NREL (National Renewable Energy Laboratory) USA has set a new record for PV cell efficiency. They have achieved a cell conversion efficiency of 44%, compared to currently commercial available silicon PV cell of 20%. This record has won them a coveted R&D 100 award from R&D Magazine for a world-record multijunction solar cell. The cell composes of 3 layers of semiconductor materials to form multiple junctions inside the PV cell to maximize the absorption of the sun energy. Low-cost lens was mounted on the surface of the cell to gather sunlight and direct to each cell. This lens serves as a "concentrator" to focus more sunlight to reach the surface of the cell.
The semiconductor materials used here are not the conventional silicon. They are called the III-V semiconductors, which are the group 3 and group 5 elements in the chemistry periodic table. Common III-V materials used in PV cell are GaAs, InAs, InP, AlAs, GaN and combination of them, like AlGaAs, GaInAs, etc. Different material and combination will respond to different wavelength of the sun spectrum. For example, GaAs has peak spectral response at ~ 850nm which is infrared, AlAs peaked at ~ 570nm which is in the green spectrum.
Using different III-V materials to form a multiple-junction PV cell will broaden the spectral response allowing more sunlight to be absorbed in a single cell, thereby improving the efficiency of the PV cell.
The cost of production for III-V semiconductor PV cell is still very high. Currently, they are mostly used in the high-end type of PV system, like CPV (concentrated photovoltaic) system which has parabolic-disc look and capable to generate few kilo-watts of power with only 1 disc.
To read more about this please go to http://www.nrel.gov/news/features/feature_detail.cfm/feature_id=2055