What Is Cadmium Telluride?

Cadmium telluride (CdTe) is a binary compound semiconductor formed from cadmium and tellurium, crystallizing in the zinc-blende structure and belonging to the II-VI family of materials. Its direct bandgap of approximately 1.45 eV at room temperature places it near the theoretical optimum for single-junction photovoltaic conversion under the AM1.5 solar spectrum, making it one of the most photovoltaically efficient materials discovered to date. CdTe is also a strong optical absorber: a layer just 1 to 3 micrometers thick captures more than 90 percent of above-bandgap photons, which is why thin-film CdTe modules require far less semiconductor material than crystalline silicon cells of equivalent area. Beyond photovoltaics, CdTe is used as an infrared optical material and as a substrate for growing mercury cadmium telluride (HgCdTe) detectors. Its prominence in energy technology has made it one of the most intensively studied thin-film semiconductors since the 1950s, when initial photovoltaic experiments with II-VI compounds began.

Material Properties and Bandgap

CdTe is a direct-bandgap semiconductor, meaning electrons and holes recombine without requiring a phonon to conserve momentum, which gives the material a high optical absorption coefficient in the range of 10⁴ to 10⁵ cm⁻¹ for photons near its bandgap. The bandgap of 1.45 eV corresponds to absorption of photons with wavelengths shorter than about 855 nanometers, covering most of the visible spectrum and a portion of the near-infrared. Unlike indirect bandgap materials such as crystalline silicon, CdTe absorbs nearly all available solar photons within a few micrometers of the surface, enabling the thin-film device architecture. CdTe also has relatively high electron and hole mobilities for a polycrystalline material and a long minority-carrier lifetime when properly passivated, both of which are critical for efficient carrier collection in a solar cell. The material melts at 1041 °C and can be deposited by close-space sublimation, vapor transport deposition, sputtering, or electrodeposition.

Thin-Film Solar Cell Structure and Fabrication

A standard CdTe solar cell is a superstrate device in which light enters through a glass substrate coated with a transparent conducting oxide such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO). A thin CdS layer (100–200 nm) forms the n-type heterojunction partner, followed by the CdTe absorber layer (2–5 μm) and a back contact. A critical fabrication step is the CdCl₂ treatment, in which CdTe films are exposed to cadmium chloride vapor or solution at approximately 400 °C; this process recrystallizes grain boundaries, passivates defects, and dramatically increases minority-carrier lifetime, typically raising device efficiency by several percentage points. As documented by the U.S. Department of Energy's Solar Energy Technologies Office, production CdTe modules now exceed 19 percent efficiency, and a research consortium coordinated by the National Renewable Energy Laboratory (NREL) is targeting 24 percent cell efficiency by 2025 through alloying and contact engineering. A recent review in Solar Energy Materials and Solar Cells surveys advances including CdSeTe alloy absorbers and alternative buffer layers that push beyond the historic efficiency ceiling.

Market Position and Environmental Considerations

CdTe photovoltaics are the second most deployed thin-film technology globally, after amorphous silicon, and the largest U.S. manufacturer, First Solar, operates gigawatt-scale production facilities. CdTe modules have among the lowest carbon footprint, shortest energy payback time (under one year in high-irradiance locations), and lowest water consumption of any photovoltaic technology on a lifecycle basis. Cadmium is toxic, but in CdTe it is tightly bound in a stable compound with a melting point over 1000 °C; release requires extreme heat or acid. The PMC review of flexible CdTe solar cells discusses both the environmental profile and the growing interest in flexible lightweight CdTe on polymer substrates for building-integrated photovoltaics.

Applications

Cadmium telluride has applications in a wide range of photonic and energy technologies, including:

• Utility-scale and commercial photovoltaic power generation as the dominant thin-film module technology
• Building-integrated photovoltaics using flexible CdTe films on polymer or metal foil substrates
• Infrared optical windows and lenses for thermal imaging systems operating in the 2 to 30 micrometer range
• Substrates for epitaxial growth of mercury cadmium telluride (HgCdTe) infrared detector arrays
• Research platforms for studying II-VI semiconductor physics and heterojunction interface chemistry