A rhodanine flanked nonfullerene acceptor for solution-processed organic photovoltaics

Article abstract of DOI:10.1021/ja5110602

A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a nonfullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing poly(3-hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave power conversion efficiencies (PCE) up to 4.1%. Transient and steady state optical spectroscopies indicated efficient, ultrafast charge generation and efficient photocurrent generation from both donor and acceptor. Ultrafast transient absorption spectroscopy was used to investigate polaron generation efficiency as well as recombination dynamics. It was determined that the P3HT:FBR blend is highly intermixed, leading to increased charge generation relative to comparative devices with P3HT:PC₆₀BM, but also faster recombination due to a nonideal morphology in which, in contrast to P3HT:PC₆₀BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better performance than P3HT:PC₆₀BM devices, used as control, demonstrating that this acceptor shows great promise for further optimization.

Full text of DOI:10.1021/ja5110602

Article
pubs.acs.org/JACS
A Rhodanine Flanked Nonfullerene Acceptor for Solution-Processed
Organic Photovoltaics
Sarah Holliday,*,† Raja Shahid Ashraf,† Christian B. Nielsen,† Mindaugas Kirkus,† Jason A. Rohr,‡,†
̈
Ching-Hong Tan,† Elisa Collado-Fregoso,† Astrid-Caroline Knall,† James R. Durrant,†,§ Jenny Nelson,‡
and Iain McCulloch†,∥
†Department of Chemistry and Centre for Plastic Electronics and ‡Department of Physics and Centre for Plastic Electronics, Imperial
College London, London SW7 2AZ, United Kingdom
§SPECIFIC IKC, Swansea University, Baglan Bay Innovation Centre, Port Talbot, Swansea SA12 7AX, United Kingdom
∥Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
23955−6900
S
* Supporting Information
ABSTRACT: A novel small molecule, FBR, bearing 3-ethylrhodanine
flanking groups was synthesized as a nonfullerene electron acceptor for
solution-processed bulk heterojunction organic photovoltaics (OPV). A
straightforward synthesis route was employed, offering the potential for large
scale preparation of this material. Inverted OPV devices employing poly(3-
hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave
power conversion efficiencies (PCE) up to 4.1%. Transient and steady state
optical spectroscopies indicated efficient, ultrafast charge generation and
efficient photocurrent generation from both donor and acceptor. Ultrafast
transient absorption spectroscopy was used to investigate polaron generation
efficiency as well as recombination dynamics. It was determined that the
P3HT:FBR blend is highly intermixed, leading to increased charge generation
relative to comparative devices with P3HT:PC60BM, but also faster recombination due to a nonideal morphology in which, in
contrast to P3HT:PC60BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that
prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better
performance than P3HT:PC60BM devices, used as control, demonstrating that this acceptor shows great promise for further
optimization.
device failure; (iii) high synthetic costs; and (iv) limited scope
for synthetic control over electronic and structural properties.5,6
For these reasons, research has recently focused on the
development of acceptor materials that can be used to replace
fullerenes. The most common design involves small molecules
with electron deficient units: perylene diimide,7−10 diketopyr-
rolopyrrole,11−13 benzothiadiazole,14−16 and dicyanovinyl17−19
being common structural templates. Numerous solution-
processable, synthetically versatile and highly absorbing
materials have been reported, but the performance in devices
generally lags behind that of the fullerenes. Very few
nonfullerene acceptors give power conversion efficiencies over
3%, even with high performance donor polymers, and fewer still
can exceed the comparison device with fullerene.20−23
Furthermore, synthesis of these materials is frequently lengthy
and low yielding, and thus offers little advantage over fullerenes
in terms of synthetic scalability, an important aim for the
production of cheap and printable solar cells.
INTRODUCTION
■
Since the introduction of fullerenes to the field of organic
photovoltaics (OPV) in the early 1990s, C60 and C70 derivatives
such as phenyl-C60/70 butyric acid methyl ester (PC60/70BM)
have been the prevailing electron acceptor materials for bulk
heterojunction OPV devices.1,2 The long-lived success of
fullerenes arises from their unrivaled electron acceptor
properties. These properties include (i) high electron mobility;
(ii) delocalization of the lowest unoccupied molecular orbital
(LUMO) over the whole surface of the molecule to allow
three-dimensional electron transfer and transport; (iii)
reversible electrochemical reduction to yield stable reduced
charged species; and (iv) formation of domains from solution
deposition that are on the appropriate length-scale for charge
separation, with both pure and mixed phases created.3,4 Despite
these advantageous properties, however, fullerene acceptors
have some limitations. These include (i) weak absorption and
poor tunability of absorption over the range of intense regions
of the solar spectrum, limiting their contribution to the
photocurrent; (ii) morphological instability in thin film blends
over time, leading to macroscopic crystallite formation and
Received: November 4, 2014
© XXXX American Chemical Society
A
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The small molecule FBR (Scheme 1) was designed to be
synthetically simple and versatile, with wide scope to modify
aryl dialdehyde intermediate 3. Subsequent Knoevenagel
condensation with 3-ethylrhodanine yielded the product FBR.
The material is thermally stable up to ca. 300 °C as confirmed
by thermogravimetric analysis (see the Supporting Informa-
tion) and soluble in common organic solvents such as
chloroform and toluene. Although the synthesis is reported
on a small (laboratory) scale, all of the precursor materials are
relatively inexpensive and the reactions are sufficiently
straightforward that larger scale production should be easily
possible. This demonstrates an important advantage over
fullerenes, as well as many nonfullerene acceptors for which
multistep syntheses are often required.
Scheme 1. Synthesis of Rhodanine Flanked Acceptor FBR
Density functional theory (DFT) calculations were carried
out on the molecular structure using Gaussian at the B3LYP/6-
31G* level of theory. Figure 1 illustrates that the highest
the structural, electronic, and morphological properties through
chemical design. Fluorene was chosen as a rigid aromatic core
to facilitate transport and preserve conjugation along the length
of the molecule, which allows versatile energy level tuning as
well as offering the potential to vary the pendant alkyl chains to
tune the solubility and crystallinity. Incorporation of
benzothiadiazole extends the conjugation, thus improving
charge transport, as well as affording electron deficient
character to the outside of the molecule. To cap the ends of
the molecule, 3-ethylrhodanine was chosen as an electron
withdrawing flanking group. Rhodanine derivatives are
commonly used in dye chemistry for the creation of strong
push−pull chromophores,24−26 and recently in the field of OPV
there have been several high performance small molecule
donors published bearing rhodanine groups.27−30 There are
very few reports, however, of rhodanine units being used in the
capacity of the acceptor material.31 As well as affording electron
withdrawing character to the periphery of the molecule via its
ketone and thioketone groups, rhodanine allows for the further
control over solubility by altering the imide alkyl chain. In this
paper, the ethyl group is employed in order to inhibit the
hydrogen bonding associated with the unalkylated rhodanine.
This increases the solubility in common organic solvents while
avoiding overly bulky alkyl groups on the periphery of the
molecule. In order to facilitate electron transfer, the molecule
was designed to have solubilizing alkyl chains on the electron-
rich center of the molecule in order to leave the outer, electron-
withdrawing portion of the molecule sterically available as is the
case in fullerene molecules.
Figure 1. Minimum energy conformation of FBR (with methyl
replacing octyl groups) calculated using Gaussian (B3LYP/6-31G*) to
visualize the LUMO and HOMO distribution.
occupied molecular orbital (HOMO) of FBR is delocalized
over the whole molecule, while the lowest unoccupied
molecular orbital (LUMO) is concentrated onto the more
electron deficient periphery. It is expected that this large
electron-accepting area on the outer, sterically exposed part of
the molecule will benefit charge transfer. Meanwhile the
molecular oscillator strength, and thus the extinction
coefficient, should benefit from the extent of overlap between
HOMO and LUMO.
UV−vis absorption spectroscopy (Figure 2) reveals a
maximum absorption of FBR at 488 nm in solution and 509
nm in the thin film. Compared with the maximum absorption
of PC60BM (ca. 300 nm), FBR absorbs in a region of the
RESULTS AND DISCUSSION
■
Materials Synthesis and Optoelectronic Properties.
The synthesis of FBR is documented in the Supporting
Information and summarized in Scheme 1. Suzuki coupling of
9,9-dioctylfluorene-2,7-diboronic acid pinacol ester 1 with
bromo-2,1,3-benzothiadiazole-4-carboxaldehyde 2 afforded the
Figure 2. Normalized UV−vis absorption spectra of FBR in
chloroform solution compared with the thin film absorption (spun
from chloroform solution) of FBR and PC60BM.
B
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Table 1. Optoelectronic Properties of FBR and PC60BM
a
b
a
c
c
c
λmax (nm)
ε (104 M−1 cm−1
)
Egopt (eV)
Egelec (eV)
HOMO (eV)
LUMO (eV)
FBR
509
300
3.46
0.49
2.14
2.14
2.06
−5.70
−5.87
−3.57
−3.84
PC60BM
1.75
b
a
c
Measured in thin film, spin-cast from 5 mg/mL chloroform solution. Measured in chloroform solution. Measured by cyclic voltammetry in
dichloromethane solution with tetrabutylammonium hexafluorophosphate electrolyte.
spectrum with much higher solar flux32 which offers a great
advantage in terms of contributing to the photocurrent via
absorption. Figure 2 shows a normalized UV−vis absorption
spectra, while Table 1 shows that the extinction coefficient of
FBR at its maximum absorption wavelength, measured in
solution, is 1 order of magnitude greater than that of PC60BM
at its maximum absorption wavelength in the visible region.
Due to the high degree of symmetry of PC60BM, many of the
low energy transitions are forbidden which severely limits
absorption in the visible region.33,34 This further demonstrates
the capacity for this small molecule to yield a greater
photocurrent than C60 fullerene acceptors.
0.2 eV.36 FBR fulfills this requirement for wide bandgap
polymers such as poly(3-hexylthiophene) (P3HT), with an
improved Voc expected relative to PC60BM due to the higher
lying LUMO.
Structural Properties. To study the structural properties of
the new acceptor, differential scanning calorimetry (DSC) and
X-ray diffraction (XRD) were performed on the neat material,
the results of which are shown in Figure 4. On the first heating
The electrochemical behavior of the acceptors was studied by
cyclic voltammetry (CV) in dichloromethane solution. The
reduction cycle of FBR is shown in Figure 3 in comparison with
Figure 4. (a) DSC thermogram of FBR upon first and second heating
and cooling cycles, measured at 10 °C/min under nitrogen. (b) XRD
of neat FBR film dropcast from chlorobenzene, as cast (blue) and
annealed (black) at 110 °C for 15 min in air.
cycle by DSC (Figure 4a), a sharp melting endothermic
transition is observed at around 200 °C which signifies some
degree of molecular order within the bulk material. A broad
shoulder is present in the onset of the melt, indicating the
possible presence of conformational polymorphs. After melting,
the material then appears to become kinetically trapped in the
amorphous phase, with no recrystallization observed upon
cooling and a featureless second heating and cooling cycle
consistent with an amorphous material. XRD data supports the
observation that dropcast films of FBR are amorphous even
after annealing, given that there are no reflections consistent
with crystallinity on the length scale of the measurement’s
accuracy.
The amorphous nature of FBR can be understood with
reference to the molecular structure of FBR as modeled using
Gaussian (B3LYP/6-31G*). A large dihedral angle of 35° is
calculated between the fluorene core and the benzothiadiazole
unit as shown in Figure 5, giving the molecule a nonplanar 3D
structure overall. It may be expected that this nonplanar nature
would act favorably to prevent the growth of large acceptor
domains. One of the major problems with nonfullerene
acceptor units such as perylene diimides is the strong tendency
to crystallize. This leads to difficulties in controlling the
nanostructure of the bulk heterojunction which can result in
self-trapping of excitons and, ultimately, poor device perform-
ance. There is increasing evidence that nonplanar acceptors
perform better than their planar counterparts for this
reason.9,10,37,38 Furthermore, the twisted nature of FBR should
hypothetically provide the potential for charge transport in
more than one dimension, making it more analogous to the
fullerenes with their relatively isotropic transport.
Figure 3. First reduction cycles of FBR and PC60BM measured by
cyclic voltammetry in dichloromethane solution with tetrabutylammo-
nium hexafluorophosphate electrolyte.
that of PC60BM. Within the reduction limits of the solvent,
FBR undergoes at least two reversible reduction events.
Reversibility demonstrates that the reduced species is not
electrochemically unstable which is important for the opera-
tional stability of devices, while the presence of more than one
reduction event shows that there are multiple low-lying excited
states, a property which has previously been observed to
facilitate charge separation.35 The HOMO and LUMO values
were calculated from the onset voltages of oxidation and
reduction, respectively, as presented in Table 1. It may be noted
that the electrochemical bandgap measured by these methods
corresponds well with the optical bandgap measured from the
onset of absorption in the thin film. The LUMO energy of FBR
(−3.57 eV) is relatively low due to the presence of strong
electron withdrawing units, but higher lying than that of
PC60BM (−3.84 eV). In order to maximize the achievable Voc,
it is desirable that the LUMO level of the acceptor is as high-
lying as possible while still allowing for electron transfer from
the donor. It is generally assumed that the minimum LUMO−
LUMO offset required for efficient electron transfer is at least
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PC60BM as the acceptor, measured under simulated AM1.5G
illumination at 100 mW cm−2. The photovoltaic device
properties are summarized in Table 2. The FBR device
Table 2. Summarized Photovoltaic Characteristics of
P3HT:acceptor (1:1) Devices
Jsc (mA cm−2
)
Voc (V)
FF
PCE (%)
P3HT:FBR
7.95
9.07
0.82
0.59
0.63
0.66
4.11
3.53
P3HT:PC60BM
produces a lower short circuit current (7.95 mA cm−2) than the
fullerene device, which may be partially accounted for by the
difference in thickness between devices. A range of thicknesses
were evaluated, with an optimal thickness of 80 nm found for
the P3HT:FBR active layers. The best P3HT:PC60BM devices
were thicker with active layers of ca. 150 nm which is one
reason for the higher photocurrent. The lower Jsc of the
P3HT:FBR device is, however, offset by the improved open
circuit voltage (0.82 V) which is attributed to the higher lying
LUMO of FBR relative to PC60BM. This results in a PCE of
4.11%, which is currently among the highest efficiencies for
nonfullerene based devices with P3HT. It also clearly
outperforms the comparison PC60BM cell with PCE of 3.53%.
From Figure 6b, it can be seen that the maximum EQE
intensity for P3HT:FBR devices is 65% at ca. 500 nm, but the
EQE profile illustrates how the largely overlapping absorption
profiles of polymer and acceptor limit the amount of
photocurrent that can be harvested across the spectrum. It is
anticipated that by replacing P3HT with a polymer of
complementary absorption to FBR, a greater photocurrent
could be obtained, offering the possibility for even higher
device efficiencies with this acceptor. It is noted that the EQE
of the P3HT:PC60BM device is higher even at 510 nm, where
both P3HT and FBR have their peak absorption. This is likely
due to the increased thickness of the P3HT:PC60BM films, as
well as the greater degree of crystallinity in the P3HT as these
devices were annealed for longer and at a higher temperature.
Charge Separation and Recombination Dynamics.
Photoluminescence quenching (PLQ) experiments were
carried out on P3HT:FBR and P3HT:PC60BM blend films as
well as neat films, as shown in Figures 7 and Supporting
Figure 5. Dihedral planes of FBR from the minimum energy
conformations calculated using Gaussian (B3LYP/6-31G*).
OPV Device Fabrication and Evaluation. To evaluate the
potential of the rhodanine flanked acceptor in bulk
heterojunction solar cells, devices were prepared with P3HT
as the donor polymer. As well as being one of the most
thoroughly studied and understood polymers in the field of
organic photovoltaics, P3HT has the potential to be
39
synthesized via low cost and high throughput methods,
making it a promising donor partner. Furthermore,
P3HT:PC60BM blends provide a good benchmark for
comparison of device data. An inverted architecture was chosen
for the devices due to the improved environmental stability and
ease of fabrication over conventional architectures.40,41 Devices
were fabricated with the configuration glass/ITO/ZnO/
P3HT:acceptor (1:1)/MoO3/Ag. The active layers were
deposited by spin-coating from CHCl3:o-DCB (4:1) under
ambient conditions, while annealing (15 min at 110 °C),
electrode deposition, and device testing were carried out in an
inert environment. For comparison, P3HT:PC60BM devices
were fabricated in parallel, according to previously optimized
procedures. In comparison to P3HT:PC60BM, optimum
performance for P3HT:FBR devices was obtained for thinner
photoactive layer and more modest thermal annealing. Full
device fabrication details, along with solvent concentrations and
blend ratio optimization details, are given in the Supporting
Information. Figure 6 shows the current density−voltage (J−V)
characteristics for devices with P3HT as the donor and FBR/
Figure 7. Photoluminescence quenching efficiency of P3HT in (a)
P3HT:FBR blend and annealed P3HT:FBR blend (100 °C 15 min)
excited at 600 nm; (b) P3HT:PC60BM blend excited at 532 nm.
Information Figure S8. For the P3HT:FBR blend film, both
P3HT and FBR photoluminescence is strongly quenched (96%
and 99%) respectively, indicative of efficient exciton separation
following excitation of either donor or acceptor. This is
consistent with the film energetics that constitute a Type II
heterojunction, which is efficient in both electron and hole
transfer. Thermal annealing resulted in modest reduction in
Figure 6. (a) J−V curves and (b) EQE spectra of P3HT:FBR (blue)
and P3HT:PC60BM (black) devices.
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PLQ from both P3HT and FBR (by 93 and 95%, respectively),
indicating that this thermal annealing drives a modest increase
in the phase segregation of donor and acceptor. This PLQ
remains relatively strong compared to P3HT:PC60BM, where
the P3HT emission is only quenched by 80% (see Figure 7b).
This relatively modest PLQ in P3HT:PC60BM blends has been
correlated with strong phase segregation in this blend,42
resulting in the formation of pure P3HT domains on the 5−
10 nm length scale. The higher PLQ for the P3HT:FBR is
indicative of a more intimately mixed blend morphology.
The charge generation process was measured with ultrafast
transient absorption spectroscopy. Transient spectra are shown
in the Supporting Information (Figure S9). Figure 8a shows the
recombination losses of P3HT:FBR are consistent with a
thinner photoactive thickness for optimum device efficiency
(with faster recombination increasing losses during charge
transport). They are also consistent with the increase in VOC in
FBR devices relative to PC60BM being less than the decrease in
electron affinity (as faster recombination losses reduce VOC).44
Microsecond transient absorption data employing relatively
selective P3HT or FBR excitation (at 532 and 355 nm,
respectively) resulted in equivalent yields of long-lived polarons
per absorbed photons (Figure S10). This confirms efficient
charge generation from both P3HT and FBR excitons,
consistent with our PLQ and EQE data.
Electron Mobility Measurements. The electron mobility
of FBR was determined from space-charge-limited current
(SCLC) measurements,45 where the active layer blend was
sandwiched between two electron selective contacts, namely,
ITO/TiO2 and Ca/Al. The electron mobility of the P3HT:FBR
blend was then compared to a similar device with a
P3HT:PC60BM blend. The blend thicknesses were measured
using a profilometer. The current density was measured
between −5 and 5 V in steps of 0.05 V, and the resulting J−
V curves were fitted using a numerical solver,46 where the
charge carrier mobility, the injection barrier heights and the
trap density was allowed to vary. The resulting J−V curves are
shown for forward bias on a log−log scale in Figure 9. It is
Figure 8. Rise and decay of P3HT:FBR and P3HT:PC60BM blend
polaron signals using transient absorption spectroscopy under same
excitation density and atmosphere (4 μJ cm−2, N2) (a) in picosecond
time scale, excited at 600 nm and probed at 725 nm, and (b) in
microsecond time scale, excited at 532 nm and probed at 980 nm.
transient data at a probe wavelength of 725 nm, employing
selective excitation of P3HT at 600 nm. The 725 nm
wavelength corresponds to the P3HT PL maximum; at this
probe wavelength, initially excited P3HT excitons show a
negative signal (assigned to stimulated emission) while P3HT
polarons exhibit a positive signal. For the P3HT:PC60BM, the
rise time of the initially negative signal at 725 nm, assigned to
photoinduced charge separation from P3HT excitons, is clearly
biphasic, with an instrument response limited phase and slower
phase (exponential time constant of 6 ps). The fast phase is
assigned, as previously,43 to a faster charge separation from
P3HT excitons generated in close proximity to PC60BM. The
slow phase, meanwhile, is assigned to exciton diffusion limited
charge separation from excitons generated within pure P3HT
domains. For P3HT:FBR, the rise kinetics are significantly
faster, and dominated by an instrument response limited rise,
consistent with the more intimate blend mixing indicated by
the PLQ data shown in Figure 7.
Our PLQ and ultrafast transient absorption data indicate that
exciton quenching and polaron formation is faster and more
efficient in P3HT:FBR than P3HT:PC60BM. However, the
P3HT:FBR blend also exhibits faster charge recombination
losses than P3HT:PC60BM. This is apparent in the ∼200 ps
decay phase observed in Figure 8a, which has been tentatively
assigned to a geminate recombination process. It is also
apparent on longer (microsecond) time scale transient
absorption data, as shown in Figure 8b, which show faster
decay dynamics for P3HT:FBR blends, indicative of faster
nongeminate recombination. These faster recombination losses
are also indicative of a more intermixed blend morphology with
FBR as the acceptor, although we note that the increased
geminate recombination losses may also result from the smaller
LUMO−LUMO energy offset for P3HT:FBR compared to
P3HT:PC60BM. An in depth analysis of these recombination
losses will be presented elsewhere. The faster nongeminate
Figure 9. Space-charge-limited current density−voltage curves of
electron only devices (black circles) and numerical fits (black lines) of
P3HT:PC60BM and P3HT:FBR blends.
common to fit to SCLC data with the Mott−Gurney square
law, but the assumptions made to derive the equation are
generally not true for thin devices, and the equation fails
especially in the case of traps.47 For this reason, a novel
approach of numerical fitting is used.48 The two blends were
found to yield similar values for the electron mobilities, with μe
= 2.3 × 10−5 cm2/(V s) and μe = 2.6 × 10−5 cm2/(V s) for
P3HT:PC60BM and P3HT:FBR, respectively. The injection
barrier heights (which were found to be similar for both
injection from TiO2 and Ca) were found to be 0.093 eV for
P3HT:PC60BM, and since a similar contacting scheme was used
for both blends, similar injection barrier heights were assumed
for P3HT:FBR since charge injection from the contact into the
polymer is assumed. Extended states in the form of exponential
tails into the band gap has previously been reported in
polymer:fullerene blends.49 Where the devices with
P3HT:PC60BM showed intrinsic behavior, the P3HT:FBR
showed trap behavior so that exponential tails were needed in
order to fit to the data (NT = 5.2 × 1018 cm−3 with Ech = 2kBT,
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where NT is the density of the exponential states and Ech the
characteristic energy of the exponential tail),50 which is the
reason for the low observed current density despite the similar
electron mobility. The difference in the trap nature of the two
blends can be attributed to the amorphous nature of FBR and
the crystalline nature of the P3HT. It should be noted that even
though trap states were not observed in the case of
P3HT:PC60BM, a low concentration could still be present
which would not manifest in the SCLC measurements.
Morphological Stability. In order to compare the
morphological stability of our P3HT:acceptor blends, films
were spin-cast using the same active layer solutions as used in
device preparation, and these were heated to 140 °C for a
period of several hours. Optical microscopy was used to
observe the formation of any micrometer-scale aggregation
(Figure 10). In agreement with previous studies,51 the
which may help deliver nonanisotropic electron transport, as
well as a reduced tendency to crystallize which helps to prevent
large crystalline domains from forming in the bulk hetero-
junction blend composition over extended lifetimes. A higher-
lying LUMO energy relative to PC60BM also helps to achieve a
high open circuit voltage. The acceptor is effective with P3HT,
which is one of the most inexpensive and widely used polymers
in the field of OPV and a strong candidate for OPV
commercialization. Effective quenching of photoluminescence
is observed for this blend, as well as electron mobilities that are
comparable to those observed for the P3HT:PC60BM device. It
appears from transient absorption measurements that the
system is highly intermixed, leading to increased charge
generation but faster geminate recombination relative to
P3HT:PC60BM devices. Despite this suboptimal morphology,
it significantly outperforms the comparison P3HT:PC60BM
device. Lateral diffusion of the acceptor appears to be reduced
for this acceptor relative to fullerenes, offering the potential for
improved morphological stability in devices. Lastly, the
P3HT:FBR device is well-suited to the inverted device
architecture, which offers further stability in terms of reduced
reactivity of the electrodes and interlayers relative to conven-
tional architectures.
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete experimental section including synthetic procedures,
device fabrication details, additional photoluminescence and
transient absorption spectra, electron mobility measurement
details, and atomic force micrographs. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
s.holliday@imperial.ac.uk
■
Figure 10. Morphological stability of P3HT:acceptor films studied by
optical microscopy. Films were annealed in air at 140 °C, and the
images shown are after 0 and 1 h.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P3HT:PC60BM film formed relatively large fullerene clusters
within 1 h of annealing. By contrast, no particles were visible in
the P3HT:FBR films after annealing even for 12 h. Previous
reports of fullerene aggregation have correlated such optical
micrograph images with atomic force microscopy (AFM) and
TEM results and a corresponding degradation in solar cell
performance.52 It is possible, in this respect, that the reduced
aggregation demonstrated by the rhodanine flanked acceptor
should be advantageous in terms of device stability.
We thank BASF for partial financial support, as well as EPSRC
(EP/G037515/1) and EPSRC (EP/L016702/1), and EC FP7
Projects X10D (287818) and Nanomatcell (308997) for
financial support. A.-C.K. acknowledges the Austrian Science
Fund (FWF):[T 578-N19] for financial support.
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CONCLUSIONS
■
In summary, a novel rhodanine flanked small molecule FBR
was synthesized and its potential as an electron acceptor is
demonstrated. The material is structurally simple and can be
synthesized in high yields via relatively few synthetic steps. This
gives our molecule an intrinsic advantage over fullerene
acceptors, for which high cost and lengthy purification
techniques have long been a prohibitive factor. Furthermore,
this molecule is highly absorbing in the visible region, offering
the potential for enhanced photocurrent generation in OPV
devices. FBR demonstrates several advantageous properties for
OPV applications. These include reversible reduction behavior
with the ability to reversibly accept at least two electrons.
Second, the molecule has a nonplanar molecular structure
F
DOI: 10.1021/ja5110602
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
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DOI: 10.1021/ja5110602
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