Introduction

1. Graphene Increases Solar Cell Efficiency Nathan Duderstadt, Stoney Sutton, and Kate Yoshino College of Engineering and Applied Sciences Research Experience for Undergraduates, University of Cincinnati Graduate Mentor: Ms. Yan Jin, Faculty Mentor: Dr. Vikram Kuppa Aluminum (Cathode) Lithium Fluoride Active Layer (P3HT:F8BT:Graphene) PEDOT:PSS Indium Tin Oxide (Anode) Glass Slide Solar Cell Fabrication and Testing Methods Introduction Currently, much of the world’s energy consumption comes from fossil fuels and non-renewable energy sources, which are quickly being used up. Thus, new methods for generating clean energy must be developed. Solar Energy is a viable option for renewable energy with potentially low side effects. Organic Photovoltaic Devicesare of special interest due to their unique physical properties allowing for flexible panels and its mass production capabilities. Additionally, they require fewer harmful chemicals and less precisions than inorganic, silicon based cells. The problem is that organic cells are not as efficient as the inorganic cells that are currently on the market. This project aims to address this issue and help to improve efficiency in organic photovoltaic cells. • Aluminum and Lithium Fluoride applied in by vapor deposition • Active layer applied by spin coating. • Cell (without slide) is approximately 500 nm thick • Cells were tested for performance by current-voltage testing. • X-ray diffraction and UV spectrometry tests were performed for morphology. Data Objective Ultimately, the goal of this project is to determine how graphene affects the efficiency of organic solar cells. Graphene is a two-dimensional, one atom thick layer of networked carbon that is a promising material for organic photovoltaic devices. The main focus of this study centers on the highly conductive nature of graphene. Graphene is to increase percolation for charge transport at low weight fractions, therefore increasing cell efficiency. 20 mg/ml6:4 mass ratio P3HT/F8BT polymer blend, processed in p-xylene, Data taken from two batches both fabricated on 7/2/13. Conclusion • Conductivity testing showed that the increase in graphene content resulted in increase in conductivity. • The graphene concentration increase is limited by potential short-circuiting of the device. • Results show that 0.1 mg/ml graphene concentration had the highest efficiency of concentrations tested, however, more testing is required for confirmation. • Graphene increased cell’s efficiency by charge percolation, not improvement in crystal size. 20 mg/ml6:4 mass ratio P3HT/F8BT polymer blend, processed in p-xylene Conductivity Testing Methods • Conductivity studies were performed to determine the appropriate graphene concentration that will best accomplish charge percolation in the solar cells. • Graphene was obtained by exfoliation of graphite in benzene/hexafluorobenzene solvent via sonication for both the conductivity studies and cells. The benzene solvent was evaporated off when graphene was added to P3HT in p-xylene solution at varying concentrations. • Conductivity Testing involves a simplified version of the solar cell for finding conductivity in a more cost and time efficient way. • This P3HT/F8BT/Graphene solutions were spin coated onto a the electrodes and then soft baked. (Top electrode is then applied for the vertical configuration.) • Resistance measured per sample. • Lastly, the thickness of each sample was measured in order to calculate conductivity. Literature Review • Articles consulted for this project include: • Deibel, C, and V. Dyakonov. (2010). "Polymer–fullerene Bulk Heterojunction Solar Cells," Reports on Progress in Physics, IOP, Vol. 73, No. 9, pp. 1-39. • Du, Y., et al. (2012) “Preparation and characterization of graphene nanosheets/poly(3-hexylthiophene) thermoelectric composite materials,” Synthetic Metals, Vol. 162, pp. 2102-2106. • Kim Y. et al. (2004). "Organic Photovoltaic Devices Based on Blends of Regioregular Poly(3- hexylthiophene) andPoly(9,9-dioctylfluorene-co-benzothiadiazole)," Journal of the American Chemical Society, ACS Publications, Vol. 16. pp. 4812–4818 • Li,G., Yang,Y., and R. Zhu. (2012). "Polymer Solar Cells," NATURE PHOTONICS No.6, pp.153-161. • Liu,Q., et al. (2009). "Polymer Photovoltaic Cells Based on Solution-Processable Graphene and P3HT," Advanced Functional Materials Journal, Vol. 19,No.6, pp. 894-904. • McNeill, C.R., et al. (2007).“Influence of Nanoscale Phase Separation on the Charge Generation Dynamics and Photovoltaic Performance of Conjugated Polymer Blends: Balancing Charge Generation and Separation,” Journal of Physical Chemistry C, Vol. 111, No. 51, pp. 19153-19160. • Oyer A.J., et. al. (2012). "Stabilization of Graphene Sheets by a Structured Benzene/Hexafluorobenzene Mixed Solvent," Journal of the American Chemical Society, ACS Publications, Vol 134. Pg 5018-5021. • Saricifti, N.S. (2001). “Plastic Solar Cells.” Abstracts of Papers of the American Chemical Society, Vol. 222, pp. U281-U281. • Shin, M., H. Kim, and Y. Kim. (2011). “Effect of film and device annealing in polymer:polymer solar cells with a LiF nanolayer,” Materials Science and Engineering B- Advanced Functional Solid-state Materials, Vol. 176, No. 5, pp. 382-386. • Van Bavel, S.S., et al. (2010). “P3HT/PCBM Bulk Heterojunction Solar Cells: Impact of Blend Compositions and 3D Morphology on Device Performance,” Advanced Functional Materials, Vol 20, No.9, pp. 1458-1463. • Wan, X., et al. (2011). “Graphene- A Promising Material for Organic Photovoltaic Cells,” Advanced Materials, Vol. 23, pp. 5342-5358. • Yu, D., et al. (2010). “Soluble P3HT-Grafted Graphene for Efficient Bilayer- Heterojunction Photovoltaic Devices,” ACS Nano, Vol. 4, No. 10, pp. 5633-5640. 5 μm 1μm Atomic Force Microscope Image of 300 mesh Graphene solution with 0.045 mg/ml concentration. Acknowledgements We would like to thank NSF for the funding of this project under NSF Grant # DUE-0756921 for Type 1 Science, Technology, Engineering, and Mathematics Talent Expansion Program  (STEP). Special thanks also goes to Ms. Yan Jin, Dr. Vikram Kuppa, and Mr. Fei Yu for many hours of help and guidance.