Supporting Information Ultrafast Singlet Fission in a Push-Pull Low ...
Supporting Information
Ultrafast Singlet Fission in a Push-Pull Low-Bandgap Polymer Film
Yukitomo Kasai,† Yasunari Tamai,† Hideo Ohkita,*,†,‡ Hiroaki Benten,† and Shinzaburo Ito†
†
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan ‡ Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
-S1-
Sample
Preparation.
Poly[4,6-(dodecyl-thieno[3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-
dioctoxylbenzo[1,2-b:4,5-b]dithiophene)] (PTB1) was purchased from Luminescence Technology Corp. with Mw = 83,000 and PDI = 3.7, and used without further purification.
For the assignment
of PTB1 polaron, phenyl-C61-butyric acid methyl ester (PCBM, Frontier Carbon, E-100H) was blended as an electron acceptor with PTB1.
Thin films of pristine PTB1 and PTB1/PCBM (50 :
50 wt%) blend were prepared onto sapphire substrates by drop-casting from a chloroform solution. Iodine (Nacalai Tesque) was doped in the PTB1 film in order to assign PTB1 polaron.
For the
assignment of PCBM radical anion, a polystyrene film doped with PCBM (30 wt%) and N,N,N’,N’tetramethyl-p-phenylene diamide (TMPD, Wako) (20 wt%), which serves as an electron donor, was prepared onto a quartz substrate by spin-casting from a chlorobenzene solution.
Spectroscopic Measurements.
UV–visible absorption and photoluminescence (PL) spectra were
measured with a spectrophotometer (Hitachi, U-3500), and a fluorescence spectrophotometer (Horiba Jobin Yvon, NanoLog) equipped with a photomultiplier tube (Hamamatsu, R928P) and a liquid-nitrogen-cooled InGaAs near-IR array detector (Horiba Jobin Yvon, Symphony II) under ambient atmosphere.
The PL decay was measured by the time-correlated single-photon-counting
(TCSPC) method (Horiba Jobin Yvon, FluoroCube). were 640 and 750 nm, respectively. ps.
The excitation and detection wavelengths
The total instrument response function is an fwhm of ca. 280
A weak excitation power was used to prevent single–singlet exciton annihilation.
Femtosecond transient absorption data were collected with a pump and probe femtosecond transient spectroscopy system.
This system consists of a regenerative amplified Ti:Sapphire laser
(Spectra-Physics, Solstice), and two transient absorption spectrometers (Helios and FSP-1000-IR). Helios (Ultrafast Systems) was used for a spectral range from 400 to 1700 nm (visible–near-IR region), and FSP-1000-IR (Unisoku Scientific Instruments) was used for a spectral range from 1300 to 2500 nm.
The amplified Ti:sapphire laser provided 800 nm fundamental pulses at a repetition
rate of 1 kHz with an energy of 3.4 mJ and a pulse width of 100 fs (fwhm), which were split into two optical beams with a beam splitter to generate pump and probe pulses.
One fundamental
beam was converted into pump pulse at 400 nm with a second harmonic generator (Ultrafast Systems, Apollo-SS) or other wavelengths with an ultrafast optical parametric amplifier (SpectraPhysics, TOPAS), which was modulated mechanically with a repetition rate of 500 Hz (Helios) or 250 Hz (FSP-1000-IR).
For visible–near-IR measurements (Helios), the other fundamental beam
was converted into white light pulses employed as probe pulses in the wavelength region from 400 to 1700 nm.
The polarization direction of the linearly polarized probe pulse was set at a magic
angle of 54.7° with respect to that of the pump pulse in order to cancel out the orientation effects on -S2-
the dynamics. The temporal evolution of the probe intensity was recorded with a CMOS linear sensor (Ultrafast System, SPEC-VIS) for the visible measurement and with an InGaAs linear diode array sensor (Ultrafast System, SPEC-NIR) for the near-IR measurement.
For the measurements
from 1300 to 2500 nm (FSP-1000-IR), the other fundamental beam was converted with an ultrafast optical parametric amplifier (Spectra-Physics, TOPAS) employed as a monochromatic probe pulse, which was modulated mechanically with a repetition rate of 500 Hz.
The linearly polarized pump
pulse was depolarized in order to cancel out the orientation effects on the dynamics.
The temporal
evolution of the probe intensity was recorded with an InGaAs photodiode (Hamamatsu, G5853-21) which was cooled at −20 °C with a cooling controller (Hamamatsu, C1103-04).
The transient
absorption spectra and decays were collected over the time range from −1 ps to 7 ns. Typically, 2500 laser shots were averaged at each delay time to obtain a detectable absorbance change as small as ~10−4. Microsecond transient absorption data were collected with a highly sensitive microsecond transient absorption system.
An Nd:YAG laser (Elforlight, SPOT-10-200-532) was used as an
excitation source, which provides nanosecond pulses at a repetition rate of 10 Hz. The excitation wavelength was 532 nm.
A tungsten lamp (Thermo-Oriel, 66997) with an intensity controller
(Thermo-Oriel, 66950) was used as a probe light source. The probe wavelength was selected by two monochromators (Ritsu, MC-10C) and appropriate optical cut-off filters equipped before and after the sample to reduce scattered light and emission. The probe light was detected with a preamplified Si photodiode (Costronics Electronics).
The detected signal was sent to the main
amplification system with an electronic band-pass filter (Costronics Electronics) to improve the signal-to-noise ratio, which was collected using a digital oscilloscope (Tektronix, TDS2022) synchronized with a trigger signal of the laser pulse from an Si photodiode (Thorlabs, DET10A). The instrument response was of the order of 60 ns. Transient absorption decays were collected over the time range from sub-micro to milliseconds, averaging 3000 laser shots on each delay time scale, yielding a sensitivity of 10−6 to 10−4, depending on the measuring time domain. The sample films were sealed in a quartz cuvette purged with N2.
Note that the transient
absorption spectra and dynamics were highly reproducible even after the several times measurements.
In other words, the laser irradiation had negligible effects on the sample
degradation at least under this experimental condition.
-S3-
Assignments of PTB1 Absorption Bands.
The absorption band at 1.9 eV film is assigned to the
S1 band because it exhibits a mirror image to the fluorescence band.
On the other hand, the second
absorption peak at 3.1 eV does not exhibits a mirror image to the fluorescence band.
Therefore,
we assign the absorption at 3.1 eV to the S2 band. Assignments of PTB1 Singlet Excitons and Polarons.
Figure S1a shows the transient absorption
spectra of PTB1 in a toluene solution measured from 0 ps to 7 ns.
Immediately after the
photoexcitation at 680 nm, a broad absorption band was observed at around 1400 nm.
This
absorption band decayed with a lifetime of 1.1 ns, which is consistent with the PL lifetime evaluated by the time-correlated single-photon-counting (TCSPC) method (Inset of Figure S1a). We therefore assign this absorption band to singlet excitons of PTB1.S1–S4 excitation, a new absorption band was observed at around 1100 nm.
At 7 ns after the laser
The new absorption band was
still observed even in the microsecond time domain and decayed faster under an O2 atmosphere as shown in Figure S1b.
Thus, the long-lived absorption band is attributed to triplet excitons.
Figure S1. (a) Transient absorption spectra of PTB1 in a toluene solution measured at 0 (black), 10 (red), 100 (green), and 7000 (blue) ps after the laser excitation. The excitation wavelength was set at 680 nm with a fluence of 15 µJ cm−2. The inset shows the PL decay curve of PTB1 in toluene. The black and gray circles represent the PL decay and the instrument response function for the TCSPC method, respectively. The red line in the inset represents the best fitting curve with one exponential function. (b) Transient absorption spectra of PTB1 in a toluene solution excited at 532 nm under 15 µJ cm−2 measured at 1 (black), 5 (red), and 20 (blue) µs after the laser excitation. The inset shows the transient absorption decays at 1100 nm in Ar (black) and in air (red).
Figure S2a shows the transient absorption spectra of the PTB1/PCBM blend film excited at 680 nm. The singlet absorption band was also observed but rapidly decayed in a few picoseconds, which is in good agreement with the efficient PL quenching efficiency of ~90% (data are not shown). -S4-
Instead, an absorption peak and an absorption onset were observed at around 1100 nm and up to 2500 nm, respectively.
At 10 ps after the laser excitation (green circles), the singlet exciton band
completely disappeared, and the long-lived absorption bands became more pronounced. nanosecond time domain, the new band decayed slowly.
In the
The decay dynamics of the new band
measured at 2500 nm was coincidence with that at 1100 nm, suggesting that these two absorption bands can be ascribed to the same transient. Triplet exciton formation through the ISC can be ignored because spin flip in organic materials is generally negligible in such a short time domain as shown later.
The singlet fission (SF) is also negligible because of a low excitation photon energy
of 1.82 eV in this measurement, which is lower than the threshold energy mentioned in the manuscript.
Moreover, as shown in Figure S2b, the new spectrum is consistent with the steady-
state absorption spectrum in a PTB1 film doped by iodine.
Therefore, the new absorption band is
assigned to PTB1 polaron generated through charge transfer with PCBM because PCBM radical anion has no absorption peak at the short-wavelength IR region as shown in Figure S2c.
Note that
no absorption of PCBM radical anion is observed for the blend film because of its small absorption coefficient: ε = 9000 M−1 cm−1 at 890 nmS5 and ε = 2600 M−1 cm−1 at 2500 nm calculated from ∆OD.
Figure S2. (a) Transient absorption spectra of a PTB1/PCBM blend film measured at 0 (black), 1 (red), 10 (green), 100 (blue), and 7000 (orange) ps after the laser excitation. The excitation wavelength was set at 680 nm with a fluence of 15 µJ cm−2. (b) Transient absorption spectrum of a PTB1/PCBM blend film measured at 7 ns after the laser excitation (black) and steady-state absorption spectrum of a PTB1 film doped by iodine. (c) Transient absorption spectra of a polystyrene film doped with PCBM and TMPD measured at 0 (black), 1 (red), 5 (green), and 10 (blue) ps after the laser excitation. The excitation wavelength was set at 400 nm with a fluence of -S5-
40 µJ cm−2. The characteristic absorption bands observed at around 1000 and 600 nm are attributed to PCBM radical anion and TMPD radical cation, respectively.S5
-S6-
Spectral Simulation.
Figures S3, S4, and S5 show some examples of the spectral simulation of
the transient absorption spectra.
The spectral template of singlet excitons was obtained from the
transient absorption spectra of a PTB1 pristine film measured at 0 ps excited at 680 nm (1.82 eV), which is below the SF threshold photon energy.
That of triplet excitons was obtained from the
transient absorption spectra of a PTB1 pristine film measured at 7 ns excited at 800nm (1.55 eV). As shown in the figures, the transient absorption spectra can be well reproduced by the sum of singlet and triplet absorption templates.
Figure S3. Transient absorption spectra of a PTB1 pristine film (open circles) measured at (a) 0, (b) 10, (c) 100, and (d) 1000 ps after the 800-nm excitation. The black lines represent spectra simulated by the sum of singlet (red line) and triplet (blue line) excitons. The exciton density was estimated to be
Ultrafast Singlet Fission in a Push-Pull Low-Bandgap Polymer Film
Yukitomo Kasai,† Yasunari Tamai,† Hideo Ohkita,*,†,‡ Hiroaki Benten,† and Shinzaburo Ito†
†
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan ‡ Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
-S1-
Sample
Preparation.
Poly[4,6-(dodecyl-thieno[3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-
dioctoxylbenzo[1,2-b:4,5-b]dithiophene)] (PTB1) was purchased from Luminescence Technology Corp. with Mw = 83,000 and PDI = 3.7, and used without further purification.
For the assignment
of PTB1 polaron, phenyl-C61-butyric acid methyl ester (PCBM, Frontier Carbon, E-100H) was blended as an electron acceptor with PTB1.
Thin films of pristine PTB1 and PTB1/PCBM (50 :
50 wt%) blend were prepared onto sapphire substrates by drop-casting from a chloroform solution. Iodine (Nacalai Tesque) was doped in the PTB1 film in order to assign PTB1 polaron.
For the
assignment of PCBM radical anion, a polystyrene film doped with PCBM (30 wt%) and N,N,N’,N’tetramethyl-p-phenylene diamide (TMPD, Wako) (20 wt%), which serves as an electron donor, was prepared onto a quartz substrate by spin-casting from a chlorobenzene solution.
Spectroscopic Measurements.
UV–visible absorption and photoluminescence (PL) spectra were
measured with a spectrophotometer (Hitachi, U-3500), and a fluorescence spectrophotometer (Horiba Jobin Yvon, NanoLog) equipped with a photomultiplier tube (Hamamatsu, R928P) and a liquid-nitrogen-cooled InGaAs near-IR array detector (Horiba Jobin Yvon, Symphony II) under ambient atmosphere.
The PL decay was measured by the time-correlated single-photon-counting
(TCSPC) method (Horiba Jobin Yvon, FluoroCube). were 640 and 750 nm, respectively. ps.
The excitation and detection wavelengths
The total instrument response function is an fwhm of ca. 280
A weak excitation power was used to prevent single–singlet exciton annihilation.
Femtosecond transient absorption data were collected with a pump and probe femtosecond transient spectroscopy system.
This system consists of a regenerative amplified Ti:Sapphire laser
(Spectra-Physics, Solstice), and two transient absorption spectrometers (Helios and FSP-1000-IR). Helios (Ultrafast Systems) was used for a spectral range from 400 to 1700 nm (visible–near-IR region), and FSP-1000-IR (Unisoku Scientific Instruments) was used for a spectral range from 1300 to 2500 nm.
The amplified Ti:sapphire laser provided 800 nm fundamental pulses at a repetition
rate of 1 kHz with an energy of 3.4 mJ and a pulse width of 100 fs (fwhm), which were split into two optical beams with a beam splitter to generate pump and probe pulses.
One fundamental
beam was converted into pump pulse at 400 nm with a second harmonic generator (Ultrafast Systems, Apollo-SS) or other wavelengths with an ultrafast optical parametric amplifier (SpectraPhysics, TOPAS), which was modulated mechanically with a repetition rate of 500 Hz (Helios) or 250 Hz (FSP-1000-IR).
For visible–near-IR measurements (Helios), the other fundamental beam
was converted into white light pulses employed as probe pulses in the wavelength region from 400 to 1700 nm.
The polarization direction of the linearly polarized probe pulse was set at a magic
angle of 54.7° with respect to that of the pump pulse in order to cancel out the orientation effects on -S2-
the dynamics. The temporal evolution of the probe intensity was recorded with a CMOS linear sensor (Ultrafast System, SPEC-VIS) for the visible measurement and with an InGaAs linear diode array sensor (Ultrafast System, SPEC-NIR) for the near-IR measurement.
For the measurements
from 1300 to 2500 nm (FSP-1000-IR), the other fundamental beam was converted with an ultrafast optical parametric amplifier (Spectra-Physics, TOPAS) employed as a monochromatic probe pulse, which was modulated mechanically with a repetition rate of 500 Hz.
The linearly polarized pump
pulse was depolarized in order to cancel out the orientation effects on the dynamics.
The temporal
evolution of the probe intensity was recorded with an InGaAs photodiode (Hamamatsu, G5853-21) which was cooled at −20 °C with a cooling controller (Hamamatsu, C1103-04).
The transient
absorption spectra and decays were collected over the time range from −1 ps to 7 ns. Typically, 2500 laser shots were averaged at each delay time to obtain a detectable absorbance change as small as ~10−4. Microsecond transient absorption data were collected with a highly sensitive microsecond transient absorption system.
An Nd:YAG laser (Elforlight, SPOT-10-200-532) was used as an
excitation source, which provides nanosecond pulses at a repetition rate of 10 Hz. The excitation wavelength was 532 nm.
A tungsten lamp (Thermo-Oriel, 66997) with an intensity controller
(Thermo-Oriel, 66950) was used as a probe light source. The probe wavelength was selected by two monochromators (Ritsu, MC-10C) and appropriate optical cut-off filters equipped before and after the sample to reduce scattered light and emission. The probe light was detected with a preamplified Si photodiode (Costronics Electronics).
The detected signal was sent to the main
amplification system with an electronic band-pass filter (Costronics Electronics) to improve the signal-to-noise ratio, which was collected using a digital oscilloscope (Tektronix, TDS2022) synchronized with a trigger signal of the laser pulse from an Si photodiode (Thorlabs, DET10A). The instrument response was of the order of 60 ns. Transient absorption decays were collected over the time range from sub-micro to milliseconds, averaging 3000 laser shots on each delay time scale, yielding a sensitivity of 10−6 to 10−4, depending on the measuring time domain. The sample films were sealed in a quartz cuvette purged with N2.
Note that the transient
absorption spectra and dynamics were highly reproducible even after the several times measurements.
In other words, the laser irradiation had negligible effects on the sample
degradation at least under this experimental condition.
-S3-
Assignments of PTB1 Absorption Bands.
The absorption band at 1.9 eV film is assigned to the
S1 band because it exhibits a mirror image to the fluorescence band.
On the other hand, the second
absorption peak at 3.1 eV does not exhibits a mirror image to the fluorescence band.
Therefore,
we assign the absorption at 3.1 eV to the S2 band. Assignments of PTB1 Singlet Excitons and Polarons.
Figure S1a shows the transient absorption
spectra of PTB1 in a toluene solution measured from 0 ps to 7 ns.
Immediately after the
photoexcitation at 680 nm, a broad absorption band was observed at around 1400 nm.
This
absorption band decayed with a lifetime of 1.1 ns, which is consistent with the PL lifetime evaluated by the time-correlated single-photon-counting (TCSPC) method (Inset of Figure S1a). We therefore assign this absorption band to singlet excitons of PTB1.S1–S4 excitation, a new absorption band was observed at around 1100 nm.
At 7 ns after the laser
The new absorption band was
still observed even in the microsecond time domain and decayed faster under an O2 atmosphere as shown in Figure S1b.
Thus, the long-lived absorption band is attributed to triplet excitons.
Figure S1. (a) Transient absorption spectra of PTB1 in a toluene solution measured at 0 (black), 10 (red), 100 (green), and 7000 (blue) ps after the laser excitation. The excitation wavelength was set at 680 nm with a fluence of 15 µJ cm−2. The inset shows the PL decay curve of PTB1 in toluene. The black and gray circles represent the PL decay and the instrument response function for the TCSPC method, respectively. The red line in the inset represents the best fitting curve with one exponential function. (b) Transient absorption spectra of PTB1 in a toluene solution excited at 532 nm under 15 µJ cm−2 measured at 1 (black), 5 (red), and 20 (blue) µs after the laser excitation. The inset shows the transient absorption decays at 1100 nm in Ar (black) and in air (red).
Figure S2a shows the transient absorption spectra of the PTB1/PCBM blend film excited at 680 nm. The singlet absorption band was also observed but rapidly decayed in a few picoseconds, which is in good agreement with the efficient PL quenching efficiency of ~90% (data are not shown). -S4-
Instead, an absorption peak and an absorption onset were observed at around 1100 nm and up to 2500 nm, respectively.
At 10 ps after the laser excitation (green circles), the singlet exciton band
completely disappeared, and the long-lived absorption bands became more pronounced. nanosecond time domain, the new band decayed slowly.
In the
The decay dynamics of the new band
measured at 2500 nm was coincidence with that at 1100 nm, suggesting that these two absorption bands can be ascribed to the same transient. Triplet exciton formation through the ISC can be ignored because spin flip in organic materials is generally negligible in such a short time domain as shown later.
The singlet fission (SF) is also negligible because of a low excitation photon energy
of 1.82 eV in this measurement, which is lower than the threshold energy mentioned in the manuscript.
Moreover, as shown in Figure S2b, the new spectrum is consistent with the steady-
state absorption spectrum in a PTB1 film doped by iodine.
Therefore, the new absorption band is
assigned to PTB1 polaron generated through charge transfer with PCBM because PCBM radical anion has no absorption peak at the short-wavelength IR region as shown in Figure S2c.
Note that
no absorption of PCBM radical anion is observed for the blend film because of its small absorption coefficient: ε = 9000 M−1 cm−1 at 890 nmS5 and ε = 2600 M−1 cm−1 at 2500 nm calculated from ∆OD.
Figure S2. (a) Transient absorption spectra of a PTB1/PCBM blend film measured at 0 (black), 1 (red), 10 (green), 100 (blue), and 7000 (orange) ps after the laser excitation. The excitation wavelength was set at 680 nm with a fluence of 15 µJ cm−2. (b) Transient absorption spectrum of a PTB1/PCBM blend film measured at 7 ns after the laser excitation (black) and steady-state absorption spectrum of a PTB1 film doped by iodine. (c) Transient absorption spectra of a polystyrene film doped with PCBM and TMPD measured at 0 (black), 1 (red), 5 (green), and 10 (blue) ps after the laser excitation. The excitation wavelength was set at 400 nm with a fluence of -S5-
40 µJ cm−2. The characteristic absorption bands observed at around 1000 and 600 nm are attributed to PCBM radical anion and TMPD radical cation, respectively.S5
-S6-
Spectral Simulation.
Figures S3, S4, and S5 show some examples of the spectral simulation of
the transient absorption spectra.
The spectral template of singlet excitons was obtained from the
transient absorption spectra of a PTB1 pristine film measured at 0 ps excited at 680 nm (1.82 eV), which is below the SF threshold photon energy.
That of triplet excitons was obtained from the
transient absorption spectra of a PTB1 pristine film measured at 7 ns excited at 800nm (1.55 eV). As shown in the figures, the transient absorption spectra can be well reproduced by the sum of singlet and triplet absorption templates.
Figure S3. Transient absorption spectra of a PTB1 pristine film (open circles) measured at (a) 0, (b) 10, (c) 100, and (d) 1000 ps after the 800-nm excitation. The black lines represent spectra simulated by the sum of singlet (red line) and triplet (blue line) excitons. The exciton density was estimated to be
