The enhanced efficiency to 3.6% based on organic dye as donor and Si/TiO2 acceptor bulk hetero-junction solar cells

Article abstract of DOI:10.1016/j.jphotochem.2014.07.003

The efficient organic dyes 2-{4-[2-(2-hydroxybenzylidene)hydrazino]phenyl} ethylene-1,1,2-tricarbonitrile and 2-{4-[2-(4-hydroxybenzylidene)hydrazino] phenyl}ethylene-1,1,2-tricarbonitrile have been synthesized, characterized and fabricated as hetero-junction solar cell materials. We use B3LYP/6-31G*, B3LYP/6-31G** and HF/6-31G** level of theories to optimize the ground state geometries. The absorption wavelengths have been computed by using time dependent density functional theory which is in good agreement with the experimental data. The hetero-junction solar cell devices have been fabricated by organic-inorganic heterojunction (dye/Si/TiO ₂) and measured the efficiency by applying the incident power 30, 50 and 70 mW/cm². The maximum efficiency 3.6% for dye2 has been observed. We shed light on the electronic and charge transport properties. Moreover, the stability and external quantum efficiencies have been measured.

Full text of DOI:10.1016/j.jphotochem.2014.07.003

Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The enhanced efficiency to 3.6% based on organic dye as donor and
Si/TiO2 acceptor bulk hetero-junction solar cells
Abdullah G. Al-Sehemia,b,c,∗, Ahmad Irfana,∗, Mohrah Abdullah M. Al-Melfia,
Ahmed A. Al-Ghamdid, E. Shalaand
a Department of Chemistry, Faculty of Science, King Khalid University, PO Box 9004, Abha 61413, Saudi Arabia
b Unit of Science and technology, Faculty of Science, King Khalid University, PO Box 9004, Abha 61413, Saudi Arabia
c Center of Excellence for Advanced Materials Research, King Khalid University, PO Box 9004, Abha 61413, Saudi Arabia
d Faculty of Science, Department of Physics, King Abdulaziz University, PO 80203, Jeddah 21589, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
The
efficient
organic
dyes
2-{4-[2-(2-hydroxybenzylidene)hydrazino]phenyl}ethylene-1,1,2-
Received 15 November 2013
Received in revised form 25 June 2014
Accepted 9 July 2014
tricarbonitrile and 2-{4-[2-(4-hydroxybenzylidene)hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile
have been synthesized, characterized and fabricated as hetero-junction solar cell materials. We use
B3LYP/6-31G*, B3LYP/6-31G** and HF/6-31G** level of theories to optimize the ground state geometries.
The absorption wavelengths have been computed by using time dependent density functional theory
which is in good agreement with the experimental data. The hetero-junction solar cell devices have been
fabricated by organic–inorganic heterojunction (dye/Si/TiO2) and measured the efficiency by applying
the incident power 30, 50 and 70 mW/cm2. The maximum efficiency 3.6% for dye2 has been observed.
We shed light on the electronic and charge transport properties. Moreover, the stability and external
quantum efficiencies have been measured.
Available online 17 July 2014
Keywords:
Organic–inorganic hybrid solar cells
Electronic properties
Fabrication
Efficiency
Density functional theory
Time dependent density functional theory
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
In organic–inorganic hybrid solar cells generally organic and
inorganic parts have been combined with the aim to gain the
No doubt the efficiency of conventional solar cells made from
inorganic materials reached up to 24% [1] and 25% [2] but this tech-
nology is very expensive. New ways to achieve low cost, easy to
fabricate and environmental friendly solar cells alternatives to inor-
ganic semiconductors are required. The organic solar cells achieved
10% efficiency [1] but inorganic materials are more stable than
the organic counterparts [3]. The dye-sensitized solar cells (DSSCs)
[4–7] and hybrid solar cells using the bulk heterojunction concept
with different nanoparticles such as TiOx [8], ZnO [9], CdSe [10,11],
CdS [12], PbS [13], CuInS2 [14,15] are attracting attention in recent
years. The organic–inorganic (Si) hybrid solar cells have been also
studied and proved an efficient technology [16a]. The efficiency has
been obtained from 1.15 to 10.3% by varying the donor (organic
material) and shape of the Si [16b–16g].
advantages coupled with both material groups [17]. The solar cell
devices having inorganic materials could support to overcome the
photo-induced degradation of the conjugated organic materials.
Moreover, the excitons which absorbed in the inorganic materi-
als would lead to the photogeneration of charge carriers [18]. Still
the efficiencies of hybrid solar cells are lower than that of poly-
mer: fullerene devices but the issue of stability is major problem
in this technology. On other hand, the most important issues in
organic-nanoparticles hybrid solar cells are related to the nanopar-
ticle surface chemistry and the nanomorphology of the photoactive
layer.
Recently, we have synthesized and characterized the
2-{4-[2-4-methoxybenzylidenehydrazino]phenyl}ethylene-1,1,2-
tricarbonitrile,
2-{4-[2-(3,4-dimethoxybenzylidene)hydrazino]
phenyl}ethylene-1,1,2-tricarobonitrile, 2-{4-[2-(4-nitrobenzy-
lidene)hydrazino)]phenyl}ethylene-1,1,2-tricarbonitrile, 2-{4-[2-
p-chlorobenzylidenehydrazino]phenyl}ethylene-1,1,2-tricarbon-
itrile
and
2-{4-[2-p-bromobenzylidenehydrazino]phenyl}
∗
Corresponding author at: Corresponding authors at: King Khalid University,
ethylene-1,1,2-tricarbonitrile hydrazones [19]. Previously, we also
found that by substituting the donor groups ICT can be improved
[7]. Thus in the present study the donor group OH has been
Chemistry, Abha 61413, Saudi Arabia. Tel.: +096672418632; fax: +0096672418426.
E-mail addresses: agmasq@gmail.com (A.G. Al-Sehemi), irfaahmad@gmail.com
(A. Irfan).
http://dx.doi.org/10.1016/j.jphotochem.2014.07.003
1010-6030/© 2014 Elsevier B.V. All rights reserved.

2
A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
substituted at ortho (2-{4-[2-(2-hydroxybenzylidene)hydrazino]
phenyl}ethylene-1,1,2-tricarbonitrile (dye1) and para positions (2-
{4-[2-(4-hydroxybenzylidene)hydrazino]phenyl}ethylene-1,1,2-
tricarbonitrile (dye2). It is expected that OH at para position
would be more favorable to stabilize the dye. Moreover, probably
as a result of the superior electron donating character of the
OH group at the para position would more direct the electrons
toward the bridge. In the best of our knowledge, no these dyes
were studied previously with respect to hetero-junction solar cell
materials.
standard (TMS). The following abbreviations are used s—singlet,
t—triplet, q—quartet, d—doublet, m—multiplet; J is the coupling
constant (Hz).
2.1.2. General procedure for hydrazones
We have synthesized the hydrazone based sensitizers by the
same method as in previous studies [19]. The hydrazone derivatives
would be prepared through direct condensation between the cor-
responding aromatic aldehydes and phenyl hydrazine. Equimolar
quantities of phenylhydrazine and the aldehydes would be boiled
in ethanol for an hour. The precipitated hydrazones would be fil-
tered, washed and dried. The pure hydrazones would be obtained
after recrystallization from ethanol.
With the aim to design and synthesize efficient stable materi-
als having high efficiency, we focused on organic hybrid solar cells
(dye/Si/TiO2) which would have no concerns like organic solar cells,
organic-nanoparticle hybrid solar cells and conventional solar cells
(inorganic). The organic hybrid solar cells would have the advan-
tages, i.e., improved stability due to inorganic part compared to the
organic solar cells. This technology would be low cost and environ-
mental friendly as compared to the inorganic/conventional solar
cells. In first step donor–bridge–acceptor organic materials with
improved intra-molecular charge transfer (ICT) were designed, see
Fig. 1. The ground state geometries have been optimized by using
density functional theory (DFT) at B3LYP/6-31G*, B3LYP/6-31G**
and HF/6-31G** level of theories. The absorption spectra have
been computed by using time dependent density functional theory
(TDDFT) at three different levels of theories mentioned above in gas
phase and solvents (CHCl3, CH3CN and C2H5OH). The FTIR spectra
were computed and compared with the experimental evidences.
After that the ionization potentials, electron affinities, reorganiza-
tion energies and chemical descriptors have been discussed. We
shed light on the charge transfer behavior of the newly designed
systems as well as structures-properties relationship. This study
deals in depth study of the hetero-junction solar cell materials,
their fabrication, solar cell device characterization, FTIR, stability,
efficiency, charge transport behavior and DFT investigations.
We have divided the manuscript as follow: first section deals
with the computational and experimental methodology which has
been adopted in the present study. In next section, we have dis-
cussed the charge transfer properties and chemical descriptors.
The main focus of this study is detailed investigations about the
fabrication techniques, stability and efficiency measurements.
2.1.3. Preparation of 2-((2-phenylhydrazono)methyl)phenol (1a)
Salicylaldehyde (5.3 ml, 0.05 mol) was added to phenyl
hydrazine (4.91 ml, 0.05 mol) and added 30 ml of ethanol abso-
lute as solvent. The flask was related with condenser under
reflux system and solution boiled at 170–180 ◦C for 1 h then
transfer solution to beaker and stilled in room temperature or
cooled, the precipitated hydrazones would be collected and fil-
tered, washed with ethanol then dried. The product crystallization
from ethanol gave yellow crystals of title compound (2-((2-
phenylhydrazono)methyl)phenol) m.p 133–135 ◦C. ı H (DMSO,
500 MHz) 6.78–7.56 (9H, m, Ar–H), 8.2 (1H, s, CH N), 10.42 (1H,
s, OH), 10.55 (1H, s, NH); ı C (DMSO, 500 MHz) 111.70–144.72
(10C–Ar), 155.64 (CH N).
2.1.4. Preparation of 4-((2-phenylhydrazono)methyl)phenol (2a)
The 4-hydroxybenzaldehyde (6.1 g, 0.05 mol) was added to
phenylhydrazine (4.9 ml, 0.05mole) and added 30 ml of ethanol
absolute as solvent. The flask was related with condenser under
reflux system and solution boiled at 170–180 ◦C for 1 h then
transfer solution to beaker and stilled in room temperature
or cooled, the precipitated hydrazones would be collected and
filtered, washed with ethanol then dried. The product crystal-
lization from ethanol gave yellow crystals of title compound
(4-((2-phenylhydrazono)methyl)phenol m.p 164 ◦C. ı H (DMSO,
500 MHz) 6.69–7.49 (9H, m, Ar–H), 7.79 (1H, s, CH N), 9.68 (1H,
s, OH), 9.80 (1H, s, NH); ı C (DMSO, 500 MHz), 112.92, 115.63,
118.11, 126.88, 127.14, 128.65, 137.11and 145.68(10C–Ar), 157.67
(CH N).
2. Methodology
2.1. General experimental methods
2.1.5. General procedure for dyes
The new chromospheres were prepared by direct tricyanoviny-
lation of hydrazones. A solution of the requisite hydrazone 1
(0.01 mol) and tetracyanoethylene (TCNE) in DMF (20 ml) was
stirred at 60–90 ◦C for 8–12 h or boiled at high temperature up to
400 ◦C for 8 h. The solvent was removed and the residual solid was
collected and recrystallized from toluene/petroleum ether mixture.
The hydrazone derivatives were prepared through direct con-
densation between the corresponding aromatic aldehydes and
phenyl hydrazine. Equimolar quantities of phenylhydrazine and
the aldehydes were boiled in ethanol for an hour. The precipitated
hydrazones were filtered, washed and dried. The pure hydrazones
were obtained after recrystallization from ethanol (detail can be
found in supporting information).
Infra-red (FTIR) spectra of crystalline compounds were deter-
mined using a Thermo scientific smart omni-transmission. 1H and
13C nuclear magnetic resonance (NMR) spectra were recorded on
a Bruker at 500 MHz Ultra ShieldTM at room temperature in deu-
trated dimethyl sulfoxide (DMSO-d6) (CH3SOCH3 have two signals
in 1H–NMR at ␦ 2.52 singlet, 3.38 singlet). The UV–vis spectra
were recorded with UV-1800 Shimadzu. Melting points (mps) were
determined with a Stuart SMP11 without correction. Ultrasonic in
which used an Elmasonic S 60H.
2.1.6. Preparation of 2-{4-[2-(2-hydroxybenzylidene)hydrazino]
phenyl}ethylene-1,1,2-tricarbonitrile (dye1)
The 2-((2-phenylhydrazono)methyl)phenol (1a) (2.12 g,
0.01 mol) was dissolved in 20 ml DMF as solvent then added
1.28 g of TCNE direct gave dark color. The solution was boiled
at 400 ◦C under reflux for 6 h. The solvent was removed and the
residual solid collected and recrystallized from toluene/petroleum
ether mixture to get dye1 (0.72 g, 23% yield) m.p 225–227 ◦C. ı
H (DMSO, 500 MHz) 6.87–7.99 (8H, m, Ar–H), 8.45 (1H, s, CH N),
10.2 (1H, s, OH exchange with D2O), 11.84 (s, 1H,NH exchange
with D2O); ı C (DMSO, 500 MHz) 78.69 and 142.28 (C C), 114.01,
114.24 and 114.79 (3xC N), 112.72, 115.87, 119.25, 120.16,
125.27, 128.86, 132.78, 136.90, 150.94 and 156.39 (10C–Ar),
162.27(CH N).
2.1.1. Physical data
For nuclear magnetic resonance (NMR) spectra, chemical shifts
and expressed in ppm on the ı-scale relative to the internal

A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
3
Fig. 1. The synthesized organic dyes in the present study.
2.1.7. Preparation of 2-{4-[2-(4-hydroxybenzylidene)hydrazino]
2.1.9. Solar cell device characterization
phenyl}ethylene-1,1,2-tricarbonitrile (dye2)
For solar cell device characterization a combination of software
and hardware has been used. Before initiate of characterization pro-
cess for our fabricated solar cell device, the light and circuit setup
was tested by measuring the I–V curve of a reference 2 × 2 cm cal-
ibrated solar cell made of Si from Oriel. The solar reference cells
come with a certificate of calibration accredited by NIST to the ISO-
17025 standard and is traceable both to the National Renewable
Energy Laboratory (NREL), and to the International System of Units
(SI).
Au back contact of thickness about 100 nm was evaporated on
a fabricated solar cell sample by use of thermal evaporation tech-
nique in working pressure of 10−6 mbar. A device area of (20 × 9)
mm was identified by an evaporation mask. The masked solar
cell device was tested under a solar simulator (Oriel Sol3A Class
AAA Solar Simulators with illuminated area 8 × 8 in) at AM1.5 with
30, 50 and 70 mW/cm2 illumination conditions adjusted and cal-
ibrated by use of a Skye SKS1110 sensor. Finally, the I–V curve
was registered by use of Keithley Model 4200-SCS semiconductor
characterization system using a scan rate of 1 mV/s.
The 4-((2-phenylhydrazono)methyl)phenol (2a) (2.12 g,
0.01 mol) was dissolved in 20 ml DMF as solvent then added
1.28 g of TCNE direct which gave dark color then the solution
was boiled at 320–350 ◦C under reflux for 8 h. The solvent was
removed and the solid residual was collected and recrystallized
from toluene/petroleum ether mixture to get violet crystals of
(dye2) m.p >300 ◦C. ␦ H (DMSO) 6.84–7.99 (8H, m, Ar–H), 8.06
(1H, s, CH N), 10.01 (1H, s, OH exchange with D2O), 11.75 (1H,
s, NH exchange with D2O); ı C (DMSO) 77.60 and 145.50 (C C),
114.22, 114.41 and 114.87 (3xC N), 112.75, 115.77, 119.03,
125.15, 128.86, 132.80, 136.58 and 151.29 (8C–Ar), 159.50
(CH N).
2.1.8. Solar cell device fabrication
The Si wafer was immersed for 10 s in HF solution (water/HF
49%) for etching purpose. Then it was rinsed in a mixture of ethanol,
acetone and de-ionized water with molar ratios 1:1:3, respectively,
to eliminate the fluoride ions. The substrates were rinsed with dry
nitrogen gas. Recently we showed that the nanoporous TiO2 pre-
pared by sol–gel method from the mixture of H2O2 and HNO3 has
better porosity about 32% and showed only pure anatase phase
with energy gap of 3.2 eV [20a]. The sol–gel method is a favorable
method to prepare nanocrystalline/amorphous porous materials
[20b]. The estimated porous size is about 30–40 nm (diameter).
The SEM image shows deep porous sponge-like structure (see Sup-
porting information and Ref. [20a]). The sol–gel solution was used
to deposit a TiO2 films on glass and silicon substrates by spin-
coating method. All films were prepared at spin-coated at rate
2000 rpm (revolution per minute) for 1 min on pre-cleaned sub-
strates (glass and SiO2 etched-silicon), then dried in oven (60 ◦C
for 10 min). For building solar cell device; about 20 nm nanoporous
TiO2 film has been deposited on SiO2 etched-Si substrate having
porosity 32% [20a]. Then about 400 nm of dye was deposited on
the top of TiO2 film. By inserting nanoporous TiO2 between the
dye (active layer) and metal electrode enable the active layer to
harvest more light and increasing optical absorption in four ways:
increasing the surface area and roughness, increasing the charge
collection efficiency, helping as a blocking layer for holes, and
reducing the recombination rate [21a]. Gold has been used as a
top metal electrode by use of a suitable mask to control the shape
of the electrode.
2.2. Computational details
The DFT [22] is good approach to reproduce the experimental
data and to predict the properties of interests. The B3LYP has been
proved an efficient approach to reproduce the experimental data
for the small molecules [23]. Moreover, by using the large fraction
of HF exchange charge transfer states have been discussed previ-
ously [24]. The TD-B3LYP functional has been applied to compute
and reproduce the absorption wavelengths of different organic dyes
(indigo, azobenzene, phenylamine, hydrazone, and anthraquinone)
and 0.12 eV average deviation was observed for hydrazone dyes
[25]. Recently, it was pointed out that the B3LYP was good choice to
reproduce the excitation energies for hydrazone based dyes. It was
found that B3LYP (polarizable continuum model (PCM), methanol)
is better, accurate and more reasonable choice than BHandHLYP,
LC-BLYP and CAM-B3LYP (PCM, methanol) to reproduce the exper-
imental data (Details can be found in Ref. [26]). In the present study
we have computed the excitation energies by using B3LYP func-
tional. The effect of solvents (CHCl3, CH3CN and C2H5OH) has been
studied on the absorption spectra.
Geometry optimizations for all dyes have been performed
using DFT at the B3LYP/6-31G*, B3LYP/6-31G** [27–30] and
Hartree–Fock (HF) at HF/6-31G** [31] level of theories. The vibra-
tional modes were examined by using the Chemcraft program [32].
The absorption spectra and energy gap for dyes were calculated
in different solvents (PCM) at TD-B3LYP/6-31G* level of theory
[33–35]. The ionization potentials, electron affinities and reorga-
nization energy of all dyes have been performed by using the
B3LYP/6-31G* level of theory [36,37]. The PCM [38–41] has been
used for evaluating bulk solvent effects at all stages. All of the cal-
culations were performed by using Gaussian-09 program package
[42].
The dye thin films were prepared by conventional thermal
evaporation technique at a pressure of about 10−6 mbar. The pow-
der was loaded into a molybdenum cell with nozzle of 2 mm
in diameter on the top. The flat glass; glass coated-TiO2; and
silicon substrates were located above 20 cm from the dye. The
films were deposited at room temperature and the rate of depo-
sition was 0.2 nms−1. The low deposition rate was adjusted for
better film quality and crystallinity [21b]. The dye layer was
prepared by thermal evaporation on the etched surface of Si
substrate.

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A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
Fig. 2. The HOMOs energies, LUMOs energies and energy gaps of dyes in different solvents (left), gas phase (right) and acceptor materials (in eV).
3. Results and discussion
to overcome the exciton binding energy as well as dissociation of
excitons.
3.1. Electronic properties and absorption spectra
The dye2 displayed a clearly red-shifted absorption wavelengths
compared to dye1, probably as a result of the superior electron
donating character of the OH group at the para position which
more directed the electrons toward the bridge and the tricyano
moiety is more electron withdrawing in comparison with an dye1
when the OH group is at the ortho position. Interestingly, for both
dyes the absorption maxima in the most polar solvent (C2H5OH)
were slightly red-shifted in comparison with those observed in the
least polar one (CHCl3). This behavior indicates possible partial pro-
tonation of the donor group due to the interaction with the polar
protic medium.
By substituting the hydroxyl group at ortho position (dye1)
showed absorption band at 514 nm in chloroform and 521 nm
in CH3CN. No significant effect has been observed in absorp-
tion spectra toward red shift by changing chloroform to CH3CN.
While substituting hydroxyl group at para position (dye2) showed
absorption band at 531 nm in chloroform. The dyes were mea-
sured in various solvents having different polarity, see Table 1.
The trend of absorption spectra toward red shift of dye1 and dye2
in different solvents is CHCl3 < CH3CN < CH3CH2OH. The maximum
absorption spectra computed at TD-B3LYP/6-31G* level of theory
in chloroform are 524 and 568 for dye1 and dye2, respectively.
The maximum absorption wavelengths in acetonitrile are 529 and
574 eV for dye1 and dye2, respectively. In ethanol, the maximum
absorption wavelengths at 529 and 574 nm for dye1 and dye2,
respectively, have been discerned which are in reasonable agree-
ment with the experimental evidences.
The computed highest occupied molecular orbitals (HOMOs)
energies (EHOMO), lowest unoccupied molecular orbitals (LUMOs)
energies (ELUMO) and HOMO–LUMO energy gaps (Egap) have been
illustrated in Fig. 2. In solvents, the EHOMO level elevated as com-
pared to gas phase. The EHOMO of dye1 in CHCl3, CH3CN and C2H5OH
are −0.21, −0.29 and −0.29 eV higher than that of the EHOMO with-
out solvent, respectively. The EHOMO of dye2 in CHCl3, CH3CN and
C2H5OH are −0.18, −0.24 and −0.23 eV are elevated compared to
the EHOMO without solvent, respectively. The solvents have no sig-
nificant effect to elevate or diminish the ELUMO. The root cause of
reduced Egap in solvents is the elevation of EHOMO. The effect of
CH3CN and C2H5OH solvents on the EHOMO and ELUMO is similar to
lift up or reduce resulting corresponding Egap. The larger Egap in
CHCl3 revealing that all the studied dyes would be red shifted in
CH3CN and C2H5OH solvents.
The EHOMO and ELUMO of Si are −5.43 and −3.92 eV [43] while
the EHOMO and ELUMO of TiO2 are −7.40 and −4.20 eV, respectively
[44]. The Nested band alignment in the donor and acceptor frontier
molecular orbitals was observed by considering the Si as acceptor.
The successful operation of a photovoltaic device requires a stag-
gered band alignment heterojunction which allocate electrons to
transport to the cathode and holes to the anode. By considering the
average values both for Si and TiO2, the valance band energy has
been found −6.41 eV while the conduction band energy −4.06 eV
(Si/TiO2). It is expected that Si/TiO2 acceptor changed the alignment
to staggered band alignment heterojunction, see Fig. 2, Tables S1
and S2.
3.2. FTIR spectra
In hybrid solar cells, excitons formed in the donor material
are dissociated at the donor–acceptor (D–A) interface. The force
required to overcome the exciton binding energy is provided by
the energy level offset of the LUMO of the donor and the conduc-
tion band edge of the acceptor material. We found energy level
offset 0.67 and 0.77 eV for dye1 and dye2, respectively, to overcome
the exciton binding energy. It is revealing that in dye1 less force is
required to overcome the exciton binding energy as compared to
dye2. For dissociation of excitons formed in the acceptor material,
the energy offset of the HOMO of the donor and the valence band
edge of the acceptor material is required. The energy level offset
0.25 and 0.57 eV for dye1 and dye2, respectively, was noticed to
dissociate of excitons. In dye1 less force would be required for dis-
sociation of excitons as compared to dye2. It is expected that dye1
would be more efficient because of less force would be required
Vibrational spectral assignments have been performed on
the recorded FTIR spectrum based on the theoretical predicted
wavenumbers by DFT/B3LYP/6-31G*, B3LYP/6-31G** and HF/6-
31G** level of theories. The computed wavenumbers by DFT are
in good agreement with the experimental values. The FTIR spectra
have been calculated for free molecules in vacuum while experi-
ments have been performed for solid samples. For this reason scale
factors in theoretical calculations were applied, i.e., 0.937 for DFT
calculations and 0.860 and 0.850 for dye1 and dye2, respectively,
for HF calculations.
The FTIR spectra exhibited three important absorption bands;
the first band has been observed at 3261 and 3221 cm−1 for the
(ꢀN H) mode for dye1 and dye2, respectively. In the present case,
DFT calculations gave band at 3361 and 3380 cm−1 at B3LYP/6-31G*

A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
5
Table 1
Calculated (calc) and experimental (exp) absorption spectra and oscillator strength (f) in different solvents.
CHCl3
Exp
ꢁ
CH3CN
Exp
ꢁ
EtOH
Exp
ꢁ
Calc
Calc
Calc
Calc
(f)
ꢁ
(f)
(f)
ꢁ
(f)
(f)
ꢁ
(f)
Dye1
Dye2
514
338
289
531
335
282
0.1
524
448
360
568
409
341
0.57
0.21
0.1
0.74
0.22
0.006
521
339
287
545
333
252
0.17
0.02
0.004
0.39
0.21
0.02
529
362
0.56
0.09
539
350
298
553
333
290
0.31
0.15
0.04
0.57
0.06
0.01
529
452
362
574
415
347
0.56
0.21
0.09
0.72
0.23
0.002
0.02
0.004
1.35
0.24
0.08
574
415
347
0.72
0.23
0.002
and 3379 and 3396 cm−1 at B3LYP/6-31G**. The HF calculations
gave band at 3462 and 3258 cm−1 for dye1 and dye2, respectively.
The NH stretching wavenumber is red shifted in FTIR from the com-
Table 2
The vertical and adiabatic ionization potentials (IPv/a), vertical and adiabatic elec-
tronic affinities (EAv/a), hole and electron reorganization energies (ꢁ(h/e)) of
hydrazone dyes (in eV) at the B3LYP/6-31G* level of theory.
puted wavenumber which indicates the weakening of the N
H
Dyes
IPv
IPa
EAv
EAa
ꢁ(h)
ꢁ(e)
bond resulting in hydrogen bonding. The strong and sharp sec-
ond band appears in the region of 2219–2215 cm−1 which was
attributed to the cyano group. The B3LYP/6-31G* and B3LYP/6-
31G** calculations gave these modes at 2201 and 2200 cm−1 while
HF gave at 2224 and 2223 cm−1 for dye1 and dye2, respectively.
The third absorption band in the region of 1600 and 1610 cm−1
ascribed for the C N in dye1 and dye2, respectively. The DFT cal-
culations gave band at 1570 and 1581 cm−1 at B3LYP/6-31G* level
of theory and 1566 and 1577 cm−1 at B3LYP/6-31G** level of the-
ory. The HF calculations gave this mode at 1549 and 1627 cm−1 for
dye1 and dye2, respectively. The absorption of OH group appear in
dye1 and dye2 in the range 3143–3443 cm−1 (experimental FTIR
measurements), from 3139 to 3577 cm−1 for DFT and from 3261 to
3564 cm−1 for HF method. The N H stretching vibration showed
only one band at 3335 cm−1 [45]. The sharp band has been observed
at 3265 cm−1 in the FTIR spectrum. The DFT calculations showed
band at 3361 and 3379 cm−1 for B3LYP6-31G* and B3LYP6-31G**,
respectively. The HF calculations gave band at 3462 cm−1. Effect
of hydrogen bonding on an O–H stretching vibration has been
Dye1
Dye2
7.26
6.98
7.49
7.15
2.26
2.10
1.89
1.82
0.387
0.359
0.674
0.555
Table 3
The chemical descriptors of dye1 and dye2 computed at different levels of theories
(in eV).
Dyes
ꢃ
ꢄ
ω
S
ꢅ
ωi
B3LYP/6-31G*
Dye1
Dye2
B3LYP/6-31G*
Dye1
Dye2
HF/6-31G**
Dye1
Dye2
4.78
4.56
1.39
1.27
8.22
8.17
0.69
0.64
9.21
11.75
30.51
54.13
4.78
4.56
1.38
1.27
8.27
8.17
0.69
0.64
6.78
10.53
16.61
43.50
3.99
3.80
4.14
4.04
1.92
1.79
2.07
2.02
8.53
10.69
8.78
14.14
3.3. Chemical descriptors
appeared in the region 3500–2500 cm−1 [45]. The ꢂO
mode is
H
The vertical ionization potentials (IPv), adiabatic ionization
potentials (IPa), vertical electron affinities (EAv), adiabatic elec-
tron affinities (EAa), hole reorganization energies ꢁ(h) and electron
reorganization energies ꢁ(e) have been tabulated in Table 2. Pre-
viously, it was concluded that higher EAv would be favorable for
the generation of free hole [49]. The better hole transfer materials
have smaller value of ionization potentials [50] while better elec-
tron transfer materials have higher electron affinities. The high EA
of dye1 is revealing that it would be more appropriate to generate
free electrons and holes. The calculated hole reorganization ener-
gies of dye1 and dye2 are smaller than the electron reorganization
energies revealing that these dyes might be better hole transfer
materials as well.
interference with N H stretching mode and observed broad band
in the region about 3143 cm−1. The DFT calculations gave band at
3139 and 3152 cm−1 for B3LYP/6-31G* and B3LYP/6-31G** level of
theories, respectively, while the HF gave at 3261 cm−1
.
For the existence of benzene rings in a structure, the C H and
C C vibrations are varying and depending on the number and
C
type of substitutions. The C C stretching vibration showed in the
region (1577 4) cm−1 and (1579 6) cm−1 [46]. In the present
case the ꢂC mode of aromatic rings occured at 1656 cm−1. The
C
ꢂC mode appeared at 1576 and 1573 cm−1 at B3LYP/6-31G* and
C
B3LYP/6-31G** level of theories, respectively. The C N stretching
vibration has been reported in the range 2235–2215 cm−1 [46]. The
sharp and strong band for ꢂC
mode appeared at 2219 cm−1 in
N
The electronegativity (ꢃ), hardness (ꢄ), electrophilicity (ω),
softness (S) and electrophilicity index (ωi) at the B3LYP/6-31G*,
B3LYP/6-31G**, and HF/6-31G** level of theories have been pre-
sented in Table 3. It has been observed that the trend of the
electronegativity (ꢃ), hardness (ꢄ), electrophilicity (ω), softness (S)
and electrophilicity index (ωi) at all the level of theories is similar.
The ꢃ and ω are larger for dye1. The ꢄ and S for dye1 have the high-
est values. The significant effect to reduce the ωi has been observed
in dye1.
FTIR spectra. The DFT calculations gave ꢂC
mode in the range
N
2178–2201 cm−1 at B3LYP/6-31G* and B3LYP/6-31G** level of the-
ories while HF showed band in the range 2217–2224 cm−1
.
The ꢂC N mode has been reported in the range 1605–1665 cm−1
by Asiri et al. [47]. In present case the strong band has been
observed at 1601 cm−1 in the FTIR spectra (C N stretching mode).
The DFT calculations gave same mode at 1570 and 1566 cm−1 at
B3LYP/6-31G* and B3LYP/6-31G** level of theories, respectively.
In HF calculations this mode has been observed at 1549 cm−1. The
C
O stretching mode in literature has been observed in the range
1000–1300 cm−1 [45]. The ꢂC mode was observed at 1294 cm−1
O
3.4. Efficiency
in FTIR while calculations showed this band at 1250 and 1206 cm−1
for DFT and HF, respectively. The C C in aromatic rings showed
band at 1598, 1588, 1494, 1456 cm−1 [48]. The computed modes at
different level of theories have been tabulated in Tables S3 and S4
(Detail can be found in supporting information and Fig. 3).
The external quantum efficiency (EQE) can be measured by using
the following equation.
EQE = ꢄabs × ꢄdiff × ꢄdiss × ꢄtr × ꢄcc
(1)

6
A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
80
70
60
50
40
30
20
10
0
60
Dye 1
Dye 2
50
40
30
20
10
0
4000
3500
3000
2500
2000
1500
1000
500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
Wavenumber cm-1
Fig. 3. The FTIR spectra of dye1 and dye2.
Table 4
The short circuit current density (Jsc) is directly linked with the
The organic–inorganic hybrid solar cell device performance parameters.
absorption yield (ꢄabs). In present case, not only the inorganic part
is improving the absorption yield but the organic materials are also
covering most of the absorption spectra revealing it can absorb
more solar spectrum region. The next important parameter is ꢄdiff
describes the ability of an exciton to diffuse to a donor-acceptor
(D–A) interface. This factor is inversely related to the rate of recom-
bination within the photoactive material. The coplanarity between
the benzene near anchoring group having LUMO and the bridge
(N N) is broken in studied dyes, i.e., 43◦–53◦ out-of-plane distor-
tion, thus the, consequently hampering the recombination reaction.
Ultimately this might enhance the ability of excitons to diffuse
efficiently at the interface. The third parameter is the exciton disso-
ciation yield (ꢄdiss). As the electron is still bound within the exciton,
the energy offset formed at the D–A interfaces is required to provide
a driving force which releases the electron and allows conduction
to occur. This energy offset must be larger than the excitonic bind-
ing energy in the material to facilitate charge transfer. This energy
is typically in the range of 0.1–0.5 eV [51,52]. In present case 0.25 eV
energy is required for dye1 which is in the above mentioned range.
The fourth parameter describes the efficiency of charge carrier
transport throughout the device (ꢄtr). In organic materials, charge
transport occurs via a process of hopping between energy states
and is affected by traps and recombination sites in the photoactive
film. The success of this transport depends greatly on the mobility
of the associated semiconductors [53]. The recombination would be
hampered and ultimately this would also enhance hopping/charge
transport. Moreover, the dye1 has the high electron affinity which
would improve the electron transport toward cathode.
The last important parameter describes the efficiency of charge
collection at the electrodes (ꢄcc). The success of this step is greatly
dependent on the electronic composition of the device. For suc-
cessful injection of electrons into the cathode the magnitude of the
conduction band edge energy level of the acceptor material with
respect to the vacuum level must be lower than the work function
of the metal. For successful injection of holes into the anode, the
magnitude of the HOMO level of donor material must be higher
than the work function of the transparent anode. The higher Jsc of
dye2 is might be due to the above mentioned reasons. The higher
FF value of dye2 is due to the less recombination occurring at the
D–A interface.
Complexes Incident power
(mW/cm2)
Voc (V) Jsc (mA/cm2) FF
Efficiency (%)
Dye1
30
50
70
30
50
70
0.55
0.58
0.60
0.45
0.49
0.53
2.72
4.72
5.83
2.73
5.67
7.80
0.46
0.49
0.56
0.51
0.58
0.61
2.33
2.72
2.76
2.10
3.22
3.58
Dye2
It has been found that when the electron affinity increases generally
the Voc increases.
The relationship between diagonal band gap of the hetero-
junction and Voc has been studied and no linear relationship was
observed [55]. In the present study, the diagonal band gap of
dye2 and dye1 have been observed, i.e., 2.10 and 1.78 eV, respec-
tively. The Voc have been observed 0.53 and 0.60 V, Jsc 7.80 and
5.83 mA/cm2 while the FF 61.0 and 56.0% with 70 mW/cm2 inci-
dent power. We didn’t observe the linear relationship between the
diagonal band gap and Voc which is good agreement with previous
study [55]. The larger diagonal band gap has been observed for dye2
resulting higher Jsc and FF.
Previously, a simple relationship between the HOMO of the
donor material and the Voc of the device was derived. Similarly, here
linear relationship between the HOMO energies/ionization poten-
tial and the Voc has been found. It was pointed out that if the LUMO
energy level of donor material would be less than −3.92 eV, it can
improve the efficiency of organic solar cell devices [56]. In present
case, the LUMO of dye1 is smallest revealing that this would be the
efficient donor.
The poly (3-hexylthiophene) is being used as efficient donor
material in solar cell devices due to the enhanced absorption,
environmental stability and higher hole mobility [57]. We have
observed that the hole reorganization energy of dye2 is smaller
than other counterpart revealing that it would be good donor mate-
rial. Fig. 4 showed the current–voltage characteristics curves with
open-circuit voltage and short-circuit current under different illu-
mination power, i.e., 30, 50 and 70 mW/cm2. The photovoltaic
properties of heterojunction organic–inorganic hybrid solar cells
(dye/Si/TiO2) based on dye1 and dye2 have been shown in Table 4.
The dye1 cell gave a short-circuit photocurrent density (Jsc) of
2.72 mA/cm2, an open-circuit voltage (Voc) 550 mV, and a fill factor
(FF) of 46% with the illumination power (Pin) 30 mW/cm2, corre-
sponding to an overall conversion efficiency of 2.33%. The same
device showed 2.72% efficiency with Jsc 4.72 mA/cm2, Voc 580 mV
and FF of 49% when Pin is 50 mW/cm2. By applying Pin 70 mW/cm2,
Brabec et al. proposed an effective band gap model for bulk het-
erojunction cells; they correlated the maximum value of Voc and
the energy difference between the HOMO level of the donor and
the LUMO level of the acceptor [54]. A linear relationship between
electron affinity and Voc was observed. In the present study, we
have correlated the electron affinity of the donor material and Voc
.

A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
7
Fig. 4. The current-voltage characteristics curves with open-circuit voltage and short-circuit current under different illumination power left (dye1) and right (dye2).
the Jsc, Voc and FF further improved to 5.83 mA/cm2, 600 mV
and 56%, respectively, resulting efficiency enhanced to 2.76%. The
dye2 gave Jsc 2.73 mA/cm2, Voc of 450 mV and FF of 51% with Pin
30 mW/cm2 leading power conversion efficiency 2.10%. The same
system correspond Jsc 5.67 mA/cm2, Voc of 490 mV and FF of 58%
with Pin 50 mW/cm2 consequential efficiency 3.22%. We observed
3.58% efficiency when the Pin was 70 mW/cm2 which improved
the Jsc, Voc and FF to 7.80 mA/cm2, 530 mV and 61%, respectively.
It can be noticed that by increasing the Pin from 30 mW/cm2 to
70 mW/cm2 Jsc, Voc and FF improved resulting enhanced the effi-
ciency, see Table 4.
were used which are (i) constant temperatures exposure and
(ii) temperature illumination stresses. In the first test, accelerated
thermal stability was performed by store the dye1 and dye2 cells
in the oven at 80 ◦C for 240 h exposure period. Fig. 5 shows the
temperature dependent of the characteristic parameters of the
fabricated dye1 and dye2 cells. The change with temperature of
VOC and photovoltaic efficiency of dye2 was found to be less than
that found in dye1. The efficiency of dye1 solar cell during 10 days
of aging at 80 ◦C showing a ∼15% decrease while dye2 solar cell
shows only 3% indicated very good stability.
In the temperature illumination stresses test which used to
examine the photo-stability of the cells. Solar cells were exposed
to continuous light soaking in a solar simulator tell the temper-
ature of the cell reach 60 ◦C. Fig. 6 illustrates the time evolution
of the photovoltaic parameters where both cells show very good
stabilities. The overall performance of the dye2 cell shows a very
3.5. Stability
Preliminary measurement results of the stability of dye1 and
dye2 solar cells were reported. Here two basic accelerated tests
Fig. 5. Temporal evolution of dye1 and dye2 solar cells characteristic parameters under sequential thermal aging at 80 ◦C.

8
A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
Fig. 6. Temporal evolution of dye1 and dye2 solar cells characteristic parameters under continuous light soaking at 60 ◦C.
than dye1 cell. The EQE spectrum of dye2 cell had a broad plateau
region compared to the spectra of dye1 cell. Therefore, the cell made
from dye2 can efficiently convert incident light to current in the
region 350–650 nm.
Finally, the maximum EQE value for dye1 was observed at
525 nm while for dye2 at 550 nm. On other hand, the maximum
absorption wavelength for dye1 and dye2 were found 514–529 and
531–574 nm by varying the solvents, respectively. These results
showed that EQE and absorption spectra are almost similar. We
observed some differences between the EQE and absorption spec-
tra in the range of 350–450 nm, it is might be due to that the
EQE were measured for cell while the absorption spectra in
solvent.
4. Conclusions
The root cause of reduced energy gap in solvents is due to the
elevation of HOMO energy level. The dye2 showed evidently red-
shifted absorption wavelengths compared to dye1, probably due
to the superior electron donating character of the OH group at
the para position. The absorption maxima in the most polar sol-
vent were slightly red-shifted in comparison with those observed
in the least polar one. The Si/TiO2 acceptor might change the align-
ment to staggered band alignment heterojunction which would
enhance the photovoltaic efficiency. The distortion in organic dye
would hamper the recombination reaction which might enhance
the ability of excitons to diffuse efficiently at the interface as well
as improve charge carrier transport throughout the device. The
smaller hole reorganization energy of dye2 revealed that it would
be good donor material. The dye2 cell shows a fabulous stabil-
ity while dye1 solar cell had decreased notably after 10 days of
aging. It is expected that the cell made from dye2 can efficiently
convert incident light to current in the region 350–650 nm. The
bulk-heterojunction solar cell fabricated with dye2 film exhib-
ited conversion efficiency of 3.6%. Moreover, the computed DFT
Fig. 7. External quantum efficiency (EQE) for the fabricated dye1 and dye2 solar cell
devices (as indicated).
good stability while dye1 solar cell had decreased notably after 10
days of aging, due to the loss in Voc and a lack of attenuation by
Jsc
.
3.6. External quantum efficiency
External quantum efficiency (EQE) spectra of the dye1 and dye2
cells are shown in Fig. 7. The maximum EQE value at 525 nm of the
dye1 was approximately 45%, while the maximum EQE value for
cell made from dye2 is 38% located at 550 nm. Generally, adsorbed
spectrum of the dye2 cell is relatively higher than that of dye1 cell.
In the region 330–480 nm the dye2 solar cell absorbed more spectra

A.G. Al-Sehemi et al. / Journal of Photochemistry and Photobiology A: Chemistry 292 (2014) 1–9
9
absorption and FTIR spectra were found in good agreement with
the experimental evidences.
(e) A.G. Al-Sehemi, M.A.M Al-Melfi, A. Irfan, Struct. Chem. 24 (2013) 499–506;
(f) A. Irfan, A.G. Al-Sehemi, J. Mol. Model. 18 (2012) 4893–4900;
(g) R. Jin, A. Irfan, Comput. Theor. Chem. 986 (2012) 93–98;
(h) A. Irfan, N. Hina, A.G. Al-Sehemi, A.M. Asiri, J. Mol. Model. 18 (2012)
1208–4199;
Acknowledgement
(i) A. Irfan, A.G. Al-Sehemi, A.M. El-Agrody, J. Mol. Struct. 1018 (2012) 171–175;
(j) A. Irfan, F. Ijaz, A.G. Al-Sehemi, A.M. Asiri, J. Comput. Electron. 11 (2012)
374–384;
(k) A. Irfan, J. Zhang, Y. Chang, Chem. Phys. Lett. 483 (2009) 143–146;
(l) A.R. Chaudhry, R. Ahmed, A. Irfan, A. Shaari, A.G. Al-Sehemi, Mat. Chem. Phys.
138 (2012) 468–478;
We thank King Abdul Aziz city of Science and Technology
(KACST) for their financial support to Mohrah Abdullah M. Al-Melfi
by the grant No. GSP-12-24.
(m) A. Irfan, A.G. Al-Sehemi, A.M. Asiri, J. Theor, Comput. Chem. 11 (2012)
631–640;
Appendix A. Supplementary data
(n) A. Irfan, R. Jin, A.G. Al-Sehemi, A.M. Asiri, Spectrochim. Acta, A: Mol. Biomol.
Spectrosc. 110 (2013) 60–66;
(o) A. Irfan, H. Aftab, A.G. Al-Sehemi, J. Saudi Chem. Soc. (2011), http://dx.doi.
org/10.1016/j.jscs.2011.11.013;
(p) A.G. Al-Sehemi, A. Irfan, Arabian J. Chem. (2013), http://dx.doi.org/
10.1016/j.arabjc.2013.06.019.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2014.07.003.
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