Synthesis and optical properties of a new triphenylamine-p-phenylenevinylene-small molecule with applications in high open-circuit voltage organic solar cells

Article abstract of DOI:10.1039/c5nj01525a

A new oligomer-type para-phenylenevinylene end-capped with triphenylamine groups (TPAPV) was synthesised and its optical properties thoroughly investigated. Fluorescence quenching studies showed that an important fraction of the photogenerated excitons fo

Full text of DOI:10.1039/c5nj01525a

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DOI: 10.1039/C5NJ01525A.
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ARTICLE
Synthesis and optical properties of a new triphenylamine-p-
phenylenevinylene-small molecule with applications in high open-
circuit voltage organic solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Rui Meira,a Pedro M. M. Costa,b Roberto E. Di Paolo,b Jorge Morgado, a,c Luís Alcácer,a João P.
Bastos,d David Cheyns,d Ana Charasa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new oligomer-type para-phenylenevinylene end-capped with triphenylamine groups (TPAPV) was synthesised and its
optical properties thoroughly investigated. Fluorescence quenching studies showed that an important fraction of the
photogenerated excitons formed in TPAPV is quenched by [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in blend
films, therefore indicating efficient exciton dissociation and potential applications in organic photovoltaic (PV) cells. Planar
heterojunction organic photovoltaic cells containing TPAPV as electron-donor were fabricated and devices with structure
ITO/TPAPV(6nm)/C60(50 nm)/BCP(10 nm)/Ag exhibited an open-circuit voltage (VOC), short-circuit current (JSC), Fill Factor
(FF), and power-conversion efficiency (PCE) of 0.92 V, 2.9 mA/cm2, 47.8 %, and 1.23 %, respectively.
investigating electroactive small-molecules-based devices have
driven numerous works and have resulted in device
performances up to values well-comparable with most
Introduction
Triarylamines have been considered one of the best building
blocks to design organic semiconductors for a significant
number of opto-electronic devices, such as organic light-
emitting diodes (OLEDs),1,2 organic field effect transistors,3,4
electrochromic devices,5 etc. Due to the combination of good
hole transport properties (µh = 10-2 – 10-3 cm2V-1s-1) via radical
cationic species arisen from the electron-donating nature of
the nitrogen center atom and high light-emitting efficiency
typically observed for the arylamine-based oligomers and
polymers, a wide variety of chemical structures containing the
triarylamine block has been studied as hole-transport layer
(HTL) in multilayer OLEDs,6-8 as sensitizers in dye-sensitized
solar cells,9 and as electron-donor in organic photovoltaic cells
(OPVs).10,11 In particular, the search for small molecular donors
for OPV devices has attracted significant efforts, since they do
not hold the drawbacks of polymers, namely batch-to-batch
variations and ill-defined chemical structures, which can affect
device reproducibility and efficiency. Also, conversely to
electroactive polymers, small-molecules, being structurally
well-defined and monodisperse, are ideal models to unravel
structure-properties relationships. Such benefits of
promising
ones
demonstrated
for
polymer-based
counterparts. In the particular case of OPVs, small-molecule-
based cells have surpassed already 8% in thermally evaporated
structures,12 and 9 % in power-conversion efficiency for
optimized solution-processed BHJ PV cells.13 The search for
improving PV cells performance has also encouraged studies to
understand the processes that govern the photophysical
behaviour of the electroactive materials. To this respect, small
molecules and oligomers are also the best candidates since
they can be model compounds for large molecular weight
materials.
Here, we present the synthesis of a new
π-conjugated
triphenylamine-containing small molecule composed of a
para-phenylene-vinylene trimer as the central unit (Scheme 1),
a study on its optical properties with emphasis on time-
resolved fluorescence analysis, and show applications in
organic photovoltaic devices with a planar heterojunction
architecture. Hole mobility of the new compound was also
measured through hole-only defices and thin field-effect
transistors (TFTs). Time-resolved fluorescence spectroscopy
has been shown to be an useful tool for investigating many of
the mechanisms involved in energy transfer processes in end-
capped oligophenylene-vinylene oligomers, such as solvent
relaxation and/or conformational relaxation of the conjugated
backbone,14 intrachain energy transfer in solutions,15 or
interchain energy transfer in solutions or films.16 We chose to
fabricate planar heterojunction OPVs, since for such
architecture, the device operation is not dependent on the
intricate morphology of the donor:acceptor blend as for BHJ
cells, and therefore they are good prototypes for investigating
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(16H, m), 0.99 (6H, t, J = 7.46 Hz), 0.92 (6H, t, J = 6.99 Hz). 13C
(ppm): 151.25, 136.97, 131.79,
materials individual performance. Furthermore, planar
heterojunction OPVs are attractive for being easy to fabricate NMR (100 MHz, CDCl3)
δ
127.92, 127.60, 126.69, 124.22, 121.09, 110.35, 71.79, 39.77,
through utilizing thermal evaporation to deposit the organic
materials from the gas phase, allowing precise control of layer
thicknesses. However, they often show power conversion
efficiencies (PCEs) lower than their BHJ counterparts (due to
an unfavorable balance between the optical absorption length
and the exciton diffusion length) but they potentially exhibit
high charge collection efficiencies owing to reduced
nongeminated (bimolecular) charge recombination rates.17
This loss mechanism refers to charge annihilation caused by
recombination of polarons of opposite charge meeting on their
way towards the electrodes and therefore is minimized in the
pure-layers (all donor and all acceptor) of planar
heterojunctions.
30.97, 29.28, 24.27, 23.13, 14.14, 11.35.
4,4’-[(2,5-(2-ethylhexyl)oxy-1,4-phenylene)di(E)ethene-2,1-
diyl]dibenzotriphenylamine (TPAPV) A flask charged with
4
(0.326 g, 0.47 mmol), and 4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)triphenylamine (0.382 g, 1.03 mmol) was
charged with Pd(PPh3)4 (4 %, mol) in a glove box with nitrogen
atmosphere. A degassed mixture of 7.0 ml of toluene and 4.5
ml of Et4NOH (20 % wt. in H2O) was added and the mixture
was refluxed under vigorous stirring for 12 h, under inert
atmosphere (N2). The reaction was quenched by adding water
and the organic layer was separated with CH2Cl2, washed with
brine, dried over anhydrous MgSO4, and concentrated under
reduced pressure. The product was purified by column
chromatography with SiO2 (70/30, v/v, CH2Cl2/Hexane) as a
Experimental
1
yellow solid (0.348 g). Yield: 73 %. H NMR (400 MHz, CD2Cl2),
Materials
δ
(ppm): 7.59 (8H, s); 7.56-7.52 (2H, d, J = 16.56 Hz); 7.54-7.52
(4H, d, J = 8.78 Hz); 7.30-7.26 (8H, m); 7.23-7.19 (2H, d, J =
16.56 Hz); 7.18 (2H, s); 7.13-7.12 (8H, d, J = 1.25 Hz); 7.11-7.10
(4H, d, J = 1.25 Hz); 7.06-7.02 (4H, tt, J = 7.28 and 1.13 Hz);
3.99-3.98 (4H, d, J = 5.52 Hz); 1.87-1.81 (2H, quint, J = 6.02 Hz);
1.65-1.33 (16H, m); 1.02-0.98 (6H, t, J = 7.53 Hz); 0.94-0.91
(6H, t, J = 7.03 Hz). 13C NMR (100 MHz, CDCl3): 206.37, 151.21,
147.64, 139.41, 136.39, 129.26, 128.08, 127.36, 126.83,
126.72, 126.65, 124.44, 123.76, 123.08, 123.01, 39.83, 30.93,
30.58, 29.26, 24.25, 23.13, 13.90, 11.10. MALDI-TOF MS
(matrix 2,5-DHB)), m/z (Da): 1025.54 (M+). Calcd. for
C74H76N2O2: 1025.43.
All reagents were purchased from commercial suppliers
(SIGMA-Aldrich and STREM) and used as received and without
further purification unless otherwise stated. All the solvents
used under inert atmosphere were dried by refluxing over a
suitable drying agent under nitrogen and deoxygenated before
use by freeze-pump-thaw cycling.
Synthesis
TPAPV was synthesised through a Suzuki Pd(0)-catalyzed cross-
coupling reaction between 4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)triphenylamine and
tetramethyl-1,3,2-dioxaborolan-2-yl)triphenylamine
prepared from 4-bromotriphenylamine following reported
procedures18 and
was prepared through a Wittig reaction
from 4-bromobenzaldehyde and 1,4-bis((2-ethylhexyl)oxy)-2,5-
xylenebis(triphenyl)phosphonium bromide ( ) using the same
1,4-bis((hexyl)oxy)-2,5-
4
(Scheme 1). 4-(4,4,5,5-
was
4
3
procedure
reported
for
xylenebis(triphenyl)phosphonium bromide (Scheme 2).19,20
4,4’-[(2,5-(2-ethylhexyl)oxy-1,4-phenylene)di(E)ethene-2,1-
diyl]dibromobenzene (4) 2.5 ml of a 0.8 M NaOEt/EtOH
solution, freshly prepared from 0.047 g (2.03 mmol) of Na in
2.5 ml of dried EtOH, were added dropwise to a solution of 3
(0.850 g, 0.81 mmol) and 4-bromobenzaldehyde (0.316 g, 1.71
mmol) in 10 ml of CH2Cl2, under N2. The mixture was stirred at
room temperature for 12 h. Then, 10 ml of water were added
and the organic layer was extracted with CH2Cl2, washed with
brine, dried over anhydrous Na2SO4, and concentrated under
reduced pressure. The product was purified by column
chromatography with SiO2 (97/3, v/v, CH2Cl2/EtOAc) as a
yellow solid (0.340 g). Yield: 60 %. 1H NMR (300 MHz, CDCl3),
δ
(ppm): 7.50-7.45 (4H, d, J = 8.88 Hz + 2H, d, J = 16.05), 7.39
(4H, d, J = 8.50 Hz), 7.11 (2H, s), 7.09 (2H, d, J = 16.05 Hz), 3.96
(4H, d, J = 5.48 Hz), 1.86-1.79 (2H, quint, J = 5.95 Hz), 1.62-1.30
Scheme 1. Synthetic route of TPAPV.
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Device fabrication
Thin film transistors of TPAPV were prepared by subliming
TPAPV under vacuum (ca. 2
10-6 mbar) over a 625 nm thick
×
dielectric layer of silicon dioxide (SiO2) treated with HMDS
(1,1,1,3,3,3-hexamethyldisilazane). Underneath this dielectric,
a highly-doped silicon layer acted as gate contact. Source and
drain contacts, made of 20 nm-thick molybdenum oxide
(MoO3) coated with a 40 nm thick aluminium layer, were
deposited over the TPAPV layer (110 nm thick) using a shadow
mask, defining a channel with a length of 100 micron and
width of 3.5 mm. Hole-only devices were prepared by
subliming TPAPV under vacuum over a glass/ITO substrate
coated with a PEDOT:PSS layer (40 nm thick). A top contact of
MoO3/Al was deposited defining pixel areas of 4 mm2. Organic
PV cells were fabricated with the planar heterojunction
architecture ITO(indium tin oxide)(140 nm)/TPAPV/C60/BCP(10
nm)/Ag (120 nm) by depositing the organic layers from vapor
Scheme 2. Synthetic route to 3: i) NaOHaq., 2-ethylhexylbromide, Hexane, TBABr;
reflux, 12 h, ii) HBr, HCHO, AcOH, 70 °C, 4h; iii) PPh3, DMF, 80 °C, 12 h.
Measurements
NMR spectra were recorded on a Brucker “Advance II”
spectrometer (300 MHz or 400 MHz) in CDCl3 or CD2Cl2. 1H and
13C NMR spectra are referenced to the residual solvent
impurity peak of the solvents. Multiplicities were abbreviated
as: singlet (s), doublet (d), triplet (t), quartet (q), quintet
(quint), multiplet (m), doublet of doublets (dd), doublet of
triplets (dt), triplet of doublets (td). Matrix-assisted laser
desorption ionization (MALDI) mass spectra were performed
in a high vacuum chamber (base pressure < 1
×
10-6 Torr). The
deposition rates for organic layers were kept between 0.1-1.2
Å/s. The active area of the devices was 13 mm2. Current–
voltage (I–V) characteristics were measured under inert
atmosphere (N2). Power conversion efficiencies (PCEs) of the
devices were measured with a solar simulator (Abet) with
simulated AM 1.5G illumination, calibrated using a Fraunhofer
certified cell at 100 mW/cm2.
on
a
Voyager-DE™ PRO Biospectrometry Workstation
analyzer (Applied Biosystems) by
MALDI/TOF/MS
LAREQUIMTE. Thermal analysis was performed by Differential
Scanning Calorimetry (DSC) in a 2920 Modulated DSC TA
Instruments. Cyclic voltammetry (CV) measurements were
performed in
a
Solartron potentiostat using 0.1
M
tetrabutylammonium
tetrafluorborate (TBABF4)/CH3CN
supporting electrolyte, at a scan rate of 50 mV/s. A saturated
calomel reference electrode (SCE) calibrated against ferrocene,
Fc/Fc+ (0.41 V), a platinum wire as counter electrode and a
platinum disk as working electrode were used. The compound
TPAPV was drop cast from toluene solutions onto the working
electrode. As the energy level of Fc/Fc+ is at 4.80 eV below the
Results and Discussion
Synthesis
The isomer E,E of TPAPV was isolated in good yield (73 %)
although other geometric isomers were formed according to
NMR-1H spectra for the mixture. The (E,E) trimer was purified
by chromatography in SiO2 and its structure was confirmed by
the presence of two doublets in the NMR-1H spectrum
vacuum level we calculate HOMO (eV) =
− (Eonset,ox (eV) + 4.41)
and LUMO (eV) = - (Eonset,red (eV) + 4.39).21 UV/Visible
absorption spectra were recorded in a Beckman DU-70
spectrophotometer. Photoluminescence spectra were
measured on a SPEX Fluorolog 212I spectrophotometer, at the
right angle geometry, S/R mode, and corrected for
instrumental wavelength dependence, optics and detector
wavelength dependence. Photoluminescence decays were
measured using the picosecond time-correlated single-photon
counting (TCSPC) technique, and the obtained fluorescence
decays analyzed as described in ref. 22. A mode-locked
Ti:sapphire Tsunami laser was used as a pulsed excitation
source. Measurements were carried out by exciting the
samples with vertically polarized light and setting the emission
polarizer at the magic angle. Sample signals were collected
until 5000 counts had been accumulated at the maximum of
the decay.
1
assigned to the four H of the vinylene groups, at 7.56-7.52
and 7.23-7.19 ppm with J = 16.56 Hz. Thermal analysis of
TPAPV (DSC) at a heating rate of 10 K/min in the range of 0-
350 °C shows a well-defined melting peak at 183.8 °C and do
not show exothermal transitions which could indicate thermal
degradation under that temperature range (Figure S1 in
Supplementary Info).
Electrochemical properties
TPAPV electrochemical properties were characterised through
cyclic voltammetry and optical properties were characterised
by UV−Vis absorption and fluorescence spectroscopy. The
HOMO and LUMO energies were estimated from the onset
values measured for the oxidation (p-doping) and reduction (n-
doping) processes, respectively, by cyclic voltammetry, and
compared with values calculated for the optimized ground
state geometric structure for TPAPV in the gas phase
determined by quantum-chemical methods using density
functional theory (DFT). The obtained voltammograms are
shown in Figure 1. As the anodic and cathodic peaks are not
fully reversible, fresh films were used for the oxidation
The optimized ground state geometric structures and frontier
levels (HOMO and LUMO) for the molecule TPAPV in the gas
phase were determined by density functional theory (DFT,
B3LYP method with 6-31G* basis set) using
a
SPARTAN software (Wave function, Inc.). Atomic Force
Microscopy (AFM) analysis were performed on
NanoObserver from Concept Scientific Instruments.
a
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(anodic) and for the reduction (cathodic) scans. The estimates solid films, where in these last packing and ordering effects
for HOMO and LUMO values are -5.26 eV and -2.54 eV, should lead to -electron delocalization over neighbour
π
respectively. The HOMO energy is higher than values found for molecular orbitals. To better elucidate the optical properties-
conjugated polymers based on the triphenylamine monomer structure relationships, we have further investigated the
units which range at -5.6 – -5.8 eV.23 This is attributed to the photophysical properties of TPAPV.
strong contribution of the central block of three phenylene
rings for the HOMO, as predicted by DFT calculations (Figure Photophysical studies
2).
Figure 3 shows the absorption and fluorescence spectra of
TPAPV in dilute toluene solution and in solid film spin cast
from concentrated toluene solutions (40 mg/ml). The
fluorescence spectrum in solution exhibits vibrational
resolution similar to that observed for other related
phenylenevinylene trimers end-capped with carbonyl groups
instead of triphenylamines.22 This is in accordance with
previous works, showing that the first excited singlet state of
phenylenevinylene (PV) trimers is not significantly affected by
the incorporation of triphenylamine-type end-caps. The
fluorescence quantum yield measured for TPAPV in toluene at
293 K was 0.70 ± 0.01, also very similar to that for the
aforementioned PV trimers without TPA end-groups (0.71-
0.72).16,24
The absorption spectrum in film is broadened relative to the
one in solution, this being probably associated to the presence
of a greater variety of conformational species in film with
Figure 1. Cyclic voltammograms of TPAPV in film deposited over the Pt disc working
electrode.
more planar conformations (larger effective conjugation
lengths) absorbing at longer wavelengths. The emission
spectrum is red-shifted with respect to the solution spectrum,
as typically found for conjugated compounds, this being
attributed to solid-state interactions and effects of molecular
ordering in the solid state.
Figure 3. Normalized absorption and fluorescence spectra of TPAPV in toluene and in
solid film, at room temperature. The emission spectra were obtained with excitation at
415 nm.
Figure 2. Optimized ground-state (S0) geometric structure of TPAPV and respective
HOMO and LUMO wavefunctions representations as determined at the B3LYP/6-31G*
level of DFT.
Unlike HOMO, the LUMO is more delocalized over the C=C
backbones of the central trimer and shows little contribution
of triphenylamine end-caps. The HOMO and LUMO energies
calculated by DFT, being -4.78 eV and -1.81 eV, respectively,
are both higher than those estimated by cyclic voltammetry
for TPAPV in solid films. This shift should be mainly related to
Solvent effect In order to investigate the variation of electronic
distribution from the ground state to the first excited state, a
systematic study of the absorption and fluorescence spectra of
TPAPV in solvents of different polarizabilities was carried out.
The changes in Stokes shifts with the solvent polarizability
the orbitals energy differences between the gas phase and provides information on the solvent reorganization around the
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excited state and on variations on charge distribution between function of the solvent polarizability was close to zero, indicating a
the electronic states involved in the transitions.25 We found small dipole moment change (< 1 Debye) upon photoexcitation and
that the solvent effects are less pronounced in the absorption therefore, a negligible solvent reorganization around the excited
spectra than in the fluorescence spectra (Table 1), although state should be expected. Hence, we conclude that the differences
both spectra show clear sensitivity to the solvent. Also, the between the electronic distribution of the HOMO and the LUMO
emission spectra show a progressive red-shift and reduction of orbitals, calculated by DFT, should not affect significantly the dipole
the vibronic structure with increasing solvent polarity (Figure 4 moment variation nor the solvent reorganization upon photo-
and Table 1). Thus, using a Lippert-Mattaga plot,24 we found that excitation.
the slope of the linear regression used to fit the Stokes shifts as a
Table 1. Maximum absorption and emission wavelengths of TPAPV in different solvents.
b
nD
Solvent
εa
ηc
(cP)
λabs
(nm)
λem
(nm)
*
Stokes Shift
(nm)
DMSO
DMF
47.2
36.7
21.0
7.68
4.81
2.38
2.05
2.03
2.02
2.01
1.88
1.48
1.43
1.36
1.41
1.48
1.50
1.44
1.44
1.42
1.44
1.38
1.991
0.802
2.415
0.575
0.580
0.586
3.351
2.342
0.734
1.508
0.313
425
422
417
418
418
418
416
415
411
413
410
481
475
468
471
474
471
466
465
464
464
462
56
53
51
53
56
53
50
50
53
51
52
Acetone
HF
Chloroform
Toluene
Hexadecane
Tetradecane
MCH
Undecane
Hexane
aDieletric constant (pure solvent), bRefractive Index (pure solvent), cViscosity (pure solvent), * Maximum emission wavelength of the first vibronic band, identified
through spectra deconvolution with Gaussian functions.
contribution to the fluorescence decays. Therefore, the longer
time is assigned to the lifetime of the compound. The shorter
time appears as a decay-time at the beginning of the emission
band (positive pre-exponential coefficient) and as a rise-time
at the lower energy part of the band (negative coefficients).
Similar short components have been observed in other
conjugated systems and various explanations have been given,
in particular associated to intrachain or interchain energy
transfer, solvent relaxation, and conformational relaxation.26,27
Figure 4. Normalized fluorescence spectra of TPAPV in different solvents, at room
temperature (λexc. = 415 nm).
Time-resolved fluorescence studies Time-resolved fluorescence
measurements with picosecond time resolution of TPAPV were
performed by collecting the fluorescence intensity at the
beginning (450 nm), at close to the maximum (490 nm), and at
the lower energy side (540 nm) of the emission band, in two
solvents, toluene and methylcyclohexane (MCH). Figure 5
shows the fluorescence decays obtained in MCH. For each
solvent, the simultaneous analysis of the three decays were
fitted to double exponential functions, showing a short
component (toluene: 14 ps, MCH: 26 ps) and a long
component (toluene: 936 ps, MCH: 926 ps). The pre-
exponential coefficients of the longer decay components are
Figure 5. Fluorescence decays of TPAPV in MCH at 293 K, measured with excitation at
positive at the three wavelengths and represent the largest 425 nm and 6.13 ps/channel. From the simultaneous fit of the three decays, the times
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and pre-exponential coefficients at each emission wavelength are obtained. Weighted
residuals and autocorrelation functions (AC) are represented together with χ2 values of
the fits. The time resolution of the experimental set-up is ca. 3 ps.
it may perform as electron-acceptor for TPAPV and cause
exciton dissociation. Regarding the neat film, the average
lifetime for the first excited state is 464 ps (0.69×595 ps +
0.31 172 ps). In comparison, for the blend film, the average
×
Photophysical studies of TPAPV in blend films with PC61BM
To anticipate potential applications of TPAPV in OPV cells as
electron-donor, excited state-related processes of TPAPV in
solid films, prepared from blends with [6,6]-phenyl-C61-butyric
acid methyl ester (PC61BM) were investigated. For a better
understanding of films of blends behaviour, neat films of
TPAPV were also studied. For neat films, we found that, when
exciting at 415 nm, a strong reabsorption of the first vibronic
emission band occurs, as inferred from the observed relative
lifetime is 45 ps, i.e. an order of magnitude lower. This means
that an important fraction of the photogenerated excitons
formed in TPAPV is quenched by PC61BM in blend films. We
therefore conclude that, similarly, TPAPV may perform as an
efficient electron-donor in heterojunction PV cells with the
fullerene PC61BM as electron-acceptor.
Hole mobility studies
reduction in intensity of this vibronic band when compared The hole mobility of TPAPV was determined using two
with that of the second vibronic peak, at 523 nm (Figure 3). On different device structures: a thin film transistor (TFT) and a
the other hand, in the low energy region of the emission band, hole-only device fabricated as described. From the transfer
where the reabsorption is absent, the red shift of the spectrum curve measured for TFTs in the saturation regime (Vsd=-100 V,
is clear, which should be related with the emission of species Vg down to -100 V), a field-effect hole mobility of 1.3×
10-5
of more extended effective conjugated lengths. We also note cm2/V.s was obtained. The hole mobility extracted from hole-
that photoexcitations at different wavelengths, ranging at the only devices was obtained by fitting the current-voltage curves
absorption maximum and at the low energy edge of the to the equation:
absorption band, do not induce significant changes in the
emission spectrum. Time-resolved fluorescence measurements
9
8
(ꢇ − ꢇ )ꢊ
ꢈꢉ
carried out for films of TPAPV with PC61BM (blend films
prepared from 10:1 TPAPV:PC61BM solutions) and without
PC61BM (neat films) were compared. The fluorescence decays
were measured at 293 K, with excitation at 410 nm and
collecting the emission at 550 nm. Since PC61BM does not
show emission at 550 nm, only TPAPV contributes for the
collected fluorescence. Therefore, fluorescence extinction at
this wavelength is only due to quenching of photogenerated
excitons in TPAPV. The measurements for the fluorescence
decays in neat films were fitted to a sum of two exponential
ꢀꢁꢂꢃꢂ
=
εꢄεꢅꢀꢆ
ꢋꢌ
in the space-charge limited current (SCLC) regime.31 Assuming
r = 3 (a typical value for organic conjugated systems)32 and L=
110 nm (thickness of the TPAPV layer), ꢀh was determined as
4.0
10-6 cm2/V.s. The difference between the two values of ꢀh
is attributed to the sample morphology as the measured
mobility in the hole-only device is normal to the substrate and
in the TFT device the transport is parallel to the dielectric
substrate surface. Since chain orientation and packing on
surfaces treated with organic moieties is different from the
bulk33 (the surface of TFT was treated with HMDS as described
in Experimental) charge mobility along the substrate surface is
expected to be different from that perpendicular to the
substrate.
ε
×
terms, with two decay times, τ1 = 595 ps and τ2 = 172 ps, which
are well different from those found for TPAPV in toluene
solution (936 ps). The pre-exponential coefficients of the decay
times in films are: 0.69 (τ1) and 0.31 (τ2). This type of variance
in excited state dynamics from solution to film has been
reported for various conjugated polymers and oligomers, and
has been attributed to a combination of photophysical
processes involving, for example, interchain and non-radiative
energy transfer mechanisms.27,28 The fluorescence decays and
corresponding fitting curves obtained for films of TPAPV are
shown in Supplementary Information (Figure S1). In blend
films of TPAPV:PC61BM, prepared from spin casting solutions in
toluene, the fluorescence decays required a fit of a sum of
three exponential terms, τ1 = 404 ps (pre-exponential
coefficient: 0.04), τ2 = 112 ps (0.17) and τ3 = 12 ps (0.79), to
obtain a good fitting. While the two first decay times compare
well with those observed in the neat film, a new short time
appears (12 ps), similar to that found in toluene solution. Such
short time cannot be assigned to conformational relaxation,
since this is unlikely to occur in solid films. Thus, we assign this
component to the dissociation process of the photogenerated
excitons in TPAPV phase due to the presence of the electron-
acceptor PC61BM. Since the HOMO and LUMO levels for
PC61BM are estimated as -6.1 eV and -3.75 eV,29,30 respectively,
Applications in planar heterojunction PV cells
To investigate the performance of TPAPV in OPV cells, planar
heterojunction cells were fabricated using fullerene C60 (Nano-
C) as the electron-acceptor. As mentioned above, we chose to
fabricate planar heterojunction architectures since cells
performance reflects more clearly materials characteristics.
The device structure is ITO (140 nm)/TPAPV/C60/BCP (10
nm)/Ag (120 nm), where ITO is Indium Tin Oxide and BCP
represents a thermally evaporated thin layer of bathocopruine
(Aldrich) as buffer layer. The thicknesses of TPAPV and C60
were varied to assess the cell performance dependence on the
respective active layer thickness. The TPAPV layer was firstly
varied from 3 to 18 nm, while keeping the C60 layer at 50 nm,
then the fullerene C60 layer was varied for the best performing
TPAPV thickness. The topography of the TPAPV layer in the
devices, investigated by AFM, showed a relatively flat surface
with a Rrms of ca. 0.31 nm (Figure S3 in Supplementary Info).
6 | J. Name., 2012, 00, 1-3
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Table 2 shows the devices performance and Figure 6 shows the studies performed for TPAPV:PC61BM blends showed
J-V curves measured for the cells.
encouraging results.
Table 2. Device characteristics under simulated AM1.5G solar illumination with an
intensity of 100 mW/cm2.
Conclusions
Donor/acceptor
JSC
VOC
FF
PCE
(%)
A
new small molecule containing an all-trans p-
(mA/cm2)
(V)
(%)
phenylenevinylene core end-capped with triphenylamine
groups (TPAPV) was synthesised through a Suzuki coupling
reaction with a good yield (73 %). Time-resolved fluorescence
studies performed on TPAPV:PC61BM blend films
demonstrated strong fluorescence quenching of excitons
photogenerated in TPAPV (decrease in one order of magnitude
of first excited state lifetime) and therefore suggest that
TPAPV may find applications in heterojunction PV cells with
PC61BM as the acceptor component. Planar heterojunction PV
cells fabricated using TPAPV and fullerene C60 showed VOC
values of 0.91-0.95 V which are among the highest for small-
molecule organic cells.
TPAPV (3 nm)/C60 (50 nm)
TPAPV (6 nm)/C60 (50 nm)
TPAPV (12 nm)/C60 (50 nm)
TPAPV (18 nm)/C60 (50 nm)
TPAPV (6 nm)/C60 (40 nm)
TPAPV (6 nm)/C60 (30 nm)
2.88
2.89
2.80
2.64
2.88
2.70
0.92
0.92
0.92
0.95
0.91
0.93
47.7
47.8
46.9
43.4
46.5
46.5
1.21
1.23
1.17
1.05
1.18
1.12
Acknowledgements
This work was supported by Fundação para a Ciência e
Tecnologia (Portugal), FCT, under the projects PTDC/CTM-
NAN/1471/2012, UID/EEA/50008/2013, UID/QUI/00100/2013,
and M-ERA.NET/0001/2012. The authors thank The National
NMR Network IST-UL (contract RECI/QEQ-QIN/0189/2012).
Figure 6. Current density–voltage (J–V) curves measured for planar heterojunction cells
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Fluorescence quenching studies enabled to preview potential applications of a new small
molecule as electron-donor in organic solar cells. Results of planar heterojunction cells are also
shown.