Strain relaxation and domain enlargement via phase transition towards efficient CsPbI₂Br solar cells

CsPbI2Br perovskite is emerging as an attractive photovoltaic material due to its excellent stability against heat and illumination. However, a huge tensile stress of about 162 MPa is created during the CsPbI2Br film formation, resulting in a high defect density, severe non-radiation recombination loss and instability of CsPbI2Br PSCs. Herein, an alpha -> delta -> alpha phase transition growth (PTG) approach is developed for the deposition of CsPbI2Br films that allows relaxation of the relative tensile strain by 62 +/- 4% and increases the domain size by 800%. The CsPbI2Br films realize a low defect density of 5.59 x 10(15) cm(-3) and a reduced Urbach energy. PSCs prepared by the PTG approach achieve a PCE of 16.5% with a remarkably high V-oc of 1.36 V. These CsPbI2Br PSCs retain 90% of their initial efficiency after storage under 10-15% relative humidity (RH) conditions for 1200 h and 90% of their initial efficiency after heating at 65 degrees C for 800 h.