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UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics

UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics American Physical Society March Meeting 2011 Dallas, TX Emilee Sena , Justin Peel, Shreya Nathan,   Devin Wesenberg, Marianne Wallis,   Thorsteinn Adalsteinsson, Brian McNelis, Richard Barber

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UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics

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  1. UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics American Physical Society March Meeting 2011 Dallas, TX Emilee Sena, Justin Peel, Shreya Nathan,  Devin Wesenberg, Marianne Wallis,   Thorsteinn Adalsteinsson, Brian McNelis, Richard Barber Santa Clara University

  2. Overview • Description of organic photovoltaics • How the devices work • Device fabrication • Device measurement • Active layer molecules • UV-vis spectroscopy

  3. + Charge Generator e- Charge Acceptor A - Multijunction What Are Organic Photovoltaic Devices? ħω A Multijunction Bulk Heterojunction

  4. + Charge Generator e- Charge Acceptor A A - Multijunction What Are Organic Photovoltaic Devices? ħω ħω ħω Bulk Heterojunction

  5. + Charge Generator e- Charge Acceptor A A - Multijunction What Are Organic Photovoltaic Devices? ħω ħω e- Bulk Heterojunction

  6. + Charge Generator e- Charge Acceptor A A - Multijunction What Are Organic Photovoltaic Devices? ħω ħω e- Bulk Heterojunction

  7. + Charge Generator e- Charge Acceptor A A - Multijunction What Are Organic Photovoltaic Devices? ħω ħω e- e- Bulk Heterojunction

  8. Why Study Them? • Advantages over inorganic (silicon) solar cells: • Inexpensive • Flexible • Lightweight • Manufacturable • Why aren’t they used now?

  9. Research Goals • Understandand improve • Device lifetime • Degradation • Power conversion efficiency

  10. The Project • Physics: • Sample Fabrication • Measurement of Device Performance • Physical Chemistry: • Spectroscopic Analysis • Organic Chemistry: • Molecule Synthesis

  11. How Are the Devices Made? • Indium-Tin-Oxide (ITO) • Indium-Tin-Oxide (ITO): “transparent” conductor (Cross section)

  12. How Are the Devices Made? • Indium-Tin-Oxide (ITO) (Cross section)

  13. How Are the Devices Made? • Indium-Tin-Oxide (ITO) • PEDOT:PSS • Indium-Tin-Oxide (ITO) • PEDOT:PSS PEDOT (poly(3,4-ethylenedioxythiophene)) PSS (poly(styrenesulfonate))

  14. How Are the Devices Made? • Indium-Tin-Oxide (ITO) • PEDOT:PSS PEDOT (poly(3,4-ethylenedioxythiophene)) PSS (poly(styrenesulfonate))

  15. How Are the Devices Made? • Indium-Tin-Oxide (ITO) • PEDOT:PSS • Active layer (or) P3HT (poly(3-hexylthiophene)) PCBOD ([6-6]-phenyl C61 butyric acid octadecyl ester) PCBM ([6-6]-phenyl C61 butyric acid methyl ester)

  16. How Are the Devices Made? • Indium-Tin-Oxide (ITO) • PEDOT:PSS • Active layer • LiF

  17. How Are the Devices Made? • Indium-Tin-Oxide (ITO) • PEDOT:PSS • Active layer • LiF • Al

  18. How Are the Devices Measured? I recorded A V varied & recorded Solar simulator illuminates from below and I-V curves are generated

  19. Device Performance Power Conversion Efficiency: ɳ = [(IV)max/area]/(Pin/area) Pin = 100 mW/cm2 |I*V|MAX (V,I)

  20. Device Degradation

  21. Device Degradation

  22. The Active Layer • Electron donor and acceptor molecules • Light-induced electron transfer PCBOD + (or) P3HT PCBM

  23. Meet the Molecules! • PCBM: [6,6]-phenyl-C61-butyric-acid-methyl-ester • PCBOD: [6,6]-phenyl-C61-butyric-acid- octadecyl-ester • Alkyl chain improves solubility, lifetime C60 PCBM PCBOD

  24. UV-Vis Spectroscopy: Why Do We Use It? • Insight into: • Absorption of light energy to excite electrons • Changes in chemical environment • Absorption changes due to degradation

  25. 750 nm 300 nm • How Does a Typical Spectrum Look? • Ultraviolet and visible range • Differential spectrum • Time evolution • MATLAB for analysis

  26. How Does a Typical Spectrum Look?

  27. Finding the Characteristic Time (τ)

  28. Exponential Fit, Solve for Characteristic Time (τ): Ln [Amax – A (t) ] = -t / τ + constant Result for 554 nm: τ= 1/0.57 = 1.75 days IV data: τ= 1/0.56 = 1.79 days

  29. Variable Factors • Weight percent • Blend stoichiometry • Annealing temperature • Most significant improvement in device performance

  30. Annealing Anneal on hotplate (up to 380°C) (HEAT) Continue fabrication process with LiF and Al layers

  31. Effects of Annealing: Device Performance 0.4 Molar Fraction PCBOD:P3HT Blend

  32. 0.16 Molar Fraction PCBM:P3HT Blend Not Annealed; PCEmax = 0.64 % Annealed at 220°C; PCEmax = 1.17% λmax = 555 nm Initial Spectrum Differential Spectrum 555 nm 450 nm

  33. Effects of Annealing: Hypotheses • Decreased quenching • Morphological changes • Crystallite formation • Aggregation • More stable structure

  34. Summary • Organic photovoltaics use light energy for electron transfer to produce current • UV-vis spectroscopy can be used to indicate changes in structure and chemical environment, how absorption changes with degradation • Annealing at high temperatures improves device performance

  35. Funding • Santa Clara University's BIN-REU - UCSC BIN-RDI, NASA Grant NNX09AQ44A • SCU, Science Technology and Society Grants • Grant from IntelliVision Technologies Acknowledgements Michael Sena

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