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Measuring the mobility of organic thin films

Measuring the mobility of organic thin films. Johannes Vanpaemel. Outline. Introduction Theory Results Conclusion Planning. Introduction: Aim. Obtain the charge carrier mobility of organic films from their I-V characteristics Hole-only Electron-only

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Measuring the mobility of organic thin films

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  1. Johannes Vanpaemel imec confidential 2009

  2. Measuring the mobility of organic thin films Johannes Vanpaemel

  3. Outline • Introduction • Theory • Results • Conclusion • Planning

  4. Introduction: Aim • Obtain the charge carrier mobility of organic films from their I-V characteristics • Hole-only • Electron-only • Sandwich organic layer between two electrodes • Choice of electrodes depends on their function • Inject/block a specific carrier Johannes Vanpaemel imec confidential 2010

  5. Introduction : Current density • By applying a voltage over an organic layer, an electric field is formed within • Current density J is induced in layer • Built-inpotential drop Vbiacrosslayerdue to differenceworkfunctions

  6. Introduction: Current injection in solids • When a voltage is applied, carriers are injected into the organic layer • Potential barrier Δ • Depending on Δ and the applied voltage, charges are injected into the layer • Δ < 0.3eV : bulk-limited current • Δ > 0.3eV : injection-limited current Johannes Vanpaemel imec confidential 2010

  7. Theory: I-V Characteristics (Δ < 0.3eV) • Typically, two regimes are discerned • Linear @ low voltages (Regime 1) • Quadratic (Regime 2) • deviates @ higher voltages • Cross-over point Vx • Depends on built-in voltage, thickness layer, thermal free carriers Johannes Vanpaemel imec confidential 2010

  8. Theory: Equations • Due to the high electric field (thin films) only the drift current is considered: • μ : mobility • n(x) : charge carrier concentration • E(x) : electric field • The electric field obeys the Poisson’s equation • ε : dielectric constant Johannes Vanpaemel imec confidential 2010

  9. Theory: Ohmic Regime (at low voltage) • Injection charge negligible compared to thermal charge • Electric field is nearly uniform (Poisson) • Current continuity equation yields • J : current density [J/m2] • µ : the mobility [m2/Vs] • n0 : thermal charge density [m-3] • L : thickness organic layer [m] Johannes Vanpaemel imec confidential 2010

  10. Theory: Space-charge limited current (SCLC) • Above cross-over voltage Vx, ninj dominates n0 • Electric field alters due to space charge • Simultaneous solving Poisson’s equation and the current continuity equation yields • ε : dielectric constant [F/m] Johannes Vanpaemel imec confidential 2010

  11. Theory: Poole-Frenkel (PF) mobility • At high electric fields (E>104 V/cm), the mobility is field dependent • µ0 : zero-field mobility [m2/Vs] • γ : field activation parameter [(m/V)1/2] • E : electric field [V/m] • Exact solution by simultaneously solving Poisson, current continuity and PF equation • No analytical solution possible • Need for an approximation Johannes Vanpaemel imecconfidential 2010

  12. Theory: Approximation* • Plug PF equation in SCLC equation and use average electric field • Electric field • Average electric field •  Johannes Vanpaemel imec confidential 2010

  13. Theory: Fitting • Goal: Obtainμ0 and γ • εr=ε/ε0 is set to 3 • Applied voltage needs to becorrectedbyVbi • Notpossible to determineexperimentally @ IMEC • Strategy: optimizearchitecture in order thatVbi = 0 V • Thickness is determinedbyquartzcrystal • Calibratedbymeans of ellipsometry

  14. c Johannes Vanpaemel imec confidential 2009

  15. c Johannes Vanpaemel imec confidential 2009

  16. x Johannes Vanpaemel imec confidential 2009

  17. Thickness dependent symmetry • c Johannes Vanpaemel imec confidential 2009

  18. ITO/PEDOT:PSS/MoOx(5nm)/CuPc(100nm)/MoOx(5nm)/Au(100nm) • c Johannes Vanpaemel imec confidential 2009

  19. Results: ITO(UV ozone)/CuPc/Au(100nm) • Vbi = 0.5V used to obtain I-V characteristics • Below 0.8V, measurements show a large fluctuation (not shown in graph) • Reverse bias shows injection limited current Johannes Vanpaemel imec confidential 2010

  20. Results: ITO(UV ozone)/CuPc/Au(100nm) • Thickness dependence corrected for PF factor • After correction, dependence shows L-3behaviour as expected from the formula for SCLC Johannes Vanpaemel imec confidential 2009

  21. Results: ITO(oxygen plasma)/CuPc/Au(100nm) • SCLC with Vbi= 0.5V • Φm(ITO UV ozone) = 4.73eV • Φm(ITO ox.pl.) = 4.79eV • Thickness dependence after correction not L-3 • Is it space charge limited? • Good fits and comparable results with ITO(UV ozone) Johannes Vanpaemel imec confidential 2010

  22. Results: AFM ITO(UV ozone)

  23. Resuls: Thickness issues • Quartz crystal in Lesker system gives an average value of the thickness • However, AFM measurements show a rough surface morphology • What is the correct electrical thickness and how can it be measured? • Strategy : introduce interlayer to reduce the surface roughness • e.g. PEDOT:PSS is smoother than ITO Johannes Vanpaemel imec confidential 2010

  24. Results: AFM picture PEDOT:PSS/CuPc

  25. Results: ITO/PEDOT:PSS/CuPc/.../Au(100nm) • I-V does not fit well with SCLC equation • Large variation in obtained μ0 and Vbi • PEDOT:PSS is not a good hole injector • Introduce injection layer on top of PEDOT:PSS Johannes Vanpaemel imec confidential 2010

  26. Results: ITO/MoOx(5nm)/CuPc/Au(100nm) • Comparable results as obtained with ITO • Vbi is equal to that with ITO on the bottom • MoOx injects as well as ITO • Reported work function 5.3-5.5 eV • Suitable candidate for injection interlayer Johannes Vanpaemel imec confidential 2010

  27. Results: ITO/PEDOT:PSS/MoOx(5nm)/CuPc/Au(100nm) • I-V characteristics are similar as without PEDOT:PSS layer • Rms of surface is 6.8nm • Roughness at bottom diminished by PEDOT:PSS • Crystalline growth of CuPc inevitably results in rough surface at the top Johannes Vanpaemel imec confidential 2010

  28. Results: ITO(UV ozone)/CuPc/MoOx(5nm)/Au(100nm) • No Vbi due to introduction of MoOx interlayer • Reduction of a fitting parameter • I-V curve shows quasi-antisymmetricbehaviour Johannes Vanpaemel imec confidential 2010

  29. Results: Comparison with literature • M

  30. Conclusion Johannes Vanpaemel imec confidential 2010

  31. Planning • Thickness dependence J-V • Investigate temperature dependence of parameters • Find a way to determine the electrical thickness • Influence of thickness can be relatively large (L3) • Electron-only devices Johannes Vanpaemel imec confidential 2010

  32. Johannes Vanpaemel imec confidential 2009

  33. Built-in voltage Vbi • Difference between the workfunction of the two electrodes • At metal/organic interface, surface dipoles are observed, which can alter the effective workfunction • Not able to determine experimentally @ IMEC • Applied voltage has to be corrected by Vbi • Strategy : optimize architecture to obtain Vbi = 0V Johannes Vanpaemel imec confidential 2010 33

  34. Measuring I-V characteristics Surface resistance ITO ~20 Ω/□ Voltage drop over ITO significant when J > 102 A/m2 Johannes Vanpaemel imec confidential 2010 34

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