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Microlens Array Light Trapping CdTe Solar Cells for use in Concentrator Photovoltaics Student: Patrick Margavio, Mechanical and Aerospace EngineeringFaculty Advisor: Dr. Hailung Tsai, Mechanical and Aerospace Engineering Why Solar? Current energy usage of the worlds 6.5 billion people is 13 Terawatts 1 The sun outputs 120,000 TW Globally extractable wind power is 4 TW With current efficiencies, fraction of land required to supply present energy consumption is 1.35% of Earths landmass 2 Amount required for food production is 13% cultivation, 26% pasture 2 Why Concentrate Solar Energy? There are two basic strategies to compete with fossil fuels Make solar inexpensive Copper indium gallium diselenide, dye sensitized solar Commercially available single and polycrystalline silicon Solar houses on campus Maximize efficiency Combine solar cell (like multijunction GaAs) with solar concentrator Mirrors, Luminescent Solar Concentrators Solar cell is 75% of cost of system Whats a Microlens Array? A microlens array is a collection of microscopic lenses assembled in an orderly pattern Focuses light into a periodic pattern Two purposes for us Light less likely to reflect off top surface Incident light intensity increased locally Efficiency increases with increased intensity It is expensive to create microlens arrays Decrease expense by creating a Foturan glass mold and then using it to make many cheap silicon films Mold is created by laser machining G code used to computer control process is shown After laser machining Foturan glass is baked at 500 0 C and then at 600 0 C. After baking, sample is etched with HF acid After etching sample is annealed for another hour Making the Microlens Array Mold: What is Light Trapping? There are two basic ways to trap light within a solar cell Reducing the amount of light reflected away from top surface Examples below scatter incoming light to reduce reflection 3 Preventing light from leaving once it has entered CdTe Cell Fabrication: Transparent Conducing Oxide (TCO) purchased from Pilkington (TEC C10) 150 nm CdS film chemically deposited on TCO 5 m CdTe film deposited on CdS Sample dipped in CdCl 2 Sample rinsed with methanol Making the Silicon Microlens Array: The silicon gel used is Polydimethylsiloxane (PDMS) PDMS is combined with a curing agent The PDMS mixture is poured over the glass mold Sample is placed in a vacuum chamber (right) to remove bubbles from mixing process Next PDMS is cured at 70 0 C for one hour in a furnace to solidify To left, a microscope image of the silicon film is visible. Below, the glass mold is shown in the silicon gel Acknowledgements: Dr Hailung Tsai Dr Cheng-Hsiang Lin Material Research Center Intelligent System Center References: 1)P. V. Kamat. J. Phys. Chem. C 111, 2834-2860 (2007) 2)A. Luque. Journal of Applied Physics 110, 031301 (2011) 3)V. V. Iyengar, B. K. Nayak, M. C. Gupta. Solar Energy Materials and Solar Cells 94, 2251-2257 (2010) Demonstration Model We will implement our solar cell design to power a small electronic device A two square inch solar cell can produce around 10 to 11 W of power (based on commercially available silicon solar technology) The current output can be a limiting factor for solar cell designs We chose a 7 inch digital picture frame, which fits this power output to power with the 2 inch solar cell design