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Cadmium Telluride

Graphic showing the five layers of a cadmium telluride PV cell: aluminum, p- layer,  n- layer, transparent conductive oxide, and glass.DOE supports innovative research focused on overcoming the current technological and commercial barriers for cadmium telluride (CdTe) solar cells. Below are a list of current projects, summary of the benefits, and discussion of the production and manufacturing techniques used for this solar technology.

Background

CdTe solar cells are the second most common photovoltaic (PV) technology in the world marketplace after crystalline silicon, currently representing 5% of the world market. CdTe thin-film solar cells can be manufactured quickly and inexpensively, providing a lower-cost alternative to conventional silicon-based technologies. The record efficiency for a laboratory CdTe solar cell is 22.1% by First Solar, while First Solar recently reported its average commercial module efficiency to be 16.1% at the end of 2015. 

Research Directions

Current projects seek higher cell efficiencies by increasing crystal quality, improving doping control, and increasing the minority carrier lifetime. Manufacturers are also investigating the possibility of materials reuse and recycling as a way to mitigate concerns on toxicity and materials scarcity.

Learn more about the DOE SunShot awardees and the projects involving CdTe below.

Benefits

The benefits of CdTe thin-film solar cells include:

  • High absorption: Cadmium telluride is a direct-bandgap material with bandgap energy of about 1.45 defined (eV), which is well matched to the solar spectrum and nearly optimal for converting sunlight into electricity using a single junction.
  • Low-cost manufacturing: Cadmium telluride solar cells use low-cost manufacturing technology to produce low-cost cells.

Production

The most common CdTe solar cells consist of a simple p-n heterojunction structure containing a p-doped CdTe layer matched with an n-doped cadmium sulfide (CdS) layer, which acts as a window layer. This structure is similar to the heterojunction in CIGS cells. As with most thin-film solar technologies, carrier collection is accomplished by drift, or field-assisted collection.

Typical CdTe thin-film deposition techniques include: close-spaced sublimation, vapor-transport deposition, physical-vapor deposition, sputter deposition, electrodeposition, metal-organic chemical-vapor deposition, spray deposition, and screen-print deposition.

CdTe solar cells are completed by adding a high-quality transparent conductive oxide (TCO)—usually fluorine-doped tin oxide (SnO2:F)—and a back electrical contact—typically a metal or carbon paste with copper (Cu). One disadvantage to using Cu in the back contact is the gradual diffusion of Cu atoms into the CdTe and CdS layers, which creates defects and promotes Cu accumulation at the CdTe/CdS junction.

Overall CdTe solar cell performance was significantly improved after the discovery of a cadmium chloride (CdCl2) vapor treatment. This annealing process is accomplished in the presence of oxygen at temperatures near 390°C after the CdTe layer is grown on the CdS layer and prior to the back-contact deposition. The CdCl2 treatment has positive effects on the CdTe solar cell, such as growth of larger CdTe grains and the passivation of defects.

For more information on cadmium telluride solar cells, visit the Energy Basics website.