P1254325947qFNzu

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
4
(No Transcript)
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 ltlt ?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
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
Aggregated Species
Isolated Species
FF 8
FF 72
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
Good Solvent
Bad Solvent

• 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

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 ltlt ?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)
23
(No Transcript)
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 gt 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