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The U.S. Department of Energy (DOE) Solar Office 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. First Solar also reported its average commercial module efficiency to be approximately 18% at the end of 2020. 

Research Directions

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

Learn more about the DOE Solar Office 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 that can be tuned from 1.4 to 1.5 (eV), which is nearly optimal for converting sunlight into electricity using a single junction.
  • Low-cost manufacturing: Cadmium telluride solar cells use high throughput manufacturing methods to produce completed modules from input materials in a matter of hours.

Production

The most common CdTe solar cells consist of a p-n heterojunction structure containing a p-doped CdTe layer matched with an n-doped cadmium sulfide (CdS) or magnesium zinc oxide (MZO) window layer.

Typical CdTe thin-film deposition techniques include vapor-transport deposition and close-spaced sublimation. The incorporation of selenium into the absorber layer has become common in recent years due to its ability to produce higher quality absorber layers with bandgap values that are better matched to the solar spectrum that that of pure CdTe.

CdTe absorber layers are generally grown on top of a high-quality transparent conductive oxide (TCO) layer—usually fluorine-doped tin oxide (SnO2:F). Cells are completed using a back electrical contact—typically a layer of zinc telluride (ZnTe) followed by a metal layer or a carbon paste that also introduces copper (Cu) into the rear of the cell. One disadvantage to using Cu in the back contact is the gradual diffusion of Cu atoms into the CdTe layer, which creates defects and often results in Cu accumulation at the junction.

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 after the CdTe layer is grown on the window layer but prior to the back-contact deposition. The CdCl2 treatment has positive effects on the CdTe solar cell, such as the growth of larger CdTe grains and the passivation of defects and grain boundaries.

Additional Information

Find out more about the solar office’s photovoltaics research.

Learn how photovoltaics work.