Remarkable Stability and Reproducibility Enhancement in CH3NH3SnI3 Solar Cells: Role of SnF2 Additive and Advanced Cell Encapsulation
Eisuke Ito a, Hideo Yamagishi a, Shogo Ise a, Yoshihiro Miyamoto a, Kiyomi Tsukagoshi a, Tetsuhiko Miyadera b, Koji Yamada c, Hiroyuki Yoshida d
a CEREBA, Central 5-2, 1-1-1 Higashi, Tsukuba, 305-8565, Japan
b AIST, Tsukuba, Ibaraki 305-8565, Japan
c Nihon Univ., Izumi-cho, Narashino, Chiba 275-8575, Japan
d Chiba Univ., 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan
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, Eisuke Ito, presentation 131
Publication date: 7th November 2016

  We have succeeded to fabricate methyl ammonium tin iodide (MASnI3) hybrid perovskite solar cells with remarkable reproducibility and long term stability of more than 500 hours under continuous illumination of 1 sun. The oxidation of the MASnI3 layers was prevented by controlled addition of SnF2 and fine sealing procedures. So far the detailed role of SnF2 additive for the stabilization and efficiency enhancement of MASnI3 solar cells has not been fully understood. In this study, the effect of SnF2 additive was investigated by using photoemission spectroscopy (UPS), low energy inverse photoemission spectroscopy (LEIPS), and Hall-effect measurement.

  The UPS and LEIPS spectra reveal intense gap states in pristine MASnI3 films reflecting formation of defects due to oxidization of Sn2+ to Sn4+. As a result, the valence band maximum was located near the Fermi level. The formation of the gap states caused significant increase of conductivity and carrier density in the film. In contrast we observed reduction of the gap states density after addition of SnF2 to MASnI3. SnF2 is assumed to act as effective reducing agent that promotes reverse conversion of Sn4+ to Sn2+. Also, XRD gave strong evidence of the decrease of Sn4+ defects upon SnF2 addition. The Hall effect measurement indicated p-type conductivity of the as-prepared MASnI3 films. In contrast, the as-prepared MASnI3 doped with SnF2 demonstrates n-type conductivity. After exposure of the doped MASnI3 films to air we observed rapid conversion of their conductivity to strong p-type. This clearly indicates that SnF2 additive can be effectively used to eliminate Sn4+ acceptor states in the MASnI3 perovskite.

  Also, our studies reveal that the alignment MASnI3 energy levels with respect to the energy levels of TiO2 and PTAA is unfavorable for carrier transfer in the cases when MASnI3 films were prepared without SnF2 additive. The energy levels of the SnF2 doped MASnI3 showed better matching with those of the TiO2 and PTAA layers that indicates importance of the careful elimination of the gap states for achieving high device performance. Our results demonstrate a strong evidence that the combination of fine sealing and the reverse conversion of Sn4+ to Sn2+ controlled by SnF2 doping opens a way toward highly stable and environmentally friendly Sn perovskite solar cells.

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