With the EU PVSEC conference approaching great breakthroughs are expected on solar cells efficiency with new record highs and also in technological innovation from the most prestigious research institutes in Europe. A Germany based company, Fraunhofer ISE sets the first two records for the highest efficiency in a solar cell both using multiple junctions with cells made from III-V materials on crystalline silicon. Monolithic multi-junction solar cells are the drivers of development of the silicon solar cells and will dominate the field because they lead to significantly higher efficiency values in transforming sunlight to electrical power (photovoltaic phenomenon). An efficiency of 36% is achievable says Andreas Bett, a company official.
Both devices, used three active layers manufactured to absorb a wide range of spectrum. A layer of gallium indium phosphide (GaInP) is placed at the top to absorb visible light in 300 to 660 nanometers range, an aluminum gallium arsenide (GaAs) layer turned to infrared light in the range of 600 to 840 nanometers, and a silicon cell absorbing lower wavelength light at approximately 1200nm. The research center, achieved an efficiency of 34.1% using direct wafer bonding method where the III-V layer is first deposited on a GaAs substrate and all of them, then pressed together. The III-V layers attach to the silicon and form one unit and then the substrate is removed with a chemical process. Fraunhofer ISE had set the previous efficiency record for the same technology, which was 33.3%. The head of the department of III-V photovoltaics and concentrator technology at the company, said that a new cell structure for GaInP layer but also improvements to the deposition process, were the key innovations that freed the hands of the scientists and enabled them to increase efficiency.
Cost too high
While such an efficiency is an important breakthrough for the institute and the research community as well, the need of GaAs substrate, but other costly technologies as well, are making it too expensive for mass production. The company’s other record involves fewer steps in the process of cell making and therefore lower production costs. This technology simply deposits the III-V layers directly to the silicon cell, bypassing the GaAs substrate. This technique provides an efficiency of 22.3% when it was first published in December, while by now the same efficiency reaches 24.3% without increasing the cost. The potential, senior scientists of the institute state, is comparable to that of the wafer-bonded cells. The challenges to produce a cost effective process for cells production still remain and are highly dependent on the research development of the silicon substrates. Other research centers dealing with such complicated issues like National Renewable Energy Laboratory in United States are focusing on more complicated techniques to achieve cost reduction by making the substrate reusable. “For cost-effective solar cell production, new deposition machines with a higher throughput and deposition area will be required.”
France and Germany, the game changers
In collaboration with France’s Center for Nanoscience and Nanotechnology, Fraunhofer claim to have developed an ultra-thin solar cell based on Gallium arsenide (GaAs), which is a compound of elements belonging to the so-called III-V materials, with an efficiency of 19.9%. Those results, as explained by the researchers, obtained through a new process of manufacturing that uses a 205 nm thick GaAs absorbing layer combined with a nanostructured back mirror. The concept behind this is to construct a nanostructured back mirror to generate multiple resonances that overlap one another in the solar cell. This process is called Fabry-Perot and guided-mode resonance. Those resonances are considered to trap light in the absorber for longer time. In line with longer photovoltaic effect, they also improve optical absorption of the cell. It is also enhanced over a larger range of spectrum which fits the solar spectrum starting from visible and reaching even infrared. A soft nano-imprint lithography technique was used to fabricate the silver back mirror through the application of a sol-gel film of titanium dioxide. The fabrication of the mirrors is essential to be in the nanometer scale. It is also stated by the R&D department that this solar cell technology can reach 25% efficiency in short term, also, without further efficiency loss, the size can get further thinner. The results of the research were published in “III-V Photovoltaics and Concentrator Technology,” in the scientific journal Nature Energy.
Old method again on test
Last year, scientists at the United States National Renewable Energy Laboratory (NREL) began working with a process called dynamic hydride vapor phase epitaxy (D-HVPE) – which many in the solar R&D community thought outdated and inefficient – and found it could greatly reduce production time for cells, significantly reducing costs. At the time, Kelsey Horowitz of the NREL’s Strategic Energy Analysis Center said, with further optimizations to the process and economies of scale the production cost for III-V solar cells could fall to between $0.20 and $0.80/W.
The NREL has now published details of the first of those optimizations. In a paper published in Nature Communications, the scientists report enhanced growth rate for a base layer of around 23 seconds, compared to more than eight minutes under the previous process. “If we can get the costs down like we think we can, that opens up a huge number of markets where these devices would be useful,” said senior NREL scientist Aaron Ptak. “Anywhere you want a high-efficiency device that’s thin, light and flexible – electronics charging cases, electric vehicles, building-integrated PV, rooftops, drones.”
One thing is for sure, solar industry is a multi-billion one and as soon as demand for high efficient and low cost PV modules increases, research and development will never stop to impress us.