Solar Cell Efficiency Levels Continue to Rise

April 20, 2011 by Jeff Shepard

Solar Junction, a developer of high efficiency multi-junction cells for the concentrated photovoltaic (CPV) market, claims that it has set a world-record for 43.5% efficiency on a commercial-ready production cell. This achievement was, in part, supported under the U.S. Department of Energy (DOE) PV Incubator Program, managed through DOE’s National Renewable Energy Laboratory (NREL). The cell’s efficiency was confirmed by NREL’s Measurement and Characterization Laboratory. The 5.5 x 5.5mm production cell tops the current record by 1.2% and is significantly higher than the average efficiency gain achieved by previous record holders. The Solar Junction cell measured a peak efficiency of 43.5% at greater than 400 suns and still maintained an efficiency as high as 43% out to 1,000 suns.

Solar Junction’s cells incorporate the company’s proprietary adjustable spectrum lattice-matched, A-SLAM™ technology, which enables the company to more optimally partition the solar spectrum for maximum efficiency and greater reliability. Increases in CPV cell efficiencies are a key driver for improving CPV economics, with each cell efficiency gain leveraged and multiplied in value by the components that account for the remaining 80% of total system costs.

"In the time I’ve been on Solar Junction’s board, the company has hit all of its aggressive efficiency milestones on target," said Dr. Forest Baskett, General Partner at New Enterprise Associates. "That ability to deliver and execute sets it apart from the pack and positions it for swift market gains."

Bosch Solar Energy AG announced that it has achieved 19.6% efficiency for a large-area passivated emitted and rear cell (PERC), monocrystalline silicon solar cell measuring 156 x 156 mm. Fraunhofer ISE confirmed the degree of efficiency of 19.6% being reached for the monocrystalline solar cells with screen printed metallisation in 156 x 156 mm format.

"This is a great success for the engineering team," said Dr Volker Nadenau, Chief Technology Officer at Bosch Solar, "especially since we’ve exclusively used industrial production steps to manufacture those record-breaking cells. We now have to push hard in order to make our products based on this design as soon as possible ready for production."

In order to reduce the cost of solar electricity, Fraunhofer ISE is working hard on increasing solar cell efficiency. Their goal is to convert as much sunlight as possible into electricity using new solar cell concepts based on industrial-level production processes. Especially for silicon solar cells, industry-tested processes, like screen-printing, combined with new cell architecture, such as back contacts, holds enormous potential for increasing efficiencies and lowering costs at the same time. At the Photovoltaic Technology Evaluation Center PV-TEC of the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, researchers have been pressing ahead to explore such new concepts for several years now. For back-contact silicon solar cells, efficiencies up to 20.2 % have been achieved.

For the so-called MWT-PERC solar cells, two different approaches for increasing efficiency were applied. In the Metal Wrap Through (MWT) concept for solar cells, the external front-contacts are transferred to the backside of the solar cell. As a result, more sunlight is incident on the front surface and the solar cell efficiency increases. In the Passivated Emitter and Rear Cell, or PERC, concept, both the optimized reflection of the solar cell backside as well as passivation of the rear surface contribute to higher efficiencies. The screen-printed aluminum back-contact is connected to the p-type silicon material using local laser alloying – the so-called Laser Fired Contact (LFC) process. Just recently, PV-TEC at Fraunhofer ISE manufactured large-format PERC solar cells (edge length 156 mm, with solder contacts on the backside) from monocrystalline Czochralski silicon with efficiencies reaching 19.3%.

Decisive for even higher efficiencies is the reduced amount of shading that is gained through the MWT concept. For the MWT-PERC solar cells produced, the efficiency increased up to 19.4%. Through the use of high quality float-zone silicon material, the Fraunhofer researchers increased the efficiency even further to 20.2% – the highest efficiency measured up to now for large format solar cells manufactured with cost-effective and industrially applicable screen printing, diffusion and thermal oxidation processes. The edge length of the solar cells corresponds to the original wafer size of 125 x 125mm2. Solder contacts of both polarities are located on the backside and used for connecting the cells in the module. The know-how for manufacturing MWT-PERC solar cells is already being transferred to several German solar cell manufacturers.

The PV-TEC team also achieved efficiencies of 20.0% for Back-Contact Back-Junction (BC-BJ) solar cells based on n-type monocrystalline float-zone silicon material. The development could be substantially accelerated by the excellent cooperation with Fraunhofer ISE’ ETAlab. Presently this type of solar cells has an aperture area of 37 x 45mm², however, all of the used technologies can be transferred to the production of larger formats. BC-BJ solar cells not only have both polarities located on the backside, as in MWT-PERC solar cells, but also the emitter. Thus, the shading losses on the front side can be reduced even further. This results in a very high efficiency and an enormous cost saving potential. For this solar cell concept, both the metallization with Al-alloyed emitter as well as the structuring steps are carried out exclusively with screen-printing technology.

Both types of solar cells were produced using the industry-level equipment available at PV-TEC. The cell efficiencies were confirmed by the certified solar cell calibration laboratory CalLab PV Cells.