Role of Organic Counterion in Lead- and Tin-Based Two-Dimensional Semiconducting Iodide Perovskites and Application in Planar Solar Cells

Hybrid
halide perovskites are emerging semiconducting materials,
with a diverse set of remarkable optoelectronic properties. Besides
the widely studied three-dimensional (3D) perovskites, two-dimensional
(2D) perovskites show significant potential as photovoltaic (PV) active
layers while exhibiting high moisture resistance. Here, we report
two series of new 2D halide perovskite solid solutions: (HA)­Pb1–xSnxI4 and (BZA)2Pb1–xSnxI4 (x =
1, 0.75, 0.5, 0.25, 0), where HA stands for the organic spacer histammonium
and BZA stands for benzylammonium cations. These compounds are assembled
by corner-sharing octahedral [MI6]4– units
stabilizing single-layered, anionic, inorganic perovskite sheets with
organic cations filled in between. The optical band gaps are heavily
affected by the M–I–M perovksite angles with the band
gap steadily decreasing when the angle approaches 180°, ranging
from 2.18 eV for (BZA)2PbI4 to 2.05 eV for (HA)­PbI4. We find an anomalous trend in electronic band gap in the
mixed compositions (HA)­Pb1–xSnxI4 and (BZA)2Pb1–xSnxI4. When Sn substitutes for Pb to form a solid solution, the
band gap further decreases to 1.67 eV for (HA)­SnI4. The
minimum band gap is at x = 0.75 at 1.74 eV. For BZA,
the irregular trend is more intense, as all the intermediate compounds
(BZA)2Pb1–xSnxI4 (x = 0.75, 0.5, 0.25)
have even slightly lower band gaps than (BZA)2SnI4 (1.89 eV). DFT calculations confirm the pure Pb and Sn compounds
are direct band gap semiconductors. Relatively shorter photoluminescence
(PL) lifetime in (BZA)2PbI4 than (HA)­PbI4 is observed, suggesting faster recombination rates of the
carriers. Solution deposited thin films of (HA)­PbI4 and
(BZA)2PbI4 show drastically different orientations
with (HA)­PbI4 displaying a perpendicular rather than parallel
growth orientation with respect to the substrate, which is more favorable
for PV devices. The higher potential in PV applications of the HA
system is indicated by device performance, as the champion air stable
planar device with the structure ITO/PEDOT:PSS/2D-perovskite/PCBM/Al
of (HA)­PbI4 achieves a preliminary power conversion efficiency
(PCE) of 1.13%, featuring an open-circuit voltage (VOC) of 0.91 V.