Bulky Organic Cations with Large Steric Hinderance Inhibit Structural Degradation and Enhance Open Circuit Voltage in Perovskite Solar Cells

Despite the growth in power conversion efficiencies of lead halide perovskite solar cells to over 25% in recent years, interfaces at the active layer/charge transport layers prove to be a key factor in limiting charge extraction. Organic ammonium cations such as PEA⁺ commonly used for passivation of interfacial defects can cause reconstruction of the cubic perovskite phase upon thermal exposure, affecting charge extraction and long-term device performance. Here, we have explored molecules with extended π-conjugation length in the form of ammonium halide salts such as 2-([2,2'-bithiophen]-5-yl)ethan-1-aminium iodide (2TI), 2-(3′′′,4′-dimethyl-[2,2′:5′,2″:5″,2′″-quaterthiophen]-5-yl)ethan-1-ammonium iodide (4TmI) to explore the structural changes and defect passivation induced at the perovskite/Hole Transport Layer interface. Our XPS studies suggest lowering of Iodine and undercoordinated lead defects on perovskite films with passivation. However, lattice distortion is triggered by formation of low dimensional Ruddlesden Popper phase as 2TI penetrates the bulk on thermal treatment. On the other hand, large steric hinderance in 4TmI inhibits octahedral tilting in the inorganic [PbX₆]^4- layer by preventing the ammonium head from replacing the A site cations in the perovskite lattice. The limitation on the reconstruction of the interface along with defect passivation translates to enhanced charge extraction and reduced nonradiative charge recombination. These mechanisms will be confirmed by GIWAXS and Photoluminescence measurements. Differences in layer uniformity and grain morphology for varying steric sizes of the cations are highlighted in SEM images. Using conjugated 2TI and 4TmI molecules acting as a shielding layer over the perovskite film, we have fabricated solar cells showing highest open circuit voltage of 1.09 with over 20% efficiency.