The Effect of Structural Dimensionality on Carrier Mobility in Lead-Halide Perovskites

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-A-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 A-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/cm² to less than 1 mA/cm². 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 A-site cations play an important role in measured charge carrier mobility, and in the performance of perovskite solar cells.