**5. Conclusions**

In this chapter, the structure characteristics and optoelectronic properties of InGaN/GaN multiple quantum wells solar cells with In content about 0.3 and 0.4 are analyzed and studied, the phase separation of GaN and InN are not observed in the InGaN alloy materials with all kinds of In content when the InGaN alloys materials are packaged by InGaN/GaN multiple quantum wells heterojunction. The open circuit voltage of InGaN/GaN multiple quantum wells solar cells with In content about 0.3 and 0.4 are measured by 2 V and 1.8 V, respectively, when InGaN alloys materials are irradiated by monochromatic light under the same white light source. The InGaN/GaN multiple quantum wells are used as solar cells excitation region, the fill factor of this solar cell is about 0.6. The external quantum efficiency of this solar cell is 40% at the wavelength 420 nm, but external quantum efficiency drops to only 10% at the wavelength of 450 nm.

The performance of InP/InGaAsP quantum wells solar cells can be greatly improved by integration of dielectric or metal nanoparticles into the surface of the tandem solar cells devices structure in order to couple incident light into the lateral propagation paths which can confine the slabby waveguide formed by the multiple quantum wells intrinsic layer. This approach can also improve the inherent conflict in achieving both efficient photon absorption which mandated a thick multiple quantum wells layer and efficient collection of the photo-generate carriers which required a thin multiple quantum wells layer, and could further realize the high optoelectronic conversion efficiency predicted for InP/InGaAsP quantum wells tandem solar cells.

It has been known that quantum wells can tailor bandedges absorption, which also provides the flexibility to currently match InGaP/GaAs tandem solar cells under any defined spectrum. The device conversion efficiency has been achieved to 30.6% under the concentrator spectrum. This is a new record for any nanostructure solar cells devices. Strain balanced quantum wells solar cells with optimized conventional top solar cells should achieve the conversion efficiency of 34%. A new high efficiency consisting of quantum wells tandem solar cells have been proposed in both the GaAs and InGaP solar cells. This solar cells devices structure has the advantage to achieve higher efficiency comparable to the current lattice mismatched multijunction tandem solar cells.
