Triplet state diffusion in organometallic and organic semiconductors

Transcription

1 Triplet state diffusion in organometallic and organic semiconductors Prof. Anna Köhler Experimental Physik II University of Bayreuth Germany From materials properties To device applications

2 Organic semiconductors allow for attractive displays... OLED display by Sony

3 ... and lighting producs and solar cells Lighting windows by Osram Flexible solar cells on fabric

4 ...and electronic reader devices... The E ink reader by Plastic Logic, fabricated in Dresden

5 What makes organic semiconductors so attractive? Opto electronic properties of semiconductors + Mechanical properties of plastic conduction absorption emission transistors, solar cells, light emitting diodes flexible robust light weight novel products soluble new fabrication technologies

6 The different physics of organic semiconductors I Energetics R R R R R R + + Inorganic crystal amorphous organic film strong coupling : bands weak coupling : localised states high dielectric constant: low dielectric constant : large e h distance small e h distance ( 0.3 nm) weak binding strong binding ( 0.4 ev) weak exchange energy high exchange energy ( 0.7 ev)

7 The different physics of organic semiconductors II Dynamics Energetic disorder Electron phonon coupling E + initial + final Density of states

8 What happens in an organic LED? Ca ITO V glass 100 nm Operate LED Energy π π Conduction states LUMO HOMO Valence states Place spin 1/ electrons and spin 1/ holes in π and π* orbitals

9 What happens in an organic LED? V Ca ITO glass Operate LED 100 nm Energy S 1 S 0 ISC X They form two types of states ΔE T 1 1 spin = 0 : Singlet state emission allowed (fluorescence) 3 spin = 1 : Triplet state emission forbidden (phosphorescence) Energy π π OR Energy π π Singlet S 1 Triplet T 1 Place spin 1/ electrons and spin 1/ holes in π and π* orbitals

10 Triplet state photophysics For OLEDs with phosphorescent host guest systems Y.Sun + S. Forrest, Nature 006 QE ext =19 %, 30 lm/w for white OLED For solar cells using triplet excitons (W.Y. Wong, Macromol. Chem. Phys. 008) We need to know: Energetics: How big is the exchange energy, and how can we modify it? Dynamics: How is triplet state energy transferred and what controls it?

11 What do we know about the exchange energy It depends on the electron hole wavefunction overlap In π conjugated polymers with π π* transition, it is 0.7 ev In associated shorter oligomers, it raises up to 1.3 ev A. Köhler, AFM 004 To reduce the exchange energy, spatially separate electron and hole use n π* transitions and non conjugated linkages use charge transfer states Brunner, Van Dijken, JACS 004 Zhang, Köhler JCP 006 p

12 Triplet state energy transfer Triplet diffuses via exchange interaction Simultaneous transfer of two electrons Depends on wavefunction overlap Along a chain or between chains? Depends on electron phonon coupling Chain length dependence? Depends on Donor Acceptor energies Dependence on energetic disorder?

13 Our workhorse: Pt containing model systems Photoluminescence (a.u.) Monomer T 1 S 1,0,5 3,0 3,5 4,0 4,5 Energy (ev) monomer polymer Absorption (a.u.) Polymer Energy S 1 T 1 S 0 Strong Spin orbit coupling strong phosphorescence Some conjugation is preserved along the chain

14 Temperature dependence of phosphorescence intensity Phosphorescence intensity Monomer Polymer Different temperature dependence for polymer and monomer not due to internal conversion (Wilson, Köhler et al. JACS (001)) Temperature (K) Triplet exciton mobility is increased in polymer

15 Temperature dependence of phosphorescence lifetime 10 0 c log (1/τ) (μs -1 ) K E A ~ exp kt Polymer E a = 60 mev Monomer E a = 100 mev K /T (K -1 ) There is a temperature activated high energy branch, and a transition temperature below which the thermal activation changes (consistent with the Holstein Small Polaron model)

16 Where does this temperature dependence come from D + A Consider exchange transfer as a double electron transfer Markus theory describes electron transfer (at high temp) DA D + A The transfer rate is given by: W if J if E exp kt a Ea = λ ΔG λ Energy E a λ Activation energy ΔG 0 λ = the reorganisation energy (electron phonon coupling) Configuration Coordinate

17 Triplets and Marcus theory For a self reaction, ΔG 0 =0 (neglecting energetic disorder) E pot W if J if E exp kt a Ea = λ ΔG λ λ ΔG* ΔG 0 =0 Q j W if J if exp 4kT λ Ea = λ 4 The rate of electron transfer k if depends on the coupling between two sites V if the reorganisation energy λ.

18 Triplets and Marcus theory For a self reaction, ΔG 0 =0 (neglecting energetic disorder) E pot W if J if E exp kt a Ea = λ ΔG λ λ ΔG* ΔG 0 =0 Q j W if J if exp 4kT λ Ea = λ 4 Wavefunction overlap, good along chain The rate of electron transfer k if depends on the coupling between two sites V if the reorganisation energy λ.

19 The reorganization energy Can we experimentally determine the reorganisation energy? E pot E rel f E rel i i + f E rel f + λ = i rel E + E = E rel f rel E rel i Q j The reorganisation energy λ for the triplet transfer relates to the geometric relaxation energy associated with optical transitions We can derive the activation energy for energy transfer just by analysing the absorpton and emission spectra! Brédas et al, Chem. Rev. 004, 104, 4971; Markvart & Greef, JCP, 004, 11, 6401

20 The reorganization energy Can we experimentally determine the reorganisation energy? E pot E rel f E rel i i + f E rel f + λ = i rel E + E = E rel f rel E rel i S= Huang Rhys parameter Q j For optical transitions E I rel 0 n = = E = rel, j S j n e n! S j hω S j j Brédas et al, Chem. Rev. 004, 104, 4971; Markvart & Greef, JCP, 004, 11, 6401

21 1. mode hω j S E rel j j Phosphorescence (a.u.) Polymer E Polymer = hω j rel S j j E a =50 mev = 100 mev W if J if E a = E exp kt Erel a Phosphorescence (a.u.) Monomer Energy (ev) E = Monomer: rel S j j hω E a =90 mev j = 180 mev

22 The activation energy for triplet diffusion 10 0 c Polymer E a = 60 mev From Analysis of optical spectra from temp. dep. of phosporescence 50 mev Polymer 90 mev Monomer 60 mev Polymer ~100 mev Monomer log (1/τ) (μs -1 ) K Monomer E a = 100 mev K E a = E rel /T (K -1 ) W if J if E exp kt a

23 The different physics of organic semiconductors II Dynamics Energetic disorder Electron phonon coupling E σ σ + + initial final Density of states

24 How to consider the effect of disorder on the transport Holstein small polaron theory, modified by Emin, + effective medium approximation D. Emin, Adv. Phys. 4, 305, (1975) I. I. Fishchuk et al., PRB 67, 4303 (003) 8 1 ~ exp T k T k E W B B a e σ 1 ~ exp T k W B e σ High Temperature Low Temperature ( ) + T k W B i j i j ij ~ exp ε ε ε ε ( ) T k E T k T k E W B a i j B i j B a ij 16 ~ exp ε ε ε ε Multiphonon hopping Phonon assisted tunneling I. I. Fishchuk et al., PRB (008)

25 And what happens if we increase the energetic disorder? log(w e ), (W e in μs -1 ) W a/l=10 e ν 0 =3x10 1 sec -1 J 0 =50 mev E a =50 mev Ea ~ exp k T B /T (K -1 ) σ kbt σ/e a The two regimes, multiphonon hopping and phonon assisted tunneling, are no longer distinct! The exp ( 1/T ) dependence dominates the energy transfer W e ~ exp 1 σ kbt I. I. Fishchuk et al., PRB 008

26 Test against experimental data log 10 (W e ), (W e in μs -1 ) 0,0-0,5-1,0-1,5 -,0 -,5 input parameters from experiment a E a =60 mev, J= mev, ν 0 =3x10 1 s -1 Marcus model (eq. 1) fitting parameters σ=3 mev, a/l=9.6 transition temperature 80 K Miller-Abrahams model (eq. 13) High Temp.: Multiphonon hopping Adiabatic, multiphonon hopping Low Temp.: Phonon assisted tunneling 1000/T (K -1 ) log 10 (W e ), (W e in μs -1 ) Marcus model (eq. 1) Miller-Abrahams model (eq. 13) b -.5 transition temperature 80 K (1000/T(K))

27 a) luminescence intensity (arb. units) 3,0,5,0 1,5 1,0 0,5 wavelength (nm) ,0 PF Ph,5 DF PF Ph DF PF DF Ph H 3 C CH 3 n H 3 C CH 3,0 1,5 1,0 0,5 0,0 0,0,0,5 3,0 3,5 4,0 4,5 Energy [ev] absorption (arb. units) Does our model for triplet diffusion also apply to organic polymers? Organic model compound: Polyfluorene polymer, trimer and dimer

28 Yes, it does apply! normalized integrated phosphorescence intensity 1,0 0,8 0,6 0,4 0, 0, T [K] polymer trimer dimer 1/τ [ms -1 ] E-3 E a 109 mev 141meV 15 mev polymer trimer dimer /T [K -1 ]

29 Summary and Conclusion How is triplet state energy transferred and what controls it? It is transferred via a multiphonon hopping process above T T and a single phonon tunneling process below T T It is controlled by wavefunction overlap, the amount of geometric reorganization energy and energetic disorder Triplets diffuse faster along polymer chains than between them Triplets diffuse much faster in polymers than in oligomers

30 Acknowledgements Experimental Physics: Lekshmi Sudha Devi Sebastian Hoffmann Jo Wilson NingZhang Synthetic chemistry: Muhammad Khan, Oman + students Ullrich Scherf, Wuppertal + students Peter Strohriegel, Bayreuth+ students Discussions: Richard Friend, UK Heinz Bässler, Marburg Theoretical calculations: Ivan Fishchuk, Kiev Andrey Kadashuck, Kiev