Nanowires (yellow) are grown directly on a metalic electrode (green), then covered with an organic polymer (blue). The wires help shuttle electrons directly to the electrode while a gold conductor (gold) attracts the holes away from it.  (Source: University of California, San Diego, Jacobs School of Engineering)
Electron ferrying nanowires may boost solar cell efficiency by hundreds of percent.

Research in solar energy collection has seen a flurry of results in recent months. Some companies are working on making panel installation easier. Others are pursuing novel techniques that could produce flexible photovoltaic sheets that can simply be printed with an inkjet printer.

While these technologies focus on improving the usefulness of solar panels themselves, the advancement of photovoltaic technology itself is not at a standstill. Last year, Harvard researchers announced a silicon nanowire which is not only cheap and durable, but photovoltaic, unlocking various possibilities for portable or wartime "disposable" collectors.

One of the most important properties of any solar collector is its efficiency. By improving efficiency, more power can come from less surface area. With large collector sites scheduled to come online in the next few years, even a single digit gain in conversion efficiency could make a large impact on the output of a system.

Part of the problem with current panel technology is something called electron-hole recombination. Rather than photon absorbing electrons and their hole counterparts finding their ways to their respective electrodes, they float around until they recombine, returning to being uncollectable and unusable. With current mass produced solar cells hovering in the mid-teens in solar energy conversion, recombination is more likely than not to occur.

Published in the online journal NanoLetters this February, researchers at the University of California at San Diego's Jacobs School of Engineering have found a way to use nanowires to theoretically increase organic photovoltaic efficiency. When compared to a non-nanowire control cell, they observed a six to seven hundred percent gain in forward bias current in the nanowire-laden cell.

The UCSD cell is not the first to use nanowires in its construction, but it is the first published work to use nanowires grown directly upon the electrode. The nanowires in this case help capture and ferry freed electrons and holes to their specific collection electrodes, preventing them from recombining.

Part of the success of the UCSD's photodiode is in having found a method for growing the nanowires on an untreated metal surface. Most nanowire growth techniques involve special substrates, often seeded with catalyst materials like gold nanodrops. For their solar cell research, the group used an electrode made of indium tin oxide and grew nanowires of indium phosphide on it. The procedure, however, can be used on other untreated metals. It could help pave the way for next-generation collectors, as the technique is performable on cheap metals of most any shape or form.

Though the paper, titled "InP Nanowire/Polymer Hybrid Photodiode" does not specify actual conversion efficiency -- being a proof-of-concept project -- the large jump in forward bias current output seems to speak volumes about how much of an improvement it may be over standard organic polymer photovoltaics. It also opens doors to new types of collectors, perhaps "grown" right into buildings and vehicle sheet metals. Though solar energy is not without its drawbacks, constant research and improvement will certainly keep it a player in the global power production picture.

"There is a single light of science, and to brighten it anywhere is to brighten it everywhere." -- Isaac Asimov

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