Generation gap
Efficiency versus cost for the three generations of solar cells. First-generation
cells, which are based on expensive single-crystal silicon wafers, account for about 85% of
devices sold today. Second-generation cells, which comprise thin films of silicon and other
semiconductors, are cheaper but less efficient. Meanwhile, third-generation devices made from
more advanced technologies and materials are at an earlier stage of development but promise
high efficiencies at low costs. The horizontal lines show the theoretical upper limits of the
efficiencies (from bottom to top) of a standard solar cell, an advanced cell exposed to
unconcentrated sunlight, and an advanced cell subject to sunlight concentrated by a factor of
46,200 – the maximum possible. The dashed diagonal lines are loci of constant cost per unit
power, measured in dollars per peak watts. Because of natural variations in the solar power that
reaches a given area across the day/night cycle and changes in cloud cover, the average
electrical power produced by a solar cell in a year is about 20% of its peak rating. A price of
$1 per peak watt translates to a unit electrical cost of about $0.05 per kWh over the
30-year lifetime of a typical solar cell.