Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
Publication date: 18th December 2016
Formamidinium lead iodide (FAPbI3) has a broader absorption spectrum and improved thermal stability than the more famous methylammonium lead iodide system, thus providing great promise for its integration into photovoltaic (PV) devices. Of the two main FAPbI3 polymorphs, by far the most interesting for PV applications is the high temperature metastable α-phase (trigonal perovskite) modification – over its low temperature δ–phase (hexagonal non-perovskite) counterpart – because of its superior optical and electronic properties1; from its long carrier lifetime (484 ns), to the roughly one order of magnitude increase in conductivity (1.1x10-7 (Ωcm)-1), when compared to methylammonium PbI3 (MA PbI3). FAPbI3 is well known to undergo a δ- to α-phase transition above 150 ºC,2 however the real-world effects of transforming this material in oxygen-rich environments are yet to be explored. In this report, phase-sensitive Raman scattering and x-ray diffraction (XRD) methods are employed to investigate the structural properties of δ- and α-phase FAPbI3 transformed via thermal annealing under atmospheric conditions. The kinds of information the two techniques yield are notably different, with Raman scattering probing surface structure, rather than the overall average, like in XRD. Raman is able to fingerprint both δ- and α-phases readily,1 however it also reveals the formation of non-trivial PbO polymorph surface deposits after thermal treatment. Parallels can be drawn here to the related MAPbI3 system, which is prone to oxygen intercalation.3 In fact, PbO compound Raman bands where actually inadvertently contained in the Raman data reported by Han et al.1, for the low temperature delta-phase, even before undergoing thermal annealing. We conclude that such a susceptibility to lead oxide surface incorporation must be considered when designing and fabricating stable optical devices based on the thermally transformed metastable α -FAPbI3 materials.
[1] Q. Han et al., “Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties”, Adv. Mater. 28, 2253 (2016).
[2] N. J. Jeon et al., “Compositional engineering of perovskite materials for high-performance solar cells”, Nature 517, 476 (2015).
[3] W. Kong et al., “Oxygen Intercalation Induced by Photocatalysis on the Surface of Hybrid Lead Halide Perovskites”, J. Phys. Chem. C 120, 7606 (2016).​
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