Stephan Sagmeister, Kerstin Hummer, Peter Puschnig, and Claudia AmbroschDraxl

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Excitonic Effects in Organic Semiconductors
Phenylenes versus Thiophenes
Stephan Sagmeister, Kerstin Hummer, Peter
Puschnig, and Claudia Ambrosch-Draxl Institute
for Physics, University of Graz.
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Synopsis
- OLEDs - Open Questions - Literature -
Materials - BSE - Results
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Optical Light Emitting Diodes (OLEDs)
Understanding physics of organic semiconductors
Why do we calculate their properties?
PPP-based materials show blue light emission
Tailoring of optical properties
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Open Questions
- nature of lowest optical transitions - exciton
lt--gt free e-h pair - experiments BE from kBT
--gt 1eV
?
?
?
??
Dependence on - type of molecule - size of
molecule - inter-chain interaction (pressure)
?
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Literature
Ab-initio calculations with BSE
BE of PA 0.60 eV / 0.05 eV (!)eV for 1D/3D
5, 1 BE of PT 0.96 eV / 0.23,0.20 eV
1D/3D 2, 4 BE of PPV 0.6-0.7 eV / 0.2 eV
for 1D/3D (interchain coupling) 3 3D packing
important Reasons enhanced screening and larger
distribution of exciton in 3D
1 P. Puschnig and C. Ambrosch-Draxl, Phys. Rev.
Lett. 89, 56405 (2002) 2 J.-W. van der Horst
et. al., Phys. Rev. Lett. 83, 4413 (1999) 3 A.
Ruini, M. J. Caldas, G. Bussi, and E. Molinari
Phys. Rev. Lett. 88, 206403 (2002) 4 G. Bussi,
A. Ruini, E. Molinari, M.J. Caldas, P. Puschnig,
and C. Ambrosch-Draxl, Appl. Phys. Lett.
80, 4118 (2002) 5 M. Rohlfing and S. Louie,
Phys. Rev. Lett. 81, 2312 (1998) -gt J. Chem.
Phys.
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Polymers - lattice structure - herring bone
arrangement - building blocks infinite chains
Molecular Crystals - building blocks molecules
PPP
Biphenyl
Bithiophene
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The Bethe-Salpeter Equation (BSE)
RPA
Singlet
Triplet
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Bithiophene Dielectric Function
z-polarization
y-polarization
0.79 eV
1.07 eV
2.13 eV
1.92 eV
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y-polarization
z-polarization
Biphenyl Dielectric Function
Features
e-h interaction - lowest optical transition
z-pol. - strongly bound excitons - SBEz 0.72
eV - TBEz 2.26 eV
pressure - decreased bandgap - red-shifted RPA
spectrum - decrease of SBE and TBE - blue-shift
in absorption spectra
WHY?
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y-polarization
Biphenyl
z-polarization
pressure
- pressure reduces bandgap - RPA spectrum
red-shifted
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y-polarization
z-polarization
Biphenyl
- pressure enhances dielectric
screening - pressure increases e-h
distribution gt reduction of BE
-??Eg(0kb,50kb) lt ?SBE(0kb,50kb) gt blue-shift
of singlet spectrum - considering a pressure
dependent self-energy the effect is reduced
but blue-shift remains
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Summary
Dependence on type of molecule eV
2T 2P 2A -----------------------------------
-------------------- SBEz 0.79 0.72 0.9 TBE
z 2.13 2.26 2.3 pol. z
z y of low. transition
Dependence on pressure eV 2P 2P(20kb)
2P(50kb) -----------------------------------------
-- SBE 0.72 0.65 0.30 TBE
2.26 1.81 1.73
Dependence on size of molecule eV 2T
PT 2P
PPP ----------------------------------------------
------------------ SBE 0.79 0.17-0.23
0.72 to come soon. TBE 2.13
2.26 to come soon.
taken from G. Bussi, A. Ruini, E. Molinari,
M.J. Caldas, P. Puschnig, and C. Ambrosch-Draxl,
Appl. Phys. Lett. 80, 4118 (2002)
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Acknowledgements
Austrian Science Fund, Project No. P16227
EU RT network EXCITING, Contract No.
HPRN-CT-2002-00317
Claudia Ambrosch-Draxl Kerstin Hummer Peter
Puschnig Roland Resel Georg Heimel
Institute for Physics, University of
Graz Institute for Solid State Physics, Graz
University of Technology
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Thank you for your attention!
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