Frequency and electric field controllable photodevice: FYTRONIX device

Article abstract of DOI:10.1016/j.physb.2017.05.046

Al/p-Si/BODIPY/Al diode was fabricated by forming BODIPY organic layer on p-Si having ohmic contact. The electrical and photoresponse properties of the prepared diode were investigated in detail. The current-voltage (I-V) measurements were performed under dark and various illumination intensities. It is observed that the photocurrent under illumination is higher than the dark current. The transient measurements indicate that the device exhibits both photodiode and photocapacitor behavior. We called this device as FYTRONIX device. The photoresponse behavior of the FYTRONIX device is controlled simultaneously by frequency and electric field. The FYRONIX device can be used as a photoresponse sensor in optoelectronic applications.

Full text of DOI:10.1016/j.physb.2017.05.046

Author’s Accepted Manuscript
Frequency and electric field controllable
photodevice: FYTRONIX DEVICE
A. Tataroğlu, Abdullah G. Al-Sehemi, Mehmet
Özdemir, Resul Özdemir, Hakan Usta, Ahmed A.
Al-Ghamdi, W.A. Farooq, F. Yakuphanoglu
www.elsevier.com/locate/physb
PII:
S0921-4526(17)30272-7
http://dx.doi.org/10.1016/j.physb.2017.05.046
PHYSB309970
DOI:
Reference:
To appear in:
Physica B: Physics of Condensed Matter
Received date: 14 May 2017
Revised date: 23 May 2017
Accepted date: 24 May 2017
Cite this article as: A. Tataroğlu, Abdullah G. Al-Sehemi, Mehmet Özdemir,
Resul Özdemir, Hakan Usta, Ahmed A. Al-Ghamdi, W.A. Farooq and F.
Yakuphanoglu, Frequency and electric field controllable photodevice
FYTRONIX
DEVICE, Physica B: Physics of Condensed Matter
http://dx.doi.org/10.1016/j.physb.2017.05.046
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Frequency and electric field controllable photodevice:
FYTRONIX DEVICE
A. Tataroğlua, Abdullah G. Al-Sehemib,d, Mehmet Özdemire, Resul Özdemire, Hakan
Ustae, Ahmed A. Al-Ghamdif, W. A. Farooqg, F. Yakuphanogluh*
aDepartment of Physics, Faculty of Science, Gazi University, Ankara, Turkey
bDepartment of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box
9004, Saudi Arabia
cResearch Center for Advanced Materials Science, King Khalid University, Abha 61413, P.O. Box
9004, Saudi Arabia
dUnit of Science and Technology, Faculty of Science, King Khalid University, Abha 61413,P.O.
Box 9004, Saudi Arabia
eDepartment of Materials Science and Nanotechnology Engineering, Kayseri, Turkey
fDepartment of Physics, Faculty of Sciences, King Abdul Aziz University, Jeddah, Saudi Arabia
gDepartment of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi
Arabia
hDepartment of Physics, Faculty of Arts and Sciences, Firat University, 23169 Elazig, Turkey
*Corresponding author. fyhanoglu@firat.edu.tr (F.Yakuphanoglu)
Abstract
Al/p-Si/BODIPY/Al diode was fabricated by forming BODIPY organic layer on p-Si having ohmic
contact. The electrical and photoresponse properties of the prepared diode were investigated in
detail. The current-voltage (I-V) measurements were performed under dark and various
illumination intensities. It is observed that the photocurrent under illumination is higher than the
dark current. The transient measurements indicate that the device exhibits both photodiode and
photocapacitor behavior. We called this device as FYTRONIX device. The photoresponse behavior
of the FYTRONIX device is controlled simultaneously by frequency and electric field. The
FYRONIX device can be used as a photoresponse sensor in optoelectronic applications.
Keywords: BODIPY; organic thin film; Photodiode; Photoresponse device
1

1. Introduction
BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-sindacene) is a highly electron-deficient
molecular building block, which is considered as “porphyrin's little sister”. Since its
discovery in 1960s, it has fostered a significant improvement in structural versatility in the
field of organic chemistry, and a great number of molecules and polymers has been
developed and studied in a variety of applications such as chemosensors, fluorescent
switches, electroluminescent devices, and biochemical labeling [1-5]. To this end, although
the structural and electronic properties of BODIPY π-core offer unique advantages such as
structural co-planarity, high dipole-moment and energetically low lowest
energy unoccupied molecular orbital (LUMO) level, its semiconducting characteristics has
remained a relatively unexplored research field. Only, in the recent years, interest in
semiconductor compounds embedding BODIPY as key π-building block has increased. To
this end, some of us has recently reported the first BODIPY-based n-type molecular
semiconductor in the literature, BDY-3T-BDY, which carries BODIPY π-acceptor units at
molecular termini and terthiophene donor unit as the central π-core. The low LUMO
energy level (-3.7 eV) and good solubility of this semiconductor has enabled the solution-
based fabrication of n-channel organic field-effect transistors (OFETs) [6-7]. in bottom-
gate/top-contact device geometry with electron mobilities of 3×10-4 cm2/V·s and
impressive Ion/Ioff ratios of 107-108. To the best of our knowledge, this was the first
demonstration of n-channel semiconductivity for a BODIPY-based compound
demonstrating that π-extension along BODIPY’s meso-position leads to electron-transport,
which is in sharp contrast to our earlier findings and other groups’ observations as well that
π-extension through aromatic substitutions on BODIPY’s 2,6-positions leads to hole-
transport [6-11]. In these studies, BODIPY core has proven itself as a promising π-
2

conjugated building block with superior solution processibility and good charge-transport
behavior. Therefore, further electrical and optical characterizations to investigate these
BODIPY compounds are highly desirable to reveal their full potential in various
optoelectronic applications [13].
In this paper, we fabricated Al/p-Si/ BDY-3T-BDY /Al diode and studied its
electrical and photoresponse properties. These properties were investigated by
illumination-dependent current-voltage and the frequency-dependent admittance
measurements.
2. Experimental details
2.1. Synthesis processes of BDY-3T-BDY
2.1.1. Synthesis of 5-bromo-2-thiophenecarbaldehyde
To a solution of 2-thiophenecarboxaldehyde (6.0 g, 53.5 mol) in anhydrous CHCl3
(125 mL), N-bromosuccinimide (10.4 g, 58.9 mmol) was slowly added, and then the
reaction mixture was stirred for overnight at room temperature. The reaction mixture was
extracted with CHCl3, and the organic phase was washed with deionized water, dried over
Na2SO4, filtered, and evaporated to dryness to give the crude product. Column
chromatography was performed on silica gel using CHCl3 as the eluent, which affords the
1
pure product as a colorless oil (10.0 g, 98%). H NMR (400 MHz, CDCl3): δ 7.20 (d, H, J
= 4.0 Hz), 7.53 (d, H, J = 8.0 Hz), 9.79 (s, H).
2.1.2. Synthesis of 8-(2-bromothien-5-yl)-3,5-dimethyl-4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene (BDY-T-Br)
3

Trifluoroacetic acid (TFA) (4 drops) was added under nitrogen to a solution of 5-
bromo-2-thiophenecarbaldehyde (1) (1.05 g, 5.49 mmol) and 2-methylpyrrole (2) (1.0 g,
12.42 mmol) in degassed CH2Cl2 (330 mL), and the mixture was stirred at ambient
temperature overnight. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (1.25 g, 5.49
mmol) was then added to this solution, and the reaction mixture was stirred for additional 3
h. Finally, N,N-diisopropylethylamine (i-Pr)2EtN (3.9 g, 30.08 mmol) and boron trifluoride
diethyl etherate (BF3·Et2O) (2.72 g, 19.05 mmol) were added, and the reaction mixture was
stirred for 2 h. The reaction mixture was poured into water and extracted with CH2Cl2. The
organic phase was dried over Na2SO4, filtered, and evaporated to dryness to give a crude
product. The crude was purified by column chromatography on silica gel using
CH2Cl2:Hexanes (2:1) as the eluent, which yields the pure product as a crystalline red solid
(1.0 g, 48% yield). m.p. 132-133 °C. 1H NMR (400 MHz, CDCl3): δ 2.68 (s, 6H), 6.32 (d,
2H, J=4.0 Hz), 7.05 (d, 2H, J = 4.0 Hz), 7.19 (m, 2H). 13C NMR (100 MHz, CDCl3): 15.0,
116.9, 119.7, 130.1, 130.6, 131.7, 133.2, 133.9, 136.1, 158.2.
2.1.3. Synthesis of 5,5''-Bis(3,5-dimethylBODIPY-8-yl)-2,2':5',2''-Terthiophene (BDY-
3T-BDY)
The reagents BDY-T-Br (0.72 g, 1.88 mmol), 2,5-bis(trimethylstannyl)thiophene
(0.36 g, 0.90 mmol), and Pd(PPh3)2Cl2 (0.02 g, 26.7 µmol) were suspended in anhydrous
toluene (45 mL) and the reaction mixture was heated at 110 °C under nitrogen for 48 h.
Then, the reaction mixture was cooled down to room temperature and evaporated to
dryness. The crude product was filtered and washed with methanol to give a dark crude
solid, which was purified by column chromatography on silica gel with CHCl3:Hexanes
(1:1) as the eluent to give final pure product as a dark red solid (0.33 g, 54% yield).
1
Tdecomposition > 250 °C. H NMR (400 MHz, CDCl3): δ 2.68 (s, 6H), 6.35 (m, 2H), 7.15(m,
4

13
2H), 7.22-7.42 (m, 3H). C NMR (100 MHz, CDCl3): 15.0, 119.5, 119.7, 124.4, 124.5,
125.0, 125.2, 125.7, 130.1, 132.6, 132.8, 133.8, 134.7, 140.4, 157.6, 157.7, 158.1.
MS(MALDI-TOF) m/z (M+): calcd. for C34H26B2F4N4S3: 684.14, found: 684.96 [M+H]+,
664.88 [M-F]+, 602.78 [M-4×CH3-F]+, 583.70 [M-4×CH3-2×F]+. Anal.calcd. for
C34H26B2F4N4S3: C, 59.67; H, 3.83; N, 8.19, Found: C, 59.83; H, 4.07; N, 7.89.
The synthesis of BDY-3T-BDY is shown in Scheme 1 and it’s synthesis was
performed in three steps in accordance with our previous reported procedure [1].
Scheme 1. Synthesis of BDY-3T-BDY
2.2. Fabrication of Al/p-Si/ BDY-3T-BDY /Al Diode
The diode was prepared using p-type silicon wafer and BDY-3T-BDY organic
material. Firstly, a p-type silicon substrate was chemically cleaned using some chemical
baths. After cleaning procedure, Al ohmic contact was prepared to p-Si wafer by thermal
evaporating of Al and then, it was annealed at 570 oC for 5 min in nitrogen atmosphere.
After ohmic contact procedure, the solution of BDY-3T-BDY organic compound
o
was prepared and the solution was drop coated on p-type silicon wafer and dried at 50 C
for 20 min. Al metal as a top contact was prepared on the organic layer having a diode
contact area of 7.85×10-3. Electrical measurements of the diode were carried out by using a
FYtronix FYED-500 Electronic Device characterization system at room temperature.
5

3. Results and Discussion
3.1. Current-voltage (I-V) characteristics of the diode
The current-voltage (I-V) characteristics of the fabricated diode are shown in Fig.1.
As seen in Fig.1, the current of the diode in lower forward bias voltages is increased
exponentially. This indicates that the diode exhibited a rectifying behavior. This behavior
can be analyzed by the conventional thermionic emission (TE) theory. According to TE
theory, the relationship between current (I) and voltage (V) for forward biases of V >
3kT/q is given as [13-14],

q(V  IR )





s
I  I exp
1
(1)
o


nkT




where I0 is the reverse saturation current, V is the applied bias voltage, Rs is the series
resistance, n is the ideality factor, k is the Boltzman’s constant, and T is the absolute
temperature. I0 is extracted from the straight line intercept of ln(I) at V=0 and is given by,
q
kT




I  AAT 2exp 
b 
(2)

0
where A is the diode area, Φb is the zero-bias barrier height, and A* is the effective
Richardson constant which is equal to 32 A/cm2.K2 for p-type Si. The Φb value is obtained
from Eq. (2). The ideality factor (n) is extracted from the slope of the linear region of lnI-V
plot, and its value should be 1 for ideal Schottky diode. From Eq. (1), the ideality factor
can be written as


q
dV




n 
(3)
kT d(ln I)


6

The photoresponse properties of the Al/p-Si/BODIPY/Al diode was evaluated by
measuring the I-V characteristics under illumination. Fig. 1 shows the semi-logarithmic
forward and reverse bias I-V plots of the diode in dark and under various illumination
intensities. As seen in Fig. 1, the diode shows a good rectifying behavior. Under
illumination, photocurrent in reverse bias is higher than the dark current, and its value
increases with increasing illumination intensity. This indicates that the diode exhibits a
photoconducting behavior. This also explains that the resistance of the diode decreases
with illumination. When a diode is illuminated, the electron-hole pairs are generated in the
depletion region due to absorption of photons. The photogenerated charge carriers such as
electrons and holes will create an additional current flow across the junction, and they must
be added to the dark current [15-21]. In addition, under forward bias, no differences were
observed between the dark and photocurrent.
Fig. 1. The semi-logarithmic current-voltage (I-V) plots of the diode.
The value of ideality factor and barrier height for dark was found to be about 3.1 and 0.82
eV, respectively. This barrier height is found to be slightly increased from the vacuum
7

level energy difference between Aluminum work function (4.3 eV) and the LUMO energy
level of BDY-3T-BDY (-3.7 eV) probably due to the formation of interface dipoles.) For
all illumination, the values of these parameters was found to be about 3.5 and 0.80 eV. The
obtained n value is greater than 1. This can be attributed to the existence of the interface
layer and interface states, barrier inhomogeneous, and series resistance [1,2,22-24].
Additionally, Norde method is an alternative method to find out the magnitude of
barrier height. According to this method, Norde’s function, F(V), is plotted against voltage
(V) and is expressed as [25,26],
V
kT
q
I




F(V) 

ln

AA*T2


(4)
where γ is an integer greater than the ideality factor. The barrier height is given by
Vmin kT
b  F(Vmin ) 


q
(5)
where F(Vmin) is the minimum value of F(V) and Vmin is the corresponding voltage. The
series resistance is given by
kT(  n)
Rs 
(6)
qImin
Fig. 2 shows plots of F(V) for dark and 100 mW/cm2. As seen in Fig. 2, the F(V)
plots give a minimum point.. The barrier height values for dark and 100 mW/cm2 were
found to be 0.86 and 0.84 eV, respectively. These values are in good agreement with the
values obtained from the I-V method. For these conditions, the series resistance values
were found to be 171 kΩ and 122 kΩ. This indicates that the series resistance decreases
with illumination.
8

Fig. 2. Plots of F(V) vs. V of the diode.
The photoresponse properties and photoconduction mechanism of the diode can be
analyzed by the following relation [27,28],
Iph  APm
(7)
where Iph is the photocurrent, A is a constant, m is an exponent and P is the illumination
intensity. The m value is extracted from the slope of log (IPh) versus log (P) plot. Values of
m between 0.5 and 1 indicate the presence of a continuous distribution of localized states.
A higher value of m indicates a lower density of the unoccupied trap level. A higher value
of m indicates a lower density of the unoccupied trap level [28-31]. Fig. 3 shows the
variation of photocurrent with various illumination intensities. The value of m is found to
be 1.18. This m value indicates that the photoconduction mechanism of the diode exhibits a
linear behavior.
9

Fig. 3. Plot of log(Iph) vs. log(P) of the diode (at -4 V).
3.2. Transient photocurrent/photocapacitance/photoconductance-time characteristics
To analyze the photoresponse stability of the diode, time-dependent transient
photocurrent measurements were used. Fig. 4 shows the transient photocurrent-time plots
of the diode. The photocurrent increases under illumination, and then decreases quickly
when the illumination is turned off. The increase in photocurrent is due to the increase in
the number of photogenerated charge carriers. The decrease in photocurrent is attributed to
the trapping of the charge carriers in the deep levels [32-36].
Fig. 4. Transient photocurrent-time plots under various illuminations.
10

The transient photocapacitance and photoconductance are a measure of the density of
photogenerated carriers in the space charge region. Fig. 5(a) and (b) show the transient
photocapacitance-time and photoconductance-time plots of the diode at 10 kHz,
respectively. As seen in Fig. 5(a), in case of turning off the illuminating, the
photocapacitance increases rapidly to a certain value. In case of turning off, the
capacitance suddenly come back to initial value. Moreover, the photocapacitance value
increases with increasing illumination intensity. As seen in Fig. 5(b), the photoconductance
value decreases with increasing illumination intensity. These measurements suggest that
the diode exhibits a photocapacitive behavior.
Fig. 5. Transient (a) photocapacitance-time (b) photoconductance-time plots at 10 kHz and
under various illuminations.
3.3. Capacitance/conductance-voltage (C/G-V) characteristics
The capacitance-voltage (C-V) and conductance-voltage (G-V) plots at various
frequencies are given in Fig. 6(a) and (b), respectively. As seen in these figures, in reverse
bias region, the capacitance value decreases, while the conductance value increases with
increasing frequency. In forward bias region, they do not change with frequency. The
frequency dependence is attributed to the existence of interface states in the interface of the
diode. At low frequencies, the interface states are very sensitive to the alternating current
11

signal (ac). But, their sensitivity decreases at high frequencies (f≥500 kHz). Therefore, the
interface states cannot follow the ac signal at high frequencies and cannot contribute to the
total capacitance of the diode [13,37-42].
Fig. 6. (a) capacitance-voltage (C-V) and (b) conductance-voltage (G-V) plots at various
frequencies.
The series resistance (Rs) of the diode is determined by the following the equation
[37,43],
Gma
(8)
Rs 
2
G2  C

ma 
ma
where Cma and Gma are the measured capacitance and conductance, respectively. The Rs-V
plots at various frequencies are given in Fig. 7. As seen in Fig. 7, the Rs value decreases
with increasing frequency. This behavior of Rs is result from the frequency dependence of
interface charges. Also, in reverse bias region, the Rs-V plots give a peak. The magnitude
of peaks is attributed to the particular distribution of interface states [40-44].
12

Fig. 7. The series resistance-voltage (Rs-V) plots at various frequencies.
The measured capacitance and conductance are affected by series resistance and
corrected by the following equations [33,43,45],

Gm2  (Cm )2

Cm
CADJ

(9)
a2  (Cm )2

Gm2  (Cm )2
a2  (Cm )2

a
GADJ

(10)
and
2
2


a  Gm  Gm  (Cm) Rs
(11)
where CADJ and GADJ is the corrected capacitance and conductance, respectively. Fig. 8(a)
and (b) show CADJ-V and GADJ-V plots at various frequencies, respectively. From Fig. 8(a),
it is observed that the value of CADJ decreases with increasing frequency. As seen in Fig.
8(b), the value of GADJ increases with increasing frequency. Furthermore, the GADJ-V plots
gives a peak in the reverse bias voltage. This peak is attributed to the existence of series
resistance and interface states.
13

Fig. 8. (a) CADJ-V and (b) GADJ-V plots at various frequencies.
The interface state density (Nss) can be determined by using Hill-Coleman method
[46]. According to this method, the Nss is given by,
2
(Gm /)max
Nss 
(12)
qA

((Gm /)max Cox)2  (1 Cm /Cox)2

where (Gm/ω)max is the maximum measured conductance and Cox is the oxide capacitance.
The variation of Nss with frequency is given in Fig. 9. As seen in Fig. 9, the Nss value
decreases with increasing frequency. This behavior of Nss is attributed to the decrease in
numbers of interface charge carries.
14

Fig. 9. Variation of Nss with frequency
4. Conclusions
The electrical and photoresponse properties the Al/p-Si/BODIPY/Al diode with
organic layer have been investigated by using I-V, C/G-V characteristics. The fabricated
diode indicates a good rectifying behavior and exhibits photovoltaic behavior. The diode
exhibited both photodiode and photocapacitor behavior. The obtained experimental results
suggest that the prepared diode can be used as a photodiode in optoelectronic applications.
Acknowledgements
The authors extend their appreciation to the International Scientific Partnership
Program ISPP at King Saud University for funding this research work through ISPP#
0046.
15

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