Modified triphenylamine-dicyanovinyl-based donor-acceptor dyes with enhanced power conversion efficiency of p-type dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201200402

To dye for: Two new dyes are synthesized by structural modifications of one of the best dyes for NiO p-type dye-sensitized solar cells, which is based on a triphenylamine-dicyanovinyl donor-acceptor system. An additional thiophene unit near the anchoring group can greatly retard charge recombination while enhancing the absorption coefficient to significantly improve the photoconversion efficiency by 50 %. Copyright

Full text of DOI:10.1002/asia.201200402

DOI: 10.1002/asia.201200402
Modified Triphenylamine-Dicyanovinyl-Based Donor–Acceptor Dyes with
Enhanced Power Conversion Efficiency of p-Type Dye-Sensitized Solar Cells
Linna Zhu,[a] Hongbin Yang,[a] Cheng Zhong,[c] and Chang Ming Li*[a, b]
Since the 1990s, great efforts have been devoted to dye-
sensitized solar cells (DSCs) for harvesting electricity from
solar energy.[1–3] At present, most of the studies focus on n-
type DSCs by using sensitized n-type semiconductors such
as TiO2 as a photoelectrode. As their inverse model, p-type
DSCs that inject holes into the valence band of a p-type
semiconductor such as NiO from the excited state of a sensi-
tizer have rarely been studied, and their overall efficiencies
(0.41% at most) are much lower than those of their n-type
counterparts.[4–10] p-DSCs have great potential to be used in
inexpensive tandem devices for high photoconversion effi-
ciencies.[11] Thus, there is a great need to investigate and de-
velop more efficient p-DSCs to match the performance of n-
DSCs for advanced p/n tandem devices.
In p-DSCs, after photoexcitation, the holes in an organic
dye are transported from its acceptor to the donor part,
after which they are injecting through the carboxylic group
into the valence band of NiO, in which the dye molecules
play a critical role as sensitizer. Fast recombination between
the injected hole in the semiconductor and the reduced dye
Scheme 1. a) Chemical structures of P1 and two new dyes, T1 and T2,
and electrolyte preventing efficient hole collection and dye
regeneration is the main limitation for high p-DSC perfor-
mance.[12–14] Recently, dye molecules based on triphenyla-
mine and dicyanovinyl donor–p–acceptor (D-p-A) struc-
tures have been developed to improve the p-DSC perfor-
mance, of which 4-(bis-{4-[5-(2,2-dicyanovinyl)thiophene-2-
yl]phenyl}amino)benzoic acid (P1, Scheme 1) shows the
highest photoconversion efficiency (h) of approximately
0.15%.[6] To improve h, various dyes have been synthesized
and b) the design approaches for p-type dyes.
based on the aryl amine structure. Different linkers are also
used to enhance the absorption and retard the charge re-
combination, but h values of only 0.06% have been ach-
ieved.[7] Yen et al. have increased the number of anchoring
groups to facilitate the hole injection, resulting in h of 0.08–
0.09%.[15] However, to date most dyes based on the aryl
amine structure still show h lower than 0.1%.
Herein we describe an approach to design and synthesize
two new D-p-A dyes T1 and T2 (Scheme 1) as sensitizers
for NiO-based p-type DSCs, in which the distance between
the acceptor part of the dye and the NiO semiconductor is
increased over P1 structure by two means (Scheme 1): a thio-
phene ring is inserted between the carboxylic group and tri-
phenylamine for T1, while bithiophene is introduced as
a bridge between the triphenylamine and dicyanovinyl
groups to give T2.[16] Results show that the position of the
additional thiophene unit in the structure strongly affects
both charge recombination and hole injection in the photo-
electrode. This work could provide a valuable methodology
to synthesize new dye molecules for significant improvement
of the performance of p-type DSCs.
[a] L. Zhu,+ H. Yang,+ C. M. Li
School of Chemical and Biomedical & Center for Advanced Biona-
nosystems
Nanyang Technological University
70 Nan-yang Drive, Singapore 637457 (Singapore)
[b] C. M. Li
Institute for Clean Energy & Advanced Materials
Southwest University, Chongqing 400715 (P. R. China)
E-mail: ecmLi@ntu.edu.sg
[c] Dr. C. Zhong
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University, Wuhan 430072 (P. R. China)
[+] These authors contributed equally to this work.
The dye molecules were synthesized by Suzuki coupling
between brominated triphenylamine and thiophene boronic
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200402.
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Table 1. Optical and electrochemical properties of dyes.
acid before finally undergoing
Knoevenagel reaction with ma-
lononitrile. More details are
Dye labs [nm] (e
[104 mÀ1 cmÀ1])[a]
lem
E0À0
Excited-state life-
time [ns][c]
Eonset ox [V vs.
EHOMO
ELUMO
[nm]
[eV][b]
Fc+/Fc][d]
[eV][e]
[eV][f]
given in the Supporting Infor- P1 337
464
(3.72)
488
(4.30)
485
(5.19)
654
719
741
2.24
2.18
2.09
2.66
3.17
2.91
0.43
0.42
0.38
À5.54
À5.53
À5.49
À3.30
À3.35
À3.40
(3.83)
mation. The prepared products
were fully characterized by 1H
and 13C NMR spectroscopy and
high-resolution mass spectrom-
etry. P1 was also evaluated for
comparison.
T1
T2
369
(3.35)
363
(2.83)
[a] Absorption spectra were measured in acetonitrile. [b] E0À0 was determined from the intersection of absorp-
tion and emission spectra in acetonitrile. [c] Excited-state lifetime was determined from the transient emission
spectra. [d] Oxidation potentials of dyes were measured in acetonitrile with 0.1m Bu4NPF6 with a scan rate of
50 mVsÀ1 (Figure S2 in the Supporting Information). [e] EHOMO was calculated by (5.1+Eonset ox vs. Fc+/Fc)).
UV/Vis and fluorescence
emission spectra of the dyes
[f] ELUMO was calculated by EHOMO +E0À0
.
measured
in
acetonitrile
(0.03 mm) are shown in Fig-
ure 1a. Pronounced red shift of the absorption edge and
light absorption enhancement of T1 and T2 relative to P1
are observed due to the additional thiophene rings, which
are for efficient light harvesting. T2 shows the most batho-
chromic shift and the strongest absorbance due to its longer
bithiophene linker. Detailed optical parameters and the
electrochemical properties obtained from the measured
cyclic voltammograms are summarized in Table 1. After ad-
ditional thiophene units are introduced into the P1 structure,
the HOMO levels of both T1 and T2 are increased, while
the LUMO levels are decreased. Figure 1b shows the molec-
ular-orbital energy diagram of the dyes, energy level of NiO
electrode, and redox potential of electrolyte. The HOMO
level of all dyes is below the top of the valence band of NiO
(À5.04 V), which is above that of the redox mediator
(À4.15 V), and thus all the dyes possess sufficient driving
force for hole injection and dye regeneration. The excited-
state lifetime of the dyes measured in acetonitrile solution
(Figure S4 in the Supporting Information) shows T1
(3.17 ns)>T2 (2.91 ns)>P1 (2.66 ns), indicating that the in-
tramolecular recombination of the excited dye molecules T1
and T2 is delayed by the additional thiophene units relative
to P1. Therefore, after photoexcitation, T1 could offer the
best charge separation and injection at the metal oxide/dye
interface, since a slower exciton decay is more favorable for
charge separation and injection.[18]
Density functional theory (DFT) calculations were per-
formed in Gaussian 09 at the B3LYP/6-31+G(d) level,[19]
showing that the HOMOs of these dyes are all distributed
evenly over the entire p-conjugation system, while the
LUMOs are mainly localized on the acceptor part distant
from the group anchored on the electrode to retard the
charge recombination process. Nevertheless, the LUMOs
are also partially located on the thiophene rings adjacent to
the acceptor, but unable to surpass the triphenylamine part,
thus resulting in a larger distance of HOMO from LUMO
in T1 due to the additional thiophene ring on the carboxylic
group, while that in T2 is almost the same as P1. Thus, only
T1 has the elongated distance between LUMO of the dye
and the anchoring group to suppress the charge recombina-
tion for improving the overall device performance signifi-
cantly.
Figure 2 shows the incident photon-to-current conversion
efficiency (IPCE) and the calculated absorbed photon-to-
current conversion efficiency (APCE) spectra of the cells.
Compared to P1, the devices sensitized with T1 or T2 have
broader photoresponse. However, the maximum IPCEs ob-
tained at 480 nm under same conditions are 32%, 29%, and
21% for T1, P1 and T2, respectively, which is not in agree-
ment with the absorption of NiO films loaded with the three
dyes (as shown in Figure S1 in the Supporting Information),
indicating that inserting an additional thiophene ring into
the P1 structure has a significant effect on the charge-trans-
fer process at the NiO/dye–electrolyte interface. The APCE
could well measure the whole charge injection and charge
collection efficiency of a device, which is defined as IPCE/
light harvesting efficiency (LHE, defined as 1–10A, where A
Figure 1. a) UV/Vis absorption and normalized emission spectra of P1,
T1, and T2 in acetonitrile. b) Molecular orbital energy diagram of the
dyes, energy level of NiO electrode, and redox potential of electrolyte.
The energy level of NiO and redox mediator is taken from Ref. [17].
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Triphenylamine-Dicyanovinyl-Based Donor–Acceptor Dyes
Figure 2. IPCE and APCE spectra of NiO DSCs sensitized with P1, T1,
and T2.
is the absorbance of the photoelectrode).[20–22] It is observed
in Figure 2 that the APCE of T1 device is higher than that
of P1, whereas that of T2 device is much lower than that of
P1. For p-DSCs sensitized with different dyes, considering
the same charge-transfer resistance of NiO electrodes, the
APCE values actually reflect the net effect between hole in-
jection at the NiO/dye–electrolyte interface and the subse-
quent charge recombination.
The typical current–voltage characteristics of p-DSCs sen-
sitized with T1, T2, and P1 (Figure S3 in the Supporting In-
formation) show that T1 also outperforms the other two
dyes in regard to the short-circuit current (Jsc) and open-cir-
cuit photovoltage (Voc), which are in good agreement with
the IPCE results. The Jsc (3.27 mAcmÀ2) and Voc (122 mV)
obtained for T1 are also the highest among the three dyes,
to achieve a photo conversion efficiency of 0.113% under
AM 1.5 solar simulator illumination. The solar cells made
from P1 and T2 under the same conditions show Jsc of 2.64
and 2.53 mAcmÀ2, and Voc of 104 and 94 mV, respectively
(Table S2 in the Supporting Information).
Figure 3. a) Electrochemical impedance spectra on NiO electrodes sensi-
tized with the P1, T1, and T2 dyes with 0.07 V in the dark. b,c) Depend-
ence of Rrec (b) and hole lifetime (c) on the applied bias voltage.
To better understand the charge-transfer mechanism gov-
erning the device performances, impedance spectra (IS) at
different bias voltages in the dark and at open-circuit volt-
age bias under illumination were performed. In the dark
with external bias, holes are injected from the external, fol-
lowed by recombination at the metal oxide/electrolyte inter-
face via oxidation of IÀ ions in the electrolyte. Thus recom-
bination resistance (Rrec) at the metal oxide/electrolyte inter-
face and the free charge lifetime could be obtained by ana-
lyzing the IS with the transmission line model.[23,24] Nyquist
plots of NiO electrodes sensitized with different dyes with
0.07 V bias are shown in Figure 3a, in which the semicircle
diameter for the measured devices is T1>T2>P1, indicat-
ing a larger resistance of T1 against interfacial hole transfer
from the NiO valence band to IÀ than for P1- and T2-based
devices. The Rrec values derived from impedance data with
the transmission line model are plotted in Figure 3b as
a function of bias voltage. The Rrec of the three dyes de-
creases with increased bias voltage due to the increase in
the valence-band hole concentration. The fact that the Rrec
of T1 is larger than that of T2 and P1 at the same external
bias suggests a higher energetic barrier for recombination of
hole in valence band with IÀ in T1-sensitized cells at the
same hole concentration. Considering that the dye loading
amount of the three dyes are close to each other
(32 nmolcmÀ2 for P1 and T1, 28 nmolcmÀ2 for T2), we can
argue that T1 could more efficiently block IÀ from ap-
proaching the NiO surface for charge recombination, proba-
bly due to more uniform molecular packing of the T1 dye
onto the NiO surface. Thus, the modified structure in T1 re-
sults in the largest recombination resistance. The hole life-
time is plotted in Figure 3c as a function of bias voltage,
which is acquired by the inversion of frequency of the maxi-
mum imaginary impedance component at the medium fre-
quency (1/2pfmax) of the semicircle. Obviously, the T1-sensi-
tized electrode has the longest hole lifetime, consistent with
the highest recombination resistance measured above, and
the effects together result in the highest Voc of T1-sensitized
p-DSCs.
Under illumination at open-circuit voltage bias, there is
no net current flow through the cell. All the injected holes
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Chang Ming Li et al.
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from the excited dyes are recaptured by IÀ before being ex-
tracted to the external circuit. Meanwhile, the reduced dye
is regenerated by I3 . Thus, the semicircle of the IS in the in-
outperforms T2 and P1 with photon conversion efficiency
up to 0.113%, which improves that of P1 by 50%. This
work clearly shows that in triphenylamine-dicyanovinyl-
based donor–acceptor dye molecules, the increased conjuga-
tion length near the anchoring group could significantly im-
prove the device performance, while the increased bridge
ligand between donor and acceptor has a negative effect on
the device efficiency. It is noteworthy that a rationally de-
signed tandem p/n DSC should provide much higher power
conversion efficiency, which is currently under investigation
in our lab.
À
termediate frequency range (Figure S6 in the Supporting In-
formation) also represents the transfer resistance at the
NiO/dye-electrolyte interface, which is the apparent combi-
national effect of hole injection, charge recombination, and
the reduced dye regeneration processes. The apparent
charge-transfer resistance at the NiO/dye–electrolyte inter-
face of the devices obtained from the semicircle of the IS
measured at the open-circuit voltage under illumination are
41.1 (P1), 36.7 (T1), and 57.7 W (T2), which are smaller than
that measured at open-circuit voltage bias in the dark (47.
(P1), 147.8 (T1), and 65.5 W (T2)). Similar to the n-DSCs,
the disparity of charge transfer resistance at NiO/dye-elec-
trolyte interface in the dark and under illumination origi-
nates from the difference of the local IÀ concentration.[25] In
the dark, IÀ is generated at the counter electrode and pene-
trates into NiO film by diffusion. Under illumination, IÀ is
formed “in situ” by dye regeneration at the NiO/electrolyte
interface. As discussed above, higher LUMO level of the
dyes (À3.3 (P1), À3.35 (T1), and À3.4 eV (T2), respectively)
than the redox mediator (À4.15 eV) suggests sufficient driv-
ing force for dye regeneration.[17] Thus the IÀ concentration
could be determined by the reduced dye concentration,
which is relevant to the injection ability of the excited dyes.
Consequently, better hole injection of T1 compared to P1
and T2 results in higher IÀ concentration and hole concen-
tration in the valence band of NiO, and further leads to
lower charge-transfer resistance of T1 under illumination,
which could also be confirmed by its longest exited-state
lifetime stated above. Higher recombination resistance in
the dark and longer hole lifetime of T2-based DSCs than for
P1 are shown in Figure 3b,c. However, the charge-transfer
resistance of T2 under illumination is larger than for P1,
suggesting lower hole injection of T2 than P1, and as
a result bringing about lower efficiency of T2; similar results
have been observed in other p-type dyes.[15]
Acknowledgements
This work is partially financially supported by Singapore National Re-
search Foundation Grant under NRF-CRP2-2007-02, and Institute for
Clean Energy & Advanced Materials, Southwest University, Chongqing,
P.R. China.
Keywords: charge carrier injection
·
donor–acceptor
systems · dye-sensitized solar cells · electronic structure ·
organic materials
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Received: May 4, 2012
Published online: && &&, 0000
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Solar Cells
To dye for: Two new dyes are synthe-
sized by structural modifications of
one of the best dyes for NiO p-type
dye-sensitized solar cells, which is
based on a triphenylamine–dicyano-
vinyl donor–acceptor system. An addi-
tional thiophene unit near the anchor-
ing group can greatly retard charge
recombination while enhancing the
absorption coefficient to significantly
improve the photoconversion effi-
ciency by 50%.
Linna Zhu, Hongbin Yang,
Cheng Zhong, Chang
Ming Li*
&&&& — &&&&
Modified Triphenylamine-Dicyano-
vinyl-Based Donor–Acceptor Dyes
with Enhanced Power Conversion Effi-
ciency of p-Type Dye-Sensitized Solar
Cells
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ÝÝ These are not the final page numbers!