From 2D to 1D Electronic Dimensionality in Halide Perovskites with Stepped and Flat Layers Using Propylammonium as a Spacer

Two-dimensional (2D) hybrid halide perovskites are promising in optoelectronic applications, particularly solar cells and light-emitting devices (LEDs), and for their increased stability as compared to 3D perovskites. Here, we report a new series of structures using propylammonium (PA⁺), which results in a series of Ruddlesden-Popper (RP) structures with the formula (PA)₂(MA)_n-1Pb_nI_3n+1 (n = 3, 4) and a new homologous series of "step-like" (SL) structures where the PbI₆ octahedra connect in a corner- and face-sharing motif with the general formula (PA)_2m+4(MA)_m-2Pb_2m+1I_7m+4 (m = 2, 3, 4). The RP structures show a blue-shift in bandgap for decreasing n (1.90 eV for n = 4 and 2.03 eV for n = 3), while the SL structures have an even greater blue-shift (2.53 eV for m = 4, 2.74 eV for m = 3, and 2.93 eV for m = 2). DFT calculations show that, while the RP structures are electronically 2D quantum wells, the SL structures are electronically 1D quantum wires with chains of corner-sharing octahedra "insulated" by blocks of face-sharing octahedra. Dark measurements for RP crystals show high resistivity perpendicular to the layers (10¹¹ ω cm) but a lower resistivity parallel to them (10⁷ ω cm). The SL crystals have varying resistivity in all three directions, confirming both RP and SL crystals' utility as anisotropic electronic materials. The RP structures show strong photoresponse, whereas the SL materials exhibit resistivity trends that are dominated by ionic transport and no photoresponse. Solar cells were made with n = 3 giving an efficiency of 7.04% (average 6.28 ± 0.65%) with negligible hysteresis.