Synthesis and photophysical properties of semiconductor molecules D1-A-D2-A-D1-type structure based on derivatives of quinoxaline and dithienosilole for organics solar cells

Article abstract of DOI:10.1016/j.orgel.2016.10.010

A novel small molecule with D1-A-D2-A-D1 structure denoted as DTS(QxHT₂)₂ based on quinoxaline acceptor and dithienosilone donor units was synthesized and its optical and electrochemical properties were investigated. The thin film of DTS(QxHT₂)₂ showed a broad absorption profile covering the solar spectrum from 350?nm to 780?nm with an optical bandgap of 1.63?eV. The energy levels estimated from the cyclic voltammetry indicate that this small molecule is suitable as donor along with PC₇₁BM as acceptor for the fabrication solution processed bulk heterojunction solar cells for efficient exciton dissociation and high open circuit voltage. The organic solar cells based on optimized DTS(QxHT2)2:PC₇₁BM active layers processed with chloroform and DIO/CF showed overall power conversion efficiency of 3.16% and 6.30%, respectively. The higher power conversion efficiency of the solar cell based on the DIO/CF processed active layer is attributed to enhanced short circuit photocurrent and fill factor may be related to better phase separation between donor and acceptor in the active layer and more balanced charge transport, induced by the solvent additive. The power conversion efficiency of the organic solar cell was further improved up to 7.81% based on active layer processed with solvent additive, using CuSCN as hole transport layer instead of PEDOT:PSS and mainly attributed to increased fill factor and open circuit voltage due the formation of better Ohmic contact between the active layer and the CuSCN layer.

Full text of DOI:10.1016/j.orgel.2016.10.010

Organic Electronics 39 (2016) 361e370
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and photophysical properties of semiconductor molecules
D -A-D -A-D -type structure based on derivatives of quinoxaline and
1
2
1
dithienosilole for organics solar cells
M.L. Keshtov a , D. Yu Godovsky , S.A. Kuklin , A. Nicolaev , J. Lee , B. Lim , H.K. Lee ,
,
**
a
a
a
b
b
b
c
,
d
e, *
E.N. Koukaras , Ganesh D. Sharma
a
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Str., 28, Moscow, 119991, Russia
LG Chem Research Park, Daejeon, 104-1, Moonji-dong, Yuseong-gu, 305738, South Korea
Nanotechnology and Advanced Materials Laboratory, Department of Chemical Engineering, University of Patras, Patras, 26500 GR, Greece
Molecular Engineering Laboratory, Department of Physics, University of Patras, Patras, 26500 GR, Greece
Department of Physics, The LNM Institute of Information Technology, Jamdoli, Jaipur, Rajasthan 302031, India
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 September 2016
Received in revised form
A novel small molecule with D1-A-D2-A-D1 structure denoted as DTS(QxHT2)2 based on quinoxaline
acceptor and dithienosilone donor units was synthesized and its optical and electrochemical properties
were investigated. The thin film of DTS(QxHT ) showed a broad absorption profile covering the solar
2 2
spectrum from 350 nm to 780 nm with an optical bandgap of 1.63 eV. The energy levels estimated from
the cyclic voltammetry indicate that this small molecule is suitable as donor along with PC71BM as
acceptor for the fabrication solution processed bulk heterojunction solar cells for efficient exciton
dissociation and high open circuit voltage. The organic solar cells based on optimized DTS(QxHT2)
6
October 2016
Accepted 6 October 2016
Keywords:
2:PC71BM active layers processed with chloroform and DIO/CF showed overall power conversion effi-
D1-A-D2-A-D1 based small molecule
Bulk heterojunction solar cells
Solvent additive
ciency of 3.16% and 6.30%, respectively. The higher power conversion efficiency of the solar cell based on
the DIO/CF processed active layer is attributed to enhanced short circuit photocurrent and fill factor may
be related to better phase separation between donor and acceptor in the active layer and more balanced
charge transport, induced by the solvent additive. The power conversion efficiency of the organic solar
cell was further improved up to 7.81% based on active layer processed with solvent additive, using CuSCN
as hole transport layer instead of PEDOT:PSS and mainly attributed to increased fill factor and open
circuit voltage due the formation of better Ohmic contact between the active layer and the CuSCN layer.
Power conversion efficiency
©
2016 Elsevier B.V. All rights reserved.
1. Introduction
improvement has been achieved in the power conversion efficiency
PCE) from less than 2% in 2008 to over 10% in the past three years,
(
In the last decade, organic solar cells (OSCs) have attracted
notable attention as renewable energy sources due to their unique
advantages such as low cost and large area flexible devices via
solution processing technique [1]. The most successful OSCs to date
are based on a bulk heterojunction (BHJ) active layer which consists
of a donor and acceptor [2]. Significant progress has been made in
both material development and device optimization (morphology
of active layer and interface engineering), and dramatic
using both conjugated polymers [3] and small molecules [4] with
the aid of the synergistic optimization of chemical structure design
and device engineering. In comparison to polymer donors, conju-
gated small molecules possess well defined structure to reduce the
structural variation in terms of irregularities, molecular weight and
polydispersity [5]. Moreover, small molecules can be more easily
designed to meet ideal specifications of good donors for BHJ-OSCs
such as strong absorption, suitable highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels, high mobility and solubility [6]. Different strategies
can be used to design semiconductor molecules, comprising of (a)
formation of conjugated donor-acceptor (D-A) structures for
achieving broad absorption in visible and near-infrared ranges [6],
*
Corresponding author.
** Corresponding author.
E-mail addresses: keshtov@ineos.ac.ru (M.L. Keshtov), gdsharma273@gmail.
com, sharmagd_in@yahoo.com (G.D. Sharma).
http://dx.doi.org/10.1016/j.orgel.2016.10.010
1566-1199/© 2016 Elsevier B.V. All rights reserved.

362
M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
and (b) introduction of planar structures to achieve high crystal-
linity, increasing charge carrier mobility, and as a result, short-
circuit current. These concepts have been used by us in the syn-
thesis of a new semiconductor molecule with a D1-A-D2-A-D1
were measured under AM1.5 solar simulator at 100 mW/cm2 and
data were collected using computer controlled Keithley source
meter.
The hole mobilities of the active layers were determined by
fitting the dark current to the model of space charge limited current
(SCLC) in the hole only device with configuration ITO/PEDOT:PSS/
active layer/Au. The active layers were deposited under the same
conditions as the corresponding solar cells.
structure, DTS(QxHT
dithienosilole (DTS) as a central donor (D
hexyl-bithiophene units as acceptor (A), and terminal groups (D
respectively. DTS has a simple and planar structure, increasing the
-stacking packing and charge transport. Quinoxaline is a relatively
2
)
2, which combines favorable properties of
), quinoxaline (Qx) and
),
2
1
p
simple and planar acceptor, which contributes to intramolecular
charge transfer and reduced bandgap of semiconductor molecules.
3. Results and discussion
Two terminal groups of thiophene (D
1
) cover the entire visible
2 2
3.1. Synthesis of DTS(QxHT )
range, while alkyl substituents provide compound solubility and its
self-organization in solid state. DTS has been widely used as a
building block in high efficiency organic solar cells [7]. Quinoxaline-
containing D-A copolymers in PSC composition have lead to effi-
ciencies exceeding 6% [8]. These results suggest that D-A small
molecules based on quinoxaline acceptor and other donor units can
be effective semiconductor compounds for producing OSCs with
good characteristics. On the other hand, to the best of our knowl-
edge, the small molecule with DTS donor and quinoxaline acceptor
has not been explored for OSC applications. In continuation to our
research work in designing low bandgap polymers and small
molecules, we have designed a small molecule with D1-A-D2-A-D1
with quinoxaline acceptor and dithienosilone donor units with
Synthetic route to 2,6-bis[2,3-(3’-octyloxyphenyl)-8-(5-hexyl-
2,2’-bithiphen-5-yl)quinoxaline-5-yl]-4,4-bis(2-ethylhexyl) -4H-
silolo[3,2-a: 4,5-a '] dithiophene DTS(QxHT ) (19) comprising of 16
2 2
stages is shown in Scheme 1. Key “building blocks” - (8) [9], (12) [9],
(18) [10] were obtained according to methods described in litera-
ture. Cross-coupling of quinoxaline derivative 8 with dithienosilole
18 led to formation of new dibromide 13 with 37% yield. Further
reaction of compound 13 with bithiophene derivatives 12 led to
2 2
formation of the desired compound DTS(QxHT ) with 65% yield
being dark purple solid substance. Composition and structure of
intermediate compounds 4, 6e8, 10e18 and the desired product
1
DTS(QxHT
2
)
2
have been characterized by elemental analysis and H
hexyl-bithiophene units as terminal units, denoted as DTS(QxHT
and its optical and electrochemical properties were investigated in
detail. We have used DTS(QxHT as donor along with PC71BM as
electron acceptor for the fabrication of the solution processed
organic bulk heterojunction solar cells and after the optimization of
2
)
2
NMR spectroscopy (Fig. 1).
2 2
In particular, in the aromatic range of the DTS(QxHT ) com-
2
)
2
pound proton spectrum at
responding to two protons of the quinoxaline fragment; singlet at
7.92 ppm belongs to the hydrogen atom of the dithienosilole
cycle. Signals at 7.80, 7.20, 7.12 and 6.75 are doublets belonging to
bithiophene terminal fragments. The rest of the signals at 7.6,
7.62, 7.30e7.21 and 7.01e6.95 are protons of octyloxyphenyl sub-
stituents. There are two triplets at 3.91 and 4.02 ppm in the
aliphatic spectrum range, related to the CH groups directly con-
nected to oxygen atoms. Triplet signal at 2.85 ppm belongs to CH
protons of terminal hexyl substituents terminal groups directly
bonded with the thiophene fragments. At 1.80e0.75 ppm there
d 8.17 ppm there is a broad singlet cor-
d
DTS(QxHT
2
)
2
to PC71BM weight ratio, the device processed from
d
chloroform solvent showed PCE of 3.16%. The PCE has been raised
up to 6.30% when a concentration of 3v% of DIO was added to the
host chloroform solution as a solvent additive. Moreover, when
PEDOT:PSS is replaced by CuSCN as HTL in combination with DIO/
CF processed active layer a further enhancement of all photovoltaic
parameters is realized leading to an increase in the PCE to 7.81%.
d
d
2
d
2
d
2
. Experimental details
are other signals of aliphatic substituents, and the integrated in-
tensity ratio of aliphatic to aromatic fragments is in good consis-
Synthesis and characterizations of DTS(QxHT
2
)
2
and interme-
2 2
tency with proposed structure of DTS(QxHT ) small molecule.
diate compounds are described in the supplementary information.
3.2. Thermal properties
2.1. Device fabrication and characterization
2 2
The thermal stability of the DTS(QxHT ) small molecules was
The devices were fabricated on indium tin oxide (ITO) coated glass
investigated by thermogravimetric analysis (TGA) and decompo-
substrates and cleaned via a route solvent ultrasonic cleaning, sub-
sequently with detergent, de-ionized water, acetone, and isopropyl
alcohol in ultrasonic bath for 10 min each. Subsequently, the pre-
cleaned ITO coated glass substrates were treated with UV-ozone for
sition temperatures (T
5% mass loss) for DTS(QxHT
(Fig. 2a), indicating high thermal stability of these molecules,
adequate for application in organic solar cells. The melting tem-
d
defined as the temperature corresponding
ꢀ
2
)
2
were determined to be 407
C
20 min. A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
2 2
peratures for DTS(QxHT ) was determined by differential scanning
(
PEDOT:PSS) solution (Clevious P VP AI 4083, H. C. Stark, Germany)
calorimetry (DSC). In the DSC curves a sharp endothermic peak
was then spin-coated onto the pre-cleaned ITO coated glass sub-
strates, and then dried by baking in an oven at 110 C for 15 min to
indicating the melting temperature (T
are 126 C (Fig. 2b).
m
) is observed for molecule
ꢀ
ꢀ
get PEDOT:PSS films with a thickness of ~40 nm. The active layer
blend layer was prepared by spin coating the chloroform solution of
3.3. Optical and electrochemical properties
2 2
DTS(QxHT ) and PC71BM (with different weight ratios with con-
centration 18 mg/mL) on top of the PEDOT:PSS layer and dried to
evaporate the residual of solvent completely. The ultra-thin poly
2 2
Fig. 3 shows the normalized absorption spectra of DTS(QxHT )
in chloroform solution and thin film cast from chloroform solution.
In solution DTS(FHT exhibits broad absorption bands in the
[
(
(9,9-bis(30-(N,N-dimethylamino)-propyl)-2,7-fluorene)]-alt-2,7-
9,9-dioctylfluorene) (PFN) layer was deposited by spin casting at
2 2
)
shorter wavelength range of 300e450 nm and in the longer
wavelength range of 470e720 nm centered at 596 nm, can be
2
9
000 rpm for 30 s from a 0.02% (w/v) methanol solution. Then,
0 nm aluminum (Al) was thermally deposited in a vacuum under a
assigned to the
p-p* transitions in the conjugated chain and
pressure of 10e5 Pa as a top electrode and the effective area was
measured to be 16 mm . The current-voltage (J-V) characteristics
intramolecular charge transfer (ICT) in D-A units. In thin film, the
absorption profile is broader, and the main ICT absorption band is
2

M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
363
2 2
Scheme 1. Synthetic route of DTS(QxHT ) .
moved toward a longer wavelength that may be related to better
molecules packing and strong inter-chain stacking in solid
state thus giving rise to electronic delocalization. The optical
bandgap (E go pt) of DTS(FHT
the DTS(FHT
Wide absorption spectra and narrow optical bandgap for
2
)
2
, found from the absorption onset of
p
-p
2 2
) absorption spectra, is estimated about 1.63 eV.

364
M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
Fig. 1. 1H NMR spectra of DTS(QxHT
) small molecule.
2 2
2 2
Fig. 2. (a) TGA curves Ar at scan rate 10 deg/min and (b) DSC curves of at scan rate 10 deg/min of DTS(QxHT ) .
DTS(FHT
2
)
2
suggests notable prospects for their application in OSC.
exciton dissociation in OSC [12]. The higher electrochemical
bandgap than optical bandgap may be attributed to the electron
transfer for the oxidation and reduction needs to overcome the
energy barrier at the electrode/solution interface [13].
The electrochemical cyclic voltammetry (CV) was performed to
measure the HOMO, LUMO energy levels and bandgap (E
DTS(QxHT
and 1.74 eV, respectively, which are close to values of ideal mate-
rials [11] for BHJ-OSCs. The deeper HOMO favors high open-circuit
voltage and chemical stability of OSCs in air. On the other hand, the
ec
g
) of
2
)
2
(as shown in Fig. 4) and were found to be ꢁ5.42, -3.68
3.4. Theoretical calculations
difference between the LUMO of DTS(QxHT
2 2
) and the LUMO of
Furthermore, we have performed a theoretical study on the
DTS(QxHT ) molecular structure within the framework of density
2 2
(
PC71BM) is greater than 0.3 eV, which is sufficient for efficient

M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
365
lowest allowed vertical electronic excitation, employing the PBE,
M06, and B3LYP functionals. In Table 1, in addition to the frontier
orbitals' energy levels, we also provide the optical gap the main
contributions to the first excitation as well as the wavelength of the
first excitation and of the excitations with the largest oscillator
strengths.
The HOMOeLUMO (HL) gap calculated using the hybrid B3LYP
functional is notably smaller, by 0.37 eV, than that using the meta-
hybrid M06 functional, however, the calculated optical gaps only
differ by 0.11 eV. In Table 1 we also provide the character of the first
allowed excitations only for contributions larger than 4%. The first
excitation, as calculated by each of the functional clearly exhibits a
single-configuration character. In Fig. 5, we have plotted the iso-
surfaces (isovalue ¼ 0.02) of the HOMO and LUMO of DTS(QxHT
2 2
) .
The HOMO and LUMO both extend over the main body, however
the LUMO exhibits an increase contribution from the pyrazine of
the Qx moiety as well as from the sulfur atoms of the DTS moiety.
2 2
Fig. 6 shows the UV/Visual absorbance spectra the DTS(QxHT )
molecular structure calculated at the TD-DFT/M06 level of theory,
both accounting for solvent effects for CF and in gas phase. The
computed theoretical spectrum overestimates the wavelengths
compared to the experimental spectrum by ~70 nm in the long
wavelength region, and ~35 nm in the shorter wavelength region.
Fig. 3. The normalized absorption spectrum of the compound DTS(QxHT
roform solution and thin film cast from chloroform solution.
2 2
) in chlo-
3.5. Photovoltaic properties
OSC
DTS(QxHT
2 2
investigate the photovoltaic performance of DTS(QxHT )
with
:PC71BM/PFN/Al were fabricated and characterized to
as donor
the
device
structure
ITO/PEDOT:PSS/
2 2
)
in BHJ active layer. First of all, we optimized the D/A weight ratio in
chloroform solvent and found that the weight ratio 1:2 showed the
best performance. The current evoltage characteristics of the
2
optimized OSCs under AM1.5 G irradiation with 100 mW/cm in-
tensity are shown in Fig. 7a and the corresponding photovoltaic
parameters are compiled in Table 2. The OSC based on the opti-
mized active layer processed with chloroform showed a PCE of
2
3
.16% with Jsc of 7.85 mA/cm , Voc of 0.96 V and FF of 0.42. The high
V
oc may be related to the deeper HOMO energy level of
DTS(QxHT , since the Voc of OSCs based on BHJ active layer is
2 2
)
proportional to the difference of the LUMO level of electron
acceptor and HOMO level of the donor. The PCE of the device
fabricated with the presence of 3 v% DIO improved up to 6.30% with
2 2
Fig. 4. Cyclic voltammograms of DTS(QxHT ) compounds in acetonitrile 0.1M
Bu NCIO at scanning rate 100 mV .
4 4
ꢁ
1
Jsc of 12.08, Voc of 0.90 and FF of 0.58. The increase in the PCE has
been mainly attributed to the enhancement of both Jsc and FF, can
be contributed by the more ordered molecular self-assembly and
better nano-phase separation and D/A networks and therefor more
effective charge separation and transportation [23], which is
confirmed from the AFM measurements discussed later on. The
IPCE spectra of the devices (Fig. 7b) closely resemble the absorption
spectra of the corresponding active layers (Fig. 8) and also indicates
functional theory (DFT) and time-dependent density functional
theory (TD-DFT) [14e18]. Details on the computations are given in
the Supplementary Information.
TD-DFT excited state calculations were performed to calculate
2 2
the optical gaps of DTS(QxHT ) using the same functionals and
basis set on the corresponding ground state structures. The UV/Vis
spectra were calculated using the B3LYP [17] and M06 [18] func-
tionals. The M06 meta-hybrid functional was chosen since it pro-
vides leveled performance over transition types [21,22]. We provide
results using all three functionals, which can additionally be used
for comparison with the literature. All of the follow up calculations
were performed using the Gaussian package [19]. The first round of
geometry optimizations was performed using the Turbomole
package [20].
2 2
that both DTS(QxHT ) and PC71BM contributes to the exciton
generation and thereby photocurrent. Moreover, the IPCE spectra of
the optimized device with DIO additive showed a broad response as
compared to that of the as cast device, that also confirm the in-
crease in Jsc. The integration of IPCE spectra yields the estimated Jsc
2
2
of 7.79 mA/cm and 11.96 mA/cm for the as cast and DIO solvent
additive based devices, respectively, which are in good agreement
with the measured values.
Fig. 8 shows the UV-visible absorption spectra of the blend film
cast with and without solvent additive. As shown in Fig. 8 the main
ICT absorption peak observed in the as cast blend film is slightly
The main body of the structure, that consists of dithienosilole
(
DTS) and quinoxaline (Qx) moieties, is somewhat planar with
dihedral angles between these moieties, as well as the thiophenes,
2 2
blue shift as compared to the pristine DTS(QxHT ) thin film.
ꢀ
ꢀ
in the range of ~2 e15 , depending on the functional and the
presence of solvent. We have calculated the HOMO and LUMO
energy levels and the optical gaps, defined here as the energetically
However, when the film is processed with DIO, absorption in
shorter wavelength region is unchanged but induces a redshift and
increased intensity in the longer wavelength region, which can be

366
M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
Table 1
Calculated properties of DTS(QxHT
2
)
2
. Specifically, HOMO and LUMO energies (eV), HOMOeLUMO gap (eV), HL, Optical gap (eV), OG, with corresponding oscillator strengths, f,
the wavelengths of the first excitation and excitations with the largest oscillator strengths, the main contributions to the first excited state, and the dipole moment (D), m.
HOMO (eV)
LUMO (eV)
HL (eV)
OG (eV)
l1st/max (nm)
f
Main contributions
m (D)
2 2
DTS(QxHT )
PBE
ꢁ4.07
ꢁ3.05
1.03
1.03
1.28
1.23
1.79
1.73
1.90
1.84
966
1010
1.26
1.52
1.73
1.97
1.73
1.98
H / L (96%)
H / L (98%)
H / L (98%)
H / L (97%)
H / L (94%)
H / L (94%)
2.77
3.21a
2.92
3.38a
2.74
3.04a
4.29a
ꢁ3.26
a
a
a
a
a
a
a
a
ꢁ
B3LYP
M06
ꢁ4.63
4.83a
ꢁ4.92
5.13a
ꢁ2.54
2.09
2.09
693/444/428/400/389
715/492/444/426/392
653/420/405/376/370
674/565/420/404/382
a
a
a
a
a
a
ꢁ
ꢁ2.73
ꢁ2.46
2.46
2.46
a
a
a
a
ꢁ
ꢁ2.67
a
Values when solvent effects are taken into account for chloroform.
2 2
Fig. 5. Frontier orbitals i.e. HOMO and LUMO of DTS(QxHT ) .
respectively, indicating the balanced charge transport in the device,
which is consistent with the increase in FF. The electron mobility of
the active layer is mainly determined by the n-type PC71BM in the
active layer, which show slight increase by DIO additive indicating
that the packing behavior of PC71BM may be adjusted by the solvent
additive due to the high boiling point of DIO [24] and attributed to
the better nanoscale phase separation for electron transport. The
increase in hole mobility leads to the decrease in the electron to
hole mobility, results a balanced charge transport, after the solvent
additive. These effects increase the power conversion efficiency.
To further get information about the performance enhancement
upon the DIO additive for active film, transmission electron mi-
croscopy (TEM) images of the active layer films processed with and
without DIO additive were investigated and shown in Fig. 10. As
shown in Fig. 10, the bright and dark regions in the TEM images
correspond to the donor and PC71BM rich domains, respectively
[
25]. The as cast film shows relatively smooth morphology with
2 2
Fig. 6. Theoretical UV/Vis absorption spectrum of DTS(QxHT ) (calculated using the
M06 functional).
surface which is not sufficient for the efficient exciton dissociation
as well as for charge transport within the active layer and resulted
low Jsc and FF. When the active layer is processed with DIO, the TEM
shows fibrous features with a better phase separation, which is
favorable for exciton dissociation and charge transport.
In order to get information about the relationship between the
PCE and the packing behavior and crystallinity of the active layer of
DTS(QxHT
diffraction (XRD) of the thin films processed with and without DIO
additives (Fig. 11). Both CF and DIO/CF cast DTS(QxHT
films showed a reflection peak at 2
to the d-spacing of DTS(QxHT
CF, the reflection intensity of the DTS(QxHT
2 2
attributed to more ordering of DTS(QxHT ) . This increases the
light harvesting efficiency of the device and improves the Jsc of the
device processed with DIO.
In order to get information about the charge transport proper-
ties within the active layer processed with different conditions, the
space charge limited current (SCLC) was employed to measure the
2 2
) with the solvent additive, were investigated by X-ray
)
2
:PC71BM
hole and electron mobilities with
a device structure ITO/
2
ꢀ
q
¼ 4.58 which is corresponds
PEDOT:PSS/active layer/Au and ITO/Al/active layer/Al, respectively.
The current evoltage characteristics of hole and electron only de-
vices are shown in Fig. 9a,b, respectively, The hole and electron
2
)
2
. As compared to the film cast from
:PC71BM was
2
)
2
ꢁ
5
2
significantly enhanced, indicating that the degree of crystallinity of
the active layer, particularly DTS(QxHT ) was improved, which is
in agreement with the TEM images that the surface morphology
was superior and phase separation was improved. The increased
mobilities without additive are about 4.54 ꢂ 10
cm /V and
ꢁ
4
2
2
.64 ꢂ 10 cm /V, respectively. However, when the active layer is
2 2
processed with DIO additives, the hole and electron mobilities are
increased up to 1.45 ꢂ 10ꢁ cm /V and 2.76 ꢂ 10
4
2
ꢁ4
2
cm /V,

M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
367
Fig. 8. Normalized absorption spectra of DTS(QxHT
and DIO/CF solutions.
2
)
2
:PC71BM thin films cast from CF
photogenerated excitons are dissociated onto free charge carriers at
high Veff, and Jphsat is only limited by the maximum exciton gen-
eration rate (Gmax) and thus Jphsat ¼ qLGmax, where q is the
elementary charge and L is the thickness of active layer. Gmax for
21
3
21
3
these devices are as follows: 7.36 ꢂ 10 cm /s, 9.8 ꢂ 10 cm /s for
the as cast and DIO/CF processed based devices, respectively. The
increased value of Gmax for the solvent additive based device
compared to that of the as cast counterpart suggests an increased
overall exciton generation in the device based on the solvent ad-
ditive processed active layer, which originates from the enhanced
absorption in the active layer processed with DIO/CF solvent as
indicated by the absorption spectral response (Fig. 8). It should be
noted that Jphsat is increased by only 34% with the solvent additive,
Jsc increased by 54%, indicating different P
c c
for these devices. P is
determined as P
c
¼ Jsc/Jphsat were 0.74 and 0.83 for the devices
Fig. 7. (a) Current-voltage (J-V) characteristics under illumination and (b) IPCE spectra
of the organic solar cells based on DTS(QxHT ) :PC71BM active layers processed under
2 2
different conditions.
fabricated with active layers CF and DIO/CF processed, respectively.
This shows that the solvent additive contributed to both light ab-
sorption and exciton dissociation in the active layer.
Although the PEDOT:PSS as hole transport layer (HTL), the
organic solar cells based on either polymers or small molecules
have shown significant improvement in the PCE of these devices.
Table 2
Photovoltaic parameters of the organic solar cells based on optimized
DTS(QxHT
2
)
2
:PC71BM active layers.
Since the HOMO energy level of DTS(QxHT
2
)
2
is around ꢁ5.42 eV,
sc (mA/cm2)
V
FF
PCE (%)
R
(
U
cm
ꢁ2
)
R
(U
cm
ꢁ2
)
Device
J
oc
s
sh
PEDOT:PSS form a barrier with active layer for hole extraction due
to the mismatch of HOMO energy levels [27]. Moreover, the hy-
groscopic and acidic nature of PEDOT:PSS are the major cause for
the long term device stability due to the degradation of the active
layer and ITO electrode. Thus, a pH neutral HTL is preferable to
avoid the interfacial chemical reactions. It has also been reported
that PEDOT:PSS is not a good electron blocking layer and can trap
electrons and thus reduce the device performance [28]. In order to
overcome the above drawbacks, recently copper thiocyanate
(CuSCN) has been used as an efficient solution processable HTL due
to its high optical transparency over the visible to near infrared
CF cast
DIO/CF cast
CuNCS/DIO/CF 12.46
7.85
12.08
0.96 0.42 3.16
0.90 0.58 6.30
0.98 0.64 7.81
13.45
10.74
10.23
721
813
925
crystallinity is beneficial to the charge transport, resulting in an
improvement in both Jsc and FF. Moreover, the increase in full width
at half maxima (FWHM) of small molecules in the active layer also
confirms the increase in the crystalline domain.
In order to get information on the influence of the solvent ad-
ditives on the electronic properties and light absorption of the
devices, the photocurrent density (Jph) is plotted against the
effective voltage (Veff), shown in Fig. 12, and the saturation photo-
2
ꢁ1 ꢁ1
region, high hole mobility (0.01e0.1 cm V
s ) and deep valance
band edge (~ꢁ5.35 eV). We explored the possibility that the overall
PCE can be improved using the DTS(QxHT ) :PC BM active layer
2
2
71
current (Jphsat) and exciton dissociation probabilities (P
measured. Jph is defined as Jph ¼ J -J , where J and J
rent densities under illumination and in dark, respectively. Veff is
defined as Veff ¼ V - Vappl, where V is the voltage at which Jph is
zero and Vappl is applied voltage. Generally, it is assumed that all the
c
) [26] were
are the cur-
by using CuSCN as HTL. As the active layer we have used DIO/CF
L
D
L
D
processed DTS(QxHT ) :PC71BM thin film. The J-V characteristics
2
2
of the device are shown in Fig. 7a. The device showed overall PCE of
2
o
o
7.81% with Jsc of 12.46 mA/cm , V of 0.98 V and FF of 0.64 which
oc
are higher than that for PEDOT:PSS as HTL. When the CuSCN is used

368
M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
2 2
Fig. 9. Dark current-voltage characteristics of (a) hole only and (b) electron only devices based on DTS(QxHT ) :PC71BM active layers processed CF and DIO/DF solutions.
2 2
Fig. 10. TEM images of CF and DIO/CF cast DTS(QxHT ) :PC71BM films.
relative to PEDOT:PSS could be related to the better alignment
between the HOMO level of DTS(QxHT
2
)
2
(ꢁ5.42 eV) with the va-
lance band edge of CuSCN (ꢁ5.35 eV) at the active layer/CuSCN
interface, which act as electron blocking layer.
c
Moreover, we estimated the P using the variation of Jph with Veff
as shown in Fig. 12. As shown in Fig. 12, the value of Jphsat tends to
saturate at lower value of Veff compared to the device based on
c
PEDOT:PSS. The value of P for the device is 0.88, although the value
21
3
of Gmax is about only 9.7 ꢂ 10 cm /s which is similar to the device
based on PEDOT:PSS, indicating the exciton generation rate is
almost same for both the devices, since the absorption spectra of
PEODT:PSS and CuSCN are identical. The higher value of Pc for
CuSCN HTL as compared to that for PEDOT:PSS may be due to the
formation of better ohmic contact at the active layer/CuSCN inter-
face and holes are extracted more efficiently, leading to a sup-
pression of charge recombination and an increase in FF. As Veff
corresponds to the internal field for extraction of the charge car-
riers, with low internal field is needed to sweep out the charge
2 2
Fig. 11. X-ray diffraction patterns of CF and DIO/CF cast DTS(QxHT ) :PC71BM films.
as HTL, the FF has been increased from 0.58 to 0.64, which may be
attributed to the better ohmic contact between active layer and
CuSCN owing to the valance band edge of CuSCN is deeper than
PEDOT:PSS. On the other hand, the increase in the Voc with CuSCN
carriers. Finally, the estimated series resistance (R ) and shunt
s
ꢁ
2
resistance (Rsh) with CuSCN as HTL (R ¼ 10.23
s
U
U
cm
and
ꢁ
ꢁ
2
ꢁ2
Rsh ¼ 10.74
Rsh ¼ 813
U
U
cm ), compared to PEDOT:PSS (R ¼ 10.74
s
cm and
2
cm ), also confirm the formation of better ohmic

M.L. Keshtov et al. / Organic Electronics 39 (2016) 361e370
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M.L Keshtov, S.A.Kuklin, D.Y.Godovsky, A.Yu. Nikolaev thank
Russian Science Foundation for financial support for experimental
research, study of spectral and photovoltaic characteristics (RSF,
project 14-13-01444).
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3