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TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π -CON JUGATED POLYMERS

TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π -CON JUGATED POLYMERS. M artin Weiter , Heinz Bä ß ler Institute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Marburg, Germany. Outline: introduction transient photoconductivity

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TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π -CON JUGATED POLYMERS

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  1. TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π-CONJUGATED POLYMERS Martin Weiter, Heinz Bäßler Institute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Marburg, Germany • Outline: • introduction • transient photoconductivity • 3. charge photogeneration • 4. effect of doping • 5. reexcitation of GP‘s • 6. conclusion LAMINATE meeting Norrköping, September 2004

  2. motivation focus on photoconductivity in Π-conjugated materials:the origin of the optical charge generation and following charge transport • What is the origin of observed transient photocurrent? • What is the rate limiting field dependent step for photoionization in conjugated polymers: exciton dissociation into geminate e-h pair or full dissociation into free charge carriers? • Is the photoionization tractable in terms of Onsager theory? phenyl‑substituted poly‑phenylenevinylene methyl‑substituted ladder typepolyphenylene LAMINATE meeting Norrköping, September 2004

  3. 1. 2. charge photogeneration – classic Onsager concept Two step process: 1. creation of e-h pairs - dependent on photon energy - independent on field and temperature 2. dissociation of e-h pairs - dependent on field and temperature LAMINATE meeting Norrköping, September 2004

  4. charge generation at variance with Onsager concept: motivation D. Hertel, E. V. Soh, H. Bässler and L. J. Rothberg, Chem. Phys. Lett 361, (2002), 99 M.A. Stevens, C. Silva, D.M. Russell, R.H. Friend, Phys. Rev. B 63 (2001) 165213. W. Graupnerat all, Phys. Rev. Lett. 81 (1998) 3259. M. Gailberger, H. Baessler, Phys. Rev. B 44 (1991) 8643. S. Barth, H. Baessler, Phys. Rev. Lett. 79 (1997) 4445. S. Barth, H. Baeassler, U. Scherf, K. Muullen, Chem. Phys.Lett. 288 (1998) 147. J. Pan, U. Scherf, A. Schreiber, R. Bilke, D. Haarer, Synth. Met. 115 (2000) 79. Measured by different techniques at variety of materials LAMINATE meeting Norrköping, September 2004

  5. question What is the rate limiting field dependent step for photoinization in conjugated polymers? ? Exciton dissociation into geminate e-h pair? Full disssociation into free charge carriers ? OR LAMINATE meeting Norrköping, September 2004

  6. photocurrent Ito / (semitr. Al) Al h n time photoconductive Time of Flight method Scope • electrodes (ITO, Al, semitransparent Al, SiO, Mg) • wide range of excitation light wavelengths • different field strength with both polarity • temperature influence • recombination LAMINATE meeting Norrköping, September 2004

  7. experiment 1 MeLPPP methyl‑substituted ladder typepolyphenylene • electrodes (ITO+semitansparent Al, Al) • laser wavelengths 453 nm (2.7 eV), 355 nm (3.5 eV), • different field strength with both polarity • temperature 150K – 320 K LAMINATE meeting Norrköping, September 2004

  8. transient photocurrents RC constant Transit time • MeLPPP: • Hole trasporting material • low degree of disorder • week T and F dep. of mobility. • No electr. transport observed Duration field independent Carrier transport is controlled by charge carrier generation rather by their transport Ito:Al-MeLPPP-Al,100 nm thick T=295K, 355 nm, 5ns 40 uJ/pulse (6e12 photons) LAMINATE meeting Norrköping, September 2004

  9. transient photocurrents • source of the transient photocurrent must be the final dissociation of metastable geminately bound electron‑hole pairs (GPs) whose existence has been well established by • delayed field collection experiment • delayed fluorescence • thermally stimulated luminescence • two color photoconduction NGP-number of GPs which survived until time t kesc-rate constant of their subsequent escape ηdiss -the initial yield of dissociation of singlet excitations, Nph-produced by the laser and ηescis the fraction of GP, ηdiss·Nph is the number of GP which escape geminate recombination. LAMINATE meeting Norrköping, September 2004

  10. charge generation vs. light intensity • GP dissociation yielding quickly moving holes and trapped electrons. The latter create a negative space charge that act as recombination centers. • meaningful information on monomolecular dissociation of singlet excitons requires experiments with laser intensities sufficiently low that bimolecular reaction of neither singlet excitons nor charge carrier can occur Consistent with: D. Hertel, Yu. V. Romanovski, B. Schweitzer, U. Scherf and H. Bässler, Synth. Met. 116, (2001), 1391 A. Haugeneder, M. Neges, C. Kallinger, W. Spirkl, U. Lemmer, …, J. Appl. Phys 85, (1999), 1124 LAMINATE meeting Norrköping, September 2004

  11. charge generation vs. electric field Q=CV/e LAMINATE meeting Norrköping, September 2004

  12. charge generation - yield • yield normalizing Q to the number of singlet pairs generated • field dependence of photocurrent maximumis very similar to the field dependence of the number of charges collectedthe field dependence of the escape rate of GP from initial coulombic well must be vanishingly small squares: 4.1 μJ/pulse, circles: 44 μJ/pulse LAMINATE meeting Norrköping, September 2004

  13. charge generation vs. temperature LAMINATE meeting Norrköping, September 2004

  14. ultrafast absorption spectroscopy Additional information: the number of GP generated within time domain of 200 fs to 500 ps which is too short for their subsequent escape from the coulombic potential V. Gulbinas at all: Phys. Rev. Lett. 89, (2002), 107 401 LAMINATE meeting Norrköping, September 2004

  15. charge generation - yield • It confirms that • the rate limiting field assisted step is the dissociation of relaxed singlet excitons into GPs rather than their subsequent escape from coulombic well, • dissociation of relaxed singlet excitons proceeds on a 200 fs to 0.5 ns time scale while the dwell time of GPs extends to the microsecond regime • approximately 10% of initially generated GP are liable to complete dissociation at a field of 3 MV/cm. LAMINATE meeting Norrköping, September 2004

  16. conclusion1 the field dependence of the intrinsic photogeneration in the π‑conjugated polymer MeLPPP is controlled by the initial step of the dissociation of a relaxed singlet exciton into a geminate pair rather than its escape from the coulombic potential M. Weiter, H. Bässler, V. Gulbinas, U. Scherf, Chem. Phys. Lett.379 (2003) 177. LAMINATE meeting Norrköping, September 2004

  17. (Ph‑PVV) experiment 2 phenyl‑substituted poly‑phenylenevinylene • electrodes (ITO+semitansparent Al, Al) • laser wavelengths 450 nm (2.7 eV), 355 nm (3.5 eV), • different field strength with both polarity • temperature 150K – 320 K • doping by TNF LAMINATE meeting Norrköping, September 2004

  18. transient photocurrents Ph-PVV • -hole mobility 2×10-6 V/(cm2s) •  Transit time (2-40) μs • j(t) shape invariant with light intensity, field, polarity • transient is a convolution of time dependent charge carrier generation, transport and discharge LAMINATE meeting Norrköping, September 2004

  19. temperature dependence at E=2.2·105 V/cm Q is weakly temperature dependent features activation energy of about 0.07 eV for T>200 K at lower temperatures Q tends to saturate model: V. I. Arkhipov, E. V. Emelianova and H. Bässler, Chem. Phys. Lett. 372, (2003), 886 LAMINATE meeting Norrköping, September 2004

  20. comparison of the yield of photogeneration • the photoresponse in PhPPV is larger than in MeLPPP and is associated with weaker field dependence • The doping increase the photogeneration by about a factor of 2 Taking into account previous experimental results : in this case the field assisted and rate-limiting step for photogeneration must be the subsequent escape of the pair from its coulombic well LAMINATE meeting Norrköping, September 2004

  21. conclusion2 • There are two processes occurring in the bulk of conjugated polymer that can give rise to photogeneration: • the field assisted dissociation into geminate pairs that subsequently can separate fully • sensitized photogeneartion at either non‑intentional or intentional dopants that can act as electron scavengers M. Weiter, V.I. Arkhipov, H. Bässler, Synth. Met. 141 (2004) 165 LAMINATE meeting Norrköping, September 2004

  22. h1 Al electrode Double shot: Time delay ITO + semitr. Al h2 Scope „Double shot“ photocurrent technique experiment 3 (Ph‑PVV) To elucidate whether or not there are longer-lived geminate e-h pairs that can be dissociated optically by a second laser pulse rather than decay by geminate pair recombination. Why? LAMINATE meeting Norrköping, September 2004

  23. transient photocurrents Double shots photocurrents at el field 7.3 105 V/cm. The energy of the first pulse at 3.49 eV was 35 J/pulse, the energy of the second pulse at 2.74 eV was 0.7 J/pulse. LAMINATE meeting Norrköping, September 2004

  24. transient photocurrents At low electric fields and low light intensities the result signal is not the superposition of both signals but the photocurrent is controlled by charge carrier generation (and their transport) originate on dissociation of metastable geminately bound electron‑hole pairs (GPs) caused by the reexciting signal. At higher intensities of electric field and/or higher laser intensities the influence of bimolecular recombination is major. LAMINATE meeting Norrköping, September 2004

  25. final conclusion • What is the origin of observed transient photocurrent? • What is the rate limiting field dependent step for photoionization in conjugated polymers: exciton dissociation into geminate e-h pair or full dissociation into free charge carriers? • Is the photoionization tractable in terms of Onsager theory? 1. The observed current transient is a convolution of time dependent charge generation (dominant at MeLPPP) , transport and discharge. LAMINATE meeting Norrköping, September 2004

  26. final conclusion • What is the origin of observed transient photocurrent? • What is the rate limiting field dependent step for photoionization in conjugated polymers: exciton dissociation into geminate e-h pair or full dissociation into free charge carriers? • Is the photoionization tractable in terms of Onsager theory? 2. There are two processes occurring in the bulk of conjugated polymer: the field assisted dissociation into geminate pairs that subsequently can separate fully (significant for MeLPPP) and sensitized photogeneration at either non‑intentional or intentional dopants that can act as electron scavengers (significant for PhPPV). LAMINATE meeting Norrköping, September 2004

  27. final conclusion • What is the origin of observed transient photocurrent? • What is the rate limiting field dependent step for photoionization in conjugated polymers: exciton dissociation into geminate e-h pair or full dissociation into free charge carriers? • Is the photoionization tractable in terms of Onsager theory? 3. We emphasize that in neither case the Onsager description is applicable because in the intrinsic case the field assistance in the primary event of exciton breaking is disregarded in the Onsager formalism, while in the other case GP disssociation is aided by the finite energy of the charge carrier - usually the hole - that resides on a segment of the conjugated polymer next to the counter charge. LAMINATE meeting Norrköping, September 2004

  28. LAMINATE meeting Norrköping, September 2004

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