Tuning the Surface Potential of Hybrid Perovskite Active Layers Through Interfacial Engineering Using Fluorinated Compounds

<br/>Despite the tremendous increase in their overall power conversion efficiency over the past decade, organic-inorganic hybrid perovskite solar cells are still plagued by issues related to their long-term device stability.¹ In particular, the hydrophilicity of the hybrid organic-inorganic perovskite surface due to the ionic nature of the perovskite crystal plays a key role in determining device stability.² In order to improve device stability and performance, the localization of a thin interfacial monolayer can be used to reduce the wettability of the perovskite and also to modify the surface potential of the active perovskite layer.^1,3 For instance, fluorocarbons possess strong hydrophobicity and exhibit specific binding interactions with the perovskite surface.² Because of this, they may be good candidates for passivating the surface while also optimizing its surface potential.² We investigate self-assembled monolayers on top of the CH₃NH₃PbI₃ surface using various fluorinated molecules with different heteroatoms including tin, phosphorous, and boron. The chemical nature of the fluorinated monolayers, their interaction, orientation, and thickness were investigated by contact angle measurement, X-ray diffraction, X-ray photoelectron spectroscopy, and polarization-modulation infrared reflection absorption spectroscopy. The formation of stable monolayers with specific orientation on top of the perovskite surface was achieved without altering the bulk crystallinity of the active layer as evidenced by XPS. In addition to the increased hydrophobicity of the CH₃NH₃PbI₃surface, the surface potential of CH₃NH₃PbI₃ was easily tuned over a 160 mV range upon SAMs deposition. This work clearly shows that chemically engineering the composition of the monolayer allows for modifying the perovskite work function, and hence the electrical behavior of the active layer.<br/><br/>¹ Christian M. Wolff <i>et al.,</i> ACS Nano 2020, 14, 1445−1456<br/>² Weixin Huang <i>et al</i>., Chem. Mater., 2016, 28, 303–311.<br/>³ Subha Sadhu <i>et al., </i>Mater. Horiz., 2019, 6, 192-197