Abstract

1. Wet Chemistry Based Copper Oxide and Zinc Oxide Nanowire Photovoltaic Cells S. MacNaughton1, D. F. DeMeo2, Sameer Sonkusale1, Thomas E. Vandervelde2 1Nanoscale Integrated Circuits and Sensors Laboratory, 2Renewable Energy and Applied Photonics Laboratory ECE department, Tufts University, Medford, MA, USA Band Structure Abstract Fabrication Results Solar cells are a promising, green energy source; however, cost and efficiency limitations prevent widespread adoption. We propose a solar cell design that is cost-effective both in production and materials. Here, the junction is made of copper (I) oxide and zinc oxide, which are oxides of earth-abundant metals. Furthermore, we utilize a wet chemistry fabrication process, making the production of such cells inexpensive and easily scalable. The process involves growing copper nanowires, depositing zinc, oxidizing, and depositing a top contact, detailed in the fabrication section. This is a greener manufacturing method of solar cells where no harmful compounds or excessive energy is used in fabrication. Currently results are pending. Right: Copper nanowire growth is successful and consistant Band diagram of the pn junction between ZnO and CuO Band diagram of the pn junction between ZnO and Cu2O Left: Zinc on copper nanowires The actual material is a mix of CuO and Cu2O Conclusion Much progress has been made on developing the process to fabricate these solar cells. The largest difficulty is to overcome short circuiting the layers when depositing the top contact. As can be seen we see a diode response before the top contact layer is applied, however once the top contact is in place the cell only exhibits a resistive response. Future work includes further testing and fabrication to obtain a working sample. There is also much work to be done in optimizing each layer. The ITO transparent conducting oxide can be optimized for increased transmittance and conductance. We eventually hope to replace this with a cheaper, more abundant TCO such as TiO2 or ZnO. Motivation and Significance Right: EDS showing the presence of both zinc and copper • Nanowire, earth abundant oxide solar cells offer several advantages over traditional bulk silicon: • Inexpensive materials - copper, zinc and their oxides are earth-abundant and easily refined elements and molecules. • Inexpensive fabrication - the electrochemical fabrication techniques presented here are used extensively in industry due to their ease of operation and cost-effectiveness at all scales of production. • Hot carrier conversion - the nanowire arrangement of this cell allows for more energy to be harvested from each photon absorbed. • Increased absorption - the nanowires create a dark surface which improves absorption in the cell. Top View Thermal oxidation causes interdiffusion across the Cu/Zn boundary. Therefore ZnO is directly electroplated onto the copper oxide nanowires. Zn evaporated onto CuO nanowires References Comparison to traditional Si solar cell Results Rakhshani, A.E., Preparation, characteristics and photovoltaic properties of cuprous oxide--a review. Solid-State Electronics, 1986. 29(1): p. 7-17. McKubre, M.C.H. and D.D. Macdonald, The Dissolution and Passivation of Zinc in Concentrated Aqueous Hydroxide. Journal of the Electrochemical Society, 1981. 128(3): p. 524-530. C. Wadia, A. P. Alivisatos, and D. M. Kammen, "Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment," Environmental Science & Technology, vol. 43, pp. 2072-2077, Mar 15 2009. S. Ishizuka, K. Suzuki, Y. Okamoto, M. Yanagita, T. Sakurai, K. Akimoto, N. Fujiwara, H. Kobayashi, K. Matsubara, and S. Niki, "Polycrystalline n-ZnO/p-Cu2O heterojunctions grown by RF-magnetron sputtering," physica status solidi (c), vol. 1, pp. 1067-1070, 2004. An early sample of the nanowire cell. Although the nanowire geometry will suffer from a decreased quantum efficiency due to the spacing between the individual wires, it will be able to convert more energy from each incident photon that strikes the active area. This is called harvesting hot carriers. A hot carrier is a photon with an energy above the band gap of the material. Normally these will be absorbed and the excess energy above the band gap will be lost thermally, but with the decreased distance to the contacts in the nanowire geometry more of this excess energy will be converted into electricity. • Entire cell is less than 15μm thick. • Cell weighs ≈15mg/cm2 • Material costs are <$1USD per m2 IV curve of Flat cell on glass Thermalization of hot carrier http://nanolab.ece.tufts.edu/ Contact: tvanderv@ece.tufts.edu http://reap.ece.tufts.edu