Microlens Array Light Trapping CdTe Solar Cells for use in Concentrator Photovoltaics

Microlens Array Light Trapping CdTe Solar Cells for use in Concentrator Photovoltaics

Student: Patrick Margavio, Mechanical and Aerospace Engineering Faculty Advisor: Dr. Hailung Tsai, Mechanical and Aerospace Engineering

Why Solar?• Current energy usage of the world’s 6.5 billion people is 13 Terawatts1

• 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 Earth’s landmass2

• Amount required for food production is 13% cultivation, 26% pasture2

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 systemWhat’s 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 5000C and then at 6000C.

• 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 reflection3

• 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 µm CdTe film deposited on CdS

• Sample dipped in CdCl2

• 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 700C 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 outputto power with the 2 inch solar cell design