III-V PV Technology 2

Continuing from III-V PV Technology 1:

Epitaxy is a deposition method to make high quality III-V materials, where for example, the GaAs crystalline lattice is grown one layer at a time on a germanium substrate, and adopts the substrate's crystal lattice structure.  Epitaxy, which is done in high vacuum conditions, prevents impurities, but dopants can be added.  It results in compact materials without vacancy defects.  Metal-organic chemical vapour deposition (MOCVD) is the usual epitaxy method for depositing III-V semiconductor layers.  It is an expensive process.

The main difficulty of making III-V materials is to ensure a match/similarity in their lattice constants.  When there is mismatch between the interfaces of materials, not every valence electron is able to bond with neighbouring atoms.  This variance in lattice constants can be seen in the phase diagram of semiconductors (see diagram above).  It can also be seen that the triple junction with GaInP, GaAs and Ge is quite lattice matched.  GaAs has the same lattice constant as Ge, but with a higher band gap, which reduces coordination defects.  The top material GaInP is a III-V alloy designed to have a band gap of 1.8eV and having a similar lattice constant to ensure full lattice matching.

It must be noted that III-V materials have sharp band gaps and high absorption coefficients (see diagram above).  For the triple junction example, the bottom cell generates more current than the other cells, which is more ineffective than quadruple or multi-junctions of 5-6 solar cells.  Less energy is wasted as heat for higher stacked multi-junctions, but the lattice matching may become mismatched.  If mismatched, the junctions are called metamorphic multi-junctions.  Buffer layers that have profiling in the lattice constant must be used.

In general, III-V PV technologies are costly, and so are used in space applications or concentrator technology, where sun light is focused on one solar cell.



Reference:
5.1 - III-V PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=NCRoe-S17e8