Enhancing the Short Circuit Current Density Due to the Influence of the Si-buffer Layer at the Hetero-Interface of Strained Epi-Si1-xGex (x ? 0.10) Heterojunction Solar Cell
M. Ajmal Khan a b, R. Sato c, K. Sawano c, Y. Ishikawa d
a National Institute of Technology, Fukushima College, Iwaki, Fukushima, 970-8034, Japan
b Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan
c Advanced Research Laboratories, Tokyo City University, 8-15-1 Todoroki, Setagaya-ku, Tokyo158-0082, Japan
d Nara Institute of Science and Technology
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Oral, M. Ajmal Khan, presentation 126
Publication date: 7th November 2016

Semiconducting epi-Si1-xGex alloys has promising features as a solar cell material and field-effect transistors [1-2]. The band gap of epi-Si1-xGex can be tuned in between 0.66 and 1.12 eV as a function of Ge composition (x)[3]. About 40% efficiency is expected from 1.8 eV/1.4 eV/0.9–1.0 eV, tandem solar cell, under concentrated sunlight [3-4,5].  However, using Si1-xGex has serveral challenges: first to overcome the reduction in short circuit current density, Jsc due to the lattice mismatch, the threading dislocations (TDD) can propagat to the active layer.  We also need to improve the open circuit voltage, Voc due to the increased dark current from smaller bandgap. And second is the current matching issue for tandem structure SiGe bottom cell (0.9–1.0 eV). Until now very limited work has been done about the low composition of Ge, 10 % in strained epi-Si1-xGex /c-Si and insertion of Si buffer layer  to grow low TDD heterojunction (HET) solar cell and to get spectral shift toward the long wavelength side for better current matching [5-6]. Therefore, in this work we choose 10% Ge to make fully strained n-Si0.93 Ge0.07 epilayer on the over layer of Si buffer layer grown by MBE and then it was used to make a-Si:H (HET) solar cell using CVD. Thanks to the c-Si buffer layer at the hetero-interface, where the performance of the cells was improved from 2.3 % efficiency with Jsc, 7.90 mA/cm2, Voc, 433 mV and Fil Factor (FF), 0.666, to the efficiency, 3.5 % for the epi-SiGe with Jsc, 15.46 mA/cm2, Voc, 418 mV and with FF, 0.539. This result indicates that one on hand we found slight decrease in Voc and FF but on other side the Jsc were drastically enhanced to 15.46 mA/cm2.

References

[1] Said, et al., Thin Solid Films 337, 85 (1999).

[2] Shiraki, et al., Surf. Sci. Rep. 59, 153 (2005).

[3] Friedman, et al., J. Cryst. Growth 195, 409 (1998).

[4] R. Varache, et al., Solar Energy Materials and Solar Cells 141 (2015) 14–23

[5] Hadi, et al., Mater. Res. Soc. Symp. Proc. Vol. 1447 © 2012 Materials Research Society.

[6] Oshima, et al., Japanese Journal of Applied Physics 54, 012301 (2015).



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