Photovoltaic performance of novel push-pull-push thienyl-Bodipy dyes in solution-processed BHJ-solar cells

Article abstract of DOI:10.1039/c3nj01436c

This work explores the synthesis of extended dipyrromethene dyes engineered from thiophene-linked triphenylamine modules in order to favor pronounced charge transfer within the dyes and to enable their use in solar cells. We found that these soluble dyes

Full text of DOI:10.1039/c3nj01436c

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Photovoltaic performance of novel push–pull–
push thienyl–Bodipy dyes in solution-processed
BHJ-solar cells†
Cite this: New J. Chem., 2014,
38, 1701
Alexandra Sutter,a Pascal Retailleau,b Wei-Ching Huang,c Hao-Wu Lin*c and
Raymond Ziessel*a
This work explores the synthesis of extended dipyrromethene dyes engineered from thiophene-linked
triphenylamine modules in order to favor pronounced charge transfer within the dyes and to enable
their use in solar cells. We found that these soluble dyes absorb up to 760 nm in solution and 850 nm in
thin films. An effective charge transfer absorption band was found at around 460 nm. In the crystal
lattice, molecules are organized in layers separated by 3.50 Å with pronounced p–p stacking and Sꢀ ꢀ ꢀS
interactions. The electroactivity of the dyes indicates that electron injection in PC71BM is feasible. Bulk
heterojunction solar cells based upon dyes 1 and 2 in an optimized device structure ITO/Ca/1 or
2:PC71BM/MoO3/Ag provide a power conversion efficiency of about 1.5% after thermal annealing.
Received (in Montpellier, France)
18th November 2013,
Accepted 5th February 2014
DOI: 10.1039/c3nj01436c
www.rsc.org/njc
A key factor for the success of OSCs is the nature of the dyes
which absorb the photons, allow diffusion of the excitons in the
1. Introduction
The fabrication of solar energy conversion devices is a major thin layer and finally promote charge separation at the inter-
contemporary interest of the general scientific community due face with the acceptor in the bulk-heterojunction (BHJ).7
to their promise as low-cost and renewable energy sources.1 Many dyes have been screened including oligothiophenes,8
Production of organic solar cells (OSCs) by solution-processing diketopyrrolopyrroles (DPP),9 mixed DPP-TAT dumbbell-shaped
techniques is a promising technology for cheap and flexible scaffoldings (TAT for triazatruxene),10 squaraines,11 hexabenzo-
panels.2 Usually the photoactive layer is made of a blend of a coronenes,12 merocyanines,13 donor–acceptor oxoindanes,14
small and highly colored organic molecule or a soluble low thiadiazolo-bithienyls,15 benzodithiophenes,16 benzothia-diazoles,17
band-gap polymer considered as the electron donor and a and Bodipy-thienyls.18,19
soluble fullerene derivative able to accept the photo-excited
Borondipyrromethene (Bodipy) dyes are attractive for use in
electron.3 Note that small molecules can be co-evaporated with organic photovoltaic cells (OPVs)18–20 and dye-sensitized solar cells
the acceptor under ultra-high vacuum to form the photoactive (DSSCs)21 mostly because many chemical tools allow tailoring at
layer. In both cases nanophase-segregation between the donor will of the optical and redox properties.22 Consequently, several
and the acceptor is mandatory to provide high efficiency.4 substituted Bodipy dyes were tested as sensitizers in BHJ solar cells
Impressive photoconversion efficiencies of 8.9%5 to above with photoconversion efficiencies spanning from 0.9 to 4.7%. Some
10%6 were recently reached using small molecules and both of these Bodipys were constructed with vinyl side arms in the
technological approaches.
3,5-substitution positions, ensuring extended delocalization path-
ways in the ground state and intense absorption in the visible
portion of the electromagnetic spectra. In some cases, these vinyl
groups were elaborated with strong donor groups like triazatruxene
providing good charge transport in the active layer and a good match
with the solar spectrum.20b Interestingly, mixing of similar dyes
allowed extension of the photon absorptivity of the device and
ensured better photon to current conversion.18b The use of
thiophene-grafted Bodipys markedly improved the efficiency, mostly
due to high mobility of hole and electron inside the thin photoactive
layer.19 It has further been demonstrated that a very high open-
circuit voltage could be achieved by linking triphenylamine (TPA)
fragments in the 2,6-susbtitution positions of a Bodipy framework.20c
a
´
Laboratoire de Chimie Organique et Spectroscopies Avancees (ICPEES-LCOSA),
´
`
´
UMR 7515 au CNRS, Ecole Europeenne de Chimie, Polymeres et Materiaux,
´
Universite de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France.
E-mail: ziessel@unistra.fr
b Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles,
ˆ
UPR2301-CNRS, Batiment 27, 1 avenue de la Terrasse,
91198 Gif sur Yvette Cedex, France
c Department of Materials Science and Engineering, National Tsing Hua University,
Hsinchu, Taiwan. E-mail: hwlin@mx.nthu.edu.tw
† Electronic supplementary information (ESI) available. CCDC 939187. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3nj01436c
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Scheme 1 Keys: (i) [Pd(PPh3)4] (6 mol%), toluene, 110 1C, overnight; (ii) (a) POCl3 (1.1 eq.), DMF (1.4 eq.), 0 1C, 1 h, (b) compound A (1 eq.), 1,2-
dichloroethane, 85 1C, overnight; (iii) compound C (1 eq.), compound B (3 eq.), piperidine, p-TsOH, toluene, reflux, Dean-Stark apparatus, overnight; (iv)
(a) 3-(2-methoxyethoxy)prop-1-yne (4 eq.), EtMgBr (3.5 eq.), THF, 60 1C, 2 h, (b) compound 1 (1 eq.), THF, 60 1C, overnight.
Herein we describe novel extended Bodipy dyes built from CHCl3 (d = 7.26 ppm) for 1H NMR, and CDCl3 (d = 77.2 ppm) for
both TPA-thiophene (push) modules in a conjugated situation 13C NMR, while the signal from the B within the borosilicate of
with a 3,5-substituted Bodipy (pull) skeleton. The idea was to the NMR tubes was used for 11B NMR. Electronic absorption
increase the electronic density on the thiophene site in order to spectra were recorded using a Shimadzu UV-3600 dual-beam
favour charge transfer with the electron withdrawing Bodipy grating spectrophotometer with a 1 cm quartz cell. Steady-state
module without perturbing to a large extent the electrochemical emission and excitation spectra were recorded on a HORIBA
gap (HOMO/LUMO). The target dyes were prepared as shown in Jobin-Yvon FluoroMax 4P spectrofluorimeter. All fluorescence
Scheme 1. The rationales for the fluorine group substitution rely spectra were corrected. The fluorescence quantum yield (Fexp
on our previous work, where we increase the steric bulkiness was calculated from eqn (1):
around the boron center to avoid the aggregation process and
)
F½1 ꢁ expðꢁAref ln 10Þꢂn
increase the solubility and chemical and photochemical stability
Fexp ¼ Fref
(1)
of the dyes.23
2
Fref ½1 ꢁ expðꢁA ln 10Þꢂnref
Here, F denotes the integral of the corrected fluorescence spectrum,
A is the absorbance at the excitation wavelength and n is the
refractive index of the medium.
2 Experimental part
2.1 General methods
All chemicals were obtained from commercial sources (Sigma
Tetramethoxydiisoindomethene-difluoroborate (fF = 0.51 in
Aldrich, Alfa Aesar) and used, unless otherwise stated, without dichloromethane) was used as the emission reference for both
further purification. All anhydrous reactions were performed under compounds emitting above 650 nm.24
a dry atmosphere of argon using standard Schlenk techniques.
Potentials are determined by cyclic voltammetry in deoxyge-
THF was distilled over sodium and benzophenone under an nated CH2Cl2 solutions, containing 0.1 M TBAPF6, at a solute
argon atmosphere. 1,2-Dichloroethane was distilled over P2O5 concentration range of ca. 1 mM and at rt. Potentials are given
under an argon atmosphere. DMF and thiophene were distilled versus the saturated calomel electrode (SCE) and standardized vs.
over potassium hydroxide under argon.
ferrocene (Fc) as internal reference assuming that E1/2 (Fc/Fc+) =
1H NMR, 13C NMR and 11B NMR spectra were recorded on +0.38 V (DEp = 60 mV) vs. SCE. The error in half-wave potentials
BRUKER spectrometers with reference chemical shifts taken as is ꢃ10 mV. Where the redox processes are irreversible, the peak
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potentials (Eap or Ecp) are quoted. All reversible redox steps of compound 1, at room temperature. Diffraction data were
result from one-electron processes unless otherwise specified. collected using a Rigaku MM007 HF copper rotating-anode
generator with Osmic confocal optics and a rapid II Curved
2.2 Preparative work
Image Plate at low temperature, 193 K. A total of 173 images
Materials. [Pd(PPh3)4],25 A,26 B,27 Bodipy C,17,18 and 3-(2-methoxy- with 51 rotation per image and two minute exposure per degree
ethoxy)prop-1-yne18 were synthesized according to the indicated of oscillation were measured according to an o-scan profile
literature procedures. The preparation of trimethyl(thiophen-2- data strategy derived by the CrystalClear software package.28
yl)stannane was adapted from ref. 26 by replacing tri-n-butyl- Intensities were integrated using FS_Process as implemented in
stannylchloride by trimethylstannyl-chloride.
the CrystalClear suite, then corrected for Lorentz-polarization
Synthesis of 1. To a solution of Bodipy C (199.8 mg, effects, and scaled using symmetry-equivalent reflections using
0.44 mmol) in toluene (40 mL) were added 5-(4-diphenylamino- FS_Abscor. The structure was solved by direct methods
phenyl)-2-carbaldehydethiophene B (473.5 mg, 1.33 mmol), (SHELXS-97)29 and refined on F2 by means of full-matrix least-
piperidine (1 mL) and one crystal of p-toluenesulfonic acid. squares methods (SHELXL-97).24 All non-hydrogen atoms were
The resulting mixture was stirred at reflux overnight using a allowed anisotropic thermal motion. Aliphatic and aromatic C–H
Dean-Stark apparatus. After being cooled, the organic product hydrogen atoms were included at calculated positions, with C–H =
was extracted into CH2Cl2, and washed with water and brine. 0.98 Å, and 0.95 Å respectively and were refined with a riding
The organic layer was dried over anhydrous MgSO4 and evaporated model and with Uiso set to 1.5 (resp. 1.2) times that of the
under vacuum. The crude product was purified by silica gel attached C atom. The structure showed the presence of three
chromatography (from 60/40 to 50/50 petroleum ether/CH2Cl2) disordered dichloromethane solvent molecules with refined
and recrystallized by slow evaporation of dichloromethane from a occupancy factors less than one but the overall modelling was
mixture of CH2Cl2–EtOH. The resulting crystals were collected and not satisfactory, requiring resort to the SQUEEZE procedure of
washed with pentane to afford a green compound (431.1 mg, 86%). Spek and van der Sluis.30 An extra 229 e per cell were indicated,
3
1H NMR d (ppm): (300 MHz, CDCl3): d (ppm) 7.85 (d, J = 8.1 Hz, corresponding to 2.7 molecules of CH2Cl2. Their contribution to
2H), 7.42–7.55 (m, 6H), 7.01–7.36 (m, 32H), 6.60 (s, 2H), 1.47 (s, 6H). the diffraction pattern was removed and modified Fo2 written to
13C NMR (300 MHz, CDCl3): d (ppm) 152.3, 147.8, 147.4, 146.3, a new HKL file. The number of electrons thus located was
141.3, 140.8, 138.2, 134.9, 133.5, 130.7, 129.4, 129.2, 127.9, 126.7, included in the formula, formula weight, calculated density,
124.7, 123.4, 123.3, 123.2, 118.2, 117.7, 94.7, 14.9. EI-MS m/z (%) m and F(000). ORTEP drawings were made using ORTEP-III31
1124.1 (100). Anal. for C65H48BF2IN4S2: found C, 69.31; H, 4.21; N, as implemented within PLATON29 and packing studies were
4.68; calcd for C65H48BF2IN4S2: C, 69.40; H, 4.30; N, 4.98.
carried out using MERCURY.32
Synthesis of 2. In a Schlenk flask, ethylmagnesiumbromide
The crystallographic data† are summarized as follows: a
(702.8 mL, 0.80 mmol) was added to a stirred solution of dark blue prism of dimensions 0.44 ꢄ 0.18 ꢄ 0.12 mm,
3-(2-methoxyethoxy)prop-1-yne (96.3 mL, 0.70 mmol) in anhydrous 65H48BF2N4S2, 2.7 (CH2Cl2), M = 1354.20, triclinic, space group
C
THF (5 mL). The mixture was stirred at 60 1C for 2 h under argon. P-1, a = 12.6116 (5), b = 13.0839 (6), c = 20.2570 (14) Å,
The resulting anion was then transferred with a canula to a solution a = 80.833(6)1, b = 77.449(6)1, g = 80.043(6)1, V = 3187.7(3) Å3,
of 1 (225.9 mg, 0.20 mmol) in anhydrous THF (5 mL). After the Z = 2, Dcalcd = 1.411 g cmꢁ3, F(000) = 1375, m = 7.039 mmꢁ1
,
reaction mixture was stirred at 60 1C overnight under argon, water 14 957 collected reflections (2.251 r y r 451), ꢁ10 r h r 11,
was added, and the solution was extracted with CH2Cl2. The organic ꢁ12 r k r 12, ꢁ18 r l r 18, 22 781 independent reflections
phase was washed with water, dried over anhydrous MgSO4 and (Rint = 0.0481), goodness-of-fit on F2: S = 1.188, R1 = 0.089 and wR2 =
evaporated under vacuum. The crude product was purified by silica 0.209 for all 5140 reflections, R1 = 0.059 and wR2 = 0.168 for 5141
gel chromatography (from pure CH2Cl2 to 98/2 CH2Cl2/MeOH) and observed reflections [I 4 2s(I)], refining 676 parameters and 41
recrystallized by evaporation in CH2Cl2/EtOH to afford a green rigid-bond restraints (DELU with default esd), semi-empirical
compound 2 (198.2 mg, 75%) after washing with pentane. absorption correction from multi w-scans (Tmin = 0.20, Tmax
=
3
1H NMR (300 MHz, CDCl3) d (ppm): 8.00 (d, J = 15.9 Hz, 2H), 0.46), final electron density between ꢁ0.386 and 0.418 eÅꢁ3
.
7.85 (d, J = 8.3 Hz, 2H), 7.52 (d, 3J = 8.6 Hz, 4H), 6.98–7.33
2.4 Device preparation
(m, 32H), 6.61 (s, 2H), 4.23 (s, 4H), 3.64–3.71 (m, 4H), 3.28–3.35
(m, 4H), 3.16 (s, 6H), 1.45 (s, 6H).13C NMR (300 MHz, CDCl3) d The OPV devices were fabricated on indium tin oxide (ITO) coated
(ppm) 151.5, 147.8, 147.2, 145.9, 141.5, 139.9, 138.2, 136.1, glass substrates (sheet resistance B10 O sqꢁ1). The substrates
135.24, 131.8, 130.8, 129.4, 127.8, 127.3, 126.7, 124.7, 123.4, were cleaned in an ultrasonic bath with de-ionized water, acetone,
119.7, 118.5, 71.7, 68.3, 59.4, 58.8, 15.1. EI-MS m/z (%): 1312.2 and methanol for 15 min, respectively. The 1 nm Ca layers were
(100). Anal. for C77H66BIN4O4S2: found C, 70.22; H, 4.86; N, deposited onto ITO glass substrates in a high vacuum chamber
4.01; calcd for C77H66BIN4O4S2: C, 70.42; H, 5.07; N, 4.27.
with a base pressure of B8 ꢄ 10ꢁ7 Torr, and the deposition was
performed at a rate of 1 to 2 Å sꢁ1 with the substrates held at room
temperature. A blend solution of solar active Bodipy donors and
2.3 Crystallographic section
The structure presented herein was solved from a dark blue PC71BM (purchased from Nano-C) was prepared using chloroform
single crystal suitable for X-ray diffraction and obtained by slow as solvent with different ratios and a total concentration of
diffusion of ethanol into a saturated dichloromethane solution 15 mg mLꢁ1. The active layers were spin-coated on the substrates
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in a glove box under an anhydrous nitrogen atmosphere. The
samples were then transferred to a vacuum chamber for top
electrode deposition. Devices were encapsulated using a UV-cured
sealant (Everwide Chemical Co., Epowide EX) and a cover glass
under the anhydrous nitrogen atmosphere after fabrication and
were measured in air. The active area of the cells had an average
size of 5 mm2 (intersect area between the Ag cathode and the
ITO anode) and was carefully measured device-by-device using
a calibrated optical microscope.
2.5 Characteristics measurement
Current density–voltage characteristics were measured using a
SourceMeter Keithley 2400 under AM 1.5G simulated solar illumina-
tion from a xenon lamp solar simulator (Abet Technologies). The
incident light intensity was calibrated as 100 mW cmꢁ2 using a
NREL-traceable KG5 filtered Si reference cell. The external quantum
efficiency (EQE) spectra were taken by illuminating chopped
monochromatic light with a continuous-wave bias white light
(from halogen lamp, intensity B100 mW cmꢁ2) on the solar
cells. The photocurrent signals were extracted using a lock-in
technique using a current preamplifier (Stanford Research
System) followed by a lock-in amplifier (AMETEK). The EQE
measurement is fully computer controlled and the intensity of
monochromatic light is carefully calibrated using an NIST-
traceable optical power meter (Ophir Optronics). Thicknesses
and extinction coefficients (k) of the thin films were determined
using spectroscopic ellipsometry (J. A. Woollam Inc. V-VASE).
Atomic force microscopy (AFM) images were taken using a
Veeco Nanoscope 3100 atomic force microscope. The electron
and hole mobilities for the Bodipy:PC71BM blended films were
carried out by using the SCLC method. The hole only device was
configured as follows: ITO/MoO3 (1 nm)/Bodipy:PC71BM
(80 nm)/MoO3 (10 nm)/Al (80 nm), while the electron only device
was configured as follows: ITO/Mg (5 nm)/Bodipy:PC71BM
(80 nm)/Ca (5 nm)/Al (80 nm). Current density–voltage char-
acteristics of SCLC devices were also measured using a Source-
Meter Keithley 2400. The annealing process was performed by
placing the encapsulated devices on a temperature controlled
hot plate.
Fig. 1 Partial 1H NMR spectra of dye 1 (a) and dye 2 (b) in CDCl3 at rt.
Proton NMR spectra were diagnostic for the state of boron sub-
stitution. It has been clearly established that by boron substitution
with an ethynyl residue the vinyl protons a (2H) at 8.02 ppm ( JH–H
=
16 Hz) are deshielded in 2 by 0.54 ppm, compared to the same
protons at 7.48 ppm in the BF2 compound 1. In contrast, the
doublet at 7.86 ppm (2H, JH–H = 8 Hz) assigned to the AB quartet of
the phenyl proton b in the pseudo meso position remained
unchanged in both compounds (Fig. 1). Furthermore, the high
coupling constants ( JH–H = 16 Hz) for the vinylic protons are in
keeping with a trans conformation of the double bonds. Finally,
the b-pyrrolic protons c remain unchanged at 6.6 ppm.
3.2 X-ray characterization
X-ray diffraction on a blue single crystal confirms that the
structure of compound 1 shown in Fig. 2 incorporates a nearly
flat 3,5-vinylbisthienyl-Bodipy framework as recently described
elsewhere19 in a related compound (see Fig. 3): it features an
even more nearly-planar Bodipy platform (maximum deviation
from the mean least-squares plane is 0.107 (8) Å for the carbon
3 Results and discussion
3.1 Synthesis and characterization
The synthetic protocol used to prepare the novel dyes is sketched in
Scheme 1. The first step produced compound A by cross-coupling
of 4-(bromophenyl)diphenylamine and 2-trimethyltinthiophene.
Formylation of A under Vilsmeier–Haack conditions provided
regio-selectively the aldehyde B in excellent yield under mild
conditions. Next, a Knoevenagel reaction using forcing conditions
provided the deep-green divinyl derivative 1 in an average yield of
86%. The use of the Grignard reagent of 3-(2-methoxyethoxy)prop-1-
yne provided dye 2 in about 75%. All compounds were purified by
column chromatography. The molecular structures were assigned
by NMR spectroscopy, elemental analysis and mass spectro-
Fig. 2 ORTEP view of compound 1. Thermal ellipsoids are plotted at
the 30% level. B1–N3, B1–N2, B1–F1, B1–F2, N3–C1, N2–C8, N3–C4 and
N2–C5 bond lengths are 1.525(14), 1.516(14), 1.376(14), 1.389(13), 1.370(10),
1.370(10), 1.349(11) and 1.368(11) Å, respectively and the FB–F, N–B–F,
and N–B–N angles are 107.2(10), 111.0(11)/110.9(11)/110.6(10)/109.2(10)
scopy and these data unambiguously confirm those assigned. and 107.9(10)1.
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Fig. 3 A superposition of compound 1 and compound 3,5-vinylbisthienyl-
Bodipy18 (in green) generated by least-squares fitting of their iodophenyl-
Bodipy groups.
Fig. 5 Part of the crystal structure viewed down the molecular stacking
direction. Molecules from n ꢁ 1, n and n + 1 layers are in pink, grey and
light green respectively.
arms overlapped and an SꢀꢀꢀS distance of 4.03 Å. As well, p–p
stacking interactions occur where the molecules at 1 ꢁ x, 1 ꢁ y,
1 ꢁ z and 2 ꢁ x, ꢁy, 1 ꢁ z in one layer extend one arm towards their
equivalent molecule in the next layer, resulting in thiophene groups
overlapping the outer five-membered ring of the Bodipy with a
centroid–centroid distance of 3.69 ꢃ 0.03 Å (Fig. 5).
3.3 Spectroscopic characterization
Dyes 1 and 2 display similar spectroscopic features (Fig. 6) in
THF, with three intense absorption peaks in the 350–550 nm
and 600–750 nm range and extinction coefficients of about
70 000 Mꢁ1 cmꢁ1 for the former peaks and 90 000 Mꢁ1 cmꢁ1 for
Fig. 4 Lattice view highlighting the layered structure parallel to the
(3 3 ꢁ4) plane (cyan dotted lines). Phenyl units from the triphenylamine
substituents interspersed between the layers are shown as balls.
atom C5) and a near-orthogonally-attached (89.01) iodobenzene
group with +C9 B1 I1 of 4.21.
Similarity extends to the sulphur atoms from the first thiophene
groups of both arms, which face one another. In the crystal, the flat
molecules assemble in a layered structure, parallel to the plane
(3 3 ꢁ4) with an interplanar separation of 3.50 ꢃ 0.06 Å (Fig. 4).
Within a layer, the V-shaped molecules are aligned along the [1 0 ꢁ1]
direction with the iodophenyl group pointing here towards a phenyl
from the triphenylamine unit of one of the two arms, unlike the
former complex crystal structure in which the iodobenzene group
was inserted between the arms of a neighbor, interacting with the
BF2 group. As a direct consequence, this interaction maintains the
thiophene and the phenyl ring bonded to it coplanar with Bodipy
platform whereas the other arm is tilted from the mean plane by an
angle of circa 9.51. The triphenylamine unit in interaction with the
iodo atom adopts a close to staggered conformation with respect to
the mean plane, with the outer phenyl groups interposed between
molecular layers. This contrasts with the other triphenylamine unit,
where with the phenyl groups are oriented in an eclipsed way.
Relative to the mean plane defined by the N atom and its three
bound C atoms, the torsion angles of each of the aromatic rings
range from 28.6 to 43.31 for the latter group and from 38.6 to 45.01
for the former one, but are similar to the values observed for the
diphenylaminophenyl units, 37.0 (7) r 50.5 (7)1.33 Between adjacent
layers, inversion related molecules at general position and at 2 ꢁ x,
1 ꢁ y, 1 ꢁ z lie pairwise head-to-tail, the Bodipy platform lying over
Fig. 6 Absorption (blue trace), emission (green trace) and excitation
spectra (red traces) of dye 1 at lexc @ 640 nm and lem @ 800 nm (a)
the inside phenyl of the ‘flat’, eclipsed triphenylamine unit, with the and dye 2 (b) at lexc @ 650 nm and lem @ 780 nm in THF, at rt.
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Fig. 7 Thin-film absorption spectra of 1 and 2.
the latter. The higher energy transition at 391 nm is due to the
styryl fragments,34 while that at 719 nm is the S0 - S1
transition of the Bodipy core with a clear vibronic sequence
(Dn = 1100 cmꢁ1) characteristic of the dipyrromethene frame-
work. The unusual transition observed at 462 nm is not present
in most of the styryl derivatives,34 but present when dimethyl-
amino residues are used for the construction of the dye, and
has been assigned to a charge transfer transition.35 Weak
fluorescence at 767 nm for 1 and 758 nm for 2 was detected with
an estimated quantum yield of around 3 to 4%. This fluorescence is
Fig. 9 Cyclic voltammetry of: (a) dye 1 with ferrocene at 0.38 V (blue
dashed line) and 2 (red continuous line); (b) dye 1 calibrated with ferrocene
and PC61BM (blue dashed line) and 2 with ferrocene and PC61BM (red
continuous line). Fc accounts for ferrocene and * for PC61BM.
independent of the excitation wavelength and in both cases the
excitation spectra perfectly match the absorption spectra, excluding
the presence of aggregates (Fig. 6). The thin-film (thickness of
about 50 nm) absorption spectra of 1 and 2 exhibit red-shifts, with
lmax = 766 nm and 760 nm, respectively (Fig. 7). The red-shift and
broadened absorption is possibly due to the p–p interaction in the
solid state as seen in the crystal packing. The broad thin-film
absorption of 1 and 2 which covers the photo-harvesting range
from UV to near-IR is promising for application in photovoltaic
devices. Optical constants for 1 and 2 thin films are shown in
Fig. 8. The extinction coefficients, k, are consistent with the
absorption spectra with high values from 300 nm to 800 nm.
Fig. 8 Optical constants (refractive index, n, and extinction coefficient, k)
of 1 and 2 thin films.
Table 1 Electrochemical data for relevant compoundsa
0
0
Optical-gapb (eV)
0
0
Dyes
E (ox, soln) (V), DE (mV)
E (red, soln) (V), DE (mV)
Electro-gap (eV)
1
+0.59 (70), +1.03 (70)
+0.59 (70), +1.03 (70)
ꢁ0.96 (70), ꢁ1.73 (irr.)
1.55
—
1.58
—
1 + PCBM*
ꢁ0.70 (60)*, ꢁ0.96 (70)*, ꢁ1.08 (70),
ꢁ1.59 (70)*, ꢁ1.80 (irr.)
2
+0.54 (60), +0.62 (70), +0.99 (60),
+0.54 (60), +0.62 (70), +0.99 (60),
ꢁ1.07 (60), ꢁ1.80 (irr.)
1.61
—
1.62
—
2 + PCBM*
ꢁ0.70 (60)*, ꢁ0.95 (70)*, ꢁ1.07 (60),
ꢁ1.58 (70)*, ꢁ1.78 (irr.)
a
Potentials determined by cyclic voltammetry in deoxygenated dichloromethane solution, containing 0.1 M TBAPF6, [electrochemical window
from +1.7 to ꢁ2.2 V], at a solute concentration of ca. 1.5 mM and at rt. Potentials were standardized versus ferrocene (Fc) as internal reference and
converted to the SCE scale assuming that E1/2 (Fc/Fc+) = +0.38 V (DEp = 60 mV) vs. SCE. Error in half-wave potentials is ꢃ10 mV. For irreversible
b
processes the peak potentials (Eap) are quoted. Determined from the absorption spectra from the onset with the tangent at the low energy side.
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3.4 Redox properties
Cyclic voltammetry was used to determine the HOMO/LUMO
levels of the dyes in solution, with PC61BM and ferrocene used
as calibrants. Ferrocene is used because its Fermi level in the
vacuum is well-known and PC61BM because this acceptor is
used in the real solar cell device. Owing to the low solubility of
PC71BM in dichloromethane, PC61BM was preferred for the
calibration. Under similar conditions but at low concentration
(about 10ꢁ4 M) and at 40 1C, PC71BM is easier to reduce than
PC61BM by 120 mV. Both dyes display a rich electrochemistry
induced by the presence of various electroactive modules
(Table 1, Fig. 9). For both dyes, two reversible oxidation waves
at about +0.60 V and +1.00 V were observed. The first of these
waves is dielectronic whereas the second process is monoelec-
tronic. No splitting of the waves was observed for dye 1 whereas
for the more bulky dye 2 the first oxidation wave was clearly
split into two waves at +0.54 and +0.62 V (Fig. 9a). In light of
previous electrochemical studies achieved with similar thienyl-
derivatives (D and E in Chart 1) engineered from styryl-
derivatives, it clearly appeared that the first oxidative redox
potential is assigned to one pyrrole-vinyl-thiophene arm while
the second potential (usual similar or very close to the first
potential) is assigned to the second pyrrole-vinyl-thiophene
arm. In the case of dissymmetric side-arms (e.g. F in Chart 1),
two distinguish potentials are clearly observed.19,36 We have
previously observed that the shifts of these redox potentials
reflect the differing degrees to which the principal donor
couples to the extended Bodipy unit.37 DFT calculations
indicate that both HOMO and HOMO(ꢁ1) are spread over
much of the molecule and include contributions from the
Bodipy and donor residues. As such, it is inaccurate to ascribe
Fig. 10 Current density–voltage characteristics (under AM 1.5G,
100 mW cmꢁ2 illumination) and EQE spectra (inset) of the 1:PC71BM devices.
Table 2 Performance parameters of devices
Device
VOC (V)
JSC (mA cmꢁ2
)
FF
PCE (%)
1:PC71BM (1 : 0.67)
1:PC71BM (1 : 1)
1:PC71BM (1 : 1.5)
0.72
0.72
0.71
1.3
3.2
5.5
0.26
0.28
0.31
0.24
0.65
1.2
2:PC71BM (1 : 0.67)
2:PC71BM (1 : 1)
2:PC71BM (1 : 1.5)
0.68
0.62
0.56
3.0
4.3
5.5
0.30
0.32
0.29
0.61
0.85
0.89
1:PC71BM (1 : 1.5)a
2:PC71BM (1 : 1.5)a
0.72
0.51
5.5
8.9
0.29
0.34
1.1
1.5
a
150 1C annealing for 10 min.
the first oxidation step as being localized on either the TPA or
the Bodipy unit.
In both cases, two reduction waves were observed at around
ꢁ1.00 V typical of radical anion formation on the Bodipy core
and a second irreversible wave observed at around ꢁ1.70 V was
assigned to the Bodipy dianion.38 Interestingly, when com-
pared with related dyes e.g. 3,5-vinylbisthienyl-Bodipy B19 and
as would be expected by the fact that the TPA fragment imports
Chart 1 Various thienyl-Bodipy derivatives extracted from ref. 18 (for D
and E) and 35 (for F).
Fig. 11 Current density–voltage characteristics (under AM 1.5G, 100 mW cmꢁ2
illumination) and EQE spectra (inset) of the 2:PC71BM devices.
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PC61BM (Fig. 9b). With the exception of the last reduction
potential, all redox processes were highly reversible (ipa/ipc
E
1) and exhibited the characteristic shape (DEp = 60–70 mV) of a
Nernstian process.
3.5 Solar cells and characterization
Ultraviolet photoelectron spectroscopy in air was utilized to
measure the energy levels of 1 and 2 in the thin-film state
(Fig. S1, ESI†). The HOMO levels of 1 and 2 were found to be
ꢁ5.14 eV and ꢁ5.02 eV, respectively. These values together with
the HOMO–LUMO gap estimated from the absorption spectra
were used to acquire the LUMO levels of the compounds
(ꢁ3.50 eV for 1 and ꢁ3.37 eV for 2). The LUMO levels of these
compounds are at least 0.2 eV higher than those of fullerenes
(ꢁ3.7 to ꢁ4.3 eV), ensuring efficient charge transfer between 1
and 2 with fullerenes under irradiation.
Fig. 12 Current density–voltage characteristics of the 1:PC71BM and
2:PC71BM devices after annealing for 10 min at 150 1C.
For photovoltaic characterization, we adopted the inverted
additional electronic density, the first oxidation potential in the structure utilizing ultra-thin 1 nm Ca as a cathode and MoO3 as
novel dyes was anodically shifted (easier oxidation) by 50 and a hole extraction layer. The structure has been used in our
100 mV respectively for 1 and 2. Whereas, the reduction previous Bodipy cells and showed promising performance.20c
potentials were anodically shifted (more difficult reduction) The optimized device structure was configured as follows: ITO/
by 70 mV and 180 mV respectively for 1 and 2. Note that Ca (1 nm)/1 or 2:PC71BM (B80 nm)/MoO3 (7 nm)/Ag (150 nm).
there is a significant driving force (about 0.25 eV) for photo- The details of optimization are provided in the experimental
induced electron transfer from the photoexcited dyes toward part. The active layers of the devices were spin-cast from a
Fig. 13 Carrier mobility of (a) 1:PC71BM, (b) 2:PC71BM BHJ without annealing and (c) 1:PC71BM, (d) 2:PC71BM BHJ after annealing for 10 min at 150 1C.
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chloroform solution of the mixture of Bodipy dyes and PC71BM. (NSC-102-2221-E-007-125-MY3, NSC-101-2112-M-007-017-MY3)
Fig. 10 shows the current density to voltage ( J–V) characteristics and for financial support of this work. Professor Jack Harrowfield
the corresponding external quantum efficiencies (EQEs) of the (ISIS in Strasbourg) is warmly acknowledged for a critical
1:PC71BM cells. There is negligible difference in the Voc value in reading of this manuscript prior to publication.
the cells with various donor to acceptor ratios (Table 2). However, the
Jsc and EQE values increased significantly with the increase of the
PC71BM ratio. The best cell with 1:PC71BM = 1 : 1.5 showed a Voc of
Notes and references
0.71 V, a Jsc of 5.5 mA cmꢁ2, a fill factor (FF) of 0.31 and an overall
PCE of 1.2%. Fig. 11 shows the J–V and EQE characteristics of the
2:PC71BM cells. The Voc, FF and PCE are lower than that of 1:PC71BM
devices. However, as shown in Fig. 12, interestingly, upon annealing
the devices at 150 1C for 10 min, the 1:PC71BM cell showed no
difference from the device without annealing, but Jsc and PCE were
considerably increased in the 2:PC71BM devices. This can be ratio-
nalized by the carrier mobility in the active layers. The carrier
transportation properties were investigated using the space-charge-
limited-current (SCLC) method. As shown in Fig. 13, the hole
mobilities in the BHJ were all increased after annealing. In the
2:PC71BM thin film, the electron and hole mobilities were initially
very unbalanced. Nevertheless, after thermal annealing, the hole
mobility was enhanced by 2 orders of magnitude and matched the
mobility of electrons. On the other hand, the 1:PC71BM thin film
exhibited balanced mobilities without thermal annealing. This may
explain why the performance of the 2:PC71BM devices increased
upon annealing while that of the 1:PC71BM cell remained constant.
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In short, we have synthesized new green Bodipy dyes incorporating
two thienyl-vinyl units bridging a Bodipy core with two 4-diphenyl-
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which does not require post-functionalization of the dyes. The X-ray
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