Secondary crystallization strategy for highly efficient inorganic CsPbI2Br perovskite solar cells with efficiency approaching 17%

Inorganic CsPbI2Br perovskite solar cells (PSCs) have a tremendous development in last few years due to the trade-off between the excellent optoelectronic properties and the relatively outstanding stability. Herein, we demonstrated a strategy of secondary crystallization (SC) for CsPbI2Br film in a facile planar n-i-p structure (ITO/ZnO-SnO2/CsPbI2Br/Spiro-OMeTAD/Ag) at low-temperature (150 °C). It is achieved through the method of post-treatment with guanidinium bromine (GABr) atop annealed CsPbI2Br film. It was found that the secondary crystallization by GABr can not only regulate the crystal growth and passivate defects, but also serve as a charge collection center to effectively collect photogenerated carriers. In addition, due to the excess Br ions in GABr, the formation of the Br-rich region at the CsPbI2Br perovskite surface can further lower the Fermi level, leading to more beneficial band alignment between the perovskite and the hole transport layer (HTL), while the phase stability was also improved. As a result, the champion cell shows a superb open-circuit voltage (Voc) of 1.31 V, a satisfactory power conversion efficiency (PCE) of 16.97% and outstanding stabilities. As far as we know, this should be one of the highest PCEs reported among all-inorganic CsPbI2Br based PSCs.