Ultrathin nanolayer constituted by a natural polysaccharide achieves “egg-box” structured SnO2 nanoparticles toward efficient and stable perovskite solar cells

Notorious nonradiative recombination and retarded charge dynamics corrodes the performance of perovskite solar cells (PSCs). Accordingly, a natural polysaccharide sodium alginate (SA) is introduced into SnO2 colloid solution, developing "egg-box" structured SnO2 nanoparticles wrapped by ultrathin SA nanolayer to achieve a bottom-up holistic passivation strategy. The active oxygen-containing functional groups with lone pair electrons of SA can coordinate with uncoordinated Sn4+/Pb2+. Meanwhile, hydroxyl groups can form hydrogen bonds with I of perovskite. Additionally, Na+ can fill the cation vacancies in perovskite and electrostatically interacts with the halogen ions, hindering the formation of iodine Frenkel defects. Based on the synergistic effects of various functional groups, SA-modification can simultaneously inhibit the agglomeration of SnO2 nanoparticles, improve carrier dynamics, release tensile stress and promote crystallization of perovskite. Finally, the SA-optimized methylammonium-free (MA-free) device fabiricated entirely in ambient air achieves a champion efficiency up to 23.61%. The optimized devices without encapsulation exhibit better thermal and moisture stability, with the PCE retaining 80.2% of its initial efficiency after 1080 h of thermal aging at 60 °C and 91.1% of its initial efficiency after 1440 h at 25% relative humidity, respectively.