Every perovskite solar cell undergoes changes in temperature during operation and most fabrication routes include one or more thermal annealing steps. For methylammonium lead iodide (MAPI), the archetype of metal halide perovskite solar cells, both the formation and growth of the crystals are strongly interlinked with the phase transition temperature at 328 K. At this temperature, the crystal changes from a polar tetragonal to a non-polar centrosymmetric cubic structure. To date, the majority of research related to the thermal treatment of perovskite solar cells focuses on the growth of crystals from precursor chemicals during layer formation or the thermal decomposition at elevated temperatures. So far, the influence of temperature on the formation of the microstructure within perovskite grains such as formation and evolution of polar domains remains unclear.
Here, we study the effects of annealing temperature and duration on the formation and transformation of ferroelectric domains and show the influence of domain patterns on the optoelectronic microstructure and macroscopic device performance of solar cells. Annealing of MAPI layers at 100°C yields flat grains with an average diameter of > 1 μm. After fast grain growth in the first few minutes, further annealing does not change the layer morphology anymore. However, ferroelectric domains as revealed by piezoresponse force microscopy (PFM) show vastly different orientations and domain shapes during this thermal treatment. The investigation of the corresponding solar cells elucidates the influence of changes in the sub-grain microstructure on the JV-characteristics, power conversion efficiencies and the hysteretic behavior of solar cells.