Decoupling Interlayer Spacing and Cation Dipole on\nExciton Binding Energy in Layered Halide Perovskites

Layered\nhalide perovskites (LHPs) are emerging semiconductor materials\ndue to their superior environmental stability compared to that of\ntraditional halide perovskites. While LHPs have tunable optoelectronic\nproperties, quantum and dielectric confinement effects due to organic\nspacer layers limit their application. Recent attempts to mitigate\nthe high exciton binding energy (<i>E</i><sub>b</sub>) of\nLHPs by organic cation engineering have been demonstrated; however,\nsystematic studies to decouple the influence of interlayer spacing\nand molecular dipole are very limited. Here, we designed a new class\nof organic spacer employing a malononitrile (MN) functionality giving\na calculated dipole moment of 7.9 D. Malononitrile phenethylammonium\n(MNPEA) was successfully incorporated into lead iodide-based LHPs\nthin films and as single crystals. Comparing the MNPEA-based LHP to\nphenethylammonium (PEA) and biphenethylammonium (BPEA), selected as\nreference cations to elucidate the influence of increased dipole moment\nwhile excluding the contribution of increased interlayer distance,\nclarified the effect of the large organic dipole. Binding energies, <i>E</i><sub>b</sub>, estimated by temperature-dependent photoluminescence\nspectroscopy for MNPEA<sub>2</sub>PbI<sub>4</sub>, PEA<sub>2</sub>PbI<sub>4</sub>, and BPEA<sub>2</sub>PbI<sub>4</sub> were 122, 354,\nand 183 meV, respectively. Moreover, the similar interlayer spacing\nof BPEA<sub>2</sub>PbI<sub>4</sub> and MNPEA<sub>2</sub>PbI<sub>4</sub> (21.04 and 21.36 Å, respectively) confirms the importance of\ndipole in tuning the optoelectronic properties. Photovoltaic devices\nwith <i>n</i> = 1 LHPs demonstrated a higher fill factor\nand open circuit voltage with MNPEA<sub>2</sub>PbI<sub>4</sub> compared\nto the reference layered perovskites, likely due to the favored charge\ndissociation and transport afforded by the malononitrile-based cation.