A group of scientists from the University of Illinois at Urbana-Champaign and the Arizona State University in the United States has developed a III–V silicon epitaxial tandem solar cell based on gallium arsenide phosphide (GaAsP). It claims that the cell offers improved performance thanks to fewer defects at the interface between layer defects, and an improved top cell carrier collection.
The researchers built the device with an electron-blocking layer made of aluminium gallium arsenide phosphide (AlGaAsP), instead of more commonly used gallium indium phosphide (GaInP), that was placed on the back of the cell. Furthermore, a GaAsP spacer layer was added to put more distance between the cell active region and defects in the graded buffer.
According to the group, these interfaces are structurally perfect – which they confirmed using transmission electron microscopy (TEM) – with a strong reduction in dark-line defects in electron beam-induced currents. As a result, the steady-state photoluminescence of the cell built with the AlGaAsP was three times higher than an equivalent cell relying on GaInP.
The improved carrier collection obtained through this step was then combined with texturing and a hydrogenated amorphous silicon (a–Si:H) passivation of a silicon back surface. A back reflector made of indium tin oxide (ITO) and silver was used for the silicon bottom cell.
The demonstrated cell, according to the academics, had an efficiency of 25.0% and has closely matched short circuit current of 18.8 milliampere per square cm.
The researchers stressed that their study demonstrates the importance of considering factors other than reducing threading dislocation density (TDD), a long unresolved problem in this type of cell because of the large lattice mismatch, for achieving high efficiencies for III-V solar cells. Threading dislocations are structural defects that can propagate through the III-V layers and lead to high leakage currents.
“Reductions in TDD will nevertheless play an important role in pushing GaAsP/Si tandems to 30% efficiency and beyond,” the academics concluded.
The solar cell is described in the paper Current-Matched III–V/Si Epitaxial Tandem Solar Cells with 25.0% Efficiency, published in Cell Reports Physical Science.
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Source: pv magazine