Alternative simple method to realize p-type BaSi 2 thin films for Si heterojunction solar cell applications
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.191
Alternative simple method to realize p-type BaSi2 thin films for Si heterojunction solar cell applications Kazuma Takahashi1, Yoshihiko Nakagawa1, Kosuke O.Hara2, Isao Takahashi1, Yasuyoshi Kurokawa1,and Noritaka Usami1 1 Graduate School of Engineering, Nagoya University, Nagoya, 464-8603, Japan
2 Center for Crystal Science and Technology, University of Yamanashi, Kofu, 400-8511, Japan
Abstract:
A novel preparation method of B-doped p-type BaSi2 (p-BaSi2) is proposed to realize heterojunction crystalline Si solar cells with p-BaSi2. The method consists of thermal evaporation of BaSi2 on B-doped amorphous Si (a-Si). In this study, the effect of a-Si interlayers and substrate temperature during BaSi2 evaporation on the electrical characteristics and crystalline quality of the evaporated films were investigated. While no cracks were found in the BaSi2 films formed using hydrogenated a-Si deposited by plasma enhanced chemical vapor deposition (PECVD), the films formed with sputtered a-Si have cracks. In addition, BaSi2 films formed with a 600 C substrate temperature using PECVD aSi showed p-type characteristics. After a post-deposition anneal at 800 C for 5 minutes, the film hole density was measured at 1.3×1019 cm-3 and boron was found to be uniformly distributed throughout the film. These results show that the proposed method using PECVD is promising to obtain p-BaSi2 thin films with high hole density for p-BaSi2/n-type crystalline Si heterojunction solar cells.
INTRODUCTION: To fabricate solar cells with higher conversion efficiency (η) at lower cost, various alternative materials to Si such as perovskites, Cu(In,Ga)Se2, and CdTe [1-3] have attracted considerable attention. In our previous research, we have focused on BaSi2, which has suitable physical characteristics for solar cell applications such as a bandgap of 1.3 eV, minority carrier lifetime more than 10 μs, and absorption coefficient of 3.0×104 cm-1 at a photon energy of 1.5 eV [4-11]. Recently, as one of the promising applications of BaSi 2, we have proposed to utilize p-type BaSi2 (p-BaSi2) as a hole selective material for Si heterojuntion solar cells based on calculation results of a device simulation (Afors-Het ver. 2.5) [12,13]. In theory, holes of minority carriers generated in n-type crystalline Si (n-c-Si) can be transmitted, while electrons of majority carriers are repelled from the p-BaSi2/n-c-Si heterojunction due to high conduction band offset, because BaSi2 has the electron affinity
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of 3.2 eV and bandgap of 1.3 eV [14]. P-BaSi2/n-c-Si heterojunction solar cells with η of 9.9% have been previously demonstrated [15]. P-type BaSi2 has been previously prepared by using B, Al, Ag and In as accepter dopants [16-26]. In the case of
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