Date: Jul 15, 2010 Author: Joan Zimmermann Source: MDA (
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by Joan Zimmermann/jzimmermann@nttc.edu
An emerging material system for solar cells offers greater efficiency, affordability, and a greener manufacturing process.
EPIR Technologies, Inc. (Bolingbrook, IL), has received a number of SBIRs from MDA and other DOD agencies to develop materials and substrates for infrared sensors, including the search for cheaper, more robust and radiation-hardened sensors for space operations. Amongst the most important of these successful efforts is growing mercury cadmium telluride (MCT), the dominant high performance infrared detector material, on top of cadmium telluride (CT)-on-silicon. Achieving this breakthrough is helping dispense with the fragile and expensive cadmium zinc telluride (CZT) substrates for subsequent MCT growth.
EPIR Technologies' molecular-beam epitaxy device is used to grow crystals of cadmium telluride on silicon substrates.
MCT and CZT are well-matched in crystal structure; thus CZT substrates have been its principal "growth director" for a number of years. However, CZT is difficult to grow in large-area formats, is very expensive, and is of limited commercial availability, making the U.S. vulnerable to a cutoff in its supply. The founder of EPIR Technologies, Dr. Siva Sivananthan, grew his company in part on the strength of this dilemma.
To address the problem, EPIR turned to successfully growing CT-on-silicon as a substrates for MCT growth, leading to larger substrate areas and lower cost for MCT production. EPIR's approach has produced state-of-the-art CT-on-silicon crystals with a very high yield. This process can readily be scaled to larger areas since 12-inch-diameter silicon wafers are available, as compared to the much smaller CZT areas of 7 × 7 cm that are commercially available. Most importantly, CT-on-silicon by itself happens to be well suited to both infrared (IR) detection and solar photovoltaic applications.
Recognizing the potential for CT-on-silicon, EPIR and Sunovia Energy Technologies, Inc., have joined in a partnership to develop ultra-high-efficiency, single crystal CT-on silicon for multijunction solar cells at a much lower cost than competing processes, thanks in part to high-throughput manufacturing. EPIR's environmentally friendly technique uses commercially available, inexpensive silicon substrates. CT-based multijunction solar cells, in which multiple, stacked solar cells absorb differing wavelengths of sunlight, use less than 1/1000th the amount of cadmium and telluride than their polycrystalline thin-film cousins. Chemical vapor deposition is a technique typically used in making crystals like gallium arsenide, a so-called III-V semiconductor, referring to the elements' placements in the periodic chart.
Instead of using toxic gases such as arsine and phosphine for these III-V materials, the II-VI material CT can be grown via a process called molecular beam epitaxy that does not involve such dangerous elements, greatly reducing the costs of safety and handling. Conventional III-V-based multijunction solar cells are grown on germanium substrates, which are much more expensive than the silicon substrates being used by EPIR for its multijunction solar cells. Also, EPIR's II-VI based cells are expected to have efficiencies similar if not greater than their III-V counterparts. According to Dr. Christoph Grein, vice president of research and development at EPIR, this CT-on-silicon multijunction cell approach for concentrator solar photovoltaics is expected to achieve a levelized cost of electricity below the retail price of electricity in the United States, otherwise known as
grid parity.
EPIR also uses molecular beam epitaxy in its single-element infrared detector product line. The MCT detector material can be grown on either CZT or CT-on-silicon for single detector devices and arrays. EPIR is also developing rapid-detection nanosensors (biosensors) for biological warfare agents in buildings and in heating, ventilation, and air-conditioning (HVAC) systems. The nanosensors use an optically based sensor technology to detect extremely low concentrations of multiple biological agents with very low false-positive rates.
In a recent move under a three-year Phase III SBIR with the Army, EPIR and Sunovia have teamed to develop key precursor substrates for IR imaging cameras and systems. The agreement between the companies will help further the commercialization of the companies' MCT and CT-on-silicon infrared and solar cell technology, and will reinforce the companies' position as key developers and suppliers of advanced night-vision materials for the U.S. Army.
