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−1PbnI3n+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−2Pb2m+1I7m+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.