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Polycrystalline Thin-Film Materials and Devices R&D

NREL has significant and long-term capabilities in both cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) thin-film PV research and device development. Currently, NREL has separate groups performing research in CdTe and CIGS technologies; each group consists of about 10 researchers, postdocs, and students. 

CdTe Research

CdTe-based thin-film solar cell modules currently represent one of the fastest-growing segments of commercial module production. This is due partly to the simplicity of the two-component absorber layer (i.e., CdTe contains only cadmium and tellurium) and the ability of bulk cadmium telluride source material (in the form of high-purity powders) to be reconstructed into the CdTe thin films needed to produce PV modules. During the 20+ years of research undertaken by the CdTe Group, much effort has been directed at producing CdTe structures that allow more light to penetrate the top layers of the device (the transparent conducting contacts and cadmium sulfide [CdS] layers) to achieve high efficiency. This understanding has been transferred to commercial processes for use in producing higher-performance modules.

Graphic showing the five layers that comprise CdTe solar cells. Graphic showing the five layers that comprise CIGS solar cells.

(Left). 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 and demonstrates a conversion efficiency of 17.0%.

(Right). Schematic illustration of a typical CIGS substrate thin-film PV device. In this design, the layers of the device are deposited onto a glass, metal, or polymer substrate. Sunlight enters through the top layer of the device (the transparent conducting oxide) and produces electrical current and voltage in the lower layers. The world-record NREL CIGS device is based on this substrate structure and demonstrates a conversion efficiency of 20.0%.

CIGS Research

CIGS-based thin-film solar cell modules currently represent the highest-efficiency alternative for large-scale, commercial thin-film solar cells. Several companies have confirmed module efficiencies exceeding 13%. Most of these companies are using ideas and intellectual properties that were developed by the NREL CIGS Group during the past 20+ years of research. Central to this understanding was the group's development of the "three-stage process." This process enables the formation of a CIGS thin-film layer that is of the proper composition and structure to allow the charges generated by sunlight (i.e., electrons and holes) to exist long enough in the CIGS layer of the device so that they can be separated and collected at the front and back contacts. This separation and collection is critical for demonstrating high conversion efficiency.

This section summarizes these additional topics:

We Address Industry's R&D Challenges

Our R&D addresses key challenges:

CdTe Technology

  • High-Temperature Transparent Conductors and Buffer Layers.The cadmium telluride layer of the highest-performance devices is deposited at ∼500°–650°C. Because this layer is deposited onto an existing transparent conductor layer, the industry requires alternative types of layers that can tolerate this temperature while allowing high transmission. The NREL CdTe Group has developed both Cd2SnO4and Zn2SnO4layers for this use.
  • CdTe Junction Functionality. Although it is relatively easy to produce 10%-efficient cadmium telluride devices, scientists have only recently come to understand how the device forms during various process steps. This understanding provides a critical foundation for industry on which to develop cost-effective processes that yield high-performance, stable CdTe modules.
  • Cell Reliability. The cadmium telluride PV industry needs very rapid screening tests that can predict module reliability to time periods of 25–30 years. To develop these tests, it is essential to understand the failure mechanisms in both active and non-active regions of the device. The group has established novel and long-term activities in this study area.

CIGS Technology

  • Understanding Effects of Material and Process Choices.The group has been key in helping industry understand how cost-effective, industrially relevant process choices can impact the ultimate performance and reliability of CIGS modules.
  • Earth-Abundant Materials Research. Such investigations relate to replacing rare or expensive materials (the use of which may hinder future industry expansion) with alternatives that are broadly available.
  • Cell Reliability for Advanced Device Designs.Although the present CIGS designs can meet the required reliability goals for rigid modules, new CIGS products and markets are envisioned (e.g., flexible products) that may require a higher level of reliability to meet deployment requirements. The group is actively pursuing new materials for inclusion into the cell structure that will meet these requirements.

We Have Special Capabilities and Tools

CIGS Cluster Tool

The CIGS cluster tool in our Process Development and Integration Laboratory has capabilities that can benefit your research and development.

We use the following as we develop advanced thin-film cell technology:

  • Two 1.5" x 1.5" and one 3" x 3" close-spaced sublimation (CSS) systems for CdTe deposition
  • Two 3" x 3" co-evaporators with electron-impact ionization spectrometer (EIES) rate control for CIGS deposition
  • 6" x 6" CSS (CdTe) and co-evaporator (CIGS) tools that are integrated with appropriate sputtering and analysis capabilities to study interface formation and industrially relevant processes (i.e., CdTe and CIGS PDIL Tools)
  • 12" x 12" system for ZnO and ZnO:Al deposition
  • 1.5" x 1.5" and 3" x 3" sputter system for Cd2SnO4, Zn2SnO4, and CdS deposition.
  • Suite of cell testing techniques, including current-voltage and quantum efficiency testing of superstrate and substrate thin-film devices
  • Extensive collaboration with NREL Measurements and Characterization and Materials Theory Groups to study functionality of existing materials and devices, and develop ideas for new product avenues.

We Have Deep Expertise with Multijunctions

Our researchers have invented and transferred many aspects of thin-film PV technology to related industries, and we develop cells with improved performance, reliability, and cost effectiveness. Our interconnected R&D tasks create a path toward the industry's requirements of higher-performance, reliability-certified cells that are optimized for real-world concentrator systems.

Specific and sustained R&D by our group has been recognized for its technical innovation and market value through numerous prestigious awards:

We Partner with Industry

We transfer our technological advances to major industrial players through licensing and high-value cooperative research and development agreements (CRADAs). Our basic criteria for CRADA projects are (1) a potential for a significant impact on the industry, (2) advancement of the technology, and (3) a strong connection to our core competency of multijunction cells.

We work with numerous university and industry partners in the module manufacturing and supply-side aspects of the thin-film PV industry.

Working with Us

Visit Working with Us to learn more about NREL's PV partnership opportunities. Contact us for specific information on NREL's R&D in the area of polycrystalline thin-film materials and devices.