PV Cell Type and Efficiency
Two categories of PV cells are used in most of today's commercial
PV modules: crystalline silicon and thin film. The crystalline
silicon category, called first-generation PV, includes
monocrystalline and multicrystalline PV cells, which are the most
efficient of the mainstream PV technologies and accounted for about
84% of PV produced in 2008 (Bartlett et al. 2009). These cells
produce electricity via crystalline silicon semiconductor material
derived from highly refined polysilicon feedstock. Monocrystalline
cells, made of single silicon crystals, are more efficient than
multicrystalline cells but are more expensive to manufacture.
The thin-film category, called second-generation PV, includes PV
cells that produce electricity via extremely thin layers of
semiconductor material made of amorphous silicon (a-Si), copper
indium diselenide (CIS), copper indium gallium diselenide (CIGS), or
cadmium telluride (CdTe). Another PV cell technology (also second
generation) is the multijunction PV cell. Multijunction cells use
multiple layers of semiconductor material (from the group III and V
elements of the periodic table of chemical elements) to absorb and
convert more of the solar spectrum into electricity than is
converted by single-junction cells. Combined with
light-concentrating optics and sophisticated sun-tracking systems,
these cells have demonstrated the highest sunlight-to-electricity
conversion efficiencies of any PV technologies, in excess of 40%.
Various emerging technologies, known as third-generation PV, could
become viable commercial options in the future, either by achieving
very high efficiency or very low cost. Examples include
dye-sensitized and organic PV cells, which have demonstrated
relatively low efficiencies to date but offer the potential for
substantial manufacturing cost reductions.
The efficiencies of all PV cell types have improved over the past
several decades, as illustrated in Figure 3.6, which shows the best
research-cell efficiencies from 1975 to 2008. The highest-efficiency
research cell shown is a multijunction concentrator at 41.6%
efficiency. Other research-cell efficiencies illustrated in the
figure range from 20% to almost 28% for crystalline silicon cells,
12% to almost 20% for thin film, and about 5% and to 11% for the
emerging PV technologies organic cells and dye-sensitized cells,
respectively.