True, traditional photovoltaics get the same energy out of every color photon as long as it has enough energy to clear the bandgap. The rest of that energy is lost to vibrations in the semiconductor as heat instead of electrical energy. BUT - here's another area where nanostructured devices might help, because it takes much longer for an energetic electron to emit those vibrations in a structure on the quantum scale (i.e., nanometer scale). These energetic electrons can sometimes be collected at their high energy or kick up another electron across the bandgap through Auger recombination. This has been demonstrated a few times for nanometer-sized semiconductor particles called quantum dots and is called "hot electron capture" or "multiple exciton generation" in the scientific literature.
What I'm trying to say is that nanorod structures have the potential of increasing the maximum efficiency because they might be able to collect those higher energy photons and pull out more electricity than a conventional photovoltaic cell.
Hopefully this will be practical someday...the multijunction cells are just way too expensive how they are currently made.
This could really be a fascinating technology -- although technically it's "nanowires" and not nanotubes. As an experimentalist, I really hope that when it comes to actually growing these things it is feasible; it also might be difficult to make contact to the nanowires after you've made them to collect the electricity. Nonetheless, I think that nanostructured devices (while expensive at the moment) may be the solution to making high efficiency photovoltaics possible.
Sometimes it's surprisingly easy to grow nanowires/nanorods just by flowing gas over a material and a substrate in a tube furnace, so cost may turn out to be fairly low. Patterning these by photolithography (how computer chips are made) would definitely be too expensive, along with molecular beam epitaxy or atomic layer deposition. My hope is that a simple inexpensive thermal process would work to grow these or other photovoltaic nanorods.
The reason that so many stories are posted about solar energy is that it's our one scaleable renewable energy that could eventually displace a significant fraction of the fossil fuel energy that we currently use and spew CO2 into the air. For a really interesting lecture about world energy and alternatives check out Dr. Nate Lewis' presentation at http://nsl.caltech.edu/energy.html (the video is probably the best). Who knows, one of these breakthroughs if it works well could change a lot for us.
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What I'm trying to say is that nanorod structures have the potential of increasing the maximum efficiency because they might be able to collect those higher energy photons and pull out more electricity than a conventional photovoltaic cell.
Hopefully this will be practical someday...the multijunction cells are just way too expensive how they are currently made.
This could really be a fascinating technology -- although technically it's "nanowires" and not nanotubes. As an experimentalist, I really hope that when it comes to actually growing these things it is feasible; it also might be difficult to make contact to the nanowires after you've made them to collect the electricity. Nonetheless, I think that nanostructured devices (while expensive at the moment) may be the solution to making high efficiency photovoltaics possible. Sometimes it's surprisingly easy to grow nanowires/nanorods just by flowing gas over a material and a substrate in a tube furnace, so cost may turn out to be fairly low. Patterning these by photolithography (how computer chips are made) would definitely be too expensive, along with molecular beam epitaxy or atomic layer deposition. My hope is that a simple inexpensive thermal process would work to grow these or other photovoltaic nanorods. The reason that so many stories are posted about solar energy is that it's our one scaleable renewable energy that could eventually displace a significant fraction of the fossil fuel energy that we currently use and spew CO2 into the air. For a really interesting lecture about world energy and alternatives check out Dr. Nate Lewis' presentation at http://nsl.caltech.edu/energy.html (the video is probably the best). Who knows, one of these breakthroughs if it works well could change a lot for us.