**2. InGaN/GaN MQWs solar cells**

InGaN alloys semiconductor materials as active layers of light emitting diodes have been widely investigated, which emission wavelengths cover from red to near ultraviolet spectral regions. [13] Recently, InGaN alloys as a new solar cells materials have been interested by their tunable energy band gaps which vary from 0.7 eV to 3.4 eV, covering almost the whole sunlight spectrum, and also their superior photovoltaic characteristics, such as high carrier mobility, direct energy band gap, high optical absorption coefficient near the band edge, high drift velocity and low radiation resistance. [14]

Although InGaN alloys material solar cells offer tremendous potential for photovoltaic applications, there are only a few references on InGaN alloys solar cells. Furthermore, most of reported InGaN alloys solar cells usually have In contents no more than 15% and band gaps about 3.0 eV or larger, therefore, external quantum efficiency is nearly not exist at wavelengths longer than 420 nm. [15] An earlier theoretical calculation has been indicated that the use of a special active material in solar cells can obtain conversion efficiency greater than 50%, that material is InGaN alloys whose In content exceed 40%. [16]Additionally, group III-nitride multi-junction solar cells with ideal band gaps for maximum conversion efficiency must be incorporated InGaN alloys with higher In contents. [13]

In this section, the optoelectronic properties of InGaN alloys solar cells are researched by merging into InGaN/GaN multiple quantum wells (MQWs), attempt to alleviate the phase separation issue of InN and GaN in a certain extent, and demonstrate the photovoltaic performance of solar cells operating in wavelengths longer than 420 nm.
