Sunovia and EPIR accelerating development of Cd-on-Si solar cells
Source: Semiconductor Today, 9 April 2008
http://www.semiconductor-today.com/news_items/2008/APRIL/SUNOVIA_090408.htm
Renewable energy firm Sunovia Energy Technologies Inc of Sarasota, FL and EPIR Technologies Inc of Bolingbrook, IL, USA, which develops II-VI materials and devices for infrared (IR) sensor and imaging in applications including night-vision, missile tracking and space exploration, say they have developed new solar cell materials that they believe will rival the performance of the most efficient existing multi-junction solar cells, but at significantly lower cost.
The firms aim to achieve this by combining cadmium telluride (CdTe) and silicon (Si) – currently the most prevalent photovoltaic (PV) semiconductors on the market – in a multi-junction solar cell that leverages the economies of scale and manufacturing infrastructure associated with the materials.
Until now, semiconductors for PV applications have essentially come in two forms: high-efficiency, high-cost solar cells created from multi-junction III-V compounds, and lower-cost solar sells made from crystalline silicon and amorphous thin films. While II-VI solar cell materials such as CdTe have shown promise, the lack of suitable low-cost substrates on which to deposit high-quality single-crystal thin films have relegated these very efficient materials to polycrystalline films having low-efficiency on heavy glass substrates.
However, in contrast to the efficiency of about 16% attained under the best conditions (and less than 10% in real-world modules) at different laboratories for amorphous or polycrystalline CdTe solar cells, EPIR’s calculations for single-crystal CdTe solar cells give an efficiency of 24% using realistic assumptions and numbers characteristic of CdTe of typical crystal quality for material grown on silicon by high-throughput molecular beam epitaxy (MBE) deposition methods developed by EPIR.
This efficiency was calculated assuming no anti-reflection coating and no back mirroring under a one-sun intensity with a terrestrial solar spectrum and with the standard global tilt of 37º (AM1.5G). The calculation was performed for a cell having a thin top layer of n-type CdS on 4 microns of p-type CdTe (the most common CdTe solar cell configuration). For a CdTe solar cell of proprietary design, EPIR has calculated an efficiency above 26%. The firm also believes that a maximum efficiency of more than 30% is achievable for optimized two-junction CdTe/Si solar cells in which both the CdTe and the Si act as solar energy absorbers. These detailed calculations indicate that the upper limits of energy conversion efficiencies of solar cells employing group II-VI semiconductor materials such as CdTe rival those of present solar cells using corresponding group III-V semiconductor materials such as gallium arsenide (GaAs), but with much greater manufacturability and significantly lower cost.
The high-throughput manufacturing technologies for growing CdTe on Si have been proven by the University of Illinois at Chicago and transferred by EPIR into night-vision sensors over the past decade. The technologies that have enabled the commercialization of night-vision sensors are now being used to accelerate the development of next-generation, multi-junction solar cell materials.
The companies believe the above results show that EPIR’s CdTe/Si, and more generally II-VI based technology, has the potential to displace high-cost III-V based technology, particularly in CPV systems for power plants. The results also indicate that technologies based on EPIR’s high-throughput MBE deposition technique for growing epitaxial crystalline CdTe or CdTe/Si could displace existing amorphous or polycrystalline CdTe cells and Si cells, respectively.
EPIR reckons that the manufacturability of II-VI based IR focal plane arrays for IR imaging shows that MBE-grown II-VI materials have reached a maturity level worthy of solar cell production. The firm has now completed a major facility for manufacturing CdTe on silicon and for the development of improved solar cells based on II-VI materials.
The materials, which have been proven in night-vision IR systems for many years, will initially be applied to concentrator PV (CPV) systems, targeting reduced price points from what is currently available. The firms believe that this will enable solar-based electricity generation at prices equal to and even below current grid-supplied power from non-renewable sources.
Also, as the deposition processes are further refined and increased in throughput, low-cost high-efficiency solar cells for specialty and terrestrial applications using flat-plate systems (as opposed to CPV systems) will be commercialized.
The firms aim in future to apply new materials and technologies in ultra-high-volume deposition processes to create thin films on flexible substrates for residential, commercial and utility power generation products with very high production capacities and high-speed time-to-market deployments.
The companies expect that revenues from IR materials will begin this year, with revenues from advanced CPV materials following after the IR technology is transferred to EPIR’s next-generation solar cell manufacturing facilities (currently in development).