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-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.
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.
We Address Industry's R&D Challenges
Our R&D addresses key challenges:
- 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.
- 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.
- 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.
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.