GaAs organic heterojunction for solar energy application

Journal Pre-proof Ac Conductivity and Impedance Spectroscopy Study and Dielectric Response of MgPc/GaAs Organic Heterojunction for Solar Energy Application

M. Benhaliliba, T. Asar, I. Missoum, Y.S. Ocak, S. Özçelik, C.E. Benouis, A. Arrar PII:

S0921-4526(19)30680-5

DOI:

https://doi.org/10.1016/j.physb.2019.411782

Reference:

PHYSB 411782

To appear in:

Physica B: Physics of Condensed Matter

Received Date:

13 August 2019

Accepted Date:

10 October 2019

Please cite this article as: M. Benhaliliba, T. Asar, I. Missoum, Y.S. Ocak, S. Özçelik, C.E. Benouis, A. Arrar, Ac Conductivity and Impedance Spectroscopy Study and Dielectric Response of MgPc /GaAs Organic Heterojunction for Solar Energy Application, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411782

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Journal Pre-proof Research Highlights

 Ag/MgPc/GaAs/Au-Ge Organic Heterojunction for Solar Energy is made by low cost spin coating technique  An Impedance Spectroscopy Study and Dielectric Performance have studied  Ac current conductivity σAc against frequency (lnω) of MgPc/GaAs OHJ is well detailed.  Conduction mechanisms control of the transport charge inside the MgPc/GaAs OHJ are evidenced  Cole-Cole plots give many explanations of such OHJ dielectric behavior.

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Ac Conductivity and Impedance Spectroscopy Study and Dielectric Response of MgPc/GaAs Organic Heterojunction for Solar Energy Application * 1,2M.

Benhaliliba, 3,4T. Asar, 1,5I. Missoum, 6Y.S.Ocak, 3,4S. Özçelik 1,2C.E. Benouis and 1,2A. Arrar

1Film

Device Fabrication-Characterization and Application FDFCA Research Group USTOMB, 31130, Oran, Algeria. 2Physics Faculty, USTOMB University POBOX 1505 31130, Mnaouer Oran Algeria. 3Physics Department, Faculty of Sciences, Gazi University, 06500, Ankara, Turkey. 4Photonics Application and Research Center, Gazi University, 06500, Ankara, Turkey 5Department of Physics , Saad Dahleb University, Blida Algeria. 6Dicle UniversityDicle University, Education Faculty, Science Department, 21280 Diyarbakir, Turkey Diyarbakir Turkey *Corresponding

Author

* [email protected]

Abstract The study concerns the C/ω-V performances and examination of dielectric response of MgPc/GaAs organic heterojunction (OHJ) structure of solar energy applications. Throughout this work, many characterizations have been achieved and various parameters have been extracted. C/ω-V and G/ω-V plots are subject to give much knowledge about the mechanism and behavior inside the OHJ. It is revealed that capacitance of OHJ increases with voltage defining deep depletion (dd), depletion (dep) and accumulation (ac) regions. Besides, Ac conductivity at room temperature increases with frequency in particular within the forward biasing voltage, reaching a high point of 28x10-7 S/cm at 5V. Real and imaginary terms of complex dielectric constant or well known by permittivity, ε versus lnω are studied inside the 1.5V-2.5V range. The real and imaginary parts of the impedance are found to be frequency dependence within the 1.5V and 2.5V bias range. Ac current conductivity σAc against frequency (lnω) of MgPc/GaAs OHJ inside 1.5V-2.5V bias voltage range is well detailled; the average of conductivity about 8x10-7 S/cm is then recorded. The impedance spectroscopy study is evidenced by the complex impedance M where real and imaginary part are M’ and M’’. Profile of M’ and M’’ as function of voltage exhibits peaks for 3kHz-300kHz frequency range. It is indicated that M’’ and M’ are roughly comprised between 0.08 and 1.12 and 0 and 2 respectively.Conduction mechanism is then determined by lnσ-lnω plots. The Cole-Cole diagram displays different curves of impedance as distinct semicircle for measured frequencies. Keywords: Organic heterojunction (OHJ); MgPc/GaAs; C/ω-V and G/ω-V characteristics; Dielectric constant; Cole-cole sketch.

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Journal Pre-proof 1. Introduction A large amount of papers is devoted to organic materials, allowing a better understanding of their characteristics as well as the physical methods, which take place in materials and devices. Organic electronic device topic becomes more and more a field of multidisciplinary applications composed by physics, chemistry, electronics, biologists and material science. Organic as metalphthalocyanine (MPc), Poly(3-hexylthiophene)

(P3Ht),

Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV) and pentacene have been used in thin films and electronic devices for optoelectronic and solar cell applications [1-4]. Many substrates are used in such heterojunction as n-type Si, p-type Si, InP and GaAs [5-8] and many fabrication routes are employed like spin coating, thermal evaporation, dip coating and matrix assisted pulsed laser evaporation (MAPLE) [9-12]. Phthalocyanine molecules own many interesting characteristics such optical response, electronic transport, catalytic properties which are required to employ them in scientific and technological applications. Furthermore, magnesium phthalocyanine (MgPc) molecule is considered as an encouraging material since it exhibits a high photoconductivity and electrical conductivity compared to other organic material.

Organic

material semiconductr MgPc presents a photoabsorption coefficient of 2 x 105 cm-1 and an energy gap from 1 eV to 2.5 eV as mentioned in literature [13, 14]. Tang et al. have used organic materials in fabrication of solar cells [15] and Burroughes have made light emitted diodes from organic molecules [16] while Fuchigami et al. have successfully prepared polythienylenevinylene thin-film transistor [17]. Many methods to characterize organic semiconductors, like capacitance measurement including C-V characteristics [18], C-V curve as a function of frequency [19], and deep level transient spectroscopy are mentioned in literature [20].

Further, many papers reported on organic heterostructures like

Au/(CuSe-polyvinyl alcohol)/n-Si (MPS) Schottky barrier structures which are fabricated by ultrasonic assisted method [21]. Another device based on Au/HgS-PVA/n-Si (MPS) structure is manufactured where HgS nanoparticles are obtained using a facile ultrasound-assisted route by polyvinyl alcohol PVA as capping agent as reported previously by Sevgili et al. [22]. The optical band gap is of 2.4 eV and a blue shift of 0.3 eV due to the quantum confinement of charge carriers in a small nanostructure is observed as earlier mentioned [22]. Authors have reported a detailed and more useful research on dielectric behavior of Au/HgS-PVA/n-Si (MPS) structure within the 1kHz-5MHz frequency range by using admittance (C-V and G/ω-V) measurements [22].

The Au/n-Si structures with and without

(CoSO4–PVP) organic interlayer are fabricated on n-Si wafer and electrical characteristics of them are analyzed by using I-V, C-V, and G-V characteristics in forward and reverse bias voltages and experimental results are compared with each other as mentioned by Altindal et al. [23]. A mixed 3

Journal Pre-proof copper/PVA nanocomposites as an interface layer for fabrication of Al/Cu-PVA/p-Si Schottky structures is investigated by Akhlagi et al. [24]. This last researcher exhibits the effect of (Cu-PVA) interlayer, utilized for Schottky barrier diodes (SBD) structure as an interfacial layer, on the electrical parameters and conduction mechanism, both the Al/p-Si (MS) and Al/(Cu-PVA)/p-Si (MPS) structures are fabricated on the same Si wafer using the reverse and forward bias of the I-V and C/G-V measurements at room temperature. Throughout their paper, the electric and dielectric parameters of the fabricated SBD structure have been entirely characterized [24]. Through this study, we fabricate the electronic device based on organic magnesium phthalocyanine MgPc onto inorganic GaAs substrate by the spin coating process. Our device is classified as organic heterojunction having a structure of Ag/MgPc/GaAs/AuGe which finds application in solar energy application. Ac conductivity and dielectric measurements have been described for a wide variety of organic semiconductors in order to understand the mechanisms of conduction processes in these materials and the type of dielectric behavior. To know better the potential barrier at the MgPc/GaAs contact, room temperature C-V measurements in the dark are explored. The effect of MgPc/GaAs interface is explained in order to understand the photoelectric conversion properties. The objective of this research is to examine the capacitance and conductance versus voltage characteristics at different frequencies and dielectric response and analyze Cole-Cole plots obtained from electrical impedance spectroscopy measurements in OHJ device based on MgPc material. Consequently, keeping in view the high demand and importance of organic polymer material, we report in this work the dielectric and impedance spectroscopy at room temperature. The dielectric parameters such as real and imaginary parts of dielectric permittivity ε’, ε″, angle loss tanδ and Ac conductivity σ are as well as C-V and G-V characteristics with frequency studied and discussed. Up to our best knowledge, rare are papers which report on C/ω-V, G/ω-V and dielectric constants properties of organic material like MgPc made as heterojunction onto GaAs substrate. 2.

Fabrication of MgPc/GaAs OHJ The Ag/MgPc/GaAs/AuGe OHJ is fabricated using (100) n-type GaAs substrate by spin-coating

process using a spincoat G3P-8 at 2000 rpm for 1 min and dried at 115 °C for 3 min. Ag contact is thermally evaporated under 10-6 Torr. MgPc products are provided from Sigma-Aldrich. n-GaAs/AuGe is sold from the commerce and Ag is evaporated by NVBJ-300 NANOVAK® thermal evaporation system under low pressure. n-GaAs is used as substrate with (111) crystal orientation, having 1.7x10-3–2.37x10-3 ohm.cm of resistivity and 350 µm of thickness. Initially, the substrates are rinsed in 4

Journal Pre-proof 5H2SO4 + H2O2 + H2O solution for 60 s to eliminate surface damage layer and organic contamination and then in H2O + HCl solution. After, the substrates are then washed in dionized water and finally dried by nitrogen (N2). An Au–Ge (88:12) pellet (with 1/800 diameter 1/800 length) supplied by from Kurt Lesker is evaporated on n-GaAs at 3x10-6 Torr and annealed at 450 °C in N2 atmosphere for 3 min to make good ohmic contact. Once the clean process is achieved, 0.02 g of MgPc is dissolved in 25 ml of chloroform; the obtained blue solution is poured on suitable substrate and the film is produced by spin-coating route via a spin coating system (spin coat G3P-8) at 2000 rpm for 1 min and dried at 115 °C for 3 min. The devices are characterized by HP 4192 A LF impedance analyzer as described before [25]. 3.

Results and discussion

3.1. Capacitance and conductance characteristics Figs. 1 and 2 show the experimental C/ω-V and semi-logarithmic G/ω-V characteristics of the Ag/MgPc/GaAs/Au-Ge organic heterojunction in the voltage and frequency ranges of -2V, +5 V and 3 kHz-300kHz at room temperature, respectively. The availability of the inversion, depletion, and accumulation regions are obvious for all applied frequencies from the figures. Mirzanezhad et al. reported that behaviors of Au/ (CuSe-polyvinyl alcohol)/n-Si (MPS) Schottky barrier structures are like a common metal-insulator/oxide-semiconductor (MIS/MOS) structures [21]. So in applications, both the value of capacitance and conductance increases with decreasing frequency especial due to a special density distribution of Nss or that is to say formation of traps in interfacial layer/semiconductor interface, polarization processes, Rs of the interfacial layer and MIS/MOS type structure [21]. No important variation of capacitance in the reverse bias voltage region in terms of applied frequencies while it increases abruptly with a decrease in frequency in the forward bias voltage range. Two broaden peaks, around 20 nF and 70 nF, of capacitance of MgPc/GaAs OHJ are recorded, around 0.6 V and 3.25 V respectively and a valley is also observed around 1V at the edge of depletion and deep depletion areas as displayed in figure 1 for the whole range of frequency. The C-V curve of MgPc/GaAs OHJ defines 3 regions, accumulation, depletion and deep depletion as seen in figure 1. From 1/C²-V ( not shown here) plots, we extract several parameters such as donor density Nd which is found to be 8x1017 cm-3 for 300 kHz, barrier height of 0.4 V and potential in built Vbi of 1.3 V. Similarity with others works should be cited like capacitance-voltage plot for the P3HT diode in both forward and reverse bias demonstrated a Vbi of 1.2 V and 0.55 V respectively as reported earlier [14]. 5

Journal Pre-proof Based on the C-V characteristics of MgPc/GaAs OHJ, we conclude that some parameters increase like Nd for the frequency 20 kHz, Vbi up to 30 kHz and b up to 40 kHz. While a decay of the same magnitudes is detailed in the 30 kHz-100 kHz, 40 kHz-100 kHz and 50 kHz-80 kHz ranges respectively. Such variation in Nd and Φb values might be due to a particular density distribution of interface states and to interface layer. Besides Vbi parameter obeys to the similar variation of that of Nd and Φb because the charge carrier transport inside space charge zone “SCZ” in OHJ is induced by a diffusion mechanism. Besides, at 300kHz, the extracted values of Cmax, Gmax, Cox and Nss are found to be 35 nF, 1x10-4 S, 35 nF and 1550x1013 (1/ eVxcm²) respectively.

The C/ω-V curve of organic

semiconductor increased toward the minor frequency which indicates the contribution of defects to capacitance as mentioned earlier [26]. Ago, I.S. Yahia et al. reported that the capacitance increases with increasing the applied voltage, the peaks of C-V are shifted to the negative biasing voltage side with increasing frequencies [27]. The figure 2 shows the conductance-voltage measurements in dark and room temperature conditions of MgPc/GaAs OHJ. The conductance G/ω-V, within 3 kHz-300 kHz, is increasing when frequency decreases in the forward bias voltage range. It is indicated that the maximum of conductance (Gmax) and capacitance of oxide layer Cox decrease with a rise of frequency while energy distribution variation of the interface state densities (Nss) obeys to an exponential growth within 3kHz-300 kHz range. Nss parameter starts to diminish from 200 kHz and attains a value of 1.55x1016 (1/eVxcm²) at 300 kHz. Probably, localized interface states occur at M/S interface layer. Due to presence of Nss and their lifetime, the OHJ behave a different behaviour than of that of ideal junction. Nss parameter can easily follow the alternating signal in current at low frequency and produces an excess of capacitance which depends on relaxation time of surface states and frequency of alternating signal. But, Nss parameter doesn’t follow the alternating signal in current at high frequency. Ş. Altındal mentioned that the interface states can follow the Ac field at lower frequencies but cannot at higher ones [28]. While G-V present only one visible peak at 2 V for 3kHz, the peak vanishes when frequency grows up. It is reported in litertature different state of C-V and G-V peaks which vanish at high frequencies and its intensity depends on density distribution of Nss and their relaxation time. This difference in C-V and G-V behaviour might due to inorganic layer insertion in the device. At minor frequency, charges follow the Ac signal in traping centers which produce gradually the capacitance and conductance in the device. Similar trends have been recorded by Mirzanezhad et al. [21]. This fact is explained by the intensity of series resistance which occurs in accumulation zone and simultaneously the parameter Nss can be important in deep depletion and depletion zones. Our OHJ is built by metal, organic layer and semiconductor substrate so Rs is the sum of their contributions. 6

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3.2. Dielectric response and Ac conductivity Conduction mechanism and electrical transport inside semiconductior material are governed by free charges carriers. When electric field is applied inside semiconductor, the conduction density current and displacement current are noted jc and jD, are respectively expressed as [29];

  jc  E

(1)

And displacement current jD is expressed as[29];

  E jD   t The ratio of JD over Jc, when sinusoidal alternating bias voltage is applied, is then;

(2)

  E i E  t

(3)

j D   JC 

(4)

For dielectric material biased by sinusoidal voltage, σ tends to zero, so JD is rewritten [29];

iE  j D

(5)

   'i "

(6)

Where ɛ′ and ɛ″ correspond to the real and imaginary parts of complex dielectric constant or permittivity ε. In vacuum, it takes the value of 8.84x10-12 F/m from 1/4πε0=9x109 Nxm²/C². The imaginary part of dielectric constant (ε”) or the loss factor is a measure of how dissipative or lossy a material to an external electric field. The value of (ε”) is always greater than zero and is usually much smaller than (ε’). So, the behavior seems to superposition of displacement current and conduction current [29];     "E  i ' E  j D

(7)

Where conductivity depends on frequency ω=2πf and imaginary part of dielectric constant ε”.

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Journal Pre-proof So, an imperfect capacitor can be represented by the parallel association of a perfect capacitor and a resistance. Capacitance of such capacitor is then expressed as [29];

 C0 0

C

(8)

Where C0 is the capacitance of capacitor placed in vacuum, and the complex admittance A is then given by [29];

A  iC

Ai I

C 0

0

C 0

0

(9)

( 'i " )

(10)

( ' 'i ' )V

(11)

The phase of current with respect of voltage is [30, 31],

tan  

 ''  cot '

(12)

The dielectric loss tangent (tan δ) of a material indicates the inherent electromagnetic energy loss due to several physical processes. Loss angle tangent represents how lossy the material is for Ac signal. The properties of OHJ are determined by Ac impedance spectroscopy using d’impédance "HP 4192 A LF impedance analyzer over a frequency range from 3kHz to 300 kHz.

The current density j can be

associated to the complex admittance (Y), electric field E and Dc conductivity as follows [29],

(i ' ' ' )E  j D

(13)

( 'i ' ' ) E  j D YE  j D

 '    ' ' 0

,

(14) (15)

 ' '    ' 0

(16)

Where Y is admittance is the inverse of impedance M, Y=1/M, and loss current conductivity or Ac conductance is σ’ and conductivity due to charging current or susceptance is σ’’. Besides, we can rewrite σAc in terms of permittivity of free space εo, ω, and the dielectric loss factor tan δ, [32]

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 '      ' tan  AC

0

(17)

Where δ= π/2-, dielectric constant ε’ in terms of frequency against bias voltage at room temperature of MgPc/GaAs OHJ is sketched in figure 3.

As reported in literature, that strong frequency and

voltage dependencies of ε’ and ε’’ are recorded particularly at minor frequencies in both accumulation and depletion regions [22]. As displayed in figure 3, theε’ values stay approximately unaffected in the reverse bias range while a slight peak is demonstrated. The magnitude of this peak increases with decreasing frequencies as indicated by arrow and shift towards the accumulation region since the charges at MgPc/GaAs interface can be reorganized. After a distinct voltage about 3V, both capacitance and dielectric constant ε’ decay rapidly. plotted in the figure 4.

Dielectric loss ε’’ as a function of voltage is

The relative dielectric constant is expressed as [29],

   'i ' tan  r

r

r

(18)

Where ε’rtan is loss factor. The variation of tan versus bias voltage of MgPc/GaAs OHJ at several frequencies is plotted in figure 5. This sketch shows a peak of 130 at around 1.2 V for lower frequency ( 3-7 kHz). Resistor has a resistance R as;

1  ''  C 0 R 0

(19)

The angle loss or dielectric loss tangent variation of MgPc/GaAs organic heterojunction is described in figure 5.

The variation of tan  versus voltage looks like a Gaussian variation within 0-2 V range in

particular for low frequencies recording a top of 130 at 1.2 V for 7 kHz. This fact might due to the dipolar reorientation and charge effects stopping from trapping of charge carriers in the surface states as Batoo et al. reported later [33]. The electronic, ionic, and dipolar polarization contribute to the dielectric material polarization as reported in literature. Electronic and ionic polarization occurs during a very short interval of time, while the dipole polarization needs rather extended time. In case of polar polymers, the dielectric constant values decreased with increasing frequency because when the applied field frequency increased, it is assumed that the permanent dipoles cannot reorient themselves fast enough, and therefore dielectric constant decreases [34]. 9

Journal Pre-proof Capacitance and dielectric loss reveal a decrease with an increase in frequency and increase with increase in temperature as cited by Anuroop et. al for inorganic material [35]. Figure 6 describes the variation of the impedances M’ and M’’ parameters versus biasing voltage of MgPc/GaAs OHJ at various frequencies. M’ profile sketches roughly a Gaussian distribution inside -2V, 1V which is tight for the forward bias range. The M-V plots in the reverse bias turn out the occurrence of peak values for the impedance (2.1 for M’ and 1.08 for M’’) and impedances fall nearly with increasing of frequency. But, frequency has no dependence on the M variation, after a descent of M’, within the forward biasing voltage greater than 1.2 V, so curves look like combined together as shown in fig. 6a. M” profile exhibits two peaks, located respectively at 1.04 and 0.6, whose intensity decreases when voltage shift to forward bias as observed in fig. 6b. Alternating current conductivity σAc versus bias voltage is plotted in figure 7.

It shows variation of alternating current conductivity σ as a function of voltage

within -2V, 5V in terms of frequency. Very low value of σ within reverse voltage and the profile is changing in forward voltage where we record a high value of σ of 27.5x10-7 S/cm at 5V for 300 kHz. It is mentioned in literature that conductivity is bigger, around 0.02 S/m, for Ag/Zn-DLC/Si device [36]. The achieved Cole-Cole plots contain a set of semicircles which represent the mechanism throughout the interface and electron transport from organic material to semiconductor. A plot of dielectric constant ε’ versus lnω shows that it reaches its maximum value one third of a cycle simultaneously with voltage reaches its maximum value of 2.5V as displayed in figure 8 inside voltage range of 1.5 V to 2.5 V step by 0.1 V at room temperature. A sharp decay of dielectric loss ε’’/V-lnω, within the 8-12.5 Hz range, is shown in fig.8. The curve seems to be exponential decay from 520 to the lower value of ε” parameter up to 10.5 Hz, beyond this limit, the profile of ε’’ becomes as straight path as seen in the inset of figure 8. Overall, the values of Tan fall within 8-11.5Hz as voltage sweeps inside 1.5 V-2.5V. Dielectric constant ε’ ( real part) is sketched as a function of frequency ( semilogarithmic scale) for the 1.5V -2.5 V bias voltage range. It is seen that ε’ increases, from 2 to 10, with applied voltage within 8-12.5 Hz range as depicted in fig.8. A plot of dielectric constant ε’ versus lnω shows that it reaches its maximum value one third of a cycle simultaneously with voltage reaches its maximum value of 2.5V as displayed in fig.8. The dielectric constant ε’ of zinc-containing diamond-like carbon film follows nearly the similar shape [36]. It is revealed from both figures 8 that the dielectric constant, ε’ and dielectric loss ε”, drop to a constant value at high frequency for whole 1.5V-2.5V bias voltage range. It is seen that at small frequencies, the high value of dielectric constant, ε’ is attributed to the contribution of charge carrier accumulation at the interface of contact and MgPc organic material. We found the same trends as those reported in literature. It is mentioned that for slight frequency dielectric constant is 10

Journal Pre-proof increasingly higher and conversely for high frequency the values are smaller. It is revealed that at small frequency the electrode polarization can influence the ε(ω) variation. Consequently, the charge carriers, which accumulate at the contact layer-interfacial layer, have a response to applied external field and ε (ω) values increase. While the ionic and electronic polarization contributions are evident and dipoles become frozen with no effective contribution to the dielectric constant ε [36]. In literature they report that the dielectric constant value ε′ reduces with the frequency increments; however the Ac electrical conductivity σAc increases, depending on the nature of reducing polarization and series resistance (Rs) effect [22]. A peak is observed in the ε″-V and tanδ-V curves in the inversion region and the plots show a decay of the amount of this peak with the frequency increase, shifting towards the accumulation region that can be attributed to a frequency dependent dielectric relaxation [22]. The figure 8 describes that ε’ and ε’’ parameters are both strongly frequency-dependent. The capacity of dipoles to turn themselves under the effect of minor frequencies with respect higher frequencies is the important cause of the change in dielectric parameters of HJ. Figure 9 displays the profile of tanδ vs. lnω of MgPc/GaAs organic heterojunction within 1.5V-2.5V bias voltage range. The impedance spectroscopy study is evidenced by the complex impedance M expressed as [37-38]; M  M 'iM ' '  Rs  i

1 C s

(20)

To explain relaxation regime of M ( M’ and M’’) parameter as a function of lnω are plotted in fig.10 a and b. It is observed that M’ and M’’ parameters tend to zero for minor frequency, displaying the insignificant low contribution of electrode polarization to M′ variable. An exponential growth of M’ variable versus lnω of MgPc/GaAs OHJ is recorded with an increase in voltage till 11.5 Hz ( log scale) then saturation is reached at 0.023 within 11.4-12.2 frequency gap and then a decay happens as indicated in figure 10 a. At suitably strong frequencies, the dipole molecules cannot turn themselves, indicating that the electrical variable of M reaches the maximum value corresponding M = 1/ε owing to the dielectric relaxation process. On the other hand, in the inversion region, M″ shows a peak shifting towards the positive voltages with an increment in frequency, which is confirmation for the contribution of space charge polarization to dielectric relaxation. Similar trends have been previously mentioned [39].An exponential growth of M’’ versus lnω of MgPc/GaAs OHJ is observed for all applied voltages from 1.5V to 2.5V up to 11.5 Hz (log scale value). After saturation happen and continues to increase from 12.2 Hz (log scale) with a decrease in voltage as seen in fig.10 b. The profile of ε’ and ε’’ functions of MgPc/GaAs organic heterojunction yields to determine the real and 11

Journal Pre-proof imaginary parts of complex electric impedance, M’, and M’’ as depicted in Figure 10 (a) and (b) in the same 8-12.5 frequency (semilog scale) range within 1.5 V-2.5 V range. So, both M’and M’’ are also strongly frequency dependent.

A frequent character in organic semiconductors is a frequency

dependency of the Ac conductivity that almost rises linearly with frequency as depicted in fig.11. Ac current conductivity σAc against frequency (semilog scale) is plotted in figure 11 for 1.5V-2.5V bias voltage range. Within the 8-10.5 ( log scale) range there is nearly no effect of biasing on measured conductivity of OHJ, the average of conductivity is then about 8x10-7 S/cm and suddenly falls to the half value around 11.5 Hz and then rises back to a higher value (around 19.5x10-7 S/cm at 2.5V) with voltage within 11.5-12.5 frequency range. To recognize which conduction mechanisms control the transport charge inside the MgPc/GaAs OHJ, we plot from figure 11 the ln(σAc)-ln(ω) diagram in order to separate the linear part of the curves at extreme measured voltages 1.5 V and 2.5 V. The conduction mechanism type is related to exponents s1 and s2 in the following expressions [40],

    A  B s1

s2

0

(21)

Where σ0 is the frequency independent Dc part of Ac conductivity, A and B are constant, ω is the angular frequency (ω= 2πf). In particular, within high frequency range (lnω>11 Hz), It is clear that the value of σAc rises with increasing frequency due to the reduction of interfacial polarization. The growth in σAc causes an increase in the current and then results in the energy loss tanδ. This occurrence of σAc can be attributed to the decrease in series resistance (Rs) as reported prior [41, 42]. The hoping of charge carriers from one trap site to another, which has an energy level in the band gap of GaAs, causes an increment in Ac electrical conductivity. Elsewhere, σAc values still almost constant around 8x10-7 S/cm for any value of voltage within the low frequency range (8
12

Journal Pre-proof plot. Fig.11A shows ln(σAc)-ln(ω) plot of the MgPc/GaAs organic heterojunction device. According to the sketch of the figure 11A, the plot has two distinct linear regions at 1.5V and 2.5V with different slopes.

Region I presents two curves linearly fitted with equations y=0.9x-15.04 and y=0.019x-14.32

respectively at 2.5V and 1.5V. While region II with strong slope are displayed with equations fit y=0.54x-21.09 and y=0.23x-17.21 at respectively 2.5 V and 1.5V. Based upon these results, the values of s1 are obtained as 0.0197 and 0.902 at 1.5V and 2.5V, respectively. The values of s2 are also obtained as 0.2328 and 0.5457 at 1.5V and 2.5V, respectively. These results indicate that there are different conduction mechanisms through MgPc/GaAs film. The dimensionless frequency exponents between 0 and 1 corresponding to low/intermediate frequency region is considered to be a result of the interaction among charge carriers and trap states [22]. The Cole-Cole diagrams, in which the imaginary M’’ and real M’ impedances, are plotted in figure 12 at discrete excitation frequencies within 3kHz-300kHz. Additionally, semicircle width is the value of the recombination (junction) resistance and the maximum imaginary impedance corresponds to the product from which the electron lifetime was calculated. However, the decrease in the radii of M” vs.M’ diagram describes the change in conductivity of space charge region SCR in the OHJ device. When the electrical impedance characteristics in OHJ are uniform, a Cole-Cole plot defines roughly a semicircle shape which is tighted for lower value of M’ (M’< 0.4) and for any frequency. Whereas M’ value becomes greater (> 0.2), M’’ plots describe a distinct semicircle from 3 kHz to 300 kHz. When M’ varies within 0-2 range M’’ scan the plane reaching the high value of 1.1. Similar trends for further devices are reported earlier [36]. As seen in figure 12, impedance shapes are characterized by the appearance of a single semicircular arc whose radii of curvature increases with increasing frequency. Since the polarization is dominant, we have several semicircles because of the space charge and orientation polarization in organic materials.

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Conclusion The MgPc/GaAs organic heterojunction is fabricated successfully by both spin coating and thermal evaporation techniques and properties are well studied. As reported before, the (123)-oriented monoclinic system is revealed by X-ray pattern. The transparency of thin films based on Mg-Phthalocyanine is as high as 67 % within the visible range. Absorbance measurements reveal that Mg-Phthalocyanine is a wide band gap semiconductor. An AFM analysis demonstrates that such material in thin film form is composed by nanograins having an average size of 22.76 nm, and the RMS is of 10.5 nm. C/ω-V and G/ω-V characteristics, within 3kHz-300kHz spectral range, are the main tool to give more interesting parameters. The complex impedance measurement provides us information about the resistive (real part) and reactive (imaginary part) components in the material. The complex impedance plot known as the Cole-Cole plot can give semicircles, depending upon the electrical properties of the material. Extracted parameters are barrier height, donor density, potential in built, dielectric constant and loss angle. Dielectric magnitudes as a function of bias voltage and spectral frequency are well studied and details are described. Besides, ε’ and ε’’ are both strongly frequency-dependent for the whole biasing voltage range. The average of conductivity is about 8x10-7 S/cm, a high value of 27.5x10-7 S/cm at 5V for 300 kHz is indicated. Parameter Nss decay is detected from 200 kHz and attains 1.55x1016 (1/eVxcm²) at 300 kHz. From Ac conductivity versus frequency plots, different conduction mechanisms through HJ are described. The dimensionless frequency exponents s1 and s2 are comprised between 0 and 1 which is corresponding to low/intermediate frequency region. This fact is considered to be a result of the interaction among charge carriers and trap states. Remarkable characteristics of OHJ based on MgPc offer appropriate solar energy application.

Acknowledgments The work is inserted in the ANVREDET project n°18/dg/2016 “projet innovant caractérisation de films semiconducteurs nanostructurés et de cellule solaire” https://www.anvredet.org.dz It is also a part of PRFU 2018 PROJECT under contract B00L02UN310220180011. http://www.prfu-mesrs.dz/ 14

Journal Pre-proof https://orcid.org/inbox?lang=en

http://www.livedna.net/?dna=213.14146. This work is also supported by the

Ministry of Development of Turkey under project number: 2016K121220.

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[41]E. Maril, S.O. Tan, S. Altindal, I. Uslu, Evaluation of electric and dielectric properties of metal–semiconductor structures with 2% GC-doped-(Ca3Co4Ga0.001Ox) interlayer, IEEE Trans. Electron Devices 65 (2018) 3901–3908, https://doi.org/10.1109/ted.2018.2859907. [42]S. Demirezen, A. Kaya, S.A. Yerişkin, M. Balbaşi, I. Uslu, Frequency and voltage dependent profile of dielectric properties, electric modulus and Ac electrical conductivity in the PrBaCoO nanofiber capacitors, Results Phys. 6 (2016) 180–185, https://doi.org/10.1016/j.rinp.2016.03.003. [43]A.A.M. Farag, A. Ashery, M.A. Salem, Electrical, dielectric characterizations and optoelectronic 17

Journal Pre-proof applications of epitaxially grown Co/n-CuO/p-Si heterojunctions, Superlattices and Microstructures 135 (2019) 106277.

.

18

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80

60

Capacitance (nF)

ac

3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

40

dep

20

dd 0

-2

-1

0

1

2

3

4

5

Voltage (V)

Fig.1. C-V plots of Ag/MgPc/GaAs/Au-Ge organic heterojunction for 3kHz-300kHz frequency range. The accumulation (ac), depletion (dep), and deep depletion (dd) regimes are clearly described in C–V curve. Inset shows the cross sectional of MgPc/GaAs heterojunction.

19

Journal Pre-proof G-V 6x10-6 3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

5x10-6

Conductance (S)

4x10-6

3x10-6

2x10-6

10-6

0 -2

-1

0

1

2

3

4

5

Voltage (V)

Fig.2. The variation of conductance verus bias voltage of MgPc/GaAs organic heterojunction for 3kHz-300kHz frequency range. Dielectric Constant, ' 12 3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

Dielectric Constant, '

10

8

6

4

3kHz

300 kHz

2

0

-2

-1

0

1

2

3

4

5

Voltage (V)

Fig.3. Dielectric constant ε’ (F/m) against bias voltage of MgPc/GaAs organic heterojunction at different frequencies. 20

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Dielectric Loss, '' 1000 3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

Dielectric Loss, ''

800

600

400

200

0 -2

-1

0

1

2

3

4

5

Voltage (V)

Fig.4. Dielectric loss ε’’ (F/m) versus V measured of Tan MgPc/GaAs organic heterojunction diode. 140 3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

120

Tan

100 80 60 40 20 0 -2

-1

0

1

2

3

4

5

Voltage (V)

Fig.5. Tanδ versus bias voltage of MgPc/GaAs organic heterojunction.

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M'

2,1 3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

a

1,8 1,5

M'

1,2 0,9 0,6 0,3 0,0 -2

-1

0

1

2

3

M'' Voltage (V)

4

5

1,2

b

3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

1,0

M''

0,8

0,6

0,4

0,2

0,0 -2

-1

0

1

2

3

4

5

Voltage (V)

Fig.6. Real and imaginary parts of impedance M’ (Ω) (a) and M’’ (Ω) (b) versus voltage of MgPc/GaAs organic heterojunction.

22

Journal Pre-proof ac

30 3 kHz 5 kHz 7 kHz 10 kHz 20 kHz 30 kHz 40 kHz 50 kHz 70 kHz 80 kHz 90 kHz 100 kHz 200 kHz 300 kHz

ac (x 10-7 S.cm-1)

25

20

15

10

5

0 -2

-1

0

1

2

3

4

5

Voltage (V)

Fig.7. Variation of measured Ac-conductivity versus voltage of MgPc/GaAs organic heterojunction.

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Journal Pre-proof Dielectric Constant, '

Dielectric Constant, '

10

2.5 V

1.50 V 1.60 V 1.70 V 1.80 V 1.90 V 2.00 V 2.10 V 2.20 V 2.30 V 2.40 V 2.50 V

a

8

6

4

2

1.5 V 8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

Ln() (Hz)

Dielectric Loss, '' 600 45

1.50 V 1.60 V 1.70 V 1.80 V 1.90 V 2.00 V 2.10 V 2.20 V 2.30 V 2.40 V 2.50 V

2.5 V 40

500

Dielectric Loss, ''

35

400 30

1.5 V 300

25 10.5

10.6

10.7

10.8

10.9

11.0

200

b

100

0 8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

Ln() (Hz)

Fig.8. Plotting of dielectric constant ε’ (F/m) (a), ε’’ (F/m) (b) against ln ω of MgPc/GaAs organic heterojunction diode within 1.5V-2.5V bias voltage range.

24

Journal Pre-proof Tan 10

Tan

8

6

1.5 V

1.50 V 1.60 V 1.70 V 1.80 V 1.90 V 2.00 V 2.10 V 2.20 V 2.30 V 2.40 V 2.50 V

4

2

2.5 V 8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

Ln() (Hz)

Fig.9. Profile of tanδ versus ln ω of MgPc/GaAs organic heterojunction within 1.5V-2.5V bias voltage range.

25

Journal Pre-proof M' 0.030 1.50 V 1.60 V 1.70 V 1.80 V 1.90 V 2.00 V 2.10 V 2.20 V 2.30 V 2.40 V 2.50 V

0.025

M'

0.020

0.015

2.5 V

0.010

0.005

a 1.5 V

0.000

8

9

10

11

12

Ln() (Hz)

M'' 0.18 1.50 V 1.60 V 1.70 V 1.80 V 1.90 V 2.00 V 2.10 V 2.20 V 2.30 V 2.40 V 2.50 V

0.16 0.14

M''

0.12 0.10 0.08

1.5 V

0.06 0.04

2.5 V

0.02

b

0.00 8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

Ln() (Hz)

Fig.10. M’ (Ω) (a) , M’’ (Ω) (b) against lnω of MgPc/GaAs organic heterojunction diode within 1.5V-2.5V bias voltage range.

26

Journal Pre-proof ac 20 1.50 V 1.60 V 1.70 V 1.80 V 1.90 V 2.00 V 2.10 V 2.20 V 2.30 V 2.40 V 2.50 V

18

ac (x 10-7 S.cm-1)

16 14 12

2.5 V

10 8 6 1.5 V

4 8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

Ln() (Hz)

Fig.11. Ac current conductivity σAc against frequency (semilog scale) of MgPc/GaAs organic heterojunction within 1.5V-2.5V bias voltage range.

-12,6 Region I

Region II

1.5 V 2.5 V

-12,8

ln(ac) (.cm)

-1

-13,0 -13,2

y = 0.5457x-21.093

-13,4 -13,6 y = 0.902x-15.049

-13,8

y = 0.2328x-17.214

-14,0 -14,2

y = 0.0197x-14.322

-14,4 11

12

13

14

15

16

ln() (Hz)

Fig. 11A. The ln(σAc)-ln(ω) plot of the MgPc/GaAs organic heterojunction device. Two distinct regions are displayed and linear fit equations are indicated for each region at 1.5 V and 2.5 V. The variables y and x denote ln(σAc) and ln(ω) respectively. 27

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Cole-Cole 1.2

1.0

M''

0.8

0.6

20 kHz

0.4

30 kHz

0.2

0.0

0.0

0.2

3 kHz

40 kHz

5 kHz

50 kHz

100 kHz

7 kHz

70 kHz

200 kHz

10 kHz

80 kHz

300 kHz

0.4

0.6

0.8

90 kHz

1.0

1.2

1.4

1.6

1.8

2.0

M'

Fig. 12.

The Cole-Cole plot of MgPc/GaAs OHJ, M’’(Ω) versus M’ (Ω) at several frequencies ( 3kHz-300kHz).

28

Journal Pre-proof Dear Editor Physica B Journal

Oct, 09 2019

This paper has no conflict interest

Ac Conductivity and Impedance Spectroscopy Study and Dielectric Response of MgPc/GaAs Organic Heterojunction for Solar Energy Application * 1,2M.

Benhaliliba, 3,4T. Asar, 1,5I. Missoum, 6Y.S.Ocak, 3,4S. Özçelik 1,2C.E.

Benouis and 1,2A. Arrar

1Film

Device Fabrication-Characterization and Application FDFCA Research Group USTOMB, 31130, Oran, Algeria. 2Physics Faculty, USTOMB University POBOX 1505 31130, Mnaouer Oran Algeria. 3Physics Department, Faculty of Sciences, Gazi University, 06500, Ankara, Turkey. 4Photonics Application and Research Center, Gazi University, 06500, Ankara, Turkey 5Department

6Dicle

of Physics , Saad Dahleb University, Blida Algeria. UniversityDicle University, Education Faculty, Science Department, 21280 Diyarbakir, Turkey Diyarbakir Turkey *Corresponding

Author

* [email protected]

With best regards,

Mostefa Benhaliliba Prof USTOMB