Additive-Assisted Electronic Defect Passivation in Lead-Free Tin Perovskite Solar Cells: Suppression of Sn²⁺ Oxidation and I^– Losses

Tin-based perovskite solar cells (Sn-PSCs) have been considered promising alternatives to low-toxic PSCs. However, the rapid and faster crystallization process in Sn perovskite has formed poor crystalline films with a severe electronic defect such as oxidation of Sn2+ and anionic vacancies (losses of I–). The minimization of the electronic defects is remarkably challenging for further progress in Sn-PSCs. Herein, we report a bulky guanidine-based 1,3-diaminoguanidine monohydrochloride (DAGCl) bifunctional additive as an electronic defect passivation agent in the precursor solution of three-dimensional (3D) FASnI3 perovskite. Specifically, the 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 spectroscopy analysis. Moreover, the Cl– ion formed coordination with undercoordinated Sn2+ ions and stabilized the 3D FASnI3 perovskite structure with effective suppression of oxidation of Sn2+ to Sn4+. Suppressing the oxidation of Sn2+ and loss of I– guarantee a better Sn2+/I– stoichiometric ratio of 1:2.34 for DAGCl (3 mol %)-contained perovskite than pristine perovskite (1:1.92). Consequently, the dual aspect of the DAGCl additive strongly affects the charge carrier dynamics by reducing the charge carrier recombination centers of the perovskite films and significantly increases the charge carrier lifetime by about 2.7 times. The PSCs with an optimized concentration of DAGCl showed a power conversion efficiency (PCE) of 8.92% with a significant enhancement in VOC up to 0.61 V from that of the pristine PSC (0.50 V). Additionally, PSCs with the DAGCl additive retained 95% of its initial PCE after 1100 h and showed a good light-soaking stability.