Ultrafast dynamics of photoexcitations in conjugated polymers

ELSEVIER

Synthetic

ULTRAFAST

DYNAMICS

Metals

84 (1997)

OF PHOTOEXCITATIONS

889-890

IN CONJUGATED

POLYMERS

G.R.Hayesa, I.D.W.Samuelb and R.T.Phillips Cavendish Laboratory, Madingley Road, Cambridge, CB3 OHE, United Kingdom aPresent Address: Institut de Micro- et Optoklectronique, &ok Polytechnique F&d&ale, Lausanne, Switzerland bPresen.t Address: Department of Physics, University of Durham, South Road, Durham, DH13LE, United Kingdom Abstract Measurements are reported of the time-resolved photoluminescence (PL) from unoriented films of poly@-phenylenevinylene) [PPV], poly[2-methoxy,5-(2’-ethyl-hexyloxy)-p-phenylenevinylene] [MEH-PPV] and a cyano-substituted derivative of PPV, CN-PPV. The PL features observed for PPV and MEH-PPV are consistent with the ultrafast formation and subsequent energy migration of intrachain excitons followed by their decay through radiative and non-radiative processes. The dynamics of the PL in CN-PPV is substantially altered compared to that of PPV and MEH-PPV films and also to that of CN-PPV in solution. A transient high energy feature is observed that is not seen in continuous-wave (CW) measurements and a long-lived low energy feature which corresponds to the CW PL. These results are consistent with the formation of inter-chain excitations in CN-PPV films. Keywordr:

Time-resolved

fast spectroscopy, poly@henylenevinylene)

Results are presented of femtosecond PL spectroscopy measurements, using the upconversion techniquel, of PPVl, MEH-PPV1 and CN-PPV2 films and of CN-PPV2 in solution. The excitation energy was 3.06 eV and the pulse width was approximately 200 fs. The temporal resolution of the experiment was 200 fs and the spectral resolution was 18 meV. In PPV the PL has been attributed to the radiative decay of singlet intra-chain excitons. Several picosecond and subpicosecond PL studies have been performed on PPV~V~-~ and MEH-PPV1v4 to observe the energy relaxation of the excitons. The observed behaviour has been explained by describing the polymer chain as a series of smaller sub-units whose size depends upon the degree of chain alignment and sample purity. Exciton migration can occur prior to their decay, via Fijrster transfer, from poorly conjugared and hence higher energy chain segments to more conjugated and thus lower energy segments. Figure 1 shows the PL spectrum of MEH-PPV at various times after photoexcitation. There are three main features of the data that we wish to highlight. The Fist is the extremely rapid rise in the luminescence extending across the entire spectral region. After this, a rapid decay of the high-energy luminescence tail is observed within a few hundred femtoseconds and a slower redshift and narrowing of the luminescence peaks, which occurs on a picosecond timescale. The recognisable vibronic structure seen in CW measurements becomes apparent within a few p&seconds. Thirdly, the overall decay of the luminescence can be seen. The ultrafast PL rise is attributed to the formation of excitons followed by their subsequent vibrational relaxation onto the lowest vibrational level of the first excited electronic state as has been suggested for PPV5p6. Our results show that this formation and subsequent vibrational relaxation occurs in less than 200 fs. The broad high-energy PL tail that exists at very short times after photoexcitation is believed to be caused by the radiative decay of excitons from short-conjugated chain segments. The rapid removal of the high-energy PL tail is explained by exciton migration from higher-energy sites that 0379-6779/97/$17.00 80379-6779(96)04197-5

PII

0 1997

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and derivatives, semiconducting

films

occurs on a sub-picosecond timescale due to the presence of a large density of lower lying sites to shich the exciton can hopl. This migration then becomes a slower diffusion because as the excitons lose more and more energy there are fewer and fewer energetically available sites to which the exciton can migrate. This migration causes the redshift and narrowing of the luminescence peaks. The overall decay of the hnninescence is due to radiative and non-radiative decay processes.

Fig. 1. Time-resolved PL spectrum of an MEH-PPV film at various times after excitation (in picoseconds). Inset: CW emission (left) and CW absorption (right) of an MEH-PPV film. Similar spectral features to those observed for MEH-PPV films are also seen for PPV films. The temporal evolution of the PL at the peak of the CW PL (2.25 eV for PPV and 2.2 eV for MEH-PPV) has also been measured. The decay curves can be fit adequately by a single exponential with a measured decay time of 33Ozt30 ps for PPV and 145tiO ps for MEH-PPV. These results suggest that more efficient non-radiative decay channels are present in MEH-PPV compared to PPV. The decay times that we observe are fully consistent with recent measurements of the PL

GA Hayes et al. /SjmtheticMetals 84 (1997) 889-890

890

that for transition dipoles that are randomly oriented the higher lying level has the higher oscillator strength9. Therefore our results suggest that the high energy PL feature may be due to the radiative decay of inter-chain excitations from their higher-lying level. The strength of the signal at early times after excitation shows that it is more strongly coupled to the ground state than the lower-lying level.

0.0

2.6 2.8 Energy,% Fig. 2. Time-resolved PL spectrum of a CN-PPV fii at various times after excitation (in picoseconds). Inset: CW emission (left) and CW absorption (right) of a CN-PPV film. 2.0

2.2

quantum efficiencies of 0.27ti.02 for PPV films and 0.10-0.15 for MEH-PPV films prepared by the same method as our samples7. The reduced decay time of MEH-PPV compared to PPV may be due to the reduced crystallinity of MEH-PPV in comparison to PPV. In figure 2 the time-resolved spectra of a CN-PPV film are shown. The differences in the time-resolved spectra of CNPPV films compared to those of PPV and MEH-PPV films are quite striking. A high energy PL feature, centred at about 2.35 eV, exists at early times after excitation as the spectra at 0 ps and 1 ps show. The high energy feature is not observed in the CW emission spectrum, which is plotted in the inset for comparison (note the scale on the x axis is different in the two graphs) nor is it seen in the time-resolved spectra of PPV and MEH-PPV films. Measurements of the time dependence of the PL at 2.3 eV show that the high energy feature decays rapidly with a time constant of 6fl ps. The spectra measured at 100 ps and 500 ps show that, at long times after excitation, a low energy PL feature is observed that is centred near 1.8 eV. The low energy feature corresponds to that measured in the CW emission spectrum. Measurements of the low energy feature using time-correlated single-photon counting* have shown that it has a characteristic decay time of 5.6kO.2 ns which is much in excess of the decay times measured for PPV and MEH-PPV films. It may appear that our timeresolved data is incompatible with the CW emission spectrum however, as the decay of the high energy feature is much faster than that of the low energy feature when the time-integrated PL is measured only the low energy feature is observed. In order to elucidate the origin of the unusual timeresolved PL spectra of CN-PPV films further measurements were performed of CN-PPV in dilute solution the results of which are plotted in figure 3. The results are similar to those observed in PPV and MEH-PPV films, i.e. an extremely rapid luminescence rise is observed, followed by a redshift and narrowing of the luminescence peaks. It is important to note that in the timeresolved PL spectra of the CN-PPV solution only a single PL peak is observed which corresponds to that seen in the CW emission spectrum. In dilute solution the polymer chains are well separated and can be considered as isolated units. Therefore we consider that the substantially altered PL features seen from CNPPV fhs is due to inter-chain effects. Recently it has been propsed that inter-chain excitations, such as physical dimers or excimers, are readily formed in CNPPV* films. Inter-chain excitations are widely reported for organic molecules and their formation leads to a splitting of the exciton level into a higher- and lower-lying level. The allowed optical transitions dpeends upon the orientation of the transition dipole moments of the constituent molecules. It has been shown

Fig. 3. Time-resolved PL spectrum of CN-PPV in solution at various times after excitation (in picoseconds). Inset: CW emission (left) and CW absorption (right) of CN-PPV in solution. Ultrafast PL measurements of a model oligomer, consisting effectively of three repeat units of the polymer, have also been performed and provide further insight into the nature of spectra showed similar the excitations2. The time-resolved features to those observed in CN-PPV in solution and PPV and MEH-PPV films, i.e. only a single PL feature was observed. The PL peak was measured to be 2.35 eV. Therefore an alternative explanation of origin of the high energy PL is that it is due to the radiative decay of intra-chain excitons located on chain-segments of a similar length to that of the oligomer. In both scenarios, the low energy feature is attributed to the radiative decay of interchain excitations. Further information has been obtained about the relaxation dynamics of inter-chain excitations in CN-PPV2 by measuring the temporal evolution of the PL at different energies. In conclusion, femtosecond time-resolved PL measurements have uncovered a wealth of information concerning the relaxation and decay dynamics of excitations in PPV and its derivatives. In particular, results obtained for CNPPV show marked differences to those obtained for PPV and MEH-PPV films and for CN-PPV in solution indicating that the emitting species has a different character. This is attributed to enhanced inter-chain interactions in CN-PPV. CN-PPV has a high PL quantum efficiency (0.35rtO.03). Therefore these results have very important implications in the development of efficient light-emitting devices by the control of inter-chain interactions, References (1) G.R.Hayes, I.D.W.Samuel and R.T.Phill.ips, Phys. Rev. B, 52 (1995) R11569 (2) G.R.Hayes, I.D.W.Samuel and R.T.Phillips, lo be published in Phys. Rev. B (3) I.D.W.Samuel et al., Synfh. Met.,.% (1993) 281 (4) I.D.W.Samuel et al., Chem Phys. Left. 213 (1993) 472 (5) R.Kersting et al., Phys. Rev. L&t.., 70 (1993) 3820 (6) M.Yan et al., Phys. Rev. B, 49 (1994) 9419 (7) N.C.Greenham et al., Chem. Phys. Left. 241(1995) 89 (8) I.D.W.Samuel, G.Rumbles and C.J.Collison, Phys. Rev. B, 52 (1995) R11573 (9) R.S.Knox, J. Phys. Chem. 98 (1994) 7270