Charge Injection and Transport in Conjugated Polymers

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1 Charge Injection and Transport in Conjugated Polymers George Malliaras Department of Materials Science and Engineering, Cornell University & Cornell NanoScale Facility

2 Outline Brief introduction to PLEDs Charge transport: theory and experiments Charge injection: theory and experiments Outlook

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6 Chemical structure of conjugated polymers n n PPP PPV S n O S O n C 8 H 17 C 8 H 17 n P3HT PEDOT PFO

7 Carbon as a semiconductor Hybridization: sp 2 and p Z CH 2 =CH 2 Particle in a box: ħ 2 π 2 E n = n 2 2mL 2 n=1,2,3,... E G LUMO HOMO ħ E G 2 π 2 2maN

8 Tuning of optical properties Blue Red R.E. Gill et. al., Adv. Mater. 6, 132 (1994). Covion

9 PLED structure and operation Ca e - LUMO Cathode ITO h + Anode HOMO Electroluminescence: charge injection, charge transport, recombination

10 Outline Brief introduction to PLEDs Charge transport: theory and experiments Charge injection: theory and experiments Outlook

11 Hierarchy of transport models J = e n µ E + e D dn/dx First order correction: Space charge effects J = (9/8) ε ε 0 µ V 2 /L 3 Disorder: Energetics Influence on mobility Manifold filling: Charge density dependence of mobility Charge generation: Electric field dependence of charge density Localized states µ=µ(e,t) µ=µ(n) n=n(e)

12 Space charge effects Log(J) V 0 = (8/9) e N 0 L 2 /ε ε 0 J SCL V 0 Low voltages: Ohm s law J OHM = e Ν 0 µ V/L J OHM High voltages: Space charge limited current J SCL = (9/8) ε ε 0 µ V 2 /L 3 Log(V) Lampert and Mark, Current Injection in Solids (Academic Press,1970).

13 Energetics of semiconductors v=µ E Conduction band - Shallow trap Deep trap + ε Valence band x Figure of merit: mobility, µ (cm 2 /V sec)

14 Energetics of amorphous semiconductors Extended states Localized states N. Mott, Nobel Lecture (1977)

15 Time-of-flight (TOF) Light pulse V µ= L 2 t TR V R OSC P. M. Borsenberger, D. S. Weiss, Organic photoreceptors for Xerography (Marcel Decker, Inc., New York, 1998).

16 Non-dispersive hole transport in TFB ITO/PEDOT:PSS(CH8000)/TFB(6.4µm)/Al Photocurrent (ma) H 17 C 8 C 8 H MV/cm 295K N n Hole mobility (cm 2 V -1 s -1 ) 10-2 ITO PEDOT:PSS(60nm) Au(20nm) K 0.1 MV/cm Thickness (µm) Time (µs) H.H. Fong, A. Papadimitratos, and G.G. Malliaras, Appl. Phys. Lett. 89, (2006).

17 Electric field dependence of mobility Hole mobility (cm 2 V -1 s -1 ) K 320K 295K 280K 260K 240K 220K [Electric field (V/cm)] 1/2

18 Molecularly dispersed polymers Solid solutions of conjugated molecules in inert host + Hopping sites are well-defined Control over the average distance between hopping sites

19 Gaussian disorder model LUMO ε ε σ = 0.1 ev x HOMO Energetic disorder Positional disorder DOS

20 Gaussian disorder model (II) Density of states: DOS(ε) = (2 π σ 2 ) -0.5 exp[-(ε 2 /2σ 2 )] ε i ε j R ij Hopping rate: { exp[-(ε j-ε i)/kt] ; ε j>ε i ν ij = ν 0 exp-(2 γ a R ij /R ij ) 1 ; ε j >ε i Mobility: µ=µ 0 exp[-(2σ/3kt) 2 ] exp{c [(σ/kt) 2 -Σ 2 ] E 0.5 } H. Bässler, Phys. Stat. Sol. (b) 175, 15 (1993).

21 Gaussian disorder model (III) Carriers relax at: σ 2 /kt µ ~ exp[-(σ/kt) 2 ] H. Bässler, Phys. Stat. Sol. (b) 175, 15 (1993).

22 Electric field dependence of mobility (II) µ 0 (cm 2 V -1 s -1 ) σ = 65.9 ± 0.5 mev µ = 0.19 cm 2 V -1 s (1000/T) 2 (1/K 2 ) β (x10-3 ) (cm/v) 1/ Σ = 2.2 C 0 = 2.8 x (σ/kt) 2 µ=µ 0 exp[-(2σ/3kt) 2 ] exp{c [(σ/kt) 2 -Σ 2 ] E 0.5 }

23 High mobility in conjugated polymers Organic Crystals Rubrene Pentacene C H 3 N TPD N CH 3 cm 2 /V sec a-si PFO MEH-PPV O N n C 8 H 17 C 8 H 17 n O n H 17 C 8 C 8 H 17 TFB

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25 Correlated disorder model ε LUMO x HOMO Deeper valleys are also wider. µ=µ 0 exp[-(σ/kt) (σ/kt) (e a E/kT) 0.5 ] D.H. Dunlap, P.E. Parris and V.E. Kenkre, Phys. Rev. Lett. 77, 542 (1996).

26 Correlated disorder model (II) Black/white: sites with energy above/below the mean. S.V. Novikov, J. Polym. Sci. Part B: Polym. Phys. 41, 2584 (2003).

27 Correlated disorder in polymers Correlations in site energy arise from fluctuations in: -Conjugation length -Density S.V. Rakhamanova and E.M. Conwell, Appl.Phys. Lett. 76, 3822 (2000).

28 Mobility vs. change density σ=0.1ev W.F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P.A. Bobbert, P.W.M. Blom, D.M. de Leeuw, and M.A.J. Michels, Phys. Rev. Lett. 94, (2005).

29 Manifold filling E x<<1 σ 2 /kt x=0.5 DOS x=1 Mobility (cm 2 /Vsec) Doping Ratio A. Troup et al., J. Non-Crystalline Solids 35, 151 (1980). Y. Shen, K. Diest, M.H. Wong, B.R. Hsieh, D.H. Dunlap, and G.G. Malliaras, Phys. Rev. B. 68, 81204(R) (2003).

30 Mobility vs. charge density (II) C. Tanase, E.J. Meijer, P.W.M. Blom, and D.M. de Leeuw, Phys. Rev. Lett. 91, (2003).

31 Charge density vs. electric field Field ionization of impurities leads to n=n(e) B.A. Gregg, J. Phys. Chem. B 108, 45 (2004).

32 Charge density vs. electric field (II) J SCL (9/8)εε 0 µ 0 V 2 exp[0.89(v/e 0 L) 0.5 ]/L 3 JL 3 /(V appl -V bi ) 2 (macm/v 2 ) Ca Al Au (98 nm) Au (168 nm) ((V appl -V bi )/L) 0.5 ((V/cm) 0.5 ) G.G. Malliaras, J.R. Salem, P.J. Brock, and J.C. Scott, Phys. Rev. B. 58, R13411 (1998).

33 Charge transport First order correction: Space charge effects J = (9/8) ε ε 0 µ V 2 /L 3 Disorder: Energetics Localized states Influence on mobility µ=µ(e,t) Manifold filling: Charge density dependence of mobility µ=µ(n) Charge generation: Electric field dependence of charge density n=n(e)?

34 Outline Brief introduction to PLEDs Charge transport: theory and experiments Charge injection: theory and experiments Outlook

35 Injection vs. transport Is the flow limited by the valve or the hose? Water hose and valve Semiconductor contacts Is the current limited by injection or transport?

36 Hole injection in TFB Current (A/cm 2 ) SCLC ITO PEDOT:PSS (CH8000) Au Electric field (V/cm)

37 Hierarchy of injection models Mechanism: Thermionic emission Tunneling J = A exp(-φ/kt) J = A E 2 exp(-b φ3/2 /E) First order corrections: Barrier lowering J ~ exp(e 0.5 ) Recombination with image force J ~ µ Disorder: Gaussian disorder J ~ exp(e 0.5 )

38 Thermionic emission and tunneling E c Lampert and Mark, Current Injection in Solids (Academic Press,1970).

39 Energetics of conjugated polymers experiment (UPS/IPES) calculation experiment (UPS/IPES) calculation DOS DOS Energy w.r.t E F Energy w.r.t E F VET NOV TES TAM EN TVM V J. Hwang et al., J. Phys. Chem. C, in press.

40 Energetics at the contact F8 IP : 5.7 ev E g : 3.35 ev E vac Ionization energy (IE) and electron affinity (EA) measured from HOMO and LUMO edges w.r.t vacuum level Intensity E F E F E vac *5.1 EA=2.50 E vac LUMO edge UPS IPES 2.65 IE= Energy w.r.t E F * Work function of PEDOT:PSS substrate measured on separate sample produced in same batch. E F PEDOT:PSS 0.65 F8 (energies in ev) HOMO edge VET NOV TES TAM EN TVM V

41 Hole injection barriers for TFB contacts E vac E vac E vac 0.25eV Au 4.75eV TFB ITO 4.85eV TFB PEDOT:PSS TFB 5.5eV 5.5eV 5.15eV 0.6eV 5.5eV 0.75eV 0.65eV 0.6eV VET NOV TES TAM EN TVM V

42 Hole injection in TFB Current (A/cm 2 ) SCLC ITO PEDOT:PSS (CH8000) Au Electric field (V/cm)

43 Barrier lowering Lampert and Mark, Current Injection in Solids (Academic Press,1970).

44 Field dependence of injection Current Density (A/cm 2 ) C 7C -38C 10-7 PEDOT:PSS/PFO E 0.5 (V/cm)

45 Low mobility J = N e µ E exp(-ϕ B /kt) P.R. Emtage and J.J. O Dwyer, Phys. Rev. Lett. 16, 356 (1966)

46 Recombination with image charge J = Cexp(-ϕ B /kt) -en 0 S(E) Surface recombination as a hopping process in the image charge potential. No current flow at zero field. C = 16πεε 0 N 0 (kt) 2 µ/e 2 S(0) = 16πεε 0 (kt) 2 µ/e 3 + _ J.C. Scott and G.G. Malliaras, Chem. Phys. Lett. 299, 115 (1999)

47 Dependence of injection on mobility MV/cm J SCL J INJ J (A/cm 2 ) Mobility (cm 2 /Vsec) Y. Shen, M.W. Klein, D.B. Jacobs, J.C. Scott, and G.G. Malliaras, Phys. Rev. Lett. 86, 3867 (2001).

48 Gaussian disorder E F σ 2 /kt LUMO J Inj [ exp( γ x ) w ( x )] de[ Bol( E) g( U ( x ) E) ] = e ν dx0 2 0 esp 0 a 0 First hop is the rate limiting step V.I. Arkhipov, E.V. Emelianova, Y.H. Tak, and H. Bässler, J. Appl. Phys. 84, 848 (1998).

49 Gaussian disorder (II) Field dependence resembles thermionic emission V.I. Arkhipov, E.V. Emelianova, Y.H. Tak, and H. Bässler, J. Appl. Phys. 84, 848 (1998).

50 Gaussian disorder (III) Energetic disorder due to charge-dipole interactions different at the interface S.V. Novikov, and G.G. Malliaras, Phys. Rev. B 73, (2006).

51 Gaussian disorder (III) Hole injection efficiency eV 0.75eV ITO Au Electric field (MV/cm)

52 Charge injection Mechanism: Thermionic emission Tunneling J = A exp(-φ/kt) J = A E 2 exp(-b φ3/2 /E) First order corrections: Barrier lowering J ~ exp(e 0.5 ) Recombination with image force J ~ µ Disorder: Gaussian disorder J ~ exp(e 0.5 )?

53 Opportunities There is rich physics to be explored in organic light emitting diodes Conjugated polymers that show ideal transport characteristics and high mobilities have become available Charge injection in TFB is poor (and poorly understood) opportunity for major improvements in OLED performance

54 Challenges Picture of metal/organic interfaces from spectroscopy is only now getting incorporated in injection models Injection expected to be spatially inhomogeneous due to correlated disorder J ~ exp(e 0.5 ) ubiquitous, temperature range rather small need other tests for theories

55 Acknowledgments Postdocs Hon Hang Fong Maria Nikolou Aram Amassian Na Young Shim Graduate students Jason Slinker Matthew Lloyd Dan Bernards John DeFranco Alexis Papadimitratos Yee-Fun Lim Vladimir Pozdin Chung-Han Wu Visiting Scientists Kiyoshi Morimoto (Panasonic) Satoyuki Nomura (Hitachi) DOE funded collaboration: Princeton Jaehyung Hwang Antoine Kahn Georgia Tech Eung-Gun Kim Jean-Luc Brédas General Electric Anil Duggal Also: University of New Mexico Dave Dunlap Frumkin Institute Sergey Novikov VET NOV TES TAM EN TVM V