Effects of oligothiophene π-bridge length on physical and photovoltaic properties of star-shaped molecules for bulk heterojunction solar cells

Article abstract of DOI:10.1039/c4ta01933d

The preparation of four different star-shaped donor (D)-π-acceptor (A) small molecules (N(Ph-1T-DCN-Me)₃, N(Ph-2T-DCN-Me)₃, N(Ph-2T-DCN-Hex)₃and N(Ph-3T-DCN-Hex)₃) possessing various oligothiophene π-bridge lengths and their use in solution-processed bulk heterojunction small molecule solar cells is reported. Optical and electrochemical data show that increasing oligothiophene π-bridge length leads to a decrease of the optical band gap due to a parallel increase of the highest occupied molecular orbital (HOMO) level. Furthermore, subtle modifications of a molecular π-bridge length strongly affect the thermal behavior, solubility, crystallization, film morphology and charge carrier mobility, which in turn significantly change the device performance. Although the moderately increasing oligothiophene π-bridge length uplifts the HOMO level, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the short circuit current (J_sc) and fill factor (FF). The study demonstrates that such an approach can represent an interesting tool for the effective modulation of the photovoltaic properties of the organic solar cells (OSCs) at a moderate cost.

Full text of DOI:10.1039/c4ta01933d

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Materials Chemistry A
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Luponosov, D. Baran, S. N. Chvalun, M. . Shcherbina, A. V. Bakirov, P. V. Dmitryakov, S. Peregudova, N.
Kaush-Busies, S. Ponomarenko, T. Ameri and C. J. Brabec, J. Mater. Chem. A, 2014, DOI:
10.1039/C4TA01933D.
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Effects of oligothiophene πꢀBridge Length on Physical and Photovoltaic
Properties of StarꢀShaped Molecules for Bulk Heterojunction Solar Cells
Jie Min*1，Yuriy N. Luponosov2, Derya Baran1, Sergei N. Chvalun2,3, Maxim A.
Shcherbina2,4, Artem V. Bakirov3, Petr V. Dmitryakov3, Svetlana M. Peregudova5,
Nina KauschꢀBusies6, Sergei A. Ponomarenko2,7, Tayebeh Ameri1, Christoph J.
Brabec1,8
1Institute of Materials for Electronics and Energy Technology (IꢀMEET), Friedrichꢀ
AlexanderꢀUniversity ErlangenꢀNuremberg, Martensstraße 7, 91058 Erlangen,
Germany
2Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of
Sciences, Profsoyuznaya st. 70, Moscow 117393, Russia
3Scientific and Research Centre Kurchatov Institute, 1 Kurchatov square, Moscow,
123182 Russia
4Moscow Institute of Physics and Technology, 4 Institutsky line, Dolgoprudny,
Moscow region, 141700 Russia
5Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
Vavilova St. 28, Moscow, 119991, Russia
6Heraeus Precious Metals GmbH & Co. KG, Conductive Polymers Division (Clevios),
Chempark Leverkusen Build. B202, Dꢀ51368 Leverkusen, Germany
7Chemistry Department, Moscow State University, Leninskie Gory 1ꢀ3, Moscow
119991, Russia
8Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058
Erlangen, Germany
Eꢀmail: Min.Jie@ww.uniꢀerlangen.de (J. Min)
Abstract
The preparation of four different starꢀshaped donor (D)ꢀπꢀacceptor (A) small
molecules (N(Phꢀ1TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀHex)3 and
N(Phꢀ3TꢀDCNꢀHex)3) possessing various oligothiophene πꢀbridge lengths and their
1

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use in solutionꢀprocessed bulk heterojunction small molecule solar cells is reported.
Optical and electrochemical data show that increasing oligothiophene πꢀbridge length
leads to a decrease of the optical band gap due to a parallel increase of the highest
occupied molecular orbital (HOMO) level. Furthermore, subtle modifications of a
molecular πꢀbridge length strongly affect the thermal behavior, solubility,
crystallization, film morphology and charge carrier mobility, which in turn
significantly change the device performance. Although the moderately increasing
oligothiophene πꢀbridge length uplifts the HOMO level, it nevertheless induces an
increase of the efficiency of the resulting solar cells due to a simultaneous
improvement of the short circuit current (Jsc) and fill factor (FF). The study
demonstrates that such an approach can represent an interesting tool for the effective
modulation of the photovoltaic properties of the organic solar cells (OSCs) at a
moderate cost.
1. Introduction
During the last decade, bulk heterojunction (BHJ) organic solar cells (OSCs) utilizing
conjugated polymers or small molecules have attracted attention due to the low
fabrication costs achievable via solutionꢀprocessing methods.[1ꢀ6] In particular,
intensive research on small molecule OSCs has been carried out resulting in a
remarkable improvement of the photovoltaic performance by judicious molecular
design, morphology control and device optimization.[4ꢀ7] They are emerging as
promising substitutes for conjugated polymers owing to the advantages of simple
2

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synthesis, definite molecular structure and weight, high purity, and reproducible
photovoltaic performance.[8] The current research is focused on obtaining high
molecular physical and photovoltaic properties by the design and synthesis of novel
pushꢀpull molecular systems built by connecting various electron donating (donor)
and electron capturing (acceptor) moieties through a πꢀconjugating spacer (DꢀπꢀA).
Such an approach can be used to lower the optical band gap to broaden molecular
optical absorption and assist the formation of favorable morphologies for high
photovoltaic performance.[9] It is also apparent that desirable highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels
can be obtained by coupling a wide variety of donor and acceptor units.[10] A large
number of DꢀA small molecules, such as linear molecules,[4, 5, 8, 11, 13ꢀ15] starꢀshaped
molecules[12, 16ꢀ19] and other organic dyes,[20] have been extensively studied and
exhibited considerable molecular photovoltaic properties. More recently, using this
class of compounds, the best power conversion efficiency (PCE) of OSCs reached 8%
for single junction solar cells,[4, 14] and 10% for doubleꢀjunction tandem OSCs,[14]
endowing organic small molecules with a great application potential for the
generation of low cost solar cells.
However, compared to πꢀconjugated polymers, relatively short conjugated backbone
of DꢀA small molecules reduces the tendency for the formation of interpenetrating
networks and mixed morphologies when blended with fullerene derivatives.[21, 22]
Furthermore, the limited conjugation length makes charge transport in these materials
primarily intermolecular with a negligible intramolecular component, which increases
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charge recombination in OSCs causing low photocurrent and low fill factor (FF).[9]
Overcoming these obstacles requires to enhance intramolecular πꢀπ stacking
interactions and facilitate charge transport and collection in DꢀA small molecule
OSCs, as well as to further increase the PCE. A promising approach is to incorporate
two dimensional (2ꢀD) or starꢀshaped conjugated structures into DꢀA conjugated
systems.[4, 14, 17, 23] These structures can enhance intramolecular πꢀπ interactions
extending the conjugated surface of small molecules. Substantial progress has
recently been made in small molecule OSCs by introducing alkylthienylꢀcoꢀvinyl
conjugated side chains in an oligothiophene for the formation of a 2ꢀD conjugated
structure, achieving a PCE of ca. 4%.[23a] Remarkably, the PCEs surpassing 8% have
also been reported more recently by employing alkylthienyl and alkylbithienyl
substituted benzodithiophene (BDT) moieties in the core of AꢀπꢀDꢀπꢀA type linear
molecules.[4] In addition, moderate increase of the molecular size and πꢀbridge length
within structurally wellꢀdefined DꢀA small molecules is another smart strategy of
facilitating charge transport and improving molecular properties by the extension of
the conjugated backbone and corresponding modification of the electronic
properties.[24, 25] For instance, Bazan et al. have described the effects and merits of
increasing molecular size within structurally wellꢀdefined DꢀA linear molecules on
relevant physical and photovoltaic properties.[24] In addition, Wang et al. showed that
the absorption maximum redꢀshifts, the optical bandgap decreases, photovoltaic
properties gradually improve for 2,7ꢀcarbazole based DꢀA conjugated linear
molecules with different number of the terminal thiophene units from one, two to
4

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three.[25] However, the research devoted to the development of DꢀA starꢀshaped
molecules with different molecular size as well as varying πꢀbridge length is
inadequate, especially when compared to the amount of research devoted to linear
molecules.[24,25] Therefore, the effect of πꢀbridge length in starꢀshaped molecules can
possibly provide a subtle way of controlling film morphology and device performance,
providing more detailed insights into their structureꢀproperty relationships.
In recent years, triphenylamine (TPA) based starꢀshaped molecules have been
developed as an interesting class of semiconducting materials. They were used in
OSCs because of their good solubility and high hole mobility.[26, 27] Roncali and coꢀ
workers firstly reported a series of TPAꢀbased starꢀshaped molecules with a DꢀπꢀA
architecture.[28ꢀ30] Increasing the number of electron withdrawing groups in this series
of compounds caused a deepening of the HOMO energy level and a decrease in the
optical band gap with corresponding increase in open circuit voltage (Voc) and short
circuit current (Jsc). Hereafter, Li et al. introduced vinylene bridges between TPA and
oligothiophene fragments to improve the molecular effective conjugated bridge length
and photovoltaic properties.[31] Recently, modifying alkyl side chains’ position and
length to reduce steric hindrance and improve the strength of molecular πꢀconjugated
17]
bridges, our group reported a series of TPAꢀbased starꢀshaped molecules,[16,
including N(Phꢀ2TꢀDCNꢀMe)3 and N(Phꢀ2TꢀDCNꢀHex)3 (see Figure 1 for
molecular structures). The resulting OSC based on N(Phꢀ2TꢀDCNꢀMe)3 provided a
PCE of 4.76%, which is the highest reported value for the OSCs based on starꢀshaped
molecules. However, the influence of molecular size and/or πꢀbridge length within
5

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structurally wellꢀdefined TPAꢀbased molecules on the properties relevant to OSCs,
was not systematically studied. Obviously, it would be interesting to explore the
physical and photovoltaic properties of some DꢀπꢀA conjugated starꢀshaped small
molecules with various molecular sizes by introducing oligothiophene πꢀbridges of
different lengths into each branch of the molecule. We envisioned that extending
conjugated oligothiophene πꢀbridges into TPAꢀbased starꢀshaped molecules could
substantially enhance absorption, alleviate the film morphology and improve charge
carrier mobility, which are indispensable for high performance photovoltaic devices.
Thus, two new TPAꢀbased starꢀshaped molecules N(Phꢀ1TꢀDCNꢀMe)3 and N(Phꢀ3Tꢀ
DCNꢀHex)3 were designed and synthesized with thiophene (1T) and terthiophene (3T)
as πꢀconjugated bridges, as shown in Figure 1. Combined with N(Phꢀ2TꢀDCNꢀMe)3
and N(Phꢀ2TꢀDCNꢀHex)3 with bithiophene (2T) as πꢀconjugated bridges, we
systematically investigated the influence of oligothiophene πꢀbridges of varying
conjugation length on relevant physical and photovoltaic properties of starꢀshaped
small molecules with a DꢀπꢀA architecture. It should be noted that N(Phꢀ2TꢀDCNꢀ
Me)3 and N(Phꢀ2TꢀDCNꢀHex)3 are simultaneously introduced to exclude the
influence of alkyl chain groups in the investigation. The detailed characterization of
their thermal, optical and electrochemical properties, solubility, film structure and
morphology, charge transport, and photovoltaic performance are presented.
6

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NC
NC
NC CN
S
CN
CN
S
NC
S
S
CN
S
S
N
N
S
S
S
NC
NC
CN
CN
N(Ph-1T-DCN-Me)3
N(Ph-2T-DCN-Me)3
NC
CN
S
NC
CN
NC
S
CN
S
S
S
S
S
CN
S
S
NC
S
N
N
S
S
S
S
CN
CN
S
NC
NC
N(Ph-3T-DCN-Hex)3
N(Ph-2T-DCN-Hex)3
Figure 1. Molecular structures of the investigated TPAꢀbased starꢀshaped molecules.
2. Results and discussion
2.1 Synthesis and Characterization of the StarꢀShaped Molecules
Preparation of TPAꢀbased starꢀshaped molecules (Figure 1) is outlined in
Scheme 1. Synthesis of N(Phꢀ2TꢀDCNꢀMe)3 and N(Phꢀ2TꢀDCNꢀHex)3 were
described in detail previously.[16] The two new TPAꢀbased molecules, N(Phꢀ1TꢀDCNꢀ
Me)3 and N(Phꢀ3TꢀDCNꢀHex)3 were prepared similar to the previously developed
synthetic approach based on preparation of the starꢀshaped triketones followed by a
7

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Knövenagel condensation with malononitrile under a microwave irradiation.[16, 17]
Synthesis of N(Phꢀ1TꢀDCNꢀMe)3 and N(Phꢀ3TꢀDCNꢀHex)3 consists of six
consecutive reaction stages (see Scheme 1, and experimental part in Supporting
Information). First, the preparation of ketone precursors (2a, 2d) by the acylation of
magnesium derivatives of thienyl (1a) and terthienyl (1d) bromides with
corresponding alkanoyl chlorides, using lithium manganese chloride as a catalyst, was
carried out. Second, the protected ketones (3a, 3d) were prepared by reactions of 2a
and 2d with 2,2ꢀdimethylꢀ1,3ꢀpropanediol or ethylene glycol, respectively, at the
presence of pꢀtoluenesulfuric acid. It is interesting to note that the protection of 2a
with ethylene glycol was found to be unsuccessful in this case because of the
extremely high instability of 1,3ꢀdioxolane (DOX) groups during the isolation and
purification procedures. This problem was successfully overcome by the use of more
stable 5,5ꢀdimethylꢀ1,3ꢀdioxane (DMOX) protective groups. The third stage included
the synthesis of pinacol boronic esters of the corresponding protected ketones (4a and
4d) via the reaction of lithium derivatives of compounds 3a and 3d with 2ꢀ
isopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane (IPTMDOB). On the fourth stage
the synthesis of starꢀshaped compounds 5a and 5d was carried out via Suzuki crossꢀ
coupling between tris(4ꢀbromophenyl)amine and compounds 4b or 4d, respectively.
In the next stage, deprotection of 5a and 5d by treatment of their THF solutions with
hydrochloric acid gave the poorly soluble starꢀshaped ketones 6a and 6d in high
yields. Finally, N(Phꢀ1TꢀDCNꢀMe)3 and N(Phꢀ3TꢀDCNꢀHex)3 were obtained by
Knövenagel condensation reaction of compounds 6a and 6d with the excess of
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malononitrile in pyridine using a microwave heating.
1
13
GPC, Hꢀ and CꢀNMR spectroscopy, elemental analysis, high resolution massꢀ
spectroscopy and MALDIꢀTOF studies were used to characterize the molecular
structure and purity of these molecules (see Supporting Information, Figures S1 ꢀ
Figure S23). N(Phꢀ1TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀHex)3, and
N(Phꢀ3TꢀDCNꢀHex)3 were readily soluble in common organic solvents such as THF,
chloroform, dichloromethane, etc. Since N(Phꢀ2TꢀDCNꢀMe)3 demonstrates
somewhat lower solubility at room temperature, N(Phꢀ3TꢀDCNꢀHex)3 based on
terthiophene bridges and hexyl terminal chains was introduced for the sake of
comparison with the bithiophene bridgeꢀbased N(Phꢀ2TꢀDCNꢀHex)3, taking into
account the influence of alkyl terminal chains.
X
1) Mg
HO-(CH2
)
2-OH
S
X
n
PG
O
2) RCOCl,
Li2MnCl4
1) BuLi
2) IPTMDOB
PG
or
O
S
S
S
S
Br
HO-CH2-C(CH3)2-CH2-OH
S
n
R
B
R
n
N(Ph-Br)3
R
n
THF, 0 - +23 o
C
n
THF
p-TosH,
benzene, reflux
n
O
N
-78 - +20 o
C
5an = 1; R=Me; PG=DiOX
Pd(PPh3)4, Na2CO3
toluene/ethanol
n = 2; R=Hex; PG=DOX reflux
1a n = 1
2an = 1; R=Me
3an = 1; R=Me; PG=DiOX
3bn = 2; R=Me; PG=DOX
n = 2; R=Hex; PG=DOX
n = 3; R=Hex; PG=DOX
4an = 1; R=Me; PG=DMOX
4bn = 2; R=Me; PG=DOX
PG
n = 2; R=Me; PG=DOX
n = 2; R=Hex; PG=DOX
5b
5c
5d
n = 2
n = 2
n = 2; R=Me
n = 2; R=Hex
1b
1c
1d
2b
2c
X =
*
3c
3d
4c
HCl
n = 3; R=Hex; PG=DOX
R
THF, reflux
n = 3
2dn = 3; R=Hex
4dn = 3; R=Hex; PG=DOX
O
6an = 1; R=Me
n
X =
n = 2; R=Me
n = 2; R=Hex
n = 3; R=Hex
6b
6c
6d
S
R
*
CH2(CN)2
pyridine
O
O
;
DMOX
where: DOX =
O
O
NC
X =
CN
N(Ph-1T-DCN-Me)3
N(Ph-2T-DCN-Me)3
N(Ph-2T-DCN-Hex)3
N(Ph-3T-DCN-Hex)3
105 0
C
X
* *
*
*
R
*
Scheme 1. Synthesis of starꢀshaped molecules.
2.2 Thermal Properties and Solubility
Thermal properties of the four TPAꢀbased molecules were studied by
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).
Figure S24 ꢀ Figure S27 (see Supporting Information) show the results of TGA of the
studied compounds in the air and nitrogen flow correspondingly. One can see that in
the inert atmosphere the compounds possess very high thermostability (Figure S24),
starting to decompose only above 370°C (see the temperature of 5% weight loss ꢀT5%,
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N2 in Table 1). The comparison of the loss of weight at very high temperatures
indicates unambiguously that this is due to decomposition of the aliphatic tails
together with the dicyano groups. The lowest weight loss was observed for N(Phꢀ2Tꢀ
DCNꢀMe)3 (19%). It is a bit higher for N(Phꢀ1TꢀDCNꢀMe)3 (21%) due to the
increased proportion of methyl ends vs. oligothiophene fragmetns (2T in the former
case and 1T in the latter). For compounds with hexyl tails the weight loss was 35ꢀ40%,
which is also in good agreement with the molecular composition (weight fraction of
the aliphatic tails together with the dicyano groups). In the air flow all the compounds
start losing weight at 300°C, totally decomposing without leftover at 550°C, as shown
in Figure S25.
First heating DSC scans of N(Phꢀ1TꢀDCNꢀMe)3 (Figure S25) are characterized by
two endothermal peaks at 160°C (ꢀH1 = 4.7 J/g) and 276°C (ꢀH2 = 60.7 J/g).
Analysis of the temperature behavior of N(Phꢀ1TꢀDCNꢀMe)3 revealed that the first
transition is crystalꢀcrystal transformation (see Figure S30), while the second one
corresponds to the sample isotropization. It should be underlined that after annealing
at 260ºC and subsequent cooling, high temperature crystal modification appeared to
be stable down to room temperature: compare curves 2 and 3 on Figure S30b .
Parameters of its crystal lattice will be presented later on. Other materials studied are
characterized by glass transition only (see Figure S27). Its temperature decreases with
increasing the length of aliphatic tails, as shown in Table 1. Moreover, an increase of
the length of oligothiophene bridge leads to higher glass transition temperature of the
material due to increased rigidity of the branches of the molecule.
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Solubilities of the four compounds were determined in oꢀdichlorobenzene (ODCB) at
[32]
room temperature by a reported method
and are listed in Table 1. The four
compounds have a solubility sequence of N(Phꢀ2TꢀDCNꢀMe)3 < N(Phꢀ1TꢀDCNꢀ
Me)3 < N(Phꢀ3TꢀDCNꢀHex)3 < N(Phꢀ2TꢀDCNꢀHex)3, opposite to the sequence of
glass transition temperatures. N(Phꢀ2TꢀDCNꢀMe)3 has the lowest solubility (<3
mg/mL) because the short methyl chains as solubilizing groups result in strong πꢀπ
interactions. In contrast, N(Phꢀ2TꢀDCNꢀHex)3 has the highest solubility (20 mg/mL
in ODCB) because of weak πꢀπ interactions due to the effect of long hexyl chains
manifesting itself in twisted conformation between oligothiophene bridges and
acceptor units. In addition, N(Phꢀ1TꢀDCNꢀMe)3 has higher solubility than N(Phꢀ2Tꢀ
DCNꢀMe)3 (6 mg/mL vs. <3 mg/mL), which is attributed to the reduced
oligothiophene bridges. The solubility of N(Phꢀ3TꢀDCNꢀHex)3 underlines that simply
extending the oligothiophene bridges leads to a relatively lower solubility and strong
πꢀπ interactions. Saturated alkyl chains (eg. hexyl chains) often cause the conjugated
backbone to twist, which reduces the effective conjugation length and consequently
increases the optical band gap. Therefore, a moderate extension of the oligothiophene
πꢀbridge can possibly improve the effective conjugation length and optical absorption
as discussed below.
Table 1. Thermal properties and solubility of four TPAꢀbased molecules.
a
Solubilityb (mg/mL)
a
Compound
Tg , °C
ꢀCp , J/g·K ꢀT5%, N2, °C
c
N(Phꢀ1TꢀDCNꢀMe)3
N(Phꢀ2TꢀDCNꢀMe)3
128
159
64
0.27
0.21
0.25
395
453
401
6
<3
20
N(Phꢀ2TꢀDCNꢀHex)3
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72
0.29
388
12
N(Phꢀ3TꢀDCNꢀHex)3
b
aParameters of glass transition in the compounds studied by DSC method; In ODCB
c
at room temperature, determined by the method in ref.[32]; Second heating of the
once melted material.
2.3. Photophysical and Electrochemical Properties
The UVꢀvis absorption spectra of the small molecules in dilute ODCB solutions and
films are shown in Figure 2. Samples used for the measurement of film absorption
spectra were prepared by doctorꢀblading from ODCB solutions. The corresponding
optical data are summarized in Table 2. In ODCB solution, N(Phꢀ1TꢀDCNꢀMe)3
exhibits two distinct absorption peaks, the UV absorption peak at 363 nm in the
shorter wavelength region corresponds to the πꢀπ* transition, and the visible
adsorption peak at 495 nm is attributed to the intramolecular chargeꢀtransfer (ICT)
transition between the TPAꢀthiophene donor unit and the dicyanovinyl (DCN)
acceptor unit. With an increasing πꢀconjugated bridge length of each branch the
absorption peaks of N(Phꢀ2TꢀDCNꢀMe)3/N(Phꢀ2TꢀDCNꢀHex)3 and N(Phꢀ3TꢀDCNꢀ
Hex)3 solution are redshifted significantly to 517/517 and 522 nm, respectively,
despite of the tendencies of absorption maxima (λmax) and absorption onset (λonset) to
the saturation. Probably, the longer oligothiophene πꢀbridges linkage between TPA
and DCN eliminate torsional interactions and increase the intensity of the ICT
transition. In addition, the shorter wavelength region peaks redshift from 363 nm to
382/382 nm and 416 nm for N(Phꢀ2TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3/N(Phꢀ2Tꢀ
DCNꢀHex)3 and N(Phꢀ3TꢀDCNꢀHex)3, respectively, which is attributed to the
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stronger localized πꢀπ* transition arising from the longer oligothiophene πꢀbridge
lengths. The difference in position and intensity of the absorption spectra are mainly
due to the different structures of the πꢀbridges in the molecules. Comparative analysis
of solution and films’ photophysical properties revealed that absorption spectra of
N(Phꢀ1TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3, and N(Phꢀ2TꢀDCNꢀHex)3 become
broader and their absorption peaks are significantly redꢀshifted. However, the
absorption peak of N(Phꢀ3TꢀDCNꢀHex)3 is negligibly shifted between solution and
film due to the weak aggregation and intermolecular interactions of the molecules in
the solid state.[33] Among these starꢀshaped molecules, λmax and λonset of N(Phꢀ2Tꢀ
DCNꢀMe)3 are most significantly redꢀshifted by 23 nm and 111 nm, respectively,
indicating the efficient πꢀstacking and strong intermolecular interactions in the film.[16]
A reduced solid state band gap of 1.74 eV is achieved for N(Phꢀ2TꢀDCNꢀMe)3 based
on bithiophene bridges, 0.31 eV lower than that for N(Phꢀ1TꢀDCNꢀMe)3 based on a
simple thiophene bridge. This may be attributed to the increased effective conjugation
length and improved electronic communication between TPA and DCN units. In
addition, λonset of N(Phꢀ2TꢀDCNꢀHex)3 is significantly blueꢀshifted by 54 nm as
compared to N(Phꢀ2TꢀDCNꢀMe)3, corresponding to a minor difference in planarity
caused by the steric hindrance of the terminal chains. Finally, compared to the other
three compounds, the absorption intensity of N(Phꢀ3TꢀDCNꢀHex)3 in the shorter
wavelength region is higher. This suggests that longer oligothiophene πꢀbridge
effectively releases the distortion induced by the long alkyl terminal chains.
13

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The electrochemical properties of the four TPAꢀbased starꢀshaped molecules and their
HOMO and LUMO energy levels were examined by cyclic voltammetry (CV) (see
Table 2 and Figures S28 ꢀ Figure S29 in the Supporting Information). From the
value of the onset reduction potentials (φred) and the onset oxidation potentials (φox),
the LUMO and HOMO energy levels of these molecules were calculated as shown in
Table 2, according to the equations of LUMO = ꢀe(φred + 4.40) (eV) and HOMO = ꢀ
e(φox + 4.40) (eV).[34] The LUMO energy levels were determined to be ꢀ3.46 eV for
N(Phꢀ1TꢀDCNꢀMe)3, ꢀ3.41 eV for N(Phꢀ2TꢀDCNꢀMe)3, ꢀ3.41 eV for N(Phꢀ2Tꢀ
DCNꢀHex)3, and ꢀ3.40 eV for N(Phꢀ3TꢀDCNꢀHex)3 by the aboveꢀmentioned equation,
revealed that the measured LUMO energy levels of these four molecules are very
close. Therefore, the incorporation of various oligothiophene πꢀbridges has little effect
on their LUMO level differences. However, it is interesting to note that the HOMO
energy levels are significantly different due to the effect of πꢀconjugated bridges with
various electron donating characters.[35] Owing to the strong electronꢀdonating
terthiophene units as πꢀbridges, which usually increases the HOMO energy level,
N(Phꢀ3TꢀDCNꢀHex)3 exhibtis a high HOMO energy level of ꢀ5.26 eV. And it is 0.34
eV higher than that of N(Phꢀ1TꢀDCNꢀMe)3 (ꢀ5.60 eV) because of its relatively weak
electronꢀdonating thiophene bridges. In addition, the HOMO energy levels of
N(Phꢀ2TꢀDCNꢀMe)3 and N(Phꢀ2TꢀDCNꢀHex)3 are in between the HOMO energy
levels of N(Phꢀ1TꢀDCNꢀMe)3 and N(Phꢀ3TꢀDCNꢀHex)3, as shown in Table 2.
Earlier, BHJ OSCs based on N(Phꢀ2TꢀDCNꢀMe)3 and N(Phꢀ2TꢀDCNꢀHex)3 with
[6,6]ꢀPhenylꢀC71ꢀbutyric acid methyl ester (PC71BM) acceptors exhibited high Voc of
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0.98 V. Therefore, a comaprable high Voc is expected for N(Phꢀ1TꢀDCNꢀMe)3 and a
slightly low Voc is expected for N(Phꢀ3TꢀDCNꢀHex)3, becasue Voc is mainly
determined by the difference between the HOMO of the donor and the LUMO of the
acceptor.[35]
N(Ph-1T-DCN-Me)
N(Ph-2T-DCN-Me)
N(Ph-1T-DCN-Me)
N(Ph-2T-DCN-Me)
3
3
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
a
b
3
3
N(Ph-2T-DCN-Hex)
N(Ph-2T-DCN-Hex)
3
3
N(Ph-3T-DCN-Hex)
N(Ph-3T-DCN-Hex)
3
3
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 2. UVꢀvis absorption spectra of N(Phꢀ1TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3,
N(Phꢀ2TꢀDCNꢀHex)3, and N(Phꢀ3TꢀDCNꢀHex)3 in ODCB solution (a) and in the
film (b).
Table 2. Optical and electrochemical properties of the TPAꢀbased molecules.
UVꢀvis absorption spectra
cyclic voltammetry
solutiona
λmax
filmb
pꢀdoping
nꢀdoping
opt
EC
Compounds
λmax
(nm)
502
541
525
522
λonset Eg
φox/HOMO
(V)/(eV)
φred/LUMO Eg
(nm)
495
(nm) (eV) c
(V)/(eV)
(eV)
2.14
1.91
1.93
1.86
605
711
657
677
2.05
1.74
1.89
1.83
1.20/ꢀ5.60
0.92/ꢀ5.32
0.94/ꢀ5.34
0.86/ꢀ5.26
ꢀ0.94/ꢀ3.46
ꢀ0.99/ꢀ3.41
ꢀ0.99/ꢀ3.41
ꢀ1.00/ꢀ3.40
N(Phꢀ1TꢀDCNꢀMe)3
N(Phꢀ2TꢀDCNꢀMe)3
d
517
d
N(Phꢀ2TꢀDCNꢀHex)3
N(Phꢀ3TꢀDCNꢀHex)3
517
522
b
c
aMeasured in ODCB solution. Cast from ODCB solution. Bandgap estimated from
opt
the onset wavelength (λonset) of the optical absorption: Eg = 1240/ λonset. dData from
15

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Ref. [16].
2.4. XꢀRay diffraction
Here, we further investigate the effects of oligothiophene πꢀbridge length on the
packing of the molecules in solid state by XRD analysis. At room temperature
N(Phꢀ1TꢀDCNꢀMe)3 is highly crystalline, as is shown by DSC (Figure S26) and Xꢀ
Ray scattering (Figure S30) studies. Annealing at 260°C allowed to get a stable
crystal phase of triclinic symmetry with parameters a = 11.47 Å, b = 18.17 Å,
c = 18.30 Å,
α = 98.3°, β = 52.2°, γ = 130.7° (see Table_S1 and Figure 3). Such
mutual arrangement provides the optimum πꢀπ interaction between the neighbouring
molecules. Calculated density of the crystalline phase appeared to be
ρ
calc = 1.227 g/cm3 which is higher than experimentally observed value of the density
exp ~ 0.98 g/cm3. However, such discrepancy is reasonable as real crystals can
ρ
possess a number of defects upsetting the equilibrium arrangement of the molecules.
Moreover, the samples can also contain the regions of the quenched amorphous phase
having substantially lower density. Melting the N(Phꢀ1TꢀDCNꢀMe)3 compound leads
to its irreversible transformation to the amorphous state – crystalline phase does not
recover on slow cooling after heating to 290ºC. Only glass transition is observed at
128ºC (see Table 1).
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Figure 3. Packing of the molecules of compound N(Phꢀ1TꢀDCNꢀMe)3 in the triclinic
crystal lattice with parameters a = 11.47 Å, b = 18.17 Å, c = 18.30 Å, = 98.3°,
= 52.2°, = 130.7°.
α
β
γ
Wideꢀangle measurements of XꢀRay scattering revealed that all samples but N(Phꢀ1Tꢀ
DCNꢀMe)3 do not possess crystalline order. At room temperature, a wide halo was
observed on the corresponding scattering curves (Figure S31). However,
comparatively narrow reflection on curve 4 corresponding to dꢀspacing of 5.55 Å
should be noted. This is ascribed to the remnants of the ordering of crystalꢀsolvates.
However, it should be noted that crystalline modification of N(Phꢀ2TꢀDCNꢀMe)3
compound can be obtained by the crystallization from solution. It has a melting
temperature of Tm = 248ºC (ꢀH = 60.7 J/g). Temperature behaviour of such samples is
rather similar to that of N(Phꢀ1TꢀDCNꢀMe)3 compound. In the melted material,
crystal phase does not reappear even after slow cooling with 2ºC/min cooling rate.
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Figure 4. Molecular modelling of the packing of N(Phꢀ3TꢀDCNꢀHex)3 molecules in
2D hexagonal columnar mesophase. Black solid circles correspond to 2D hexagonal
columnar packing with the diameter of cylinders D = 23.7Å. Grey solid are the
envelopes of molecular discs (Dm = 30.6 Å).
In contrast to the crystalline N(Phꢀ1TꢀDCNꢀMe)3, and to the amorphous N(Phꢀ2Tꢀ
DCNꢀMe)3 and N(Phꢀ2TꢀDCNꢀHex)3, which do not reveal any reflections in smallꢀ
angle Xꢀray scattering (SAXS) patterns (Figure S32, curve 1), N(Phꢀ3TꢀDCNꢀHex)3
at room temperature is characterized by a set of three reflections with the ratios of
2
2
2
corresponding dꢀspacings d1 : d2 : d3 = 1 : 3 :4 (Figure S32, curve 2). They can be
indexed by 2D hexagonal columnar phase with cylinder diameter D = 23.7 Å.
Moreover, wide halo is observed at s = 0.16 Åꢀ1. Its origin relates to the Fourier
transform of single cylinder of columnar mesophase. With the temperature increasing,
mesophase reflections of compound N(Phꢀ3TꢀDCNꢀHex)3 shift to small angles,
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diameter of the columns increased to 23.9 Å at 120°C (Figure S32, curve 3). Such a
change corresponds to thermal expansion coefficient β = 1.3·10ꢀ4 Kꢀ1. As the diameter
of the molecule envelope is 30.6 Å, substantial correlation between the relative
positions of the molecules in neighboring molecules should be suggested (Figure 4).
There is no order in the packing of the molecules along the column axis (because of
the absence of diffraction maxima on wideꢀangle XꢀRay scattering patterns), as well
as no ordering between the neighboring columns, summarizing the pattern of
molecular packing is irregular in each transverse section due to the steric necessity of
mutual packing of long πꢀconjugated bridges, while column centers form 2D
hexagonal lattice. Appearing steric limitations lead to the azimuthal irregularities in
stacking of the molecules along the column, as every molecule should adjust its
position not only with its top and bottom neighbors inside the cylinder, but with its
side neighbors as well. An interesting feature of the columnar mesophase in
compound N(Phꢀ3TꢀDCNꢀHex)3 are the specific relative intensities of the reflections:
second reflection (11) is 7.5 times stronger than the first one (10), and the third one
(20) is of the same intensity (I10 = I20). Thus, with temperature increasing to 140°C
(Figure S32, curve 4), reflection 10 disappears while the other ones are still observed,
and only at 160°C isotropization takes place (Figure S32, curve 5). Such observations
can also be explained by a specific radial distribution of electron density in the
cylinders of columnar mesophase leading to the shift of the position of the first
diffraction peak to the minimum of Bessel function of an individual column. Thus, the
composition of the column and lattice factors causes the intensity of 10 reflection to
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be substantially low. The columnar mesophase formed in the samples of N(Phꢀ3Tꢀ
DCNꢀHex)3 looks to be very promising for the directed charge transport along the
columns as the oligothiophene fragments of neighboring molecules are arranged to
allow the best π−π stacking. However, a good alignment of the columns is needed for
the good semiconductor properties to manifest their selves. A separate work on the
properties of oriented samples should be carried out.
Thin films formed by the mixtures of starꢀshaped molecules with PC71BM have also
been studied using XꢀRay reflectivity and Grazing Incidence XꢀRay reflectivity
methods. Thickness of all blended films calculated from the positions of reflectivity
minima (see Figure S33) is ca. 500 Å. Substantial roughness 2(~ 5 Å) of the surface
of studied samples does not allow to provide unambiguous analysis of the distribution
of electron density along the substrate director. Grazing incidence diffraction patterns
(Figure S34) of all blends reveal three maxima at s1 = 0.672 Åꢀ1, s2 = 1.346 Åꢀ1 and
s3 = 1.957 Åꢀ1 corresponding to scattering from liquidꢀlike phase of PC71BM
(s1 : s2 : s3 = 1 : 2 :3, curve 5 on Figure S34). The conclusion can be made that in
blended films microsegregation takes place. Thiopheneꢀbased materials are
amorphous, and PC71BM balls possess shortꢀrange order in their mutual arrangement.
2.5. Film Morphology
Compared to their covalent polymeric counterparts, small molecules can significantly
and directly influence many important material parameters such as solubility,
crystallinity, phase behavior, microscopic morphology, which ultimately determine
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the device performance of OSCs. Normally, the size of crystalline domains in donorꢀ
acceptor films is one of the critical parameters determing device efficiency.[35]
Therefore, to fabricate efficient BHJ solar cells using small molecules as donor
materials, it is crucial to develop some methods of controlling its tendency to
crystallize. In our previous study, modifying the alkyl solubilizing groups attached to
the starꢀshaped molecules appears to be an excellent approach to control the film
morphology of the blend.[16]
Here, to further examine the effect of using these four starꢀshaped molecules based on
various πꢀbridge length on the film morphology of the blend, atomic force microscope
(AFM) was employed to probe the film morphological properties. The topographic
AFM images of the blended films determined by the optimized OSCs in Ref. [16], are
given in Figure 5. All the blended films are rather smooth with an average surface
roughness of 0.55 nm, 0.65 nm, 0.46 nm and 0.53 nm for N(Phꢀ1TꢀDCNꢀ
Me)3:PC71BM, N(Phꢀ2TꢀDCNꢀMe)3:PC71BM, N(Phꢀ2TꢀDCNꢀHex)3:PC71BM, and
N(Phꢀ3TꢀDCNꢀHex)3:PC71BM films, respectively. However, in all cases, the blend
film morphologies were slightly different. It is evident from AFM studies that the
variations in the molecular size and πꢀbridge length can cause changes in the
microstructure of blended films. Quantitative investigation of the surface morphology
based on the analysis of height correlation functions[35ꢀ37] (see Figures S35ꢀS38)
revealed that N(Phꢀ2TꢀDCNꢀMe)3:PC71BM, and N(Phꢀ2TꢀDCNꢀHex)3:PC71BM
blends are heterogeneous films. They are characterized by the two specific scales of
the local and correlation length, one of them corresponding to the slope, diffuse blobs
21

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of the matter (supposedly, due to the phase segregation of the starꢀlike compound and
PC71BM), and the other one ascribed to very high but extremely localized peaks.
Contrary, the blends of N(Phꢀ1TꢀDCNꢀMe)3:PC71BM and N(Phꢀ3TꢀDCNꢀ
Hex)3:PC71BM are more homogeneous and can be described by only one correlation
length. These nanoscaleꢀ aggregated domains are beneficial to the exciton charge
separation, charge transportation and in turn enhance performance of the OSCs.[16, 38ꢀ
40]
Therefore, judicious choice of an oligothiophene πꢀbridge at an appropriate length
is of crucial importance for fineꢀtuning the structureꢀblended film morphology
relationship.
Figure 5. Tapping mode AFM surface scans (5×5 µm2) of blended films without any
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thermal annealing of a) N(Phꢀ1TꢀDCNꢀMe)3:PC71BM, b) N(Phꢀ2TꢀDCNꢀ
Me)3:PC71BM, c) N(Phꢀ2TꢀDCNꢀHex)3:PC71BM, and d) N(Phꢀ3TꢀDCNꢀ
Hex)3:PC71BM (D:A; 1:2 wt%).
2.5. Hole Mobility
To further gain insight into the effect of πꢀbridges length within structurally wellꢀ
defined starꢀshaped molecules on the charge transport, space charge limited current
(SCLC) method was used to map out hole mobilities of the pristine and blended films.
The structures of hole only devices were ITO/PEDOT:PSS/pristine or blended films
(D:A; 1:2 wt%)/MoO3/Ag.[16] We calibrated the hole mobility (ꢀh) by fitting the
currentꢀvoltage curves and measuring three different thicknesses of each films over
eighteen diodes (Figure S39 – Figure S41, Supporting Information). As shown in
Figure S41, the hole only mobilities of pristine and blended films just have moderate
variations at different thicknesses of semiconductor layers. In addition, the charge
carrier mobilities of blended films with PC71BM as acceptors exhibit a similar trend
of the pristine TPAꢀbased star shaped molecules. The results indicate a nonꢀ
correlation between the charge carrier mobilities of pristine and blended films and the
related films’ thicknesses, and a correlation between the molecular physical properties
and BHJ devices’ transport properties. The related average values of hole mobilities of
pristine and blended films are shown in Figure 6 and Table 3. The pristine films in a
range of film thicknesses for N(Phꢀ1TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3, N(Phꢀ2Tꢀ
DCNꢀHex)3 and N(Phꢀ3TꢀDCNꢀHex)3 give hole only mobilities of 3.67×10ꢀ5,
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3.99×10ꢀ3, 3.02×10ꢀ4, and 6.40×10ꢀ4 cm2V−1s−1, respectively. Obviously, N(Phꢀ1Tꢀ
DCNꢀMe)3 shows the lowest hole mobility in these pristine films, which is two orders
of magnitude worse than that of N(Phꢀ2TꢀDCNꢀMe)3. It suggests that a short πꢀ
conjugated thiophene bridge is not beneficial to the intramolecular electron transport,
which can directly limit exciton diffusion and charge transport in OSCs. In addition,
the hole mobility of N(Phꢀ2TꢀDCNꢀHex)3 is one order of magnitude worse than that
of N(Phꢀ2TꢀDCNꢀMe)3, which can most likely be attributed to weak conjugated
backbone resulting from the long hexyl terminal chains. In order to compensate the
negative effects of long terminal chains, introducing an electron linker to the
conjugated main chains is also effective in maintaining a coꢀplanar structure, and
releasing the distortion induced by the alkyl terminal chains. Therefore, by further
extending the πꢀconjugated bridges from bithiophene to terthiophene units and
improving the bridgeꢀplanarization, the two times higher hole mobility measured in
the N(Phꢀ3TꢀDCNꢀHex)3 diodes, as compared to that of N(Phꢀ2TꢀDCNꢀHex)3. It
suggests that long πꢀconjugated terthiophene bridges can improve the bridgeꢀ
planarization and facilitate intramolecular charge transport. However, it is interesting
that the hole mobility of N(Phꢀ3TꢀDCNꢀHex)3 is significantly lower than that of
N(Phꢀ2TꢀDCNꢀMe)3, as shown in Figure 6. This may be attributed to the twisted
conjugated branch and reduced effective conjugated length caused by steric hindrance
of long hexyl terminal chain. While this tendency of N(Phꢀ3TꢀDCNꢀHex)3 to give a
low hole mobility in pristine film agrees well with the absorption measurements that
N(Phꢀ3TꢀDCNꢀHex)3 has a relatively broad optical band gap.
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Among the blended films with these four starꢀshaped molecules, N(Phꢀ2TꢀDCNꢀ
Me)3:PC71BM exhibits the highest hole mobility of 5.72×10ꢀ4 cm2V−1s−1, 45 times
higher than that of N(Phꢀ1TꢀDCNꢀMe)3:PC71BM. The high hole mobility of N(Phꢀ
2TꢀDCNꢀMe)3:PC71BM, which arose from the longer conjugation length along the
molecule, is promising with respect of enhanced photocurrent generation and reduced
recombination loss. In addition, the hole mobilities of N(Phꢀ2TꢀDCNꢀHex)3:PC71BM
and N(Phꢀ3TꢀDCNꢀHex)3:PC71BM are 8.45×10ꢀ5 and 2.06×10ꢀ4 cm2V−1s−1,
respectively, which is consistent with the hole mobilities of the pristine films. These
results illustrate that intramolecular charge transfer instead of intermolecular transport
in small molecule domains plays a decisive role for the charge transport of blended
film. Therefore, we expect that these differences in film morphology and charge
transport in blended films will have a significant impact on the device performance as
discussed below.
0.01
1E-3
1E-4
Pristine films
Blended films
1E-5
3
P
(
N
3
)
e
M
3
3
)
x
e
)
)
e
x
e
M
-
-
N
C
D
H
H
-
N
C
-
N
C
D
N
C
D
-
T
1
-
D
-
T
3
T
2
-
h
-
T
2
-
-
-
h
P
(
N
h
h
P
(
N
P
(
N
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Figure 6. Hole mobilities of pristine and blended films based on the four TPAꢀbased
molecules with an average value for a range of thicknesses.
Table 3. Hole mobilities determined from the SCLC measurements.
Thicknesses of
pristine films
(nm)
Thicknesses of
blended films
(nm)
Hole Mobilitya
(ꢀh) [cm2V−1s−1]
Hole Mobilitya
(ꢀh) [cm2V−1s−1]
Molecules
3.67×10ꢀ5
3.99×10ꢀ3
3.02×10ꢀ4
6.40×10ꢀ4
60/90/130
70/100/120
70/90/110
60/80/110
1.26×10ꢀ5
5.72×10ꢀ4
8.45×10ꢀ5
2.06×10ꢀ4
N(Phꢀ1TꢀDCNꢀMe)3
N(Phꢀ2TꢀDCNꢀMe)3
N(Phꢀ2TꢀDCNꢀHex)3
N(Phꢀ3TꢀDCNꢀHex)3
100/160/190
80/130/180
90/140/190
90/130/200
aThe reported mobility data are average values of each pristine and blended films for a
range of thicknesses.
2.6. Photovoltaic properties of BHJ devices based on these compounds
To study the effect of oligothiophene πꢀbridge length on the photovoltaic properties,
bulk heterojunction OSCs with a configuration of ITO/PEDOT:PSS/TPAꢀbased
molecules : PC71BM (1:2, wt%) /Ca(15nm)/Ag(80nm) were fabricated, and the
detailed device fabrication process is described in the Experimental Section.. The best
current densityꢀvoltage (JꢀV) curves are shown in Figure 7a and the corresponding
photovoltaic performance is summarized in Table 4.
As expected, OSCs based on these small molecules show relatively high Voc (>0.9 V),
due to their lowꢀlying HOMO energy levels. Encouragingly, OSC based on N(Phꢀ1Tꢀ
DCNꢀMe)3 exhibited a high Voc up to 1.08 V. With the increasing πꢀconjugated
oligothiophene bridge length, OSCs based on N(Phꢀ2TꢀDCNꢀMe)3/N(Phꢀ2TꢀDCNꢀ
Hex)3 and N(Phꢀ3TꢀDCNꢀHex)3 showed the lower Voc of 0.98/0.98 V and 0.93 V,
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respectively. These results are consistent with their HOMO level variations (Table 2).
Simultaneously, OSCs based on N(Phꢀ1TꢀDCNꢀMe)3, N(Phꢀ2TꢀDCNꢀMe)3, N(Phꢀ
2TꢀDCNꢀHex)3, and N(Phꢀ3TꢀDCNꢀHex)3 provide a Jsc of 5.97 mA/cm2, 8.87
mA/cm2, 8.07 mA/cm2, and 8.51 mA/cm2, respectively, which correlates with the
trend in their absorption spectra, optical band gaps and transport properties. The FF
exhibits the similar trends of Jsc, as shown in Table 4. Absorption spectra and
electronic structure of the components are obviously important for device operation,
but the role of film morphology cannot be ignored, since the charge separation takes
place at the donorꢀacceptor interface. As discussed, we found that the changes in the
Jsc and FF values of the devices are also consistent with both changes in the film
morphologies as well as the hole mobilities. In the case of N(Phꢀ1TꢀDCNꢀ
Me)3:PC71BM device, both poor film quality and the charge transport result in a
significant drop in the Jsc and FF, as shown in Figure 5a and Figure 6. This is
attributed to strong recombination losses, presumably because of the inferior hole
mobility of the active layer. In contrast, the N(Phꢀ2TꢀDCNꢀMe)3:PC71BM device
exhibits an increased Jsc of 8.87 mA/cm2 and an improved FF of 54.3%, mainly due to
a higher hole mobility and a better charge collection as a result of the effective
conjugated length in the molecule and formation of nanoscale domains in the active
layer. In addition, compared to N(Phꢀ2TꢀDCNꢀHex)3:PC71BM device, a slightly
higher Jsc of 8.51 mA/cm2 and a FF of 50% are observed for the N(Phꢀ3TꢀDCNꢀ
Hex)3:PC71BM device, which is again attributed to a favorable film morphology as
well as high hole mobility. Finally, the PCEs of these devices are 2.30% for N(Phꢀ1Tꢀ
27

Journal of Materials Chemistry A
Page 28 of 41
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DOI: 10.1039/C4TA01933D
DCNꢀMe)3, 4.72% for N(Phꢀ2TꢀDCNꢀMe)3, 3.41% for N(Phꢀ2TꢀDCNꢀHex)3 and
3.96% for N(Phꢀ3TꢀDCNꢀHex)3, respectively. The external quantum efficiency (EQE)
of the corresponding devices is shown in Figure 7b, Jsc calculated from the EQE is
5.46 mA/cm2 for N(Phꢀ1TꢀDCNꢀMe)3, 8.27 mA/cm2 for N(Phꢀ2TꢀDCNꢀMe)3, 7.53
mA/cm2 for N(Phꢀ2TꢀDCNꢀHex)3, and 7.76 mA/cm2 for N(Phꢀ3TꢀDCNꢀHex)3,
respectively, well consistent with the values of Jsc measured under simulated AM 1.5.
60
50
40
30
20
10
0
N(Ph-1T-DCN-Me)
3
a
b
0
-2
-4
-6
-8
N(Ph-2T-DCN-Me)
3
N(Ph-1T-DCN-Me)
N(Ph-2T-DCN-Me)
3
N(Ph-2T-DCN-Hex)
3
3
N(Ph-3T-DCN-Hex)
3
N(Ph-2T-DCN-Hex)
N(Ph-3T-DCN-Hex)
3
3
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
700
Voltage (V)
Wavelength (nm)
Figure 7. Current densityꢀvoltage (IꢀV) curves (a) and EQE spectra (b) of solar cells
based on N(Phꢀ1TꢀDCNꢀMe)3:PC71BM, N(Phꢀ2TꢀDCNꢀMe)3:PC71BM, N(Phꢀ2Tꢀ
DCNꢀHex)3:PC71BM, and N(Phꢀ3TꢀDCNꢀHex)3:PC71BM (1:2, wt%), respectively,
under the illumination of AM 1.5, 100 mWcmꢀ2.
Table 4. Photovoltaic properties of starꢀshaped molecules:PC71BM (1:2, wt%)
devices, under the illumination of AM 1.5, 100 mW/cm2.
Voc
Jsc
(mAcmꢀ2)
5.97
FF
PCE
(%)
Donor: PC71BM
(V)
(%)
1.08
0.98
0.98
0.93
35.6
54.3
43.1
50.0
2.30
4.72
3.41
3.96
N(Phꢀ1TꢀDCNꢀMe)3
N(Phꢀ2TꢀDCNꢀMe)3
N(Phꢀ2TꢀDCNꢀHex)3
N(Phꢀ3TꢀDCNꢀHex)3
8.87
8.07
8.51
28

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DOI: 10.1039/C4TA01933D
3. Conclusions
In summary, the effects of oligothiophene πꢀbridge length on the properties of
starꢀshaped molecules were evaluated by comparing four TPAꢀcontaining compounds
with different πꢀbridge lengths. Material physical properties of the four compounds
revealed that the oligothiophene πꢀbridges between D and A units have a significant
impact on intramolecular and intermolecular interactions, which subsequently affect
various solidꢀstate properties such as film absorption, blend morphology and charge
carrier mobility, thereby influencing the photovoltaic properties of devices. Within
this series, N(Phꢀ2TꢀDCNꢀMe)3 shows the highest performance among the four
compounds due to the broader absorption and higher hole mobilities in pristine and
blend films. Therefore, the utility of conjugation length engineering on small
molecules is of crucial importance to elucidate structureꢀproperty relationships.
Insight into such relationships will be beneficial for the judicious choice of new
molecules based on appropriate conjugation length for use in organic photovoltaics as
well as other electronic devices.
4. Experimental Section
Materials: nꢀButyl lithium (1.6 M solution in hexane), magnesium, isopropoxyꢀ
4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane
(IPTMDOB),
tetrakis(triphenylphosphine)palladium (0) Pd(PPh3)4, pꢀtoluenesulfonic acid (pꢀTosH),
tris(4ꢀbromophenyl)amine, malononiltile, heptanoyl chloride, acetyl chloride, 2,2ꢀ
29