5 September 2017
GaInP/GaAs/Si triple-junction solar cell by NREL/ISFH achieves 35.4% efficiency
Germany’s Institute for Solar Energy Research Hamelin (Institut für Solarenergieforschung Hameln, ISFH) and the US National Renewable Energy Laboratory (NREL) have jointly achieved a confirmed one-sun solar energy conversion efficiency of 35.4% with a mechanically stacked GaInP/GaAs/Si triple-junction solar cell. The bottom cell with POLO (poly-silicon on oxide) passivating contacts was fabricated by ISFH, while the monolithic GaInP/GaAs (gallium indium phosphide/gallium arsenide) top cell was fabricated at NREL.
The efficiency is a close second to the record of 35.9% for III-V/Si tandem cells, reported in late August by NREL in collaboration with the Swiss Center for Electronics and Microtechnology CSEM (Centre Suisse d’Electronique et de Microtechnique) and EPFL (École Polytechnique Fédérale de Lausanne) in Switzerland for a triple-junction GaInP/GaAs/Si solar cell.
Notably, ISFH’s silicon bottom cell is optimized for converting the full one-sun solar spectrum. On its own the efficiency of this type of silicon solar cell with POLO junctions reaches 25%. Combination with the NREL GaInP/GaAs top cell therefore raises the efficiency by about a further 10%.
The efficiency of the mechanically stacked GaInP/GaAs//Si solar cell is even close to the record of 37.9% for a pure III-V triple-junction solar cell. This comparison shows the suitability of silicon wafer-based solar cells (which provide a mature and inexpensive basis for over 90% of today’s photovoltaic devices) for tandem applications. To keep cell interconnection within the module format, a monolithic tandem device is desirable. Mimicking this with two-terminal (2T) interconnection, the efficiency of the serial-connected GaInP/GaAs//Si cell triple-junction solar cell is 31.1%.
Although this is already excellent, there is a deficit of 4.3% in efficiency compared with the measurement without serial connection. The difference is due to the current mismatch between sub-cells, such that in serial-connected stacked solar cells every sub-cell must carry the same current, and hence the sub-cell with the smallest current limits the current of the cell stack. This is a basic challenge for all serial-connected stacked solar cells, regardless of the top-cell absorber material (perovskites, III-Vs,…) with only two contacts (2T).
A promising approach to bypass this constraint even for monolithic stacked solar cells is to use an interdigitated back-contact (IBC) POLO cell with an additional front contact as the bottom cell. The rear base contact serves as a third terminal for the collection of excess charge carriers or the injection of missing charge carriers.
Further insights into this concept will be presented at the 33rd European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC 2017) in Amsterdam, The Netherlands (25-29 September) in a plenary talk about the collaborative work by ISFH and NREL.