Fig. 28. Scanning electron microscope pictures of m-TiO₂ layers with the prepared perovskite solution heat-treated at various temperatures (a) 60 °C, (b) 80 °C, (c) 100 °C, (d) 120 °C, (e) 150 °C, (f) 175 °C, and (g) 200 °C. (h) Side view SEM image of the sample annealed at 150 °C. Black scale bars parallel to (a) 5 μm, (b–g) 1 μm, and (h) 200 nm¹⁰² Optical micrographs of the CH₃NH₃PbI_3−xCl_x perovskite layer (i).²¹⁵ AFM pictures of perovskite layer toughened with the one-step technique (j), multi-step way (k) (reproduced with permission from ref. 218. Copyright 2015 AAAS). Scanning electron microscope picture of an unannealed hot-cast layer (l) and an annealed hot-cast layer (m), displaying that the perovskite crystals produce while annealing (reproduced with permission from ²¹⁷ Copyright 2015 Elsevier).