In situ TEM analysis of structural changes in metal-halide perovskite solar cells under electrical bias

Organic-inorganic metal-halide perovskite solar cells are emerging as a promising photovoltaic technology to harvest solar energy, with latest efficiencies now surpassing 22%1 - an impressive increase from the first reported value of 3% in 2009.2 In addition to low manufacturing costs, the optical properties of such cells can be tailored to form efficient tandems when combined with high-efficiency silicon solar cells.3 A typical perovskite cell structure as investigated here is based on a methylammonium lead trihalide absorber (MAPbI3) that is placed between hole- (Spiro-OMeTAD) and electron-selective contacts (a fullerene-based material).While new record efficiencies are frequently reported, the commercial application of this solar cell technology remains hindered by issues related to thermal and operational stability. Different mechanisms that are still debated modify cell properties with time, temperature, illumination and general operating conditions.4 In order to correlate applied voltage (V) and resulting current (I) to changes in active layer chemistry and structure on the nanometre scale, we performed both ex situ and in situ transmission electron microscopy (TEM) experiments, involving (scanning) TEM (STEM) imaging, selected-area electron diffraction, energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Samples were prepared by focused ion beam (FIB) milling, with exposure to air during transfer to the TEM minimised to