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C.A. Schiller 1 , M. Multerer 1 , U. Würfel 2 1 Zahner-Messsysteme, Kronach

C.A. Schiller 1 , M. Multerer 1 , U. Würfel 2 1 Zahner-Messsysteme, Kronach 2 ISE Freiburg Materials Research Centre FMF Germany. Impedance- and Spectro-Electrochemical Measurements on Organic LED and Solar Cell Materials: The Potential Dependent Properties of the Bi-Layer PDOT:PSS-P3HT.

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C.A. Schiller 1 , M. Multerer 1 , U. Würfel 2 1 Zahner-Messsysteme, Kronach

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  1. C.A. Schiller1, M. Multerer1, U. Würfel2 1Zahner-Messsysteme, Kronach 2ISE Freiburg Materials Research Centre FMF Germany Impedance- and Spectro-Electrochemical Measurements on Organic LED and Solar Cell Materials: The Potential Dependent Properties of the Bi-Layer PDOT:PSS-P3HT

  2. Outline • The meaning of the polymer films under test: ITO- PEDOT:PSS - P3HT as a model system for OSC and OLED multilayer systems. • Experimental set-up and measurement campaign. • Discussion of the results. • Impedance- and photo-electrochemical modeling.

  3. OSC andOLED Band schemes of a typical OSC (bulk hetero-junction type, left) and an OLED (three layer type, right). p: hole transport layer (e.g. PEDOT:PSS), n: electron transport layer (e.g. left: PCBM as e--acceptor. right: Alq3 [tris(8-hydroxyquinoline) aluminum]), BG: active band-gap material for photon absorption (left, e.g. P3HT as e--donor) respectively electron-hole recombination under light emission (right, e.g. Ir(ppy)3 = fac tris(2-phenylpyridine) iridium).

  4. Measurement Sample & Cell Section scheme of the measurement sample (not to scale) in the photo-electrochemical cell PECC2 (left) and the PECC2 picture (right). PDOT:PSS: 170 nm hole transport layer, P3HT: 130 nm active band-gap material for photon absorption, applied by successive spin coating. Area: 3.2 cm2. Layer annealed at 110°C, 4h, N2. Electrolyte volume kept under N2.

  5. Electrochemical workstation UV/VIS- spectrometer Lightsource supply potentiostat Tungsten lightsource Photo-electrochemical reference and measurement cell Feedback sensor for automatic intensity control Actuator for automatic change between reference- and measurement cell The Automatic Series Measurement Campaign: Set-Up and the Working Principle Details of the UV-VIS-NIR Absorption Spectroscopy (AS)

  6. Measurements Overview

  7. Components of the Automatic Series Measurement Campaign SER

  8. Pretreatment Phases within the SER 35 steps from -0.1V to +0.75V and back

  9. Spectral Resolved Techniques within the SER 35 steps from -0.1V to +0.75V and back

  10. Extinction Spectra Series vs. Cell Voltage Referenced to Ag/AgCl. P3HT-PEDOT:PSS Film in Acetonitrile / TBA-PF6, under N2.

  11. Stability and Reversibility of the Film under Test

  12. Impedance Spectra Series vs. Cell Voltage Referenced to Ag/AgCl of a P3HT-PEDOT:PSS Film in Acetonitrile / TBA-PF6.

  13. Model Fit of the Impedance Spectra Series vs. Cell Voltage. Impedance modulus (left) and phase angle diagram (right). Symbols: Experimental samples. Solid lines: Model Fit.

  14. Model Fit of the Impedance Spectra Series. Modeling & fit of the reduced state spectra (left) and the model used (right). 1,2: inert hole transport layer with 3, porous distribution. 4,5: active layer redox Faraday impedance, 6 film assigned diffusion, 7 layer capacity with 8, porous distribution. 9: electrolyte resistance.

  15. Model Fit of the Impedance Spectra Series. Modeling & fit of the oxidized states (left) and the model used (right). 1,2: (slightly oxidized) hole transport layer with 3, porous distribution. 4,5: (almost) fully oxidized active layer. 6: electrolyte resistance.

  16. Summary & Conclusion • A model system for organic LED and solar cells was investigated with coupled electrochemical impedance and light absorption spectroscopy in dependence of the potential. • A complex sequence of different techniques was applied in a fully automatic, reproducible way. • Impedance models were assigned to the different phases of the multilayer system over the complete course of oxidization states. • This encourages to similar investigations evaluating systems of practical interest in OSC & OLED fields with additional diffusion barriers and electron/hole injection enhancing layers.

  17. ISE Freiburg Materials Research Centre FMF Thank You for Your Attention ! Special thanks for their help to Steffen Berger,Steffen Fröba,Werner Strunz, Birger Zimmermann

  18. Model Fit of the Impedance Spectra Series vs. Cell Voltage. Complex plane diagrams of the lowest (left) and highest (right) oxidation states. Symbols: Experimental samples. Solid lines: Model Fit.

  19. EIS, IMPS & IMVS Joint Model Fit of the Reduced State. TRIFIT-Modeling (solid lines) of the data (symbols) and the model used (inlay right). 1,2: inert hole transport layer with 3, porous distribution. 4,5: active layer redox Faraday impedance, 6 film assigned diffusion, 7 layer capacity, 8 photocurrent source with 9, porous distribution. 10: electrolyte resistance.

  20. OSC Under Test light substrate Glass-Cr-Al-Cr-(P3HT-PCBM-BLEND)-PEDOT:PSS-Au

  21. EIS IMPS IMVS The Three “Facts” of a Solar Cell: Voltage–Current-Intensity • No information about the photoelectric process through EIS • Two additional force-response couples can be used for further transfer function analysis: • No information about the photoelectric process through EIS • Two additional force-response couples can be used for further transfer function analysis: • Dynamic Photocurrent vs. Intensity IMPS • Dynamic Photovoltage vs. Intensity IMVS • Each transfer function emphasizes different parts of the system!

  22. The Aim of IMPS / IMVS when optimizing SC • The competition between photo-charge recombination and charge diffusion affects the efficiency of OSC & DSSC. • Dynamic photocurrent vs. intensity IMPS: • electron diffusion time constant c dominates • at short-circuit under illumination. • Dynamic photo-voltage vs. intensity IMVS: • electron recombination time constant r • dominates at OCP under illumination. • The ratio 1 - c/ r determines the efficiency. • Dynamic network analysis: join EIS, IMPS & IMVS results to find a common model.

  23. CIMPS: The measured intensity is used as force signal information. The actual intensity is controlled with a photo-sensor at the site of the cell and stabilized by means of a feedback loop. Comparison Between Conventional IMPS/IMVS and CIMPS/CIMVS Conventional IMPS: The LED supply current is used as force signal information. The actual intensity and modulation isuncertain.

  24. Organic solar cell (OSC) and light emitting devices (OLED) usually consist of a sequence of stacked layers, each with different conductance properties. In particular OSC of the bulk-hetero-junction type need a carefully composed sequence of materials providing the work-function gradient necessary for a sufficient transport selectivity for the photo-induced charges to the appropriate electrodes. In some popular types of OLED and OSC, the layer sequence starts with the metallic conductive transparent cathode material, followed by a blend from poly(3,4-ethylenedioxythiophene) together with poly(styrene sulfonate) (PEDOT:PSS) as an efficient hole-conductor (barrier against Indium diffusion, good hole injector into ITO). In the case of OSC the photo-active layer is then often a mixture of regioregular poly(3-hexylthiophene) (P3HT) as donor with (phenyl-(6, 6_)-C61)-butyric acid methyl ester (PCBM) as photo-electron acceptor forming the bulk hetero-junction layer. In the present work, the bi-layer PEDOT:PSS-P3HT on Indium-Tin-Oxide (ITO) conductive glass was investigated in a three electrode photo-electrochemical cell. The layer was polarized between –0.2V and +0.75V in small potential steps under charge and steady state control. The procedure was accompanied by online EIS and UV-VIS-NIR absorption spectroscopy in the range between 350nm to 950nm in order to monitor the changes in layer properties with the oxidation state. Impedance models, covering the different oxidation states will be discussed. The changes in light absorption spectra will be assigned to the phase changes in the samples.

  25. Comparison of semiconductor characteristics

  26. Electrochemical Workstation LED light source 466nm with photo-sense amplifier Light source control potentiostat. OSC under test (feedback control photo- sensor hidden behind the cell) Set-Up for General Photo-Electrochemical-, EIS/IMPS/IMVS Measurements

  27. Spectroscopy for Ex Situ Material and Electrode investigation

  28. Consider DSSC: the characteristic time constants c and r are both depending strongly on the illumination intensity P: Consider OSC: the high frequency decay of the photocurrent response characteristic for the diffusion length and time constants appears typically around some 10-100 KHz. c 1/P r 1/P The relationship between LED supply current and light output is in a frequency range beyond some 100Hz neither linear, nor frequency independent nor true in phase ! One is interested on the frequency behavior of the OSC, but the frequency characteristic of the LED may even dominate the spectra ! The efficiency determining quotient c/r must get wrong, if the actual intensity changes between both measurements due to LED drift or degradation ! Why is it Important to Prefer CIMPS/CIMVSInstead of Conventional IMPS/IMVS ? DSSC & OSC: a joint modeling of EIS-, photocurrent and photo-voltage spectra must fail, if LED frequency response, scale factor uncertainties and drift between the measurements prevents comparability !

  29. Aluminium Photoactive layer Polymer Anode ITO Substrate OSC Principle 2 Acceptor Donor

  30. Linear Potential Scan Phase within the SER

  31. Polarization Phase within the SER

  32. EIS Phase within the SER

  33. AS Phase within the SER

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