Sunovia & EPIR Announce Breakthroughs For Infrared Sensors and Next-Generation Solar Cells

April 09, 2008 by Jeff Shepard

Sunovia Energy Technologies, Inc. announced technology and manufacturing breakthroughs by EPIR Technologies, Inc. (EPIR). EPIR states that its research and development efforts have formed a cornerstone for products that enabled the U.S. military’s night vision superiority.

It is claimed that the breakthroughs address cost reductions that further enable infrared (IR) sensors for use in large commercial markets and provide a new paradigm for multi-junction solar cells. It is claimed that the new solar cell technology will have high efficiencies at lower costs by leveraging less expensive IR system materials and manufacturing processes that have been developed with more than $30 million dollars of investment over the past 25 years.

Sunovia and EPIR believe that a very successful strategy is to leverage the ongoing commercialization and continued sale of advanced IR sensors while directly transferring these IR technologies to the commercialization of next-generation solar cells. A portion of the core technologies behind EPIR’s developments are disclosed in U.S. Patent No. 6,657,194, "Multispectral Monolithic Infrared Focal Plane Array Detectors".

Specifically, EPIR claims that its breakthrough is a MCT IR focal plane array (FPA) grown directly on a thin cadmium telluride (CdTe) epilayer, which in turn was grown directly on a silicon (Si) read out integrated circuit (ROIC). Thus, the infrared focal plane array that generates the electrical signals to be converted into a digital picture was directly and monolithically connected to the ROIC that interprets those signals to create a picture, without a need for externally applied contacts and interconnects. This monolithic integration of an MCT FPA with its ROIC simultaneously realized a key differentiator within IR detector technology and formed a proof of concept for the fabrication of a novel high efficiency, two-junction and two-terminal solar cell.

One of the major challenges overcome was the cleaning of the Si ROICs without damaging the contacts or resorting to temperatures high enough to damage the ROICs. Another was the direct deposition of high quality single crystal CdTe directly on the Si ROICs with excellent current collection by the ROICs despite the small fractional area available for growth on ROICs. Yet others were the deposition of high quality single crystal MCT on the CdTe and the dopant activation and device processing to create an FPA, all at temperatures low enough not to harm the ROIC. Overcoming these challenges clearly demonstrated the ability to fabricate multijunction, two-terminal high- efficiency solar cells such as CdTe/Si cells or other more complex cells based on CdTe/Si and went far beyond that demonstration. EPIR claims that this breakthrough and much other work on IR FPAs has clearly demonstrated the ability to fabricate such solar cells with the necessary current matching and efficient current collection, without a buffer layer between the Si and the CdTe, with or without a thin zinc telluride tunneling barrier as needed for current matching.

Monolithic integration such as that achieved by EPIR is said to obviate all of the deficiencies associated with the usual bump binding between the contact on each pixel of an FPA and the corresponding ROIC contact, eliminates complex and low-yield processes, eliminates thermal mismatches and thus allows much larger formats and greater resolution, creates compact systems with lower heat loads, and reduces costs by increasing yield. Non-monolithic FPAs, whether grown on Si or on cadmium zinc telluride (CZT) suffer from many deficiencies because of the necessary bump-bonding This bump-bonding is said to require extensive packaging, to be susceptible to vibration failures and is plagued by parasitic capacitances and inductances that degrade the sensitivity and bandwidth of the FPAs.

The material grown and the monolithic devices fabricated were subjected to a number of tests and passed all of them. For example, the measured carrier recombination time, which is the primary measure of material quality and must be long enough to allow the currents generated by incident light to be efficiently collected, was said to be excellent.

Also the measured dynamic FPA impedance at the standard operating temperature of 80K reached 10(6) Ωcm(2) at zero bias for growth on Si and 10(5) Ω-cm² for growth on ROICs.