Slow surface passivation and crystal relaxation with additives to improve device performance and durability for tin-based perovskite solar cells

We investigated the doping effect of bulky organic cations with ethylenediammonium diiodide (EDAI2) and butylammonium iodide (BAI) as additives to passivate surface defects, to control the film morphology and to improve the crystallinity for inverted planar FASnI3 perovskite solar cells. The addition of BAI altered significantly the orientation of crystal growth and improved the connectivity of the crystal grains, but the existence of pinholes in the surface of the pristine FASnI3 films was unavoidable; this effect impeded further improvement of the device performance (PCE 5.5%), which is nevertheless superior to that of a pristine FASnI3 film (PCE 4.0%). The addition of EDAI2 had the effect of curing the pinhole problem, passivating the surface defect states, preventing Sn2+/Sn4+ oxidation and inducing slow relaxation of the crystal structure. In the presence of the EDAI2 additive (1%), the FASnI3 device attained the best initial efficiency, 7.4%, and the device performance continuously increased as a function of duration of storage; the maximum PCE, 8.9%, was obtained for a device stored in a glove box for over 1400 h with only slight degradation for storage beyond 2000 h. The observed slow passivation of surface defects and relaxation of crystal strain were verified with X-ray diffraction, X-ray photoelectron spectroscopy and photoluminescence decay techniques.