Design and fabrication of dioxyphenylcoumarin substituted cyclotriphosphazene compounds photodiodes

Article abstract of DOI:10.1016/j.physb.2017.03.025

The present study introduces cyclotriphosphazene compounds substituted by dioxyphenylcoumarin as a photodiode application. Firstly, 7,8-dihydroxy-3-(3-methylphenyl)coumarin (1b) has been obtained by conventional as well as microwave assisted methods. Novel optoelectronic device characteristics for both mono and disubstituted dioxyphenylcoumarin bearing cyclotriphosphazene compounds (HCP-2 and HCP-4) have been synthesized from the reactions of cyclotriphosphazene containing dioxybiphenyl (HCP-1 and HCP-3) with compound 1b, respectively. The structures of compounds HCP 1–4 were identified by using elemental analysis, ¹H, ¹³C-APT, ³¹P NMR and 2D HETCOR NMR and FT-IR spectroscopy methods. The Al/HCP-2/p-Si/Al and Al/HCP-4/-p-Si/Al photodiodes properties have been investigated from current-voltage (I?V) and capacitance-voltage (C?V) measurements. The electrical parameters of the prepared diodes such as ideality factor n and series resistance R_s were investigated in dark and at room temperature from (I?V) curve and Nord's method. As can be seen, the Al/HCP-2/p-Si/Al diode of high rectification ratio RR and with ideality factor greater than unity. The influence of light illuminations on the diode shows that the device can be used as photodiode with good efficiency. The barrier height ?_b and series resistance R_s have been calculated from the capacitance-voltage (C?V)and conductance-voltage (G?V) measurements under various applied frequencies from 10?kHz to 1?MHz. the high difference in the results of barrier height obtained from I-V and C-V calculations confirm the influence of series resistance and localized states on transport of charge carriers and the photodiode performance.

Full text of DOI:10.1016/j.physb.2017.03.025

Author’s Accepted Manuscript
Desing and fabrication of dioxyphenylcoumarin
substituted
photodiodes
cyclotriphosphazene
compounds
Elsayed Elgazzar, A. Dere, Furkan Özen, Kenan
Koran, Abdullah G. Al-Sehemi, Ahmed A. Al-
Ghamdi, Ahmet Orhan Görgülü, F. El-Tantawy, F.
Yakuphanoglu
www.elsevier.com/locate/physb
PII:
S0921-4526(17)30138-2
http://dx.doi.org/10.1016/j.physb.2017.03.025
PHYSB309869
DOI:
Reference:
To appear in:
Physica B: Physics of Condensed Matter
Received date: 1 March 2017
Revised date: 15 March 2017
Accepted date: 16 March 2017
Cite this article as: Elsayed Elgazzar, A. Dere, Furkan Özen, Kenan Koran
Abdullah G. Al-Sehemi, Ahmed A. Al-Ghamdi, Ahmet Orhan Görgülü, F. El-
Tantawy and F. Yakuphanoglu, Desing and fabrication of dioxyphenylcoumarin
substituted cyclotriphosphazene compounds photodiodes, Physica B: Physics o
Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.03.025
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Desing and fabrication of dioxyphenylcoumarin substituted
cyclotriphosphazene compounds photodiodes
Elsayed Elgazzara, A. Dereb, Furkan Özenc, Kenan Koranc, Abdullah G. Al-Sehemid-f
Ahmed A. Al-Ghamdig, Ahmet Orhan Görgülüc , F. El-Tantawya, F. Yakuphanoglub*
aDepartment of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt
bDepartment of Physics, Faculty of Science, Fırat University, Elazığ, Turkey
cDepartment of Chemistry, Faculty of Science, Fırat University, Elazığ, Turkey
dDepartment of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box 9004,
Saudi Arabia
eResearch Center for Advanced Materials Science, King Khalid University, Abha 61413, P.O. Box
9004, Saudi Arabia
fUnit of Science and Technology, Faculty of Science, King Khalid University, Abha 61413,P.O. Box
9004, Saudi Arabia
gDepartment of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
Abstract
The present study introduces cyclotriphosphazene compounds substituted
by dioxyphenylcoumarin as a photodiode application. Firstly, 7,8-dihydroxy-3-(3-
methylphenyl)coumarin (1b) has been obtained by conventional as well as microwave
assisted methods. Novel optoelectronic device characteristics for both mono and disubstituted
dioxyphenylcoumarin bearing cyclotriphosphazene compounds (HCP-2 and HCP-4) have
been synthesized from the reactions of cyclotriphosphazene containing dioxybiphenyl (HCP-
1 and HCP-3) with compound 1b, respectively. The structures of compounds HCP 1-4 were
1
13
31
identified by using elemental analysis, H, C-APT, P NMR and 2D HETCOR NMR and
FT-IR spectroscopy methods. The Al/HCP-2/p-Si /Al and Al/HCP-4/-p-Si/Al photodiodes
(
)
properties have been investigated from current-voltage and capacitance-voltage
( ) measurements. The electrical parameters of the prepared diodes such as ideality
factor and series resistance were investigated in dark and at room temperature from
( ) curve and Nord’s method. As can be seen, the Al/HCP-2/p-Si /Al diode of high
rectification ratio and with ideality factor greater than unity. The influence of light
illuminations on the diode shows that the device can be used as photodiode with good
1

efficiency. The barrier height and series resistance have been calculated from the
(
)
capacitance-voltage and conductance-voltage ( ) measurements under various
applied frequencies from 10 kHz to 1 MHz. the high difference in the results of barrier height
obtained from I-V and C-V calculations confirm the influence of series resistance and
localized states on transport of charge carriers and the photodiode performance.
Keywords: Photodiode, Sensor, Cyclotriphosphazene; Dioxycoumarin-phosphazene;
Corresponding author: fyhan@hotmail.com (F.Yakuphanoglu)
1. Introduction
Phosphazene derivatives are a class of cyclic and polymeric molecules which contain
alternating phosphorus and nitrogen atoms bound by alternating single and double bonds in
their skeleton [1]. Substituted cyclophosphazenes with phosphorus-carbon bonds have been
synthesized from hexacyclotriphosphazenes by Friedel Crafts substitutions and by reactions
with nucleophiles such as primary, secondary amines, aryloxides or alkoxides reagents [2-4].
One of the most important property for cyclophosphazene compounds and its derivatives is
the substitution reactions exchange between organic groups and phosphorus atoms.
Cyclophosphazenes with nucleophilic substitution reactions have several advantages with
high thermal and chemical stability. When organic groups are bonded at each phosphorus
atom, the phosphazene skeleton is stabilized against hydrolytic attack [1,5]. The
phosphazenes, particularly the substituted phosphazene derivatives containing organic or
inorganic side groups, have generated considerable research interest in recent years because
of their unusual physical and chemical properties such as organic light emitting diodes and
fluorescence sensor [6-8], cathode material for rechargeable lithium batteries [9] and flame
retardants [10].
2

In this research we study the cyclotriphosphazene compounds as a new photoconducting
material. Although some novel cyclotriphosphazene structures and their physical properties
have been reported [11-14], there is no study about the optoelectronic properties of the
cyclotriphosphazene compounds bearing dioxyphenylcoumarin groups. The aim of this work
is to determine how the photodiode prepared from both mono and di substituted
dioxyphenylcoumarin bearing cyclotriphosphazene compounds (HCP-2 and HCP-4) change
and how its characteristics alter regarding with the number of coumarin attachment.
2. Experimental techniques
2.1. Materials and methods
N3P3Cl6 (HCP) was provided from TCI and was used by the recrystallization from n-hexane.
The 2,2'-dihydroxybiphenyl and cesium carbonate have been procured from Merck and
Sigma Aldrich, respectively. All the organic solvents used in the synthesis and purification of
the compounds were provided from Merck.
1H, 13C and 31P-NMR spectra were recorded in dimethylsulphoxide-d6 and chloroform-d
1
solutions on a Bruker DPX 400 MHz spectrometer using TMS (an internal reference for H-
31
NMR) and 85% H3PO4 (an external reference for P-NMR). The micro analysis and FT-IR
spectra of compounds were measured using a Leco 932 CHNS-O apparatus and on Perkin
Elmer FT-IR spectrometer, respectively.
2.2. Synthesis
2-(2,3,4-trimethoxyphenyl)-1-(3-(methylphenyl))acrylonitrile (1a) was synthesized and
purified according to methods in the literature [15-17]. Cyclotriphosphazene compounds
containing dioxybiphenyl HCP-1 and HCP-3 were synthesized as defined by Carriedo et al.
[18,19].
3

2.2.1. Synthesis of 7,8-dihydroxy-3-(3-methylphenyl)coumarin (1b)
sealed vessel containing mixture of 2-(2,3,4-trimethoxyphenyl)-1-(3-
A
a
(methylphenyl))acrylonitrile (1a) (0.4 g, 1.3 mmol), an appropriate silica gel (15 g) and
pyridinium hydrochloride (5 g) was stirred under argon atmosphere. The reaction was
continued for 25 minutes at 320 W under microwave irradiation. After cooling to room
temperature, it was treated with acidified with 1 M HCl (150 mL) and then the product was
isolated by filtration. The product was dissolved in acetone (3x30 mL) and it was isolated
with using filtration through silica gel. The solvent was evaporated on a rotary evaporator.
The solid washed with water many times and then dried under vacuum. The crude product
was resolved in ethyl acetate (15 mL) and precipitated with n-hexane (250 mL). The crude
solid was purified to column chromatography using chloroform:hexane and the target product
1b was obtained as a grey solid. Yield=75% (0.26 g). Anal. Calc. for C16H12O4 (MW:
268.26): C, 71.64; H, 4.51; Found C, 71.62; H, 4.50%. FT-IR (KBr, cm-1): 3477, 3316 νO-H
,
1
3052 νC-H(Ar), 2917, 2857 νC-H(Al),1676 νC=O, 1610, 1589, 1509 νC=C. H NMR (400 MHz,
DMSO-d6): δ 2.27 (3H, s, H18), 6.86 (1H, d, J=8.4 Hz, H4), 7.13 (1H, d, J=8.4, H5), 7.21
(1H, d, J=7.2 Hz, H17), 7.33 (1H, t, J=7.6 Hz, H16), 7.53 (1H, d, J=8.8 Hz, H15), 7.55 (1H, s,
H13), 8.11 (1H, s, H9), 9.44 (1H, s, H7), 10.17 (1H, s, H8). 13C-NMR (400 MHz, DMSO-d6): δ
21.19 C18, 113.16 C6, 113.28 C4, 119.89 C5, 122.71 C10, 125.64 C13, 128.57 C15, 129.02 C17,
129.23 C16, 131.61 C12, 134.99 C14, 138.01 C2, 142.06 C9, 143.08 C1, 149.20 C3, 161.17 C11.
2.2.2. Synthesis procedure of 2,2-bis[spiro(7,8-dioxy-3-(3-methylphenyl)coumarin)]-
4,4,6,6-bis[spiro(2',2"-dioxy-1',1"-biphenylyl)]cyclotriphosphazene (HCP-2)
HCP-1 (0.3 g, 0.52 mmol) and Na2CO3 (0.44 g, 4.16 mmol) were dissolved in 50 mL of dry
xylene (mixture of isomers) in a 150 mL three-necked reaction flask. 7,8-dihydroxy-3-(3-
4

methylphenyl)coumarin (1b) (0.28 g, 1.04 mmol) in 25 mL of xylene was slowly added to the
0
0
stirred solution at 0 C. After the reaction mixture was continued for 25 minutes at 0 C, it
was refluxed in xylene for 48 h. After stopping the reaction, the mixture was filtered to
separate the formed sodium chloride. Xylene was evaporated at rotary evaporator. The solid
product was resolved in 15 mL of xylene and then precipitated into n-hexane (200 mL). The
crude solid was filtered and then dried. The crude product was purified to column
chromatography using chloroform as mobile phase. The target product HCP-2 was obtained
as pure white solid. Yield=40% (0.16 g). Anal. Calc. for C40H26N3O8P3 (MW = 769.57
g/mol): C, 62.43; H, 3.41; N, 5.46. Found: C, 62.51; H, 3.48; N, 5.49%. FT-IR (KBr, cm-1):
3027 and 3063 C-H(Aromatic), 2923 C-H(Aliphatic), 1736 C=O, 1583, 1605 and 1633 C=C, 1174
31
and 1193 P=N, 977 
. P NMR (CDCl3) δ/ppm: 36.60 (2P, t, PA(O2C12H8)), 24.65 (1P,
P-O-Ph
d, PB(O4C16H10)). 1H NMR (CDCl3) δ/ppm: 2.63 (3H, s, H22), 7.02-7.73 (23H, m, H3, H4, H5,
13
H6, H11, H12, H13, H17, H19, H20 and H21). C NMR (CDCl3) δ/ppm: 147.81 C1, 128.49 C2,
126.51 C3, 129.74 C4, 129.94 C5, 122.03 C6, 146.93 C7, 146.93 C8, 126.06 C9, 116.40 C10,
122.32 C11, 108.77 C12, 139.91 C13, 138.16 C14, 159.02 C15, 138.74 C16, 125.62 C17, 134.42
C18, 128.65 C19, 130.12 C20, 129.14 C21 and 21.96 C22.
2.2.3. Synthesis procedure of 2,2,4,4-bis[spiro(7,8-dioxy-3-(3-methylphenyl)coumarin)]-
6,6-bis[spiro(2',2"-dioxy-1',1"-biphenylyl)]cyclotriphosphazene (HCP-4)
HCP-3 (0.35 g, 0.76 mmol) and Na2CO3 (1.28 g, 12.16 mmol) were dissolved in 50 mL of
dry xylene (mixture of isomers) in a 150 mL three-necked reaction flask. 7,8-dihydroxy-3-(3-
methylphenyl)coumarin (1b) (0.81 g, 3.04 mmol) in 25 mL of xylene was slowly added to the
0
0
stirred solution at 0 C. After the reaction mixture was continued for 25 minutes at 0 C, it
was refluxed in xylene for 48 h. After stopping the reaction, the mixture was filtered to
separate the formed sodium chloride. Xylene was evaporated at rotary evaporator. The solid
5

product was resolved in 15 mL of xylene and then precipitated into n-hexane (200 mL). The
crude solid was filtered and then dried. The crude product was purified to column
chromatography using chloroform as mobile phase. The target product HCP-4 was obtained
as pure white solid. Yield=46% (0.3 g). Anal. Calc. for C44H28N3O10P3 (MW = 851.62
g/mol): C, 62.05; H, 3.31; N, 4.93. Found: C, 62.12; H, 3.38; N, 4.99%. FT-IR (KBr, cm-1):
3060 C-H(Aromatic), 2917 C-H(Aliphatic), 1738 C=O, 1585, 1604 and 1633 C=C, 1185 and 1210

P=N, 948 P-O-Ph.
31P NMR (CDCl3) δ/ppm: 35.94 (1P, d, PA(O2C12H8)), 23.92 (2P, t,
1
PB(O4C16H10)). H NMR (CDCl3) δ/ppm: 2.44 (6H, s, H22), 7.02-7.73 (22H, m, H3, H4, H5,
13
H6, H11, H12, H13, H17, H19, H20 and H21). C NMR (CDCl3) δ/ppm: 147.51 C1, 128.34 C2,
126.81 C3, 130.91 C4, 130.02 C5, 121.90 C6, 146.64 C7, 146.57 C8, 127.19 C9, 116.77 C10,
122.65 C11, 108.58 C12, 139.71 C13, 138.22 C14, 158.80 C15, 138.96 C16, 125.62 C17, 134.33
C18, 128.46 C19, 129.84 C20, 129.14 C21 and 21.53 C22.
2.3. Fabrication of the diodes
Firstly, the native oxide layer on p-Si substrates was removed by etching of HF. The Si
substrates were chemically cleaned (methanol, acetone and deionized water baths) for 10-15
min and dried with nitrogen. The solutions of HCP-2 and HCP-4) organic materials and were
coated on p-Si having aluminium (Al) ohmic contact by drop coating method. After coating
o
procedure, the films were dried at 50 C for 10 min. Aluminium metal was evaporated on p-
Si wafer to form ohmic contact. After evaporation procedure, the substrates were annealed at
570 oC for 5 min in nitrogen atmosphere. The top electrode of the diode was prepared by Al
metal on the films with a diode contact area of 7.85x10-3. The photoresponse properties of the
diodes were carried out using FYtronix Electronic Device characterization system (Scheme
1) in dark and illumination conditions. The intensity of solar light is controlled using a
FYtronix Electronic Device characterization system.
6

Scheme 1. FYTRONIX Electronic Device characterization system
3. Results and discussion
3.1. Synthesis
7,8-dihydroxy-3-(3-methylphenyl)coumarin (1b) has been obtained at good yield that from
the reaction with compound 1a of pyridinium hydrochloride with using of silica gel as
support material by microwave irradiation under solventless conditions. The reactions of
HCP-1 with 1.1 equiv. of 7,8-dihydroxy-3-(3-methylphenyl)coumarin (1b) in the presence of
Na2CO3 in dry xylene (mixture of isomers) gave 2,2-bis[spiro(7,8-dioxy-3-(3-
methylphenyl)coumarin)]-4,4,6,6-bis[spiro(2',2"-dioxy-1',1"-
biphenylyl)]cyclotriphosphazene (HCP-2). The reactions of HCP-3 with 2.1 equiv. of 1b
gave
2,2,4,4-bis[spiro(7,8-dioxy-3-(3-methylphenyl)coumarin)]-6,6-bis[spiro(2',2"-dioxy-
1',1"-biphenylyl)]cyclotriphosphazene (HCP-4).
1
13
The structures of compounds were identified by using elemental analysis, H, C-APT,
31P (only phosphazene compounds) and 2D HETCOR NMR and FT-IR spectroscopy
methods. General presentation of compounds is shown in Scheme 2.
7

CH
3
CH
3
CH
CH
O
O
CH
3
3
CH
O
O
3
3
OH
8
O
1) NaOH
2) EtOH
O
2
O
CN
HO
7
O
9
MW
3
4
1
6
11
10
CH
3
O
1) Py.HCl
2) H3O+
13
16
CH
3
18
5
12
17
14
15
CN
H C
3
( 1a )
( 1 )
( 1b )
3
1
4
2
HO
O
2
3
O
CH
3
HO
O
O
1 : 2
O
O
P
N
P
N
A
P
( 1b )
1 : 1
A
N
N
22
CH
O
O
P
Cl
Cl
O
O
3
N
B
A
P
P
O
15
14
O
A N
8
B
17
9
Cl Cl
18
19
O
7
O
16
21
10
P
13
12
A
11
N
P
N
20
( HCP-1 )
( HCP-2 )
OH HO
Cl
P
Cl
+
A
A
N
Cl
Cl
3
4
3
2
( HCP )
HO
O
2
O
CH
3
HO
1
O
O
O
O
1 : 1
P
( 1b )
P
A
N
N
N
A
N
O
22
Cl
P
P
Cl
Cl
O
1 : 2
O
P
P
CH
B
N
B
3
O
15
14
N
8
B
B
Cl
17
9
18
19
O
7
O
16
21
10
13
12
11
( HCP-3 )
20
( HCP-4 )
O
O
CH
3
Scheme 2. General synthetic procedures of reactions.
The nitrile peaks (-C≡N) in the structure of 1a were not observed at FT-IR spectra of 1b.
The -OCH3 groups in the compound 1a were converted to the -OH functional groups. The
presence of -OH stretching vibration in the FT-IR spectra of 1b proves that -OCH3 groups in
the compound 1a and converted to the -OH groups. The OH stretching vibration for 1b was
showed at 3316 and 3477 cm-1. The -C=O stretching vibrations, which are exhibit at 1676
1
cm–1, is characteristic for coumarin compound. The –OCH3 protons were not observed at H
1
13
NMR spectrum of 1b. H and C-NMR spectra of 1b were showed in Fig. 1. The –OH
protons in the structure of 1b (7 and 8 number protons in the Scheme 1) were exhibited at
9.44 and 10.17 ppm.
8

Fig. 1. (A) 1H-NMR spectrum and (B) 13C-NMR spectrum of 1b (DMSO-d6).
The OH stretching vibrations in the structure of compound 1b were not observed in the
FT-IR and 1H-NMR spectra of compounds HCP-2 and HCP-4. The aromatic -CH stretching
frequencies at the FT-IR of HCP-2 and HCP-4 were showed between 3027 and 3063 cm–1.
The carbonyl group in the structures of compounds HCP-2 and HCP-4 was showed at 1736
and 1738 cm–1, respectively. The -P=N stretching frequencies, which are observed between
1174 and 1210 cm–1, are characteristic of phosphazene compounds. The P-O-Ph stretching
vibration for HCP-2 and HCP-4 was exhibited at 977 and 948 cm-1.
The 31P NMR spectra for HCP-2 and HCP-4 are showed in Fig. 2 (AB2 system). There
are two peaks in the 31P NMR spectra of dioxyphenylcoumarin substituted HCP-2 and HCP-
4 compounds. The 31P NMR spectra for HCP-2 give two sets of peaks around δ = 36.60 and
24.65 ppm in a doublet-triplet. At HCP-4 give two sets of peaks around δ = 23.92 and 35.94
ppm in a doublet-triplet.
9

Fig. 2. (A) 31P-NMR spectrum of HCP-2 and (B) 31P-NMR spectrum of HCP-4 (in CDCl3).
1
13
The characteristic peaks in the H and C-NMR spectra of HCP-2 and HCP-4 were
given in the Experimental section in detail. The 13C APT NMR spectra of HCP-2 are given in
Fig. 3. The carbonyl carbon peaks (C=O, C15) for HCP-2 and HCP-4 was showed at 159.02,
158.80 ppm, respectively. The methyl carbon (-CH3, C22) for HCP-2 and HCP-4 was
observed at 21.39 and 21.53 ppm, respectively.
1
In addition, the ratio of the protons integral height in the H-NMR spectra supports the
proposed structures. In addition, the places of primary, secondary and tertiary carbon atoms
were supported with 13C-APT NMR technique.
10

Fig. 3. 13C APT NMR spectrum of HCP-2 (in CDCl3)
3.2. I-V characteristics for Al/HCP-2/p-Si /Al and Al/HCP-4/p-Si /Al diodes
The diode characteristics for mono and di substituted dioxyphenylcoumarin bearing
cyclotriphosphazene compounds (HCP-2 and HCP-4) are shown in Fig.4. It is well known
that substitution doping process is very important in increasing free charge carriers [20]. As
can be seen, the Al/HCP-2/p-Si/Al diode of high rectification behaviour with rectification
ratio of at 3 volt. Schottky diode parameters such as ideality factor and
barrier height were calculated from (I-V) characteristics curve in dark and at room
temperature by using the thermionic emission theory [21-23]:
(
)
) [ ( ( ) )]
(1)
(
11

-5
-10
-15
-20
-25
HCP-2
HCP-4
-6
-4
-2
0
2
4
6
V (volt)
Fig. 4. – Characteristics of Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes in dark and at
room temperature.
where I is the current through a Schottky diode, is the applied voltage, q is the electronic
charge, k is the Boltzmann constant, is the absolute temperature, is the series
resistance, is the ideality factor can be calculated by using the following equation [24,25]:
( ) , and
Io is the saturation current given by [26,27]:
(
)
(2)
where, is the effective Richardson's constant equal (32 for p-type silicon), is the
effective diode area and is the zero bias effective barrier height is expressed by the following
relation [28]:
( ⁄ ) (
)
(3)
⁄
According to Fig.4 it can be observed that the forward current If of HCP-2 compound with
greater value compared with HCP-4 compound this behaviour is believed to be due to mono
substituted dioxyphenylcoumarin of high chemical molecular interaction and high electron
12

density [29,30]. The diode parameters of Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al devices have
been estimated and presented in Table.1. Al/HCP-2/p-Si/Al shows very small saturation current
compared to Al/HCP-4/p-Si/Al. It is also evaluated that the ideality factor n with values greater
than unity attributed by the presence of deep levels and interfacial states between organic
material and semiconductor. In order to determine the values of series resistance and shunt
resistance of the photodiode in dark and at room temperature we can use the equation [31]:
(4)
where, is the junction resistance of the diode. Fig.5 shows ( - ) plots for Al/HCP-2/p-
Si/Al and Al/HCP-4/p-Si/Al diodes. and for the synthesized diodes were calculated and
tabulated in Table 1. It is noted from Fig.5 that, the Al/HCP-2/p-Si/Al diode of low series
resistance and high shunt resistance which indicates that the diode of mono substituent
dioxyphenylcoumarin bearing cyclotriphosphazene compound is mostly favoured by larger
electron rich substituent and this is related to the arrangement of dioxyphenylcoumarin
groups [30-32].
2.5x1010
2.0x1010
1.5x1010
HCP-2
HCP-4
1.0x1010
5.0x109
0.0
-6
-4
-2
0
2
4
6
V (volt)
Fig. 5. Plot of vs. V for Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes in dark and at
room temperature.
13

On the other hand, the barrier height and series resistance of the photodiode can be
determined by other theory known as Nord’s method introduced by the equation [33]:
( )
( )
(
)
(5)
where, γ is the first integer greater than ideality factor . I(V) is the current obtained from the
I–V characteristics. The barrier height of the prepared diode can be obtained from equation
[34]:
( )
(6)
( )
( )
where the minimum point of demonstrated in Fig.6 and is the corresponding
voltage. The series resistance of the junctions can be calculated as follows [35]:
(
)
(7)
(
)
where I is the corresponding current. and of the synthesized diodes were determined
and listed in Table 1. It worth to note that, the Al/HCP-2/p-Si/Al diode of low series
resistance compared to other one which is consistent with the results obtained from I-V
characteristics.
3.0








2.8







2.6









2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6






























































































































HCP-2
HCP-4


















































0
1
2
3
4
5
V(volt)
14

Fig. 6. ( ) versus plots for Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes in dark and at
room temperature.
3.3. Photoresponse properties of Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes
The study of photoconductivity for both mono and di substituted dioxyphenylcoumarin
bearing cyclotriphosphazene compounds (HCP-2 and HCP-4) under different power
intensity plays a key role in optoelectronic applications [36]. It is seen in Fig.7 (a and b) that,
the prepared diodes with high photocurrent response and Al/HCP-2/p-Si/Al diode with high
photoconductivity and more of free carriers affected by incident light [37].
-5
-10
(a)
HCP-2
-15
in dark
10mW/cm2
30mW/cm2
60mW/cm2
-20
80mW/cm2
100mW/cm2
-25
-6
-4
-2
0
2
4
6
V(volt)
15

-6
-8
-10
-12
-14
-16
-18
-20
-22
(b)
HCP-4
in dark
10mW/cm2
30mW/cm2
60mW/cm2
80mW/cm2
100mW/cm2
-6
-4
-2
0
2
4
6
V(volt)
Fig. 7 (a and b). – plots (a) Al/HCP-2/p-Si/Al and (b) Al/HCP-2/p-Si/Al diodes under
various light intensities.
Fig. 8 (a and b) shows the influence of light intensity on the prepared device conductivity. It
can be seen that the current of the photodiode increase through incident of light and remain
constant until turning off light reach its initial value. This is dependent on the presence of
charge carriers trapped in localized states within band gap [38]. The trapped charge carriers
overcome the optical gap by increasing illumination power intensity suggests that these states
have significant influence on the transport mechanism [39].
8.0x10-9
7.0x10-9
6.0x10-9
(a)
HCP-2
5.0x10-9
10 mW/cm2
30 mW/cm2
4.0x10-9
60 mW/cm2
80 mW/cm2
100 mW/cm2
3.0x10-9
2.0x10-9
1.0x10-9
0.0
0
5
10
15
20
25
30
35
Time (sn)
16

1.6x10-5
1.4x10-5
1.2x10-5
1.0x10-5
8.0x10-6
6.0x10-6
4.0x10-6
(b)
(HCP-4)
10 mW/cm2
30 mW/cm2
60 mW/cm2
80 mW/cm2
100 mW/cm2
2.0x10-6
0.0
0
5
10
15
20
25
30
35
Time (sn)
Fig. 8 (a and b). Phototransient current plots of (a) Al/HCP-2/p-Si/Al and (b) Al/HCP-4/p-
Si/Al photodiodes.
It is observed from this behaviour that power intensity activate the transport of charge
carriers [40]. The photodiode conductivity affected by trap centres response to the influence
of various illuminations intensities this can be described according to the following relation
[41]:
(8)
where is the photocurrent, A is a constant, is the illumination intensity and m is an
exponent. According to Fig.9, the exponent m for Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al
diodes were determined from the slope with values 0.6 and 1.34 respectively. The values of
exponent m enhance the presence of trap centers within optical band gap which increase the
photodiode conductivity [42].
17

-8
-10
-12
-14
-16
-18
-20
-22
-24
HCP-2
HCP-4
2.0
2.5
3.0
3.5
4.0
4.5
5.0
lnP (mW/cm2)
Fig. 9. versus of Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes.
(
)
3.4. , ( ) and ( ) characteristics of the prepared photodiodes
Fig. 10 (a and b) shows the influence of applied frequency varies from 10 kHz to 1MHz on
capacitance-voltage ( ) and corrected capacitance-voltage ( ) of Al/HCP-2/p-
Si/Al diode. It can be seen from Fig.10 (a) that the capacitance is constant in forward region
and decrease with increasing frequency in the reverse bias voltages [43]. Moreover, a peak
appears at nearly -3.5 volt and shifts to higher voltage with applied frequency. This behaviour
can be explained by the change in transport of electric charge in depletion region or may be
due to recombination process influenced by applied alternating current voltage [44]. Fig.10
(b) indicates the ( ) plots for Al/HCP-2/p-Si/Al diode. The capacitance of the
photodiode affected by series resistance so it can be corrected by using the following
equations [45,46]:
(
)
[
]
=
(9)
(
)
[
(
) ]
=
(10)
where is the corrected capacitance, is the series resistance, is the angular
frequency and and are the measured capacitance and conductance respectively [47]. It
18

is noted that the corrected capacitance remains constant in forward region. In addition, there
is a peak appears at -1.5 volt in the reverse bias shifts irregularly to higher voltage [48].
1.00E-011
0.00E+000
-1.00E-011
-2.00E-011
-3.00E-011
-4.00E-011
-5.00E-011
-6.00E-011
10 kHz
50 kHz
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
(a)
-6
-4
-2
0
2
4
6
V (volt)
1.05x10-10
9.00x10-11
7.50x10-11
6.00x10-11
4.50x10-11
3.00x10-11
10 kHz
50 kHz
(b)
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
1.50x10-11
0.00
-6
-4
-2
0
2
4
6
V (volt)
Fig. 10 (a and b). (a) and (b) characteristics for Al/HCP-2/p-Si/Al diode.
Fig.11 (a and b) demonstrates the variation of capacitance-voltage ( ) and corrected
capacitance-voltage ( ) of Al/HCP-4/p-Si/Al diode with applied different frequencies
[49]. It is noteworthy that ( ) and ( ) varies independent of applied frequency
may be due to the change of the geometric and dielectric properties of the photodiodes
[50,51].
19

2.0x10-10
1.8x10-10
1.6x10-10
1.4x10-10
1.2x10-10
1.0x10-10
8.0x10-11
6.0x10-11
4.0x10-11
10 kHz
50 kHz
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
(a)
2.0x10-11
0.0
-6
-4
-2
0
2
4
6
V (volt)
10 kHz
50 kHz
2.0x10-9
1.5x10-9
1.0x10-9
(b)
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
5.0x10-10
0.0
-5.0x10-10
-6
-4
-2
0
2
4
6
V (volt)
Fig. 11 (a and b). (a) and (b) characteristics for Al/HCP-4/p-Si/Al.
(
)
The conductance-voltage and corrected conductance-voltage ( ) for
Al/HCP-2/p-Si/Al diode have been measured as a function of frequency and are given in Fig.
12 (a and b). According to Fig. 12 (a), the conductance changes with applied frequency. It is
observed that the conductance become constant in forward bias and starts to increase at -0.35
volt in the reverse bias [52]. On other hand, the conductance of the photodiode is affected by
series resistance can be expressed using the following equation [53]:
(
)
[
]
(11)
(
)
20

where
is the corrected conductance. Fig. 12 (b) shows the variation of corrected
conductance-voltage ( ) independence of applied various frequencies which confirms
the effect of series resistance and change of charge carriers distribution in the depletion
region with applied various frequencies [54].
1.0x10-3
10 kHz
50 kHz
(a)
8.0x10-4
100 kHz
200 kHz
300 kHz
400 kHz
6.0x10-4
500 kHz
600 kHz
700 kHz
4.0x10-4
800 kHz
900 kHz
1 MHz
2.0x10-4
0.0
-6
-4
-2
0
2
4
6
V (volt)
5.0x10-5
0.0
10 kHz
50 kHz
-5.0x10-5
-1.0x10-4
-1.5x10-4
-2.0x10-4
-2.5x10-4
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
(b)
-6
-4
-2
0
2
4
6
V (volt)
Fig. 12 (a and b). (a) and (b) characteristics for Al/HCP-2/p-Si/Al diode.
(
)
Fig. 13 (a and b) demonstrate the change of conductance-voltage and corrected
conductance-voltage ( ) of Al/HCP-4/p-Si/Al diode with applied frequency [55]. It can
(
)
be seen that both of and ( ) varies with applied frequency in the reverse bias
voltage and become constant in the forward bias voltage [55].
21

(a)
2.0x10-3
1.5x10-3
1.0x10-3
10 kHz
50 kHz
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
5.0x10-4
0.0
-6
-4
-2
0
2
4
6
V (volt)
2.0x10-3
1.5x10-3
1.0x10-3
10 kHz
50 kHz
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
(b)
5.0x10-4
0.0
-6
-4
-2
0
2
4
6
V (volt)
Fig. 13 (a and b). (a) and (b) characteristics for Al/HCP-4/p-Si/Al diode.
The series resistance of the prepared diodes can be determined from measured
capacitance and measured conductance using the following equation [56]:
(12)
( )
where, and are the measured capacitance and conductance, respectively. Fig. 14 (a
(
)
and b) demonstrates the variation of series resistance-voltage with applied different
frequencies for Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes respectively. It can be seen
that the series resistance of the two devices decrease with applied frequency [57]. Fig. 14 (a),
shows a peak appears at -0.35 volt in reverse bias and shifts to higher voltage. The
broadening of the peak increase with frequency suggests the influence of interface states to
22

applied frequency and the increase of charge carriers [58]. On other side, Fig.14 (b), indicates
two peaks appears at -1.15 volt and -0.15 volts in reverse bias and shift to positive voltage
with increasing frequency [59].
6x105
10 kHz
50 kHz
100 kHz
5x105
(a)
200 kHz
HCP-2
4x105
300 kHz
400 kHz
500 kHz
600 kHz
3x105
700 kHz
800 kHz
2x105
900 kHz
1 MHz
1x105
0
-1x105
-6
-4
-2
0
2
4
6
V (volt)
5x104
4x104
3x104
2x104
10 kHz
50 kHz
100 kHz
200 kHz
300 kHz
400 kHz
500 kHz
600 kHz
700 kHz
800 kHz
900 kHz
1 MHz
(b)
HCP-4
1x104
0
-6
-4
-2
0
2
4
6
V (volt)
Fig. 14 (a and b). ( ) characteristics for (a) Al/HCP-2/p-Si/Al and (b) Al/HCP-4/p-
Si/Al diodes.
The capacitance of a Schottky barrier diode can be determined from the equation [60]:
( )
(13)
23

where is the doping concentration, the built-in voltage, the applied voltage and is
⁄
the static dielectric constant and
. Fig. 15 (a and b) shows the
⁄
( ) plot measured at 1MHz frequency for Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al
diode respectively. The built-in voltage has been calculated from an extrapolation of the
⁄
⁄
linear ( ) to the – axis and is estimated from the slope of ( ) curve
[61]. The barrier height of the synthesized diodes can be investigated from the following
relation [62]:
[
]
(14)
where ( = 182 × 1019 cm−3) is the density of states in the valence band of silicon [63].
The values of barrier height of Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al diodes have been
(
)
estimated from measurements and are listed in Table 1. The results indicate that
there are high differences in the values of barrier height obtained from I-V and C-V
calculations [64]. This can be explained by the change of energy level at interface states or
the change of trapped charges due to the existence of deep levels and localized states at the
interfaces of organic-semiconductor contact [65].
24

7.0x1022
6.5x1022
6.0x1022
5.5x1022
5.0x1022
4.5x1022
4.0x1022
3.5x1022
3.0x1022
(a)
HCP-2
-6
-4
-2
0
2
4
6
V (volt)
1.2x1021
1.0x1021
8.0x1020
6.0x1020
4.0x1020
(b)
HCP-4
2.0x1020
0.0
-6
-4
-2
0
2
4
6
V (volt)
Fig. 15 (a and b). ( )for (a) Al/HCP-2/p-Si/Al and (b) Al/HCP-4/p-Si/Al diodes.
Table 1. Schottky parameters for Al/HCP-2/p-Si /Al and Al/HCP-4/p-Si/Al diode obtained
from ( ) characteristics, Nord’s equation and ( ) plots.
(I-V)
Nord’s model
(C-V)
photodiode
Characteristics
Measurements
215
( )
( )
( )
( )
( )
( )
2.29
1.71
0.87
2960
0.2
5
330
2.3
0.97
0.24
Al/HCP-2/p-Si /Al
0.79
8883
1.6
3.8
3.47
0.82
0.25
Al/HCP-4/p-Si /Al
25

4. Conclusions
Mono and di substituted dioxyphenylcoumarin bearing cyclotriphosphazene compounds
(HCP-2 and HCP-4) were prepared from the reaction of HCP-1 and HCP-3 with compound
1b, respectively. The photodiode characteristics of Al/HCP-2/p-Si/Al and Al/HCP-4/p-Si/Al
devices were investigated from and measurements. Electrical properties of the
synthesized diodes and Schottky parameters such as ideality factor , series resistance and
barrier height have been determined in dark and at room temperature from I-V
characteristic curve and Norde’s equation. The Al/HCP-2/p-Si/Al diode indicates a high
rectification ratio and high shunt resistance. The photoresponse properties of the diodes
indicate that the photodiodes exhibited a high photocurrent response. It is evaluated that the
fabricated diodes can be used a photosensor for solar tracking systems.
Acknowledgements
The authors are grateful to the Research Fund of the TUBITAK for their support (for the
synthesis of compounds) with the project No-110T652 and the photochemical applications
supported by the Firat University Research Fund (Project no: FF.16.19). Also, authors would
like to acknowledge the support of the King Khalid University for this research through a
grant RCAMS/KKU/002-16 under the (Research Center for Advanced Materials Science) at
King Khalid University, Kingdom of Saudi Arabia.
26

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