Halide perovskites have become a promising material class for the new generation of photovoltaic applications. The rapid growth in device efficiency of this emerging field attracts great interest for lightweight and cheap solar devices. Perovskite-based solar cells have reached 23.7% efficiencies,^[1] thus surpassing established technologies such as CIGS and CdTe. Nonetheless, core understanding of the fundamental properties of this material, especially in regard to its anomalous ionic movement, is still developing. The ion migration is responsible for the well-known hysteresis in device operation and affects the long-term stability of the material.^[2] In our previous work, we have shown that there is a significant impact of the microstructure (i.e. grain boundary density) on the device performance and its stability in maximum power point tracking.^[3] Films with a higher density of grain boundaries (GBs) exhibited more hysteresis, to which ion migration is strongly correlated.^[4] This points to a vital role of the microstructure of the material on the ion migration.

In this work, we used micro-photoluminescence (PL) to directly link the microstructure of methylammonium lead iodide (MAPbI₃) to its ionic movement. It has been shown that the accumulation of photogenerated charge carriers can create local electric fields, which in turn induces ionic motion. We highlight two reported PL phenomena related to ion migration: 1) the filling/creation of a trap state affecting the PL quantum yield^[5] and 2) the destabilization of the lattice by the removal of ions which alters the bandgap and induce PL spectral shifts.^[6] Resolving the PL signal in space and in time with high-resolution wide-field imaging, comparing a thin film and a single crystal of MAPbI₃, we studied the impact of GBs on ion migration. We established that ion migration was at play by observing a spatial redistribution of PL quantum yield, as well as blue shifts of the PL spectra, evolving on time-scales ranging from ms to minutes. Furthermore, as both these phenomena became greatly limited in the film compared to the single crystal, we believed that the GBs inhibit the movement of the ions. Moreover, simultaneously observing blue shifts of the PL spectra and changes in PL quantum yield at different rates suggests we are directly resolving the motion of two different ionic species. Although our study focuses on the PL signal alone, we trust that this real-time demonstration of ion migration in different structures will provide useful insight into device improvement where monolithic grains are vital for the highly efficient and stable devices.

[1]          NREL, Vol. 2019, National Renewable Energy Laboratory,  2018. Retrieved from https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181221.pdf. Accessed on 01/02/2019.

[2]          K. Domanski, et al., Energy & Environmental Science 2017, 10, 604.

[3]          J. P. Correa‐Baena, et al., Advanced Materials 2016, 28, 5031.

[4]          H. J. Snaith, et al., The journal of physical chemistry letters 2014, 5, 1511.

[5]          D. W. deQuilettes, et al., Nature communications 2016, 7, 11683.

[6]          A. Merdasa, et al., The Journal of Physical Chemistry C 2016, 120, 10711.