In the lattice-matched system, the conversion efficiency of InGaP(1.86eV)/
In0.01Ga0.99As(1.40eV)/ Ge (0.65eV) triple-junction tandem cell has been improved up to
31-32% (AM1.5G). However, the band gap combination is not optimized to utilize
photons of the entire sunlight spectrum. The optimized band gap combination can be
obtained by introducing lattice-mismatched system. Numerical analysis shows that
efficiency up to 39% (AM1.5G) is possible with an ideal band gap combination; achieved
by replacing the In0.01Ga0.99As middle cell with In0.16Ga0.84As (1.2eV) cell. However, the
use of the lattice-mismatched In0.16Ga0.84As middle cell poses a serious problem that
many dislocations generate due to 1% lattice mismatch between the middle cell and Ge
bottom cell. These dislocations are accountable for deterioration of the solar cell
performance. To reduce the dislocation density, the graded buffer layers have been often
introduced between upper cell layers and substrate as one of the effective technologies.
Another approach is the thermal cycle annealing (TCA) process that could be used to
reduce the defect density in GaAs/ Si lattice-mismatched system. I found that the TCA
process was useful to improve the electrical properties of lattice-mismatched
In0.16Ga0.84As solar cells, grown on GaAs substrates. Moreover, the reduction in the
defect densities was confirmed in the same region of the device. Further study has been
performed to understand the mechanism behind this behavior and to obtain the optimum
TCA conditions such as the cycle temperature and the annealing time.

I have taken the lattice-mismatched solar cells, which are composed to III-V compound
semiconductor as an object of my study. In order to realize good quality solar cells in my
research, I have acquired the crystal growth techniques. Moreover I have experience to
evaluate the crystal quality and structural defects in semiconductor materials using X-ray
diffraction (XRD), Transmission Electron Microscopy (TEM), Electron Beam Induced
Current (EBIC) and so on. If the opportunities to study compound solar cells are obtained
in the future, I would like to apply these techniques to advanced solar cell research.
Personally I am interested in new type solar cells of low dimensional structures that are
well known as third generation solar cells. Higher conversion efficiency is expected
against the conventional solar cells by low dimensional structures such as quantum dots
(QDs) or quantum wells (QWs). I think that it is possible to apply growth or evaluation
techniques to advanced solar cell research for example low dimensional structure solar
cells.

I was born on March 31, 1982, in Tottori prefecture, Japan.

In April 2002 I transferred to a third year of the undergraduate course of Toyota
Technological Institute from National Colleague of Technology because I wanted to
study about III-V multi-junction solar cells under Prof. Yamaguchi.
In March 2004, after finishing the Undergraduate Course, I continued the research on
solar cells in his laboratory for two years, as a student of the master’s course. At that time,

I was interested in the defect evaluations of III-V compound materials in nano-scale by
using Transmission Electron Microscopy (TEM). In my master’s thesis, the effects of
thermal annealing on reduction of the defect density in the InGaAs/GaAs
hetero-structures was surveyed through TEM observations in order to realize the high
efficiency III-V multi-junction solar cells.

In April 2006 I entered the doctoral course of Toyota Technological Institute to
perform advanced research. At the present, I am still engaged on the defect evaluation and
the improvement of solar cell performance in III-V multi-junction solar cells.