Structural Isomer of Fluorinated Ruddlesden-Popper Perovskites Toward Efficient and Stable 2D/3D Perovskite Solar Cells

Defectpassivation using two-dimensional (2D)-layered perovskiteswith organic spacers on 3D bulk perovskites has been proposed as aneffective strategy to improve perovskite solar cell stability andefficiency. Specifically, fluorination of the organic spacers hasbeen employed due to the resulting hydrophobic nature and the defectpassivation characteristics. In addition to the type of functionalgroups attached to the spacer molecules, conformational changes offluorine isomers on layered perovskites can provide an extended strategyto control a variety of opto-electrical properties related to theinterlayer spacing. As a model system for the structural isomer offluorinated spacers, meta-CF3 and para-CF3 groups anchored to phenethylammoniumiodide (PEAI) spacer molecules are employed to synthesize 2D perovskitesand to investigate their full potential as an interfacial modifierfor perovskite solar cells. The fluorination position change leadsto altered opto-electrical characteristics in layered perovskites.Although they possess identical functional groups, the different orientationsof the functional groups used in the perovskite layer deposited onthe 3D perovskite absorber result in distinct electrical propertiesof 2D/3D heterostructures due to dissimilar intermolecular interactions.The 2D perovskite with meta-CF3-PEAI spacersexhibits an enhancement of the charge transport in the out-of-planeorientation and an improved suppression of the trap states of 3D perovskiteswhile also providing a more favorable energy alignment for efficientcharge transfers. Theoretical simulations are consistent with theexperimental results. The structural isomers of fluorination anchoringto spacer cations alter the structural configuration of the spaceras well as the interlayer spacing that can improve the performanceand the stability of 2D/3D perovskite solar cells.