Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.299
Publication date: 18th July 2019
The use of high-throughput experimentation can be of great advantage to explore the compositional-phase landscape of halide perovskites. Here we present a study of a coevaporated CsPbI3 sample with a lateral compositional gradient which varies from Cs-rich conditions to Pb-rich conditions. The composition, crystal structure, grain size, charge carrier mobility, lifetime and photoluminescence external quantum yield (PLQY) were determined by mapping these properties across the samples. Correlation of these results provides complete structure-property relationships and shows that stable high quality g-CsPbI3 can be obtained by Cs-rich low temperature deposition, without need of a high temperature annealing step. 12% efficient solar cells are demonstrated based on these results [1].
The phase transitions of CsPbI3 have been further investigated due to the interest of increasing the stability of the solar cells and to allow possible applications of these materials in thermochromic devices [1, 2]. We use several in-situ techniques to reveal the phase transformation relations between the a, b and g (black) perovskite phases and the d (yellow) non-perovskite polymorph in thin films. Structural changes were tracked as a function of relative humidity (RH) and temperature by in-situ X-ray diffraction with a liquid-metal jet X-ray source. We find that at room temperature, the transition from the g-black to the d-yellow phase is exclusively triggered by the % of RH whereas O2 is not involved in the mechanism. The use of samples with lateral gradients allowed us to conclude that this transition is slowed down under the Cs-rich regime. Spectrally filtered in-situ optical microscopy reveals that this transformation does not affect the grain structure. An intermediate state is observed, where the optical transmission at 700 nm is significantly reduced during the atomic reordering. The reversed transition from the d (yellow) non-perovskite polymorph to the perovskite-CsPbI3 upon heating happens at the same temperature regardless if the sample is in the Cs-rich or the Pb-rich regime. This conversion is further explored atomically by in-situ heating using scanning transmission electron microscopy (STEM), revealing the transition from the d- to the a-phase in which a layered structured is observed in an intermediate step.
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