CNT Based Solar Cells MAE C187L Joyce Chen Kari Harrison Kyle Martinez

Our Approach An array of micro-sized “blocks” composed of single walled carbon nanotubes coated with photovoltaic materials and anti- reflective coating on a silicon wafer The 3 dimensional surface causes light rays to be trapped inside the cell This, combined with the anti-reflective surface reduces the percentage of reflected rays

Our Approach

Methodology The single walled nanotubes are cheaper, easier to fabricate and have better electrical properties than multi walled nanotubes Silicon based photovoltaic materials are cheaper than many alternatives, and proven to be successful and reliable E-beam lithography and chemical vapor deposition were used where applicable because we are familiar with these processes

Step 1: Silicon wafer

Clean a 2 inch silicon wafer with Acetone, Methanol and IPA De-ionized water rinse N 2 blow dry

Step 2a: E-beam Lithography: Spin Coating Photoresist

PMMA 495C2: 500 RPM for 5 seconds and 4000 RPM for 45 seconds Soft bake Sample at 180C for 90 seconds

Step 2b: E-beam Lithography: E-beam Patterning

Make a pattern of 700 blocks x 700 blocks of 40um x 40um squares with 10 um gaps Based on our limited knowledge, we would use the same parameters procedure as in Lab 1

Step 2c: E-beam Lithography: Metal Deposition

Using chemical vapor deposition we would deposit a thin layer of iron oxide on top of the patterned resist and wafer

Step 2d: E-beam Lithography: Photoresist Development and Metal Lift-Off

Soak PMMA developer for sec –Isopropanol + MIBK at 3 to 1 volume ratio Rinse in Isopropanol Blow dry with N 2

Step 3: SWCNT Growth

Place the wafer in a furnace heated to 1000C and pass an argon flow through the furnace Replace the argon flow with a methane flow of 99% purity at a flow rate of 6150cm 3 /min under 1.25 atm for 10 minutes Replace the methane flow with an argon flow and cool to room temperature

Step 4: Photovoltaic Deposition

Using molecular beam epitaxy, deposit silicon phosphorus (n-type layer) and silicon boron (p-type layer) Molecular beam epitaxy is a slow deposition of films taking place in a high vacuum

Step 5: Anti-Reflective Coating Deposition

Use a Cooke Thermal Evaporator to deposit a layer of silicon monoxide on the solar cell Program the Sigma Film Thickness mOnitor with these parameters –Density = 2.13 g/cm 3 –Tooling = 126% –Z-ratio = 0.87

Step 5: Anti-Reflective Coating Deposition Fill a long tungsten boat with SiO fragments Turn power up to 15% until boat beings to glow and stay there for 2 minutes Switch on heating until and increase dial to 30% for 30 seconds, until deposition rate is between angstroms/s Slowly increase to 35%-40% Once desirable thickness is obtained, close shutter and record thickness after 1 minute Slowly reduce boat current to zero and switch of heating unit

Cost Analysis Iron Oxide $1.00/ounce Silicon ~ $2.00/lb Silicon monoxide ~ $1.45/g Methane < $0.10/L A typical solar cell costs ~$0.05/kwh This cell uses less silicon, an expensive commodity, and should produce more energy per square meter – therefore we would expect it to cost at least the same, if not less per kwh

Estimated Efficiency A similar experiment obtained a 7% efficiency, while it is expected that a 40% efficiency is possible The addition of an anti-reflective coating can reduce the reflected light from 30% to 10%, which adds ~ 1% efficiency

Lifespan Current solar panels are rated ~ 30 years It is still unknown how long carbon nanotubes will last, but we assume their lifespan is the same as the copper wires they are replacing, if not longer This would make our solar cell life span also ~30 years

Testing Test in lab with UV light to determine kw per square meter Test at different angles to the sun to determine the correct incident angle for maximum efficiency Test in extreme temperatures, as well as in wind tunnels to determine structural stability

References Lecture Slides and Lab Handouts