Synthesis and exciton dynamics of triplet sensitized conjugated polymers

Article abstract of DOI:10.1021/jacs.5b06223

We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of

Full text of DOI:10.1021/jacs.5b06223

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Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated
Polymers
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Rolf Andernach,†a Hendrik Utzat,†a, b Stoichko Dimitrov,a Iain McCulloch,a,c Martin Heeney,a* James
Durrant,a* and Hugo Bronstein.d*
a) Centre for Plastic Electronics, Imperial College London, UK, SW7 2AZ
b) Department of Chemistry, MIT, Cambridge MA, 02139, USA
c) SPERC, King Abdullah University of Science and Technology, Thuwal, 23955ꢀ6900, Saudi Arabia
d) Department of Chemistry, University College London, UK, WC1H 0AJ
KEYWORDS Conjugated polymers, excitons, triplets, diffusion length, organic solar cells
ABSTRACT: We report the synthesis of a novel polythiopheneꢀbased hostꢀguest copolymer incorporating a Ptꢀporphyrin complex
(TTPꢀPt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin
films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple timescales and investigated the mechanism
of triplet exciton formation. During sensitization, singlet exciton diffusion is followed by exciton transfer from the polymer
backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the
triplet exciton back transfer from the Ptꢀporphyrin to the polymer and find that 60% of the complex triplet excitons are transferred
with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low
triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased tripletꢀtriplet
annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and upꢀ
conversion layers.
complex where they can subsequently decay radiatively
Introduction
(phosphorescence) due to the large spin orbit coupling of the
Photoexcited states in organic semiconductors, termed
heavy metal atom.2ꢀ3 One drawback, however, shows at high
excitons, can have two spin states: a singlet state and a triplet
current densities where reductions in efficiency have been
state. In the triplet state, the projections of the respective spin
momenta on a given axis are aligned in parallel preventing
observed due to tripletꢀtriplet annihilation.4
In polymer:fullerene bulk heterojunction solar cells,
them from occupying the same space and hence they
population of the polymer triplet state is considered a main
loss mechanism. It has been shown, that ChargeꢀTransfer (CT)
States, electronꢀhole pairs bound across the polymer:fullerene
interface, can irreversibly recombine to form the polymer
triplet excitons due to their lower energy relative to the CT
state.5ꢀ11 Furthermore, the presence of triplet excitons has been
linked to the lifetime and stability of conjugated polymers
both in neat film and in BHJ devices due to their quenching by
oxygen potentially forming highly reactive oxygen species.12
Triplet excitons can however be used for the generation of
photocurrent when energy levels are appropriately aligned. It
has been recently demonstrated for pentaceneꢀbased devices
utilising singlet fission that 100% quantum efficiencies can be
surpassed. This is possible as singlet fission allows the
conversion of one singlet into two triplet excitons each of
which is capable of generating charges.13ꢀ16 In addition, the
incorporation of tripletꢀtriplet annihilation (TTA) upꢀ
conversion layers allow the conversion of IR radiation,
normally wasted in solar cells due to their transparence in this
wavelength region, into higher energy singlet excitons that are
able to generate photocurrent.17ꢀ19 Despite the ubiquitous
presence of triplet excited states in optoelectronic devices
experience reduced Pauli repulsion. This results in an overall
lower energy of the triplet state compared to the singlet.1
While the optical transitions between singlet states are
typically allowed, the respective transitions between singlet
and triplet states are spinꢀforbidden. Therefore, singlet
excitons are normally the primary photoexcitations in organic
semiconductors as triplet states cannot be accessed by
excitation directly. Their ease of access has allowed singlet
excitons to be studied extensively in organic semiconductors.
Spectroscopic techniques are routinely employed to measure
their properties such as energy levels, lifetime and diffusion
length, and thus opened the door for organic semiconductors
to be used in optoelectronic devices.
Triplet excitons are, however, of great importance in many
optoelectronic devices. In OLEDs, a statistical ratio of 3:1
triplets to singlets is electrically generated via nonꢀgeminate
hole and electron recombination, hence potentially limiting
their efficiency due to the nonꢀemissive nature of the polymer
triplets. The incorporation of heavy metal complexes with
high phosphorescence quantum yields can allow the utilization
of up to 100% of excitons through the concept of energy
harvesting whereby all the excitons are transferred to the metal
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neither the yield, energetic position nor mobility of polymer
triplet excitons are routinely measured, partially due to the
challenges of experimentally and theoretically accessing these
states.20ꢀ21
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Understanding and controlling the properties of triplet
excitons is a key consideration to allow polymers to be used in
the next generation of optoelectronics devices. Of particular
interest is the determination of polymer triplet exciton
mobility. It has been suggested that the longer lifetime of
triplet excitons could lead to a longer diffusion length than that
of their singlet counterparts.22ꢀ26 Theoretical predictions have
in fact suggested that an enhanced diffusion length could even
be achieved with significant disorder present in the system.27
Achieving an enhanced exciton diffusion length would allow
for the construction of efficient bilayer organic photovoltaic
devices, therefore negating the use of the complicated bulk
heterojunction.22ꢀ23, 25ꢀ26 Conversely, due to the slower Dexter
type nature of the triplet energy transfer, triplet excitons have
also been suggested to possess lower mobilities than their
singlet counterparts. In tripletꢀtriplet annihilation upꢀ
conversion layers, a lower exciton mobility could be utilized
to induce higher local exciton population thus increasing the
collision encounter probability.
S1
T1
S1
T1
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S0
Figure 1: Triplet sensitization of a conjugated polymer using a
heavy metal complex
One approach to access the triplet state in the polymer from
the photoexcited singlet state is to blend the polymer with
heavy metal complexes, which allow the conversion of the
singlet to the triplet state due to the strong spin orbit coupling
of the heavy metal. For this to occur, careful consideration of
the polymer and metal complex energy levels must be taken.
The emission of the polymer must overlap with the absorption
of the metal complex to allow for efficient energy transfer.
Subsequently, the triplet energy level of the polymer must be
below that of the complex to allow for back transfer of the
triplet excitons (figure 1). This approach has been
Experimentally, there have been few reports of triplet
exciton diffusion lengths in organic semiconductors.28 The
hole transport material N,N′ꢀDi(1ꢀnaphthyl)ꢀN,N′ꢀdiphenylꢀ
(1,1′ꢀbiphenyl)ꢀ4,4′ꢀdiamine (NPD) has recently been reported
to show a triplet diffusion length of 87 nm,29 whereas the
triplets in structurally related 4,4′ꢀbis(carbazolꢀ9ꢀyl)1,1′ꢀ
biphenyl (CBP) have been reported to have diffusion lengths
of 12, 25 and 250 nm.30ꢀ32 C60 was found to possess a triplet
diffusion length of 30ꢀ35 nm,33 and pentacene, which has been
extensively studied for its ability to undergo singlet fission,
has been shown to have a triplet diffusion length of 40 nm.34
In contrast, structurally related rubrene has been reported to
have a triplet diffusion length of 4 µm in a single crystal.35 For
polymeric systems we are aware of only four literature values
of triplet exciton mobility. Samiullah et al. reported a triplet
diffusion length of almost 4 µm in a ladderꢀtype polymer.36
Rand et al. reported both the triplet diffusion length of a polyꢀ
phenylenevinylene poylmer to be 9 nm (as compared to the
4nm single diffusion length).22ꢀ23 Tamai et al. very recently
determined the triplet diffusion lengths of two fluorene based
polymers to be 0 and 41 nm.37 It is worth noting how much the
triplet properties of two such related structures can differ,
highlighting the difficulty as well as the importance of
creating the toolset necessary to systematically investigate
triplet excited state properties. For polymer based
optoelectronics devices to attain their ultimate efficiency and
stability, the nature of the triplet exciton must be understood,
especially in the next generation of OPV devices and TTA upꢀ
conversion layers where control over triplet exciton mobility is
the key to their success. Consequently, it would be of
considerable interest to possess a class of materials where
photoexcitation results in a high population of the triplet
excitons – hence allowing for their detailed study and potential
exploitation.
demonstrated previously, with the aim of producing tripletꢀ
24, 38ꢀ39
sensitized solar cells.22,
However, it has been
demonstrated that blends of typically amorphous polymers and
highly crystalline metal complexes often lead to extensive
phase segregation resulting in a nonꢀhomogenous distribution
of the metal complex.40ꢀ43 Furthermore, this aggregation can
result in undesirable exciton annihilation processes.43ꢀ44
To overcome these issues we report here the synthesis and
photophysical characterization of a novel polythiophene
polymer which includes a Ptꢀporphyrin complex in the
backbone to create a tripletꢀsensitized conjugated polymer.
Our material system allows for the tuning of the incorporation
ratio of Ptꢀporphyrin in the polymer host and for the inꢀdepth
study of both the singlet and triplet exciton formation.
Incorporation of the metal complex into the polymer backbone
prevents phase segregation whilst also simplifying film
formation by removing the necessity of blending the two
components and may also enhance the energy transfer
mechanisms via direct orbital overlap. Furthermore,
porphyrins typically show low solubility; incorporation into
the polymer backbone circumvents any potential processing
issues which can arise as a result of this.
Results and Discussion
The synthesis of the polymers is shown in Schemes 1 and 2.
Mesityldipyrromethane (1) was obtained through a Lewis acid
catalyzed condensation reaction of mesitaldehyde in an excess
of pyrrole, leading to
a white, sheetꢀlike solid. The
dipyrromethane was reacted to form the platinated 10,20ꢀ
dibromophenylꢀ5,15ꢀdimesitylporphyrin (2) in two steps.
Reaction of (1) with 4ꢀbromobenzaldehyde proceeded at low
concentrations (~5 mM), favoring the ring closure over linear
side reactions; aromatization of the porphyrin cores was
achieved using the oxidizing agent DDQ. The free base was
then reacted with PtCl2, forming the corresponding platinum
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porphyrin complex. The mesityl substituents were chosen so decreasing molecular weight at higher porphyrin dopant ratios.
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as to impart solubility to the porphyrin and the final polymer,
as well as to minimize porphyrin aggregation in the solid state,
mediated by the outꢀofꢀplane twisting of the mesoꢀsubsituents.
We believe this to be due to a slight loss in solubility for the
resulting polymers which resulted in a quicker precipitation of
the polymer chains during the synthesis. We note the lower
molecular weight of the porphyrin containing polymers may
affect their photophysical properties somewhat, but we believe
that due to the low molecular weight of the repeat units the
degree of polymerization is sufficiently high for them to be
considered polymeric. The incorporation of the platinum
H
(i)
(ii)
N
N
N
N
N
O
Br
Br
Pt
(iii)
NH HN
1
1
2
porphyrin was verified by H NMR spectroscopy (Supporting
9
Information S2), the aromatic signals of the thiophene (7.70
ppm) and phenyl groups (7.51 ppm) clearly attributed and
their integrals matching. The porphyrin peaks (8.9 – 8.0 ppm)
of the 10 wꢀ% doped polymer can be seen in comparison, with
each peak integral corresponding to four protons on the
porphyrin substructure. In the polymers containing higher
weight percentages of porphyrin a secondary smaller set of
porphyrin peaks can be observed. This is likely to be due to a
terminal porphyrin on the polymer chain as opposed to the
above species which are incorporated into the conjugated
polymer on both sides. Overall, the incorporation ratio of the
porphyrin was slightly lower than the feed ratio for all
concentrations which possibly stems from a lower reactivity of
the porphyrin monomer relative to the bithiophene species.
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Scheme 1: Synthesis of the porphyrin precursor, (i) MgBr2, TFA, DCM,
50%; (ii) a) DCM, TFA, b) DDQ, 11%; (iii) PhCN, PtCl2, reflux, 24%
We chose the medium bandꢀgap conjugated backbone
poly(phenylꢀbithiophene) (TTP) for this study in order to
ensure that all the energetic requirements for tripletꢀ
sensization could occur efficiently. The TTP polymer was
synthesized by Suzuki polymerization of 5,5’ꢀdibromoꢀ3,3’ꢀ
didecylꢀ2,2’ꢀbithiophene with 1,4ꢀbenzenediboronic acid
bispinacolic ester (Scheme 2). The porphyrinꢀcontaining
polymers (TTP-Pt) were synthesized via a Suzuki terꢀ
molecular polymerization, with the different feed ratios and
properties given in Table 1. Terꢀmolecular polymerizations
have previously been used to optimize and explore the effects
of specific monomers in conjugated polymers.45
C10H21
C10H21
(i)
S
O
O
O
Br
S
Br
B
O
B
S
S
C10H21
C10H21
TTP
N
N
N
feed ratio:
5, 10%
Br
Pt
Br
N
C10H21
N
N
S
Pt
y
S
x
N
N
(i)
C10H21
Figure 2: Absorption and Emission spectra of novel polymers
TTP-Pt
Figure 2 displays the UVꢀVIS spectra of TTP, TTPꢀPTꢀ5
and TTPꢀPtꢀ10 polymers in solution and thin films spinꢀcoated
on glass. All thin film spectra are weakly redꢀshifted compared
to the solution, likely because of solid state packing effects
commonly observed in conjugate polymers (404 nm compared
to 419 nm for TTP). The absorption maximum of the TTP
polymer and the TTPꢀPt5 and TTPꢀPt10 polymers is around
λmax = 416 nm and have similar onset of polymer absorption.
These findings indicate that the addition of the Ptꢀporphyrin
complex and the difference in molecular weight, as
determined by GPC, has no noticeable effect on the effective
conjugation length of the polymer. The Ptꢀporphyrin complex
exhibits two distinct absorption features around 405 nm and at
510 nm. The polymers show emission around 525 nm which
we assign to the fluorescence of the TTP backbone. Both, the
absorption and emission energies are consistent with
previously reported values for TTP.46ꢀ47 With increasing
content of Ptꢀporhyrin in the backbone, the fluorescence
intensity decreases (Figure 2), indicating an efficient singlet
exciton quenching, which we tentatively assign to quenching
induced by the Ptꢀporphyrin backbone.
Scheme 2: Synthesis of the TTP and PtꢀTTP polymers, (i) Pd(PPh3)4,
base, toluene, Ar, 115°C
Table 1: Properties of novel polymers
b)
c)
c)
Mn
Mw
PDI
FR
IR
IR
[kDa] [kDa]
[wꢀ
%]
[molꢀ [wꢀ
a)
a)
%]
%]
TTP
44.1
10.8
14.5
73.0
19.0
24.6
1.65 0%
1.75 5%
0%
2%
5%
0%
3%
8%
TTPꢀPt5
TTPꢀPt10
1.70 10
%
a) Determined by GPC using a polystyrene standard.; b) Feed
ratio (FR) determined by weight per repeat unit.; c) Incorporation
1
ratios (IR) determined by integration of the H NMR porphyrin
signal per repeat unit (referenced to the integral of the thiophene
aromatic signal).
TPP was isolated as a bright yellow solid with high visible
fluorescence, whereas all the porphyrin containing polymers
were visibly less fluorescent. Reasonable molecular weights
were obtained for all the resulting polymers, with a trend for
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A comparison of the transient absorption between the three
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polymers reveals a clear increase of the initial signal
amplitude with the incorporation ratio of Ptꢀporphyrin, which
shows an improved triplet generation yield in the presence of
the metal complex in the polymer backbone. In addition, a
change in the decay dynamics of the triplet exciton of TTPꢀPtꢀ
5 and TTPꢀPtꢀ10 compared to the non porphyrin containing
TTP polymer is observed. While the decay of the triplet
exciton of TTP follows a monoꢀexponential dynamics, the
relaxation of the triplet exciton in the TTPꢀPt polymers can
only be fitted with biꢀexponential functions yielding τ1ꢀTTPꢀ
Pt5 = 440 ± 3 ns and τ2ꢀTTPꢀPt5 = 3.55 ± 0.03 ꢁs; τ1ꢀTTPꢀ
Pt10 = 380 ± 3 ns and τ2ꢀTTPꢀPt10 = 3.16 ± 0.03 ꢁs. The ꢁs
decay component is not associated with any spectral evolution.
In the course of our discussion we will provide a possible
explanation for the altered exciton decay dynamics of the
TTPꢀPt polymers compared to the TTP polymer.
1.8
1.5
1.3
1.0
0.8
0.5
0.3
0.0
TTP
TTP-Pt5
TTP-Pt10
A
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600 650 700 750 800 850 900 950 1000
λ [nm]
3.0
2.5
2.0
1.5
1.0
0.5
0.0
B
TTP
TTP-Pt5
TTP-Pt10
TTP-Pt10
In order to elucidate the singlet exciton dynamics and the
triplet exciton formation we utilized ultrafast TAS to measure
thin films of TTP, TTPꢀPtꢀ5 and TTPꢀPtꢀ10. The spectra of
TTP and TTPꢀPtꢀ10 up to 5.8 ns are presented in Figure 4.
1
Nitrogen
Oxygen
1E-6
1E-5
1E-4
1E-6
1E-5
time (s)
1E-4
Figure 3: Transient absorption spectra of TTP, TTPꢀPtꢀ5 and
TTPꢀPtꢀ10 measured at 800 ns after photoexcitation (420nm) (A)
and the corresponding decay dynamics measured at 800 nm (B).
The insert in (B) shows the effect of moving from a nitrogen
atmosphere (used for all experiments unless otherwise stated) to
oxygen for TTPꢀPtꢀ10. Analogous acceleration of decay dynamics
in the presence of oxygen were observed for all three polymers.
In order to study the singlet and triplet exciton dynamics
and the mechanism of the triplet exciton formation in TTP,
TTPꢀPtꢀ5 and TTPꢀPtꢀ10, we used Transient Absorption
Spectroscopy (TAS) on multiple timescales. First, we
performed microsecond TAS of the polymer thin films spinꢀ
coated on glass to determine the triplet exciton yields and
triplet exciton decay dynamics in the three polymers studied.
Figure 3 (A) presents the TAS spectra in the visible spectral
region at 800 ns after photoexcitation of the TTP backbone
(420 nm) for TTP, TTPꢀPt5 and TTPꢀPt10. The corresponding
decays of the transient absorption are presented in figure 3
(B). For TTP, TTPꢀPtꢀ5 and TTPꢀPtꢀ10 a similarly broad
transient absorption signal at around 800 nm is observed. This
signal is strongly quenched by the presence of oxygen for all
samples, as it is shown for TTPꢀPtꢀ10 in the inset of figure 3
(B). The decay of the transient absorption of the pristine TTP
polymer was fitted well with a monoꢀexponential function
with τTTP = 501 ± 3 ns. Therefore, based on the similarity of
the spectra and the observed oxygen quenching, we assign the
signal to the transient absorption of the triplet exciton on the
TTP backbone. Previously, Ohkita et al. reported the transient
absorption maximum of the TTP polymer triplet exciton
around 700 nm which is in close agreement with our observed
value.46
Figure 4: TAS spectra of TTP and TTPꢀPtꢀ10 films at delay times
of 0.04 – 5800 ps after photoexcitation of the TTP (λexc: 420nm).
The spectra of the neat TTP polymers exhibit three
characteristic features. First, a negative, quickly relaxing
transient absorption at around 525 nm is observed,
disappearing on the 10 – 100 ps timescale. Comparison to the
steady state PL spectra (see figure 2) allows us to assign this
feature to stimulated emission from the TTP singlet exciton.
Second, a strong, fast decaying transient absorption in the red
spectral region is observed. This transient absorption signal
can be assigned to absorption of the singlet exciton of the TTP
backbone which exhibits an absorption maximum in the near
IR spectral region (see SI 3). Following the rapid decay of
these two signals, assigned to singlet exciton decay, a residual
broad transient absorption with increasing amplitude from 500
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nm to 800 nm is observed, which does not evolve over the triplet rise were fitted with monoexponential functions with a
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timescale measured (up to 5.8 ns after photoexcitation). This
broad, long lived transient absorption matches the spectral
shape of the triplet exciton absorption measured at 800 ns
delay (figure 3 (A)) after photoexcitation and can thus be
assigned to the triplet exciton on the TTP backbone.
shared time constant of 48.0±1.4 ps, indicating direct
population conversion from the singlet to the triplet exciton
manifold. This time constant relates to the singlet exciton
lifetime in the TTP polymer, in which the triplet excitons are
formed via intersystem crossing. Similar lifetimes have
previously been reported for other medium bandgap
polythiophene polymers such as P3HT.51 By taking into
account the extinction coefficient of the TTP triplet exciton ε
(800 nm) = 80.000 Mꢀ1cmꢀ1 (see SI4 for calculation of ε), we
estimated the efficiency of the ISC in the neat polymer to be
20%, calculated from the ratio of the yields of the
photoexcited singlet excitons and the generated triplet
excitons. Given that
The spectra of the TTPꢀPtꢀ10 polymer exhibit similar
features as the TTP polymer, but an additional negative,
relatively narrow transient absorption feature is observed at
around 510 nm, most prominently in the spectra at time delays
of 85 and 275 ps, as shown in figure 4 (arrow). We assign this
feature to the ground state bleach (GSB) of the Ptꢀporphyrin
(compare to GS spectrum in figure 2). We note that ISC from
singlet to triplet excited states in the Ptꢀcontaining organic
molecules and other heavy metal porphyrins have been
observed to be very fast (<200fsꢀ1ps), significantly faster than
the lifetime of this feature. We thus assign this Ptꢀporphyrin
GSB signal specifically to the presence of Ptꢀporphyrin triplet
states.48ꢀ50
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Á¥¯Ÿ
¥¯Ÿ
Ɣ
»¼¼¿¹¿»Ä¹Ï
͟
ͤ
and using our estimates for the singlet exciton decay rate
constant k0 of (48 ps)ꢀ1 = 2.1 x 1010 sꢀ1 and ISCeff = 20%, we
obtain kISC = (200±10 ps)ꢀ1 = 4.2 x 109 sꢀ1. The relatively high
ISC efficiency in the polymer can be attributed to the
incorporation of a higher homologue atom, (i.e. sulfur, in the
polythiophene polymer exhibiting a high spinꢀorbit coupling
constant) or residual palladium from the polymerization.52
Based on the assignments made, we were able to extract the
excited state population dynamics from the transient
absorption spectra. Specifically the amplitude of the broad
TTP stimulated emission band at 525 nm was used as an assay
of TTP singlet excitons. The triplet TTP absorption at 800 nm
was determined by subtracting the polymer singlet absorption
from the total photoinduced absorption at this wavelength,
assuming a negligible triplet yield early (200 fs) after
photoexcitation. The triplet photoinduced absorption was used
to estimate the TTP triplet exciton population rise in the films.
Finally, the amplitude of the narrow GSB feature of the Ptꢀ
porphyrin complex at 510 nm was used as a measure of the
triplet state population of this complex. The resulting transient
decays are presented in figure 5.
The stimulated emission decay in TTPꢀPtꢀ10 (figure 5,
orange traces) occurs on a faster timescale compared to the
neat TTP, as expected from the observed TTP fluorescence
quenching in this sample. It is striking that for this film, unlike
for the TTP film, the rise of TTP triplet absorption is
significantly delayed relative to the decay of this stimulated
emission. The decay of the TTP stimulated emission
approximately coincides with the appearance of the Ptꢀ
porphyrin GSB, whilst the rise of the TTP triplet absorption
coincides with the decay of this GSB. This is strongly
indicative of efficient TTP singlet exciton quenching by the
Ptꢀporphyrin complex on the 10’s ps timescale and a
subsequent slower back transfer of the complex’s triplet state
to the polymer (figure 6).
Singlet Exciton Transfer
~10 ps
Singlet Exciton Transfer
~1000 ps
Triplet Exciton Back Transfer
Triplet Exciton Back-Transfer
Figure 5: Dynamics of spectral features observed from transient
absorption spectra such as that shown in figure 4, comprising the
525 nm stimulated emission assigned to TTP singlet excitons
(triangles), the photoinduced absorption assigned to TTP triplet
excitons (circles) and 510 nm GSB signal assigned to Ptꢀ
porphyrin triplet states (dotted line). Lines represent least square
fits of these population dynamics using monoexponential
functions.
Figure 6: Diagram showing triplet sensitization process
PtꢀPorphyrins such as the one studied here are known to
undergo very fast and efficient ISC, thus here we assume that
after initial population of the Ptꢀporphyrin singlet, the
porphyrin undergoes extremely fast and efficient ISC,
previously estimated to be <200 fs.49ꢀ50 We also assume a
homogenous distribution of Ptꢀporphyrin quencher sites in the
film because the covalent incorporation of the Ptꢀporphyrin as
a quencher molecule mitigates phase segregation between the
materials. Therefore, we used a single exponential function to
fit the decay of the Ptꢀporphyrin GSB, which we associated
with the triplet back transfer to the TTP triplet, giving a time
We focus first on the TTP polymer (figure 5, black traces).
As the stimulated emission of the singlet exciton relaxes
(black triangles), the triplet exciton absorption rises (black
circles). Both, the stimulated emission decay and the TTP
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constant of 1087±104 ps. Due to spin conservation rules this triplet exciton concentration up until an exciton density of 2.5
x 1017 cmꢀ3 is reached. However, upon increasing the initial
triplet density to 4.6 x 1017 cmꢀ3 via using higher light
excitation density to pump the sample an accelerated
population decay is observed. Notably, the TTA in TTPꢀPtꢀ10
occurs at triplet densities which do not cause annihilation
processes in the TTP polymer. The triplet excitons in TTPꢀPtꢀ
10 are formed predominantly due to the energy transfer from
the Ptꢀporphyrin complex and are thus generated in higher
densities around the Ptꢀporphyrin centers. In turn, this
inhomogeneous distribution of excitons may result in a higher
probability of TTA in TTPꢀPtꢀ10 compared to the TTP
polymer, where in the latter the triplets are formed via ISC
homogeneously. Based on these observations, we conclude
that the polymer triplet exciton is confined in close proximity
to the Ptꢀporphyrin complex, perhaps due to a low triplet
exciton mobility. As noted earlier we observe the triplet
exciton decay for TTPꢀPt polymers is biphasic (Figure 3 b)
and its lifetime is significantly increased (> 3 µs) whereas the
triplet lifetime of the neat TTP polymer is ~500 ns only (as
determined by monoexponential fitting of its decay (Figure
3b)). The excited state lifetime of Ptꢀporphyrin complexes is
typically > 40 µs.54 This suggests that the triplets generated via
the platinum complex are in equilibrium with the triplet state
of the complex (Figure 8). The proposition that the Ptꢀ
porphyrin and TTP triplet states are in equilibrium is
consistent with the observed incomplete triplet back transfer
(60%) from the Ptꢀporphyrin to the TTP backbone. A similar
triplet state equilibria has been reported for conjugated small
molecules but has never been demonstrated in a conjugated
polymer.55ꢀ56 Using a Boltzmann distribution, we can therefore
estimate the triplet free energy level of the polymer to be
~0.06 eV below that of the Ptꢀcomplex. The phosphorescence
emission maxima of Pt ꢀTPP is 1.86 eV and thus we can
estimate the polymer triplet energy level to be ~1.93 eV. The
SꢀT gap can subsequently be estimated to be 0.65 eV which is
in accordance with studies suggesting that the exchange
energy in conjugated polymers is approximately 0.7 eV.57ꢀ58
Such drastic manipulation of local triplet exciton density and
lifetime which result in increased TꢀT annihilation events and
has significant implications for tripletꢀtriplet annihilation
upconversion layers. It is proposed that utilizing low triplet
exciton mobility materials may lead to nonꢀdiffusion
controlled TTAꢀUC materials whereby high local triplet
density is used to increase upconversion annihilation
efficiency.
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triplet energy transfer can only occur via shortꢀrange Dexter
electron exchange mechanism, which depends strongly on the
orbital offset as well as on the orbital overlap between the
electron exchanging entities. According to our results, this
process is therefore the rate limiting step in the overall process
of triplet exciton formation in the TTPꢀPt polymers. The time
constant for the overall process of TTP triplet formation is of a
similar magnitude to the previously reported time constant for
mediated triplet exciton formation in polyfluoreneꢀIridium
complex blends: 0.28 ns.53 In addition to the rate constant of
triplet population, we also estimated the efficiency of the
triplet exciton back transfer by simply taking the ratio of the
GSB of the Ptꢀporphyrin at 5.5 ns and 30 ps, revealing very
high triplet exciton generation efficiency of ~60%.
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One of the properties of triplet excitons in organic
semiconductors most relevant to optoelectronic applications is
their diffusion length. Here, we carried out transient
absorption spectroscopy on the µs timescale as a function of
excitation density to study the decay dynamics of the triplets
in the TTP polymer and TTPꢀPtꢀ10 (Figure 7). Tripletꢀtriplet
exciton annihilation is a wellꢀdocumented process taking place
at increased excitation densities when the encounter of two
triplets can lead to the generation of one singlet exciton. This
process is expectedly strongly dependent on the triplet exciton
diffusion constant of the material studied and as such can be
used for estimation of triplet exciton diffusion lengths.
Figure 7: Decay dynamics of the triplet excitons at different
initial triplet densities for TTPꢀPtꢀ10. The inset shows the
corrresponding kinetics of the neat TTP polymer. The densities
were calculated on the basis of the extinction coefficient of the
TTP triplet exciton and the film thickness.
The TTP polymer showed triplet exciton decay completely
independent from the initial triplet yield, at least in the range
of triplet exciton densities studied here (figure 7, inset). On the
basis of our triplet density dependent decay dynamics, we
were able to estimate an upper limit for the triplet exciton
diffusion in the TTP. The highest triplet density created upon
laser excitation in the TTP polymer corresponds to an initial
mean separation distance between triplets of 12.4 nm. The
absence of any TTA indicates that the triplet exciton diffusion
length in the pristine TTP polymer is below this value.
For the TTPꢀPtꢀ10 polymer we observed that the decay
dynamics of the triplet excitons is independent of the initial
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diffusion lengths using only spectroscopic methods on thin
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films. Finally, we demonstrate that this novel hostꢀguest
polymeric system can be used to manipulate the spatial
distribution of triplet excitons and their lifetime which we
believe has significant implications for both triplet sensitized
OPV devices and tripletꢀtriplet annihilation upꢀconversion
layers.
Energy Transfer
ASSOCIATED CONTENT
Supporting Information. Synthetic details and photophysical
characterization are shown. http://pubs.acs.org.”
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AUTHOR INFORMATION
ISC
Keq
1Por
Corresponding Author
3Por
3TTP
* h.bronstein@ucl.ac.uk, j.durrant@imperial.ac.uk
Author Contributions
†These authors contributed equally.
knr
ACKNOWLEDGEMENTS
We are grateful to the Imperial College JRF scheme for funding.
We are also grateful to the EPSRC, EP/I019278/1 for funding.
S0
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Figure 8: Proposed exciton transfer and equilibration processes in
TTPꢀPt polymer films.
Conclusion
We report the synthesis of a series of novel polythiopheneꢀ
based semiconducting polymers suitable for efficient polymer
triplet exciton generation. The triplet formation is achieved by
conversion of the singlet excitons using a hostꢀguestꢀapproach
which covalently incorporates heavy metal platinum porphyrin
into the conjugated backbone. This approach of incorporating
the complex into the polymer backbone allows for an efficient
generation of triplet excitons in the polymer material while
avoiding potential phase segregation between the polymer
backbone and the Ptꢀporphyrin complex in thin films.
We obtained a detailed photophysical picture of the singlet
and triplet exciton dynamics and the triplet exciton formation
mechanism using transient absorption spectroscopy. Initial
singlet photoexcitations and transferred to the Ptꢀporphyrin
which undergoes ISC. The triplets of the complex are
subsequently transferred back onto the polymer triplet energy
level with an efficiency of 60% and a time constant of
1087±104 ps, indicating a slow, Dexter electron exchange
process as the underlying mechanism. By measuring the triplet
density dependent triplet decay dynamics in the TTP polymer
we were able to define an upper limit for the triplet exciton
diffusion length of 12.4 nm. We associate the incomplete
triplet backtransfer to an equilibrium between the triplet states
of the polymer and the complex which is causing a prolonged
polymer triplet exciton lifetime in platinum containing
polymers. This results in an increase in local triplet exciton
density and increased tripletꢀtriplet annihilation events
potentially allowing for
upconversion layers.
a new method of producing
Our findings show that the sensitization of semiconducting
polymers with triplet excitons can be very efficient. This novel
materials system allows for determination of almost all the
photophysical parameters of a conjugated polymer including
estimations for the polymer triplet energy level and triplet
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