Solar cell efficiency – Four recent success stories

Freiburg - Germany (Andrewglaser)

There is still significant scope for further percentage points when it comes to energy conversion efficiency of a solar cell. Obviously, it is important to improve the efficiency factor to enhance its attractiveness. The higher the efficiency of solar cells the higher the area efficiency and possibly also the profitability. Particularly for cities, the area efficiency is an important point due to the lack of space. A couple of research facilities around the world try to increase the light yield. There is a strong competitiveness. Consequently, new efficiency records are announced regularly. In the following some examples.

25.1 percent with TOPCon Technology

The German research institution Fraunhofer ISE recently announced (September 2015) a new efficiency record for silicon solar cells with TOPCon Technology. For a both sides-contacted silicon solar cell, an efficiency of 25.1 percent has been achieved. This record is the highest efficiency achieved to date for both sides-contacted silicon solar cells. They are characterized by having metal contacts on both the front and rear sides.

“The biggest advantage of our new concept is that we can now contact the entire rear cell surface without patterning. Compared to the high-efficiency solar cell structures presently in use, we offer both a simplified manufacturing process and higher efficiencies at the same time,” explains Dr. Martin Hermle, Head of the High Efficiency Solar Cells department at Fraunhofer ISE. The special feature of the so-called TOPCon (Tunnel Oxide Passivated Contact) technology are the applied metal contacts to the rear side without patterning. TOPCon is developed by Fraunhofer ISE.

The researchers developed a selective passivated contact made of tunnel oxide that enables majority charge carriers to pass and prevents the minority carriers from recombining. The thickness of the intermediate passivation layer is reduced to one or two nanometres, allowing the charge carriers to “tunnel” through it. Subsequently, a thin coating of highly doped silicon is deposited over the entire layer of ultra-thin tunnel oxide. This novel combination of layers allows electrical current to flow out of the cell with nearly zero loss. The major solar cells in the photovoltaic industry have an aluminum-alloyed back contact covering the entire rear side. It limits the efficiency. With their latest result, they have surpassed the 25 percent mark. The research was funded within the project FORTES from the German Federal Ministry for Economic Affairs and Energy and U.S. Department of Energy.

New efficiency record for artificial photosynthesis

Solar energy is not available constantly and not everywhere. Artificial photosynthesis is one hope for storing this energy. This is what every leaf can do, namely converting sunlight to “chemical energy”. It can also take place with artificial systems based on semiconductors. First of all, the individual semiconductor components create electrical power with sunlight. The electrical power splits water into oxygen and hydrogen. Hydrogen from sunlight is still a niche within the solar market. But it does not mean that there is no progress.

An international team achieved a significant efficiency improvement for direct solar water splitting with a modified tandem solar cell. The old record of 12.4 percent was held by the National Renewable Energy Laboratory (NREL) in the USA.  After 17 years a new record value of 14 percent has now (September 2015) been broken by researchers from the Institute for Solar Fuels at the Helmholtz-Zentrum Berlin, the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, the Technical University Ilmenau and the California Institute of Technology (Caltech). Scientific facilities worldwide have therefore been researching for many years how to break the existing record of 12.4 percent.

“We have electronically and chemically passivated in situ the aluminium-indium-phosphide layers in particular and thereby efficiently coupled to the catalyst layer for hydrogen generation. In this way, we were able to control the composition of the surface at sub-nanometre scales”, explains Matthias May, active at TU Ilmenau and the HZB Institute for Solar Fuels. Admittedly, it is difficult for lay people to understand. In former times, the samples only survived some seconds before their power output collapsed. After about one year of optimization, they remain stable for over 40 hours. A long-term stability goal are 1000 hours. The hydrogen generation from sunlight using high-efficiency semiconductors with an efficiency level of 15 percent or more is assumed as being economical.

Panasonic – 22.5% efficiency with solar cells based on mass-production technology

The Eco Solutions Division at Panasonic Corp., based in Japan, announced in October 2015 a 22.5 percent efficiency on a commercial-sized prototype using solar cells based on mass-production technology. Panasonic claims, that the Japanese National Institute of Advanced Industrial Science and Technology (AIST) confirmed the result. AIST is one of the leading centers for independent verification of solar cell performance results under standard testing conditions. The 72-cell, 270-watt prototype incorporates newly developed enhanced technology that will eventually be scaled into volume production.

In addition, Panasonic Eco Solutions announced the launch of the powerful photovoltaic Module HIT® N330, the latest addition to the company’s high-efficiency heterojunction photovoltaic Module product line. HIT® N330 achieves 19.7 percent Module-level efficiency with a nominal power output of 330 watts. The new 96-cell Module returns approximately 27 percent more peak-power compared to mainstream 260-watt multicrystalline modules. The new Module will be available in the UK and other European markets from March 2016.

46% with multi-junction solar cell confirmed by French-German cooperation

Soitec and CEA-Leti, France, together with the Fraunhofer Institute for Solar Energy Systems ISE, Germany, confirmed a new record for a multi-junction solar cell in December 2014. The cell converts 46 percent of the solar light into electrical energy. Multi-junction cells are used in concentrator photovoltaic (CPV) systems. to produce low-cost electricity in regions with a large amount of direct solar radiation. It is the cooperation’s second world record within one year. It clearly demonstrates the strong competitiveness of the European photovoltaic research and industry.

Multi-junction solar cells are based on a selection of III-V compound semiconductor materials. The world record cell is a four-junction cell. One quarter of the incoming photons in the wavelength range between 300 and 1750 nm are converted precisely by each sub-cell into electricity. When applied in concentrator PV, a very small cell is used with a Fresnel lens. It concentrates the sunlight onto the cell. The new record efficiency was measured at a concentration of 508 suns and has been confirmed by the Japanese AIST (National Institute of Advanced Industrial Science and Technology).

”This is a major milestone for our French-German collaboration. We are extremely pleased to hear that our result of 46% efficiency has now been independently confirmed by AIST in Japan”, explains Dr. Frank Dimroth, project manager for the cell development at the German Fraunhofer Institute for Solar Energy Systems ISE. “CPV is the most efficient solar technology today and suitable for all countries with high direct normal irradiance.” Jocelyne Wasselin, Vice President Solar Cell Product Development for Soitec, a company headquartered in France and a world leader in high-performance semiconductor materials, is optimistic that 50 percent efficiency in the near future can be achieved.