Perovskite-based solar cells have now achieved power conversion efficiencies comparable with commercial technologies, but their stability is still cause for concern. In this work [1,2] we prepare methylammonium lead iodide cells using four different approaches (with the perovskite conversion carried out in single- and double-step in glovebox, in air or in vacuum), use focused ion beam milling to extract cross-sectional lamellae and carry out transmission electron microscopy (TEM) on the devices.For each cell, we carry out scanning TEM imaging and elemental mapping as they are heated in situ in the TEM. This is a procedure that requires fine control over the temperature and the electron dose. To that aim we exploit recent advances in TEM such as high-efficiency X-ray detectors, which collect energy-dispersed X-ray spectra with a good yield, and stable MEMS heaters, enabling the temperature to be cycled quickly and reproducibly. Moreover, we employ multivariate analysis (principal component analysis, PCA) to increase the signal-to-noise ratio of the spectral maps.We do not observe changes in the morphology or the elemental distribution in the perovskite layer for heating up to 150°C for short times (employing a heating ramp with 30’ steps every 25°C). Since the ex-situ heating of the same samples above 90°C causes a significant decay in cell performance, we attribute such decay to the degradation of the charge transport properties of the hole transporter (spiro-OMeTAD in this case). Increasing the temperature further, different decomposition patterns emerge for the perovskite layer. In samples that had not been exposed to air, elemental migration of lead and iodine results in the formation of aggregates, which EDX suggests might be PbI2, clustering on the FTO electrode. In the sample exposed to air, a different phenomenon occurs – instead of forming aggregates, the elemental species diffuse from the perovskite into the hole transporter. This is visible both as an increased contrast in the high-angle annular dark field images (HAADF) and as features in the EDX spectra; we hypothesise that the trapped moisture within the cell might be hindering the formation of PbI2 and make elemental diffusion more favourable.

[1] Divitini, G. et al. – Nature Energy 201512 (2016)

[2] Matteocci, F. et al. – ACS Applied Materials & Interfaces 7, 26176 (2015)