A novel D-A-π-A organic sensitizer containing a diketopyrrolopyrrole unit with a branched alkyl chain for highly efficient and stable dye-sensitized solar cells

Article abstract of DOI:10.1039/c2cc31998e

A novel D-A-π-A type organic dye (YCD01) incorporating a diketopyrrolopyrrole unit with a branched alkyl chain was synthesized for dye-sensitized solar cells. YCD01 showed a high conversion efficiency of 7.43% (AM 1.5, 100 mW cm^-2) with a J_sc of 13.40 mA cm ^-2, a V_oc of 0.76 V, a FF of 0.73 and an excellent stability.

Full text of DOI:10.1039/c2cc31998e

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A novel D-A-π-A organic sensitizer containing diketopyrrolopyrrole unit
with branched alkyl chain for highly efficient and stable dye-sensitized
solar cells
Sanyin Qu,ab Chuanjiang Qin,a Ashraful Islam,a Yongzhen Wu, b Weihong Zhu, b Jianli Hua,b He Tianb*
5
and Liyuan Hana*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
50 intermolecular charge transfer of DPP moiety allows for high
absorbance and is beneficial for the sensitizer to absorb at
infrared wavelength. In addition, the electron-withdrawing
properties of DPP, which is suitable for constructing the D-A-π-A
configuration, can assist the electron distribution and greatly
55 stabilize the sensitizer. However, the large π-conjugated system
of DPP demonstrates strong π-stacked aggregation on TiO2,7a
which may lead to dissipative intermolecular charge transfer and
aggravate charge recombination. Recently, some researchers
reported that by connecting long alkyl chains to the donor part of
60 sensitizers, charge recombination could be successfully
suppressed. However, aggregation of these modified sensitizers
still occurs to a certain degree.8 Therefore, we assumed that
attaching a bulky substituent in the middle part of the sensitizer
might be a more effective way to reduce π-π stacking of
65 molecules on the TiO2 film. Specially, owing to their bulk,
branched alkyl chains effectively decrease aggregation of
conjugated molecules and increase polymer solubility. With this
in mind, we introduced a branched 2-ethyl-hexyl alkyl chain to
the DPP moiety for the first time to further prohibit the dye
70 aggregation. In addition, the steric hindrance of the branched
A novel D-A-π-A type organic dye (YCD01) incorporating a
diketopyrrolopyrrole unit with branched alkyl chain was
10 synthesized for dye-sensitized solar cells. YCD01 showed high
conversion efficiency of 7.43 % (AM 1.5, 100 mW cm-2) with a
Jsc of 13.40 mA cm-2, a Voc of 0.76 V, a FF of 0.73 and an
excellent stability.
Dye-sensitized solar cells (DSCs) have attracted considerable
15 attention since Grätzel and co-workers published their seminal
report in 1991.1 Owing to the efforts of many researchers over the
past twenty years, DSCs based on Ru(II)-polypyridyl complexes
have yielded a power conversion efficiency above 11 %.2 At the
same time, the metal-free organic dyes have also been desirable
20 due to their high extinction coefficient, ease of structure
modification and low cost.3 However, organic dyes are still
limited from practical use owing to their relatively narrow
absorption bands, unfavourable aggregation on semiconductors as
well as weaker light stability compared to metal-based sensitizers.
25 Recently, a D-A-π-A configuration for organic dyes was reported
and received increasing attention with an additional acceptor
chromophore incorporated into the common D-π-A structure.4,5
By inserting the subordinate acceptor, which acts as an electron
trap to separate charge and facilitate electron migration to the
30 final acceptor, the sensitizer exhibits broad responsive spectra
and long term stability. A high efficiency of nearly 9 % under
AM 1.5 G irradiation has been achieved based on this type of
sensitizers in DSCs.
-
chain may also block I3 ion from approaching the TiO2 surface,
thus reducing the charge recombination rate.
In this work, we developed a new D-A-π-A organic sensitizer,
YCD01, bearing 2-ethyl-hexyl alkyl chain in the DPP moiety,
75 furan as the linker, cyanoacrylic acid as the acceptor moiety and
the indoline unit9 was introduced as the donor for its powerful
donor ability to further broaden the absorption spectrum and
improve the efficiency. The corresponding dye TD01 was also
synthesized as a comparison (Fig. 1). Syntheses of dyes YCD01
80
Diketopyrrolopyrroles (DPPs) are a class of commercialized
35 and easily synthesized dyes with exceptional photochemical,
mechanical and thermal stability, which have aroused lots of
interest in bulk heterojunction solar cells6 and also been
considered as a π-conjugated component for DSCs.7 The efficient
40 a Photovoltaic Materials Unit, National Institute for Materials Science,
Sengen 1-2-1, Tsukuba, Ibaraki, Japan. Fax:+81-29-859-2304;
Tel:+81-29-859-2305; E-mail: Han. Liyuan@nims.go.jp
85
b Key laboratory for Advanced Materials and Institute of fine chemicals,
East China University of Science and Technology, Shanghai, 200237, P.R.
45 China. Fax:+86-21-64252288; Tel: +86-21-64252756;
E-mail: tianhe@ecust.edu.cn
† Electronic Supplementary Information (ESI) available: Experimental
procedures for synthesis, DFT calculations, Charge Extraction and all
fundamental data of DSCs. See DOI: 10.1039/b000000x/
90
Fig. 1 Molecular structure of YCD01 and reference dye TD01.
[journal], [year], [vol], 00–00 | 1
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60
5
65
10
Fig. 2 Absorption spectra of YCD01 and TD01 in CH2Cl2 and anchored on a
transparent TiO2 film (inset).
Fig.4 Photocurrent-voltage curves of DSCs sensitized by YCD01 and
70 TD01using 0.6 M MPImI, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in the solvent
of CH3CN as the electrolyte.
S+/0-E0-0, were -0.91 V and -1.10 V. The S+/* potential of both the
two dyes were more negative than the conduction-band edge of
TiO2 (-0.5 V vs. NHE), ensuring an efficient electron injection
75 process from the excited state into the conduction band of TiO2.
To obtain an insight into the electron distribution of TD01 and
YCD01, we calculated the optimized molecular structures and
frontier orbitals of the two dyes by using the density functional
theory computational method.10 At the ground state (HOMO) of
80 these dyes, electrons are distributed on the D-A system and the
excited state (LUMO) is delocalized across the entire A-π-A
system(Fig. S2, ESI†), which suggests an inductive or electron-
withdrawing tendency from the donor unit to the cyanoacetic acid
unit.
15
20
25 Fig. 3 IPCE spectra of DSCs based on YCD01 and TD01.
and TD01 in detail are described in the ESI (Scheme 1 and 2,
ESI†).
85
DSCs were fabricated according to the literature procedure.11
The incident photon-to-current conversion efficiency (IPCE)
spectra of the DSCs based on TD01 and YCD01 are shown in
Fig. 3. YCD01 showed excellent solar light response from 350 to
625 nm with a maximum value of 80 % in the plateau region.
The UV-Vis absorption spectra of the two dyes are shown in
Fig. 2. Both dyes showed broad absorption spectra covering a
30 wide range of the visible region. The maximum absorption of
YCD01 was exhibited at 526 nm with a molar extinction
coefficient (ε) of 4.6×104 M-1cm-1, and for TD01 was exhibited
at 514 nm with ε of 4.1×104 M-1cm-1(Table S1, ESI†). The red-
shift of the absorption band of YCD01 in comparison to TD01
35 was due to the stronger donor ability of indoline moiety than
triphenylamine. When absorbed on a transparent thin TiO2 film,
TD01 showed broader absorption bands and a ~32 nm red-shift in
absorption compared to that measured in solution (Fig 2, inset).
The fine absorption bands ascribed to π-π* transition and
40 intermolecular charge transfer were vaguely visible in the
spectrum. These phenomena indicate that there was a certain
degree of π-π aggregation of TD01 on the TiO2 film. In contrast,
in the spectrum of YCD01, fine absorption peaks were readily
shown, and there was no distinct shift or extension of the
45 absorption bands, which suggests that π-π aggregation was
significantly reduced. This reduced aggregation was probably due
to the presence of the branched alkyl chain, which appeared to
have successfully prohibited the interaction of DPP molecules.
The energy levels of TD01 and YCD01 were determined by
50 cyclic voltammetry (Table S1). The ground state oxidation
potential, S+/0, of TD01 and YCD01 were 1.03 and 0.78 V,
respectively, which were sufficiently low compared to the
redoxpotential of the iodide/triiodide couple used as an
electrolyte. The S+/0 energy level of YCD01 was negatively
55 shifted for 0.25 V compared to that of TD01 due to the
introduction of the indoline moiety. The estimated excited-state
oxidate potential, S+/*, for TD01 and YCD01, calculated from
90 Taking into account the reflection and absorption losses by the
conducting glass, the IPCE in this range exceeded 90 %. The
photovoltaic performance was measured under AM 1.5 G
simulated solar light at a light intensity of 100 mW cm-2 with a
metal mask of 0.25 cm2. Under optimized conditions, with the
95 addition of 0.5 M tert-butylpyridine (TBP) in the electrolyte, the
YCD01-sensitized solar cell achieved
a prominent overall
conversion efficiency of 7.43%, with a short-circuit photocurrent
density (Jsc) of 13.40 mA cm-2, an open-circuit voltage (Voc) of
0.76 V and a fill factor (FF) of 0.73. Under the same conditions,
100 TD01 showed 5.18 % efficiency (Jsc = 11.05 mA cm-2, Voc = 0.69
V, FF = 0.68) (Fig. 4, Table S2, ESI†). YCD01 showed a higher
Jsc than TD01, which is because the introduction of indoline
donor favors light harvesting in DSCs with better absorption
spectra. In addition, the weak π-π aggregation of YCD01 on the
105 TiO2 film also improved the electron injection efficiency of the
cells.
The Voc value of YCD01 was 70 mV higher than that of TD01,
which could be attributed to the introduction of branched 2-ethyl-
hexyl chain to the DPP core. The generation of Voc is related to
110 the energy level of the TiO2 conduction band (ECB) and the
charge recombination rate in DSCs. To clarify the effect of the
branched alkyl chain on Voc, we first measured ECB with the
charge extraction method (CEM)12 (Fig. S3, ESI†) and found that
ECB was almost unaffected for two cases. Therefore, the improve
115 -ment of Voc from TD01 to YCD01 based DSCs should be
2
| Journal Name, [year], [vol], 00–00
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successfully suppressed charge recombination and improved the
60 open-circuit voltage. As a result, YCD01-sensitized DSCs
achieved a high overall conversion efficiency of 7.43% with
broad IPCE response, high Voc and long-term stability. A
systematic study of the effect of bulky alkyl group attached to the
DPP unit on the device performance and further broadening the
65 absorption spectra to the infrared region is under process.
This work was supported by Core Research for Evolutional
Science and Technology (CREST) of the Japan Science and
Technology Agency, NSFC/China (21190033) and National
Basic Research 973 Program (2011CB8068400).
5
10
Fig.5 Lifetime (τ) of electrons versus electron density. The τ in DSCs
sensitized by TD01 and YCD01 were derived by using intensity-
15 modulated photovoltage spectroscopy.
70 Notes and references
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Fig.6 Variations of photovoltaic parameters with aging time for the DSCs
30 devices sensitized by YCD01 using the 3-methoxy-propylnitrile (MPN)
as electrolyte solvent.
6
attributed to the suppression of charge recombination, which is
related to electron lifetime (τ). We derived τ as a function of
electron density by means of intensity-modulated photovoltage
35 spectroscopy (IMVS) (Fig. 5).13 The electron lifetime decreased
with increasing electron density in a manner consistent with a
power law relationship. At a fixed electron density, the electron
lifetime for the DSCs based on YCD01 was larger than that of
TD01, which suggests that the branched 2-ethyl-hexyl chain was
100 7 (a) S. Qu, W. Wu, J. Hua, C. Kong, Y. Long and H. Tian, J. Phys.
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40 more effective than the single butyl chain for blocking I3 ions
approaching the TiO2 surface for charge recombination, and thus
showed an improvement in Voc.
Finally, in order to test the stability of YCD01, we made cells
using 3-methoxy-propylnitrile (MPN) as an electrolyte solvent.
45 The YCD01-sensitized solar cell showed an initial overall
conversion efficiency of 5.19 % with a Jsc of 11.04 mA cm-2, Voc
of 0.72 V and FF of 0.65. The performance of the cell improved
during the first 100 h and retained almost the same after 1000 h
of accelerated tests under full sunlight soaking at 60 °C (Fig. 6).
50 This demonstrates that the amount of dye on the TiO2 surface
remained intact after long time soaking.
In conclusion, we designed and synthesized a new D-A-π-A
sensitizer, YCD01, with indoline as the electron donor,
diketopyrrolopyrrole as the additional acceptor, furan as the
55 linker and cyanoacrylic acid as the acceptor moiety. Attaching the
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This journal is © The Royal Society of Chemistry [year]
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Table of contents:
Novel D-A-π-A organic dye incorporating a diketopyrrolopyrrole unit with branched
alkyl chain was synthesized for dye-sensitized solar cells. DSSC showed high
conversion efficiency of 7.43 % and an excellent stability.