Semiconductor

What Is a Semiconductor?

A semiconductor is a material whose electrical conductivity lies between that of a conductor (e.g. copper) and an insulator (e.g. glass). Through targeted impurity addition (doping), the conductivity can be precisely controlled. Silicon is by far the most important semiconductor in photovoltaics.

Operating Principle in Solar Cells

Semiconductors have a band gap between the valence band and the conduction band. When a photon with sufficient energy strikes the semiconductor, an electron is lifted from the valence band into the conduction band — creating a free charge carrier and a “hole”. Through doping, an electric field is created that separates the charge carriers.

Doping

Type	Dopant	Effect
n-type	Phosphorus (5 valence electrons)	Excess electrons
p-type	Boron (3 valence electrons)	Excess “holes”

The junction between the n-doped and p-doped layers forms the p-n junction, the heart of every solar cell.

Semiconductor Materials in PV

• Silicon (Si): 95% market share, crystalline or amorphous
• Cadmium Telluride (CdTe): Thin-film
• CIGS: Copper Indium Gallium Selenide, thin-film
• Perovskite: Future technology

Practical Tip

The purity of silicon largely determines the efficiency of the solar cell. Solar-grade silicon has a purity of 99.9999% (6N), while electronic-grade silicon reaches 99.999999999% (11N).