Dye sensitized solar cells

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Dye sensitized solar cells

  1. 1. <ul><li>Flame Synthesis of TiO 2 Nanoparticles </li></ul><ul><li>and Their Application in </li></ul><ul><li>Dye Sensitized Solar Cells </li></ul><ul><li>Saro Memarzadeh </li></ul><ul><li>Department of Aerospace and Mechanical Engineering </li></ul><ul><li>University of Southern California </li></ul><ul><li>Qualifying Examination </li></ul><ul><li>May 20, 2010 </li></ul>
  2. 2. Work Published/Submitted <ul><li>Memarzadeh, S., Tolmachoff, E., Phares, D. J., Wang, H.,“ Properties of Nanocrystalline TiO2 Thin Films Synthesized by Flat Flame Stabilized on a Rotating Surface ”, Proceedings of the Combustion Institute , submitted, 2010. </li></ul><ul><li>Memarzadeh, S. ,“Mesoporous TiO2 Thin Films Prepared by Flame Stabilized on a Rotating Surface --Application to Dye Sensitized Solar Cells”, presented in 9 th US national meeting of the Combustion Institute, 2009 </li></ul>
  3. 3. Outline <ul><li>Introduction </li></ul><ul><li>Flame Stabilized on a Rotating Surface (FSRS) </li></ul><ul><li>Study on particle and film properties made by FSRS </li></ul><ul><li>Study on the DSSC characteristics of cells made from FSRS anodes </li></ul><ul><li>Future Work </li></ul>
  4. 4. Introduction <ul><li>Photo-Electrochemical Cells </li></ul><ul><li>1839. Edmond Becquerel </li></ul><ul><li> Silver halide salt solution with two electrodes merged in it </li></ul><ul><li>1873. Herman Vogel </li></ul><ul><li> Extended the photo-response of Silver Halides </li></ul><ul><li> by adding small amounts of an Aniline based dye </li></ul><ul><li>1887. James Moser </li></ul><ul><li> First dye sensitized solar cell </li></ul><ul><li>using Erythrosine </li></ul><ul><li>1964. Hishiki and Namba </li></ul><ul><li> Used dye sensitization on metal oxides (ZnO) </li></ul><ul><li> Discovered a monolayer is the most efficient </li></ul>
  5. 5. Introduction <ul><li>Fundamental Problem: </li></ul><ul><li>Limited light-capture cross-section of the dye molecule </li></ul><ul><li>The area the sensitizer molecule occupies (S) is 1 to 2 nm 2 </li></ul><ul><li>At most 8% of the available surface area is getting used! </li></ul><ul><li>Solution: Use of high surface area mesoporous films </li></ul>† Hagfeldt A., Acc. Chem. Res., 2000, 33 (5), pp 269–277
  6. 6. Introduction <ul><li>Dye Sensitized Solar Cells (DSSC) using a meso-porous anode </li></ul><ul><li>Invented by M. Gratzel and B. O’Regan in 1991 </li></ul>S electrolyte TCO TCO dye TiO 2 e - HOMO LUMO S* h  ox (I 3 - ) red (I - ) Redox mediator e - e - -0.5 0.0 0.5 1.0 E (V) maximum Voltage ~0.75 V h  10-15  m 10-20  m
  7. 7. Introduction <ul><li>Incident Photon to Current Efficiency (IPCE) </li></ul>Gratzel M., Nature 414, 338-344
  8. 8. Parameters affecting the efficiency <ul><li>For many years the research was focused on the dye molecule BUT </li></ul>Film Thickness, size and crystallinity of TiO2 particles and surface area are very important. Michael Gratzel Electrochemistry Communications 11 (2009) 909–912
  9. 9. Parameters affecting the efficiency Photovoltage Redox potential of the electrolyte Fermi level of the TiO 2 Electron density in the TiO 2 Photocurrent Charge generation and injection from the dye molecule Number of dye molecules Surface Area Porosity Particle size Film thickness Number of electrons collected at the TCO Charge diffusion And collection
  10. 10. Anode characteristics 50 nm Transparent Conductive Glass 10  m thick film of TiO 2 <ul><li>Single crystal Particles </li></ul><ul><li>Phase pure anatase </li></ul><ul><li>Thickness of ~10  m </li></ul><ul><li>Large surface area for dye absorption </li></ul><ul><li>High electron diffusivity </li></ul><ul><li>Synthesis Routes </li></ul><ul><li>Laser ablation </li></ul><ul><li>Spray pyrolysis </li></ul><ul><li>Chemical vapor deposition </li></ul><ul><li>Flame </li></ul><ul><li>Sol/Gel </li></ul>
  11. 11. Sol-Gel method <ul><li>Sol-Gel is the most commonly used method for particle synthesis </li></ul><ul><li>Easy to control particle size </li></ul><ul><li>Ability to make very small < 5nm particles </li></ul><ul><li>Narrow particle size distribution </li></ul><ul><li>Ability to control film porosity </li></ul><ul><li>but </li></ul><ul><li>Slow - low throughput </li></ul><ul><li>Amorphous particles must be treated </li></ul><ul><li>Costly cleaning process </li></ul>“ For the best performing TiO 2 electrodes, the synthesis of TiO 2 paste involves hydrolysis of Ti(OCH(CH 3 ) 2 ) 4 in water to ethanol by three times centrifugation. Finally, the ethanol is exchanged with  -terpineol by sonication and evaporation. Totally, it takes 3 days . Such a long time procedure of TiO2 paste is economically unsuitable for industrial production and has to be reduced . ” Michael Gratzel Progress in Photovolt: Res. Appl. 2007; 15:603-612
  12. 12. A method of one-step particle synthesis/film processing <ul><li>Aerodynamically shaped nozzle ( D = 1 cm) </li></ul><ul><li>Nozzle-to-disc distance ( L = 3.0 cm) </li></ul><ul><li>Diameter of rotating disc 30.5 cm (0 to 600 RPM) </li></ul><ul><li>Observed flame diameter ~ 3 cm </li></ul><ul><li>Flame-to-disc distance 0.29±0.03 cm </li></ul>Flame Stabilized on Rotating Surface (FSRS) Tolmachoff et al., Proceedings of the Combustion Institute 32 (2009) 1839–1845
  13. 13. TTIP Meso-porous film TiO 2 Vapor Nanoparticles Decomposition & oxidation Nucleation, coagulation Flame Stabilized on Rotating Surface ~2100 K 400 K 400 K 0.29±0.03 cm
  14. 14. Flame Structure (Ethylene-oxygen-argon,  = 0.4) Computations used the Sandia counterflow flame code and USC Mech II 10 -4 10 -3 10 -2 10 -1 10 0 2.7 2.8 2.9 3.0 3.1 3.2 3.3 Mole Fraction O 2 C 2 H 4 H H 2 CO H 2 O CO 2 Distance from the Nozzle, x (cm) 500 1000 1500 2000 2500 Stagnation surface T (K) Particle nucleation/ growth region 0 100 200 300 400 500 Axial Velocity v (cm/s) Laminar flame speed Particle nucleation/ growth region
  15. 15. FSRS Properties Sol-Gel Method 12  m TiO 2 film 11 % photoefficiency @ AM1.5 <ul><li>FSRS Method allows for: </li></ul><ul><li>Particle synthesis and film deposition in one step </li></ul><ul><li>Particles are Anatase </li></ul>How would meso-porous thin films made using FSRS technique would perform in a DSSC ? FSRS Method 6  m TiO 2 film 7.6 % photoefficiency @ AM1.5 Gratzel M., Journal of Photochemistry and Photobiology A: Chemistry 164 (2004) 3-14 To what degree and how can these parameters be controlled in FSRS?
  16. 16. To what degree and how can these parameters be controlled in FSRS? Michael Gratzel Electrochemistry Communications 11 (2009) 909–912
  17. 17. Experimental Details <ul><li>FSRS </li></ul><ul><li>Ethylene, Oxygen, Argon flame </li></ul><ul><li>Precursor: Titanium tetra-Isopropoxide (TTIP, Aldrich, 97%) </li></ul><ul><li>Particle size: Transmission Electron Microscopy (TEM, Akashi 002B) </li></ul><ul><li>Film morphology: Scanning Electron Microscopy (JSM-7001F ) </li></ul><ul><li>Particle Crystallinity: X-ray Diffractometer (Rigaku ) </li></ul><ul><li>Band Edge: UV-Vis Spectrometry (Shimadzu UV02401) </li></ul>
  18. 18. Particle and Film Morphology 10 nm  rad = 300 RPM 3400 PPM TTIP 1070 PPM TTIP 5660 PPM TTIP <ul><li>Particles </li></ul><ul><li>Mostly Spherical </li></ul><ul><li>Mostly single crystals </li></ul><ul><li>Occasionally sintered </li></ul><ul><li>Film </li></ul><ul><li>Typically 5  m/min </li></ul><ul><li>Net deposition rate = ~ 1  m/sec </li></ul><ul><li>Film is highly porous but uniform </li></ul>Film thickness in FSRS is controlled by the total amount of the injected precursor 5 minute 14  m
  19. 19. Particle Characterization: Diameter Flames 1a, 1b and 1c pre-injection composition 4%C2H4-26.5%O2-Ar, Phi = 0.45 <ul><li>Higher injection rates lead to larger particle sizes </li></ul>60.0 36.6 11.2 TTIP (ml/hr) 1c 1b 1a Flame No.
  20. 20. Particle Characterization: Diameter Flames 1a, 1b and 1c pre-injection composition 4%C2H4-26.5%O2-Ar, Phi = 0.45 <ul><li>Higher injection rates lead to larger particle sizes </li></ul><ul><li>Particle size in FSRS is independent of the Equivalence ratio </li></ul>60.0 36.6 11.2 TTIP (ml/hr) 1c 1b 1a Flame No. 1.27 11.2 4a 1.14 11.2 3a 0.52 11.2 1a Phi TTIP (ml/hr) Flame No.
  21. 21.  = 0 RPM Particle size can be controlled using precursor injection rate and the distributions are similar to the Sol-Gel method Particle Characterization: Diameter Gratzel M., Journal of Photochemistry and Photobiology A: Chemistry 164 (2004) 3-14
  22. 22. Crystal Phase <ul><li>Lean flames favor formation of Anatase </li></ul><ul><li>Rich flames (Oxygen deficient) produce mostly Rutile </li></ul><ul><li>Crystal Phase can be controlled </li></ul>1.27 11.2 4a 1.14 11.2 3a 0.52 11.2 1a Phi TTIP (ml/hr) Flame No.
  23. 24. Particle Characterization: Band Edge <ul><li>The band edge of the particles are independent of flame stoichiometry </li></ul><ul><li>Particle size affects the band edge </li></ul>TAUC Plot for indirect bandgap semiconductor (TiO 2 ) 1.27 11.2 4a 1.14 11.2 3a 0.52 11.2 1a Phi TTIP (ml/hr) Flame No. 60.0 36.6 11.2 TTIP (ml/hr) 1c 1b 1a Flame No. 0.86 0.70 0.52 Phi
  24. 25. FSRS and DSSC fabrication <ul><li>FSRS </li></ul><ul><li>has the potential of fabrication of an </li></ul><ul><li>efficient Electron transport media by </li></ul><ul><li>Combining particle synthesis and film deposition in a single step </li></ul><ul><li>Tuning particle size </li></ul><ul><li>Controlling crystal phase of the particles </li></ul><ul><li>Tuning absorption band edge of the particles </li></ul><ul><li>Controlling film thickness </li></ul><ul><li>How do these parameters affect the efficiency of a DSSC? </li></ul><ul><li>Fabrication Steps </li></ul><ul><li>Particle synthesis and film deposition </li></ul><ul><li>Densification </li></ul><ul><li>Sintering </li></ul><ul><li>Staining with the dye </li></ul><ul><li>Counter Electrode preparation </li></ul><ul><li>(doctor blading of platinum paste) </li></ul><ul><li>Cell Assembly (Electrolyte filling, </li></ul><ul><li>sealing) </li></ul>
  25. 26. Experimental Detail <ul><li>Cell Fabrication: </li></ul><ul><li>Anode: FSRS technique </li></ul><ul><li>Dye: B4 (N3) dye (DyeSol) </li></ul><ul><li>Electrolyte: EL-HPE (DyeSol) </li></ul><ul><li>TCO: Solaronix, 15  / □ </li></ul><ul><li>Staining Time: 8 – 12 Hrs </li></ul><ul><li>Sintered at 500 C for 30 minutes </li></ul>Testing: Absorption: UV-Vis Spectrometry (Shimadzu UV2401-PC) Solar simulator: Newport (67005), Xenon Lamp AM 1.5 filter Polarization curve: LabView 9.6
  26. 27. Post-Deposition Treatment <ul><li>Densification and Sintering </li></ul><ul><li>Improve mechanical integrity </li></ul><ul><li>Create interparticle necking , BUT </li></ul><ul><li>Reduce surface area </li></ul>Anodes FSRS method Densified using ethanol droplets Thickness 3  m
  27. 28. Effect of Particle Size (3  m cells) <ul><li>Small particles -> Larger surface areas -> Better light absorption -> high closed-circuit current (better photoefficiency). </li></ul><ul><li>Current density can be more than doubled by controlling particle size </li></ul>
  28. 29. Effect of Particle Size (3  m cells) <ul><li>Particle size has a substantial effect on short circuit current at light </li></ul><ul><li>intensities above 500 W/m 2 </li></ul><ul><li>Open circuit voltage stays relatively constant at close to AM 1.5 intensities </li></ul>
  29. 30. TiO 2 layer thickness effect Nazeeruddin M. K., J. Am. Chem. Soc., 1993, 115 (14), pp 6382–6390
  30. 31. Effect of Crystal Phase Anatase performs better due to more absorption of the dye 1.27 11.2 4a 0.52 11.2 1a Phi TTIP (ml/hr) Flame No.
  31. 32. Concluding Remarks <ul><li>FSRS can provide a way to control particle and film properties that substantially affect DSSC efficiency </li></ul><ul><li>FSRS can reduce fabrication time by about a third </li></ul><ul><li>The anodes made with FSRS technique show similar properties and characteristics in a DSSC compared to the Sol-Gel method anodes </li></ul><ul><li>High efficiency cells have been achieved </li></ul><ul><li>BUT </li></ul><ul><li>Reproducibility is an issue ! </li></ul>
  32. 33. Current Status <ul><li>Reproducible current densities of 12 mA/cm2 have been achieved </li></ul><ul><li>These cells typically show 5 to 6 % efficiency </li></ul><ul><li>Under similar conditions: </li></ul><ul><li>Thickness 12  m </li></ul><ul><li>No anti-reflection layer </li></ul><ul><li>No additional treatment </li></ul><ul><li>Same TCO, electrolyte and dye </li></ul>Ito S. et al.,Thin Solid Films 516 (2008) 4613–4619
  33. 34. Current status
  34. 35. Summary The effect of FSRS fabrication parameters on the particle and film properties The effect of these parameters on DSSC efficiency have been studied In the process of identifying reproducibility issues and removing them Develop a model which directly correlates FSRS parameters and Cell Efficiency Exploring other potentials of FSRS method Example: Multi-Layer architectures Developing a fundamental model of DSSC to help in understanding fundamental parameters Example: Diffusivity
  35. 36. Electron Transport: Theory <ul><li>I-V curve can be predicted by modeling electron density variations in the film </li></ul><ul><li>n c total number of electrons in the conduction band </li></ul><ul><li>n t total number of electrons in the trap states </li></ul><ul><li>n d total number of electrons in the conduction band in the dark </li></ul><ul><li>K trap trapping rate constant </li></ul><ul><li>K rec recombination rate constant with the electrolyte </li></ul><ul><li>K tv recombination rate constant with the valence band </li></ul><ul><li>D Electron diffusion coefficient </li></ul><ul><li>G Electron Generation </li></ul><ul><li>L Electron Loss </li></ul>
  36. 37. Electron Transport: Experiments <ul><li>Open Circuit Voltage Decay </li></ul><ul><li>Charge extraction </li></ul><ul><li>Electrochemical Impedance Spectroscopy </li></ul>Bailes M. et al.,J. Phys. Chem. B 2005, 109, 15429 15435
  37. 38. Multi-layered Architecture <ul><li>FSRS can produce multi-layer architectures </li></ul><ul><li>Front layer: small particles (<10 nm) to maximize surface area and adhesion </li></ul><ul><li>Back layer: Larger particles (~20 nm) to create electron density gradient and facilitate electrolyte transport </li></ul><ul><li>Light Scattering layer </li></ul><ul><li>Bi-layer films have shown promise </li></ul>The bi-layer was prepared by the FSRS process using a TTIP precursor injection rate of 11.2 ml/hr for 2 minutes followed by 36.6 ml/hr of injection rate.

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