Supplementary information: Enhanced charge carrier mobility in -phase polyfuorene
Paulette Prins1*, Ferdinand C. Grozema1, Benjamin S. Nehls2,3, Tony Farrell2, Ullrich Scherf2 and
Laurens D. A. Siebbeles1
1
Opto-Electronic Materials Section, DelftChemTech, Delft University of Technology, Delft, The
Netherlands
2
Department of Chemistry and Institute for Polymer Technology, Bergische Universität Wuppertal,
Wuppertal, Germany
3
Current address: Melville Laboratory, Department of Chemistry, University of Cambridge,
Cambridge, England
Dose dependence of the transient conductivity The dose dependence of the conductivity signal
observed with TRMC provides information about the decay mechanism of the charge carriers. More
specifically, when no dose dependence is observed the conductivity scales linearly with the irradiation
dose or equivalently with the charge carrier concentration. In this case the charge carriers decay via
first order processes, i.e. geminate recombination or trapping of charges. Alternatively, when the dose
normalized conductivity decreases as the irradiation dose increases, the conductivity decays faster with
increasing charge carrier concentration. In this case, second order processes play a role and the decay
of the conductivity can be attributed to (second order) charge carrier recombination.
The dose normalized conductivity for PF2/6, shown in figure S1, decays faster at higher irradiation
dose. Hence, second order charge carrier recombination plays a role in the decay of the transient
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conductivity signal for glassy polyfluorene. In accordance with this, the delayed increase in
conductivity observed for PFO is less pronounced for higher irradiation dose, i.e. a smaller fraction of
the charges from the glassy phase reaches the -phase regions when the concentration of charges in the
glassy phase is increased. The decay of the conductivity for PFO (t > 10 -6 s) is dose independent,
indicating that the charges in -phase regions decay via a first order process.
Effect of excitons on the transient conductivity Upon irradiation with high energy electrons, tracks of
ionizations are formed in the irradiated material. In hydrocarbon materials a small fraction (of the order
of a few percent [1]) of the charge carriers escapes from recombination, resulting in 'free' charge
carriers. The remaining charge carriers recombine geminately (i.e. electrons recombine with the
positive ion from which they originate) yielding singlet excitons, or non-geminately yielding both
singlet and triplet excitons. For the 3 MeV electron beam used in our experiments approximately half
of the excitons are triplets [2].
Although the excitons themselves do not contribute to the conductivity measured with the TRMC
technique, the excitons can dissociate into charge separated states or free charges that do contribute to
the transient conductivity. In order to validate that the delayed increase in the conductivity is indeed the
result of charge carrier migration to regions with an enhanced charge carrier mobility, as stated in the
main text, we will discuss the possible effects of the presence of excitons on the (delayed increase of
the) transient conductivity.
The lifetime of singlet excitons in polyfluorene is short (300 ps [3, 4] , determined at about 10 K).
Hence, charge carriers resulting from singlets are formed on the timescale of the electron pulse and
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cannot be responsible for the observed delayed increase in conductivity. The lifetime of triplet excitons,
however, can be up to about 10-5 seconds at room temperature [3].
Since the increase in conductivity was not observed for glassy phase polyfluorene, triplets may lead to
the formation of charge carriers and contribute to the conductivity signal in two ways: 1) Via triplet triplet annihilation in -phase regions. These triplets can be either initially generated in the -phase
regions or they can migrate from the glassy phase to the -phase regions. 2) Via triplet - triplet
annihilation in glassy phase regions and subsequent migration of the charges to -phase regions.
Due to the more planar conformation of the polyfluorene chains in the -phase, the electronic coupling
between adjacent fluorene units in the same polymer chain is increased as compared to the coupling in
glassy phase polyfluorene. This results in a larger bandwidth for both the valence and the conduction
band and a lowering of the bandgap in the -phase. As a consequence, the HOMO level is shifted to a
higher energy, whereas the LUMO level is shifted to a lower energy with respect to the HOMO and
LUMO levels in the glassy phase. Hence, the -phase is an energetically favorable environment for
positive charges, negative charges and excitons. As a result, when charge carriers are formed in a
region with -phase organization (via route 1)), both charge carriers are confined to the same -phase
region, with a dimension of tens of nanometers [5]. Since charge carriers that are formed via route 1)
cannot escape recombination, they cannot cause the long lived delayed conductivity signal.
Contributions to the conductivity from charge carriers formed via route 2) cannot be excluded, since
charge carriers can escape recombination in the glassy phase and migrate towards -phase regions.
Note, that although triplet - triplet annihilation is a second order process and the amount of charges
created via this process increases with irradiation dose, the observed decrease of the conductivity with
increasing irradiation dose in Fig. S1 can be the result of a more pronounced charge carrier
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recombination at higher dose. Charge carriers formed via route 2) are initially formed in the glassy
phase and have to migrate to regions with -phase organization. Hence the observed delayed increase
in the conductivity shows that the -phase is an energetically favorable environment with an enhanced
charge carrier mobility.
Temperature dependence of the transient conductivity The time-scale on which the conductivity
reaches the maximum value (tmax) is determined by the rate of the transfer of charges to -phase
polyfluorene and to the decay rate of charge carriers in the -phase, as mentioned in the main text.
Figure 2 in the main text shows that tmax is strongly temperature dependent. The time on which the
conductivity reaches the maximum is found to be thermally activated and can be described by
t max
 t0 e
Eact /k BT
, where t0 is a pre-factor and Eact is the activation energy. When tmax is normalized (for
example with respect to tmax at room temperature) both the slope and the offset of the resulting
Arrhenius plot are determined by the activation energy. An activation energy of 0.20 eV is found for
the time where the conductivity for PFO is maximum using the Arrhenius plot in figure S2.
Effect of sample density on the conductivity One of the morphological changes that can be expected
is a change in the fraction of the sample that exhibits -phase organization. If the fraction of -phase in
the more compressed sample is higher, a larger number of charges will be generated directly in the phase resulting in a less pronounced delayed increase in the conductivity. Since the mobility of charges
is higher in the -phase regions, recombination of charges is faster. When the effect of recombination
is more pronounced than the effect of the higher mobility, this results in a decrease of the conductivity.
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If the fraction of -phase in the more compressed sample is lower, the average distance between the phase regions and the charges generated in the glassy phase is larger. Consequently, a smaller fraction
of the charges generated in the glassy phase reaches the -phase (mainly due to charge carrier
recombination in the glassy phase, see above). This results in a lower conductivity and a less
pronounced delayed increase in conductivity.
Alternatively, the difference in observed conductivity for different sample density can be caused by
changes in the contact between the -phase regions and the glassy matrix. This would imply that
increased sample density reduces the contact between the regions, leading to a less pronounced
delayed increase in conductivity.
Fluorescence excitation spectra for PFO and PF2/6
A characteristic spectral feature of the -phase in PFO is a low-energy band (around 435 nm) in the
absorption spectrum, which is not present in glassy phase polyfluorene. For the solid (non-transparent)
PFO samples used in the present work it is not possible to record an optical absorption spectrum.
However in the fluorescence excitation spectrum such a characteristic absorption band should also be
present. Figure S3 shows the emission at 490 nm as a function of the excitation wavelength. In the
fluorescence excitation spectrum for PF2/6 the main absorption band of polyfluorene, around 390 nm,
is observed. The spectrum for PFO clearly exhibits an additional band at lower energy, around 430 nm.
This provides a direct indication that the PFO sample used in this work contains a considerable
fraction of -phase organization, whereas this is absent in the PF2/6 sample.
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Synthesis Unless otherwise indicated, all starting materials were obtained from commercial suppliers
(Aldrich, Fischer, EM Science, Lancaster, ABCR, Strem) and were used without further purification.
All reactions were carried out under an argon atmosphere. Analytical thin-layer chromatography
(TLC) was performed on Kieselgel F-254 pre-coated TLC plates. Visualization was performed with an
254 nm ultraviolet lamp. Silica gel column chromatography was carried out with Silica Gel (230-400
mesh) from EM Science. 1H- and 13C-NMR data were obtained on a Bruker ARX 400-spectrometer.
Chemical shifts are expressed in parts per million (δ) using residual solvent protons as internal
standards. Splitting patterns are designated as s (singlet), d (doublet), t (triplett), q (quartet), bs (broad
singlet), m (multiplet). Molecular weight determinations via Gel Permeation Chromatography (GPC)
were performed using a Spectra 100 GPC column (5 µm particles) eluted with THF at 30°C (flow rate
of 1 mL/min and concentration of the polymer: ca. 1.5 g/L. The calibration was based on polystyrene
standards with narrow molecular weight distribution. All GPC analyses were performed on solutions
of the polymers in THF at 30°C.
PF2/6: To a 100 mL flame dried Schlenk tube containing Ni(COD)2 (3.6 g, 1.3 mmol), bipyridyl (1.9
g, 1.2 mmol) and 2,7-dibrom(9,9-bis(2-ethylhexyl)fluorene (3 g, 0.55 mmol), THF (70 mL) and COD
(1.3 g, 1.2 mmol) were added via a syringe. The tube was heated for 3 d at 80°C. The reaction was
quenched with HCl in 1,4-dioxane and extracted with chloroform. The solution was washed 2 times
with 2N aqueous HCl, Na-EDTA solution and water. The solvent was removed and the residue was
dissolved in chloroform. The polymer was precipitated into methanol and extracted with ethylacetate
to give a white powder. Yield 1.8 g (50%). Mn = 152,000, PD = 1.9. 1H-NMR (400 MHz, C2D2Cl4,
25°C):? δ = 7.26 - 8.05 (m, 6H), 1.74 - 2.51 (m, 2H), 0.12 - 1.08 (m, 32 H); (all signals are rather
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broad) 13C-NMR (100 MHz, C2D2Cl4, 25°C) δ = 151.5, 140.6, 140.4, 126.4, 123.3, 120.0, 55.4,
45.1, 35.3, 34.6, 28.8, 27.6, 23.0, 14.2, 10.7.
PFO: A flame dried 100 mL Schlenk-flask was charged under argon with 2,7-dibromo-9,9-di-noctylfluorene (2.0 mmol, 1 eq), Ni(COD)2 (4.8 mmol, 2.4 eq.) and 2,2’-bipyridyl (4.8 mmol, 2.4 eq).
COD (3 mmol, 1.5 eq) and THF (40 mL) were added and the mixture heated to 80°C for three days.
The solution was poured into 2N HCl and extracted with chloroform. The organic layer was washed
with conc. EDTA solution, dried over Na2SO4 and the solvent was removed by rotary evaporation.
The residue was taken up in chloroform and precipitated into methanol. The crude solid was extracted
with ethyl acetate for two days and then reprecipitated into methanol to yield 5 in 81 % yield as yellow
plates. GPC (vs polystyrene standards in THF) (5): Mn = 190,000; Mw/Mn = 2.4 (after extraction). 1H
NMR (400 MHz, CDCl3, 25 °C): d = 7.87-7.58 (bm, 6H), 1.29-1.00 (bm, 28H), 0.82 (bt, 6H, J = 7.2
Hz, -CH3). 13C NMR (100 MHz, CDCl3, 25 °C): d = 151.8, 140.5, 140.0, 126.2, 121.5, 120.0, 55.4,
40.4, 31.8, 30.0, 29.2, 28.0, 23.9, 22.6, 14.0.
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Figure S1. Dose normalized change in conductivity after creation of charge carriers in a) PF2/6 and b) PFO for
various irradiation doses ( = 0.30 103 kg/m3). The irradiation dose (D irr ) changes from 4 to 80 Gy in the direction
of the arrow (1 Gy = 1 J/kg).
Figure S2. Arrhenius plot for the time where the conductivity is maximum (tmax) relative to tmax for room
temperature (tmaxRT) for the temperature range from 173 K to 383 K in 10 K steps.
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Figure S3. Fluorescence excitation spectrum for PFO (solid line) and PF2/6 (dashed line) measured at an emission
wavelength of 490 nm. The inset shows the difference between the two spectra.
References
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P. Prins, F. C. Grozema, J. M. Schins, S. Patil, U. Scherff, and L. D. A. Siebbeles, Phys. Rev. B 73, 045204
(2006).
[2]
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Cambridge, 1997), p. 11.
[3]
C. Rothe and A. P. Monkman, Phys. Rev. B 68, 075208 (2003).
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