Converting One-Third of the Sunlight Into Electricity

Silicon-based multi-junction solar cell consisting of III-V semiconductors and silicon. The record cell converts 33.3. percent of the incident sunlight into electricity. (Source: D. Mahler, Fh.-ISE)

Silicon solar cells dominate the global photo­voltaic market today with a share of 90 percent. With ever new techno­logical develop­ments, research and industry are nearing the theo­retical effi­ciency limit for semi­conductor silicon. At the same time, they are forging new paths to develop a new genera­tion of even more efficient solar cells. Now, Fraunhofer researchers achieved a high conversion effi­ciency of one-third of the incident sunlight. The silicon-based multi-junction solar cell uses extremely thin 0.002 mm semi­conductor layers of III-V compound semi­conductors, bonding them to a silicon solar cell. The visible sunlight is absorbed in a gallium-indium-phosphide (GaInP) top cell, the near infrared light in gallium-ar­senide (GaAs) and the longer wave­lengths in the silicon subcell. In this way, the effi­ciency of silicon solar cells can be signi­ficantly increased.

“Photo­voltaics is a key pillar for the energy trans­formation,” says Andreas Bett, Institute Director of Fraun­hofer ISE. “Meanwhile, the costs have decreased to such an extent that photo­voltaics has become an economi­cally viable competitor to conven­tional energy sources. This develop­ment, however, is not over yet. The new result shows how material consumption can be reduced through higher effi­ciencies, so that not only the costs of photo­voltaics can be further optimized but also its manu­facture can be carried out in a resource-friendly manner.” Already in November 2016, the solar researchers in Freiburg together with their industry partner EVG demonstrated an effi­ciency of 30.2 percent, increasing it to 31.3 percent in March 2017. Now they have succeeded once again in greatly improving the light absorp­tion and the charge separa­tion in silicon, thus achieving a new record of 33.3 percent effi­ciency. The tech­nology also convinced the jury of the GreenTec Awards 2018 and has been nomi­nated among the top three in the category “Energy.”

For this achieve­ment, the researchers used direct wafer bonding from the micro­electronics industry to transfer III-V semi­conductor layers, of only 1.9 micro­meters thick, to silicon. The surfaces were deoxidized in a EVG580® ComBond® chamber under high vacuum with a ion beam and subse­quently bonded together under pressure. The atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a mono­lithic device. The complexity of its inner structure is not evident from its outer ap­pearance: the cell has a simple front and rear contact just as a conven­tional silicon solar cell and therefore can be inte­grated into photo­voltaic modules in the same manner.

The III-V / Si multi-junction solar cell consists of a sequence of subcells stacked on top of each other. Tunnel diodes internally connect the three subcells made of gallium-indium-phosphide, gallium-arsenide and silicon, which span the absorp­tion range of the sun’s spectrum. The GaInP top cell absorbs radiation between 300 and 670 nm. The middle GaAs subcell absorbs radiation between 500 and 890 nm and the bottom Si subcell between 650 and 1180 nm, respectively. The III-V layers are first epi­taxially deposited on a GaAs substrate and then bonded to a silicon solar cell structure. Here a tunnel oxide passivated contact is applied to the front and back surfaces of the silicon. Subse­quently the GaAs substrate is removed, a nano­structured backside contact is implemented to prolong the path length of light. A front side contact grid and anti­reflection coating are also applied.

On the way to the indus­trial manu­facturing of III-V / Si multi-junction solar cells, the costs of the III-V epitaxy and the connecting tech­nology with silicon must be reduced. There are still great challenges to overcome in this area, which the Fraun­hofer researchers intend to solve through future inves­tigations. The new Fraun­hofer-Center for High Effi­ciency Solar Cells, presently being constructed in Freiburg, will provide them with the perfect setting for deve­loping next-gene­ration III-V and silicon solar cell techno­logies. The ultimate objective is to make high effi­ciency solar PV modules with effi­ciencies of over 30 percent possible in the future. (Source: Fh.-ISE)

Reference: R. Cariou et al.: III–V-on-silicon solar cells reaching 33% photoconversion efficiency in two-terminal configuration, Nat. Ener. 3, 326 (2018); DOI: 10.1038/s41560-018-0125-0

Link: III-V and Concentrator Photovoltaics, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany

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