Searching for Solar Breakthroughs Silicon & III-V Based PV Breakout Session #2 Silicon & III-V Based PV Colorado Rural Electric Association 7th Annual Energy Innovations Summit Denver, Colorado Dr. David Young National Renewable Energy Laboratory Golden, Colorado - USA 12 September, 2016

Globally, 47 GW of new photovoltaic panels were installed in 2015. That’s like building 54 averaged-sized nuclear reactors in one year ~860 MW/reactor 15% capacity utilization for PV 77% capacity utilization for nuclear > 90% of all PV is crystalline silicon Renewable energy world.com PV Magazine, June 2016

Crystal Silicon Solar Module Production metallurgical grade Si Sand Add Carbon & Heat Energy pure SiHCl3 or SiH4 Add HCl + heat Wire sawn wafers Waste ~1/2 in sawing Add Heat Energy (1000 °C) Add more heat energy (1500°C) Si Boule Poly-silicon feedstock Use 4X more silicon than needed for Solar cells Energy payback < 2 years System payback < 7.5 years $3.65/W to install in Colorado ½ the cost of a solar cell is in the wafer. 3-9 GJ/m2 Renewable and Sustainable Energy Reviews Volume 47, July 2015, Pages 133–141

Current Trends in Silicon Solar Cells to Decrease $/W metallurgical grade Si Sand pure SiHCl3 or SiH4 Gas-to-wafer (epitaxial growth) - Crystal Solar (CA) Wire sawn wafers Continuous-pull boules -GTAT (MT) Si Boule Direct-to-wafer technology -1366 Tech. (New York) Poly-silicon feedstock Multicrystalline wafers Small grains – high lifetimes Use 4X more than needed for Solar cells High efficiency solar cells -SunPower (CA) -SolarCity (New York) -Suniva (GA) -Panasonic (Japan) - (China, Korea, Tiawan) Bifacial module mounting to collect sunlight from both sides of the module.

Standard Al back contact silicon solar cell Leaky bucket analogy (fewer leaks = higher voltage) Dominates market Efficiency: 15 – 17% Multicrystalline wafer < $0.40/W cost Very poor back contact “Passivated” Emitter and Rear Contact (PERC) solar cell Gaining market share Efficiency: 18-20% More processing steps Better front contacts Better back contact

Heterojunction Solar Cell Reflection loss SiO2, a-Si, PolySi, MoOx, LiFx High efficiency 22-25% Few, but precise process steps Bifacial modules Small market share Solar City (Silevo) building new Giga watt factory in NY Interdigitated back contacted (IBC) Solar cell No front contacts Highest Efficiency cells 25.6% (max ~26.4%) modules 24.1% Many process steps SunPower product Using these techniques five companies/institutes have broken the 25% efficiency mark.

Crystal Solar (CA) Direct Gas to Wafer Technology Epitaxial Si layer for solar cells 24% cells 50% less silicon 50% less capital costs Much less energy/wafer Wafers grown with junction preformed Porous liftoff layer Recycled parent wafer 1366 Technologies (NY) Direct wafer process Avoids kerf loss less energy to form Wafers 19.1% cells New Factory in NY Hanwha Q-cells to purchase 700 MW of wafers. Directly form wafers from molten c-Si

Bifacial Modules Cells and modules built to allow light to enter from the front and back sides Published field tests have indicated that a 10 – 40% increase in power output is achievable.

Thin-Film GaAs Alta Devices (CA) 28.6% record cell 25% eff, flexible cells ~$100/W MOCVD growth process NREL is researching a lower-cost route to the same product. 25% eff, flexible cells Low-cost precursors High deposition rates In-line processing 100 x decrease in $/W