III-V PV Technology 1

III-V PV technology has the highest energy conversion efficiencies for 1 sun STC and concentrated sun conditions.  It is considered a "thin" technology in reference to c-Si wafers.  It is based on 3 valence electrons elements, such as aluminium, gallium and indium, and 5 valence electrons elements, such as phosphorous and arsenic.

Taking GaAs as an example, it has a slightly larger lattice constant and is significantly heavier than silicon.  It is also a direct band gap material, with a band gap transition of about 1.42eV.  The absorption coefficient of GaAs is much larger than silicon, and hence its thickness can be much thinner.  Its band gap is also relatively sharp, which means that the absorption coefficient increases rapidly above the minimum band gap energy.  SRH recombination can also be low due to the purity of the epitaxy processes for III-V film deposition.

The key importance of III-V technology is that these solar cells are multi-junctions, where more than one material with different band gaps are used (see diagram above).  Hence, the Shockley-Queisser limit is overcome.  This is because there is less excess energy dissipated as heat for the same quantity of photons.

An example of a III-V triple junction stack is shown in the diagram above.  The lowest solar cell is made of Germanium (Ge) with a band gap of 0.67eV.  The middle cell is made of GaAs with a band gap of about 1.4eV.  The top cell is made of GaInP (Gallium Indium Phosphide) with the highest band gap of 1.86eV.  The top cell is also the front window surface for the entire triple junction solar cell and absorbs the highest energy photons belonging to blue light.  Ge is meant to absorb the red light and near infrared light, which has the largest penetration depth.

The J-V curve of the triple junction is shown in the diagram above, where each component cell of the triple junction are considered to be in series with each other.  Hence, the lowest J_sc of the top cell is the output J_sc, and the V_oc is the sum of all 3 V_oc.

The band diagram will be more complicated (see diagram above) because the triple junction stack will have 5 p-n junctions/space charge regions, where 2 of the junctions are in reverse, which will act to reduce the total V_oc.  Reverse junctions can be prevented by including tunnel junctions, as seen in the diagram of the triple junction stack.  Tunnel junctions provide low electrical resistance, has high band gaps to avoid parasitic absorption losses, and is relatively thin.  Hence, the valence band at one side is in line with the conduction band on the other side of the tunnel junction, as seen in the diagram below.  The tunnel junction's depletion zone is so narrow that the slope of the conduction and valence bands become very steep.

Hence, electrons can tunnel through the tunnel junction barrier from the n-layer to the p-layer to recombine with the holes (see diagram below).  The low resistance of the tunnel junction means that there is low voltage loss.

Due to the fact that holes of the p-layer of the top cell recombine with electrons of the n-layer of the middle cell, and that holes of the p-layer of the middle cell recombine with electrons of the n-layer of the bottom cell, the recombination currents at the tunnel junctions will determine the J_sc of the triple junction, and ultimate determinant of J_sc will be the J_sc of the top cell, which is the lowest of the series connected triple junction stack.



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