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Electron Beam Induced Current Measurements of Single Nanowire Solar Cells—Development of Nanowire Tandem Junction Photovoltaics
Lukas Hrachowina1,Enrique Barrigon1,Magnus Borgström1
Within one hour, the sun supplies our planet with energy sufficient for humanity’s yearly energy consumption1. Nanowire solar cells are a sustainable complementary technology to silicon solar cells, as they have the potential to reach the efficiencies of world-record III-V solar cells while only using about 10 % of the material2. Although nanowire solar cells have demonstrated high efficiencies3-6 and proven to have benefits compared to their bulk counterparts such as superior radiation tolerance7 the next step in nanowire solar cell technology – a tandem-junction solar cell that combines different band gaps within the same nanowire – has been elusive up to now.
Here, we report on single nanowire tandem junction solar cells based on both InP/GaInP and InAsP/InP. These lattice mismatched combinations of materials are accessible because of elastic relaxation at the lateral free surface of nanowires8. The development of the nanowire tandem junction photovoltaics was supported by a setup that combines a nanoprobe system with electron beam induced current (EBIC) measurements and a light emitting diode (LED) inside a scanning electron microscope9. By the use of the nanoprobe system we show voltage addition of the two sub-cells. We are able to study both sub-junctions independently with EBIC, by either using a light bias from the LED setup to saturate the bottom-junction, or by using a voltage bias to saturate the top-junction. We believe that this type of characterization will be helpful to further optimize the efficiency of nanowire tandem junction solar cells, and to develop nanowire multi junction solar cells using more materials similar to the six-junction solar cell which has the record efficiency of 47.1 %10.
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