Di(p-methoxyphenyl)amine end-capped tri(p-thiophenylphenyl)amine based molecular glasses as hole transporting materials for solid-state dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra07226c

Three tris(thienylphenyl)amine-based molecular glass hole transporting materials were synthetized, characterized and incorporated in solid state dye-sensitized solar cells. Devices using these compounds as solid hole conductors show relatively high V

Full text of DOI:10.1039/c5ra07226c

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Di(p-methoxyphenyl)amine end-capped tri(p-thiophenylphenyl)amine based
View Article Online
DOI: 10.1039/C5RA07226C
molecular glasses as hole transporting materials for solid-state dye-sensitized
solar cells
Thanh-Tuân Bui,a* Said Karim Shah,b,c Xavier Sallenave,a Mamatimin Abbas,b Gjergji Sini,a Lionel
Hirsch,b* and Fabrice Goubarda*
aLaboratoire de Physicochimie des Polymères et des Interfaces (LPPI), Université de Cergy-Pontoise,
5 mail Gay Lussac, Neuville-sur-Oise, 95031 Cergy-Pontoise Cedex, France.
bUniv. Bordeaux, IMS, UMR 5218, F-33607 Pessac, France and CNRS, IMS, UMR 5218, F-33607
Pessac, France.
cDepartment of Physics, Abdul Wali Khan University Mardan, Khyber Pakhtunkhwa, 23200
Pakistan.
*Corresponding authors: tbui@u-cergy.fr (T.-T. B.); lionel.hirsch@ims-bordeaux.fr (L. H.);
fabrice.goubard@u-cergy.fr (F. G.);
Abstract
Three tris(thienylphenyl)amine-based molecular glass hole transporting materials were synthetized,
characterized and incorporated in solid state dye-sensitized solar cells. Devices using these
compounds as solid hole conductors show relatively high Voc thanks to good energy level matching
between them and the associated D102 indoline dye. However, they yield relatively low Jsc and FF
compared to Spiro-OMeTAD based control devices, which is due to an order of magnitude lower
conductivity with respect to Spiro-OMeTAD. Maximum solar-to-electrical energy conversion
efficiencies of 1.2 % under standard illumination condition was obtained. Stability tests of unsealed
devices in air under continuous illumination have been performed and devices based on new
compounds have kept up to 80 % of their initial efficiency.
Key words: triphenylamine, arylamine, molecular glass, hole transporting material, dye-sensitized
solar cell
1-Introduction
In recent years, the development of small molecules for photovoltaic devices has received
considerable interests. A huge number of molecules have been designed, synthetized, and
characterized for use as active materials in organic solar cells1 or as hole transporting materials
(HTMs) in hybrid organic-inorganic photovoltaics.2-4 Compared to polymers, small molecules
exhibit many outstanding advantages such as structural versatility, unique molecular weight as well
as relatively easy purification processes. Despite their tendency to crystallization, small molecules
can form stable amorphous materials above ambient temperature if their structures are meticulously
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designed.5 The most successful molecular HTM is 2,2’,7,7’-tetrakis(N,N-di-4-methoxyphenyl-
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DOI: 10.1039/C5RA07226C
amine)-9,9’-spirobifluorene (spiro-OMeTAD) which was firstly reported by Grätzel et al. in 1998.
To date, the Spiro-OMeTAD based cell efficiencies have been higher than 7 % after optimizing
doping conditions by cobalt complex and in combination with high molar extinction coefficient
organic dyes.7 Using perovskite as light harvester, Spiro-OMeTAD based cells have shown
impressive efficiencies8, even reaching 20.1% as confirmed in November 2014 by NREL. The
success of Spiro-OMeTAD is due to its stable amorphous film (relative high glass transition
temperature), relative high solubility, appropriate oxidation potential versus light harvesting
materials (dyes or perovskites), and transparency in the visible spectrum with no absorption screen
effect compared to dye. Despite offering the best performance yet achieved in a molecular hole
conductor based cell, Spiro-MeOTAD suffers from a relatively low hole mobility.7, 9 Novel HTMs
other than Spiro-OMeTAD have also been investigated and most of them are amine related
compounds,4, 10, 11 exhibiting suitable electrochemical oxidation potentials, good charge transport,
good thermal and morphological stability. Only few examples of successful non-amine derivative
HTMs have been reported,12 Among amine derivatives, triphenylamines (TPA) having pseudo-3D
conjugated architecture constitutes an attractive class of materials.13 Recently, we have reported on
several series of star-shaped TPA based materials.2, 14 Combined experimental and theoretical
studies have shown that these molecular glasses have suitable characteristics for use as HTM in
solid-state DSSCs.15
In the present work, we report the design, synthesis and photovoltaic properties of new star-shaped
p-type amorphous organic semiconductors based on tris(4-(thiophen-2-yl)phenyl)amine cores
(Figure 1). Methyl moiety has been added in the thiophene ring to increase the dihedral angles
between the adjacent rings, reducing the -conjugation pathway and leading to blueshifts in optical
absorption. Each arm of these molecules is end-capped by a di(4-methoxyphenyl)amino moiety.
These electron rich end-capping groups will be seen to modulate electrochemical properties of
targeted compounds. Methoxy groups enhance pore filling properties of these molecules by (i)
rendering them more soluble in organic solvents and (ii) making them more hydrophilic and
increasing dye-sensitized TiO2/HTM affinity. Additionally, the presence of methoxy groups in para
positions of phenyl rings of the diphenylamino moiety has been shown to favor charge transport
properties.16 We have incorporated these molecules as HTMs in solid state D102-sensitized DSSC.
The photovoltaic performances of obtained devices will be presented and discussed.
2

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DOI: 10.1039/C5RA07226C
Figure 1. Molecular structures of 1-3
2-Results and Discussion
2.1 Synthesis
The synthetic route toward studied compound is outlined in Figure 2. Compounds 1 and 3 were
obtained as reported in our preliminary work.2
Figure 2. Synthesis of 1, 2 and 3. Reagents and conditions: a) 2-(tributylstannyl)thiophene, Pd(PPh3)4,
Toluene, 120°C, 16h. b) 4,4’-dimetoxydiphenylamine, NaOtBu, Pd(dba)2, P(tBu)3, Toluene, 100°C,
72h. c) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, n-BuLi, -78°C, THF, 16h. d)
Pd(PPh3)4, K2CO3 2M, Aliquat 336, Toluene, 90°C, 72h.
N-(3-methyl-2-thiophenyl)-N,N-di(4-methoxyphenyl)amine 6 was obtained from the Pd-catalyzed
amination of 2-bromo-3-methylthiophene2 and di(4-methoxyphenyl)amine. 6 was then subjected to a
brominating reaction leading to 6-Br. Finally, 2 was constructed from tris(4-(4,4,5,5-tetramethyl-
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[1,3,2]dioxaborolane)phenyl)amine2 and 6-Br by means of a threefold Suzuki cross-coupling
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reactions. Structures of intermediates and final compounds were fully elucidated by NMR and mass
spectrometry. As expected, 1-3 have good solubility in common solvents such as dichloromethane,
chlorobenzene, toluene, etc. Their good solubility renders them suitable for solution processing of
electronic devices.
2.2Thermal properties
Thermal properties of 1-3 were investigated by thermogravimetric analysis (TGA) and differential
17
scanning calorimetry under argon atmosphere. The TGA and DSC curves of 1-3 are shown in
Supporting Information (Fig S1). The thermal decomposition (Td) and the glass transition (Tg)
temperatures were summarized in Table 1. Synthesized compounds showed significant thermal
stability with Td values superior to 300°C which are high enough for the applications in optoelectronic
devices. Glass forming properties of 1-3 were revealed by DSC. They were obtained as amorphous
materials as confirmed by DSC. All compounds exhibit relatively high glass transition temperatures
(Tg) ranging from 74 to 99 °C. 1 exhibits the highest glass transition at 99°C. The introduction of a
methyl moiety on the outer position of thiophene ring (against triphenylamine core) does not affect
the thermal properties of resulting molecules 2 (Tg = 96°C). However, the methyl moiety on the inner
position of thiophene ring (molecule 3) significantly decreases thermal stability (Td of 3 is lower by
115°C than that of 1) and glass transition temperature (Tg = 74°C). This trend was confirmed by
comparative TGA analysis of corresponding precursors 5 (Td = 373°C) and 7 (Td = 287°C).
2.3 Geometry and electronic properties of compounds 1-3
-60.4°
-41.1°
-41.3°
-42.8°
-41.4°
-48.4°
-24.3°
-55.2°
-24.9°
-47.4°
-38.7°
-41.4°
1
3
2
4

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Figure 3. Geometry of compounds 1-3 obtained at the B3LYP/6-31G(d,p) level in “gas phase”. A
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selection of dihedral angles is shown.
The geometries of compounds 1-3 are shown in Figure 3. The methyl substituents in 2 and 3 are found
to increase the dihedral angles between the adjacent rings by roughly 17° (from 42.8° to 60.4°) and
14° (from 24.9° to 38.7°) respectively, as compared to 1. However, increasing the steric hindrance in
the middle of the conjugated backbone of each arm (compound 3) is expected to prevent to a larger
degree from a compact packing as compared to the positioning of methyl groups at the end of the
arms (compound 2), where some steric hindrance is already present, This difference might be at the
origin of the lower Tg value of compound 3 as compared to 1 and 2.
A selected set of MOs corresponding to compounds 1-3 are shown in Figure 4. In the three cases, the
HOMOs are localized in the TPA and thiophene rings, exhibiting very small contributions from the
DPA groups. These HOMO distribution patterns, along with the general geometrical shape of
compounds 1-3 will be seen to strongly impact the charge-transport properties of these molecules.
HOMO and LUMO energies are given in Table 1. Due to the larger size of the lobes on the LUMOs
as compared to HOMOs, the thiophene methylation in the proximity of the TPA phenyl groups results
in strong destabilization of the LUMO of 3 as compared to 1 (decrease in -conjugation efficiency at
the Ph-Th junction), and in slight stabilization of the HOMO (decrease of HOMO anti-bonding
character at the Ph-Th junction). Inspection of HOMO and LUMO of 1, indicates stabilization for
both orbitals upon thiophene methylation near the DPA position (compound 2 with respect to 1).
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DOI: 10.1039/C5RA07226C
LUMO
HOMO
1
2
3
Figure 4. HOMO and LUMO pictograms corresponding to compounds 1-3 (B3LYP/6-31G(d,p)
level).
Table 1. Thermal, optical and electrochemical properties of investigated HTMs and that of D102 dye.
The HOMO and LUMO energies (B3LYP/6-31G(d,p) energies are given in parentheses).
(f)
(g)
IPSS (HOMO)
(V/Fc+/Fc) (eV)
EASS (LUMO)
(eV)
(a)
(b)
(c)
(c)
opt (d)
Onset (e)
MW
λmax (solid)
Compd
Tg
Td
λmax (solution)
λonset
Egap
EOxi
g/mol
(°C) (°C) nm
nm
nm
eV
1173.47 99
1215.54 96
1215.54 74
428 420
397 401
313 389
406
449
439
435
556
2.76
2.83
2.85
2.23
0.07
0.12
0.1
4.87(-4.36)
2.11(-1.09)
2.09(-1.12)
2.05(-0.93)
2.86
1
397
4.92(-4.45)
4.90(-4.39)
5.09
2
382
3
490
0.29
D102
(a) determined from DSC analyses in argon at heating rate: 20°C/min
(b) 5% weight loss in TGA analyses in argon at heating rate: 20°C/min
(c) in dilute THF solution
(d) Egopt = hc/λonset = 1240/λonset
(e) in dichloromethane solution containing TBAPF6 as supporting electrolyte (0.1 M), 100 mV/S.
(f) IPSS = (EOxiOnset + 4.8)
opt
(g) EASS = IPSS - Eg
2.4 Electrochemical and Optical properties
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Cyclic voltammetry has been employed to investigate the redox behavior of synthesized molecules
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SS
SS
and to estimate the ionization potential (IP ) and electron affinity (EA ), which, in view of
Koopmans’ theorem, are good approximations of the HOMO and LUMO energy respectively. The
cyclic voltammograms of 1-3 showing their first oxidations are shown on Figure S2 in the Supporting
Information and related data is summarized in Table 1. IPSS and EASS values were estimated by using
the ferrocene (Fc) ionization potential value (4.8 eV vs vacuum) as the standard. The first oxidation
potentials could be attributed to the formation of radical cation of the triphenylamine core. These
compounds have onset oxidation potentials of approximately 0.1 V/Fc+/Fc, which could match with
the D102 dye (0.29 V/Fc+/Fc). The IPSS values determined from the oxidation onset potential values
of 1-3 with respect to Fc+/Fc range between 4.87 and 4.92 eV. Compared to the IPSS of D102 dye
(5.09 eV, Table 1) sufficient driving force for hole injection from D102 to the HTM can be deduced
(0.22, 0.17, and 0.19 eV respectively for 1-3). This suggests that 1-3 could be potential HTM in D102-
sensitized solar cells.
Optical properties of 1-3 were examined in dilute THF solutions and in thin film solid state. UV-Vis
absorption spectra were given in Figure 5 and related data are summarized in Table 1. Absorption of
indoline dye (D102) used in the present work as light harvester in DSSC devices were also given for
comparison purpose.
A
B
1.2
1.0
0.8
0.6
0.4
0.2
0
1,.2
1,.0
0,.8
0,.6
0,.4
0,.2
0
1
2
3
D102
1
2
3
D102
300
350
400
450
500
550
600
275 300 325 350 375 400 425 450 475 500 525 550 575 600
Wavelength (nm)
Wavelength (nm)
Figure 5. UV Vis absorption spectra of 1-3 recorded in dilute THF solution (A) and thin films (B),
the D102 UV Vis absorption spectra are also given in both figures.
All compounds have similar optical behaviors in dilute solution. They have a strong absorption band
located in the UV/near visible spectral domain (375 - 425 nm). Theoretical absorption spectra (Figure
S3) indicate that the intense low energy band is due to HOMOLUMO, LUMO+1 transitions. In the
case of compound 3, the introduction of methyl group into the thiophene ring near the TPA moiety
results in increased Ph-Th dihedral angle (Figure 3) and decrea
-conjugation efficiency.
Compared to compound 1, the decreased HOMO energy and the increased LUMO energy of
compound 3 (Table 1) result in gap opening and blue-shift of the absorption band. Smaller gap
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opening in the case of compound 2 results in smaller absorption blueshift as compared to 1. The
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DOI: 10.1039/C5RA07226C
opt
optical band gaps (Eg ) of 1-3 were also estimated from the edge of the first of the absorption peak
in solution (Table 1). By subtracting the optical band gap from the IPSS, the EASS was estimated. All
spectra recorded in solid state are slightly redshifted in comparison with the spectra recorded in
solution, which is consistent with some backbone-planarization and intermolecular polarization
effects in solid state. The absorption band maximum of compound 1 in THF solution is redshifted
from 404 nm to 420 nm in drop casted film. It is important to note that, the main absorption band of
D102 dye used in this study is centered in the visible spectral region (Figure 5). The three
triphenylamine derivatives exhibit therefore no- or slight absorption overlap with the dye, thus
avoiding harmful absorption competing between HTM versus the dye.
2-5 Electrical & photovoltaic properties
In addition to thermal/morphological stabilities and a good Dye/HTM energy level matching, HTMs
have to have good charge conductivity. Conductivity of the molecules was then studied using exactly
the same doping conditions as used for solar cell devices with optimum concentrations of lithium
bis(trifluoromethylsulfonyl)imide salt (Li-TFSI) plus 4-tert-butyl pyridine (tBP). Fig. 6 shows the I-
V curves of films with two probes measurement.
Figure 6. I-V curves of the doped films of compound 1-3 spin coated from chlorobenzene.
Derived conductivity of compound 1 is 9.73x10-5 S.m-1, while compound 2 and 3 show relatively
lower conductivities with 5.3x10-7 S.m-1 and 3.33x10-6 S.m-1 respectively, compared to the value
8.3x10-4 S.m-1 of SpiroOMeTAD.11 These small conductivity values seem to stem from a collective
effect of different electronic and structural factors: (i) the intramolecular reorganization energies are
relatively large (0.39, 0.31, and 0.38 eV for 1-3 respectively, B3LYP/6-31G(d,p) level in gas phase).
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(ii) The HOMOs have missing contributions from that DPA moieties (Figure 4 and S4a), which are
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important for easy HOMO-HOMO electronic couplings between adjacent molecules. (iii) Due to the
localization of the HOMOs inside the steric barrier created by the DPA moieties (Figures 3 and S4b),
the interaction between the TPA-Th moieties (the HOMO-bearing moieties) of adjacent molecules is
expected to result in large-distance HOMO-HOMO couplings (see Figure 4b-d). Addition of methyl
groups into the thiophene ring induces distortions of the average  plane due to steric hindrance and
consequently may enlarge - stacking distance, resulting in decreased charge transport ability of 2-
3 as compared to 1.
Figure 7. J-V characteristics of ss-DSSCs using compound 1-3 as hole transport layers
tested under 100 mWcm-2 AM 1.5 G illumination.
The compounds 1-3 were subsequently incorporated in solid state D102-sensitized solar cells as hole
transporting layers. J-V curves under 100 mWcm-2 AM 1.5 G illumination are presented in Figure 7.
Best device performance parameters are listed in Table 2.
Table 2. Best device performance parameters of ss-DSSCs with compound 1-3 as hole transport
layers tested under 100 mWcm-2 AM 1.5 G illumination
HTM
Jsc (mA cm-2)
Voc (V)
FF
PCE (%)
1
2
3
3.0
1.8
0.79
0.87
0.81
0.51
0.29
0.29
1.20
0.46
0.44
1.85
Rather high values of Voc were obtained from all the compounds, reaching even to 0.87V for
compound 2. Highest Jsc (3.0 mA cm-2) and fill factor (51%) were achieved from the device based on
compound 1, yielding PCE of 1.2%. Both devices based on compound 2 and 3 gave low fill factor of
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only 29%, as well as low Jsc of around 1.8 mA cm-2. Obviously, evidenced by our conductivity
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measurements, poor charge transport properties of these latter two compounds comparing to
compound 1 hindered their ability to efficiently extract the photo-generated charges to the electrode.
It is interesting to look at performance stability of these new molecules. As reference, we also
included benchmark hole transport molecule Spiro-OMeTAD in our study. Measurement of the
devices was carried out in air, without encapsulation with continuous illuminations for more than two
hours. The evolution of device performance parameters are shown in Figure 8.
Figure 8. Stability measurement of ss-DSSCs with 1-3 and spiro-OMeTAD as hole transport layers.
Devices were tested without sealing; in air under continuous illumination at 100 mWcm-2 AM 1.5
G.
Unlike Spiro-OMeTAD device, the devices with all the new compounds did not evidence initial burn-
out. On the contrary, all of them showed an initial enhancement in device performance. Decline in
overall PCE mainly followed the trend in Jsc in all the devices, which takes into account all the
processes from charge generation to collection in a cell. Notwithstanding rather low device
performance of the devices based on these new molecules, they did show relatively high stability.
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When we compare the final performances with the highest values of these compounds, Spiro-
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OMeTAD retained 27% of PCE after 140 minutes under continuous illumination, while compound
1, 2 and 3 kept 51%, 80% and 57% respectively.
3. Conclusion
In summary, novel di(4-methoxyphenyl)amino end-capped tri(4-thiophenylphenyl)amine based
molecular glasses were synthesized and characterized in detail. The synthetic routes highlighted Pd-
catalyzed reactions and allowed the preparation of highly soluble compounds using short multistep
syntheses in relatively good yields. Thanks to their glass forming properties (Tg varies from 74 to
99°C), these molecules are able to stay in amorphous state allowing the realization of a thin film with
good contact with the nanostructured inorganic layer. Additionally, their HOMO energy levels are in
good alignment with that of D102 dye rendering them suitable for use in solid-state dye-sensitized
solar cell. Indeed, all these molecules yielded rather high Voc, reaching even 0.87 V in solid state
DSSC devices. Large variation in conductivity was observed when methyl groups were introduced in
the thiophene ring. The compound without methyl group showed highest conductivity and best
performance in ss-DSSC devices with PCE of 1.2%. Comparison of device’s stability of cells
fabricated from HTM 1-3 with that fabricated from Spiro-OMeTAD was carried out in continuous
illumination in air without encapsulation. It puts into evidence that these new molecules were rather
stable keeping more than 50% of the highest performance (even 80% for compound 2) as compared
to 27% in spiro-OMeTAD device.
Experimental section
General remarks
Materials: All used chemicals and solvents were purchased from chemical companies and used as
received, unless otherwise mentioned. Toluene was dried by distillation from sodium metal.
Purification of products was performed by column chromatography on silica gel from Merck with a
grain size of 0.04–0.063 mm (flash silica gel, Geduran Si 60) eluting with analytically pure solvents.
For analytical thin layer chromatography, “60 F254” aluminium substrates coated with silica gel from
Merck were used.
Instrumentation & Measurements: NMR (1H & 13C) spectra were recorded on a Bruker DPX-250
18
FT-NMR spectrometer. Chemical shifts are given in ppm using the residual solvent signal as
internal reference. High resolution mass spectrometry was performed by the Small Molecule Mass
Spectrometry platform of IMAGIF (Centre de Recherche de Gif). UV-Vis spectra were measured on
a Jasco (V-570) UV-Vis-NIR spectrophotometer. Thermogravimetric analysis (TGA) was carried out
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on a TA Instrument Q50 TGA under argon flow at a heating rate of 20 °C/min. The temperature of
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thermal degradation (Td) was measured at the point of 5% weight loss. Differential scanning
17
calorimetry was performed on a TA Instruments Q100 calorimeter, calibrated with indium and
flushed with argon. The oxidation potential (Eox) of the investigated compounds were measured in
THF by cyclic voltammetry, scan rate 100 mV/s, with tetrabutylammonium hexaperclorate (0.1 M)
as a supporting electrolyte in a standard one-compartment, three-electrodes electrochemical cell
under an argon stream using a “SP300 BioLogic” potentiostat. The working, reference and counter
electrodes were platinum disk (Ø = 1 mm), silver wire pseudo-reference, and stainless steel gauze
electrodes, respectively. Ferrocene was used as an internal standard, and the potentials were referred
to the reversible formal potential of this compound.
Materials Synthesis
The synthesis of 1, 3 and their different intermediates have been reported in the literature previously.2
N,N-bis(4-methoxyphenyl)-3-methylthiophen-2-amine18: In a dry schlenk equipped with magnetic
stir bar and under argon, 2-bromo-3-methylthiophene (0.5 g, 2.82 mmol), bis(4-
methoxyphenyl)amine (0.97 g, 4.23 mmol), palladium(II) acetate (81 mg, 0.36 mmol), sodium tert-
butoxide (1.08 g, 11.3 mmol) and dry toluene (10 mL) were charged. The mixture was then degassed
by argon purging for 30 min. Tri-tert-butylphosphine (57 mg, 0.28 mmol) was added via syringe. The
mixture was heated at 100°C for 3 days and allowed to cool to room temperature. The mixture was
diluted with toluene (30 mL), washed with water, dried over MgSO4 and concentrated. The crude
product was purified by flash column chromatography eluting with CH2Cl2/cyclohexane (1:3) to yield
a pale yellow oil (630 mg, 68 %).
1H NMR (250 MHz, CDCl3, ): 7.02 – 6.90 (m, 5H), 6.84 – 6.73 (m, 5H), 3.78 (s, 6H), 1.94 (s, 3H).
13C NMR (63 MHz, CDCl3, ): 156.2, 137.4, 135.6, 134.6, 129.2, 127.1, 123.4, 115.4, 56.2, 12.5.
5-bromo-N,N-bis(4-methoxyphenyl)-3-methylthiophen-2-amine (6-Br): To a solution of 6 (600
mg, 1.84 mmol) in DMF (20 mL) was added N-bromosuccinimide (328 mg, 1.84 mmol) at 0 °C. The
solution was stirred for 3h at room temperature. The total reaction was confirmed by TLC. The
solution was poured into water (20 mL) and chloroform (30 mL) was added to the mixture. After
phase separation, the aqueous layer was extracted with chloroform (2 × 20 mL). The organic layer
was washed with HCl 1N solution (4 × 20 mL), water (2 × 20 mL), dried over MgSO4 and
concentrated. The crude product was purified by flash chromatography on silica gel with
CH2Cl2/cyclohexane (1:3) as eluent to give pale yellow oil (600 mg, 80 %).
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1H NMR (250 MHz, CDCl3, ): 6.95 (d, J = 8 Hz, 4H), 6.81 (d, J = 7.8 Hz, 4H), 6.48 (s, 1H), 3.80
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(s, 6H), 1.97 (s, 3H). C NMR (63 MHz, CDCl3, ): 156.1, 137.8, 137.4, 137.1, 133.6, 128.4, 116.4,
113.1, 56.2, 11.8.
Tris(4-(5-(4,4’-dimethoxydiphenylamino)-4-methyl-2-thiophenyl)phenyl)amine18: In a dry
schlenk equipped with magnetic stir bar and under argon, 6-Br (0.5 g, 1.23 mmol), tris(4-(4,4,5,5-
tetramethyl-[1,3,2]dioxaborolane)phenyl)amine (171 mg, 0.27 mmol), dry toluene (80 mL) and
K2CO3 2M solution (20 mL) were charged. The mixture was then degassed by argon purging for 30
min. Aliquat 336 (3 drops) and Pd(PPH3)4 (59 mg) were added. The mixture was heated at 90°C for
3 days and allowed to cool to room temperature. The mixture was diluted with toluene (30 mL) and
after phase separation; the organic layer was washed with brine (2 × 20 mL), water (2 × 20 mL), dried
over MgSO4 and concentrated. The crude product was purified by column chromatography eluting
with CH2Cl2/cyclohexane (7:3) to yield a pale yellow solid (250 mg, 76 %).
1H NMR (250 MHz, CDCl3, ): 7.42 (d, J = 8.5 Hz, 6H), 7.05 (d, J = 8 Hz, 12H), 6.98 (d, J = 8.4 Hz,
6H), 6.88 (d, J = 7.9 Hz, 12H), 6.78 (s, 3H), 3.83 (s, 18H), 2.08 (s, 9H). HRMS (ESI+): calculated
for M+: 1214.4145, found: 1214.4116.
Device fabrication and characterization
For conductivity measurement, the doped solutions used for DSSC devices were spin coated on glass
substrate. Hole transport molecules were dissolved in chlorobenzene (200mg mL-1), by heating and
stirring the solution at 80 °C for 30 minutes. Optimized doping concentrations of Lithium
bis(trifluoromethylsulfonyl)imide salt (Li-TSFI) dissolved in acetonitrile (170 mg mL-1) and 4-tert–
Butylpyridine (tBp) are 1:19.3 and 1:40.8 in volume ratio respectively. Au metal stripes with width
of 0.05 mm and length of 2.5 mm were deposited. Keithley 4200 semiconductor analyser was used
to record I-V curves. For ss-DSSC devices, Fluorine doped tin oxide (FTO) Glasses were used as
working electrode. One third Part of FTO was etched away using HCl and Zn powder and cleaned by
consecutive sonication in acetone, ethanol and isopropanol for 15 minutes and dried. The substrates
were then treated in UV- ozone for 15 minutes. A thin compact layer of TiOx was deposited using
spin coating from the solution of Titanium (IV) isopropoxides (TIPP) diluted in ethanol at the
concentration of 0.05 M and HCl was added in order to have a water to TIPP molar ratio(rw) of 0.82
and a pH of 1.9. The samples were annealed at 450 °C for 20 minutes with subsequent cooling at 70
°C. The films were then treated with TiCl4 in water (220 μL /100 mL) for 30 minutes at 70 °C
followed by rinsing with water and heating again to 450 °C for 20 minutes. Mesoporous TiO2 films
were spin coated on top of the compact layer using a widely available Ti Nanooxide paste
(SOLARONIX) diluting in ethanol. The films were then slowly heated to 500 °C followed by again
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Page 14 of 16
TiCl4 treatment. The mesoporous TiO2 electrodes were placed for 1 hour in organic dye (D102)
View Article Online
DOI: 10.1039/C5RA07226C
-1
solution (0.15mg mL ) in a mixture of acetonitrile and tert-butanol (1:1). The dye coated mesoporous
films were rinsed in ethanol and dried in air overnight. 40µL doped solution was deposited onto each
film at room temperature and left for 1 minute before spin coating at 3500 RPM for 20s in air. After
spin coating, the films were kept for 24 hours in air. A 100 nm of Ag was deposited by thermal
evaporation to complete the device structure. The device area, as defined by the overlap between the
ITO and Ag electrodes, was 8.6 mm2. Current-voltage (J-V) characteristics of the devices were
examined in air under illumination of an AM1.5 set at 1000Wm-2 with a calibrated silicon radiometer
(Akoos). Stability of the ss-DSSCs without sealing was tested under continuous illumination in air
for 2 hours.
Computational details
All the calculations were performed in the frame of DFT19 theory employing the B3LYP20 functional
with Gaussian 09 program21 The geometry optimizations for all the molecules were carried out
without symmetry constraints by using the 6-31G(d,p) basis set, followed by frequency calculations.
The absorption spectra of the molecules were calculated by mean of time dependent density
functional theory method (TDDFT)22 with the 6-31G(d,p) basis set. The theoretical absorption bands
were obtained by considering a band half-width at half-maximum of 0.2 eV.23
Acknowledgements
The authors thank the Agence Nationale de la Recherche (ANR) for funding (FMOCSOLE Blanc
SIMI 9 2010 project).
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