Enhanced Stability of Ag₂BiI₅ Perovskite Solar Cells by Adding Pyridine Derivatives in Hole-Transporting Materials

Fabrication of air-stable Pb-free solar cells using the Ag2BiI5 absorber has recently attracted extensive attention. 4-tert-Butylpyridine (tBP) is mainly used as an additive in the hole transport layer (HTL). However, tBP can strongly destroy Ag2BiI5 perovskite structure due to the formation of the tBP-BiI3 complex. Herein, 2-pentyl pyridine (2-py) is selected to replace the tBP in the Spiro-OMeTAD precursor solution, which will promote the dissolution of lithium bis(trifluoromethanesulphonyl)imide salt in the HTL precursor solution but not destroy the crystal structure due to more considerable steric hindrance. By judiciously choosing 2-py as the additives, the best solar cell device achieves a power conversion efficiency (PCE) of 1.32% under standard one sun (AM1.5). The devices also show excellent long-term air stability and maintain 95% of their initial PCE even after 840 h.