ISH 2019

••• 11 ••• Innovationen Tandem solar cells: combining the best Researchers achieved a new efficiency record of 22.3 per cent for a multi-junction solar cell S ilicon solar cells dominate the photovoltaic market today but the technology approaches the theoretical maximum efficiency that can be achieved with silicon as the only absorber material. Tandem solar cells, on the other hand, combine several absorber materials, enabling a better en- ergetic use of the solar irradiance spectrum. Due to their higher efficiency po- tential, tandem solar cells have a promising future. After intensive research, scientists at Fraunhofer ISE in cooperation with partners have achieved a new efficiency record of 22.3 per cent for a multi- junction solar cell made of silicon and III-V semiconductor materi- als. The outstanding achievement is that the III-V layers were direct- ly grown on the silicon. By com- bining different semiconductor materials, solar cell researchers are attempting to surpass the theoretical efficiency limit of 29.4 per cent for a single-junction silicon solar cell, and convert sun- light into electricity even more efficiently. Promising is the com- bination of silicon material with III-V semiconductor compounds like gallium arsenide. To realise this, one approach is to first de- posit the III-V solar cell structures on gallium arsenide substrates, then transfer to a silicon solar cell using semiconductor bonding technology and lastly etch away the gallium-arsenide substrate. Another less costly approach, however, is to directly grow the III-V layers on the silicon solar cell. In this second approach, the atomic structure must be con- trolled extremely well during growth so that the gallium and phosphorous atoms arrange on the correct lattice sites at the in- terface to the silicon material. Also, the distance between the atoms in the crystal lattice must be increased in order to produce the gallium arsenide material. Researchers have been working on these challenges for over ten years. Now they have been able to greatly reduce the defect density in the III-V semiconductor layers. With the tandem solar cell made of silicon and III-V semiconductor materials, a more ener- getically efficient use of the solar spectrum is possible, compared to conventional solar cells available today. Photo: Fraunhofer ISE / A. Wekkeli