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

^aCEREBA, Central 5-2, 1-1-1 Higashi, Tsukuba, 305-8565, Japan

^bNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 1-1-1 Higashi, Ibaraki, Japan

^cNihon Univ., Izumi-cho, Narashino, Chiba 275-8575, Japan

^dChiba 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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