Antisolvent-Derived Intermediate Phases for Low-Temperature Flexible Perovskite Solar Cells

Antisolvent-assisted solution deposition seems to be one of the most efficient and reproducible methods for the preparation of high-quality perovskite films. During deposition, the perovskite precursor first forms intermediate phases when adding antisolvents, which transform into perovskite phases after annealing. However, the annealing temperature is usually as high as 100 degrees C, which is much higher than the glass-transition temperature of most polymer substrates. The key to a low annealing temperature is to control the intermediate phases. We demonstrate that only toluene (TL) drives the perovskite precursor into metastable phases while other antisolvents including chlorobenzene, ethyl acetate, and trichloromethane drive the perovskite precursor into supersaturation phases. The metastable zone is a region in which only grains grow and are involved in a further exothermic reaction to form perovskite, so TL ensures a large grain size of 480 nm and reduces the annealing temperature to as low as 80 degrees C. Through controlling the intermediate phases with TL, optimized efficiencies approach 18.42 and 16.89% for rigid and flexible planar perovskite solar cells, respectively. The flexible device treated by TL retains 85% of its initial efficiency over a period of 20 days of storage under ambient conditions. Our work provides valuable insights into the role of intermediate phases during the antisolvent-assisted solution deposition process, which are essential to the design of high-efficiency flexible perovskite solar cells at low annealing temperature.