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TEMPERATURE DEPENDENT ELECTRICAL RESPONSE OF ORANGE-DYE COMPLEX BASED

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TEMPERATURE DEPENDENT ELECTRICAL RESPONSE OF ORANGE-DYE COMPLEX BASED Powered By Docstoc
					   INTERNATIONAL JOURNAL OF ELECTRONICS AND
   International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
   0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 2, March – April, 2013, pp. 269-279
                                                                            IJECET
© IAEME: www.iaeme.com/ijecet.asp
Journal Impact Factor (2013): 5.8896 (Calculated by GISI)                 ©IAEME
www.jifactor.com




           TEMPERATURE DEPENDENT ELECTRICAL RESPONSE OF
             ORANGE-DYE COMPLEX BASED SCHOTTKY DIODE

                             Syed Abdul Moiz 1, Ahmed M. Nahhas2
       1
         (Department of Electrical Engineering, Faculty of Engineering and Islamic Architecture,
                         Umm Al Qura University, Makkah, Saudi Arabia)
       2
         (Department of Electrical Engineering, Faculty of Engineering and Islamic Architecture,
                          Umm Al Qura University, Makkah, Saudi Arabia)



   ABSTRACT

          In order to investigate the temperature dependent electrical response of Orange-Dye
   complex, Schottky diodes were fabricated from solution with spin coating method. From their
   current-voltage response it is observed that Schottky diode follows space charge limited
   current model. Therefore, by applying space charge limited current model different charge
   transport parameters such as trap factor, mobility, and threshold voltage and trap density are
   determined and their response as a function of temperature are investigated and discussed. It
   is observed that all charge transport parameters improves at elevated temperature within
   given temperature range. This study will help us to understand the nature of Orange Dye
   Complexes for their future applications.

   Keywords: Orange Dye, Organic Semiconductor, Charge Injection, Schottky diode & SCLC
   model.

   I. INTRODUCTION

           Organic semiconducting based electronic devices have already received considerable
   attention by different groups of researchers and technologists due to many advantages such as
   light weight, flexible, require simple fabrication technology, low cost, deposited on various
   substrate and many other advantages [1-4]. Despite their pronounced improvement and
   currently at the early stage of commercialisation, some of the fundamental features of charge
   transport process are still not clear and required comprehensive understanding [5-8].



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Charge transport process inside the organic semiconductor plays a vital role to define the
efficiency of electronic devices. Under the influence of applied potential, electrons and
holes are injected from metal to organic semiconductor layer and are hoped from one
location to the other location inside the organic semiconductor as charge transport
process. Both charge injection and hopping transport process are very complex in nature
and depends on many other factors, but some of such factors can be characterized as a
function of temperature [9, 10].
      Broadly speaking, the electrical response of organic semiconductor can be classified
either as injection limited or bulk limited depends on the limitation imposed by either the
barrier at metal-organic semiconductor interface or by the bulk semiconductor itself for
hopping process. Generally, the mobility of organic semiconductor is very low as
compared to other inorganic semiconductor; therefore most of the cases the charge
transport limitation is imposed by the bulk nature of semiconductor itself to defined their
electrical response [11]. In bulk limited charge transport process, injected charges
occupies organic space between electrodes for longer period of time and make space
charge region, such phenomena can be modelled by space charge limited current (SCLC)
to define their electrical response [12].
      Orange-Dye (C17H17N5O2) with Vinyl-Ehtynyl-Trimehyl-Piperiodole(VETP,
C12H19NO) as complex is emerged as novel organic semiconductor and offers many
unique properties which are highly suitable for sensors especially for humidity sensor and
photo-sensors [13-16]. Despite their importance very limited amount of information about
this complex is available in literature. Therefore in this study we investigated the
electrical response of OD-VETP complex as a function of temperature and different
charge transport parameters were evaluated and their behaviour as a function of
temperature is discussed.

II. EXPERIMENTAL

        All chemical were purchases from local market and were used as it is without any
further purification. The molecular structure of both OD and VETP are shown in Figure
1. OD has molecular weight 323 gm/mole with density 0.9 gm/cm3, while VETP has
molecular weight 0.6 gm/mole and density 0.6 gm/cm3 [14-16]. Both organic materials
are solution in water and make charge transfer complex at room temperature. In order to
make the complex 5% by weight of VETP is mixed in aqueous OD solution and was
stirred in an ultrasound container for more than 1 hour and kept them in an inert nitrogen
environment for more than 24 hours to settle downs. Meanwhile SnO2 coated glass
substrate were cleaned and OD-VETP complex were deposited by spin coating method at
1000 rpm for 30 second. From simple optical examination it was clearly observed that
grown thin film showed homogenous surface and their thickness was estimated
approximately 600 nm. For external electrical characterization, silver metal was deposited
over OD-VETP surface in spherical shape with diameter ~6 mm as electrode and then
devices were annealed at 100 oC in inert environment for more than an hour. The cross-
sectional diagram of the Schottky diode is shown in Figure 2.




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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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   Figure 1. Molecular structure of (a) Orange nitrogen Dye (OD) and (b) VETP complex

       Current-voltage responses of Schottky diode were measured with help of dc
measurement station with temperature adjusting facilities, where four-probe method were
used for electrical characterization, but for simplicity only two probes are shown in Figure 2.
For each 5 oC increments, the electrical properties of diode were measured, where
temperature measurement were carried out in the range of 25 oC to 80 oC with and
experimental temperature error of ±0.5 oC. By using hot-probe method, it was observed that
OD-VETP complex is a p-type semiconductor just like OD semiconductor.



               Silver Electrode
                                                    SnO2 Electrode

                                      OD-VETP
                                      Complex



                       Glass Substrate
         Figure 2. A schematic cross-sectional view of SnO2 / OD-VETP/ Ag diode




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III. RESULTS & DISCUSSION

        The current-voltage characteristics of SnO2/OD-VETP/Ag in forward bias is shown in
Figure 3 as a function of temperature from 25, 40, 60 and 80 oC respectively. From the
figure, it is clearly observed that the Schottky diode follows nonlinear typical diode
behaviour in the forward bias. The current passing through the device is sharply rises as a
function of temperature and at 80 oC the maximum value of current 5.5 µA is observed at 10
volts, which is nearly 20 times higher than the current passing through the Schottky diode at
25oC as same voltage. Generally, VETP is highly resistive semiconductor and therefore OD-
VETP complex shows high resistance as compared to OD itself, but still complex material is
very material for many sensor applications [13-16].
        If we define V as applied voltage then current-density (J) can be defined by SCLC
model as [17,18]

   9        V2
J = εε oθµ p 3 ,                                                                            (1)
   8        d

                      6
       Current (μA)




                      5
                                                                       80oC
                      4
                      3
                                                                       60oC
                      2
                      1                                                40oC
                                                                        25oC
                      0
                          0   2.5           5           7.5          10
                               Voltage (Volts)
 Figure 3. Current-voltage characteristics for SnO2/OD-VETP/Ag Schottky diode at 25, 40,
                                 60 and 80 oC respectively

Where εr is the relative dielectric constant for OD-VETP complex and can be approximate as
3, just like as other organic semiconductor. Similarly εo is standard dielectric constant and
equal to the 8.65x10-14 F/cm, θ is refer as trap factor, µp is the mobility of hole and d is the
thickness of OD-VETP thin film. It is unanimously accepted that the mobility of free carriers
inside organic semiconductor, which is direct function of applied electric field, can be
described by Poole-Frenkel equation as [19]


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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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µ p = µ o exp β E ,         (         )                                                   (2)

where µo and β can be defined as zero field mobility and Poole-Frenkel factor respectively.
Both above discuss models give us valuable information about charge transport mechanism
for organic semiconductor. If we incorporate mobility from equation 2 into the equation, then
equation 1 can be written after some manipulation as

                                                      V2
                                     9
                                     8
                                                        (
                                  J = εε oθµ o exp β E 3 ,
                                                      d
                                                              )                           (3)


By simple manipulation the equation (3) can be written as [20]


                                      J      9        
                                  ln  2     =  εε oθµ  + β E ,                        (4)
                                     E       8d       

        Where E is applied electric field (V/d). In order to justify the SCLC model, the plots
of ln (J/E2) vs. square root of E are drawn in Figure 4, as a function of temperature. It is
observed that independent of given temperature range the Schottky diode follows SCLC
model to define their current-voltage response. As OD-VETP complex is a p-type material,
therefore we can assume that SnO2 provide ohmic contact to OD-VETP complex, in other
way holes are injected from SnO2 into OD-VETP and forms space charge region inside
complex. From the both Figure 3 and 4 it is also clear that conductivity inside complex
sharply rises with increment of temperature.

                           -10
        ln (J/E2) (A/E2)




                           -14
                                                                          40oC
                                          80oC
                           -18
                                          60oC
                           -22

                                           25oC
                           -26
                                 50               175             300   425
                                          [Electric Field (V/cm)]1/2
Figure 4. In (J/E2) vs. E1/2 response for SnO2/OD-VETP/Ag Schottky diode as a function of
                        temperature 25, 40, 60, and 80 oC respectively


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        The SCLC can be classified into three regions, depending on voltage. These regions
are termed as (1) ohmic region, (2) trapped charge region, and (3) space charge region. It can
observed from SCLC response of OD-VETP complex that initially the current increases very
slowly and then rises sharply with increment of applied electric field at all temperature [21].
The initial region of SCLC is generally considered as high resistive ohmic region and can be
model as [6,20,21]
                                            V
                          J ohm = e N o µ     ;
                                            L                                           (5)

        Where e is the charge (1.6x 10-19 C) of hole carriers, No is the free hole density. With
the increment of voltage, a transition is observed from ohmic region to the trapped space
charge region and this transition is generally defined at some threshold voltage (VT), also
called trapped filled voltage. Figure 5 shows the response of threshold voltage as a function
of temperature for Schottky diode.




 Figure 5. Threshold voltage behaviour of SnO2/OD-VETP/Ag Schottky diode as a function
                                     of temperature

       It is observed that threshold voltage is linearly decreases as a function of temperature
with given temperature range, which indicates that the transition of ohmic region to trapped
space charge region for OD-VETP diode is also linearly decreasing with respect to
temperature. Threshold voltage for SCLC model can be further defines as [17]




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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME


                                                     eN t d 2
                                            VTFL   =          .                           (6)
                                                      2ε s

        Where Nt is defined as trap density (cm-3). Density of these traps is determined for
OD-VETP Schottky diode with the help of above equation as a function of temperature. The
trap density for SnO2/OD-VETP/Ag Schottky diode as a function of temperature is shown in
Figure 6. Traps are nothing, just localized states inside organic semiconductor having
capability to hold carrier for some period of time. These traps are generated due a large
number of reasons but can be classified as chemical or structural traps. Grains boundary,
bond defects, chains ends etc. are termed as structural defects, while traces of chemical
reactants, and incorporation of impurities materials and other environment elements are
termed as chemical traps [22]. These traps can never be eliminated for organic semiconductor
but can be minimized by careful processing during thin film growth and device fabrication
process. However, these traps are direct function of energy (or temperature), every trap state
is associated with some energy, it can capture only those carriers who have lower energy then
trap associated energy. Therefore when temperature increases the average kinetic energy of
holes are also increases and available trap density is exponentially decreased for these
energetic carriers as shown in Figure 6.

                                   7
         Trap Density (1020 m-3)




                                   6

                                   5

                                   4

                                   3

                                   2
                                       20      40                 60          80
                                             Temperature               (oC)
 Figure 6. Trap density for SnO2/OD-VETP/Ag Schottky diode as a function of temperature


        Mobility of holes is another important parameter, which play a very vital role to
define the electric response of OD-VETP complex and can easily be determine from SCLC
equation. The actual mobility deviates from ideally mobility by trap factor, and such trap
factor (θ) can be defined as [17]


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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME

                                          p0 
                                    θ =         ,                                       (7)
                                        p0 + pt 

        where po (cm-3) and pt (cm-3) are free and trapped carrier density. These carrier
density and hence trap factor is determine for OD-VETP Schottky diode and are shown in
Figure 7. Trap factor is also increases as a function of temperature, and will help us to
estimate the actual mobility of OD-VETP complex, which is shown in Figure 8. Like trap
factor, mobility is also exponential function of temperature and are sharply rises at higher
temperature. The behaviour of mobility as a function of temperature inside the complex is the
collective response of all above space charge parameters as discussed above. At low
temperature injected holes face high trap density and only small holes are hopped and
succeeded to reach another electrode, however at higher temperature a large no of trapped
carriers become part of free carriers and hopped to reach opposite electrode to give rise
higher mobility and hence current. Similarly, when applied voltage is further increasing,
holes receives an extra increasing force to overcome these traps barriers and hence a large no
of holes are capable to reach at opposite electrode to give rise higher value of current.
Therefore, at higher voltage and temperature higher current is observed for OD-VETP
Schottky diode.


                       0.625
     Trap Factor (θ)




                         0.5


                       0.375


                        0.25
                               20          40          60           80
                                      Temperature           (oC)
         Figure 7. Trap factor as a function of temperature for OD-VETP Schottky diode




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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME



                                    200
        Mobility (108 cm2/ V sec)
                                    160

                                    120

                                     80

                                     40

                                      0
                                          20   40         60       80
                                               Temperature (0C)
   Figure 8. Mobility of holes as a function of temperature for OD-VETP Schottky diode


IV. CONCLUSIONS

        In this study, we have investigated the electrical response of novel OD-VETP
complex based Schottky diode as a function of temperature from the range of 25 oC to 80 oC.
From their current-voltage properties, it was observed that Schottky diode follows space
charge limited current. Therefore, by applying space charge limited current model, different
charge transport parameters such as threshold voltage, trap density, trap factor and hole
mobility were estimated as a function of temperature. It was observed that trap density
exponentially decreases, while threshold voltage also decreases linearly as a function of
temperature. On the other hand, both trap factor and hole mobility are exponentially
improved at elevated temperature. At high temperature more and more holes are injected and
hopped with higher mobility and faced lower trap density to reach opposite electrode and
give rise higher value for current. This study will help to understand the nature of the OD-
VETP and will facilitate to efficiently utilize them for different types of organic
semiconductor based electronic devices.

ACKNOWLEDGEMENTS

       Authors are thankful to Professor Khasan S Karimov for their comments and valuable
suggestion to improve this study.




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