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Substrate CdTe Efficiency Improvements

National Renewable Energy Laboratory

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PDF Document Publication11-28PCT Application as-published (984 KB)

Technology Marketing Summary

Thin film solar cells have been the focus of many research facilities in recent years that are working to decrease manufacturing costs and increase cell efficiency. Cadmium telluride (CdTe) has been well recognized as a promising photovoltaic material for thin film solar cells because of its near-optimum bandgap of ~1.5 eV and its high absorption coefficient. Researchers at NREL have found a unique way to dramatically increase efficiencies of substrate-configured CdTe solar cells.

Substrate-configured CdTe thin film solar cells allow elimination of the typical rigid glass used to support thin film layers during deposition of superstrate-configured cells.  Instead, the substrate-configured cell’s thin-film layers may be deposited onto a material that is opaque, usually a metal, high-temperature polymer, or ceramic.  This design is believed to be advantageous for applications in aerospace (high power-to-mass ratio), building integrated photovoltaics (flexibility) and low-cost production designs (roll-to-roll processing).

One known drawback to conventional substrate-configured cells is a lower efficiency compared to superstrate-configured cells.  NREL scientists have developed and demonstrated a fabrication procedure that increases the efficiency of substrate-configured CdTe thin film solar cells from ~5% to ~11%. The main device performance parameters that improve by using this procedure is the open-circuit voltage (Voc).


Increases in efficiency for substrate-configured CdTe thin film solar cells have been demonstrated using an innovative fabrication procedure. NREL scientists have shown that, when oxygen is not incorporated into the CdTe layer (as an oxygen-free CdTe layer exhibits performance advantages for certain types of back contacts), it is important to incorporate oxygen at the CdS/CdTe interface by other means. Processes to incorporate oxygen at the interface can occur during the CdS deposition and/or during the CdCl2 heat treatment steps.  Several alternatives to incorporate oxygen have been developed at NREL. Compared to existing substrate-configured devices, NREL’s demonstrated fabrication procedure doubles the efficiency of substrate-configured CdTe thin film solar cells into the ~11% range.

The main device performance parameters that improve by using this procedure are open-circuit voltage (Voc) and Fill Factor (FF). Voc increases from typical values ≤700mV to a maximum value of over 860mV, which is believed to be the highest ever reported for any type of CdTe thin film solar cell operating at an illumination near to 1 sun.


Increase in efficiency by increasing the Voc, increase in efficiency by reduced absorption in glass superstrate, allows for use of metal substrates and related high-speed roll-to-roll processing, decrease in manufacturing cost

Applications and Industries

Terrestrial solar module manufacturers who are interested in lightweight or advanced packaging options, roll-to-roll industrial processing, building integrated photovoltaics, flexible PV for military and recreational applications, high specific power for aerospace applications (i.e., high Watts/kg), and radiation-hard applications (e.g., use in low-earth orbiting satellites).

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Application 20130327398
Thin-Film Photovoltaic Devices and Methods of Manufacture
Improved thin-film photovoltaic devices and methods of manufacturing such devices are described. Embodiments include a substrate-configured thin-film PV device (200) having a photo-absorbing semiconductor layer (230) and a window layer (240). Embodiments include devices having a CdTe photo-absorbing semiconductor layer, a CdS or CdS:In window layer, and an n-p junction residing at or proximate an interface of the photo-absorbing semiconductor and window layers. Variations include methods of manufacture wherein i) O.sub.2 is excluded from an ambient environment during deposition of the CdTe layer (102), ii) O.sub.2 is included in an ambient environment during CdCl.sub.2 treatment (103), iii) O.sub.2 is included in an ambient environment during deposition of a CdS or CdS:In layer (104), or iv) a medium-temperature anneal (MTA) having an anneal temperature of C. or less is performed (105) after deposition of the CdS layer.
National Renewable Energy Laboratory 02/27/2012
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 11-28ProductionAvailable01/08/201301/24/2014

Contact NREL About This Technology

To: Bill Hadley<>