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Singlet Fission in Luminescent and Nonluminescent II-conjugated Polymers
ELSEVIER
Synthetic Metals 101 (1999) 267-268
Singlet Fission in Luminescent
and Nonluminescent
II-conjugated
Polymers
M. Wohlgenanntolb,
W. Graupnerb, R. &terbackaa,c, G. Leisingb, D. Comorettod, Z.V. Vardeny” a Department of Physics, University of Utah, Salt Lake City, Utah 84112, USA b Institut fir Festkdrperphysik, TechnischeUniversitht Graz, Petersgasse16, A-8010 Graz, Austria ’ ifbo Akademi University, Department of Physics, Turku, Finland d Universitd degli Studi di Geneva, Dipartimento di Chimica e Chimica Industriale, Geneva, Italy Abstract We have applied the action spectrum technique to the photoinduced absorption (PA) in luminescent polymers such as PPV and mLPPP, and nonluminescent polymers such obtain the TE photogeneration quantum effiency excitation spectra. In all three cases the shows an onset at the optical gap, and in addition they also show a second photogeneration higher than the optical gap. This second onset is explained by singlet exciton fission into model to fit the spectra and obtain the TE energy as a fitting parameter. Keywords: II-conjugated exciton fission
1. Introduction
polymers,
Photoinduced
absorption
and Experimental
We have used the action spectrum spectroscopy[l] to obtain the photogeneration quantum efficiency (PQE) E spectra, for the TE PA band far from the z:iady state (StS), where E is the excitation photon energy, in luminescent polymer films such as poly(paraphenylene-vinylene) [PPV] and methyl-bridged laddertype poly(para-phenylene) [mLPPP] and nonlumiscent polymers such as poly(di-acetylene) [PDA]. 2. Results
and
spectroscopy,
Photogeneration
of triplet
excitons,
Singlet
plateau is spread out over several tenths of eV. Our model should be able to explain the shape of this broad rise and get the value of JET in spite of the broad rise. Our model follows two main ideas: 1. ET is not a single energy, but is spread out in terms of inhomogenous broadening because of the conjugation length (CL) distribution in the films. 2. SF, just like any other electronic transition, may be accompanied by emission of strongly coupled vibrations. Thus the energy E necessary to produce a TE is given by:
Discussion
viTe found that whereas the photoluminescence (PL) PQE comprises of a step-function response at E close to the optical gap, Eop, of these materials, the PQE for TE (Fig. 1) in addition contains a monotonous rise at E>E,,. It is thus apparent that TE generation occurs via two main processes. The iirst process is associated with the generation of thermalized singlet excitops and therefore has a similar PQE dependence on E as that of the PL[l]. The second process, with an onset at E>E,, is therefore due to hot excitons and occurs very quickly. We identify this second process as singlet exciton fission (SF); (Ex+Tt+TJ). The fit through the data points(Fig. 1) has been obtained by a model describing SF, that we want to introduce in this paper. The theoretical prediction for the SF PQE spectrum is a step function response with an onset at 2E~[2]. But OUI spectra (Fig. 1) do not show this sharp step function response. On the contrary, the rise prior to reaching the
band of triplet excitons (TE) as polydiacetylene (PDA), to TE photogeneration spectrum onset at excitation energies triplet pairs. We introduce a
E = ET(CL) +p hvp where ET is the p is the number tional frequency. p phonons during the Huang-Rhys
(1)
TE energy which is a function of the CL, of emited phonons and YP is the vibraThe relative strength of the emission of the electronic transition is described by formula:
(2) To quantify the inhomogenous broadening we have fitted the interband absorption band for each of the three polymers with an assymetric gaussian distribution[3]. Let us call the distribution resulting from this fit D’(E), whereas D(E) is the distribution obtained by summing up over the vibronic progressions. D(E) is then given by
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All tights reserved. PII: SO379-6779(98)000 19-3
D(E) = 2 h(p)~‘(~ +p hvp) $70
(3)
M. Wohlgenannt et al. J Synthetic Metals 101 (1999) 267-268
268
SF produces pairs of TE which are usually on neighboring chains or neighboring chain segments. Then the energy E to produce a triplet pair and vibrations is given by:
E = ET(CLI) +plhvp +
ET(CLS)
(41
+pzhvp
where the distributions ET(CLI) = &(CL2), pi(ps) is the number of emitted phonons in connection with the generation of the fist (second) TE. Next, we convoluted D(E) for the two TE to obtain the distribution Dpair discribing the inhomogenous distribution and phonon emission related to the generation of a TE pair with energy
Epair: Ep& Dpair (-?&ai~) =
D(El)D(Ep,i,
I El=0
- -WEI
(5)
We finally get the P&E by convoluting Dpoir with step function having an onset at E = ,?&i,.. PQE(E)
=
r
Dpac(Epai,)H(E
a
- Epair)dEpair
E p&.=0
photon energy (eV)
(6)
E
Dp,i,(Epair)dEpir
(7)
s E pm;?. =o E paiv D(EdD(Ep,i, =
i...;;
1
and finally:
-
(8)
EddEldEpai,
co
E JJ
E--E2
=
Ez=O
where Ez = Equation and S as two points (Fig. I.
(9)
D(E$J(&)dEld&
El==0
Epair - El 9 is the result of this model and contains ET fitting parameters. From the fits to the data 1) we get ET and S, as summarized in Table
Table I The best fitting parameters for SF into triplet pairs. ET is the TE energy S is the Huang-Rhys parameter related to SF Es is the singlet exciton energy S,, is the Huang-Rhys parameter related to o(w) obtained from a fit to the absorption spectrum polymer
PPV mLPPP PDA From
&(eV) 1.55
1.6 1.1
S 1.5 0.15 0.1
Es(eV) 2.6 2.7 1.9
sop 0.8 0.15 < 0.1
Table I we get for the three polymers (i) w 0.6 and (ii) the Huang-Rhys parameter S is ErfEs close to or larger than So, of the absorption, in all three cases.
Fig. 1 Photogeneration quantum efficiency spectra of the triplet excitors in luminescent (PPV and mLPPP) and nonluminescent (PDA) r-conjugated polymers, as obtained from the action spectra of the T -+ T* PA band in photomodulation spectrum, measured far from the StS. The bold lines through the data points are fits using a model of singlet fission. 3. Summary We have applied the action spectrum spectroscopy for obtaining the PQE of TE in three polymers, both luminescent and nonluminescent. The PQE spectra show two contributions indicating two different photogeneration processes; one contribution is due to thermalized CCcitons with a step function response at E 2: Eop, that is missing in nonluminescent polymers, whereas the other contribution comes Tom hot excitons. We identified the second contribution as singlet fission at E 2 2&-. We introduced a model to fit the SF data and obtained ET and S for three polymers. We found that &/Es z 0.6 and the Huang-Rhys parameter S is close to or larger than SC, of the absorption in all three cases. We thank U. Scherf and K. Muellen for the mLPPP samples. The work at the University of Utah was sup ported in part by the DOE, grant No. FG-03-96 ER 45490. The work was also supported by the Austrian FWF SFB, Elektroaktive StofFe and P.12806 grants, respectively. 4. References [l] [2] [3]
M. Wohlgenannt et al., submitted to Phys. Rev. Lett. R.H. Austin et al., J. Chem. Phys. 90 (ll), 1989, and Refs. therein. M. Liess et al., Phys. Rev. B 56 15712 (1997), and Refs. therein.
Synthetic Metals 101 (1999) 267-268
Singlet Fission in Luminescent
and Nonluminescent
II-conjugated
Polymers
M. Wohlgenanntolb,
W. Graupnerb, R. &terbackaa,c, G. Leisingb, D. Comorettod, Z.V. Vardeny” a Department of Physics, University of Utah, Salt Lake City, Utah 84112, USA b Institut fir Festkdrperphysik, TechnischeUniversitht Graz, Petersgasse16, A-8010 Graz, Austria ’ ifbo Akademi University, Department of Physics, Turku, Finland d Universitd degli Studi di Geneva, Dipartimento di Chimica e Chimica Industriale, Geneva, Italy Abstract We have applied the action spectrum technique to the photoinduced absorption (PA) in luminescent polymers such as PPV and mLPPP, and nonluminescent polymers such obtain the TE photogeneration quantum effiency excitation spectra. In all three cases the shows an onset at the optical gap, and in addition they also show a second photogeneration higher than the optical gap. This second onset is explained by singlet exciton fission into model to fit the spectra and obtain the TE energy as a fitting parameter. Keywords: II-conjugated exciton fission
1. Introduction
polymers,
Photoinduced
absorption
and Experimental
We have used the action spectrum spectroscopy[l] to obtain the photogeneration quantum efficiency (PQE) E spectra, for the TE PA band far from the z:iady state (StS), where E is the excitation photon energy, in luminescent polymer films such as poly(paraphenylene-vinylene) [PPV] and methyl-bridged laddertype poly(para-phenylene) [mLPPP] and nonlumiscent polymers such as poly(di-acetylene) [PDA]. 2. Results
and
spectroscopy,
Photogeneration
of triplet
excitons,
Singlet
plateau is spread out over several tenths of eV. Our model should be able to explain the shape of this broad rise and get the value of JET in spite of the broad rise. Our model follows two main ideas: 1. ET is not a single energy, but is spread out in terms of inhomogenous broadening because of the conjugation length (CL) distribution in the films. 2. SF, just like any other electronic transition, may be accompanied by emission of strongly coupled vibrations. Thus the energy E necessary to produce a TE is given by:
Discussion
viTe found that whereas the photoluminescence (PL) PQE comprises of a step-function response at E close to the optical gap, Eop, of these materials, the PQE for TE (Fig. 1) in addition contains a monotonous rise at E>E,,. It is thus apparent that TE generation occurs via two main processes. The iirst process is associated with the generation of thermalized singlet excitops and therefore has a similar PQE dependence on E as that of the PL[l]. The second process, with an onset at E>E,, is therefore due to hot excitons and occurs very quickly. We identify this second process as singlet exciton fission (SF); (Ex+Tt+TJ). The fit through the data points(Fig. 1) has been obtained by a model describing SF, that we want to introduce in this paper. The theoretical prediction for the SF PQE spectrum is a step function response with an onset at 2E~[2]. But OUI spectra (Fig. 1) do not show this sharp step function response. On the contrary, the rise prior to reaching the
band of triplet excitons (TE) as polydiacetylene (PDA), to TE photogeneration spectrum onset at excitation energies triplet pairs. We introduce a
E = ET(CL) +p hvp where ET is the p is the number tional frequency. p phonons during the Huang-Rhys
(1)
TE energy which is a function of the CL, of emited phonons and YP is the vibraThe relative strength of the emission of the electronic transition is described by formula:
(2) To quantify the inhomogenous broadening we have fitted the interband absorption band for each of the three polymers with an assymetric gaussian distribution[3]. Let us call the distribution resulting from this fit D’(E), whereas D(E) is the distribution obtained by summing up over the vibronic progressions. D(E) is then given by
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All tights reserved. PII: SO379-6779(98)000 19-3
D(E) = 2 h(p)~‘(~ +p hvp) $70
(3)
M. Wohlgenannt et al. J Synthetic Metals 101 (1999) 267-268
268
SF produces pairs of TE which are usually on neighboring chains or neighboring chain segments. Then the energy E to produce a triplet pair and vibrations is given by:
E = ET(CLI) +plhvp +
ET(CLS)
(41
+pzhvp
where the distributions ET(CLI) = &(CL2), pi(ps) is the number of emitted phonons in connection with the generation of the fist (second) TE. Next, we convoluted D(E) for the two TE to obtain the distribution Dpair discribing the inhomogenous distribution and phonon emission related to the generation of a TE pair with energy
Epair: Ep& Dpair (-?&ai~) =
D(El)D(Ep,i,
I El=0
- -WEI
(5)
We finally get the P&E by convoluting Dpoir with step function having an onset at E = ,?&i,.. PQE(E)
=
r
Dpac(Epai,)H(E
a
- Epair)dEpair
E p&.=0
photon energy (eV)
(6)
E
Dp,i,(Epair)dEpir
(7)
s E pm;?. =o E paiv D(EdD(Ep,i, =
i...;;
1
and finally:
-
(8)
EddEldEpai,
co
E JJ
E--E2
=
Ez=O
where Ez = Equation and S as two points (Fig. I.
(9)
D(E$J(&)dEld&
El==0
Epair - El 9 is the result of this model and contains ET fitting parameters. From the fits to the data 1) we get ET and S, as summarized in Table
Table I The best fitting parameters for SF into triplet pairs. ET is the TE energy S is the Huang-Rhys parameter related to SF Es is the singlet exciton energy S,, is the Huang-Rhys parameter related to o(w) obtained from a fit to the absorption spectrum polymer
PPV mLPPP PDA From
&(eV) 1.55
1.6 1.1
S 1.5 0.15 0.1
Es(eV) 2.6 2.7 1.9
sop 0.8 0.15 < 0.1
Table I we get for the three polymers (i) w 0.6 and (ii) the Huang-Rhys parameter S is ErfEs close to or larger than So, of the absorption, in all three cases.
Fig. 1 Photogeneration quantum efficiency spectra of the triplet excitors in luminescent (PPV and mLPPP) and nonluminescent (PDA) r-conjugated polymers, as obtained from the action spectra of the T -+ T* PA band in photomodulation spectrum, measured far from the StS. The bold lines through the data points are fits using a model of singlet fission. 3. Summary We have applied the action spectrum spectroscopy for obtaining the PQE of TE in three polymers, both luminescent and nonluminescent. The PQE spectra show two contributions indicating two different photogeneration processes; one contribution is due to thermalized CCcitons with a step function response at E 2: Eop, that is missing in nonluminescent polymers, whereas the other contribution comes Tom hot excitons. We identified the second contribution as singlet fission at E 2 2&-. We introduced a model to fit the SF data and obtained ET and S for three polymers. We found that &/Es z 0.6 and the Huang-Rhys parameter S is close to or larger than SC, of the absorption in all three cases. We thank U. Scherf and K. Muellen for the mLPPP samples. The work at the University of Utah was sup ported in part by the DOE, grant No. FG-03-96 ER 45490. The work was also supported by the Austrian FWF SFB, Elektroaktive StofFe and P.12806 grants, respectively. 4. References [l] [2] [3]
M. Wohlgenannt et al., submitted to Phys. Rev. Lett. R.H. Austin et al., J. Chem. Phys. 90 (ll), 1989, and Refs. therein. M. Liess et al., Phys. Rev. B 56 15712 (1997), and Refs. therein.