InGaN Solar Cells: Present State of the Art and Important Challenges

Solar cells are a promising renewable and carbon-free electric energy resource to address the fossil-fuel shortage and global warming. Energy conversion efficiencies over 40% have been recently achieved using conventional III–V semiconductor compounds as photovoltaic materials. The revision of InN bandgap to a much narrower value has extended the fundamental bandgap of the group III-nitride alloy system over a wider spectral region (from 0.64 eV for InN to 3.4 eV for GaN or 6.2 eV for AlN), raising the possibility of a variety of new applications. The tunable bandgap, predicted high radiation resistance, and strong absorption coefficient of the In
$_x$
Ga
$_{ 1-x}$
N material system are promising for high-efficiency photovoltaic systems. During the past few years, the interest in In
$_x$
Ga
$_{ 1-x}$
N solar cells has been remarkable. The development of high-performance solar cells using In
$_x$
Ga
$_{1-x}$
N materials is one of the most important goals when compared with the existing solar cells using Si and other III–V materials. Significant efforts and progress have been made toward this goal, while great opportunities and grand challenges coexist. In this paper, we present a review on the present state of the art of In
$_x$
Ga
$_{1-x}$
N-based solar cells. The most important challenges toward the high-efficiency In
$_x$
Ga
$_{ 1-x}$
N-based solar cells are discussed in the context of the recent results.