Recent advances in InN‐based solar cells: status and challenges in InGaN and InAlN solar cells

Abstract

AbstractIn this paper, we review and discuss the recent advances in InN‐based solar cells. Before the discussion on InN‐based solar cells, two major losses in a solar cell, transparency loss and excess excitation loss, are explained and the effectiveness of a multi‐junction tandem cell in reducing the both losses simultaneously is described. Then, the status and issues for multi‐junction tandem solar cells using the conventional III‐V materials (InGaP/InGaAs systems) are reviewed briefly in order to understand the advantages of InN‐based material as a multi‐junction solar cell material. It is shown that main problem in the conventional III‐V multi‐junction solar cell is the current mismatching between sub‐cells, which reduces significantly the conversion efficiency of the tandem cells. The main advantage of InGaN and InAlN as a multi‐junction solar cell is that a wanted band‐gap between 0.7 and 2.5 eV is realized by changing only their composition.This means that current matching between sub‐cells will be easily achieved using InGaN and InAlN systems. After the status of InGaN solar cell research is summarized, the results on the MOVPE growth of InGaN with an In content up to 0.4, Mg doping using Cp2Mg, and the formation of an n+‐p homo‐junction are described. The insertion of step‐graded InxGa1‐xN interlayer between the GaN and InGaN epilayer is also shown to be effective to improve the epilayer quality. The study on InAlN solar cell is on the stage of the growth of In‐rich InAlN. Single‐crystalline InAlN films with an Al content from 0 (Eg = 0.7 eV) to 0.43 (2 eV) have been successfully grown by employing atmospheric‐pressure MOVPE. The study on Mg‐doping for InAlN will be the next step. Finally, we will summarize and discuss the challenges in InGaN and InAlN solar cells. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)