Pyrazino-[2,3-f][1,10]phenanthroline as a new anchoring group of organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra05349h

Two novel donor-acceptor organic dyes (PPL-1 and PPL-2) with pyrazino-[2,3-f][1,10]phenanthroline as an electron-withdrawing anchoring group were designed and synthesized for dye-sensitized solar cells, achieving a promising power conversion efficiency of

Full text of DOI:10.1039/c5ra05349h

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Pyrazino-[2,3-f][1,10]phenanthroline as a new
anchoring group of organic dyes for dye-sensitized
Cite this: RSC Adv., 2015, 5, 46206
solar cells†
a
a
a
Li-Peng Zhang,ab Ke-Jian Jiang, Qiang Chen,ab Gang Li and Lian-Ming Yang
Received 26th March 2015
Accepted 18th May 2015
*
*
DOI: 10.1039/c5ra05349h
www.rsc.org/advances
Two novel donor–acceptor organic dyes (PPL-1 and PPL-2) with intercalation into the TiO2 in the dye adsorption process, thus
pyrazino-[2,3-f][1,10]phenanthroline as an electron-withdrawing lowering open-circuit voltages of the DSC device.5
anchoring group were designed and synthesized for dye-sensitized
Accordingly, carboxylic acid-free dyes for DSCs became the
solar cells, achieving a promising power conversion efficiency of subject of intense research in recent years. In 2011, Harima
4.04% under standard AM 1.5 conditions.
et al. reported new organic dyes with the pyridine group as both
electron acceptor and anchoring group.6a These dyes are
attached onto the surface of the TiO2 through a coordination
bond formed between the pyridine N atom and TiO2. Inspired
by the work, several dye variants based on the pyridine
anchoring group were synthesized and characterized by the
Wang group.6b,c Subsequently, various other types of anchoring
groups, such as 2-hydroxybenzonitrile,7 nitro group,8 phos-
phinic acid,9 rhodanine10 and 8-hydroxylquinoline,11 were
successfully introduced into organic dye's molecules. These
efforts undoubtedly enriches the content of sensitizers and
provides new topics about dyes for DSCs, although current
efficiencies of the non-carboxylic acid dye-based devices are far
lower than those of the carboxylic acid counterpart-based ones.
Pyrazino-[2,3-f][1,10]phenanthroline (PPL) is an interesting
structural framework extensively used in organic catalysis12 and
organic optoelectronic materials.13 As a bidentate ligand, it
chelates various metal ions to form very stable complexes.14,15 And
its electron-withdrawing nature helps extend the spectral absorp-
tion range of the molecule. Therefore, PPL might likely be one of
promising candidate motifs replacing the conventional carboxylic
acid group in the development of new organic dyes for DSCs.
In this paper, two new organic dyes, featuring pyrazino[2,3-f]-
1,10-phenanthroline as an electron-withdrawing anchoring
group, were designed and synthesized, where the donor moiety,
N,N-diethylbenzenamine or 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-
N-phenylaniline was attached at 2 and 3 positions of PPL. The
two dyes were coded as PPL-1 and PPL-2, as shown in Scheme 1.
PPL-1 and PPL-2 were easily available by a simple two-step
procedure (cf. Scheme S1†): the Friedel–Cras reaction of N,N-
diethylbenzenamine (or triarylamine) with oxalyl chloride
produces the corresponding diones, followed by the conden-
sation with 5,6-diamine-1,10-phenanthroline to afford the
target dyes.
Dye-sensitized solar cells (DSCs) have emerged alongside
conventional p–n junction solar cells as a mode of converting
sun-light to electricity, and have attracted intense attention due
to their potentially low-cost production and high conversion
efficiencies.1 In a typical DSC, dye-coated mesoporous oxide
lm electrodes (usually TiO2), counter-electrodes (usually Pt),
and electrolytes (usually Iꢀ/I3ꢀ) are “sandwiched”, where the
dye is considered to play a key role in maintaining the perfor-
mance and stability of the DSC.2,3 Among the dyes utilized,
organic dyes have been extensively investigated because of the
diversity of molecular structures, high molar extinction coeffi-
cients, ease of synthesis, as well as low cost and environmental
friendliness. Organic dyes are commonly based on a D–p–A
molecular system to narrow optical band gap and improve light
induced charge transfer, most of which utilize the carboxylic
acid moiety as both acceptor and anchoring group.4 The
carboxylic acid can well bind to the TiO2 surface through
forming a stronger ester linkage with the TiO2. This contributes
to effective photo-excited electron transfer from the dye to TiO2,
allowing the DSCs to achieve high efficiencies and long-term
stability. Although the carboxylic acid group shows remark-
able performances in DSCs, an ongoing quest for alternatives is
still desirable for various reasons. Typically, for example,
carboxylic acid-containing dyes may cause the conduction-band
potential of the TiO2 to positively shi owing to proton
aBeijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green
Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China.
E-mail: kjjiang@iccas.ac.cn; yanglm@iccas.ac.cn
bGraduate School of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available: Synthesis details and
characterization. See DOI: 10.1039/c5ra05349h
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Scheme 1 Chemical structures of phenanthroline-based organic dyes
PPL-1 and PPL-2.
Fig. 1 Normalized absorption and emission spectra of PPL-1 and PPL-
2 in dilute CH2Cl2 solution.
Fig. 1 shows the UV-vis absorption and emission spectra of
PPL-1 and PPL-2 in diluted dichloromethane. Both the dyes
exhibited similar absorption characteristics having two
absorption bands with the maximum extinction coefficient is
4.5 ꢁ 104 Mꢀ1 cmꢀ1 at 433 nm for PPL-1 and 5.8 ꢁ 104 Mꢀ1
cmꢀ1 at 427 nm for PPL-2. The band between 350 nm to 500 nm
would be assigned to a p–p* transition and the intramolecular
charge transfer (ICT) excitation from the donor to the acceptor.
The luminescence is found with a maximum at 540 and 627 nm
for PPL-1 and PPL-2, respectively. Interestingly, PPL-2 shows a
larger Stokes shi compared with PPL-1, indicating its larger
structural difference between ground and excited states and
leading to the formation of more efficient charge-transfer states,
which is favorable for the electron injection from a D–p–A type
dye to TiO2. The optical bandgap is 2.52 and 2.42 eV for PPL-1
and PPL-2, respectively, estimated from the intersection of the
normalized absorption and emission spectra. The dye loading
was measured to be 1.65 ꢁ 10ꢀ7 mol cmꢀ2 for PPL-1 and 1.43 ꢁ
10ꢀ7 mol cmꢀ2 PPL-2. Interestingly, the values are slightly
higher than those for the dyes employing carboxylic acid
anchoring group,16 indicating that pyrazino-[2,3-f][1,10]phe-
nanthroline (PPL) is a better anchoring group of the dye loading
on the TiO2 lm. To conrm the difference between the two
types of anchoring groups (i.e., PPL and carboxylic acid), a
desorption experiment was carried out. As demonstrated in the
experiment (see the ESI section and Fig. S1†), the carboxyl acid-
containing dye N719 was desorbed completely from the TiO2
lm within about 20 seconds, while the dyes PPL-1 and PPL-2
were only slightly lost aer being immersed into an aqueous
NaOH solution (0.3 w/w%) for 12 h, indicating that the binding
force of PPL dyes onto the TiO2 surface is far stronger than that
of the carboxyl acid-containing dye.
redox potentials, E+/*, determined by subtracting E0–0 from E+/
0, were ꢀ1.49 eV for PPL-1 and ꢀ1.62 eV for PPL-2. Both the
values are negative enough to allow their excited-state electron
transfer into the TiO2 conduction band (ꢀ0.5 V vs. NHE).18
For preparation of the DSC devices (For detailed informa-
tion, see ESI†), a double TiO2 electrode was prepared by the
screen printing method, where a 6 mm transparent layer and a 4
mm scattering layer were employed. The sintered TiO2 elec-
trodes were stained in the dye solution of 0.2 mM PPL-1 or PPL-
2 in chloromethane. In the solutions, 2 mM 3a,7a-dihydroxy-5b-
cholic acid (chenodeoxycholic acid) was added as a co-
adsorbent to prevent dye aggregation on the TiO2 surface. The
nal devices were prepared in a sandwich-type fashion, where
the dyed TiO2 electrode and a Pt-counter electrode were sealed
To investigate the molecular energy levels, cyclic voltamme-
try measurements (Fig. S2†) were performed in a 0.1 M
dichloromethane solution of tetra-n-butylammonium hexa-
uorophosphate using ferrocene as an internal standard. The
rst oxidation potentials (Es/+) of PPL-1 and PPL-2 were observed
to be 1.03 and 0.82 eV vs. NHE, respectively, indicating stronger
donor ability of the triarylamine. Both the potential values are
substantially more positive than that of the iodide/triiodide
couple redox (0.4 V vs. NHE),17 ensuring the regeneration of
the ground-state sensitizer for both the dyes. The excited-state
Fig. 2 Photocurrent density–voltage (J–V) curves (a), and incident
photon-to-current conversion efficiency (IPCE) (b) spectra of DSCs for
PPL-1 and PPL-2.
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Table 1 Photoelectrochemical properties of dyes PPL-1 and PPL-2, and their photovoltaic parameters of solar cells measured under AM 1.5
irradiation (100 mW cmꢀ2
)
Dye loading
PCEd [%] [mol cmꢀ2
lmaxa [nm] E [Mꢀ1 cmꢀ1
]
lem(max) [nm] Es/+ [V]b Es+/* [V] E0–0 [eV] JSC [mA cmꢀ2
]
Voc [mV] FF
]
c
Dye
PPL-1 433
PPL-2 427
45 000
58 000
540
627
1.03
0.82
ꢀ1.49
ꢀ1.62
2.52
2.42
7.83
7.72
8.08
7.88
682
678
694
685
0.70 3.74
1.65 ꢁ 10ꢀ7
1.43 ꢁ 10ꢀ7
0.69 3.61
0.72 4.04
0.71 3.83
a
b
Measured in CH2Cl2 solutions (1 ꢁ 10ꢀ5 M) at room temperature. Oxidation potentials of the dyes were measured in CH2Cl2 solutions with
tetrabutylammoniumhexauorophosphate (TBAPF6, 0.1 M) as electrolyte, Pt wires as working and counter electrode, Ag/Ag+ as reference
electrode; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV. E0–0 values
c
d
were estimated from the intersection of the absorption and emission spectra. Data in italics are the average of 6 cells, data in bold are the
best results.
ꢀ
using a hot-melt lm, and lled with an Iꢀ/I3 electrolyte
In order to gain insight into the geometrical conguration
comprising 0.68 M dimethyl imidazolium iodide, 0.03 M iodine, and electron distribution of the frontier orbitals of the two dyes,
0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40 M tert- density functional theory (DFT) calculations were made at a
butylpyridine in the mixture of acetonitrile and valeronitrile B3LYP/6-31G level, as shown in Fig. 3. In the case of PPL-1, the
(85 : 15, v/v).
electron distribution of the HOMO is homogeneously delo-
Fig. 2a shows the I–V performance for DSCs sensitized by calized over both the donor groups, further extending to the PPL
PPL-1 and PPL-2 under AM 1.5 G simulated solar light at a light ring; the electron distribution of the LUMO is mainly located on
intensity of 100 mW cmꢀ2, and the photovoltaic parameters are the PPL unit, slightly extending to both the donors. For PPL-2,
listed in Table 1. The best device for PPL-1 gave a short circuit the distribution of the HOMO is mainly delocalized on both the
photocurrent density (Jsc) of 7.83 mA cmꢀ2, an open circuit donor groups, slightly extending to the adjacent PPL ring; the
voltage (Voc) of 682 mV, and a ll factor (FF) of 0.70, corre- distribution of the LUMO is located on whole molecule except
sponding to a PCE of 3.74% with an average value of 3.61% for 6 for alkyl chains. According to these observations, the electron
samples. For PPL-2, the best device gave a PCE of 4.04% with an communication is favorable between the donor and the
average value of 3.83%, which are slightly higher than those for acceptor for both the dyes, allowing an efficient electron
PPL-1. The monochromatic incident photon-to-current conver- transfer from the dye to the TiO2 electrode under light
sion efficiencies (IPCEs) were recorded for both the dye- irradiation.
sensitized devices, as shown in Fig. 2b. Both the dyes show
In conclusion, two novel donor–acceptor organic dyes using
strong response in the region between 350 and 600 nm with the pyrazino-[2,3-f][1,10]phenanthroline (PPL) as anchoring group
highest values of about 80%.
were synthesized. Both the dyes can be efficiently adsorbed on
the TiO2 surface through the anchoring group, and the dye-
sensitized solar cells showed a highest power conversion effi-
ciency of 4.04%, indicating that PPL is a promising building
block as both acceptor and anchoring group for carboxylic acid-
free organic dyes for DSCs. Further molecularly engineering of
this type of dyes, as well as optimization of the DSC devices are
under investigation in our group.
Acknowledgements
The authors thank the National 863 Program (no.
2011AA050521) and the National Nature Science Foundation of
China (Grant nos 21174149, 51373182, 61405207, 21102150) for
nancial support of this work.
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