 2.58. This curve shows that in the first region, trap-charge limited space charge mechanism (TCLC) is dominant, whereas in the second region, the current is controlled by space charge limited current mechanism (SCLC) [18]. It is evaluated that the organic semiconductor makes a contribution to the I–V characteristics of the diode. 3.2. Illuminated current–voltage characteristics of Au/n-Si/SiO2 /MEH-PPV/Al diode The current–voltage characteristics of the n-Si/SiO2 /MEHPPV/Al diode under various illumination intensities with step of 20 mW/cm2 are shown in Fig. 4 As seen in Fig. 4, the I–V curves give an open circuit voltage (Voc ) along with a short photocurrent (Isc ). The illumination increases strongly the reverse current due to the generation of carrier charges. The open circuit voltage Voc and short-circuit current density Jsc values of the device under 100 mW/cm2 illumination were found to be 0.456 V and 7.89 × 10−8 A/cm2 , respectively. This suggests that the n-Si/SiO2 /MEH-PPV/Al diode can be operated as a photodiode. The
F. Yakuphanoglu / Synthetic Metals 160 (2010) 1551–1555
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Fig. 3. Plot of log I–log V of the n-Si/SiO2 /MEH-PPV/Al diode.
Fig. 5. Phototransient current plot of the n-Si/SiO2 /MEH-PPV/Al diode.
values of Voc and Jsc for the photodiode is considerable lower and these values satisfy a photodiode, because, the photodiodes are routinely designed to achieve a spectral response or a rapid time response [18]. In order to analyze photoconductivity mechanism of the diode, the variation of the photocurrent with illumination intensity was plotted and the photocurrent increases with increasing illumination intensity. This indicates that the light illumination increases production of electron–hole pairs. The photocurrent dependence of light intensity is expressed as [19]:
and decay times for the diode indicate that the photoconductivity rise and decay of the diode increase and decrease rapidly, when the light is turned on and off, respectively. This confirms that the number of the generated free charge carriers increases with time. Fig. 6 shows the transient photocurrent plot for the reverse and forward characteristics of the diode. The reverse current of the diode increases strongly, when the light is switched on the diode, whereas the forward current does not change significantly. This result suggests that the n-Si/SiO2 /MEH-PPV/Al diode can be used for the optical sensor applications.
Iph = BF ˛
(5)
where Iph is the photocurrent, B is a constant, ˛ is a exponent and F is the intensity of light. The ˛ value was determined from the slope of curve of Iph vs. F plotted and was found to be 1.08. The ˛ values for 0.5 and 1.0 correspond to bimolecular recombination and monomolecular recombination mechanism, respectively [20]. The obtained ˛ value for the diode suggests that the photoconductivity mechanism is controlled by monomolecular recombination. Fig. 5 shows the transient photoconductivity of the diode. After reaching a steady-state condition, the light is switched on and off and the rise and decay currents are recorded with time. The rise
Fig. 4. The current–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode under different illumination intensities with step of 20 mW/cm2 .
3.3. Effect of illumination on capacitance–voltage and interface state density properties of the Au/n-Si/SiO2 /MEH-PPV/Al diode The plots of the capacitance–frequency at 0.0 V under dark and illumination conditions are shown in Fig. 7. The capacitance of the diode increases with decreasing frequency, because the interface states can follow the alternating current (AC) signal. The capacitance of the diode increases with illumination intensity. This suggests that the illumination increases the time constant of interface charges following the AC signal. At higher frequencies,
Fig. 6. Phototransient current plots of forward and reverse characteristics of the n-Si/SiO2 /MEH-PPV/Al diode.
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Fig. 7. Plots of C–f of the n-Si/SiO2 /MEH-PPV/Al diode at V = 0 V under dark and illumination conditions. Fig. 9. Plots of G/ω–f of the n-Si/SiO2 /MEH-PPV/Al diode at V = 0 V under dark and illumination conditions.
the interface states cannot follow the alternating current signal due to non-dispersive capacitance and thus, the high frequency capacitance does not make an important contribution to the total capacitance. The interface state density gives important information about the quality of the metal–insulator layer–semiconductor junctions. In order to analyze interface state properties of n-Si/SiO2 /MEHPPV/Al diode, we have used admittance spectroscopy. For this, the equivalent circuit of metal–insulator–semiconductor capacitor is shown in Fig. 8a. Here, Cd is the depletion layer capacitance, Cit is the interface trap capacitance, Cox is the oxide capacitance and Rit is the resistance of the interfacial layer. The simplified equivalent circuit of Fig. 8 is shown in Fig. 8b [21]. In the circuit, Cp and Gp values are expressed as [21]: Cp = Cd +
Cit tan−1 (ω) 2ω
(6)
and Gp qωit Dit = ω 1 + (ωit )2
(7)
where Dit is density of the interface states, q is the charge of the electron, ω is the angular frequency, it is the time constant of the interface states. The admittance of the metal–oxide–semiconductor devices can be expressed as follows: Y = Gm + iωCm
where Cox is the capacitance measured in strong accumulation. The plots of (G/ω) vs. log f of the diode at 0.0 V under dark and illuminations are shown in Fig. 9. The plots show a peak due to the presence of interface states and the peak position shifts to lower frequencies with illumination. The interface state density of the diode can be obtained by the following relation [21–22]: Dit =
(G/ω)max 0.402qA
(10)
The Dit values with it of the diode under dark and illumination conditions were found to be 2.50 × 1010 eV−1 cm−2 with 8.27 × 10−5 s and 2.53 × 1010 eV−1 cm−2 with 5.09 × 10−5 s, respectively. The Dit value does not almost change with the illumination, whereas, the illumination decreases time constant of the interface states of the diode. Fig. 10 shows the capacitance–voltage characteristics of the n-Si/SiO2 /MEH-PPV/Al diode under dark and illumination conditions. Under dark condition, the capacitance increases and reaches a saturation when the voltage is scanned from −5 V to 5 V. Whereas, under illumination condition, illumination intensity increases capacitance and saturates to the value which is the different as in the accumulation region. The increase in capacitance of the diode with increasing light power intensity is due to the shrinking of the depletion region width under illumination.
(8)
where Gm and Cm are respectively measured conductance and capacitances and the parallel conductance Gp can be expressed by the following relation [21–22]: Gp ωGm Cox 2 = 2 ω Gm + ω2 (Cox − Cm )2
(9)
Fig. 8. Schematic diagram for interface states [21] (a) MIS capacitor with interface trap time constant, (b) simplified equivalent circuit and (c) the measured circuit.
Fig. 10. Plots of C–V of the n-Si/SiO2 /MEH-PPV/Al diode under dark and illumination conditions.
F. Yakuphanoglu / Synthetic Metals 160 (2010) 1551–1555
4. Conclusions The electronic properties of the n-Si/SiO2 /MEH-PPV/Al diode was fabricated were investigated. The n-Si/SiO2 /MEH-PPV/Al diode shows a photovoltaic device with the rectification ratio of 2.22 × 105 at ±5 V, open circuit voltage Voc = 0.456 V and short-circuit current density Jsc = 7.89 × 10−8 A/cm2 values under 100 mW/cm2 illumination intensity. The photoconductivity mechanism of the diode is controlled by monomolecular recombination. The interface state density Dit values with time constant it of the diode under dark and illumination conditions were found to be 2.53 × 1010 eV−1 cm−2 with 5.09 × 10−5 s and 2.50 × 1010 eV−1 cm−2 with 8.27 × 10−5 s, respectively. The illuminated current–voltage characteristics suggest that the nSi/SiO2 /MEH-PPV/Al diode is considered as a photosensitive diode. Acknowledgements This work was supported by Feyzi AKKAYA Scientific Activates Supporting Fund (FABED). Author wishes to thank FABED for financial support. References [1] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981.
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