Raman and photoluminescence investigation of CsSnI3 perovksite phase transitions

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Athanassios Kontos, presentation 254
Publication date: 5th February 2015

Perovskite solar cells have recently emerged as a very promising class of solid state photovoltaics attaining extremely high power conversion efficiencies. This was due to the discovery of a new generation of inorganic or/and hybrid perovskite materials that can be used as both absorbers and hole transporting media. A characteristic example is the case of the p-type direct bandgap semiconductor CsSnI₃, a perovskite material with ideal energy gap for absorbing solar light and remarkably high hole mobility. Its doping with F ions dramatically improves the photocurrent density and power conversion efficiency of the corresponding solar cells [1]. CsSnI₃ belongs to the family of perovskite trihalides RMeX₃ (X=halogen, Me=transition metal, and R=alkali metal or organic short chain) that are amenable to solution synthesis and processinh [2]. CsSnI₃ perovskites exist in two polymorphs at room temperature: one has a one-dimensional double-chain structure and is yellow in color (Y), and the other has a three-dimensional perovskite structure and is black (B) [3]. Despite its interesting characteristics, our understanding of the origin of structural and optoelectronic properties in CsSnI₃ is still limited. Additionally, many previous reports understate its high sensitivity to air, moisture, and organic solvents [4]. In this work we studied the optical properties and phase transitions in F:CsSnI₃ perovskites by variable-temperature Raman and photoluminescence (PL) spectroscopies under inert atmosphere and in comparison to XRD data. Both yellow (Y) and black (B) F:CsSnI₃ perovskites as well as the transformation from the one phase to the other were examined by varying the temperature from -100 to 170 ^oC. For the black phase several Raman peaks at low frequencies were observed with distinct differences from those of the Y orthorhombic phase. No phase transformation up to 110 ^oC was evident, where the sample starts becoming very photosensitive. Raman signal was not detected above 170 ^oC, fact that corroborates with the transition to the cubic (B) phase which is not Raman active. Microluminescence (PL) spectra obtained under the same conditions confirmed the strong dependence of the PL signal on the temperature, showing that the band gap increases upon increasing the temperature. Hysteresis of the photoluminescence emission was found upon thermal cycling between room temperature and 170 ^oC, which is attributed to the reduction in the population of the excitonic centers.

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