Pinhole-free Methylammonium Bismuth Iodide Perovskite Solar Cells Via All-Solution-Processed Multi-step Spin Coating

In a functional solar cell device, the morphology of a layer is a key to the performance of Bi-based perovskite solar cells (Bi-PeSCs). Especially, pinholes generated during spin coating degrade the solar cell performance. This paper proposes a method to prepare pinhole-free methylammonium bismuth iodide (MBI) film by multi-step spin coating, with the thickness optimized by changing the layer number between 1 and 12 layers. A strong absorption band is observed at around the wavelength of ~ 500 nm for all the MBI layers. The scanning electron microscopy (SEM) cross-section images visually depict that the MBI layers are free from pinholes at more than 6 layers, and we found that the maximum solar cell performance was reached at 8 layers of MBI with an open-circuit voltage of 0.18V, which is greater than using a single-spin coating. After that, it exhibited a decrease in its performance by increasing the number of MBI layers. The growth model and its mechanism for pinhole-free multi-MBI layers is also discussed. These results suggested an improvement of MBI layer morphology in the development of solar cell devices in the future.