Apr 23, 2009
Honda eyes nanowires for multi-junction cells
20 nm columns made of five different semiconductor materials avoid lattice mismatch and overall cell size limitations.
Honda has applied for a patent that combines compound semiconductors and nanowires in an effort to deliver the world's most efficient solar cells.
The Japanese motor giant's US patent application, published on April 2, claims to solve some of the major issues preventing progress beyond today’s leading triple-junction structures.
By producing 20 nm diameter nanowires the researchers claim that they avoid defects seen in other solar cells, and which plague approaches to raising solar conversion efficiency.
The existing 40 percent-plus efficiency record-holders stack GaInP, GaAs and germanium together in triple-junction cells to convert as many photons into electronic current as possible.
One suggestion to push conversion still further is to add a GaInNAs layer that would absorb photons with an energy around 1 eV. However, the difference between the lattice constants of this and the other materials produces defects that limit the cells' efficiency.
The Honda team describes quadruple-junction wires, starting from a GaP substrate, with five semiconductor layers deposited on top by MOCVD. Starting from the closest to the substrate, these are: GaP, Al0.3Ga0.7As, GaAs, In0.3Ga0.7As and In0.6Ga0.4As.
“A junction interface of the wire-shaped semiconductors has a very small area, which relieves the distortion resulting from the difference of the lattice constants and can prevent the occurrence of defects,” the Honda Motor Co. patent claims. This makes it easier to combine different materials.
The Honda team then suggests sputtering a silica layer onto the substrate to act as an insulator between the nanowires. Circular holes are then etched out of this to reveal the GaP substrate in a pattern that defines how the nanowires will be positioned when MOCVD is performed.
The subsequently grown GaP layer does not act as a junction in the solar cell, but instead ensures that no defects arise between the substrate and the initial nanowire growth.
After this, each semiconductor material is deposited at progressively lower temperatures, ensuring that no defects form in the previous layer due to evaporation.
At each junction interface the researchers inspected the growing wires with high-resolution transmission electron microscopy. They found that although most nanowires were around 20 nm in diameter some were as large as 50 nm.
“No defect was observed in the junction interfaces of the semiconductors irrespective of the diameter thereof,” the team writes. “Therefore the four-junction solar cell can achieve high efficiency because the carriers generated by incident light do not recombine at heterojunction interfaces.”
Honda also points out that defect formation currently limits the size of conventional triple junction cells to less than 4 cm2. An “unlimited number” of nanowires could be used to produce large-area nanowire quadruple-junction cells, by comparison.