Effects of electron-withdrawing group and electron-donating core combinations on physical properties and photovoltaic performance in D-π-A star-shaped small molecules

Article abstract of DOI:10.1016/j.orgel.2016.02.027

The first representatives of star-shaped molecules having 3-alkylrhodanine (alkyl-Rh) electron-withdrawing groups, linked through bithiophene π-spacer with electron-donating either triphenylamine (TPA) or tris(2-methoxyphenyl)amine (m-TPA) core were synth

Full text of DOI:10.1016/j.orgel.2016.02.027

Organic Electronics 32 (2016) 157e168
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Effects of electron-withdrawing group and electron-donating core
combinations on physical properties and photovoltaic performance in
D-p-A star-shaped small molecules
,
,
, d
Yuriy N. Luponosov a *, Jie Min b **, Alexander N. Solodukhin a, Oleg V. Kozlov c
,
Marina A. Obrezkova a, Svetlana M. Peregudova a e, Tayebeh Ameri b, Sergei N. Chvalun a
,
,
, f
Maxim S. Pshenichnikov c, Christoph J. Brabec b g, Sergei A. Ponomarenko a
,
, h
a Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences, Profsoyuznaya st. 70, Moscow, 117393, Russia
b Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander-University Erlangen-Nuremberg, Martensstraße 7, 91058,
Erlangen, Germany
c Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
d Faculty of Physics & International Laser Center, Lomonosov Moscow State University, Leninskie Gory 1, 119991, Moscow, Russia
e Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova St. 28, Moscow, 119991, Russia
f National Research Centre “Kurchatov Institute”, 1, Akademika Kurchatova pl., Moscow, 123182, Russia
g Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058, Erlangen, Germany
h Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow, 119991, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The first representatives of star-shaped molecules having 3-alkylrhodanine (alkyl-Rh) electron-
withdrawing groups, linked through bithiophene -spacer with electron-donating either triphenyl-
Received 14 December 2015
Received in revised form
15 February 2016
Accepted 15 February 2016
Available online xxx
p
amine (TPA) or tris(2-methoxyphenyl)amine (m-TPA) core were synthesized. The physical properties and
photovoltaic performance of these novel molecules with 3-ethylrhodanine groups were comprehensively
studied and compared to their full analogs having dicyanovinyl (DCV) units as the other type of well-
known and frequently used acceptor groups. On one hand, the former demonstrate several advantages
such as higher solubility and better photovoltaic performance in bulk-heterojunction (BHJ) organics solar
cells (OSCs) as compared to the latter. Nevertheless, the former have slightly lower optical/electro-
chemical bandgaps and higher thermooxidation stability. On the other hand, molecules of both series
based on m-TPA core along with higher solubility and higher position of HOMO energy levels have more
pronounced tendency to crystalize as compared to the TPA-based molecules. Detailed investigation of
the structure-property relationships for these series of molecules revealed that donor and acceptor unit
combinations influence both charge generation and charge transport/recombination properties, as
demonstrated by the ultrafast photoinduced absorption spectroscopy, space charge limited current
measurements and transient photovoltage technique. These results give more insight how to fine-tune
and predict physical properties and photovoltaic performance of small molecules having either alkyl-
Rh or DCV units in their chemical structures and thus providing a molecular design guideline for the
next generation of high-performance photovoltaic materials.
Keywords:
Star-shaped molecules
3-Ethylrhodanine
Triphenylamine
Dicyanovinyl
Organic solar cells
Ultrafast charge separation
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
future [1]. Organic solar cells (OSCs) excel with low production
costs, substrate and shape freedom, and the ability to manufacture
Among all available renewable energy sources, solar energy
appears to be the most promising primary energy source of the
lighter, highly flexible, and semitransparent devices, which open
avenues for new application areas such as building integrated
photovoltaics for sun shading and electricity generating glass fa-
cades [2,3]. Solution-processable small molecules for bulk hetero-
junction (BHJ) OSCs have attracted considerable attention in recent
years because of their advantages of high purity, definite molecular
* Corresponding author.
** Corresponding author.
E-mail addresses: luponosov@ispm.ru (Y.N. Luponosov), Min.Jie@fau.de (J. Min).
http://dx.doi.org/10.1016/j.orgel.2016.02.027
1566-1199/© 2016 Elsevier B.V. All rights reserved.

158
Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
weight, and low cost [4,5]. Although optimized solution-processed
OSCs based on small-molecule donor materials blended with
fullerene acceptors achieved the efficiencies over 10% [6], devel-
opment of the cheap and stable photoactive materials as well as
novel design strategies for small molecules is still a key to boost
potential for commercial applications.
study of thermal, optical and electrochemical properties, film
morphology, charge generation, charge transport and recombina-
tion properties, as well as photovoltaic performance of these mol-
ecules in comparison to their full analogs having frequently-used
DCV groups, N(Ph-2T-DCV)3 and N(Ph-OMe-2T-DCV)3, allows to
evaluate benefits and drawbacks for each of the D-A combination.
Donor-acceptor (DeA) approach is a successful tool to design
small molecules possessing broadened absorption spectra, high
hole mobility, and suitable energy levels of frontier molecular or-
bitals [7,8]. D-A molecules having star-shaped [9] or linear [4e6]
symmetric architecture are the most frequently used types of ma-
terials for solution-processed OSCs. Recent studies revealed that
the properties of small molecules both in pristine and in blended
state are very sensitive to even minimal changes in their chemical
structures, such as a length and position of alkyl group [9e12],
2. Results and discussion
2.1. Synthesis and chemical characterization
Synthesis of the star-shaped molecules with 3-ethylrhodanine
groups is outlined in Scheme 2. First, star-shaped precursors with
5,5-dimethyl-1,3-dioxane-protected carbonyl functions 3a and 3b
were prepared via Suzuki coupling between the pinacol boronic
ester of 2,2-bithiophene derivative (1) and tris(4-bromophenyl)
amine (2a) or tris(4-bromo-2-methoxyphenyl)amine (2b) respec-
tively in 81e83% isolated yields. Removing the protective groups
was successfully achieved by treatment of the acetals with 1 M HCl
to give corresponding aldehydes (4aeb) in 95e99% isolated yields.
Finally, N(Ph-2T-Rh-Et)3 and N(Ph-OMe-2T-Rh-Et)3 were prepared
length and nature of
p-bridge [13e16], heteroatom substitution
[17,18] etc. However, some basic parameters of the D-A molecules,
such as optical absorption range, energy levels, electrochemical
stability etc. are specified mainly by the nature of D and A unit
combinations. Therefore, there is an urgent need for investigating
the effect of D and A unit combination in small molecules, which
can provide a more detailed insight into their structure-property
relationships and subtle design strategies.
Recently, OSCs based on D-A small molecules containing 3-
alkylrhodanine (alkyl-Rh) groups as electron-withdrawing units
have demonstrated record high efficiencies [19]. The promising
results on the linear small molecules demonstrated that alkyl-Rh
group is an excellent acceptor unit that probably can also be use-
ful in other systems. To our best knowledge, star-shaped D-A
molecules with alkyl-Rh groups have not been reported yet,
whereas the star-shaped architecture has a number of attractive
advantages over the linear one, such as higher solubility, better
film-forming properties, less anisotropy of optical and electrical
properties, increased number of pathways for light conversion, etc.
[20e22]. There is only one recent report on the synthesis of star-
€
by Knovenagel condensation of compounds 4a and 4b with N-
ethylrhodanine in 50% and 65% isolated yields, respectively. To our
best knowledge, N(Ph-2T-Rh-Et)3 and N(Ph-OMe-2T-Rh-Et)3 are
the first representatives of D-p-A star-shaped molecules with alkyl-
rhodanine groups. Preparation of their full analogs with terminal
DCV groups, N(Ph-2T-DCV)3 [9] and N(Ph-OMe-2T-DCV)3 [26], is
described elsewhere and based on the similar synthetic approach.
1H-, and 13C NMR spectroscopy, elemental analysis, and mass-
spectroscopy were used to characterize chemical structure of the
precursors and final compounds (see Experimental part and Sup-
porting Information, Fig. S1eS6). The oligomers obtained demon-
strate moderate solubility in common organic solvents such as
tetrahydrofuran (THF), chloroform, and 1,2-dichlorobenzene
(ODCB) etc.
shaped molecule with
dicyanomethylene)-3-octyl [23].
a
modified rhodanine group, 2-(1,1-
The accurate solubility values were measured in ODCB at room
temperature (see Table 1). One can see that solubility of the star-
shaped molecules with Rh-Et groups is higher as compared to
their analogs with DCV groups. In addition, usage of m-TPA instead
of TPA core also leads to increased solubility, which is in agreement
with our previous results [25]. These observations can be explained
by decreased intermolecular interactions for the small molecules
having additional aliphatic or methoxy substitutes in their chemi-
cal structures.
Nowadays, the most attractive star-shaped molecules for OSCs
are based on triphenylamine (TPA) as the donor and dicyanovinyl
(DCV) as the acceptor unit due to their low cost, well-established
chemistry, excellent physical and chemical properties [21,24].
Recently we have systematically investigated various properties of
this type of star-shaped molecules depending on the length and
position of alkyl group as well as length of p-bridge [9,11,14]. More
recently, our works have demonstrated that usage of tris(2-
methoxyphenylamine) (m-TPA), being an analog of TPA with
methoxy substitutes, to design star-shaped molecules with DCV
groups leads to an increased molecular solubility and crystallinity
without a loss in photovoltaic performance [25,26]. Besides DCV,
benzothiadiazole [27,28] and 2-ethylhexyl cyanoacetate [29] are
frequently used acceptor blocks to design efficient TPA-based star-
shaped materials for OSCs. In addition, the effect of peripheral
acceptor group, such as DCV, indanedione, 2-ethylhexyl cyanoa-
cetate and 1,3-diethyl-2-thiobarbituric in star-shaped TPA-based
symmetrical [30] and unsymmetrical [31,32] molecules were
investigated and showed a great influence of such modifications on
their photophysical properties. Nonetheless, despite all these
promising results, there is still a vast space for material scientists to
modify and improve the physical parameters and photovoltaic ef-
ficiency of star-shaped molecules by smartly gaming D and A unit
combinations.
2.2. Thermal properties
Thermal properties of these four compounds were investigated
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The results are shown in Figs. 1 and 2, respec-
tively, and summarized in Table 1.
One can see that all oligomers under consideration possess very
high stability above 380 ꢀC both in the air (Fig. 1a) and under inert
atmosphere (Fig. 1b). However, there is some dependence of the
decomposition temperature (Td, corresponding to 5% weight losses)
upon their chemical structure. Comparison of the character and
percentage of the weight losses at 400e550 ꢀC region for all these
molecules indicates that it is caused mostly by the decomposition
of the acceptors either DCV or 3-ethylrhodanine groups. The star-
shaped molecules with 3-ethylrhodanine groups have slightly
lower Td in air as compared to those of DCV based molecules. This
fact can be explained by presence of the aliphatic group in 3-
ethylrhodanine unit, which probably decomposes first at high
temperatures in the presence of oxygen. In addition, comparison of
the residual coke at high-temperature region (450e550 ꢀC) relative
In this paper, two novel star-shaped molecules, namely N(Ph-
2T-Rh-Et)3 and N(Ph-OMe-2T-Rh-Et)3, with 3-ethylrhodanine (Rh-
Et) as the acceptor groups and either TPA or m-TPA as the donor
core were designed and synthesized (Scheme 1). Comprehensive

Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
159
Scheme 1. Chemical structures of D-
p-A star-shaped molecules investigated in this work.
Scheme 2. Synthesis of the star-shaped molecules with 3-ethylrhodanine groups.
Table 1
Solubility and thermal properties of the star-shaped molecules investigated.
Compounds
Solubility, g Lꢁ1a
First heating
Second heating
TGA (air)
TGA (N2)
Tm,
ꢀC
D
Hm, J/g
Tg, ꢀC
dCp, J/(g$K)
Td, ꢀC
Td, ꢀC
N(Ph-2T-Rh-Et)3
N(Ph-2T-DCV)3
N(Ph-OMe-2T-Rh-Et)3
N(Ph-OMe-2T-DCV)3
8
2
11
9
e
e
146
146
143
147
0.25
0.29
0.24
0.33
393
406
386
401
408
407
400
408
b
276
266
200
28
45
32
c
T
m e melting temperature;
DH
m e melting enthalpy; Tg e glass transition temperature;
D
C
p e heat capacity change at the Tg; Td e decomposition temperature (at 5% weight-
loss).
a
Measured in ODCB.
Data from Ref. [9].
Data from Ref. [26].
b
c
to the nature of branching core revealed that the molecules with m-
TPA core have larger weight losses, because of elimination of the
methoxy substitutes.
DSC scans at first heating (Fig. 2a) revealed that molecules with
DCV groups can be obtained as crystalline materials from solution
[26] as opposite to compounds with 3-ethylrhodanine groups. It is

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Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
Fig. 1. TGA of D-p-A star-shaped molecules in (a) air and (b) nitrogen.
Fig. 2. DSC first (a) and second (b) heating scans of D-p-A star-shaped molecules.
interesting to note that at the first heating N(Ph-2T-Rh-Et)3 has the
glass transition only, whereas its analog with m-TPA core, namely
N(Ph-OMe-2T-Rh-Et)3, has an exothermic phase transition above
the glass transition (180e220 ꢀC), corresponding to a crystallization
followed by melting at 276 ꢀC. This indicates that m-TPA core can
stimulate crystallization as compared to TPA core, which is in a
good agreement with our previous results [26]. However, for the
N(Ph-2T-DCV)3/N(Ph-OMe-2T-DCV)3 pair we observe an opposite
effect. N(Ph-2T-DCV)3 has sharper endothermic peak at higher
temperature and with larger value of melting enthalpy, which
indicate its higher crystallinity as compared to N(Ph-OMe-2T-
DCV)3. Nevertheless, all molecules under discussion become
amorphous at cooling from the melt as it is evidenced by the sec-
ond heating DSC scans (Fig. 2b) similar to other known TPA-based
molecules, since a propeller-like TPA core hinders crystallization
[24,26]. Interestingly, glass transition temperatures (Tg) were found
to be very similar for all four molecules investigated in this work
without pronounced reference to the type of donor or acceptor
units.
attributed to the intramolecular charge-transfer (ICT) transition
between donor and acceptor groups [31]. However, our recent work
revealed that both bands have a mixed character and the intensity
of the high-energy absorption feature is a direct measure of the
degree of conformational disorder in the star-shaped molecule [33].
The molecules having m-TPA core demonstrate slightly red-shifted
absorption spectra as compared to the analogs with TPA, especially
in the case of N(Ph-2T-DCV)3/N(Ph-OMe-2T-DCV)3 pair. This can be
explained by the fact that m-TPA core possesses stronger electron-
donating ability as compared to TPA unit, resulting from the pres-
ence of electron-rich methoxy substitutes, which in turn leads to
more pronounced ICT effect for the molecules with m-TPA core.
In thin films, similar tendencies are observed together with an
obvious red-shift and broadened absorption as compared to the
spectra in solution. In general, the spectra of the molecules with m-
TPA core tend to be more red-shifted and broadened, which is more
pronounced for N(Ph-2T-Rh-Et)3/N(Ph-OMe-2T-Rh-Et)3 pair,
where the latter has 21 nm red-shift of the absorption maximum.
The lower aggregation ability for N(Ph-2T-Rh-Et)3 in this pair as
well as in the entire series of the molecules investigated can be
explained by its amorphous nature according to the DSC data dis-
cussed above. In addition, the optical band gaps (Eogpt) of N(Ph-2T-
Rh-Et)3, N(Ph-2T-DCV)3, N(Ph-OMe-2T-Rh-Et)3 and N(Ph-OMe-2T-
DCV)3 as calculated from the onset of the film absorption are esti-
mated to be 1.77, 1.78, 1.68 and 1.72 eV, respectively. Considering
these values together with the fact that the most red-shifted ab-
sorption maxima both in solutions and in thin films were found for
the molecules having DCV groups, one can conclude that the
molecules with DCV groups have slightly stronger character of ICT
2.3. Optical and electrochemical properties
UV-Vis absorption spectra of the star-shaped molecules in dilute
ODCB solutions and in thin films are shown in Fig. 3 and the cor-
responding optical data are collected in Table 2. The shape of ab-
sorption spectra in solution is similar for all molecules investigated
(Fig. 3a). Usually the UV absorption peaks at the high-energy region
(ca. 400 nm) are ascribed to the
pep* transitions, whereas the
intensive visible absorption peaks in low-energy region are

Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
161
Fig. 3. UV-vis absorption spectra of star-shaped molecules in (a) dilute ODCB solutions and in (b) films cast from ODCB.
Table 2
Optical and electrochemical properties of the star-shaped molecules investigated.
Compounds
UV-vis absorption
Solutiona
Cyclic voltammetry
Oxidation
Filmb
Reduction
lmax (nm)
lmax (nm)
lonset (nm)
Eogpt (eV)c
4
о
d/HOMO (V)/(eV)
x
4redd/LUMO (V)/(eV)
EEgC (eV)
N(Ph-2T-Rh-Et)3
N(Ph-2T-DCV)3
N(Ph-OMe-2T-Rh-Et)3
N(Ph-OMe-2T-DCV)3
524
528
527
540
541
560
562
561
700
698
738
720
1.77
1.78
1.68
1.72
0.90/ꢁ5.30
0.92/ꢁ5.32
0.82/ꢁ5.22
0.80/ꢁ5.20
ꢁ0.98/ꢁ3.42
ꢁ0.91/ꢁ3.49
ꢁ1.02/ꢁ3.38
ꢁ0.94/ꢁ3.46
1.88
1.83
1.84
1.74
e
f
a
Measured in ODCB solution.
Cast from ODCB solution.
b
c
Bandgap estimated from the onset wavelength (ledge) of the optical absorption: Eogpt ¼ 1240/ledge
.
d
e
f
Standard formal reduction (4red) and oxidation (4ox) potentials vs. SCE.
Data from Ref. [9].
Data from Ref. [26].
between the donor and acceptor units as compared to the molec-
ular systems with alkyl-Rh groups, which is in a good agreement
with electrochemical data as discussed below.
influence the first oxidation potentials, whereas presence of addi-
tional electron-donating substitutes at TPA core, such as methoxy
group, facilitates the oxidation. These facts indicate that thieno-
phenylamine fragments are mainly responsible for the oxidation in
these molecules.
Apart the oxidation, reduction of all these star-shaped mole-
cules was found to be irreversible in both solution and solid, which
can be attributed to the instability of their anion-radicals due to
presence of the active proton at DCV and alkyl-Rh groups. It is
known that substitution of the active vinyl proton to an alkyl group
can stabilize the anion-radicals leading to the reversible or qua-
sireversible reduction processes [9,35]. In this series of molecules,
Electrochemical properties of the oligomers were investigated
using cyclic voltammetry (CV) (see Fig. 4 and Table 2). Oxidation in
ODCBeacetonitrile mixture occurs in two consecutive and fully
reversible stages for the oligomers under discussion. However, in
solid the oxidation of the molecules with alkyl-rhodanine groups
becomes irreversible as compared to N(Ph-2T-DCV)3 and N(Ph-
OMe-2T-DCV)3 (Fig. 4a). This phenomenon can be attributed to a
stacking in the solid state of rhodanine-containing oligomers [34].
Interestingly, the particular type of acceptor group does not
Fig. 4. (a) Cyclic voltammograms of films and (b) HOMO/LUMO energy levels diagrams of N(Ph-2T-Rh-Et)3 (1), N(Ph-2T-DCV)3 (2), N(Ph-OMe-2T-Rh-Et)3 (3) and N(Ph-OMe-2T-
DCV)3 (4) in comparison to PC70BM [36].

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Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
replacement of DCV by alkyl-Rh groups leads to a shift of the
reduction potentials to the cathode region and hereby hinders
reduction of the latter. Meanwhile, the reduction potentials were
found to be not depended on the nature of donor core, confirming
that the electron density is localized at the terminal electron-
deficient either DCV or alkyl-Rh groups.
film morphology cannot be ignored [14]. The lower photovoltaic
performance of N(Ph-2T-DCV)3 based devices can be attributed to
the insufficient solubility (2 mg mLꢁ1 in ODCB), probably resulting
in the nonoptimal nanomorphology, which cannot be detected by
the top atomic force microscopy (AFM) surface image (see Fig. S8).
N(Ph-2T-Rh-Et)3 and N(Ph-2T-DCV)3 based blended films show
smooth surfaces without any remarkable features at the resolution
of the AFM, but the AFM images of the N(Ph-2T-Rh-Et)3 based film
exhibit larger interconnected regions and domains compared with
N(Ph-2T-DCV)3 film image. In contrast, N(Ph-OMe-2T-Rh-Et)3 and
N(Ph-OMe-2T-DCV)3 based devices without a solubility issue
showed the comparable photovoltaic parameters with the similar
Jsc values (8.6 mA cmꢁ2 vs. 8.5 mA cmꢁ2) as well as a slight dif-
ference of FF values (50.3% vs. 46.6%). It was also found that these
two systems show similar morphology characteristic as demon-
strated by AFM measurements (see Figure S8c and S8d). This sug-
gests good dispersion of both molecules in the blend, but does not
explain the difference in their photovoltaic performance.
Finally, the PCEs of these devices are 4.0% for N(Ph-2T-Rh-Et)3,
2.4% for N(Ph-2T-DCV)3, 4.0% for N(Ph-OMe-2T-Rh-Et)3 and 3.6%
for N(Ph-OMe-2T-DCV)3. As an obvious result, photovoltaic per-
formance of the OSCs based on the molecules with alkyl-Rh groups
exceeds those of the devices with DCV groups. The external
quantum efficiency (EQE) of the corresponding devices is shown in
Fig. 5b, a broad spectral response across the wavelength range of
300e700 nm was observed for all devices. Jsc values calculated from
the EQE spectra are 8.0 mA cmꢁ2 for N(Ph-2T-Rh-Et)3, 5.7 mA cmꢁ2
for N(Ph-2T-DCV)3, 7.5 mA cmꢁ2 for N(Ph-OMe-2T-Rh-Et)3,
7.6 mA cmꢁ2 for N(Ph-OMe-2T-DCV)3, respectively, which are in
good agreement with the relevant Jsc values measured under
simulated AM 1.5, in air. The slightly lower Jsc values obtained from
the EQE measurements are caused either by device degradation
during the measurements or by an underestimated spectral
mismatch factor.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels were calcu-
lated using the first standard formal oxidation and reduction po-
tentials obtained from the CV experiments in solids (see Table 2 and
Fig. 4b). As expected, HOMO energy levels of the molecules with m-
TPA core, N(Ph-OMe-2T-Rh-Et)3 and N(Ph-OMe-2T-DCV)3, were
found to be higher as compared to their analogs having common
TPA core without additional electron-donating methoxy groups.
LUMO energy levels of the molecules with alkyl-Rh groups are
slightly higher of those calculated for their analogs with DCV units.
Considering bandgap values (Eg) of the molecules, one can conclude
that usage of DCV group or m-TPA core for the molecular design of
the small molecules leads to a slight decrease of Eg. Therefore, the
lowest Eg (1.74 eV) was found for N(Ph-OMe-2T-DCV)3, which
corresponds well with the optical data (Table 2).
2.4. Photovoltaic properties of BHJ devices
Solution-processed BHJ OSCs were fabricated using [6,6]-Phenyl
C71 butyric acid methyl ester (PC70BM) as the electron acceptor
with conventional device structure of ITO/PEDOT:PSS/star-shaped
molecules:PC70BM/ZnO(ca. 25 nm)/Al(100 nm), and the detailed
device fabrication process is described in the Experimental section.
The corresponding parameters of the devices with optimized
blending ratios and without any post-treatments are summarized
in Fig. S7 and Table 3. The best current density-voltage (J-V) curves
and external quantum efficiency (EQE) spectra of the related de-
vices are shown in Fig. 5a and b, respectively.
Due to their low-lying HOMO energy levels (see Table 2), OSCs
based on these star-shaped molecules show relatively high open-
circuit voltage (Voc) of over 0.90 V as expected. Furthermore, we
found that OSCs based on N(Ph-OMe-2T-Rh-Et)3 and N(Ph-OMe-
2T-DCV)3 showed lower Voc of 0.92 V and 0.90 V, respectively, as
compared to those of TPA based molecules. These results are
consistent with HOMO level variations, originating from the
stronger electron-donating ability of m-TPA core. OSCs based on
molecules with Rh-Et acceptor groups (N(Ph-2T-Rh-Et)3 and N(Ph-
OMe-2T-Rh-Et)3) exhibited higher Voc values as compared to the
DCV based molecules (N(Ph-2T-DCV)3 and N(Ph-OMe-2T-DCV)3),
probably due to the slightly lower electron-withdrawing ability of
Rh-Et group as well as the morphological characteristics. In addi-
tion, as compared to the N(Ph-2T-DCV)3 based devices, OSCs based
on N(Ph-2T-Rh-Et)3 provide a higher short circuit current density
(Jsc) of 9.1 mA cmꢁ2 and a better fill factor (FF) of 45.6%.
2.5. Charge transport
To gain further insights into the effects of nature of donor and
acceptor unit combinations within structurally well-defined star-
shaped molecules on the charge transport as well as the related
structure-property relationships, space charge limited current
(SCLC) method was used to map out hole and electron only mo-
bilities of the blended films. The structures of hole and electron
only devices were ITO/PEDOT:PSS/blended films/MoO3 (15 nm)/Ag
(100 nm) and ITO/ZnO/blended films/Ca (15 nm)/Ag (100 nm),
respectively. Here, the charge carrier mobilities are measured by
analyzing the dark current density-voltage (J-V) characteristics of
single carrier devices. The related average values of hole only and
electron only mobilities of blended films calculated by six diodes
are shown in Fig. 6 and Table 4.
Obviously, N(Ph-2T-DCV)3 shows the lowest hole only mobility
in these blended films. Similarly, the electron only mobility (me) of
the devices based on N(Ph-2T-DCV)3 as donor also exhibits the
Absorption spectra and electronic structure of the components
are generally important for device operation, but here the role of
lowest value of 2.5 ꢂ 10ꢁ4 cm2 Vꢁ1
s
ꢁ1. Although N(Ph-2T-DCV)3
mh ratio as compared to
Table 3
blended films possessed the comparable me
/
Photovoltaic properties of star-shaped molecules:PC70BM BHJ OSCs, under the
illumination of AM 1.5, 100 mW cmꢁ2
.
the other three systems (see Table 4), the lower charge carrier
mobilities in blended films resulted in the poor photovoltaic per-
formance as mentioned above. It should be noted here that the poor
charge transport properties in N(Ph-2T-DCV)3 blended films can be
attributed to the morphological characteristics due to the insuffi-
cient molecule solubility. In order to reduce the negative effects of
poor solubility, introducing a more soluble m-TPA donor unit to
N(Ph-2T-DCV)3 is effective in improving molecular solubility. Thus,
by further introduction m-TPA instead of TPA unit, the one order of
magnitude higher hole and electron only mobilities were measured
Donor
D:A [wt%] Voc [V] Jsc [mA cm-2] FF [%] PCEa [%]
N(Ph-2T-Rh-Et)3
N(Ph-2T-DCV)3
N(Ph-OMe-2T-Rh-Et)3 1:4
N(Ph-OMe-2T-DCV)3
1:3
1:2
0.96
0.94
0.92
0.90
9.1
6.6
8.6
8.5
45.6
39.1
50.3
46.6
4.0(3.9)
2.4(2.3)
4.0(3.8)
3.6(3.4)
b
c
1:2.5
a
Best values; the average values of PCEs calculated from over six cells are given in
brackets.
b
c
Data from Ref. [9].
Data from Ref. [26].

Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
163
Fig. 5. J-V curves (a) and EQE spectra (b) of solar cells based on N(Ph-2T-Rh-Et)3:PC70BM (1:3, wt%), N(Ph-2T-DCV)3:PC70BM (1:2, wt%), N(Ph-OMe-2T-Rh-Et)3:PC70BM (1:4, wt%),
N(Ph-OMe-2T-DCV)3:PC70BM (1:2.5, wt%), respectively.
Fig. 6. The dark J-V characteristics of these star-shaped molecules based diodes including hole (a) and electron (b) only mobilities. The solid lines represent the best fitting using the
SCLC modified Mott-Gurney model.
Table 4
Hole and electron only mobilities of blended films with four star-shaped molecules determined from the SCLC measurements.
Blended films
Thickness (nm)
mh [cm2 Vꢁ1 sꢁ1
a
]
Thickness (nm)
me [cm2 Vꢁ1 sꢁ1
a
]
me/mh
N(Ph-2T-Rh-Et)3:PC70BM
75
80
75
86
4.34 ꢂ 10ꢁ4
7.98 ꢂ 10ꢁ5
4.09 ꢂ 10ꢁ4
4.58 ꢂ 10ꢁ4
81
70
80
75
1.73 ꢂ 10ꢁ3
2.5 ꢂ 10ꢁ4
1.18 ꢂ 10ꢁ3
2.15 ꢂ 10ꢁ3
3.99
3.13
2.89
4.69
N(Ph-2T-DCV)3:PC70BMb
N(Ph-OMe-2T-Rh-Et)3:PC70BM
N(Ph-OMe-2T-DCV)3:PC70BMc
a
The mobility data reported are average values of the blended films over six diodes.
Data from Ref. [9].
Data from Ref. [26].
b
c
in the N(Ph-OMe-2T-DCV)3 diodes, as compared to that of N(Ph-2T-
DCV)3. The SCLC data are also in agreement with the photovoltaic
parameters of N(Ph-2T-DCV)3 and N(Ph-OMe-2T-DCV)3 systems.
Besides, the hole and electron mobilities of N(Ph-OMe-2T-Rh-Et)3
show slightly lower values as compared to the N(Ph-OMe-2T-DCV)3
system, as seen in Table 4. This correlates well with the difference in
their phase behavior, where ordering of N(Ph-OMe-2T-DCV)3 can
be much better as compared to molecules of N(Ph-OMe-2T-Rh-Et)3
having solubilizing alkyl group at an electron-deficient unit.
Nevertheless, N(Ph-OMe-2T-Rh-Et)3 based device exhibits better FF
of 50.3% and thus higher PCE of 4.0% (see Table 3), probably
FF of the OSCs depends on a combination of charge extraction
and charge recombination rates [37] which relates to the mobility-
lifetime product [38]. To take into account the recombination factor,
we measured the effective carrier lifetime by transient photo-
voltage (TPV) technique (Fig. 7). The carrier lifetimes for N(Ph-2T-
Rh-Et)3, N(Ph-OMe-2T-Rh-Et)3 and N(Ph-OMe-2T-DCV)3-based
blends are similar and demonstrate comparable behavior on the
sun intensity to the previously-studied star-shaped molecules [11].
Considering their J-V characteristics (Fig. 5a), it is not surprising
that these systems suffer from non-geminate recombination, even
though they show the high charge carrier mobility (Table 4). In
addition, the N(Ph-2T-DCV)3-based system shows higher carrier
lifetimes at different light intensities (from 0.1 up to 1 sun), but the
carrier mobilities are by more than a factor of 5 lower as compared
to the other three systems (Table 4). As a result, N(Ph-2T-Rh-Et)3,
resulting from better balanced charge carrier mobilities (low me/mh
ratio). Comparison of N(Ph-OMe-2T-Rh-Et)3 system to that of based
on N(Ph-2T-Rh-Et)3 shows a slightly higher hole and electron
mobility of the latter, but the former has more balanced me/mh ratio,
leading to quite similar photovoltaic performance.
N(Ph-OMe-2T-Rh-Et)3
and
N(Ph-OMe-2T-DCV)3
systems

164
Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
amount of the long-lived charges (i.e. the signal amplitude at
1.5e1.8 ns delays) to a maximal amount of charges which is pro-
vided by the given molecule (i.e. the maximal amplitude in the
series of blends with different PC70BM content, see Ref. [33] for
detailed description). The efficiency of charge separation for all
donor molecules depends strongly on both PC70BM content and the
particular donor molecule structure. At the optimal PC70BM con-
tents the blends provide up to 60% of long-lived separated charges,
which perfectly matches the values of EQE for the OSCs (Fig. 5b).
Importantly, for all donor molecules, the optimal PC70BM content
for ultrafast charge separation directly correlates with the optimal
blend composition for the OSCs (see Table 3). This indicates opti-
mized performance at both the ultrafast sub-ns timescale (i.e.
maximized amount of the long-lived charges) and the ultraslow
timescale at which the device operates.
At the optimal PC70BM concentration, the blends behave quite
similarly at the ultrafast timescales, providing about 50e60% of
long-lived charges. However, there are some important differences
in the initial growth of the PIA signal. Noticeably, for the N(Ph-2T-
Rh-Et)3 and N(Ph-OMe-2T-Rh-Et)3 molecules that demonstrate the
best performance in devices, there is a prominent delayed build-up
of the transients at a ps timescale that is more suppressed at the
other two transients. This initial growth is attributed to the
diffusion-delayed splitting of PC70BM excitons [33], so that in
blends based on N(Ph-2T-DCV)3 and N(Ph-OMe-2T-DCV)3 no
PC70BM exciton diffusion is observed. The absence of such exciton
diffusion may be caused by either of the two scenarios. First, the
PC70BM domains in BHJ blends are too large (larger than the exciton
diffusion length of ~10 nm) so that the excitons cannot make it to
the interface and therefore produce no charges [40]. Second,
intermixing of the donor and PC70BM phases is too fine, with the
typical scale that is much lower than exciton diffusion length,
which leads to almost instantaneous dissociation of the PC70BM
excitons. Large PC70BM domains cause decreased contribution of
the PC70BM excitons to the overall photocurrent. However, no signs
of this are observed in the EQE spectra (Fig. 5b) which shapes look
essentially similar for all blends. Therefore, contributions of the
donor and PC70BM to the photocurrent are comparable for all
molecules.
Fine intermixing, in turn, would not affect the EQE shape, since
both donor and PC70BM excitons split with high efficiency. How-
ever, very small domain sizes imply extremely large interfacial area
in the BHJ films and, most probably, lack of intercalated pathways to
the corresponding electrodes. Such nanomorphology leads to the
increased non-geminate recombination and decreased charge
extraction, which explains low FF and Jsc values of the devices based
on the N(Ph-2T-DCV)3 and N(Ph-OMe-2T-DCV)3 molecules (Fig. 5a,
Table 3). In contrast, in the well-performing blends based on N(Ph-
2T-Rh-Et)3 and N(Ph-OMe-2T-Rh-Et)3 the PC70BM cluster sizes
seem to be more optimal, i.e. close to ~10 nm. In this case the
clusters are hardly detectable by standard AFM due to its limited
spatial resolution and/or poor contrast between the donor and
PC70BM domains.
Fig. 7. Charge carrier lifetime as obtained by transient photovoltage technique for the
photovoltaic blends based on different molecules (indicated). The inset shows the
respective mobility-lifetime products at 1 sun illumination.
demonstrate similar mobility-lifetime products, while for the N(Ph-
2T-DCV)3 system it is considerably lower (Fig. 7, inset). Therefore,
the low FF value for the latter system is explained as unbalanced
rates of charge extraction and charge recombination, which is most
probably rooted in the non-optimal film morphology.
2.6. Ultrafast charge separation
Apart from the charge transport and recombination in BHJ
blends, another important factor that impact photovoltaic perfor-
mance is the efficiency of the initial (i.e. within the first nano-
second) photon-to-charge conversion in the BHJ active layer. To
examine the effect of donor and acceptor units of star-shaped
molecules on the early-time charge separation dynamics, the
photovoltaic blends were interrogated with ultrafast photoinduced
absorption (PIA) spectroscopy. The blends were excited with an
ultrashort visible pump pulse, which mimics the sun illumination
while the concentration of photogenerated charges was monitored
by polaron absorption (i.e. charge-induced changes in the molec-
ular IR absorption spectra due to deformation of the molecular
backbone) [39] by varying of the delay time between the pump and
probe pulses.
Representative PIA transients for the studied blends are shown
in Fig. 8. All star-shaped donors in blends with PC70BM demon-
strate similar behavior, which is consistent with our previous
findings [33]. For all blends, a fast build-up of the signal occurs due
to immediate formation of charge-transfer (CT) excitons and/or
separated charges. At later times, the PIA signal grows with time-
scale of ~2 ps, which is attributed to the hole-transfer process from
PC70BM to the star-shaped donor molecule [33]. Finally, the signals
decay at ~500 ps timescale due to the intramolecular recombina-
tion of the CT excitons and/or geminate recombination of the
initially separated charges. Both processes lead to a decrease of the
amount of the long-lived separated charges thereby impeding the
maximal attaining PCE. Note that at higher excitation densities
3. Conclusions
In summary, we synthesized the first representatives of D-p-A
star-shaped molecules with alkyl-Rh groups as acceptor units.
These molecules having either TPA or m-TPA donor core were
comprehensively studied and compared to their full analogs with
DCV units as the other type of frequently used acceptor units. Usage
of 3-ethylrhodanine group having an aliphatic tail leads to a slight
decrease of the intermolecular interactions resulting in a more
disordered phase state of the star-shaped molecules. However, this
does not result in a significant decrease of the hole and electron
(>20
tion was observed which prompted us to limit excitation intensities
is such a way to ensure the exciton density of ~3$10ꢁ3 nmꢁ3
m
J cmꢁ2) substantial non-geminate exciton-exciton annihila-
.
Maximizing the amount of long-lived separated charges is
important for the optimization of a good-working device. Insets in
Fig. 8 show the efficiency of charge separation as a function of the
PC70BM content. Efficiencies were calculated as a ratio of the

Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
165
Fig. 8. PIA transients for (a) N(Ph-2T-Rh-Et)3:PC70BM (1:3, wt%), (b) N(Ph-2T-DCV)3:PC70BM (1:2, wt%), (c) N(Ph-OMe-2T-Rh-Et)3:PC70BM (1:4, wt%), (d) N(Ph-OMe-2T-
DCV)3:PC70BM (1:2.5, wt%), respectively, after excitation at the wavelengths of 550 nm. Symbols represent the experimental data while solid lines show the best multiexponential
fits (Eq. S1), with the fit parameters given in Table S2 (see Supporting Information for more details). The insets show the dependence of the efficiency of generation of long-lived
charges on PC70BM concentration.
mobilities for their films blended with PC70BM. Meanwhile, the
aliphatic tail increases solubility of the molecules with alkyl-Rh
groups. Apart from the alkyl-Rh group, usage of m-TPA core
instead of common TPA unit can also increase the solubility of the
molecules together with their crystallinity. Despite similar
morphology of the blended films as observed by AFM, these mol-
ecules demonstrate different photovoltaic performance in BHJ
OSCs. The star-shaped molecules with 3-ethylrhodanine group
show somewhat higher Voc and FF values, and thus higher PCEs in
OSCs as compared to their analogs with DCV groups. Among these
devices, the N(Ph-2T-DCV)3 system shows the least optimized de-
vice performance resulting from the lowest mobility-lifetime
product demonstrated by SCLC and TPV techniques. From the ul-
trafast charge dynamics, we conclude that this is most likely due to
the nonoptimal nanomorphology of the DCV-based blends, which
leads to increased bimolecular recombination and less effective
carrier extraction. Therefore, alkyl-Rh acceptor group seems to be a
favorable choice to design star-shaped molecules for efficient
organic photovoltaics.
used as solvents. Pyridine was dried and purified according to the
known techniques and then used as a solvent. 5,5-dimethyl-2-[50-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,20-bithien-5-yl]-
1,3-dioxane (1), tris{4-[50-(5,5-dimethyl-1,3-dioxan-2-yl)-2,20-
bithien-5-yl]-2- methoxyphenyl}amine (3b), tris[4-(50-formyl-2,20-
bithiophene-5-yl)-2-methoxyphenyl]amine (4b) were obtained as
described in Ref. [26]. Tris(4-bromo-2-methoxyphenyl)amine (2b)
was obtained as described in Ref. [25]. N-ethylrhodanine was pre-
pared as described elsewhere [41]. All reactions, unless stated
otherwise, were carried out under an inert atmosphere.
4.2. Synthesis of the star-shaped molecules
Tris{4-[50-(5,5-dimethyl-1,3-dioxan-2-yl)-2,20-bithien-5-yl]
phenyl}amine (3a). In an inert atmosphere, degassed solutions of
tris(4-bromophenyl)amine (0.92 g, 1.91 mmol) and 1 (2.79 g,
6.87 mmol) in toluene/ethanol mixture (50/5 mL) and 2 M solution
of aq. Na2CO3 (10.3 mL) were added to Pd(PPh3)4 (238 mg,
0.2 mmol). The reaction mixture was stirred under reflux for 11 h,
and then it was cooled to room temperature and poured into
100 mL of water and 100 mL of toluene. The organic phase was
separated, washed with water, dried over sodium sulfate and
filtered. The solvent was evaporated in vacuum and the residue was
dried at 1 Torr. The product was purified by column chromatog-
raphy on silica gel (eluent toluene: ethyl acetate ¼ 20: 1) to give
4. Experimental section
4.1. Materials
Tris(4-bromophenylamine), tetrakis(triphenylphosphine)palla-
dium (0) Pd(PPh3)4, malononitrile were obtained from Sigma-
eAldrich Co. and used without further purification. THF and
toluene were purified according to the known techniques and then
pure compound 3a (1.67 g, 81%) as
M.p. ¼ 247e248 ꢀC. 1H NMR (250 MHz, CDCl3):
a
yellow solid.
d
[ppm] 0.8 (s, 9H),
1.28 (s, 9H), 3.61 (d, 6H, J ¼ 11 Hz), 3.74 (d, 6H, J ¼ 11 Hz), 5.61 (s,

166
Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
3H), 7.01e7.06 (overlapping peaks, 6H), 7.09e7.17 (overlapping
peaks, 12H), 7.46 (d, 6H, J ¼ 8.54 Hz). Calcd (%) for C60H57NO6S6: C,
66.70; H, 5.32; N, 1.30; S, 17.80. Found: C, 66.57; H, 5.33; N, 1.27; S,
17.81. MALDI-MS: found m/z 1080.63; calculated for [M]þ 1080.51.
Tris[4-(5-formyl-2,20-bithiophen-5-yl)phenyl]amine (4a). 1 M
HCl (3 mL) was added to a solution of compound 3a (0.84 g,
0.8 mmol) in THF (20 mL) and then the reaction mixture was stirred
for 4 h at reflux. During the reaction, the product was gradually
formed as an orange precipitate. After completion of the reaction
the organic phase was separated using diethyl ether, washed with
water and filtered off to give pure compound 4a (0.607 g, 95%) as
orange crystals. M.p.: 145e146 C. 1H NMR (250 MHz, CDCl3):
d [ppm] 7.14 (d, 6H, J ¼ 8.7 Hz), 7.21 (d, 3H, J ¼ 3.7 Hz), 7.25 (d, 3H,
J ¼ 3.7 Hz), 7.32 (d, 3H, J ¼ 4.3 Hz), 7.51 (d, 6H, J ¼ 8.7 Hz), 7.66 (d,
3H, J ¼ 4.3 Hz), 9.85 (s, 3H). Calcd (%) for C45H27NS6: C, 65.75; H,
3.31; N, 1.70; S, 23.40. Found: C, 65.88; H, 3.27; N, 1.66; S, 23.30.
MALDI-MS: found m/z 820.25; calculated for [M]þ 821.03.
on the computer using the ACD Labs software. In the case of column
chromatography, silica gel 60 (“Merck”) was taken.
Mass-spectra (MALDI) were registered on Autoflex II Bruker
(resolution FWHM 18,000), equipped with a nitrogen laser (work
wavelength 337 nm) and time-of-flight mass-detector working in
reflections mode. The accelerating voltage was 20 kV. Samples were
applied to a polished stainless steel substrate. Spectrum was
recorded in the positive ion mode. The resulting spectrum was the
sum of 300 spectra obtained at different points of sample. 2,5-
Dihydroxybenzoic acid (DHB) (Acros, 99%) and
a-cyano-4-
hydroxycinnamic acid (HCCA) (Acros, 99%) were used as matrices.
Elemental analysis of C and H elements was carried out using CHN
automatic analyzer CE 1106 (Italy). The settling titration using BaCl2
was applied to analyse sulphur. Experimental error for elemental
analysis is 0.30e0.50%. Solubility was measured in ODCB using
€
previously described technique [35]. The Knovenagel condensation
was carried out in the microwave synthesizer “Discovery”, (CEM
corporation, USA), using a standard method with the open vessel
option, 50 W.
Thermogravimetric analysis was carried out in dynamic mode in
30 ÷ 900 ꢀC interval using Mettler Toledo TG50 system equipped
with M3 microbalance allowing measuring the weight of samples
Tris(4-{50-[(3-ethyl-2-thioxothiazolidin-4-one)methyl]-2,20-
bithien-5-yl}phenyl)amine (N(Ph-2T-Rh-Et)3. Compound 4a
(0.480 g, 0.6 mmol), 3-ethylrhodanine (0.56 g, 3.5 mmol) and dry
pyridine (15 mL) were placed in a reaction vessel and stirred under
argon atmosphere for 14 h at 108 ꢀС using the microwave heating.
After completeness of the reaction, pyridine was evaporated under
reduced pressure and the residue was dried at 1 Torr. The crude
product was purified by column chromatography on silica gel
(eluent chloroform). Further purification by precipitation of the
product from its THF solution with toluene and hexane led to pure
product as a black solid (0.365 g, 50%). 1H NMR (250 MHz, CDCl3):
in 0e150 mg range with 1 mg precision. Heating/cooling rate was
chosen to be 10 ꢀC/min. Every compound was studied twice: in air
and in nitrogen flow of 200 mL/min. DSC scans were obtained with
Mettler Toledo DSC30 system with 10 ꢀC/min heating/cooling rate
in temperature range of þ20e290 ꢀC for all compounds. N2 flow of
50 mL/min was used. Cyclic voltammetry measurements were
carried out using solid compact layers of the oligomers, which were
prepared by electrostatically rubbing the materials onto a glassy
carbon electrode using IPC-Pro M potentiostat. Measurements were
made in acetonitrile solution using 0.1 M Bu4NPF6 as supporting
electrolyte. The scan rate was 200 mV sꢁ1. The glassy carbon elec-
trode was used as a work electrode. Potentials were measured
relative to a saturated calomel electrode (SCE). Solution CV mea-
surements were done in 1,2-dichlorobenzene/acetonitrile (4:1)
mixture of solvents for 10ꢁ3 M solutions in a standard three-
electrode cell equipped with a glassy carbon working electrode
(s ¼ 2 mm2), platinum plate as the counter electrode, and SCE
(saturated calomel electrode) as the reference electrode. The
highest occupied molecular orbital (HOMO) and the lowest unoc-
cupied molecular orbital (LUMO) energy levels were calculated
using the first standard formal oxidation and reduction potentials
obtained from CV experiments in films according to the following
d
[ppm] 1.25 (t, 9 H, J ¼ 7.00 Hz), 4.10e4.24 (overlapping peaks, 6 H),
6.85 (d, 3 H, J ¼ 8.85 Hz), 7.06e7.15 (overlapping peaks, 6 H),
7.19e7.25 (overlapping peaks, 6 H), 7.28e7.34 (dd, 6 H, J1 ¼ 3.7 Hz,
J2 ¼ 4.0 Hz), 7.52 (d, 3 H, J ¼ 8.85 Hz), 7.82 (s, 3 H). 13C NMR
(125 MHz, CDCl3):
d [ppm] 12.23, 39.86, 114.71, 118.31, 120.25,
123.52,124.46,124.78,124.89,126.35,129.62,134.44,135.30,136.18,
136.83, 145.52, 145.62, 148.19, 167.14, 191.75. Calcd (%) for
C
60H42N4O3S12: C, 57.57; H, 3.38; N, 4.48; S, 30.74. Found: C, 57.49;
H, 3.29; N, 4.41; S, 30.70. MALDI-MS: found m/z 1251.92; calculated
for [M]þ 1251.79.
Tris(4-{50-[(3-ethyl-2-thioxothiazolidin-4-one)methyl]-2,20-
bithien-5-yl}-2- methoxyphenyl)amine (N(Ph-OMe-2T-Rh-Et)3.
This compound was prepared according to the procedure described
above for N(Ph-2T-Rh-Et)3 using 4b (0.500 g, 0.5 mmol), 3-
ethylrhodanine (0.403 g, 2.7 mmol) and dry pyridine (17 mL) to
give pure product as a black solid (0.480 g, 65%). 1H NMR (250 MHz,
CDCl3):
d
[ppm] 1.28 (t, 9H, J ¼ 7.00 Hz), 3.68 (s, 9H), 4.08e4.27
equations:
HOMO ¼ ꢁe(4ox þ 4.40)(eV) [42].
LUMO
¼
e(4red
þ
4.40)(eV)
and
(overlapping peaks, 6H), 6.85 (d, 3H, J ¼ 8.9 Hz), 7.05e7.16 (over-
lapping peaks, 6H), 7.20e7.25 (overlapping peaks, 6H), 7.27e7.35
(dd, 6H, J1 ¼ 3.7 Hz, J2 ¼ 4.0 Hz), 7.82 (s, 3H). 13C NMR (125 MHz,
Absorption profiles were recorded with a Perkin Elmer Lambda-
35 absorption spectrometer from 350 to 1100 nm. AFM measure-
ments were performed with a Nanosurf Easy Scan 2 in contact
mode. Single carrier devices were fabricated and the dark current-
voltage characteristics measured and analysed in the space charge
limited (SCL) regime. The structures of hole only and electron only
devices were Glass/ITO/PEDOT:PSS/semiconductor layer/MoO3
(15 nm)/Ag (100 nm) and Glass/ITO/ZnO/semiconductor layer/Ca
(15 nm)/Ag (100 nm). The detailed procedure for SCLC measure-
ment can be found in Ref. [43].
CDCl3):
d [ppm] 12.23, 39.86, 55.90, 109.74, 118.31, 120.25, 123.52,
124.46,124.78,124.89, 126.35,129.62,134.44,135.30,136.18,136.83,
145.52, 145.62, 153.08, 167.14, 191.75. Calcd (%) for C63H48N4O6S12
C, 56.39; H, 3.61; N, 4.18; S, 28.67. Found: C, 56.35; H, 3.65; N, 4.19;
S, 28.70. MALDI-MS: found m/z 1341.88; calculated for [M]þ
1341.92.
:
4.3. Characterization
1Н NMR spectra were recorded in a “Bruker WP-250 SY“ spec-
trometer, working at a frequency of 250.13 MHz and utilising CDCl3
signal (7.25 ppm) as the internal standard. 13C NMR spectra were
recorded using a “Bruker Avance II 300” spectrometer at 75 MHz. In
the case of 1Н NMR spectroscopy, the compounds to be analysed
were taken in the form of 1% solutions in CDCl3. In the case of 13C
NMR spectroscopy, the compounds to be analysed were taken in
the form of 5% solutions in CDCl3. The spectra were then processed
4.4. Fabrication and characterization of the OSCs
All the devices were fabricated in the normal architecture.
Photovoltaic devices were fabricated by doctor-blading on indium
tin oxide (ITO)-covered glass substrates (from Osram). These sub-
strates were cleaned in toluene, acetone, and isopropyl alcohol.
After drying, the substrates were bladed with 40 nm PEDOT:PSS
(Heraeus Deutschland GmbH
& Co. KG, PEDOT PH-4083).

Y.N. Luponosov et al. / Organic Electronics 32 (2016) 157e168
167
Photovoltaic layers, consisting of four different small molecules
were dissolved in ODCB with PC70BM as acceptor with various
weight ratios, and bladed on top of PEDOT:PSS layer. The thick-
nesses of these blends are ca. 80e90 nm. After that, a ZnO layer
(25 nm) was doctor-bladed on top of the active layer. Finally, an
aluminum top electrode of 100 nm thickness was evaporated. The
typical active area of the investigated devices was 10.4 mm2. The
current density voltage characteristics of solar cells were measured
under AM1.5G irradiation on an OrielSol 1A Solar simulator
(100 mW cmꢁ2). The EQE was detected with Cary 500 Scan UV-Vis-
NIR Spectrophotometer under monochromatic illumination, which
was calibrated with a mono-crystalline silicon diode.
(SolTech). OVK acknowledges support by “Aurora e Towards
Modern and Innovative Higher Education” Programme, and by
Russian Foundation for Basic Research, research project No14-02-
31632.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.02.027.
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Acknowledgements
The authors acknowledge P. V. Dmitryakov (National Research
Centre “Kurchatov Institute”) for DSC and TGA data obtained for
alkyl-Rh molecules. We also thank Jan Anton Koster and Davide
Bartesaghi (both at Zernike Institute for Advanced Materials, Uni-
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mobility-lifetime analysis. The part of this work including the
design, synthesis and characterization of the oligomers was carried
out under financial support from the Russian Science Foundation
(grant No14-13-01380). The authors gratefully acknowledge the
support of the Cluster of Excellence ‘‘Engineering of Advanced
Materials’’ at the University of Erlangen-Nuremberg, which is fun-
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work of its ‘‘Excellence Initiative’’. This work has been also partially
funded by the Sonderforschungsbereich 953 “Synthetic Carbon
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Nuremberg for financial support (Bavarian State Grant No. 20-
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