One of the many things that has entered every day vocabulary in recent years is the prefix nano-. The term refers to the fraction 1 in 1 billion, thus nanometer refers to objects 1 billionth of a meter in length (around the size of 10 atoms) and nanotechnology refers to devices that incorporate structures of this size. Photovoltaics, the conversion of sunlight into electricity, is one of many areas in which nanotechnology is being utilized.

Keen to make photovoltaic devices increasingly efficient, scientists and engineers are investigating novel and exotic nanostructured materials in order to make solar cells ever more cost effective. One such type of device is the quantum well solar cell. This type of solar cell uses nanometer-width layers (or quantum wells) comprised of different photovoltaic materials. By carefully controlling which photovoltaic materials are used and the widths of the quantum wells, the absorptive properties of the solar cell can be tailored to make more efficient use of the incident sunlight.

These materials are crystalline in structure, meaning that they are comprised of ordered, repeating sequences of atoms (often referred to as a crystal lattice) and are commonly characterized by the distance between atoms (or lattice constant). To produce solar cells of maximum possible efficiency both of the photovoltaic materials used must be made to conform to the same lattice constant, a challenge that has been the subject of intensive research by the Quantum Photovoltaics Group in the Physics Department of Imperial College London. The result of this work is the strain-balanced solar cell, in which layers of photovoltaic material under compressive strain (atoms forced closer together than in the original crystal structure) are alternated with layers of material that are under tensile strain (atoms pulled further apart than in the original structure) – effectively giving all the layers in the solar cell the same lattice constant.

In 2007 the spin-out company QuantaSol was formed to bring these high efficiency devices into commercial production. In 2009 QuantaSol reported a 28.3% efficient quantum well solar cell; a world record for solar cells of its class. Having achieved this milestone, the company is undergoing rapid growth and product launch is expected by the end of 2010.