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CdTe portfolio offers commercial ready high efficiency solar

National Renewable Energy Laboratory

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PDF Document PublicationMktgSummary CdTe.pdf (117 KB)

Schematic illustration of a typical CdTe superstrate thin-film PV device. In this design, the layers of the device are deposited onto a glass &quot;superstrate&quot; that allows sunlight to enter. The sunlight passes through the glass and produces electrical current and voltage in the lower layers. The world-record NREL CdTe device is based on this structure.<br />
Schematic illustration of a typical CdTe superstrate thin-film PV device. In this design, the layers of the device are deposited onto a glass "superstrate" that allows sunlight to enter. The sunlight passes through the glass and produces electrical current and voltage in the lower layers. The world-record NREL CdTe device is based on this structure.

Technology Marketing Summary

At production costs less than $.80/Watt, Cadmium Telluride (CdTe) thin film technology exists as the lowest cost per watt choice for solar technology.  This characteristic is due partly to the single-phase nature of the absorber layer and the ease with which the CdTe source material can be formed into thin films required for module production.

During the 20+ years of research, NREL’s CdTe Group has directed its efforts at producing CdTe structures that allow more light to penetrate the top layers of the device to achieve high efficiency.  Theoretical efficiencies for these devices are greater than 26% and laboratory efficiencies of 17% for a solar cell have been demonstrated by NREL.

The combination of low cost and increasing efficiencies have made CdTe based thin-film solar cell modules one of the fastest-growing segments of commercial production.  Worldwide CdTe solar cell production has been predicted to increase to over 6,500 MW by 2015, 4X 2010 levels, with sales doubling during this same period to over USD $4 billion.

Description

Thin-film CdTe solar cells are one of the most promising thin-film PV devices because their bandgap of 1.45 eV is an excellent match with the solar spectrum. Since these are direct bangap semiconductors with high absorption coefficient, only very thin absorber layers are required to absorb the photons.  A simplified method for fabricating a thin-film semi-conductor heterojunction photovoltaic device includes depositing a layer of cadmium stannate and a layer of zinc stannate on a transparent substrate, both by radio frequency sputtering at ambient temperature.  This step is followed by deposition of dissimilar layers of semiconductors, such as cadmium sulfide and cadmium telluride, and heat treatment to convert the cadmium stannate to a substantially single-phase, spinel crystal structure material.  The cadmium sulfide layer may also be deposited by radio frequency sputtering at ambient temperature, and the cadmium telluride layer may be deposited by close space sublimination at an elevated temperature effective to convert the amorphous cadmium stannate to the polycrystalline, spinel structure cadmium stannate.

Benefits
  • Increase in efficiency
  • Improved production yield 
Applications and Industries
  • Low-cost solar module production (roll-to-roll processing)
More Information

The intellectual property rights covering both the novel materials and methods of producing these materials are currently available for license.  NREL has one remaining license for this method of producing high efficiency CdTel solar modules and is entertaining interested parties.

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 6,221,495
Patent
6,221,495
Thin transparent conducting films of cadmium stannate
A process for preparing thin Cd.sub.2 SnO.sub.4 films. The process comprises the steps of RF sputter coating a Cd.sub.2 SnO.sub.4 layer onto a first substrate; coating a second substrate with a CdS layer; contacting the Cd.sub.2 SnO.sub.4 layer with the CdS layer in a water- and oxygen-free environment and heating the first and second substrates and the Cd.sub.2 SnO.sub.4 and CdS layers to a temperature sufficient to induce crystallization of the Cd.sub.2 SnO.sub.4 layer into a uniform single-phase spinel-type structure, for a time sufficient to allow full crystallization of the Cd.sub.2 SnO.sub.4 layer at that temperature; cooling the first and second substrates to room temperature; and separating the first and second substrates and layers from each other. The process can be conducted at temperatures less than 600.degree. C., allowing the use of inexpensive soda lime glass substrates.
National Renewable Energy Laboratory 04/24/2001
Issued
Patent 5,909,632
Patent
5,909,632
Use of separate ZnTe interface layers to form OHMIC contacts to p-CdTe films
A method of improving electrical contact to a thin film of a p-type tellurium-containing II-VI semiconductor comprising: depositing a first undoped layer of ZnTe on a thin film of p-type tellurium containing II-VI semiconductor with material properties selected to limit the formation of potential barriers at the interface between the p-CdTe and the undoped layer, to a thickness sufficient to control diffusion of the metallic-doped ZnTe into the p-type tellurim-containing II-VI semiconductor, but thin enough to minimize affects of series resistance; depositing a second heavy doped p-type ZnTe layer to the first layer using an appropriate dopant; and depositing an appropriate metal onto the outer-most surface of the doped ZnTe layer for connecting an external electrical conductor to an ohmic contact.
National Renewable Energy Laboratory 06/01/1999
Issued
Patent 5,922,142
Patent
5,922,142
Photovoltaic devices comprising cadmium stannate transparent conducting films and method for making
A photovoltaic device having a substrate, a layer of Cd.sub.2 SnO.sub.4 disposed on said substrate as a front contact, a thin film comprising two or more layers of semiconductor materials disposed on said layer of Cd.sub.2 SnO.sub.4, and an electrically conductive film disposed on said thin film of semiconductor materials to form a rear electrical contact to said thin film. The device is formed by RF sputter coating a Cd.sub.2 SnO.sub.4 layer onto a substrate, depositing a thin film of semiconductor materials onto the layer of Cd.sub.2 SnO.sub.4, and depositing an electrically conductive film onto the thin film of semiconductor materials.
National Renewable Energy Laboratory 07/13/1999
Issued
Patent 6,169,246
Patent
6,169,246
Photovoltaic devices comprising zinc stannate buffer layer and method for making
A photovoltaic device has a buffer layer zinc stannate Zn.sub.2 SnO.sub.4 disposed between the semiconductor junction structure and the transparent conducting oxide (TCO) layer to prevent formation of localized junctions with the TCO through a thin window semiconductor layer, to prevent shunting through etched grain boundaries of semiconductors, and to relieve stresses and improve adhesion between these layers.
National Renewable Energy Laboratory 01/02/2001
Issued
Patent 6,281,035
Patent
6,281,035
Ion-beam treatment to prepare surfaces of p-CdTe films
A method of making a low-resistance electrical contact between a p-CdTe layer and outer contact layers by ion beam processing comprising: a) placing a CdS/CdTe device into a chamber and evacuating the chamber; b) orienting the p-CdTe side of the CdS/CdTe layer so that it faces apparatus capable of generating Ar atoms and ions of preferred energy and directionality; c) introducing Ar and igniting the area of apparatus capable of generating Ar atoms and ions of preferred energy and directionality in a manner so that during ion exposure, the source-to-substrate distance is maintained such that it is less than the mean-free path or diffusion length of the Ar atoms and ions at the vacuum pressure; d) allowing exposure of the p-CdTe side of the device to said ion beam for a period less than about 5 minutes; and e) imparting movement to the substrate to control the real uniformity of the ion-beam exposure on the p-CdTe side of the device.
National Renewable Energy Laboratory 08/28/2001
Issued
Patent 6,458,254
Patent
6,458,254
Plasma & reactive ion etching to prepare ohmic contacts
A method of making a low-resistance electrical contact between a metal and a layer of p-type CdTe surface by plasma etching and reactive ion etching comprising: a) placing a CdS/CdTe layer into a chamber and evacuating said chamber; b) backfilling the chamber with Argon or a reactive gas to a pressure sufficient for plasma ignition; and c) generating plasma ignition by energizing a cathode which is connected to a power supply to enable the plasma to interact argon ions alone or in the presence of a radio-frequency DC self-bias voltage with the p-CdTe surface.
National Renewable Energy Laboratory 10/01/2002
Issued
Patent 6,137,048
Patent
6,137,048
Process for fabricating polycrystalline semiconductor thin-film solar cells, and cells produced thereby
A novel, simplified method for fabricating a thin-film semiconductor heterojunction photovoltaic device includes initial steps of depositing a layer of cadmium stannate and a layer of zinc stannate on a transparent substrate, both by radio frequency sputtering at ambient temperature, followed by the depositing of dissimilar layers of semiconductors such as cadmium sulfide and cadmium telluride, and heat treatment to convert the cadmium stannate to a substantially single-phase material of a spinel crystal structure. Preferably, the cadmium sulfide layer is also deposited by radio frequency sputtering at ambient temperature, and the cadmium telluride layer is deposited by close space sublimation at an elevated temperature effective to convert the amorphous cadmium stannate to the polycrystalline cadmium stannate with single-phase spinel structure.
National Renewable Energy Laboratory 10/24/2000
Issued
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL 95-35, 95-45, 96-45, 96-48, 96-49, 96-52, 97-21, 98-39Production - This portfolio has been transferred to commercial processes currently used in producing high-performance modules.Available02/21/201202/21/2012

Contact NREL About This Technology

To: Bill Hadley<Bill.Hadley@nrel.gov>