Tech Primer

Nanowire Silicon solar cell

Solar cells made of nanometer-scale wire could well end up providing trickles of useful power to nanoelectronic devices or microscopic robots in the near future. Researchers at Harvard are trying to make this concept a commercial reality.

Structure of the nanowire

The coaxial silicon nanowire is about 300 nanometers thick and can generate about 200 picowatts of electricity which is sufficient to power a small circuit. This nanowire is made up of three layers of silicon—first, a positively charged core of crystalline silicon; second, a thin intrinsic, or neutrally charged inner shell; and lastly a negatively charged outer shell. This structure is called a p-i-n structure, and is common to any flat photovoltaic devices, but for the first time this structure is being used for coaxial wire.

Working of the cells

The functions performed by these layers are the same as those of conventional solar cells. They absorb light and capture electrons in order to generate electricity. In this process, when a photon hits the nanowire structure, it generates an electron and a hole as a pair of charges. Then, due to its geometry, the charges move radially. The electrons move outwards from the center to a contact in the shell and holes move inwards to a contact in the core. In this sort of circular geometry, electrons and holes move for a very short distance, approximately 100 nm or less, compared to a flat cell to reach the contacts that collect them and send electricity flowing from the device. The short distance to be covered makes them less likely to recombine—a process where an electron and a hole rejoin instead of exiting the device before they are collected.

Nanowire vs. Nanoscale

Nanoscale solar cells made up of inorganic and organic materials combined, tend to decompose over time. Nanowire solar cells, on the other hand, can be used with a lens to concentrate more light onto the cells, an approach that does not work in the case of nanoscale cells.

As of now, nanowire cells convert only about 3.4 percent of the light that shines upon them but this number is sure to go up. Nanowires might not replace large solar cells in the foreseeable future but they can be use in experimental nanometer-scale devices which today lacking sufficient power supply. Nanowires can also be used as logic gates, creating a simple processor to control a micro electromechanical system such as a sensor.

For further information, visit:spectrum.ieee.org/oct07/5642

— Kushal Shah