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.