Defect tolerant Cs2SnI6 double perovskite thin films with ultrahigh carrier lifetime for high efficiency solar cells

Phase pure, device quality inorganic Cs2SnI6 double perovskite thin films are grown using thermal evaporation method. Both sequential and co-evaporation methods are attempted and preliminary analysis confirmed that the samples prepared by co-evaporation technique are more stable, demonstrating better properties. Effect of variation in the CsI/SnI2 ratio on the properties of Cs2SnI6 samples prepared using co-evaporation method are studied. CsI impurity phase is found to be suppressed when the ratio is reduced to 1:1. The films possess a direct bandgap of -1.53 eV and a polygon morphology with grains of size -0.4 mu m. Depth-wise elemental analysis using XPS substantiated the uniform distribution of all the elements throughout the material without any additional phases. The sample has exceptionally high minority carrier lifetime of the order of 9.6 mu s as determined from the time-resolved photoluminescence. The work function and valence band maximum of Cs2SnI6 film are found to be located at 4.9 eV and 6.31 eV respectively. These findings highlight the superior properties of the co-evaporated Cs2SnI6 thin film for the application as an absorber layer in solar cells.