**4. InGaP/GaAs MQWs solar cells**

Concentrators have the advantage to reduce the cost of photovoltaic systems by collecting the direct photon component with inexpensive lenses. The economic benefits of the concentrators demand a lot of high efficiency solar cells and the application of a nanostructure technology to these photovoltaic materials. GaAs materials provide the highest conversation efficiency in single-junction solar cells in all concentrations. However, the bandgap of GaAs is 1.42 eV and higher than the 1.1 eV of optimal efficiency bandgap at the conditions of high concentration. [35]Strain balanced InGaAs quantum wells in the intrinsic region of GaAs single junction solar cells can extend the absorption edge in substrate devices. [36] The result of increasing short circuit current accompanied with drop in open circuit voltage and increasing efficiency prevails over comparable conventional solar cells. [37]

An optimum of similar band gap can also exist in the case of two junction tandem solar cells. The efficiency record is 30.2% for the InGaP/InGaAs tandem solar cells at 300 times concen‐ tration. [12] However, as shown in Figure 15, the bandgaps combination of 1.8 eV/1.42 eV of an InGaP/GaAs tandem solar cells is significantly higher than the optimum bandgaps combi‐ nation under 500 times concentration. Approaches have being actively pursued to lower the bandgaps of tandem solar cells, including lattice mismatched dilute nitrides InGaP/InGaAs grown on a virtual substrate. [38] These introducing dislocations can lower the voltage of the tandem solar cells, though the efficiency record has achieved about 42 % by the virtual substrate approach. [39] Strain balanced quantum wells in InGaP/InGaAs tandem solar cells may allow the absorption edge of top and bottom cell to be adjusted independently without existing relaxation and lattice mismatch. [40] If there is enough absorption in the strain balanced quantum wells of top and bottom cell, the conversion efficiency of InGaP/InGaAs tandem solar cells device could ascend along the contours in Figure 15 and be adjusted to the solar spectrum. [12]Dark line shows the bandgaps of GaInP and GaAs. The red-cross shows the bandgaps position of the strain balance quantum wells tandem solar cells and the blue star shows a proposed structure with quantum wells in two junction solar cells.

**Figure 15.** Ideal conversion efficiency contour plot for tandem solar cells under 500 times concentration
