Conjugated Polymer Photophysics : The Role of Morphology

1. Conjugated Polymer Photophysics: The Role of Morphology Rachel Jakubiak Chris Collison Christine Liberatore Lewis Rothberg University of Rochester X. Linda Chen Zhenan Bao Lucent Bell Laboratories Anoop Menon Mary Galvin University of Delaware Ming Yan Thomas Huser Lawrence Livermore Labs Supported by the National Science Foundation and TOPS ARO-MURI

2. the future of organic electronics ... Apnea monitor pillow Tunable wall paper Smart Band-aid Painted solar cells for power Electronic paper

3. MEH-PPV Spectroscopy o ( ) x o

5. Increased lifetime but lower PL Red shift in film spectrum Persistent photoluminescence! Spectral dynamics in films

6. ???? • Why are the absorption and emission not mirror images? • Why is the film spectrum redder than solution? • Why do the film spectra and quantum yields vary with processing? • Why is there a large photoinduced absorption not associated with the fluorescent state? • Why is there a long tail to the film luminescence? • Why are there spectral dynamics in the film emission? • Why does the quantum yield for fluorescence drop in the film without a concomitant drop in lifetime?

7. …the answer to all of these is ... morphology … a case study of dendritic sidegroup conjugated phenylenevinylene polymers to illustrate this and to answer these questions

8. Molecular Structures of Dendritic PPV PPVD0 PPVD1

9. PPVD0 PPVD1

10. Aggregation Quenching of Luminescence • Root cause for solid << solution • Root cause for spectroscopic variation with morphology • Excited State Decay Dynamics • Root cause for nonexponential decay in solids • Root cause for spectral dynamics

11. Absorption and Photoluminescence of the PPVDs in TCE R. Jakubiak, Z. Bao and L.J. Rothberg, Synth. Met. 114, 61 (2000)

12. 70 % Quantum Yield 8 % % TCE Mixed Solvent Studies of PPVD0 in TCE/DMSO In poor solvent: • PL red-shifts (conjugation length) • Quantum yield decreases

13. Proposed Luminescent Species in PPVD Solutions Isolated Species Aggregated Species FF = 72 % FF = 8 %

14. Application of the Two Species Model

15. Mixed Solvent Studies of PPVD1 in TCE/DMSO • Apparent loss of vibronic structure • Quantum yield decreases

16. Spectral Arithmetic for PPVD1 Assumption: Fully aggregated PPVD1 is the same as aggregated PPVD0

17. Comparison of Solution Aggregated PPVD1 and PPVD0 Aggregated PPVD1: Higher energy PL and four times greater than quantum yield than aggregated PPVD0

18. The structural difference between the two species: Proton NMR in mixed deuterated solvents 1,4 dioxane : Water Chloroform : Methanol • In poor solvent the relative intensity of the phenylene and vinylene protons is dramatically reduced. • Reflects a reduction in flexibility of the backbone, particularly phenyl ring torsion Good Solvent Bad Solvent

19. Photoluminescence of Solid State and Aggregated PPVDs • Poor solvent conditions may be a good model for film morphology and quantum yield

20. Summary of Spectroscopy • Solvent-induced aggregation seems to simulate film behavior • Separation of chromophores increases aggregated state PL yield by a factor of 4 but is still lower than isolated chains • The spectroscopy of various degrees of aggregation can be explained by a two species model • Primary effect of aggregation is sterically-induced increase in conjugation length

21. Aggregation Quenching of Luminescence • Root cause for solid << solution • Root cause for spectroscopic variation with morphology • Excited State Decay Dynamics • Root cause for nonexponential decay in solids • Root cause for spectral dynamics

22. PL Decay Dynamics of PPVD0 in Mixed Solvents • Nonexponential decay due to back transfer from polaron pairs. • Decays become nonexponential with increasing aggregation. PL from 420 nm excitation and monitored at 500 nm. R. Jakubiak, Z. Bao and L.J. Rothberg, Synth. Met. 116, 41 (2001)

24. Persistent Photoluminescence in MEH-PPV C.M. Cuppoletti and L.J. Rothberg, Synth. Met., in press.

25. PL Decay Dynamics in PPVD films at 77K

26. Time-Resolved PL of Films at 77K • Much less spectral dynamics in PPVD0 than PPVD1 • Isolated spectrum of PPVD1 evident in first 500 ps • Back transfer only occurs in aggregated regions

27. Summary of Dynamics • Nonexponential decay in bad solvent is due to back transfer (not “quenching defects”) and accounts for a sizable fraction of the luminescence • Back transfer occurs preferentially in aggregated regions • The spectral dynamics (for t > 100 ps) are an effect of inhomogeneity, not energy transfer

28. Single Chain Photoluminescence ! T. Huser, M. Yan and L.J. Rothberg, PNAS 97, 11187 (2000)

29. Conclusions • A chain separation strategy to improve yield in neat films works by reducing interchain processes … but charge transport is not necessarily preserved • A “two species” model of polymer conformations • explains the spectroscopy of aggregation • explains nonexponential and spectral decay dynamics • explains even single chain spectroscopy! • may be a useful film diagnostic • Prevalence of interchain excitations is a big problem in using photoluminescence to screen for electroluminescent materials. Also an obstacle to photopumped lasing.

30. Why are the absorption and emission not mirror images? • Torsional disorder! Emission from dynamically long segments. When you pack chains they are! • Why is the film spectrum redder than solution? • Packing induced conjugation length increase • Why do the film spectra vary with processing? • Morphology dependence of spectroscopy • What is the long tail to the film photoluminescence? • Back transfer from interchain pairs • Why are there spectral dynamics in the film emission? • Two species with different spectra and dynamics! Packed regions have long lived back transfer • Why does the quantum yield for fluorescence drop in film? • Interchain processes. Charge pairs in high quantum yield in packed regions

31. More evidence for the two species model: • Applicability to MEH-PPV: • C.J. Collison et al, Macromolecules 34, 2346 (2001) • Demonstration in trimers too short to fold: • C.J. Collison et al, Synth. Met. 119, 515 (2001) • Selective dump of one species from a mixture: • P. Wang, C.M. Cuppoletti and L.J. Rothberg, Synth. Met., in press • Applicability to shear-oriented films • Z. Bao and L.J. Rothberg, J. Cond. Matt. Phys., in press Thanks … E. Conwell, S. Atherton, NSF CTS, ARO-MURI