<p>Methylammonium
lead iodide (MAPI) is a prototypical photo absorber in perovskite solar cells
(PSCs), reaching efficiencies above 20%. However, its hygroscopic nature has prompted
the quest to find water-resistant alternatives. Recent studies have suggested
that mixing MAPI with lower dimensional, bulky-<i>A</i>-site-cation perovskites helps mitigate this environmental
instability. On the other hand, low dimensional perovskites suffer from poor
device performance, which has been suggested to be due to limited out-of-plane
charge carrier mobility resulting from structural dimensionality and large
binding energy of the charge carriers. To understand the effects of
dimensionality on performance, we systematically mixed MA-based 3D perovskites with
larger <i>A</i>-site cation,
dimethylammonium, iso-propylammonium, and t-butylammonium lead iodide
perovskites. During the shift from MAPI to lower dimensional (LD) PSCs, the
efficiency is significantly reduced by 2 orders of magnitude, with
short-circuit currents decreasing from above 20 mA/cm<sup>2</sup> to less than
1 mA/cm<sup>2</sup>. In order to explain these decrease in performance, we
studied the charge carrier mobilities of these materials using optical-pump/
terahertz-probe, time-resolved microwave photoconductivity, and
photoluminescence measurements. The results show that as we add more of the low
dimensional perovskites, the mobility decreases by a factor of 20 when it reaches
pure LD perovskites. In addition, the photoluminescence decay fitting is
slightly slower for the mixed perovskites, suggesting some improvement in the
recombination dynamics. These findings indicate that changes in structural
dimensionality by mixing<i> A</i>-site
cations play an important role in measured charge carrier mobility, and in the performance
of perovskite solar cells.</p>
Tailoring capping-layer composition for improved stability of mixed-halide perovskites
