Self-Organization of Polymers and Applications in Solar Cells

1. Self-Organization of Polymersand Applications in Solar Cells Jung Hwan Woo

2. Outline • Introduction • Self-assembly and distinctive features • Ordered air bubble in polymer film example • Polymer PV cells • Application • Highly efficient polymer PV cells using self-organized polymer • Bulk heterojunction solar cells • Future Improvements

3. Outline • Introduction • Self-assembly and distinctive features • Ordered air bubble in polymer film example • Polymer PV cells • Application • Highly efficient polymer PV cells using self-organized polymer • Bulk heterojunction solar cells • Future Improvements

4. Introduction • Self-assembly • Defined as spontaneous and reversible organization of molecular units into ordered structures • Distinctive features • Order • High order • Interactions • Weak bonds play a role • Building blocks • Nano and mesoscopic structures http://nimet.ufl.edu/

5. Introduction • Types • Self-ordered/assembled… • Nanocomposites • Semiconductor islands • Pore structures • Carbon nanotubes • Quantum wires and dots www3.interscience.wiley.com

6. Introduction • Applications • Pattern transfer • Improvements in devices • Optics and sensing • Applications related to the type of structures

7. Introduction • Different methods to template • Ordered array of colloidal particles • Templating using an emulsion • Honeycomb structures by polymer with rod-coil architecture • Self-organized surfactants, i.e. mesoporous silica • Microphase-separated block copolymers • bacteria

8. Templating Example • Ordered array of colloidal particles • Procedure • Colloidal crystals infiltrate with a fluid which fills and solidifies in the space between the crystals • Spheres removed by thermal decomposition or solvent extraction • Solidified fluid forms 3D array of pores • Main drawback • Length of pores cannot be controlled

9. Outline • Introduction • Self-assembly and distinctive features • Ordered air bubble in polymer film example • Polymer PV cells • Application • Highly efficient polymer PV cells using self-organized polymer • Bulk heterojunction solar cells • Future Improvements

10. Self-Ordered Array of Air Bubbles in a Polymer Film • Procedures include forced air flow with moist atmosphere over a volatile solvent (polystyrene) • High vapor pressure and velocity drives the temperature to 0°C • Condensed water droplets form a structured array and sinks into the solution • When new water droplets condense previous array provide a template for the next layer • Size range from 0.2 to 20 μm

11. Self-Ordered Array of Air Bubbles in a Polymer Film • Final product Srinivasarao, Science, (2001)

12. Self-Ordered Array of Air Bubbles in a Polymer Film • Optically sectioned images Srinivasarao, Science, (2001)

13. Self-Ordered Array of Air Bubbles in a Polymer Film • Hole depth profile • Discontinuity of holes seen at around 5 μm in depth Srinivasarao, Science, (2001)

14. Self-Ordered Array of Air Bubbles in a Polymer Film • Parameters • Solvent • 2D porous films obtained when CS2 is used whereas 3D films obtains for polystyrene • Air velocity • 30 m/min => 6-μm pores • 300 m/min => 0.5 μm pores

15. Self-Ordered Array of Air Bubbles in a Polymer Film • Advantages • Simple method • Size of the pores controlled by air velocity • Applications • Polystyrene can be used in beam steering devices, microlens arrays or fabrication of picoliter beakers • Photonic bandgap applications • Optical stop-bands

16. Self-Ordered Array of Air Bubbles in a Polymer Film • Why do water droplets form close packed bubbles? • Liquid droplets fail to coalesce with the same liquid in some situations • This phenomenon studied by Rayleigh in 1879 • This behavior driven by thermocapillary convection • The presence of lubricating medium (air) between two liquid droplets keeps them from coalescence.

17. Outline • Introduction • Self-assembly and distinctive features • Ordered air bubble in polymer film example • Polymer PV cells • Application • Highly efficient polymer PV cells using self-organized polymer • Bulk heterojunction solar cells • Future Improvements

18. Polymer PV Cells • Advantages • Low cost of fabrication • Ease of processing • Mechanical flexibility • Versatility of chemical structure • Disadvantages • Low efficiency

19. Polymer PV Cells • Requirements for higher efficiency • High fill factor • Ordered structure • Efficient absorption of solar radiation • Increased thickness. However, this results in higher series resistance • Lower series resistance • Ordered structure can reduce Rs

20. Outline • Introduction • Self-assembly and distinctive features • Ordered air bubble in polymer film example • Polymer PV cells • Application • Highly efficient polymer PV cells using self-organized polymer • Bulk heterojunction solar cells • Future Improvements

21. High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blendsGang Li, VishalShrotriya, Jinsong Huang, Yan Yao, Tom Moriarty, Keith Emery and Yang Yang

22. Introduction • Improvements in efficiency is required • Important parameters include • Current-voltage behavior • Fill factor (FF) • Short-circuit current (JSC) • Open-circuit voltage (VOC) • Power conversion efficiency (PCE, η) • Quantum Efficiency (EQE/IQE) • e--, h+-mobility • Series Resistance (RSA)

23. Introduction • By changing the annealing time and the growth rate of the active area, any change in these parameters are observed.

24. Sample Description • Active layer of poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) is sandwiched between metallic electrodes • Thickness of the active layer: 210-230 nm • Active device area: 0.11 cm2

25. Sample Description • Variable • Active layer growth rate • Annealing time

26. Results

27. Results

28. Results • Change in the carrier mobilities • Electron and hole mobilities must be well balanced.

29. Results • Highly ordered structure in sample #1 results in high absorption of light in comparison to sample #7 • Poorly ordered structure in sample #7 gives room for annealing to “heal” the disorder

30. Outline • Introduction • Self-assembly and distinctive features • Ordered air bubble in polymer film example • Polymer PV cells • Application • Highly efficient polymer PV cells using self-organized polymer • Bulk heterojunction solar cells • Future Improvements

31. Bulk heterojunction solar cells with internal quantum efficiency approacing 100%Sung Heum Park, Anshuman Roy, Serge Beaupre, Shinuk Cho, Nelson Coates, Ji Sun Moon, Daniel Moses, Mario Leclerc, Kwanghee Lee, and Alan J. Heeger

32. Introduction • The use of bulk heterojunction (BHJ) solar cells which involves the self-assembly of nanoscaleheterojunction significantly improved the PCE of polymer solar cells over a single junction architecture. • Relatively high performance polymer PV cells are 4-5% (from 2005-2006) • The use of low-bandgap polymers will be able to offer a better harvest of energy • In this report, BHJ solar cell composed of PCDTBT*/[6,6]-phenyl C71 butyric acid methyl ester (PC70BM) is used to improve the IQE of the cells. • * PCDTBT – poly[N-9”-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)

33. Description of Samples • Titanium oxide optical spacer and hole blocking layer present. • Optical spacer increases the photocurrent of the device by redistributing the maximum light intensity within the active charge separating BHJ layer. • The used spacer is TiOx

34. Result • Internal quantum efficiency is nearly 100%, meaning it can absorb almost all the photons • The PCE is ~6%

35. Future Improvements • Change of the polymer materials which could enhance PCE • Insertion of new functional layers

36. References • Srinivasarao, M., Collings, D., Philips, A., Patel, S., Science 292 (2001) • Li, G., Shrotriya, V., Huang, J., Yao, Y., Moriarty, T., Emery K., and Yang, Y., Nature 4 (2005) • Park, S., Roy, A., Beaupre, S., Cho, S., Coates, N., Moon, J., Moses, D., Leclerc, M., Lee, K., and Heeger A., Nature Photonics 3 (2009)

37. Questions?

38. G6Rebuttal: Self-ordered devices Jung Hwan Woo Please Prepare a Rebuttal

39. G1Self-Organization of Polymersand Applications in Solar CellsReview Edson P. Bellido Sosa

40. The presenter have defined self assembly and some of their distinctive features, types, applications and different methods to template. He showed one example of template using air bubbles in a polymer film. He explained the fabrication procedure, the parameters that have to be considered in the fabrication and how this affect the final result. He mentioned some applications of this templates and a hypothesis of why the air bubbles form a close packed structure. He also explained about the use of polymer in photovoltaic cells. He describe some of the important parameters that one have to take into consideration in PV cell fabrication. In the paper he explained they have analyzed how the solidification and annealing time affects the performance of the polymer PV cell. In other paper they have analyzed the bulk heterojunction of polymeric solar cells claiming they have obtained a quantum efficiency of 100%. The overall presentation was good. However, there was too much text and not many figures to help in understanding the topic. The connection between self ordered materials and polymeric PV cells was not very well established. In future research would be interesting to know how the ordering of polymer and the self assembly affects the PV efficiency

41. G2Review Self-ordered devices Alfredo Bobadilla

42. Self-Organization of Polymers and Applications in Solar Cells (Lecture review) • Essential concepts related to solar cells were not well illustrated, it should have been shown some illustrative schemes and equations. • A comparison with Silicon solar cells was not taken into account. I think mentioning the ‘Si solar cells’ case, which is a simpler case, would have helped to illustrate the qualitative and quantitative aspect of solar cell function. • It was not illustrated with enough detail the working principle of an organic solar cell; how or where electrons and holes are generated in the solar cell, what’s the role of each thin film layer ? Alfredo D. Bobadilla

43. Review:Self Organization of polymersJung Hwan Woo Presentation Mary Coan, G3 Chemical Engineering

44. Review • Defined Self-Assembly • Gave distinctive features • Gave examples in the form of images • Applications • Gave examples of different templates used • Procedure • Draw backs • Used images to explain how a self ordered array of air bubles in a polymer film are formed and the resulting products • Characterized parameters • Gave advantages and Applications

45. Review • Spoke about polymer PV cells. • Advantages and Disadvantages • Efficiency issues were also discussed • Gave examples of how parameters effect the device area • Spoke about bulk Heterojunction solar cells • Used to improve proformance • Touched on future improvements

46. Review • Overall the presentation was geared to a more educated audience • Undergrads who have previous knowledge of this topic and to the Graduate level • Do to the shear number of undergraduates in the audience I think it is more important to gear your presentation to the undergraduates • The presentation wasn’t too involved however more information on the solar cells may have helped the audience to understand how the polymer improves the solar cell.

47. Review • The presenter used many images to help the audience follow the presentation and understand more complex thoughts • Overall a good presentation

48. G4Review Self-ordered devices Diego A. Gomez Gualdron (MISSING)

49. G5Review Self-ordered devices Norma L. Rangel

50. Self ordered devices and applications in solar cells, Jung Hwan Woo He covered in the introduction: • Procedures and drawbacks of the fabrication processes • Advantages and applications such as steering devices and microlens, and also for photonic applications • Comparison air vs. water • Polymer PV Cells: • Low efficient, but cheap to fabricate and with good mechanical properties • Efficiency can be improved ordered structures, thicker surfaces and a reduction of the surface resistance • Solar cells paper • Improvement of the quantum efficiency nearly 100% of photons by using polymers materials and incorporating different techniques. The PCE is ~6% In my opinion the speaker did not show a relation between the self ordered systems and the solar cells, he was not confident explaining the operation of the solar cell, and I did not get well the importance of the “ordering”