Singlet Exciton Fission in Solution
Brian J. Walker
1
, Andrew J. Musser
1
, David Beljonne
2
& Richard H. Friend
1
*
1
— Cavendish Laboratory, University of Cambridge
J. J. Thomson Avenue, Cambridge, United Kingdom
2
— Laboratory for Chemistry of Novel Materials, Université
de Mons
Place du Parc 20, 7000 Mons, Belgium
Singlet exciton fission, the spin-conserving process that produces two spin triplet excited
states from one photoexcited singlet state, is a means to circumvent the Shockley-
Queisser limit in single-junction solar cells. While the process through which singlet
fission occurs is not well-characterised, some local order is thought to be necessary for
intermolecular coupling. Here, we report 200% triplet yield and triplet formation rates
approaching the diffusion limit in solutions of TIPS-pentacene. We observe a transient
bound excimer intermediate, formed by collision of one photoexcited, and one ground
state TIPS-pentacene molecule. The intermediate breaks up when the two triplets have
separated to each TIPS-pentacene molecule. This efficient system is a model for future
singlet fission materials and for disordered device components that produce cascades of
excited states from sunlight.
First proposed to account for the optical and magnetic properties of crystalline
anthracene
1
and tetracene
2,3
, singlet exciton fission
4,5
is the reverse process of triplet-triplet
annihilation
6
. In singlet exciton fission, a singlet excited state decays to form two triplet
excitons on neighbouring molecules. Because the two product triplets have a net spin of zero,
the singlet fission process conserves spin and is thus a faster means to generate triplets than
intersystem crossing for organic chromophores
7
. When the fission process is exergonic
8
this
process can be rapid and efficient, as is now established for solid films of pentacene
9,10
.
Singlet fission has gained interest as a means to circumvent the Shockley-Queisser limit in
solar cells
11
. Because singlet fission converts high-energy photons into two excitons, a singlet
fission sensitiser can be used with a low band gap solar cell to reduce losses due to relaxation.
For such a single-junction cell the maximum theoretical efficiency increases to 44%
12
, and
this potential has led to recent realisations of singlet fission-sensitised solar cells
13,14
.
Some basic aspects of the singlet fission mechanism remain unresolved. Although the
energetic requirements for singlet fission are satisfied in a range of materials, experiments
have shown that this is not the only criterion for efficient singlet fission
15
. While the nature of
singlet fission suggests that the process requires exciton coupling
16–21
, the observation of
fission in amorphous solids
22
indicates that long-range order is not required. Singlet fission
occurs for tetracene in both single crystals
7
and polycrystalline films
23
, but the triplet
population is also controlled by processes such as diffusion and annihilation that complicate
analysis
24
. While molecular solids are practical for devices, they give little insight into the
short-range molecular interactions that govern singlet fission.
To address these questions, we investigated the molecular mechanism of singlet
fission in solutions of TIPS-pentacene (Fig. 1a). Originally developed for use in field effect
transistors
25
, TIPS-pentacene (Fig. 1b) has two triisopropylsilylethynyl (TIPS) groups that
lead to favourable intermolecular orientations and high hole mobilities in the solid state
26
.
The substituents also render TIPS-pentacene far more soluble than unsubstituted pentacene
27
,
allowing our solutions to reach concentrations of 3.1% by mass (0.075 mol/L) in chloroform
before saturation.
Results and Discussion
The absorption spectrum of dilute TIPS-pentacene solution is shown in Fig. 1c. For
comparison, we also took the absorption spectrum of a solid-state film of TIPS-pentacene.
Whether the film is produced through spin or drop-casting, the solid-state absorption
spectrum has a red-shifted aggregate feature that is not present in solution. We took
absorption spectra of solutions across several orders of magnitude in TIPS-pentacene
concentration (Supplementary Fig. S1), and we observed no significant aggregates even in
samples near the saturation point. We conclude that TIPS-pentacene molecules in solution are
electronically decoupled in the ground state, even at high concentration. In a control
experiment, we investigated the TIPS-pentacene solutions at different concentrations using
diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY). These spectra show that
the diffusion constant of TIPS-pentacene (9.7 x 10
-10
m
2
/s) is unchanged across the series,
indicating that the motion of TIPS-pentacene molecules is identical for concentrations from
10
-5
mol/L to 0.075 mol/L (Supplementary Fig. S2).
Quantitative singlet fission yield
We probed the excited state dynamics of TIPS-pentacene solutions using transient
absorption spectroscopy. This pump-probe technique is a popular means of studying
photophysics, due to its versatile time resolution and its ease of comparison with ground-state
absorption spectra
28
. In transient absorption, a short pulse of light at a well-defined energy
(the “pump”) excites the sample, and after a time delay ranging from femtoseconds to
hundreds of microseconds, a pulse broad in energy but short in time (the “probe”)
interrogates the same spot. The transmitted light from the probe is compared with and
without the pump light, and resolved by both spectral wavelength and delay time. If there is a
change in the spectrum of the probe due to bleaching of the ground state transitions (“ground
state bleach”), stimulated emission, or excited-state absorption from one excited state to
another, then these will manifest as a change in the transmittance of the probe, ΔT. We report
the signal normalised by the ground state transmittance, ΔT/T, to facilitate comparison across
experimental configurations.
We show a transient absorption spectrum for a dilute TIPS-pentacene solution
(10
-4
mol/L) in Fig. 2a. The electronic delay enables measurements from 10
-9
– 10
-3
s,
although the initial signals are broadened by the temporal distribution of photons in the
excitation pulse. Across the spectral range from 0.77 to 2.39 eV, the dynamics are dominated
by features with a lifetime of 13 ns. Specifically, we observe a ground state bleach at 1.9 eV,
analogous to the 0-0 band of the ground state absorption spectrum. There are also various
excited state absorptions due to transitions to higher lying excited states (see Supplementary
Figs. S3 and S4 for TA data above 2.3 eV). A spectral cross section at the peak of the signal
intensity (2.6 ns) is shown in Fig. 2b, and because the lifetime correlates with the singlet
lifetime measured during time-resolved photoluminescence spectroscopy (see below) it is
identified as the TA spectrum of singlets,
1
TP*.
The transient absorption spectrum of concentrated TIPS-pentacene solution
(0.075 mol/L) is shown in Fig. 2c. In contrast to the absorption spectrum of ground state
TIPS-pentacene (TP), there are major differences between TA spectra at low and high
concentration of TIPS-pentacene. In the concentrated solution, there are negative features that
correspond to excited-state absorptions. As these intense features have identically long
lifetimes (6.5 μs), they are distinct from the singlets in Fig. 2c and are correlated with one
another. The concentrated solutions also have an intense, long-lived excited state absorption
feature at 2.44 eV (Supplementary Figs. S3 and S4), which has been assigned to the excited
state absorption of triplets
29
. To confirm this assignment, we measured a dilute solution of
TIPS-pentacene with the triplet sensitiser N-methylfulleropyrrolidine
30
, and the sensitiser and
TIPS-pentacene spectra were deconvoluted using singular value decomposition. The resulting
TIPS-pentacene spectrum, dominated by triplets, is shown in Fig. 2b together with the singlet
spectrum described above. Notably, the triplet control spectrum has the three infrared features
that match the concentrated TIPS-pentacene solution. As a result, we infer that the long-lived
species in concentrated TIPS-pentacene solution is the triplet,
3
TP*.
Representative kinetic traces from concentrated TIPS-pentacene are shown in Fig. 3a.
As in Fig. 2a the kinetics are convoluted with the temporal breadth of the excitation pulse,
and during this pulse the dynamics of
1
TP* and
3
TP* resemble each other. The subsequent
decay of
1
TP* is matched by further rise in
3
TP* until the singlets have fully decayed at
11 ns. From 10 ns onward, the triplet does not rise further. The
3
TP* excited state absorption
is mirrored in the ground state bleach (Fig. S5), whose intensity approximately doubles in the
interval from 2.6 ns to 10 ns. Because the ground state bleach arises from molecules in
excited states, its continued increase demonstrates that additional TIPS-pentacene molecules
are excited after the pump pulse has passed. Moreover, the bleach signals decay with a
single-exponential lifetime of 6.5 μs, which closely resembles the
3
TP* dynamics identified
above. This correspondence indicates that after 10 ns the only excitations that remain on
TIPS-pentacene are triplets. Together these observations suggest that efficient singlet fission
occurs in concentrated TIPS-pentacene solutions. We attribute the delayed appearance of the
feature at 1.65 eV to the presence of stimulated emission, and as this decays the underlying
triplet becomes apparent.
Because of the isotropic nature of TIPS-pentacene solutions and the absence of
interferences in the infrared, we are able to determine the triplet yield (see Supplementary
Section 4 for details). The key to this calculation is the relationship between singlet and
triplet excited state absorption coefficients. For this isotropic solution, photoexcited TIPS-