Crystal Structures, Rotational, Vibrational Dynamics, and Optoelectronic Properties of Two-DimensionaI Hybrid Organic-lnorganic Perovskites

Hybrid organic-inorganic perovskites (HOIPs), also known as organic metal halide perovskites (MHPs), have shown promising success in the solar cell and light-emitting diode (LED) industries by showing their remarkable photovoltaic and optoelectronic performances. The 25.2% record power conversion efficiency from the perovskite solar cells has been achieved by polycrystalline thin films produced through simple, low-cost and low-temperature methods, which have an electronic quality that is comparable to, or surpasses, that of the state-of-the-art epitaxial grown semiconductors. On the other hand, the perovskite LEDs have shown high quantum efficiencies, with a recent record of 21.6%.

The long carrier lifetime and defect tolerance are the two key characteristics that are responsible for superb optoelectronic device performance in HOIPs, and yet their origins are still under intense debate. Here we show significant differences in the charge carrier dynamics between the two-dimensional (2D) and three-dimensional (3D) HOIPs, using time-resolved photoluminescence (TRPL) measurements and time-of-flight (TOF) neutron scattering measurements.

In this thesis, (BA)2PbI4 ((C4H9NH3)2PbI4) and MAPbI3 (CH3NH3PbI3), the prototypes of 2D and 3D HOIPs, are selected to be the materials to study. The TRPL measurements showed short and nearly temperature-independent charge carrier lifetimes, within 0.1 ~ 1.1 ns, for the majority charge carriers in the 2D HOIP, (BA)2PbI4, which is in contrast with the long-lived and temperature-dependent majority carriers in the 3D HOIP, MAPbI3. In MAPbI3, the lifetimes of the majority charge carriers are 0.7 ~ 60 ns, which show a gradual decrease within each structural phase, while exhibits a significant increase at the orthorhombic-to-tetragonal phase transition when a C4 rotational mode of the MA+ molecule gets activated. The short, temperature-independent and long, temperature-dependent charge carrier lifetimes are consistent with the excitonic and polaronic pictures of the majority carriers in the 2D and 3D HOIPs, respectively.

On the other hands, our TOF neutron scattering measurements revealed two types of jump rotational modes for (BA)2PbI4: the C3 modes of the NH3, CH3 groups with smaller rotational radii and shorter relaxation time, which exist in the entire temperature range; and the C4 mode of the whole BA+ molecule with larger rotational radii and longer relaxation time, which is only activated in the high-temperature phase. The C4 mode appears right after the phase transition at 280 K upon heating, showing a characteristic relaxation time of ~ 53 ps.

Furthermore, from the TOF neutron scattering measurements, the phonon melting features were observed on both 2D and 3D HOIPs, when the phonon modes gradually lose their time coherences as the temperature increases, while the spatial correlations are still conserved. Most prominently, for the mostly-inorganic and hybrid modes, the sharp coherent phonon peaks at the base temperature become broad continuums during heating within the low-temperature phases.

Combining the TRPL measurements and the TOF neutron experiments, we conclude that the 2D HOIP, (BA)2PbI4, which is dominated by excitons, does not show correlations between PL lifetime and rotational dynamics, phonon melting. On the other hand, for the 3D HOIP in which polaronic features are prominent, the major factors which enhance the charge carrier lifetime are both the rotations of the MA+ molecules and the liquid-like vibrations of the inorganic atoms.