Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered perylene-3,4:9,10-bis(dicarboximide)s

Article abstract of DOI:10.1021/ja2080482

The photophysics and morphology of thin films of N,N-bis(2,6- diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10- bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ～100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission ( ^1*TIPS-Pn + TIPS-Pn → 2 ^3*TIPS-Pn) and charge transfer from ^1*TIPS-Pn to PDIs 1-3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state ¹(TIPS-Pn ^+?-PDI^-?) that undergoes radical pair intersystem crossing to form ³(TIPS-Pn^+?-PDI ^-?), which then undergoes charge recombination to yield either ^3*PDI or ^3*TIPS-Pn. Energy transfer from ^3*PDI to TIPS-Pn also yields ^3*TIPS-Pn. These results show that multiple pathways produce the ^3*TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.

Full text of DOI:10.1021/ja2080482

Article
pubs.acs.org/JACS
Competition between Singlet Fission and Charge Separation in
Solution-Processed Blend Films of 6,13-Bis(triisopropylsilylethynyl)-
pentacene with Sterically-Encumbered Perylene-3,4:9,10-
bis(dicarboximide)s
Charusheela Ramanan,† Amanda L. Smeigh,† John E. Anthony,‡ Tobin J. Marks,*,†
and Michael R. Wasielewski*,†
†Department of Chemistry and Argonne−Northwestern Solar Energy Research (ANSER) Center, Northwestern University,
Evanston, Illinois 60208-3113, United States
‡Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States
S
* Supporting Information
ABSTRACT: The photophysics and morphology of thin films
of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis-
(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis-
(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis-
(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied
for their potential use as photoactive layers in organic photo-
voltaic (OPV) devices. Increasing the steric bulk of the 1,7-
substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI)
impedes aggregation in the solid state. Film characterization
data using both atomic force microscopy and X-ray diffraction
showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the
density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ∼100 nm solution-processed
TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the
3
TIPS-Pn results in competition between ultrafast singlet fission (1*TIPS-Pn + TIPS-Pn → 2 *TIPS-Pn) and charge transfer
from 1*TIPS-Pn to PDIs 1−3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge
separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state 1(TIPS-
3
Pn+•−PDI−•) that undergoes radical pair intersystem crossing to form (TIPS-Pn+•−PDI−•), which then undergoes charge
recombination to yield either 3*PDI or 3*TIPS-Pn. Energy transfer from 3*PDI to TIPS-Pn also yields 3*TIPS-Pn. These results
show that multiple pathways produce the 3*TIPS-Pn state, so that OPV design strategies based on this system must utilize this
triplet state for charge separation.
result in far larger D−A contact areas than in simple D−A
bilayers, yielding OPVs that exhibit greater exciton dissociation
and charge separation efficiencies and hence greatly increased
device PCEs.1a,3 Materials-oriented research to create OPVs
having larger PCEs is currently focused on the transparent
conductive oxide anode,4 the interfaces between the device
layers,5 and the materials within the photoactive layer.6 While
many new semiconducting polymer7 and small-molecule8
donors have been developed, there has been a paucity of
effective new acceptors. The relatively high performance of
OPVs having fullerene acceptors is attributed in part to the
molecular geometries, which promote efficient mixing with the
donor polymer to form optimal molecular interfaces for exciton
separation and subsequent charge transport.9 However, full-
erene derivatives also have undesirable properties, including
INTRODUCTION
■
The photoactive layers in organic photovoltaic (OPV) cells
comprise electron-donor (D) and electron-acceptor (A)
molecules with relative molecular orbital energies suitable for
exciton creation, subsequent exciton dissociation at the D−A
interface, and then diffusion-controlled charge collection at the
OPV electrodes.1 In many current-generation OPVs, these
layers include a semiconducting polymer donor, such as poly(3-
hexyl)thiophene (P3HT) or the more recently reported
poly(thieno[3,4-b]thiophene-alt-benzodithiophene) (PTB)
class, and a fullerene acceptor, such as [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM). In optimized devices, these D−A
combinations provide power conversion efficiencies (PCEs) of
∼5% and ∼7%, respectively.2 These OPV photoactive materials
are typically solution-processed from mixed D−A solutions to
create bulk heterojunction (BHJ) active layers having inter-
penetrating electron-donor (hole-transporting) and electron-
acceptor (electron-transporting) networks. Such composites
Received: August 25, 2011
Published: November 23, 2011
© 2011 American Chemical Society
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relatively low extinction coefficients for light absorption at
visible and near-IR wavelengths6a,10 as well as the tendency to
degrade in the presence of air and moisture.11
Perylene-3,4:9,10-bis(dicarboximide)s (PDIs) (Figure 1a)
offer a number of attractive characteristics as OPV electron
of not diminishing PDI−PDI molecular interactions too greatly,
since this suppresses the charge separation efficiency.
Earlier work reported promising OPV performance when a
PDI acceptor was paired with a pentacene (Pn) donor in vapor-
deposited bilayer structures.28 Pn-based devices have received
significant attention recently because Pn is known to undergo
singlet fission (SF).29 This is a process by which a singlet
exciton is converted into two independent triplet excitons, with
the result that if both triplet excitons inject charge into a
semiconductor electrode quantitatively, the maximum theoreti-
cal thermodynamic efficiency of a single-junction solar cell (the
Queisser−Shockley limit) rises from 32% to 46%.30 Singlet
fission was first proposed in 1965 to explain delayed
fluorescence in anthracene crystals31, then invoked in
crystalline tetracene to rationalize its low fluorescence quantum
yield, and subsequently confirmed in pentacene crystals.32
Singlet fission has been observed in mixed crystals (hetero-
fission) of anthracene doped with tetracene33 and pentacene
doped with tetracene.32b More recently, crystalline p-terphenyl,
p-sexiphenyl,34 tetracyano-p-quinodimethane,17,18 1,3-diphenyl-
isobenzofuran,35 perylene,36 benzophenone,37 and rubrene38
have all been shown to undergo singlet fission with triplet
quantum yields (ΦT) ranging from 6 to 200%. Singlet fission
has also been observed in carotenoid aggregates with high
efficiency39 and in selected aromatic polymers.24,25,40 While
significant computational work has been done to predict the
optimal molecular structures for SF,41 only one designer SF
chromophore has been studied experimentally.35 In addition,
there has also been only one report on charge separation in a
pentacene/fullerene bilayer resulting from triplet excitons
produced by SF.29b While there have been a few recent reports
exploring SF in OPVs,29b,42 SF chromophores have not been
widely used in these devices because a fundamental under-
standing of the factors that control the SF efficiency in
molecular solids is lacking.
The SF rate is maximized when the sum of the energies of
the two triplet excitons (T1) is lower than that of the
vibrationally relaxed singlet state (S1) [i.e., when E(S1) >
2E(T1)]. Once SF produces two triplet excitons, T1−T1
annihilation can be a serious problem if the triplet excitons
cannot diffuse apart quickly. However, when E(S1) > 2E(T1),
the rate of T1−T1 annihilation to yield S1 and S0 is slow because
this process is endergonic. In addition to these energetic
requirements, it is clear that intermolecular electronic coupling
and orientation are also critical for efficient SF, but only two
series of systematic studies varying these conditions have been
reported.35,41b,43 Theoretical work has suggested that a cofacial,
π−π slip-stacked relationship between chromophores facilitates
high SF efficiency.29c
Figure 1. (a) Chemical structure of PDIs and (b) illustration of
cofacial π-stacking of two PDI molecules.
acceptors, including favorable orbital energetics, high extinction
coefficients in the visible spectral region,12 photostability,13 and
the capacity to self-assemble into ordered nanostructures.14
PDIs have previously been used as solution-processable, high
mobility, air-stable n-type semiconductors and are readily
modified using straightforward synthetic routes.15 Furthermore,
previous reports have demonstrated charge separation in PDI
acceptors blended with thiophene16 and fluorene17 polymer
donors. Other work has paired PDI with a liquid-crystalline
hexabenzocoronene derivative,18 where transient micro-
wave photoconductivity studies showed that exciton−exciton
annihilation competes with charge separation, limiting the
device efficiency.19 This is consistent with the fact that OPV
studies incorporating PDI acceptors have generally been limited
in comparison with studies of fullerene acceptors.1b,6c,20 The
relatively poor device performance of PDI derivatives observed
to date in solution-processed devices is attributed to the well-
known propensity of these molecules to form π-stacked
aggregates (Figure 1b).13,21 While the tendency of the PDIs
to aggregate via π-stacking interactions is advantageous in
increasing the electron mobility,22 these aggregates may
diminish OPV performance by creating unfavorable nanostruc-
tures that quench excitons within the photoactive layer.17 Such
effects are particularly pronounced in solution-processed
devices, where PDI aggregates can precipitate from solution
in a broad distribution of aggregate sizes. A recent study of the
photophysical and OPV characteristics of PDI-containing
devices fabricated with three different donor polymers showed
that the PDI acceptor is aggregated to varying degrees.17b It was
found that disrupting the PDI π-stacking actually affords
morphologies more favorable for charge extraction and that the
device efficiency is limited by fast bimolecular recombination or
by enhanced PDI exciton relaxation due to the formation of
excimer-like states in PDI H-aggregates.17a These results are
consistent with our earlier studies on discrete cofacial π-stacked
PDI oligomers, which showed that excimer-like states form at a
rate that is dependent on the degree to which two or more
PDIs can tightly π-stack in an H-aggregate geometry.23
To explore the effects of PDI π-stacking modulation on
charge separation and recombination processes in solution-
processed thin films, PDI derivatives 1−3 (Figure 2) with
variable steric encumbrance to π-stacking were synthesized,
characterized, and studied as blends with the soluble donor
6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) (Figure
2).44 This solubilized polyacene has been used previously in
organic field-effect transistors.45 However, its use in OPVs has
had modest success, and bilayer-type devices using a fullerene
acceptor exhibit PCEs of ∼0.5%.46 Comparison with other
functionalized Pn derivatives has demonstrated a very slight
improvement in device parameters with impeded pentacene
crystallinity.46a In addition, the performance of TIPS-Pn in BHJ
devices is limited because of its tendency to form Diels−Alder
Recent work has highlighted the importance of electronic
and molecular interactions at the OPV D−A interface.24 Perez
et al.25 showed that changes in the molecular structure at the
D−A interface in bilayer small-molecule OPVs result in large
variations in device open-circuit voltage (Voc) and the dark
current, and Erwin and Thompson26 recently corroborated
these results with a series of PDI acceptors. Kim et al.27 showed
that decreasing the PDI π-stacking correlates with decreased
interfacial radial ion pair recombination in bilayer-architecture
OPV devices. This same work also highlighted the importance
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Figure 2. Molecular structures and HOMO/LUMO energies, as determined by electrochemistry and optical band gap measurements, of the PDI
electron acceptors 1−3 and the TIPS-Pn electron donor.
adducts with solubilized fullerenes in solution.47 This unde-
subjected to UV/O3 treatment to improve film wetting by the organic
solvent. Equimolar 20 mg/mL solutions of TIPS-Pn and each PDI
sirable chemistry does not occur with the PDI acceptors
were mixed in toluene and spin-cast onto 0.5 mm thick glass cover
examined here.
slides to give ∼100 nm thick organic films. Films of neat TIPS-Pn and
The blend films as well as neat films of each component
PDIs 1−3 (80−90 nm thick) were also fabricated using the same
material were characterized here by transient absorption
spectroscopy over the femtosecond-to-microsecond time
procedure. The films were characterized by AFM using a Veeco
Dimension ICON PT system, XRD using a Rigaku ATX-G thin-film
range. The charge separation and recombination dynamics
are correlated with the film morphology, as determined by
atomic force microscopy (AFM) and X-ray diffraction (XRD)
measurements. The film characterization and steady-state
optical absorption and fluorescence measurements confirmed
the variable tendency to undergo π-stacking across the series
PDI 1 → 2 → 3. Transient absorption spectroscopy of TIPS-
Pn in solution and films was also investigated, revealing that
neat TIPS-Pn films undergo ultrafast singlet fission. Further-
more, sterically encumbered PDIs 2 and 3 provide improved
film morphologies and enhanced D−A electronic interactions,
suppressing SF and promoting more efficient charge separation.
Thus, the present synthetic modifications of PDI electron
acceptors can be used to address those mechanisms that limit
the performance of OPVs incorporating them.
diffraction workstation, and profilometry using a Veeco Dektak 150
surface profiler.
Photophysical Measurements. Steady-state UV−vis spectra
were obtained with a Shimadzu 1800 spectrophotometer and
fluorescence measurements with a PT1 Quanta-Master 1 single-
photon spectrofluorimeter in a right-angle configuration. Femtosecond
transient absorption (fsTA) measurements were made using a 2 kHz
Ti-sapphire laser system as detailed previously.49 The instrument
response function for the pump−probe experiments was 150 fs. The
transient spectra were obtained using 5 s of averaging at a given delay
time, and the pump intensity was maintained at 0.5 μJ/pulse (200 μm
diameter spot size at the sample). The nanosecond transient
absorption (nsTA) apparatus has been described previously;14a
however, the excitation laser used was a Spectra-Physics Lab-150
laser coupled to a Basi-scan optical parametric oscillator (OPO)
(Spectra-Physics). The OPO output was directed onto the sample and
focused to a beam (1.5 mJ/pulse; 1 cm diameter spot size at the
sample) slightly larger than the probe beam, ensuring efficient pump−
probe overlap. Fifty transients were averaged at 5 nm intervals
spanning 400−800 nm. The transient spectra were constructed by
plotting specific time points of each kinetic trace with respect to the
corresponding wavelength. Solution nsTA experiments were per-
formed in toluene, and samples were degassed by five freeze−pump−
thaw cycles. For both fsTA and nsTA, the photoinduced processes
were studied using both 532 and 650 nm excitation wavelengths.
Analysis of kinetic data was performed at multiple wavelengths using a
Levenberg−Marquardt nonlinear least-squares fit to a general sum-of-
exponentials function with a Gaussian convolution to account for the
finite instrument response.
EXPERIMENTAL SECTION
■
Synthesis. The synthesis and characterization of PDI derivatives
1−3 are described in the Supporting Information. All of the final PDI
products were purified by gradient sublimation at 350 °C/10−6 Torr.
TIPS-Pn was purified by recrystallization (2×) from acetone.
Cyclic Voltammetry and Spectroelectrochemistry. Cyclic
voltammetry was carried out using a CH Instruments model 622
electrochemical workstation. All measurements were performed in dry
dichloromethane containing 0.1 M tetra-n-butylammonium hexafluor-
ophosphate (TBAPF6) as the supporting electrolyte. The solutions
were purged with N2 to ensure an O2-free environment. Measure-
ments were obtained using a 1.0 mm diameter platinum disk working
electrode, a platinum wire counter electrode, and a silver wire
reference electrode. Spectroelectrochemistry was performed using a
platinum mesh working electrode, a platinum wire counter electrode,
and a silver wire reference electrode in a 2 mm path length cuvette.
For all electrochemical measurements, the ferrocene/ferrocenium
(Fc/Fc+) redox couple (0.46 V vs SCE in CH2Cl248) was used as an
internal reference.
RESULTS AND DISCUSSION
■
Energetics of Charge Separation. The redox potentials
of TIPS-Pn and PDIs 1−3 were measured by cyclic
voltammetry and are summarized in Table 1. These measure-
ments were used in conjunction with the optical band gap
(Eg) to characterize the HOMO/LUMO energies of the
materials,50 and these results are shown in Figure 2. The results
Film Fabrication and Characterization. Glass slides were first
cleaned with detergent, water, and acetone in sequence and then
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Table 1. Redox and Excited-State Data for the TIPS-Pn Donor and the PDI Acceptors 1−3
a
a
a
b
c
c
Eox (eV)
Ered1 (eV)
Ered2 (eV)
λabs (nm)
λem (nm)
Eg (eV)
EHOMO (eV)
ELUMO (eV)
TIPS-Pn
PDI 1
PDI 2
PDI 3
0.85
644
527
553
562
648
536
600
614
1.9
2.3
2.2
2.1
−5.3
−6.2
−6.1
−6.0
−3.4
−3.9
−3.9
−3.9
−0.55
−0.52
−0.78
−0.75
−0.72
−0.50
a
b
c
0.1 M TBAPF6 in CH2Cl2; values vs SCE. Eg = (Eabs + Eem)/2 = optical band gap. Estimated from ELUMO = 4.4 eV − Ered1 or EHOMO = 4.4 eV + Eox
and EHOMO = ELUMO − Eg
for TIPS-Pn agree with previous reports.51 The slight differences in
the reduction potentials of the acceptors are offset by the shift in
Eg, giving nearly constant LUMO energies across the series. The
DLUMO−ALUMO offset was then 0.5 eV across the series, and the
DHOMO−AHOMO offset was 0.7−1.0 eV. Spectroelectro-
chemistry of TIPS-Pn showed that TIPS-Pn+• has a strong
absorption feature at 443 nm (Figure S2 in the Supporting
Information), while the corresponding data for the PDI−•
derivatives all showed a strong characteristic absorption at 700−
750 nm.52
exhibit Bragg reflections, in agreement with previous
studies.45a,46b In the as-spun blend films, which were fabricated
and measured under identical conditions, the intensities of the
TIPS-Pn diffraction features showed a steady decline across the
series PDI 1 → 2 → 3 (Figure 3). Notably, no new features
The band-edge offsets between OPV electron donors and
acceptors provide the free energy change essential for charge
separation.1a Electron transfer from the photoexcitation-
populated donor LUMO to the acceptor LUMO at the D−A
interface is considered to be the predominant photocurrent
generation pathway in BHJ OPVs having polymer donors and
fullerene acceptors, wherein exciton formation is largely
centered on the polymer donor, which has the greatest optical
cross section.9,53 However, in the TIPS-Pn:PDI blend films
examined here, the PDI acceptors also have high extinction
coefficients,12,13 so hole transfer from the acceptor HOMO to
the donor HOMO is also a possible charge separation pathway.
The magnitude of this energy level LUMO−LUMO or
HOMO−HOMO offset must be large enough to overcome
the exciton binding energy of 0.3−0.5 eV,53 a requirement that
is clearly satisfied in the TIPS-Pn:PDI blend films (Table 1).
The TIPS-Pn:PDI 1−3 D−A series therefore meets the
energetic requirements for efficient charge separation. How-
ever, as previous work in this field has demonstrated, achieving
suitable film morphology and molecular interaction between
the donor and acceptor in the photoactive layer plays a
significant role in device function, even if the energetic
requirements for charge separation are satisfied.24b54
Thin-Film Characterization. AFM scans (Figure S3a−c)
show that solution-processed films of neat PDIs 2 and 3 have a
root-mean-square (rms) roughness that is an order of
magnitude less than those of neat PDI 1 (0.16 and 0.18 nm
vs 1.3 nm, respectively). It is further evident from the AFM
images of the blend films in Figure S3d−f that PDI 1, which is
the least sterically hindered PDI derivative examined here,
forms the least homogeneous blend films with TIPS-Pn, while
PDIs 2 and 3 form more homogeneous blend films. These
differences are not as evident in the experimental film
roughnesses because of the contribution of TIPS-Pn to the
film morphology, which resulted in blend rms roughness values
of 0.22, 0.28, and 0.19 nm for TIPS-Pn:PDI 1, 2, and 3 respec-
tively. However, large, 10−20 nm wide discontinuities in the
PDI 1 blend films are evident in the AFM image. These
differences in film morphology suggest that PDIs 2 and 3
exhibit a decreased tendency to aggregate/precipitate from
solution during film fabrication in comparison with PDI 1.
XRD measurements showed that the neat PDI acceptors
form amorphous spin-cast films while the neat TIPS-Pn films
Figure 3. Θ/2Θ XRD patterns (background subtracted) for films of
neat TIPS-Pn, 1:1 TIPS-Pn:PDI 1, 1:1 TIPS-Pn:PDI 2, and 1:1 TIPS-
Pn:PDI 3.
attributable to a discrete blend crystal structure were observed.
This indicates that across the series, there are fewer TIPS-Pn
domains in the blend films having long-range crystalline order.
As has been discussed above, unsubstituted PDIs have large
association constants for π-stacking. In the case of the TIPS-
Pn:PDI 1 blend film, the strong self-association of PDI 1 results
in segregated domains of polycrystalline TIPS-Pn and
amorphous PDI 1. Placing the phenyl and 3,5-di-tert-
butylphenyl groups on the PDI at the 1- and 7-position
progressively diminishes the PDI self-aggregation in proceeding
from PDI 2 to PDI 3. This makes it possible for monomeric
PDIs 2 and 3 to overcome the weaker association of
polycrystalline TIPS-Pn, thereby producing more homoge-
neous blend films. It should be noted that previous work has
shown a correlation between the OPV film morphology and
microstructure in polymer blends and the OPV device
performance.55
Optical Absorption and Emission Spectra. Figure 4
compares the steady-state optical absorption and fluorescence
spectra of PDIs 1−3 in solution and as thin films. The overall
red shift of the absorption spectra of PDIs 2 and 3 in solution
relative to that of PDI 1 is a well-known result of 1,7-disub-
stitution that twists the PDI core.13,23 While the absorption
spectrum of PDI 1 in the neat film is essentially superimposable
on that of 1 in solution, the fluorescence spectrum of the PDI 1
film is broad and featureless and significantly red-shifted relative
to that in solution [λem(soln) = 536 nm, λem(film) = 650 nm],
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Figure 4. Optical absorption and fluorescence spectra of (top) toluene solutions and (bottom) spin-cast films of (a) neat PDI 1, (b) neat PDI 2, and
(c) neat PDI 3.
Figure 5. MM2 geometry-optimized structures of PDIs 1−3 illustrating the increasing steric impediment to π−π interactions between adjacent
chromophores in the progression PDI 1 → 2 → 3. H atoms have been omitted for clarity.
which is characteristic of emission from an excimer-like
state.23,56 Single-crystal XRD characterization of PDI 1 showed
that the planes of the 2,6-diisopropylphenyl substituents at the
imide positions are oriented nearly perpendicular to that of the
perylene core,57 which inhibits both H- and J-aggregate
formation. The molecules must therefore be π-stacked with
the primary transition dipoles along the N−N axes of
neighboring molecules rotated significantly away from a parallel
conformation, thus explaining the absence of characteristic
H- or J-aggregate excitonic energy level splitting in the absorption
spectrum. Since the present XRD measurements on the PDI 1
films exhibited no Bragg diffraction peaks, the π-stacked PDI 1
aggregates in the as-spun solution-processed films are either
amorphous or highly disordered/nanocrystalline. In contrast,
while the film and solution absorption spectra of PDI 2 are also
very similar (λabs = 553 nm), their fluorescence spectra differ
markedly, with the film fluorescence spectrum exhibiting a
weak emission band at 614 nm assignable to the monomer and
an additional excimer-like emission band at 700 nm.23,56 Lastly,
the absorption and fluorescence spectra of PDI 3 both in solution
and in films are very similar [λabs = 562 nm; λem(soln) = 614 nm,
λem(film) = 625 nm], suggesting that this derivative does not
form strongly coupled excimer-like states in the solid film. The
XRD data for the as-spun films of PDIs 2 and 3 also argue that
the films are amorphous or highly disordered/nanocrystalline.
Thus, the steady-state optical data suggest that as the series PDI
1 → 2 → 3 is traversed, the molecules exhibit a decreased
tendency for π-stacking in the solid films, in agreement with the
AFM and XRD data. The geometry-optimized structures (Figure 5)
illustrate how the increasing bulk of the 1,7-substituents sterically
encumbers π-stacking.
Neat TIPS-Pn films absorb strongly in the 600−700 nm
region and exhibit transitions that differ significantly from those
of TIPS-Pn in solution (Figure 6a), which is expected in view of
the known tendency of this molecule to form two-dimensional
π-stacked structures.44 The XRD data on the neat as-spun films
revealed sharp diffraction peaks, indicating that the film is
polycrystalline (Figure 3). The thin-film optical spectra are
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Figure 6. Steady-state optical absorption spectra of (a) TIPS-Pn in toluene solution and as a neat film and (b) 1:1 blend films of TIPS-Pn and PDI
acceptors 1−3.
Figure 7. (a) Femtosecond and (b) nanosecond transient absorption spectra of neat TIPS-Pn films (λex = 650 nm). The insets show the transient
decay kinetics at 525 nm.
characterized by a red shift and splitting of the S0 → S1
transition, yielding absorption maxima at 646 and 696 nm.58
Broadening and intensity increases in the 549 and 587 nm
bands were also observed, as well as an increase in intensity and
a slight red shift of the 440 nm band to 449 nm. Such shifts
have been observed previously in oligoacene crystals and are
attributed to a mixture of Davydov splitting, vibronic coupling,
and coupling between different excitonic states.59 The ratio of
the integrated intensities of the blue band (449 or 440 nm) to
the red bands (500−800 nm) are similar in the film (0.15) and
in solution (0.11), indicating that while some of the intensity
increase in the 449 nm band may be due to small orientational
preferences of the polycrystalline TIP-Pn domains in the film,
for the most part the ordered polycrystalline domains have
random orientations relative to one another.
and PDI as the films become more homogeneous. Each blend
film shows essentially complete fluorescence quenching relative
to the films of the neat donor and acceptors (Figure S4), which
is suggestive of competitive electron transfer.
Transient Absorption Spectroscopy. The fsTA spectra
of TIPS-Pn in solution (Figure S5) exhibit a broad absorption
from 400 to 600 nm with a maximum at 450 nm that appears
within the time scale of the instrument response and decays
with τ = 17 0.1 ns, in agreement with previous time-resolved
fluorescence measurements.51 This feature is attributed to the
1
excited singlet state absorption (S1 → Sn) of *TIPS-Pn. The
same experiment was then carried out with the addition of
iodomethane to enhance intersystem crossing (ISC) to the
triplet state.60 The resultant T1 → Tn absorption spectrum
3
of *TIPS-Pn in solution (Figure S6) shows a strong peak
The as-spun TIPS-Pn:PDI blend films absorb across the
entire visible spectrum from 400 to 700 nm (Figure 6b). The
absorption from 400 to 600 nm results largely from the PDI
derivative, while that in the 600−700 nm range results primarily
from TIPS-Pn. In contrast to neat TIPS-Pn films, the
absorption spectrum of TIPS-Pn in the blend films exhibits a
strong absorptive feature at 650 nm that is characteristic of
amorphous TIPS-Pn58 and strongly resembles that observed in
solution (Figure 6a). The TIPS-Pn:PDI 2 blend film displays a
shoulder at 600 nm relative to the PDI 2 film that is further
enhanced in the TIPS-Pn:PDI 3 blend film; this shoulder may
result from enhanced electronic interactions between TIPS-Pn
at 505 nm and a secondary vibronic band at 470 nm and is
in excellent agreement with the previously reported triplet
excited state absorption of Pn in solution.61 The extinction
3
coefficient for the *TIPS-Pn T1 → Tn absorption, which was
calculated using the ground-state bleach feature at 590 nm (ε =
10 000 M−1 cm−1 44) as an internal standard, was determined to
be 64 000 M−1 cm−1.
The fsTA spectra of the TIPS-Pn films (λex = 650 nm; Figure 7a)
initially display an absorption at 481 nm (100 fs), which is
attributed to the S1 → Sn transition red-shifted from its 450 nm
maximum observed in solution (Figure S5a). The optical
absorbances of the TIPS-Pn films were typically 0.11 at 700 nm
391
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Journal of the American Chemical Society
Article
with a film thickness of 8.5 × 10−6 cm. The density of crystalline
TIPS-Pn obtained from its crystal structure is 1.104 g cm−3,44 which
gives a TIPS-Pn concentration of 1.73 M. These data yield an
extinction coefficient of ε = 7300 M−1 cm−1 at 700 nm for TIPS-Pn
in the films. Since the TIPS-Pn films have very little ground-state
absorption at 481 nm (Figure 6a), the observed ΔA at 481 nm and
100 fs is due entirely to the S1 concentration formed by the laser
pulse. Thus, from the TIPS-Pn ground-state extinction coefficient,
ΔA for the TIPS-Pn ground-state bleach at 700 nm, and ΔA for the
S1 → Sn transition at 481 nm and 100 fs, the calculated extinction
coefficient for the TIPS-Pn S1 → Sn transition at 481 nm is ε = 33
300 M−1 cm−1.
The 481 nm absorption is rapidly replaced by a strong
absorption at 525 nm that appears with a rise time (τR) of 1.0
0.2 ps and decays with lifetimes τD1 = 125 18 ps (0.44), τD2
=
1.05 0.17 ns (0.47), and τD3 = 80 10 ns (0.09), as
determined by fsTA and nsTA spectroscopy, respectively. The
line shape of this feature is very similar to the triplet absorption
in solution but is red-shifted by 20 nm. In recent years,
numerous reports have proposed various spectral assignments
for the triplet absorption spectrum of Pn films.29b,d−f,62 Smith
and Michl29c reviewed these studies in detail and found all of
them to agree that the triplet absorption spectrum in a film
should be red-shifted from that in solution. They further
proposed that the 540 nm absorption in the Pn film transient
spectrum reported by Marciniak et al.29e was correctly assigned
to the Pn triplet state. Recently, similar red shifts were noted
for the T1 → Tn absorption spectrum of tetracene in thin solid
films relative to that in solution.63 In our experiments,
therefore, the 20 nm red shift in the TIPS-Pn triplet absorption
upon going from solution to a polycrystalline film is reasonable.
Furthermore, previous work has shown that TIPS-Pn
fluorescence is quenched in the solid film,51 suggesting that
depopulation of the photoexcited singlet state occurs by
nonradiative processes.
Figure 8. The SF observed in neat TIPS-Pn films occurs by excitation
1
of one TIPS-Pn molecule to *TIPS-Pn, which then interacts with an
3
adjacent TIPS-Pn molecule to give two *TIPS-Pn chromophores.
1 blend fsTA spectra (Figure 10a) display a prominent 510 nm
absorption that is similar to the 525 nm absorption observed
for the neat polycrystalline TIPS-Pn film. This feature forms
with τR = 1.0 0.2 ps and is most likely a result of singlet
3
fission to produce *TIPS-Pn. The blue shift of this transient
absorption from that observed in the neat film is explained by
the observation discussed above that in the blend films, the
TIPS-Pn absorption resembles that of TIPS-Pn in solution
because of the mixing with the PDI acceptor. Therefore, the
3*TIPS-Pn absorption in the blend film accordingly lies
between that in the neat film and that in solution (Figure S6).
The absorption at 710 nm indicates that there is a competi-
tion between singlet fission and ultrafast electron transfer
Utilizing the observed ΔA = 0.0124 and ε = 33 300 M−1 cm−1
1
for *TIPS-Pn at 481 nm as a measure of the initial singlet
exciton concentration in the film (Figure 7a) and given that ΔA =
1
from *TIPS-Pn to PDI. The absorption expected for TIPS-
0.0342 for 3*TIPS-Pn formation at 525 nm (Figure 7a) and εmax
=
Pn+• at 445 nm is convoluted with the first vibronic band of
3*TIPS-Pn.
64 000 M−1 cm−1 for *TIPS-Pn, we calculated the triplet
quantum yield as 144 25%. This high 3*TIPS-Pn yield along
with its ultrafast formation provide strong evidence that
3*TIPS-Pn forms by singlet fission. The energy level diagram
in Figure 8 illustrates this process. After one TIPS-Pn is photo-
excited into the S1 state (red), it interacts with a neighboring
TIPS-Pn molecule that is still in the ground state (black) to
give two TIPS-Pn molecules in the T1 state through an overall
energy- and spin-conserving process.
3
Following selective 650 nm TIPS-Pn excitation, fsTA spectra
at 2−5.6 ns and nsTA spectra at 8−25 ns are shown for the
TIPS-Pn:PDI 1 blend film in Figure S8a,b, respectively. These
spectra show that the small amount of TIPS-Pn+•:PDI−•
formed is completely gone by ∼6 ns, while the concentration
of the initially formed 3*TIPS-Pn remains essentially
unchanged over this time range. The nsTA kinetics show an
instrument-limited rise time for 3*TIPS-Pn, as expected on the
basis of the fsTA results. Figure 10b shows that at longer times
the nsTA spectra result entirely from 3*TIPS-Pn, which decays
biexponentially with the time constants summarized in Table 3.
These results are consistent with the fact that the polycrystal-
line TIPS-Pn and PDI 1 domains remain largely segregated, as
indicated by the XRD data and steady-state optical absorption
and emission data discussed above.
In contrast to the above results, selective 650 nm
photoexcitation of TIPS-Pn in the TIPS-Pn:PDI 2 and TIPS-
Pn:PDI 3 blend films clearly shows the absorption bands of
TIPS-Pn+• at ∼445 nm and PDI−• at 750 nm,52 which appear
within the 150 fs instrument response function (Figure 10c,e)
and decrease on a time scale of several nanoseconds. FsTA
spectra at 2−5.6 ns and nsTA spectra at 8−25 ns are shown for
FsTA measurements on the neat PDI films (λex = 532 nm;
Figure 9) exhibit positive absorptions at 710 nm assignable to
1*PDI,14b,d,23 which decay in concert with the recovery of the
broad ground-state bleach centered at 500 nm. The 710 nm
absorption is significantly broader for PDIs 2 and 3 than for
PDI 1. As noted above, this breadth is characteristic of PDI
derivatives having 1,7-substituents that distort the PDI core
away from planarity.14b,d,23 These excited-state absorption
changes decay rapidly to the ground state and can be fit with
a biexponential function (Table 2). No longer-lived species
were observed.
TIPS-Pn and PDI in the blend films were selectively photo-
excited at 650 and 532 nm (Figure 10 and Figure S7, respec-
tively) to probe any differences in the overall charge separa-
tion process. Following 650 nm excitation, the TIPS-Pn:PDI
392
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Journal of the American Chemical Society
Article
Pn ground-state bleaching is evident in these spectra, as are
PDI−• 2 and PDI−• 3 absorptions at 750 nm at the earliest
times. The nsTA kinetics for PDI−• decay and formation of
3*TIPS-Pn in TIPS-Pn:PDI 2 and TIPS-Pn:PDI 3 blend films
are shown in Figure S10c,e, respectively. The time constants
for TIPS-Pn+•−PDI−• charge recombination are τ = 6 1 and
3
11
1 ns, respectively. *TIPS-Pn appears at 510 nm in
concert with PDI−• decay.
These same transient absorption changes appear when the
PDI is excited in the blend films using 532 nm laser pulses (see
Figures S7, S9, and S10b,d,f). At this excitation wavelength,
we may expect to see contributions from energy transfer from
1*PDI to TIPS-Pn and hole transfer from 1*PDI to TIPS-Pn as
well as from electron transfer from 1*TIPS-Pn to PDI. It should
be noted, however, that the transient absorption spectra
measured using the two different excitation wavelengths are
very similar, indicating that electron transfer from TIPS-Pn to
1
1*PDI or hole transfer from *PDI to TIPS-Pn is much faster
than energy transfer from 1*PDI to TIPS-Pn. The TIPS-
Pn:PDI 1 blend provides one notable exception to the similar
photophysics observed following 650 and 532 nm excitation, as
selective 532 nm PDI photoexcitation results in charge
separation with no evidence of singlet fission arising from
TIPS-Pn (Figure S7a). This implies that charge separation is
1
faster than singlet energy transfer from *PDI to TIPS-Pn.
Ultrafast 3*TIPS-Pn formation is evident in the fsTA spectra
of the TIPS-Pn:PDI 1 blend films and persists in their nsTA
3
spectra. However, the majority of *TIPS-Pn formation in the
TIPS-Pn:PDI 2 and 3 blends results from nanosecond-time-
scale radical-pair ISC (RP-ISC) within the weakly spin−spin
1
exchange-coupled (TIPS-Pn+•−PDI−•) spin-correlated radical
3
ion pair to form (TIPS-Pn+•−PDI−•), which then undergoes
charge recombination to yield either 3*PDI or 3*TIPS-Pn. The
transient spectra in Figure 10 and Figure S7 show no evidence
of the 3*PDI T1 → Tn spectrum, which has absorption features
at 480 and 530 nm in solution.65 Since the energy of *PDI is
3
at least 0.3 eV higher than that of 3*TIPS-Pn,29c if any 3*PDI is
formed upon charge recombination, rapid energy transfer from
3
3*PDI to TIPS-Pn most likely produces the observed *TIPS-
Pn. Geminate charge recombination to a triplet state in D−A
films has been observed experimentally in polyfluorene
copolymer systems66 as well as in a variety of polymer-fullerene
blends.67 This pathway has also been studied theoretically as a
general mechanism for OPV performance loss.68
3*TIPS-Pn is derived from the population of TIPS-Pn+•−
PDI−• in which the radical ion pair distances are more than
∼15 Å, an approximate distance that we have shown previously
to result in a sufficiently weak spin−spin exchange interaction
(2J) for the RP-ISC mechanism to operate.64 Thus, the
Figure 9. Femtosecond transient absorption spectra of (a) PDI 1, (b)
PDI 2, and (c) PDI 3 films excited at 532 nm. The insets show kinetic
traces and fits.
3
observed *TIPS-Pn population produced by charge recombi-
Table 2. Transient Absorption Decay Times of Neat Films of
PDIs 1, 2, and 3
nation samples the radical ion pairs that have separated to long
distances at which 2J is small, while the closer radical ion pairs
most likely recombine to the singlet ground state because they
cannot undergo ISC as a result of the fact that 2J is large at
short distances.
film
τD1 (ps) (rel. amp.)
τD2 (ps) (rel. amp.)
PDI 1
PDI 2
PDI 3
1.7 0.6 (74)
6.1 1.2 (65)
7.8 2.6 (56)
56 12 (26)
133 4 (35)
274 80 (44)
The decay of the 510 nm 3*TIPS-Pn absorption can be fit to
a sum of exponentials (Figure 11 and Table 3). The shorter-
time component is attributable to triplet−triplet annihilation. It
should be noted that for the TIPS-Pn:PDI 2 and TIPS-Pn:PDI
3 blends, the shorter-time component was retrieved from a
weighted average of two exponential fits. Previous reports have
used this method to estimate photophysical lifetimes for
TIPS-Pn:PDI 2 and TIPS-Pn:PDI 3 blend films in Figure S8c,d
and Figure S8e,f, respectively. These spectra show that the
PDI−• absorption decay at 750 nm is accompanied by the
formation of 3*TIPS-Pn over this time range. The nsTA spectra
from 19 ns to 2.5 μs are shown in Figure 10d,f. PDI and TIPS-
393
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Journal of the American Chemical Society
Article
Figure 10. (a, c, e) Femtosecond and (b, d, f) nanosecond transient absorption spectra at λex = 650 nm of 1:1 blend films of (a, b) TIPS-Pn:PDI 1,
(c, d) TIPS-Pn:PDI 2, and (e, f) TIPS-Pn:PDI 3.
Table 3. Summary of Exponential Kinetic Fits for the
Triplet-State Feature in TIPS-Pn:PDI Blend Films (1:1
Molar Ratio) Probed at 500 nm
τD1 (ns) (rel. amp.)
τD2 (ns) (rel. amp.)
TIPS-Pn:PDI 1
TIPS-Pn:PDI 2
TIPS-Pn:PDI 3
55 1 (0.68)
252 3 (0.66)
367 6 (0.62)
530 4 (0.32)
2480 15 (0.34)
2960 30 (0.38)
annihilation processes in organic films, where nonexponential
behavior is often observed.69 The longer-time component is
assigned to the decay of 3*TIPS-Pn to the ground state, and the
3
3
lifetimes follow the order *TIPS-Pn:PDI 1 < *TIPS-Pn:PDI
2 < 3*TIPS-Pn:PDI 3, a difference that can again be attributed
to the diminishing degree of TIPS-Pn and PDI phase
segregation as the blends become more homogeneous across
the series PDI 1 → 2 → 3.
The charge transfer dynamics of the blend films is therefore
characterized (Figure 12) by competition between singlet
fission within TIPS-Pn polycrystalline domains and ultrafast
charge separation to form a geminate radical ion pair state
Figure 11. Nanosecond transient absorption kinetics of TIPS-Pn:PDI
1:1 blend films monitored at 510 nm.
1
(either by electron transfer from the *TIPS-Pn state to the
1
PDI or by hole transfer from the *PDI state to TIPS-Pn)
394
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Journal of the American Chemical Society
Article
Figure 12. Multiple pathways for formation of 3*TIPS-Pn: (a) Singlet fission in adjacent TIPS-Pn chromophores and (b) charge separation at the
3
D−A interface followed by intersystem crossing (ISC) and then by charge recombination (CR) to *TIPS-Pn either directly or via energy transfer
3
(EET) from *PDI.
followed by competition between recombination to 3*TIPS-Pn
Energy Sciences, under Award DE-SC0001059. Preparation of
TIPS-Pn was supported by the Office of Naval Research under
Grant N00014-05-1-0019. NMR and MS measurements were
carried out at the Integrated Molecular Structure Education and
Research Center (IMSERC) at Northwestern University. AFM
measurements were carried out at the Nanoscale Integrated
Fabrication, Testing and Instrumentation Center (NIFTI) at
Northwestern University. XRD measurements were carried out
at the J. B. Cohen X-ray Diffraction Facility supported by the
MRSEC Program of the National Science Foundation (DMR-
0520513) at the Materials Research Center of Northwestern
University. C.R. thanks Dr. R. Ponce-Ortiz for help with XRD
measurements and Drs. S. Shafaie and J. Seymour for MS
measurements.
3
and/or to *PDI and formation of the singlet ground state. If
3*PDI is formed, it rapidly transfers energy to the lower-lying
3
3
triplet state, *TIPS-Pn. The final *TIPS-Pn decays at a rate
that is directly correlated with the degree of TIPS-Pn
crystallinity in the film, which is further dictated by the PDI
acceptor aggregation.
CONCLUSIONS
■
A series of PDI acceptors with substituents providing varying
degrees of steric impediment to π-stacking were synthesized by
functionalizing the 1,7-positions of the perylene core with bulky
aromatic groups. Optical absorption and emission spectra of the
PDI films verified the expected decrease in PDI aggregation
with increasing steric bulk. In addition, film characterization by
AFM showed that solution processing of PDIs 2 and 3 yields
smoother, more continuous films than does PDI 1. Solution-
processed blend films of PDI 1−3 with a TIPS-Pn donor
exhibit a trend in TIPS-Pn crystallinity and molecular packing
across the series that correlates closely with the trend in PDI
aggregation. Upon photoexcitation, these blend films exhibit a
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ASSOCIATED CONTENT
* Supporting Information
Experimental and synthetic details, spectroelectrochemistry of
TIPS-Pn, AFM surface characterization of PDI films,
fluorescence spectra, additional TA spectra, and complete ref
35b. This material is available free of charge via the Internet at
http://pubs.acs.org.
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