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Recent Advances in InN-based Solar Cells; Status and Challenges in InGaN and InAlN Solar Cells

Akio Yamamoto ,  Md R. Islam ,  Ting Ting Kang ,  Akihiro Hashimoto 

University of Fukui (U. Fukui), 3-9-1 Bunkyo, Fukui 910-8507, Japan


The III-nitrides InGaN, InAlN are very attractive materials for the application to super-high efficiency solar cells, since bandgap energies in the range of 0.7-2.5 eV, which are the essential requirement for the fabrication of multijunction tandem solar cells with a efficiency higher than 50%, can be obtaied only by adjusting their In composition. In this paper, we review the recent advances in InN-based solar cells and discuss the main issues in their fabrication.

The studies on InGaN cells have been started using Ga-rich materials since the growth and Mg-doping of GaN were widely studied. By using MOVPE growth, InGaN films with an In content up to 0.4 have been successfully grown without both phase separation [1]. By doping with Mg, p-type conduction and an n+-p homo-junction have been successfully realized for In0.2Ga0.8N (Eg=2.65eV) on a GaN template. The fabricated device shows a Voc =1.5 V and Jsc=0.5 mA/cm2 under AM1.5 illumination [1]. For InGaN with a higher In composition, a higher CP2Mg flow rate is needed to get p-type conduction [2]. This is due to a higher defect density in InGaN with a higher In composition. The crystalline quality of InGaN is also deteriorated with increasing Mg doping level. The insertion of step-graded InxGa1-xN interlayer between the GaN and InGaN epilayer has been found to be effective to improve the epilayer quality [3].

The study on InAlN growth has been started from In-rich side. 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 [4]. These films show a strong PL at room temperature. However, the Al composition in InAlN and its distribution are found to be markedly influenced by the parasitic reaction between TMA and NH3. The reduced pressure growth at around 76 Torr is effective to suppress the parasitic reaction, giving a reduced compositional gradient along the gas flow direction [5]. The study on Mg-doping for InAlN will be the next step.

References: [1] A. Yamamoto et al., Proceedings of the 33rd IEEE PVSC, May 11-16, 2008, San Diego, USA.   [2] Md R. Islam et al., J. Cryst. Growth (in press). [3] Md R. Islam et al., To be submitted to ICNS-8. [4] Y. Houchin et al., phys. stat. sol. (C), 5, 1571 (2008). [5] M. Tanaka et al., To be presented in the 2009 EMC, June 24-26, 2009, University Park, USA.

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Presentation: Keynote lecture at E-MRS Fall Meeting 2009, Symposium A, by Akio Yamamoto
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-10 10:56
Revised:   2009-06-07 00:48