Synthesis, materials characterization and opto(electrical) properties of unsymmetrical azomethines with benzothiazole core

Article abstract of DOI:10.1016/j.saa.2012.06.054

Optical (UV-vis and photoluminescence) properties of two soluble organic molecules based on azomethines with benzothiazole core (BTA1 and BTA2) were reported. The structures of both compounds are characterized by means FTIR, ¹H NMR, and ¹³C NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure. The investigated compounds emitted blue light. Influence of excitation wavelength and concentration on maximum and intensity of emission of BTA1 and BTA2 was found. Electrochemical properties of the compounds were studied by differential pulse voltammetry. Introduction of fluorine moieties (BTA1) resulted in lower energy band gap (E_g) of approximately ～0.5 eV, whereas BTA2 showed E_g of ～2.8 eV. The devices comprised of BTA1 with P3HT:PCBM (1:1:1) showed an open circuit voltage (V_OC) of 0.40 V, a short circuit current (J_SC) of 1.19 mA/cm², and a fill factor (FF) of 0.23, giving a power-conversion efficiency (PCE) of 0.10% under AM1.5G irradiation (100 mW/cm²).

Full text of DOI:10.1016/j.saa.2012.06.054

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, materials characterization and opto(electrical) properties
of unsymmetrical azomethines with benzothiazole core
a
b
b
Agnieszka Iwan a, , Marcin Palewicz , Michal Krompiec , Marzena Grucela-Zajac ,
⇑
Ewa Schab-Balcerzak b, Andrzej Sikora a
a Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Poland
b Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
"
Thiazole ring-containing
azomethines were obtained.
Blue photoluminescence exhibited
both azomethines.
The J-U effects in monolayer and BHJ
devices depend on compound used.
NMR and FTIR spectroscopy
confirmed the structure of the
compounds.
a r t i c l e i n f o
a b s t r a c t
Article history:
Optical (UV–vis and photoluminescence) properties of two soluble organic molecules based on azometh-
ines with benzothiazole core (BTA1 and BTA2) were reported. The structures of both compounds are
characterized by means FTIR, 1H NMR, and 13C NMR spectroscopy and elemental analysis; the results
show an agreement with the proposed structure. The investigated compounds emitted blue light. Influ-
ence of excitation wavelength and concentration on maximum and intensity of emission of BTA1 and
BTA2 was found. Electrochemical properties of the compounds were studied by differential pulse voltam-
metry. Introduction of fluorine moieties (BTA1) resulted in lower energy band gap (Eg) of approximately
ꢀ0.5 eV, whereas BTA2 showed Eg of ꢀ2.8 eV. The devices comprised of BTA1 with P3HT:PCBM (1:1:1)
showed an open circuit voltage (VOC) of 0.40 V, a short circuit current (JSC) of 1.19 mA/cm2, and a fill factor
(FF) of 0.23, giving a power-conversion efficiency (PCE) of 0.10% under AM1.5G irradiation (100 mW/
cm2).
Received 4 January 2012
Received in revised form 30 May 2012
Accepted 23 June 2012
Available online 11 July 2012
Keywords:
Schiff bases
Azomethines
Benzothiazoles
Photoluminescence
Electrochemistry
Photovoltaics
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
ionization potential of the sample and generate smectic phase.
Azomethines presented in [2,3] were investigated as liquid crystal-
Azomethines are known mainly as thermostable compounds
and often are investigated as biological materials [1]. Moreover,
some of them depend on the chemical structure exhibited photolu-
minescence or liquid crystalline properties [1]. Among different
azomethines compounds with benzothiazole rings are not wide
investigated [2,3].
line compounds. For the azomethines posses alkoxy chain (C from
12 to 16) smectic A (SmA) phase was detected, while those in the
series with OH group were non-mesogenic. For the compounds
with terminal methoxy group mesogenic properties were found
for the length of chain higher than n = 3. Enantiotropic nematic
phase was found for all investigated samples. The smectic C phase
emerged from the decanoyloxy derivative onwards [2,3].
Recently, a lot of attention is paid to find new acceptors for
application in organic bulk heterojunction (BHJ) solar cells [4–9].
Moreover, theoretical calculations for compounds with benzothia-
diazole unit were investigated [10]. In this paper, first time to our
Presence of benzothiazole ring with electron-rich sulfur atom in
the organic compound structure can influence on decrease of the
⇑
Corresponding author.
E-mail address: a.iwan@iel.wroc.pl (A. Iwan).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.06.054

A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
547
knowledge photovoltaic properties of unsymmetrical azomethines
were analyzed.
ITO was purchased from Ossila company. Surface resistance of
ITO was about 20 /square.
X
The main idea of this work was applied as acceptors small or-
ganic molecules with benzothiazole ring for bulk heterojunction
(BHJ) devices. Small compounds are attractive because of their
advantages over their polymer counterparts, which include well
defined molecular structure, definite molecular weight, and high
purity without batch to batch variations. Small organic compound
posses ability to phase separation and self-assembly of molecular
solids and for this reasons may be applied in a position to compete
with polymers as the preferred type of BHJ solar cells or may be
combine with polymers in bilayer or tandem devices to con-
structed efficient solar cells.
Synthesis of both compounds was reported in our previously
paper [10], however lack of characterization of the BTA1 and BTA2.
N-[(E,Z)-1,3-benzothiazol-2-ylmethylidene]-4-(heptadecafluoroctyl)
aniline, BTA1: 1H NMR (400 MHz, CDCl3) d 8.80 (s, 1H, N@CAH), 8.20–
8.12 (m, 1H, benzothiazole CAH), 8.02–7.95 (m, 1H, benzothiazole
CAH), 7.68 (d, J = 8.5 Hz, 2H, phenyl CAH), 7.60–7.49 (m, 2H, benzo-
thiazole CAH), 7.42 (d, J = 8.5 Hz, 2H, phenyl CAH). 13C NMR
(101 MHz, CDCl3) d 166.63, 155.77 (N@CAH), 154.00, 153.05,
135.77, 128.38 (phenyl CAH), 127.45 (benzothiazole CAH), 126.97
(benzothiazole CAH), 124.70 (benzothiazole CAH), 122.37 (benzo-
thiazole CAH), 121.50 (phenyl CAH). FTIR (KBr, cmꢁ1): 3436, 3057,
2966, 2929, 2372, 2350, 2319, 1622 (HC@NA), 1597, 1587, 1554,
1488, 1453, 1430, 1415, 1371, 1331, 1319, 1299, 1247, 1222, 1142,
1113, 1090, 1048, 1013, 959, 942, 859, 849, 807, 762, 731, 688, 658,
642, 611, 594, 559, 529. Anal. calcd for (C22H9N2F17S) (656.00): C,
40.24; H, 1.37; N, 4.27. Found: C, 40.54; H, 1.42; N, 4.46.
N-[(E,Z)-1,3-benzothiazol-2-ylmethylidene]-4-hexadecylaniline,
BTA2: 1H NMR (400 MHz, CDCl3) d 8.83 (s, 1H, N@CAH), 8.12 (ddd,
J = 8.2, 1.3, 0.6 Hz, 1H, benzothiazole CAH), 7.96 (ddd, J = 7.8, 1.3,
0.6 Hz, 1H, benzothiazole CAH), 7.54 (ddd, J = 8.2, 7.2, 1.3 Hz, 1H,
benzothiazole CAH), 7.48 (ddd, J = 7.8, 7.2, 1.3 Hz, 1H, benzothia-
zole CAH), 7.31 (d, J = 8.5 Hz, 2H, phenyl CAH), 7.25 (d, J = 8.4 Hz,
2H, phenyl CAH), 2.64 (t, J = 7.5 Hz, 2H, phenyl-CH2A), 1.69–1.58
(m, 2H, phenyl-CH2ACH2), 1.39–1.20 (m, 26H, CH2), 0.88 (t,
J = 6.9 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) d 167.89, 154.06,
152.45 (N@CAH), 147.26, 143.45, 135.58, 129.52 (benzothiazole
CAH), 126.90 (benzothiazole CAH), 126.68 (benzothiazole CAH),
124.33 (phenyl CAH), 122.26 (phenyl CAH), 121.64 (benzothiazole
CAH), 35.79 (phenyl-CH2A), 32.08, 31.56, 29.9–29.8, 29.74, 29.65,
29.51, 29.44, 22.84, 14.27 (CH3). FTIR (KBr, cmꢁ1): 3436, 3056,
2954, 2921, 2847, 2325, 1619 (HC@NA), 1586, 1554, 1508, 1487,
1464, 1416, 1318, 1236, 1157, 1119, 1013, 936, 860, 832, 798,
760, 725, 683, 555. Anal. calcd for (C30H42N2S) (462.00): C, 77.92;
H, 9.09; N, 6.06. Found: C, 78.73; H, 8.90; N, 6.00.
We propose new small organic molecules with benzothiazole
and azomethine units to use as donor–acceptor (D–A) for BHJ poly-
meric solar cells based on poly(3-hexylthiophene-2,5-diyl) (P3HT)
and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The chemical
structure of these compounds is presented in Fig. 1. The donor–
acceptor type small molecules were synthesized by condensation
reaction of benzothiazole as the electron-poor unit and amine as
the electron-rich unit with azomethine as the bridge. The structural
characterization of obtained compounds was performed by 1H, 13
C
NMR and FTIR techniques, completed with optical and electrochem-
ical investigations. The preliminary photovoltaic properties of the
small molecules were investigated by fabrication of the organic
photovoltaic devices with the configuration of ITO/PEDOT:PSS/
P3HT:small molecule/Al and ITO/PEDOT:PSS/P3HT:PCBM:small
molecule/Al under AM1.5G irradiation (100 mW/cm2).
Experimental
Materials and measurements
Benzothiazole-2-carboxaldehyde,
4-(heptadecafluoroctyl)
aniline 4-hexadecylaniline, N,N-dimethylacetamide, poly(3,4-eth-
ylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), poly
(3-hexylthiophene-2,5-diyl) (P3HT, regioregular) and [6,6]-phenyl
C61 butyric acid methyl ester (PCBM) were purchased from Sig-
ma–Aldrich and used as received. Methanol, acetone, chloroform
and n-hexane were purchased from POCH and used as received.
Fourier transform infrared (FTIR) spectra were acquired on a
560 MAGNA-IR NICOLET Spectrometer using KBr pellets. 1H-NMR
and 13C-NMR spectra were acquired on Bruker Avance 400 spec-
trometer using chloroform (CDCl3) as solvent and TMS as the inter-
(CF2)7
H2N
+
CF3
N
H
(CF2)7
CF3
N
S
BTA1
DMA, PTS
10h, 160 oC
O
N
S
H
10h, 160 oC
DMA, PTS
N
C
16H33
BTA2
N
+
H2N
S
H
C16H33
OCH3
C
O
S
n
P3HT
PCBM
Fig. 1. Synthetic route and chemical structure of azomethines BTA1 and BTA2 along with the structure of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61
butyric acid methyl ester (PCBM).

548
A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
nal standard. Elemental analyses were performed using Perkin El-
mer Analyzer 2400.
UV–vis spectra were recorded as thin films on quartz substrate
and in chloroform solution by using Jasco V670 spectrophotome-
ter. Photoluminescence was measured in chloroform solution
using a Varian Cary Eclipse fluorimeter.
agreement of the calculated and found content of carbon, nitrogen
and hydrogen in the both compounds (see Experimental Section).
1H-NMR spectra of BTA1 and BTA2 are presented in SI. The ther-
mal properties of BTA1 and BTA2 were characterized by both dif-
ferential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) and were the subject of a separate paper [10].
BTA1 exhibited liquid crystalline behavior what was confirmed
by X-ray diffraction and polarized optical microscopy [10].
Solar cells were fabricated on an indium tin oxide (ITO)-coated
glass substrate with the following structures: ITO/PEDOT:PSS/
P3HT:PCBM:small molecule/Al and ITO/PEDOT:PSS/P3HT:small
molecule/Al in air atmosphere. As reference devices, the following
structures were used: ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/
P3HT/Al. The ITO-coated glass substrate was first cleaned with
deionised water, and ultrasonicated in isopropanol for about
20 min. PEDOT:PSS (ꢀ30 nm) was spin cast (10000 turns per min-
ute, 25 s.) from aqueous solution to form a film on the ITO substrate.
A solution containing a mixture of small molecule:P3HT:PCBM
(ꢀ80 nm) in chloroform solvent with weight ratio 1:1:1 was then
spin cast on top of the PEDOT:PSS layer. Then, an aluminum elec-
trode (ꢀ100 nm) was deposited by thermal evaporation in a vac-
uum of about 5 ꢂ 10ꢁ4 Torr. Current density–voltage (J–V)
characteristics of the devices were measured using a Keithley
6517B electrometer and and DC microvoltmeter V623. Solar cell
performance measurements utilized a xenon lamp with an irradia-
tion intensity of 100 mW/cm2. The area of one photovoltaic pixel
was about 4.5 mm2.
Electrochemical measurements were carried out using Eco Che-
mie Autolab PGSTAT128n potentiostat. Platinum wire (diam.
1 mm), platinum coil and silver wire served as working, auxiliary
and reference electrode, respectively. Potentials are referenced
with respect to ferrocene, which was used as the internal standard.
Differential pulse voltammetry (DPV) was carried out with the fol-
lowing parameters: step potential 2.5 mV, modulation amplitude
50 mV, modulation time 50 ms, interval time 0.5 s. The experi-
ments were conducted in a standard one-compartment cell, in
dichloromethane (Carlo Erba, HPLC grade). Before each measure-
ment, argon was passed through the solution for at least 5 min.
0.2 M Bu4NPF6 (Acros, 98%, recrystallized twice from isopropanol
and dried at 180 °C in vacuum) was used as the supporting
electrolyte.
Optical and photoluminescence properties
The optical properties of solutions and films of BTA1 and BTA2
were investigated by UV–vis absorption spectroscopy. Uniform
films were prepared on quartz plates by spin casting from their
chloroform solution at room temperature. The UV–vis absorption
spectra of the compounds in solution and as thin films are shown
in Figs. 2 and 3 and summarized in Table 1.
The solutions of BTA1 showed one absorption band with maxi-
mum peak at 324 nm, while BTA2 present two absorption bands
with maximum peaks at 315 and 355 nm. The spectra of BTA1
and BTA2 in thin solid films show one main absorption band with
maximum peak at 335 and 353 nm, corresponding to band gaps
(Eg) of 2.59 and 2.72 eV, respectively. In comparison with the
absorption spectra of the solution, absorption spectra of the BTA1
thin films show shift to a longer wavelength region. The absorption
maximum of BTA1 film shows a red-shift by 11 nm compared with
that of solution. However, in the case of BTA2 film almost no shift
was observed compared with that of solution (see Figs. 2 and 3).
Absorption coefficient plot of the BTA1 and BTA2 thin layer is
presented in Fig. 2b, while linear approximation of absorption edge
of both compounds is shown in Fig. 2c. The energy band gap of the
investigated compounds was calculated for index r equal to 2,
which is indirect band to band transition. Differences in values of
energy gap detected from absorption spectra Eogpt: and calculated
from the absorption coefficients Eðgr¼2Þ were small. For BTA1 value
of Eðgr¼2Þ calculated from the absorption coefficient was found at
about 2.48 eV, while for BTA2 was detected at ꢀ2.51 eV.
The photoluminescence (PL) spectra of the compounds were
investigated in chloroform solution at concentration 10ꢁ4, 10ꢁ5
and 10ꢁ6 mol/L and the results are summarized in Table 2.
Emission spectra were recorded under excitation in maximum
of absorption (UV–vis) wavelengths and also under excitation in
maximum of excitation spectra wavelengths. Both compounds
exhibited PL features and emitted blue light with the PL maximum
at 496 nm. BTA1 and BTA2 under excitation in maximum of
absorption emitted blue light, however with very low PL intensity.
It was found that maximum of emission band of the both com-
pounds under excitation in maximum of excitation wavelength
was more intensive in comparison with PL maximum detected un-
der excitation in maximum of absorption wavelength, however no
shift in maximum of emission band was found. BTA1, in which the
fluorinated chain is present, exhibits higher photoluminescence
intensity (by a factor of 6) in comparison with BTA2. It is, therefore
clear that the fluorine atoms not only lower the luminescent ex-
cited state but also influences the electronic structure of the com-
pounds in such a manner that the probability of the radiative
transition is increased. The origin of this behavior is not clear at
the present time and requires further investigations. Emission
spectra of BTA1 and BTA2 in chloroform solution (c = 10ꢁ5 mol/L)
under different excitation wavelengths are presented in Fig. 3.
Effect of concentration on photoluminescence in chloroform
solution shows significant spectral changes in the maximum of
emission band (for BTA2) and PL intensity (for both compounds)
(Table 2, and in SI). Concentration of compound in solution was
1 ꢂ 10ꢁ4, 1 ꢂ 10ꢁ5 and 1 ꢂ 10ꢁ6 mol/L.
The surface morphology investigations of the compounds were
performed in air using a commercial Innova AFM system from Vee-
co company. Measurements were done in Tapping Mode and Phase
Imaging. Typical cantilever (about 40 N/m and <10 nm tip curva-
ture) was used.
Results and discussion
The general synthetic route toward the small molecules BTA1
and BTA2 is presented in Fig. 1.
The structures and purities of both compounds were confirmed
by 1H NMR, 13C NMR, FTIR and elemental analysis. Their expected
chemical constitution is clearly confirmed by spectroscopic stud-
ies. In particular in 13C NMR spectra of the azomethines the signals
in the range of 167–168 ppm, confirmed the existence of the azo-
methine group carbon atoms (HC@NA). In proton NMR spectra of
the investigated compounds the azomethine proton signal was ob-
served in the range of 8.80–8.83 ppm as it was expected. The pres-
ence of the azomethine group was also confirmed by FTIR
spectroscopy since in each case the band characteristic of the
HC@NA stretching vibrations was detected. The exact position of
this band varies in the spectral range 1619–1622 cmꢁ1 (see Exper-
imental part). The obtained proton and carbon NMR and FTIR spec-
tra collected in the Experimental part are in agreement with
spectral data predicted from the chemical formulae of the synthe-
sized compounds. Additionally, elemental analyses show good

A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
549
0.50
140
120
100
80
0.33
0.30
0.27
0.24
0.21
0.18
0.15
0.12
0.09
0.06
0.03
a
in film
0.45
BTA1
0.40
exc. = 270 nm
BTA2
BTA1
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
60
40
exc. = 324 nm
20
200 250 300 350 400 450 500 550 600 650 700 750 800 850
0
250
300
350
400
450
500
550
600
650
700
λ
[nm]
Wavelength [nm]
2,75x104
2,50x104
2,25x104
2,00x104
1,75x104
1,50x104
1,25x104
1,00x104
7,50x103
5,00x103
b
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
20
18
16
14
12
10
8
BTA2
BTA1
BTA2
exc. = 270 nm
exc. = 350nm
exc. = 310nm
6
2,50x103
0,00
4
2
1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
Energy [eV]
0
300
400
500
600
700
Wavelangth [nm]
320
c
Fig. 3. Absorption (straight line) and emission (dotted line) spectra of BTA1 and
BTA2 in CHCl3 solution (c = 10ꢁ5 mol/L) under different excitation wavelengths.
280
240
200
160
120
80
shift indicates difference in the energy loss which occurred during
transition from S0 to S1 and for the investigated compounds was
observed in the range of 4585–10703 cmꢁ1. Stokes shift is an
important parameter which indicates the differences in properties
and structures of the compounds. The Stokes shift values were
found to be the highest for BTA1 what illustrates that more energy
loss occurred during the transition from S0 to S1 in the compound
with fluorine atoms.
It is known that the concentration and kind of solvent influence
strongly on the photoluminescence properties. The mechanism of
quenching due to concentration, in organic light emitting diodes
including electro- and photoluminescence is still not wide investi-
gated [11]. In our case for the compound BTA1 with F atoms only
for the concentration 10ꢁ5 mol/L PL in chloroform solution was de-
tected. These results can be interpreted as follows. At low concen-
tration (10ꢁ6 mol/L) intermolecular distances between BTA1 are so
large that the molecules do not interact. Finally, the BTA1 can be
indicate as isolated molecules, and thus PL is unable to be detected.
With increase the concentration to 10ꢁ5 mol/L intermolecular dis-
tances between BTA1 molecules start to be lowers and PL proper-
ties are detected. With further increase in the concentration
10ꢁ4 mol/L the distance between BTA1 molecules start to be nar-
rows and supramolecular interaction between compound and
dimerization can happen and consequently BTA1 photolumines-
cence is quenched in the presence of dimers. Similar behavior
was explain in [11].
BTA2
BTA1
40
0
2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4
Energy [eV]
Fig. 2. Room temperature solid-state UV–vis absorption spectra of BTA1 and BTA2
(a), absorption coefficient (b) and linear approximation of absorption edge (c). Films
were spin casting on quartz substrate.
BTA1 exhibited PL properties only under concentration
1 ꢂ 10ꢁ5 mol/L with the emission peak at 496 nm. For the concen-
tration 1 ꢂ 10ꢁ4 and 1 ꢂ 10ꢁ6 mol/L no photoluminescence effect
was detected (see Fig. in SI). Opposite behavior was found for
BTA2. Compound BTA2 emitted light under different concentra-
tions (see Fig. in SI). However, as in the case of BTA1 the highest
PL intensity was observed for the concentration 1 ꢂ 10ꢁ5 mol/L.
At a lower concentration of BTA2 (1 ꢂ 10ꢁ6 mol/L) one PL max-
imum at about 424 nm was observed. At a higher concentration of
BTA2 (1 ꢂ 10ꢁ4 mol/L) one PL maximum at about 424 nm was
found, along with long tail. Presence of one PL band in the case
of both compounds can be attributed to the 0–0 transition. Stokes
This situation can be additionally explained by the fact that in
the case of BTA1 F atoms are presented. The fluoric substituent

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A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
Table 1
Absorption properties of BTA1 and BTA2.
Code
UV–vis
*
**
kmax (nm)
***
Eogpt: (eV)
Eogpt: (eV)
**
*
Eðgr¼2Þ& (eV)
kmax (nm)
Dkmax
BTA1
BTA2
324
315, 355
335
353
+11
ꢁ2
3.11
2.99
2.59
2.72
2.48
2.51
*
In chloroform solution.
In solid state as thin film.
**
***
Shift of absorption band in solid state in comparison with solution, ‘‘+’’ indicates red shift, ‘‘ꢁ’’ indicates blue shift, & calculated from equation
a
ꢂ E = A (E ꢁ Eg)r for r = 2.
Table 2
reduction peak potential of BTA1 is less negative than that of BTA2,
most likely due to the strong inductive effect of the perfluoroalkyl
chain (compare, for example, the Hammett rp constant for
CF2CF3:+0.52 to that of CH2CH3: ꢁ0.15 [17]). Conversely, the oxida-
tion of BTA1 occurs at less positive potentials than that of BTA2
(we believe that in both cases the imine bond is oxidized [18]),
possibly because of either intermolecular interactions with the
perfluoroalkyl chain (stabilizing the oxidized BTA1) or different
geometry of the molecule due to the different electronic influence
of the alkyl and perfluoroalkyl chains. However, this effect cannot
be explained by analyzing the electron density in the imine bond
alone: the 13C-NMR shift of the azomethine carbon in BTA1
(155.77 ppm) is larger that of the corresponding carbon in the
spectrum of BTA2 (152.45 ppm), demonstrating that the charge
density in the imine moiety of BTA1 is slightly lowers than in BTA2.
Electrochemical Eg of both compounds is in agreement with the
values determined from UV–vis measurements. BTA1 exhibited an
Eg of ꢀ2.31 and 2.59 eV, respectively, investigated electrochemi-
cally and from UV–vis spectrum, while for BTA2 the electrochem-
ical and optical Eg was found to be ꢀ2.81 and 2.72 eV. It is
interesting that electrochemical properties of aromatic compounds
with benzothiazole rings and triphenylamine moieties presented
in [16] exhibited higher value of Eg and higher value of HOMO–
LUMO levels in comparison with BTA1.
Photoluminescence properties in chloroform solution of BTA1 and BTA2
(exc. = 270 nm).
Code
BTA1
c (mol/L)
kem (nm)
Stokes shift* (cmꢁ1
)
10ꢁ6
10ꢁ5
10ꢁ4
10ꢁ6
10ꢁ5
10ꢁ4
–
–
**
496
10703
–
4585
8008
4585
**
–
BTA2
424
496
424
*
mabs
Not observed.
ꢁ
m
emis. = (1/kabs. ꢁ 1/kemis.) ꢂ 107.
**
in BTA1 is very interesting because of the combination of polar and
steric effects and the great strength of the CAF bond which confers
stability on fluoric substituted compound. Fluorine has: (i) the
highest electronegativity (3.98) of all the elements such as H, Cl,
Br, I, C, N, O, (ii) a low polarizability (5.57/1025 cmꢁ1) which confers
low intermolecular dispersion interactions, (iii) the smallest size,
after hydrogen, of all possible substituents and (iv) is monoatomic
[12]. The bulky nature of the neighboring fluoric substituents cause
steric interactions which tend to limit conformational flexibility
and hence perfluoric chains are often described as stiff [12]. This
behavior was not observed in the case of BTA2 with lack of F atoms.
Electrochemical properties
Photovoltaic properties
The electrochemical properties of BTA1 and BTA2 were studied
using differential pulse voltammetry in CH2Cl2 solution (Fig. 4 and
Table 3). Electrochemically determined HOMO–LUMO energy lev-
els and electrochemical band gaps of BTA1 and BTA2 were calcu-
lated as ꢁ4.8 ꢁ Eonset, and calculated from fits to UPS/IPES [13,14]
(Table 3).
The photovoltaic properties of BTA1 and BTA2 were preliminary
investigated by fabricating the organic photovoltaic devices with
ITO as anode, the blend with BTA1 and BTA2 and P3HT and PCBM
in weight ratio 1:1:1 as active layer, and Al as cathode. Both com-
pounds were applied as acceptors in organic photovoltaic devices.
Fig. 5 shows the current density–voltage (J–V) curves of the organic
photovoltaic devices investigated in this paper and are summa-
rized in Table 4.
The reduction peaks (BTA1: ꢁ1.65 V; BTA2: ꢁ1.88 V) are attrib-
uted to the reduction of the benzothiazole moiety [15,16]. The first
Fig. 4. (a) Differential pulse voltammetry of BTA1 (solid line) and BTA2 (dashed line) in CH2Cl2 and (b) electrochemically determined HOMO–LUMO energy levels and
electrochemical band gaps of BTA1 and BTA2 (solid line – calculated as ꢁ4.8 ꢁ Eonset, dashed line – calculated from fits to UPS/IPES).

A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
551
Table 3
Electrochemical data of BTA1 and BTA2.
E1HOMO
E2HOMO
E1LUMO
E2LUMO
Eg1
Eg2
Code
Eox,onset
Eox
Ered,onset
Ered
BTA1
BTA2
0.79
1.04
0.98
1.30
ꢁ1.52
ꢁ1.77
ꢁ1.65
ꢁ1.88
ꢁ5.59
ꢁ5.84
ꢁ5.86
ꢁ6.24
ꢁ3.28
ꢁ3.03
ꢁ2.82
ꢁ2.54
2.31
2.81
3.04
3.69
Peak potentials and onsets (from DPV) are given in Volts (vs. ferrocene), orbital energies are in eV. E1HOMO = ꢁ4.8 ꢁ Eox,onset; EL1UMO = ꢁ4.8 ꢁ Ered,onset; E2HOMO = ꢁ1.2 Eox ꢁ 4.68;
E2LUMO = ꢁ1.19 Ered ꢁ 4.78.
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
N
H
C16H33
N
S
N
H
(CF2)7
N
S
CF3
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8
U [mV]
0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
U [mV]
0
-2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-4
-6
-8
-10
-12
-14
-16
N
H
C16H33
N
H
(CF2)7
N
S
N
S
CF3
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
U [V]
0
3
6
9
12 15 18 21 24 27 30 33
U [mV]
Fig. 5. Current density–potential characteristics of the devices under AM1.5G irradiation (100 mW/cm2), left side: for BTA1, right side: for BTA2.

552
A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
Table 4
Photovoltaic properties of BTA1 and BTA2 in air atmosphere at room temperature under AM1.5G irradiation (100 mW/cm2).
Device
Jsc
(
l
A/cm2)
Voc (mV)
FF (ꢁ)
PCE (%)
ITO/PEDOT:PSS/P3HT:BTA1 (1:1)/Al
ITO/PEDOT:PSS/P3HT:BTA2 (1:1)/Al
ITO/PEDOT:PSS/P3HT:PCBM:BTA1 (1:1:1)/Al
ITO/PEDOT:PSS/P3HT:PCBM:BTA2 (1:1:1)/Al
1.47
1.56
1190
15
4.4
2.4
400
30
0.25
0.13
0.23
0.28
0.2 ꢂ 10ꢁ5
0.05 ꢂ 10ꢁ5
0.1
12.6 ꢂ 10ꢁ5
Table 5
UV–vis absorption properties of BTA1 and BTA2 blended with P3HT and PCBM.
Sample
kmax (nm)
Sample
kmax (nm)
P3HT:BTA1 (1:1)
552
523
508
511
P3HT:BTA2 (1:1)
PEDOT:PSS/P3HT:BTA2 (1:1)
P3HT:PCBM:BTA2 (1:1:1)
520
529
482
489
PEDOT:PSS/P3HT:BTA1 (1:1)
P3HT:PCBM:BTA1 (1:1:1)
PEDOT:PSS/P3HT:PCBM:BTA1 (1:1:1)
PEDOT:PSS/P3HT:PCBM:BTA2 (1:1:1)
The devices comprising BTA1 with P3HT:PCBM (1:1:1) showed
a VOC of 0.40 V, JSC of 1.19 mA/cm2, and FF of 0.23, giving a power-
conversion efficiency (PCE) of 0.10%. The device comprising BTA2
with P3HT:PCBM showed very low photovoltaic performance (Ta-
ble 4). The lower PCE of BTA2 was related to the long alkyl chain of
the amine moiety and corresponds with lower PL features and
higher Eg in comparison with BTA1 containing fluorine atoms. Fill
factor of all devices, except ITO/PEDOT:PSS/P3HT:BTA2/Al was ob-
served in the range 0.23–0.28 (see Table 4). The preliminary pho-
tovoltaic experiments suggested that compounds having polar
groups such as fluorine atoms are better for photovoltaic applica-
tions. In our case it was demonstrated that compound BTA1 with
fluorine atoms has a three times higher value of PCE in comparison
with BTA2 (Table 4). The low photovoltaic values of the investi-
gated compounds can be additionally explained by the fact that
all layers in photovoltaic devices were prepared in air atmosphere
at room temperature. Perhaps the same experiments realized in
glove box can give the more satisfactory photovoltaic results.
The differences between power conversion efficiency of devices
ITO/PEDOT:PSS/P3HT:PCBM:BTA1 (1:1:1)/Al and ITO/PEDOT:PSS/
P3HT:PCBM:BTA2 (1:1:1)/Al can be related with optical and struc-
tural properties of active photovoltaic layers.
In case of BTA1 blended with P3HT (ꢀ520 nm) red shift
(552 nm) of maximum of absorption band was detected. However,
for the mixture P3HT:BTA2 not differences in maximum of absorp-
tion band were found. For both devices with active layer based on
P3HT:PCBM and small molecules (BTA1 or BTA2) changes in max-
imum of absorption band (blue shift) was detected after imple-
mentation of PCBM. The results are presented in Table 5 and Fig. 6.
Based on the UV–vis absorption measurement we can conclude
that small molecule BTA1 has better influence on maximum and
extension of absorbance peak than BTA2. It probably caused that
in thin layer with BTA1 more excitons were created, which take
part in generation comparatively greater current density than in
layer with BTA2.
character. However, BTA1 contains fluorine atoms which could
cause better transportation of electrons between polymer P3HT
and fullerenes PCBM. In devices based on P3HT and small mole-
cules without PCBM, power conversion efficiency have the same
value of PCE, but different value of open circuit voltage was de-
tected due to interaction between the HOMO, LUMO energy levels
of applied organic materials.
0.80
0.75
0.70
a
PEDOT:PSS/P3HT:BTA1(1:1)
P3HT:BTA1 (1:1)
P3HT:PCBM:BTA1 (1:1:1)
PEDOT:PSS/P3HT:PCBM:BTA1(1:1:1)
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
350
400
450
500
550
600
650
700
750
800
850
λ [nm]
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
b
PEDOT:PSS/P3HT:BTA2(1:1)
P3HT:BTA2 (1:1)
P3HT:PCBM:BTA2 (1:1:1)
PEDOT:PSS/P3HT:PCBM:BTA2(1:1:1)
To obtain highest power conversion efficiency of organic solar
cells also important is flat surface of active layers. AFM experi-
ments were allowed to appointed such parameters as porosity of
surface (see Fig. 7 and Table 6) from which could be concluded that
active layers with BTA1 small molecules exhibited lower rough-
ness compare to layer with BTA2. It contributes to better connec-
tion between organic active layer and aluminum electrode. The
final results of that is a higher current density and power conver-
sion efficiency, VOC and FF.
350
400
450
500
550
600
650
700
750
800
850
The lower value of PCE for the devices without of PCBM is a con-
sequence of nature of the compounds BTA1 and BTA2. Probably
small molecules BTA1 and BAT2 have donor–acceptor conductivity
λ [nm]
Fig. 6. UV–vis spectra of BTA1 (a) and BTA2 (b) blended with P3HT and PCBM.

A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
553
Fig. 7. AFM images of (a) BTA1 and BTA2, (b) P3HT, P3HT:BTA1, P3HT:BTA2 (c) P3HT:PCBM, P3HT:PCBM:BTA2, PEDOT:PSS/P3HT:PCBM:BTA2, (d) PEDOT:PSS/P3HT:PCBM,
PEDOT:PSS/P3HT:BTA1, PEDOT:PSS/P3HT:BTA2 and (e) P3HT:PCBM:BTA1, PEDOT:PSS/P3HT:PCBM:BTA1, from left to right (3 m).
m ꢂ 3
l
l

554
A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
Table 6
The surface parameters of BTA1, BTA2, P3HT and various devices.
Code
Surface statistics*
Ra (nm)
Rms (nm)
Skew
1.354
0.483
3.953
0.835
0.621
Kurtosis
Surface area ratio
BTA1
BTA2
P3HT
P3HT:BTA1
21.18
65.06
5.65
3.33
17.27
0.55
3.51
8.21
0.33
1.61
1.69
9.63
9.54
29.01
99.37
10.77
5.82
23.11
0.72
8.04
12.72
0.47
2.14
2.15
11.61
11.99
4.807
5.873
26.761
15.533
4.415
3.9318
89.849
11.486
12.913
4.138
1.003
1.745
1.004
1.010
1.064
1.001
1.008
1.007
1.001
1.006
1.005
1.028
1.038
P3HT:BTA2
PEDOT:PSS/P3HT:PCBM
PEDOT:PSS/P3HT:BTA1
PEDOT:PSS/P3HT:BTA2
P3HT:PCBM
P3HT:PCBM:BTA1
PEDOT:PSS/P3HT:PCBM:BTA1
P3HT:PCBM:BTA2
PEDOT:PSS/P3HT:PCBM:BTA2
0.364
7.906
ꢁ0.843
0.680
ꢁ0.020
ꢁ0.017
ꢁ0.131
0.065
3.353
2.536
2.896
*
Values calculated for scanning field 3
lm ꢂ 3 lm (9 l
m2 scan area).
AFM study
surface PEDOT:PSS/P3HT:BTA2 shows highly oriented, linear
rectangle-shaped structures, up to 7 m in length and 1 m in
l
l
In the AFM experiments the influence of the chemical structure
of the azomethines with benzothiazole core on the surface mor-
phology of the materials was investigated. Films on the glass sub-
strate were obtained by dissolving the compound in chloroform at
room temperature to form a homogenous solution. Residual sol-
vent was removed by heating of the film. Fig. 7 shows AFM images
of BTA1, BTA2, P3HT, P3HT:BTA1, P3HT:BTA2, P3HT:PCBM,
P3HT:PCBM:BTA2, PEDOT:PSS/P3HT:PCBM:BTA2, PEDOT:PSS/
P3HT:PCBM, PEDOT:PSS/P3HT:BTA1, PEDOT:PSS/P3HT:BTA2 and
P3HT:PCBM:BTA1, PEDOT:PSS/P3HT:PCBM:BTA1. Typical parame-
ters of topography, such as roughness (Ra, Rms) along with skew
(the unbalance of height distribution maximum) and kurtosis
(the peak’s width on height distribution) for two investigated sur-
faces are presented in Table 6.
width (Fig. 7d). Each structure of PEDOT:PSS/P3HT:BTA2 contains
a number of layers with apparent height about 2 nm.
When the topography is measured, one can extract certain data
describing the properties of the surface. Typical parameter is
roughness (Ra, Rms). However, it is useful to use another parameter
such as skew (the unbalance of height distribution maximum) and
kurtosis (the peak’s width on height distribution). Mentioned
parameters for investigated surfaces are presented in Table 5.
The presented parameters are defined in equations (1–4).
XX
1
MN
Ra ¼
Mꢁ1Nꢁ1jzðxk; y Þj
ð1Þ
l
k¼0 l¼0
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
Mꢁ1Nꢁ1
u
t
XX
1
MN
2
Compound BTA1 exhibited a lower value of roughness (Ra, Rms
)
Rrms
¼
½zðxk; ylÞꢃ
ð2Þ
ð3Þ
k¼0 l¼0
in comparison with BTA2 (see Table 6). This behavior definitively
confirmed together with previously experiments (UV–vis, PL, CV,
J–V) that for photovoltaic experiments more prosperous is the
compound BTA1 with fluorine atoms.
Topography images of the BTA1 exhibited highly ordered, linear
features forming terrace-like structures with almost the same
steps (2.57, 2.57, 2.66, and 2.59 nm) (see Fig. 7 and SI). Topography
of BTA2 sample (Fig. 7a) was not as ordered as BTA1 and exhibited
much more irregular features.
In Fig. 7b topography of P3HT and P3HT:BTA1 (or BTA2) are pre-
sented. Topography of the P3HT was flat and homogenous. On the
other hand, topography of the P3HT:BTA1 reveals macromolecular,
layer-like ordering (Fig. 7b). Various heights of the steps were vis-
ible: 1, 2, 3, 4 and 5 nm. The surface of P3HT:BTA2 reveals two
1
Rsk
¼
Mꢁ1Nꢁ1
XX
3
MNR3rms
MNR4rms
½zðxk; ylÞjꢃ
k¼0 l¼0
1
Rku
¼
ð4Þ
Mꢁ1Nꢁ1
XX
4
½zðxk; ylÞꢃ
k¼0 l¼0
where:
M and N are the total number of pixels in the analyzed scan,
k and l are coordinates of a given pixel,
z(xk, yi) is the height of the surface at a given coordinate.
types of the structures: (i) linear flake-like structures 3–5
length and 1 m in height and (ii) grainy structures with 500 nm
in diameter (Fig. 7b).
lm in
l
The kurtosis parameter provides quantitative information about
the height distribution of the topography. It is provided in Table 6.
For P3HT value of kurtosis was 26.761, while for P3HT:BTA1
15.533 (these are very high values as we refer them to other sur-
faces). Definitively lover value of kurtosis was detected for
P3HT:BTA2 being at 4.415. The advantage of such approach is the
less space usage than in case of the height distribution or
Abott-Firestone graph.
For P3HT:PCBM flat surface with grainy structure was observed
(Fig. 7c). Very highly ordered flake-like rectangle structures for
P3HT:PCBM:BTA2 were visible (Fig. 7c). Orientation of the objects
was approximately homogenous (360°), although one direction 22°
seems to be a little bit more privileged than others (angular spec-
trum graph shows value 13 in this direction, while the average va-
lue was about 9). The size of the objects was 2–3 lm in length and
150–400 nm in width. Every such structure has layered structure
(4 nm thick). Layers were rectangle-shaped oriented together
within one flake.
Conclusions
In the next step of AFM investigations influence of PEDOT:PSS
on topography of the devices were investigated (Fig. 7d). The sur-
face of PEDOT:PSS/P3HT:BTA1 exhibited distinct, layered structure,
with step-height 0.4 nm. On the other hand, the topography of the
Investigated compounds BTA1 and BTA2 were soluble in com-
mon organic solvents. Their wavelength absorption maximum
was at 335–353 nm in thin film corresponding to an optical band
gap of 2.59–2.72 eV. The electrochemical band gap estimated from

A. Iwan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 546–555
555
the cyclic voltammetery data was 2.31–2.81 eV, which is close to
References
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.054.