Additive-Assisted\nElectronic Defect Passivation in\nLead-Free Tin Perovskite Solar Cells: Suppression of Sn<sup>2+</sup> Oxidation and I<sup>–</sup> Losses

Tin-based perovskite solar cells (Sn-PSCs) have been\nconsidered\npromising alternatives to low-toxic PSCs. However, the rapid and faster\ncrystallization process in Sn perovskite has formed poor crystalline\nfilms with a severe electronic defect such as oxidation of Sn²⁺ and anionic vacancies (losses of I^–).\nThe minimization of the electronic defects is remarkably challenging\nfor further progress in Sn-PSCs. Herein, we report a bulky guanidine-based\n1,3-diaminoguanidine monohydrochloride (DAGCl) bifunctional additive\nas an electronic defect passivation agent in the precursor solution\nof three-dimensional (3D) FASnI₃ perovskite. Specifically,\nthe interaction between DAGCl and the iodide ion (I^–) formed hydrogen bonding and suppressed the I^– loss of the perovskite framework, which was evidenced by X-ray photoelectron\nspectroscopy analysis. Moreover, the Cl^– ion formed\ncoordination with undercoordinated Sn²⁺ ions and stabilized\nthe 3D FASnI₃ perovskite structure with effective suppression\nof oxidation of Sn²⁺ to Sn⁴⁺. Suppressing the\noxidation of Sn²⁺ and loss of I^– guarantee\na better Sn²⁺/I^– stoichiometric ratio\nof 1:2.34 for DAGCl (3 mol %)-contained perovskite than pristine perovskite\n(1:1.92). Consequently, the dual aspect of the DAGCl additive strongly\naffects the charge carrier dynamics by reducing the charge carrier\nrecombination centers of the perovskite films and significantly increases\nthe charge carrier lifetime by about 2.7 times. The PSCs with an optimized\nconcentration of DAGCl showed a power conversion efficiency (PCE)\nof 8.92% with a significant enhancement in <i>V</i>_OC up to 0.61 V from that of the pristine PSC (0.50 V). Additionally,\nPSCs with the DAGCl additive retained 95% of its initial PCE after\n1100 h and showed a good light-soaking stability.