D-π-A-π-D type benzothiadiazole-triphenylamine based small molecules containing cyano on the π-bridge for solution-processed organic solar cells with high open-circuit voltage

Article abstract of DOI:10.1039/c2cc35754b

Two novel D-π-A-π-D structured small molecules composed of benzothiadiazole and triphenylamine were designed and synthesized. BDCTBT with cyano on the π-bridge exhibited a deep HOMO energy level, resulting in an impressive V_OC of up to 1.04 V with a PCE of 3.85%, while non-cyano substituted BDETBT yielded a V_OC of 0.94 V and a PCE of 1.99%.

Full text of DOI:10.1039/c2cc35754b

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DꢀπꢀAꢀπꢀD type benzothiadiazoleꢀtriphenylamine based
small molecules containing cyano on πꢀbridge for solutionꢀ
processed organic solar cells with high openꢀcircuit voltage
Journal: ChemComm
Manuscript ID: CC-COM-08-2012-035754.R1
Article Type: Communication
Date Submitted by the Author: 07-Sep-2012
Complete List of Authors: Zeng, Shaohang; Dalian Universtiy of Technology, School of Chemistry
Yin, Lunxiang; Dalian University of Technology, School of Chemistry
Ji, Changyan; Dalian Universtiy of Technology, School of Chemistry
Jiang, Xueying; Jilin University,
li, kechang; jilin university, chemistry
Li, Yanqin; Dalian University of Technology, School of Chemistry
Wang, Yue; Jilin University, State Key Baboratory of Supramolecular
Structure and Materails, College of Chemistry

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ARTICLE TYPE
D-π-A-π-D type benzothiadiazole-triphenylamine based small molecules
containing cyano on π-bridge for solution-processed organic solar cells
with high open-circuit voltage†
Shaohang Zeng,a Lunxiang Yin,a Changyan Ji, a,b Xueying Jiang,b Kechang Li,b Yanqin Li*a and Yue
5
Wang*b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
longer wavelength, small molecular BHJ solar cells have reached
considerable JSC up to 14 mA·cm-2.6, 8 On the other hand, the BHJ
50 devices with high VOC over 1 V have scarcely been realized using
a p-type small molecule as a donor,9 on account of the barrier of
narrowing the band-gap of small molecules without sacrificing
lowest unoccupied molecular orbit (LUMO) for effective charge
separation as well as large VOC values depending on the low-lying
55 HOMO. Therefore, careful modification of energy levels from
functional groups selection is of the utmost importance in
improvement of device properties, especially achieving reduced
HOMO without enhancing molecular band-gap. Among
conjugated polymer donors, fluorine-containing polymers exhibit
60 lower HOMO and LUMO levels as a result of the high electron
affinity nature of fluorine atom, suggesting that the addition of
electron-acceptor groups is effective in further stabilizing energy
levels.10 Moreover, cyano is a well electron-withdrawing group
which has been extensively used in organic dyes for dye-
65 sensitized solar cells.11 Noticeably, lower energy levels were
observed for dyes containing one more cyano group on their π-
bridge.12 In addition, some conjugated polymers substituted with
cyano-groups at their vinylic linkage have been reported as
photovoltaic materials.13 Consequently, we envisioned that
70 incorporating an cyano unit into the D-A conjugated small
molecules could decrease the HOMO value to obtain high VOC for
photovoltaic application.
Two novel D-π-A-π-D structured small molecules composed
of benzothiadiazole and triphenylamine were designed and
10 synthesized. BDCTBT with cyano on π-bridge exhibited a
deep HOMO energy level, resulting an impressive VOC up to
1.04 V with a PCE of 3.85%, while non-cyano substituted
BDETBT yielded the VOC of 0.94 V and the PCE of 1.99%.
Organic solar cells, as a promising alternative to traditional
15 inorganic photovoltaic devices, have attracted extensive
investigation in last two decades, due to their low-cost, light-
weight and flexible characteristics.1 Solution processed bulk-
heterojunction organic solar cells (BHJ OSCs) possess potential
for commercialization because of their high internal quantum
20 efficiency and large-scale printing technique.2 Although polymers
have been widely applied to BHJ OSCs3 and achieved impressive
progress of power conversion efficiencies (PCEs) over 8%,4 the
interests of solution-processable small molecular π-conjugated
organic donors have been accelerating for BHJ photovoltaic
25 applications in recent years, owing to the advantages of definite
structure, facile purification, high purity and well photovoltaic
performance reproduction.5 Despite Heeger and Bazan et al.
reported a competitive PCE of at around 7% for small molecular
OSCs,6 in general, the application efficiencies obtained from
30 these small molecules are relatively lower than the PCEs of
polymeric counterpart. Thus, in order to introduce small
molecular semiconductors into commercial solar cells, their
efficiencies need to be improved.
In this communication, we reported the syntheses, theoretical
calculation, photophysical and electrochemical properties, and
75 photovoltaic performances of two novel D-π-A-π-D type low
band-gap benzothiadiazole-triphenylamine based small molecular
donor materials, BDCTBT and BDETBT, one of which
contained cyano groups anchoring on π-bridge (see Fig. 1(a) for
To meet this need, two straightforward strategies have been
35 considered: 1) reducing the band-gap of molecular donors to
broaden the absorption range, which results higher short-circuit
current density (JSC) of devices, and 2) lowering the highest
occupied molecular orbit (HOMO) of molecular donors so as to
increase the open-circuit voltage (VOC
)
of applications.7
40 Obviously, developing new D-A organic small molecules is an
efficient way to maximize the photovoltaics parameters, because
this structure can not only narrow the band-gap by utilizing
intramolecular charge transfer (ICT) transition between electron
donating and withdrawing units inside molecules, but also control
45 the energy levels through employing different donor or acceptor
moieties into the molecules. So far, with the increased interests in
exploring low band-gap small molecules to absorb sunlight at
80 Fig. 1 The structure (a), ground-state geometry and electronic-density
distribution (b) of small molecules
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structure). Benzothiadiazole was selected as central acceptor
since it has been well-known as one excellent acceptor utilized in
10d, 14
low band-gap polymers for OSCs.2b,
Motivation for
choosing triphenylamines as capped donors was attributed to its
good electron-donating properties caused by easy oxidation of
nitrogen center and good charge transportation as its propeller
starburst molecular structure.15 Significantly, linear or star-shape
small molecules composed of benzothiadiazole and
triphenylamine have shown high VOC over 0.9 V for BHJ OSCs.16
5
45
Fig. 2 Photophysical properties of BDCTBT and BDETBT. (a) UV-vis
10 It is rational to add cyano groups in D-A-D type
benzothiadiazole-trphenylamine molecules to get deep-lying
HOMO and high Voc. The design of BDETBT was for the
purpose of exploring the effect of cyano on photovoltaic
properties of molecules. In addition, Fig. 1(b) presented the
15 density functional theory (DFT) calculated geometry and
electronic properties for two molecules (calculation method
supported in ESI†). In both molecules, the electron density of the
HOMO delocalized over the whole molecules, while that of the
LUMO concentrated on the central benzothiadiazole groups,
20 demonstrating that an electron transfer from the triphenylamines
groups to the central acceptors during the process of being
excited from HOMO to LUMO. From the calculated energy
levels, the HOMO and LUMO values of BDCTBT were lower
than those of BDETBT for 0.24 eV and 0.18 eV, respectively.
25 These results explained that, in theoretical calculation, cyano
groups can lower the HOMO level without significantly costing
narrow band-gap.
absorption spectra of small molecules in chloroform and in spin-coated
thin film. (b) Fluorescence emission spectra of both materials in CHCl3
(1.0×10-5 M) with increasing concentration of PC61BM (×10-5 M): 0.0(1),
50 3.0(2), 5.0(3), 8.0(4), 10.0(5), 15.0(6), 20.0(7), 25.0(8); the insets are
Stern-Volmer quenching plots for two compounds respectively.
The UV-vis absorption spectra of the synthesized molecules in
chloroform and in films were shown in Fig. 2(a) and the
corresponding optical data were collected in Table 1. Both of
55 molecules exhibited a broad absorption range covering the 300-
600 nm wavelength in solution and in film. In solution, the
maximum absorption peak (λmaxsol) of BDCTBT was 484 nm,
which was blue-shifted compared with that of BDETBT (496
nm), because the cyano groups hindered the ICT transition for
60 their electron-withdrawing effect. In comparison with their
absorption spectra in solution, the absorption band for BDCTBT
and BDETBT in films were red-shifted, extending absorption
onset to 604 nm and 619 nm, respectively, and thus showing a
strong intermolecular π-π interaction in solid state.
65
In order to investigate the electrochemical energy levels
(HOMOCV, LUMOCV and EgCV) of two compounds, the cyclic
voltammetry (CV) was employed to drop-casted film (Fig. S1,
ESI†) and related electrochemical data were summarized in Table
1. Encouragingly, BDCTBT and BDETBT presented a fairly
70 low-lying HOMOCV levels of -5.32 eV and -5.25 eV, respectively,
which guaranteed the high VOC of the organic photovoltaic
devices blending with small molecules and PC61BM. As
speculation above, comparing energy levels between two
materials, cyanos successfully deepened the HOMO level without
75 greatly reducing the band-gap.
To investigate the charge separation process between small
molecules and PC61BM, the fluorescence quenching experiments
were taken place and the fluorescence emission spectra were
shown in Fig. 2(b). When the concentration of PC61BM increased,
80 fluorescence intensity of small molecules gradually decreased,
indicating that photoinduced exciton transfer from the LUMO of
small molecular donors to that of PC61BM to form charge
separation.18 Furthermore, at low concentration of quencher, the
relationship between the fluorescence intensity and the
85 concentration of PC61BM was related to Stern-Volmer equation:
F0/F = 1 + KSV·[C], in which F0 and F represented the
fluorescence intensity in the absence and present of PC61BM,
respectively, KSV was the quenching constant, and [C] was
concentration of PC61BM. The KSV of BDCTBT and BDETBT
90 were 3320 M-1 and 3260 M-1, respectively.
Scheme 1 Synthetic route for the small molecules
30 Synthetic route for the small molecules are shown in Scheme 1.
The benzothiadiazole-based acceptor moiety (compound A) was
synthesized in six steps starting from commercial available
catechol following the procedures reported in the literature.17
Knoevenagel condensation reaction of 4-(N,N-diphenylamine)-
35 benzaldehyde and 4-bromobenzyl acetonitrile were carried out
using NaOH as base in EtOH resulting compound 1 with a yield
of 89%. Compound 2 was obtained in a yield of 47% by
Pd(OAc)2 catalyzed Heck coupling reaction between N,N-
diphenyl-4-iodoaniline and 4-bromostyrene. The donor units
40 (compound D1 and D2) were both synthesized by Miyaura
borylation in high yields. Finally, BDCTBT and BDETBT were
prepared by Pd(PPh3)4-assisted Suzuki coupling reaction in high
yields of 87% and 88%, respectively. Detailed synthetic
procedures and characterization are supported by ESI.†
Photovoltaic performances of two synthesized molecules were
preliminary investigated by fabricating solution-processsable
BHJ devices in
a
conventional device structure of
ITO/PEDOT:PSS/Donor:PC61BM(1:2,w/w)/Al. The detailed
95 device fabrication process was described in the ESI† and the
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Table 1 The photophysical, electrochemical and photovoltaic properties of BDCTBT and BDETBT
Compound
BDCTBT
BDETBT
λmaxsol/nm λmaxfilm/nm Egoptc/eV
HOMOCV/eV
-5.32
LUMOCV/eV EgCV/eV KSV/M-1 JSC/mAcm-2 VOC/V
FF
0.33
0.33
PCE/%
3.85
1.99
484
496
504
511
2.05
2.00
-3.33
-3.30
1.99
1.95
3320
3260
11.2
6.33
1.04
0.94
-5.25
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corresponding parameters of photovoltaic devices were listed in
Table 1. The device with blended BDCTBT and PC61BM as
active layer provided a VOC of 1.04 V, a JSC of 11.2 mA/cm2, a fill
10 factor (FF) of 0.33, resulting a PCE of 3.85%. In contrast, the
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delivered an inferior photovoltaic properties with a VOC of 0.94 V,
a JSC of 6.33 mA/cm2, a FF of 0.33, yielding a PCE of 1.99%.
The higher VOC value of BDCTBT was consistent with its deeper
15 low-lying HOMO level caused by the cyano groups on π-bridge.
In summary, we have explored that the utilization of cyano
groups on π-bridge can be a promising way to reduce HOMO
value for conjugated D-A type small molecular photovoltaic
materials. A BDCTBT-based BHJ device shows a preliminary
20 PCE of 3.85% with a high VOC up to 1.04 V. We envisage that
further optimization of morphology on active layer and
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The authors would like to thank the NSFC (21102013,
25 20803030), the Dalian Committee of Science and Technology
(2011J21DW001), the SRF for ROCS-SEM (No. 201001438, No.
201001439), the Fundamental Funds for the Central Universities
(DUT11LK20), and the RFDP (No.20090041120017) for
financial support.
90
30 Notes and references
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aSchool of Chemistry, Dalian University of Technology, Dalian, China.
bState Key Baboratory of Supramolecular Structure and Materails,
College of Chemistry, Jilin University, Changchun, China.
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105 E-mail: liyanqin@dlut.edu.cn; yuewang@jlu.edu.cn
† Electronic Supplementary Information (ESI) available: Synthetic
procedures, CV curves, calculation method and devices fabrication details.
See DOI: 10.1039/b000000x/
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