High-Performance Multijunction Thin Films

Researchers test the voltage output of their award-winning gallium indium phosphide/gallium arsenide tandem solar cell, which has achieved record efficiencies in converting sunlight to electricity. By producing a concentrator cell with efficiency greater than 30%, NREL scientists passed an important milestone of the PV program.

Our researchers investigate the development of higher-performance devices that take advantage of tandem or multijunction solar cells. Polycrystalline thin-film tandem cells include combining high- and low-bandgap single junctions. The device structure—whether a monolithic integration or a mechanical stack—must be considered while exploring the top- and bottom-cell materials. With a monolithic configuration and with alternative device structures, the processing limits are extremely important to the top cell, bottom cell, and tunnel junction.

Research on high-performance multijunction thin films focuses on several of today's challenges:

• Determining high-bandgap alloys based on I-III-VI₂ and II-VI compounds and other novel materials for the top cell.

• Considering low-bandgap CIS and its alloys, thin-film silicon, and other novel approaches for the bottom cell.

• Studying the difficult task of integrating the thin-film tunnel junction (interconnect) with the top cell. This work includes understanding the role of defects, how they affect the transport properties of this junction, and the diffusion of impurities into the bulk material.

• Fabricating a monolithic, two-terminal tandem cell based on polycrystalline thin-film materials that requires low-temperature deposition for several layers.

• Avoiding deterioration of the top cell when fabricating the bottom cell if a low-bandgap cell is fabricated after a high-bandgap cell with a superstrate structure, such as CdZnTe.

• Avoiding temperatures and processes that could damage the CIS bottom cell if a high-bandgap cell is fabricated on top

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