Synthesis, physicochemical and spectroscopic characterizations of azochalcone derivative thin films towards solar cells application

Article abstract of DOI:10.1016/j.jphotochem.2016.09.025

A novel azochalcone derivative (ACh) has been synthesized by the reaction of azo-ketone and 4-anisaldehyde in ethanolic NaOH. The synthetized compound was characterized by FT-IR, ¹H NMR, and Mass spectra. Thin films with various thickness of ACh were deposited by thermal evaporation technique onto glass, quartz and Si wafer substrates under vacuum of 10^?5?Torr. Structural analysis of these thin films was investigated by the field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) patterns. The optical constants such as: refractive index (n), extinction coefficient (k) and optical band gap (E_g) of pristine and annealed films have been studied in the wavelength region 200–2500?nm. Analysis of the absorption coefficient, α, spectrum shows that the most probable transition is the indirect allowed transition. The onset and fundamental optical band gaps for ACh film are found to be 2.44 and 3.37 eV, respectively. The annealing temperature changes the optical constants. Tunable structural and optical properties of ACh open up the avenue to building up the solar cell based on ACh. The electronic transport mechanisms and photovoltaic properties of the fabricated solar cell of Au/ACh/p-Si/Al have been studied.

Full text of DOI:10.1016/j.jphotochem.2016.09.025

Accepted Manuscript
Title: Synthesis, physicochemical and spectroscopic
characterizations of azochalcone derivative towards solar cells
application
Author: M.M. Makhlouf A.S. Radwan M.R.E. Aly
PII:
S1010-6030(16)30382-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.09.025
JPC 10377
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
16-5-2016
21-9-2016
23-9-2016
Please cite this article as: M.M.Makhlouf, A.S.Radwan, M.R.E.Aly, Synthesis,
physicochemical and spectroscopic characterizations of azochalcone derivative towards
solar cells application, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.09.025
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Synthesis, physicochemical and spectroscopic characterizations of
azochalcone derivative towards solar cells application
M. M. Makhlouf 1,2,*, A. S. Radwan3,4, M. R. E. Aly 5,6
1Department of Physics, Faculty of Applied Medical Sciences, Taif University, 21995, Saudi Arabia.
2Department of Physics, Faculty of Science, Damietta University, 34517 New Damietta, Egypt.
3Department of Chemistry, Faculty of Applied Medical Sciences, Taif University, 21995, Saudi Arabia.
4Chemical and Environmental Affairs Sector, Ministry of Electricity and Energy, Ismailia, Egypt
5Department of Chemistry, Faculty of Science, Port Said University, Egypt
6Department of Chemistry, Faculty of Science, Taif University, 21995, Saudi Arabia.
1

Graphical Abstract
Dark
Illumination
e-11
e-13
e-15
e-17
e-19
Incident light
Isc =142 A
Expermental data
Theoretical fit
-4
1.4x10
Au grid
-4
1.2x10
-4
1.0x10
Im =89 A
PM
To the measuring
circuit
ACh
p-Si wafer
-5
8.0x10
-5
6.0x10
Al electrode
FF =0.38
 =1.8 %
-5
O
4.0x10
power curve
-5
2.0x10
N
N
O
V =0.36 V
oc
0.0
O H
0.0
0.1
0.2
0.3
N
Voltage, V
-1.0
-0.5
0.0
0.5
1.0
Voltage(V)
2

Highlights
 Novel azochalcone dye (ACh) was synthesized and characterized
 The structure analysis of ACh dye showed a nanocrystal structure based thin films.
 The optical constants and optical band gap of ACh dye thin films were calculated.
 Hybrid Au/ACh/p-Si/Al heterojunction solar cell was fabricated.
 Electronic transport and photovoltaic characterization of the solar cell were studied.
3

Abstract
A novel azochalcone derivative (ACh) has been synthesized by the reaction of azo-ketone and
4-anisaldehyde in ethanolic NaOH. The synthetized compound was characterized by FT-IR,
1H NMR, and Mass spectra. Thin films with various thickness of ACh were deposited by thermal
evaporation technique onto glass, quartz and Si wafer substrates under vacuum of 10-5 Torr.
Structural analysis of these thin films was investigated by the field emission scanning electron
microscopy (FE-SEM) and X-ray diffraction (XRD) patterns. The optical constants such as:
refractive index (n), extinction coefficient (k) and optical band gap (Eg) of pristine and annealed
films have been studied in the wavelength region 200–2500 nm. Analysis of the absorption
coefficient, α, spectrum shows that the most probable transition is the indirect allowed transition. The
onset and fundamental optical band gaps for ACh film are found to be 2.44 and 3.37 eV,
respectively. The annealing temperature changes the optical constants. Tunable structural and optical
properties of ACh open up the avenue to building up the solar cell based on ACh. The electronic
transport mechanisms and photovoltaic properties of the fabricated solar cell of Au/ACh/p-Si/Al
have been studied.
Keywords: Chalcones; Synthesis; Thin films; Optical properties; Electrical Mechanisms; Solar cells.
*Corresponding author: M.M. Makhlouf
Email: m_makhlouf@hotmail.com; m.makhlouf@edu.tu.sa
1. Introduction
Chalcones, as one of the well-known class of organic compounds which are mostly present in
flavonoids [1]. Chemically, they are open chain of flavonoids consisting two aromatic rings having
diverse array of substitutes linked by α, β-unsaturated carbonyl groups. Chalcones possess
conjugated double bonds and a completely delocalized π-electron system on both benzene rings.
Most of chalcones can be synthesized in laboratory by simple and inexpensive method depends on
the condensation product of with aromatic aldehydes in the presence of strong base which is called
Claisen-Schmidt reaction [2, 3]. Because of the ketovinylenic group (–CO–CH=CH-) and
4

π-conjugated bridges in chalcones and their analogs, they exhibit numerous chemical and physical
properties, for instance optical and fluorescence properties [4, 5] and electric properties [6,7].
Chalcones have attracted much attention for several decades due to their potential technological
application in fields of optical and photochemical sensors [8, 9], ultrafast optical nonlinearities [10],
optical filters [11], optical switch [12], fluorescent probes [13] optical computing and optical
communication [14, 15]. The structure-property relationships of 3,4,5-trimethoxy chalcone explored
by Bing Gu et al. [16]; showed that the chalcone derivatives are highly promising in photovoltaic
devices applications. The synthetic flexibility, a strong donor-acceptor intermolecular interaction and
delocalized π-electrons system and lower dielectric constants are main features of chalcones [17].
Furthermore, chalcones have charge transfer character of the low-lying π-π* transition from the
phenyl ring to the polyene chain allows one to vary the energy of the intense transition introducing
substituents in the phenyl ring [18]. Electronic structure of chalcone derivatives was explored mostly
in their singlet π-π* states in order to analyze the absorption spectra of substituted chalcones.
Substituents in the phenyl ring conjugated with -C=C- group affect the absorption maximum with a
greater extent than those in the phenyl ring conjugated with –C(=O)– group [18]. Both electron–
donor (E-D) and electron–acceptor (E-A) substituents cause a decrease in the energy of the Sπ-π*
-
state. These promising features of chalcones allow the researchers to design new chalcone molecules
substituted for donor or acceptor groups, which gives an understanding of structure-property
relationship. Pyrrole-chalcone derivatives have been synthesized using chloro- and nitro-substituted
1
aromatic aldehyde. The products were characterized by H NMR, UV–Vis., FT-IR spectroscopic
methods and quantum chemical calculations. The UV–Vis. spectra study reveals that the compounds
are almost transparent and photosensitive in the visible region [19]. The chalcone-cyclophosphazene
compounds containing dioxybiphenyl groups were synthesized by using of K2CO3/acetone system
[20]. Komarova et al. [21] studied the solvent effect on the geometry and energies of the low-lying
excited states of chalcone and its amino-derivatives using quantum chemical approach. The solvent
5

effect on the geometries and the energy shifts due to dipole–dipole interaction with the medium. The
metal complexes based chalcones with metals (Cu, Ni, Co, Pt, Cd and Zn) possess a novel structure
of interesting features and promising optical properties [22, 23].
Azo-coupling is one of the most important reactions for combining derivatives of aromatic
rings and preparing azobenzene derivatives containing active functional groups as a precursor for
further synthesis to give different organic molecules such as: azo-amide [24] and azo-imine [25].
Hence, based upon the promising properties of chalcones and azo compounds and in continuation for
search for newer molecules for better properties an attempt has been made to synthesize azochalcone
derivative (ACh) and screening for its physicochemical features. Herein, we report a simple method
[2, 3] to synthesize azochalcone derivative from the corresponding aldehyde and ketone using Aldol
condensation. The produced ACh was investigated by studying spectroscopic and physicochemical
features.
Keeping in view the importance of thin film technology that plays an important role in the
high-tech applications such as: nano- and micro-electronics, optoelectronics, photovoltaic devices
and dye-sensitized solar cells (DSSCs), we have decided to prepare the thin films of ACh using the
thermal evaporation technique and study the structural, optical and electrical properties of these
films. Furthermore, the hybrid heterojunction solar cell based ACh/p-Si has been fabricated. The
electrical transport mechanisms of this cell were studied and its photovoltaic parameters were
evaluated.
2. Experimental details
2.1. Syntheses of azochalcone derivative, (E)-1-(4-((E)-(8-hydroxyquinolin-5-yl)
diazenyl)phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one, (ACh).
Step 1: A solution of aryl diazonium salt was prepared by adding NaNO2 (0.01 mol, 10 ml
H2O) to cold solution of 4-aminoacetophenone (0.01 mol, ) in 3 ml conc. HCl with stirring. The
6

freshly prepared solution was added with continuous stirring to cold solution of 8-hydroxyquinoline
(0.01 mol, 1.45 g) in NaOH (30%, 10 ml). The reaction mixture was stirred at 0-5 °C for 2 h, diluted
with water, and then neutralized by dil. HCl. The dark red precipitate was filtered, washed
thoroughly with H2O until neutral to litmus paper then dried well. Upon recrystallization from EtOH,
intermediate azodye 3 [26] was obtained as in (74 %) yield.
Step 2: A mixture of 4-methoxy benzaldehyde (1.36 g, 0.01mol), intermediate 3 (2.91 g,
0.01mol) and NaOH (1.2 g, 0.03mol) in EtOH (20 ml) was stirred at ambient temperature overnight.
The reaction mixture was neutralized with AcOH and the brown precipitate was filtered at the pump,
washed thoroughly with H2O and dried well. Recrystallization from EtOH afforded conjugate ACh

as fine brown yield (68%); m.p. 255 - 258 C; IR (ύ, cm1): 34503163 (OHstr.), 3059 (CHstr.),
2839 (CH3str.), 1675,1654 (COstr.,), 1598, 1568, 1503 (CNstr., CCstr., NNstr.), 1200 (Ar COstr.),
1
1023 (CH3Ostr.), 808, 782 (ArHdef); H NMR (400 MHz, DMSOd6):  9.11 (s, 1H, Ar), 8.56 (s,
1H, Ar), 8.15 (d, 1H, J 8.4 Hz, Ar), 8.147.70 (m, 7H, COCHCH, 6 HAr), 7.68 (d, 1H, J, 15.6
Hz, COCHCH), 7.47 (s, 1H, Ar), 7.12 (d, 1H, J 9.2 Hz, Ar), 7.02 (d, 1H, J 8.8 Hz, Ar), 6.41 (s, 1H,
Ar OH), 3.82 (s, 3H, OCH3); EIMS (m/z, %): 409 (M+, 38), 294 (16), 251 (11), 250 (70), 205 (30),
204 (100). C25H19N3O3 (409.44)
2.2. Thin films preparation
The ACh dye was flashed through column chromatography of silica gel (Baker, 30–60 µm)
for more purification. The homogeneous thin films of ACh were thermally prepared by a
conventional thermal evaporation technique (Edward Coating Unit E-306) onto pre-clean flat glass,
quartz and Si wafer substrates for structural, optical, electrical measurements, respectively, at room
temperature under pressure of 10-5 Torr. ACh was sublimated using quartz crucible that was
subjected to induction heating from molybdenum heater boat. The thickness of the films was
7

measured by using a quartz crystal monitor (Maxtek. Inc., TM-350, USA) controls the rate of
deposition (2.5 Å/s) and film thickness (140 nm) for structural and optical measurements.
2.3 Fabrication of solar cell
The thin film of ACh with a thickness of about 90 nm was deposited on the front surface of
p-Si wafer substrate and then the layer of ACh was over coated by a mesh Au electrode was
deposited through a special mask on the surface of the organic thin film. The back side of Si was
coated by a thick layer of Al electrode. Au and Al electrodes were thermally evaporated directly
from basket-shaped tungsten filaments. Thus, an Au/ACh/p-Si/Al heterojunction solar cell was
designed. The effective area of the solar cell is 0.12 cm2. The annealing temperature of the fabricated
heterojunction at 373 K for 2 hours is needed to enhance the performance of the junction.
3.4. Measurements
Melting points were determined on Electrothermal apparatus and are uncorrected. Flash
chromatography was carried out on silica gel (Baker, 30–60 µm). TLC Monitoring tests were carried
out using eluent (tolune: acetone: isopropanol) (2: 2: 1), plastic sheets precoated with silica gel 60
F245 (layer thickness 0.2 mm) purchased from Merck. Spots were visualized by their fluorescence
under UVlamp (λ 245 and 366 nm). NMR spectra were recorded on Bruker 400 MHz
spectrometers. IRspectra were recorded on ATRAlpha FTIR Spectrophotometer from 400 to
4000 cm1. Shimadzu GCMS-QP-1000EX mass spectrometers at 70 eV. Analysis of X-ray
Diffraction (XRD) pattern for the powder and thin films of ACh were performed at room
temperature by using Philips X-ray diffraction system (model X/ Pert Pro.) with utilized
monochromatic CuKα radiation of λ=1.5418 Å. The structural morphology of the pristine and
annealed thin films of ACh were investigated using field emission scanning electron microscope,
FE-SEM, (model FEI QUANTA 250 FEG) after coating films with thin layer of Au.
8

A JASCO (V-570 UV-VIS-NIR) double-beam spectrophotometer is used for measuring The
transmittance, T(λ), and reflectance, R(λ), spectra of the films. The T(λ( and R(λ) were measured as a
function of wavelength of the incidence of light in the spectral range 200 - 2500 nm.
The electrical and photovoltaic properties of the heterojunction device were investigated
by I–V measurements using source-meter (Keithly type 2635A). The measurements were carried out
at in dark / light conditions. The intensity of incident light from tungsten filament lamp was recorded
by a calibrated digital light meter (Lutron-Model LX-107) situated at the same level position of the
device and is calculated as 100 W/m2.
3. Results and discussion
3.1. Chemical characterization
The target ACh dye was prepared by modular synthesis starting with 8-hydroxychinoline 1
and p-Aminoacetophenone 2. Thus, diazotization of compound 2 with NaNO2/HCl followed by
coupling with compound 1 afforded smoothly the intermediate acetylated azodye derivative 3.
Taking the advantage of the acetyl tag in compound 2, ClaisenSchmidt condensation was easily
performed at ambient temperature in the presence of 3.0 equivalents of NaOH to afford conjugate
ACh in 68% yield (Scheme 1).
The FT-IR spectrum of ACh compound (Fig. S1 in Supplementary material) displayed a
broad OH stretching vibration band at 34003160 cm1. The band at 3059 cm1 refer to aromatic
and olefinic CH stretching vibrations, while that at 2838 cm1 is for symmetric vibrations of CH
bonds in the CH3 group. The stretching vibration of the CO bond was observed at 1675 cm1. This
expected bathochromic shift is attributed to conjugation of the group with cloud of the benzene
ring and the olefinic linkage. The weak band at 1675 cm1 is attributed to overtone of the CO
stretching band with the CH bending modes that is known as Fermi resonance. The bands at 1598,
9

1568 and 1503 cm1 could not be clearly assigned but they are related to the stretching vibrations of
the CN, CC and NN bonds. The ArO stretching band was noticed at 1200 cm1, while the
OCH3 band was normally observed at 1023 cm1. The hypsochromic shift of the first band is related
to strengthening of the ArO linkage by the +M effect of the etheric oxygen. Despite the two aryl
groups in the molecule are p-substituted, two out-of-plane ArH bending bands were observed at
808 and 782 cm1 due to the two groups.
In the 1H NMR spectrum of ACh compound as shown in Fig. S2 in Supplementary material, the CH3
group was observed as a singlet at  3.82 ppm. The OH signal was assigned as singlet at  6.41
ppm and this signal disappeared by deuteration. Other protons were observed in the aromatic range.
Both signals were a good evidence for the progressing of ClaisenSchmidt condensation. In addition,
the doublet  7.68 ppm with a large coupling J, 15.6 Hz is another proof of this reaction and it is
attributed to the -carbon that is less deshielded than the carbon of the olefinic linkage. This high
coupling confirms the transorientation about the olefinic group. The EI-MS of ACh compound
(Fig. S3 in Supplementary material) displayed a molecular ion peak at m/z 409 that matches the
calculated mass for the molecular formula of the molecule (C25H19N3O3). This peak was one-third as
intense as the base one at m/z 204. Despite, the major peaks could not be interpreted by common
fragmentation mechanisms of chalcones and azo-dyes, it seems that the fragmentation mode shown
in (Scheme 2) explains the peak at m/z 251.
3.2 Structural properties:
The structure analysis was carried out using XRD and FESEM for pristine and annealed ACh
thin films on the glass substrates. Fig. 1(a) shows the XRD pattern of as-synthetized powder form of
ACh; It was noted that, XRD pattern has many diffraction peaks with different intensities which
indicates that, it has a polycrystalline structure. The broad halo in the diffraction pattern is due to
amorphous character is evidenced by the XRD pattern for pristine and annealed ACh thin films as
10

shown in Fig. 1(b). A small single peak as observed around (2θ = 39.3º) can be attributed to the
nanocrystallite structure of annealed thin film as shown in Fig. 1(c).
The FE-SEM investigation reveals the formation of films with different topographies. Fig. 2
displays FE-SEM images of pristine and annealed ACh thin films. As seen in Fig. 2(a), the
morphology of a pristine film is characteristic of an amorphous structure and this character is
evidenced by the XRD pattern as seen in Fig. 1(b). Fig. 2(b) shows the structure of thermally
annealed ACh film that contains nanocrystals distributed uniformly over the image, which is
consistent with the result of XRD technique and confirmed that the structure of annealed film of
ACh is nanocrystallites dispersed in amorphous matrix structure.
3.3 Optical properties
Analysis of the absorption spectra of the semiconductor materials can be used for the
investigation of the important factors such as: determining the light-harvesting capacity, the type of
most probable electronic transitions and the value of the optical energy band gap for these materials.
Fig.3 shows the absorption spectrum for the pristine and annealed ACh thin films as a function of the
wavelength in the UV-Vis. spectral region from 200 to 700 nm. The absorption spectrum exhibits
two transition bands that can be attributed to the orbital molecular transition from bonding to
antibonding molecular orbital. The absorption bands of ACh depend on aromatic rings, C=C of
chalcone and azo functional group. The absorption band due to azo group is located in the visible
region of spectra as absorption peak at 382 nm, which is ascribed to the n→π* transition. The
chalcone group and aromatic rings have absorption peak at 241 nm in UV. region and this band is
ascribed to the absorption of the π→π* transition. Annealing temperature at 473 K decreases the
value of the absorption over the spectral range of UV-Vis. region.
The transmittance, T (), and the reflectance, R(), spectra are measured in the wavelength
ranged from 200 to 2500 nm of pristine and annealed ACh thin films are depicted in Fig.4. The T (λ)
and R (λ) spectra can be divided into two regions depending on the wavelength of incident photons,
11

λ: region I at shorter wavelengths, (  680 nm); the total sum of T
and R
is less than unity
()
()
due to absorption spectrum, this means there is no light transmitted out the ACh films and all
photons of incident light in this region almost absorbed. These films are considered to be a good
absorbers for light in UV spectrum, where minimum and maximum percentage values of T
and
()
R
are 0 – 11%, and 0 – 5%, respectively; for the pristine ACh film. Therefore, this film might be
()
used as an optical filter in the limited range of spectrum. Region II, at longer wavelengths (  680
nm), no absorption takes place and the percentage of T(λ) ( ≥ 92%) is very greater than the
percentage of R(λ) ( ≤ 8%) and their total sum approaches unity (
). In this region, the
T() R() 1
films are transparent for the light and the value of absorption spectrum approaches to zero.
Therefore, in that transparent region; we can calculate the values of refractive index, absorption
coefficient and dispersion parameters for these films. Annealing ACh films at 473 K increases the
values of T(λ) and R(λ) spectra and shifts the transmittance edge to the low wavelengths side which
reflects in the variation of the value of the optical energy band gap.
The complex refractive index, n*, of the films can be expressed in terms of real part as a
refractive index, n, and imaginary part as an extinction coefficient, k, where n* = n + ik. The n is
related to the optical dispersion parameters, while k provides a measure of dissipation rate of the
electromagnetic wave in the dielectric medium.
The values of refractive index, n, and the extinction coefficient, k, of ACh films deposited
onto thick non-absorbing quartz substrates can be computed by using the absolute values of T and R.
These absolute values are calculated according to the method reported in previous works [27, 28].
The calculated values of refractive index, n, and the extinction coefficient, k, were computed by a
program comprising a modified bi-variant search technique [29 - 31] based on Murmann's exact
equations [30, 31] and satisfy the solving of minimizing T 2 and R 2 simultaneously, where
2
T 2  T (n,k,d,) T
 o
(1.a)
cal
exp
12

and
R 2  Rcal (n,k,d,)  Rexp  o
2
(1.b)
where Tcal and Rcal are the calculated values of transmittance and reflectance using Murmann’s exact
equations, Texp and Rexp are the experimental values of transmittance and reflectance, respectively;
and d is the thickness of film. The spectral behavior of n for pristine and annealed ACh thin films is
illustrated in Fig. 5(a). It shows an anomalous dispersion in the range from 190 to 860 nm with two
dispersion peaks for ACh films; however, it can be explained according to multi-oscillator model
[31]. The spectrum of n shows a normal dispersion behavior in wavelength ranged 750 - 2500 nm
and can be explained by applying the Wemple-DiDomenico model [32, 33]. Fig. 5(b) shows the
spectral behavior of
k
for the pristine and annealed ACh thin films as a function of wavelength in
UV-Vis-NIR regions of spectra. The k spectrum has two peaks around 230 - 470 nm. Annealing
temperature decreases the value of the refractive and absorption indices all over the spectra for ACh
thin film is due to decreasing of the aggregation of azochalcone molecules in these films.
The absorption coefficient, α, is related to k through the relation: α =4πk/λ. The spectral
distribution of α for pristine and annealed ACh thin films as a function of wavelength of the incident
light are depicted in Fig.6. These films show high value of absorption spectra (α > 105 cm-1) which
are in the UV-Vis region of incident light. Annealing temperature decreases the value of α and shifts
the absorption edge toward high photon energy side of spectra. Analysis of the absorption spectrum
edge is well-known method for determining the type of the transitions and also gives information
about the values of optical energy gap, Eg, of different crystalline and amorphous materials. Tauc
[34, 35] assumed that the relation between α and incident photon energy, hv , for most amorphous
semiconductors through the following general relation:
hv  B(hv  Eg )r
(2)
Where B is a constant related to the electrical conductivity and energy level separation and r is a
number that characterize the type of transition. r = 2 or 3 for indirect allowed and forbidden
13

transitions, respectively; exhibit in most amorphous semiconductors and r = 1/2 or 3/2 for direct
allowed and forbidden transitions, respectively; exhibit in most crystalline semiconductors. The
(hv)1 2
relation between
versus photon energy, hv , is found to be the best fitting of results for a
pristine and annealed ACh thin films at r = 2, this means the most probable type of electron
transition is indirect allowed transition as illustrated in Fig.7. The value of Eg can be calculated by
extrapolating the linear portion of the plot to intercept the abscissa axis in the value of energy gap.
The values of Eg for pristine film are 2.44 and 3.37 eV for onset and optical band gap, respectively.
Annealing temperature decreases the onset and optical band gap to be 2.28 and 3.07 eV, respectively.
This decreasing in energy gaps is due to the creation of new impurity or the extension of the
localized state inside the band gap. It should be noted that the values of optical band gap obtained in
the present work are consistent with the values reported in the literatures [17, 36]. The value of
optical band gap of ACh thin films is closely matched with the optical band gap of value 2.31 eV for
mono-chalcone [17] and 3.05 eV for DPMS chalcone derivative [36].
3.3. Characteristics of azochalcone / p-Si heterojunction solar cell
3.3.1 Electric transport mechanism in forward bias direction of heterojunction solar cell
Fig. 8 shows the semi-logarithmic current-voltage (I-V) characteristics of Au/ACh/p-Si/Al
heterojunction solar cell in dark and under illumination at ambient temperature. For I-V
characteristics in dark condition, it observed that, this device exhibits a rectifying behavior due to the
injection of the charge carriers from electrode to ACh layer. Under low forward voltage bias (0 ˂ V
˂ 0.4 volt), the logarithmic current, log I, increases linearly with the voltage bias, V, until reaching a
certain value, and then it increases exponentially with applied high voltage bias (0.4 ≤ V ≤ 1 volt).
On the other hand, the log I is very small under the reverse voltage bias, and this behavior can be
understood in terms of the change of width of depletion region for the diode behavior [37]. This
nonlinearity behavior is due to the effect of series resistance, Rs, and the interface states when the
applied voltage is sufficiently large. The Rs is important parameter for downward curvature of the
14

forward I-V characteristics. On the other side, an ideal device requires the higher shunt resistance,
Rsh, which plays an important role in the values of reverse I-V characteristics. The Rs and Rsh can be
calculated by the method was stated in previous report [38]. It is showed that, Rs and Rsh were found
to be 13.4 kΩ and 1.12 MΩ, respectively. The heterojunction has high Rs value due to the amorphous
interface layer in that junction [39].
In order to determine conduction mechanism of Au/ACh/p-Si/Al heterojunction, the different
conduction mechanisms were tested. For lower voltage (0˂ V˂ 0.4 volt), we invoke the thermionic
emission model for investigating the conduction mechanism of that device. According to this model,
the current can be expressed as [40, 41]








qV
qV





1 exp 


I  IO exp
(3)


nKBT
nK BT





where IO is the saturation current, KB is the Boltzmann constant, T is the absolute temperature, q is
the charge of electron, V is the applied voltage and n is the ideality factor. The saturation current, IO,
is given by


q
KBT
*
2



(4)
IO  AA T exp 



Where A is the active area of the device, A* is effective Ridchardson constant (A* = 32 for p-Si) and

is the zero-bias barrier height. The value of n can be calculated from the slope of linear relation
between log I and V (lower voltage region) as shown in Fig. 8. It was found to be 1.75 which is
higher that the ideality value of 1.02 [42] and that may due to the presence of interfacial native oxide
layer on silicon substrate, series resistance and barrier height inhomogeneity [43]. The formation of
such a thin interfacial oxide layer is inevitable during the fabrication of the solar cell by the thermal
evaporation technique and before evaporation of ACh compound on the front surface of the p-type

Si substrate [42, 43]. On the other hand, the value of can be obtained from Eq. (4) and was found
to be 0.81 eV. At relatively higher voltage bias (0.4 ≤ V ≤ 0.1 volt), in order to determine the
15

conduction mechanism that control the device characteristic in that voltage range, the relation current
and voltage in I-V characteristic should be plotted in logarithmic scale and according to the power
I V m
law of
where m is the slope of graph log V- log I which is the dioristic value to recognize
the type of conduction mechanism. Fig. 9 shows the linear relationship between log V and log I and
its slope (m = 2.35) indicating that the space charge limited conduction (SCLC) mechanism is
dominated [44].
3.3.2 Electric transport mechanism in reverse bias direction of heterojunction solar cell
Fig. 8 shows that the reverse current of the device is small compared with the forward
current; this is due to that the current in reverse direction can be governed by current limitation
mechanism. The dominated mechanism was found as Schottky effect [45]. The reverse current, Ire,
can be explained in terms of the charge transport for this conduction mechanism as follows [45, 46]:








s
KBT
V
d
Ire  Is exp
(5)
where Is is a constant, V is the applied voltage and d is the thickness of the film and βs is the Schottky
constant which can be theoretically calculated by the following relation,
e3
(6)
s 
4 o
ε and εo are the dielectric constant of the material and vacuum, respectively. From optical study, the
value of ε is 4.76. The relation between log Ire and V1/2 is depicted in Fig. 10. The slope of this linear
relation can be used to calculate βs, experimentally and it was found as 9.5 x 10-6 eV m1/2 V-1/2, while
the theoretical value of βs can be deduced from Eq.(6) and its value was found to be 1.2 x 10-5 eV
m1/2 V-1/2. It is observed that, the experimental value of βs is closer to the theoretical one and this
support that the charge transport mechanism through device is the Schottky mechanism when the
applied voltage is reverse bias.
3.3.3 Photovoltaic properties of the solar cell
16

The I-V characteristic of Au/ACh/p-Si/Al heterojunction solar cell under white light
illumination of halogen lamp of power intensity of 100 mW/m2 is shown in Fig. 8. It can be observed
that there is a significant enhancement in the reverse current by the photo-illumination. This
photosensitive property indicates that the device has the photovoltaic behavior. The light was
harvested by the device and creates excitons in the ACh layer and electron-hole pairs in the Si
waver. These excitons are dissociated into electron-hole pairs at the ACh/Si interface. In the
presence of external applied electrical field, the electrons are transported in the Si and controlled by
the Al electrode and leave holes was accelerated toward the Au electrode along the potential barrier
at the interface. The total current of the device under illumination can be described by adding the
photo-generated current to the dark current through the device.
The power curve for the device can be obtained by varying the load resistance from 0 to
infinity at constant intensity of illumination light as shown in Fig. 11. The maximum power that can
be extracted at the point for which the largest rectangle inscribed into the I-V power curve. It can be
deduced from Fig. 11 many of photovoltaic parameters such as: the short circuit current, Isc, of 142
μA and the open circuit voltage, Voc, of 0.36 volt. The fill factor, FF, can be calculated from the
relation [47, 48]:
VM IM
FF 
(7)
VocIsc
where VM and IM are the voltage and current at the maximum power point, respectively. FF is found
to be 0.38. Furthermore, the electrical conversion efficiency, η, is calculated from the relation [47,
48]:
VocI
  FF
sc 100%
(8)
P A
in
where A is the active area of the device and Pin is the intensity of incident light. The conversion
efficiency of the fabricated Au/ACh/p-Si/Al heterojunction solar cell was calculated as 1.8 %.
Conclusion
17

A new azochalcone derivative (ACh) was synthesized and its characterization properties
1
were investigated by FT-IR, H NMR, and Mass spectra. The thin films of ACh were prepared by
thermal evaporation technique at room temperature. Structural properties of ACh thin films have
been investigated. The XRD for ACh in a powder form was a polycrystalline nature. The FE-SEM
and XRD pattern showed the existence of amorphous structure of pristine thin film while, they reveal
the nanostructure of the annealed thin film.
The optical properties of pristine and annealed ACh thin films were studied using
spectrophotometric measurements of the transmittance and reflectance in the wavelength region
200–2500 nm, from which some optical constants such as the refractive index (n), extinction
coefficient (k) and optical band gap (Eg) of the pristine and annealed films were determined.
The results of the optical absorption data showed that ACh thin films have mainly indirect onset and
optical energy band gaps are found to be 2.44 and 3.37 eV, respectively and annealing temperature
decreases these energy gaps to be 2.28 and 3.07 eV, respectively.
The results of this study indicate that ACh thin film as an organic semiconductor is a good
candidate for optoelectronic devices based on its optical band gap and optical absorption spectrum.
The ACh film is used to fabricate hybrid organic/inorganic heterojunction solar cell with a simple
structure of Au/ACh/p-Si/Al. The electrical characteristics of the solar cell have been studied by
current-voltage (I-V) in dark and under illumination at room temperature. The electric conduction
mechanisms are governed by thermionic emission and space-charge-limited current (SCLC) for low
and high applied bias, respectively. The photovoltaic parameters of the cell were calculated, the solar
cell displays a fill factor of 0.38 and a power conversion efficiency of 1.8% under illumination.
18

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23

Caption figures
Scheme1. Synthetic route for azochalcone derivative (ACh)
Scheme 2. Partial fragmentation of ACh compound.
Fig. 1. XRD patterns of ACh dye: (a) as-synthetized powder, (b) pristine film and (c)
annealed film at 473 K for 2 hours.
Fig. 2. FE-SEM images of ACh dye: (a) the pristine film, (b) and (c) the annealed
film with magnifications of 60 000 and 80 000, respectively.
Fig. 3. Optical absorption spectrum for the pristine and annealed ACh films.
Fig. 4. Optical transmittance and reflectance for the pristine and annealed ACh films.
Fig. 5. The spectral behavior of refractive index and extinction coefficient for the
pristine and annealed ACh films.
Fig. 6: Variation of absorption coefficient, α, as a function of wavelength, λ.
Fig. 7: Plot of (αhν)1/2 as a function of the incident photon energy, hν, for the pristine
and annealed ACh films.
Fig.8. Semi-log current-voltage characteristics of Au/ACh/p-Si/Al heterojunction
solar cell in dark and under illumination conditions. The inset shows schematic
of the heterojunction solar cell.
diagram
Fig. 9. Plot of forward log I vs. log V for Au/ACh/p-Si/Al heterojunction solar cell.
Fig. 10. Plot of reverse log Irev vs. V1/2 for Au/ACh/p-Si/Al heterojunction solar cell.
Fig. 11. The power curve of Au/ACh/p-Si/Al heterojunction solar cell
24

O
+
N
NH2
OH
N
a
l
N
1
C
2
O
H
2
O
N
N
OH
N
3
O
N
N
O
OH
N
ACh
Scheme1
25

Scheme 2
26

600
400
200
(c)
0
(b)
600
400
200
0
(a)
50
40
30
20
10
0
10
20
30
40
50
60
70
80
(2)o
Fig. 1
27

(a)
(b)
(c)
Fig. 2
28

2.0
1.5
1.0
0.5
0.0
Pristine film
Annealed film
300
400
500
600
700
, nm
Fig. 3
29