Chris Phoenix is providing live blog coverage for us on all the presentations from an important conference on Productive Nanosystems: Launching the Technology Roadmap...
Next up: A Comparison of Nanotechnology-Enabled Photovoltaic Materials and Devices with Near-Term Commercialization Potential
Robert J. Davis, Director, Nanotech West Laboratory, The Ohio State University
Nothing in this talk that wiggles or swims; but it's very useful.
Talk structure: Intro to photovoltaics; nanotech-enabled PV; likely entry points for nano in commercial PV in next five years, and why; overview of a PV research center.
We need to harvest power at cost comparable to fossil-fueled power plants: 6-11 cents/kWh. Solar cells need to drop cost by 75% from $5/peak watt.
PV cell operation: I/V curve is like a diode, with a sharp "knee" at the origin. Under illumination it shifts down and to the right. You want to take off power at the knee; otherwise, you'll waste either voltage or current.
Crystalline and polycristalline silicon is pervasive but expensive at >$5/Wp just for the cell itself; there's been a recent worldwide silicon feedstock shortage; but even without this, can it ever meet cost targets?
Amorphous silicon, thin film deposited: 8% efficiencies, but it might be improved by a-Si:Ge heterojunctions.
Thin film II-VI compounds, CdSe, CdS, CIS, CIGS. These get around 10% efficiencies (polycrystalline) and are lowest cost at this time.
This talk will focus on nanotech in absorber layers, electrode layers.
Absorber development: Multijunction cell based on III-V compounds: use epitaxial [thin, precise] layers of AlGaAs, InGaP, GaAs, InGaAs to approach or exceed 40% efficiency. [BTW, II-VI and III-V refers to the column of the periodic table.]
Also, quantum-dot based absorbers, and nanoparticle precursors for CIGS and other films.
Multijunction cell: GaInP junction, 1.8 eV; GaAs at 1.42 eV; window layer, transparent graded layer to step to InGaAs 1.0 eV. These were recnetly published and are beginning to sample commercially. Details: You need quantum mechanical tunnel junctions to transmit holes and electrons between the layers. Also, you have to manage defects and crystal-lattice strain. Also, these cells are used in concentrator applications so you need to manage extreme thermal issues.
These type of cells were developed for spaceflight applications, but are now being shipped for concentrator applications (500 to 1000 suns). The nice thing about concentrator is that you can use tiny (cm^2) cells. Some cells are being grown on Ge:SiGe wafers for better mechanical properties. Also, nanodots are being added to increase IR usage.
Solar cells based on quantum dots in a matrix are low efficiency (2%).
Nano-ink is used to deposit II-VI. I didn't catch why, but this is likely too expensive for anything but military applications.
Electrode development: Transparent conductive oxides are expensive, hard to put on glass, hard to control stoichiometry (material ratio). But there's considerable work on single-wall nanotubes in polymer. This material is also useful in touch screens and electrostatic protection, so it's probably going to be developed usefully.
So, nanotube electrodes are probably going to be the main early entrance of nanotech into PV. Also, multi-junction may not be pure nano, but may provide an entry point for nanoparticles.
Ohio Wright Center is working on solar cells; trying to capitalize on auto-manufacturing expertise in building big-ticket items out of metal, glass, advanced polymers.
... Yep, he was right, no productive nanosystems here. But a solid interesting talk.
Chris Phoenix
Tags: nanotechnology nanotech nano science technology ethics weblog blog
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