Interconversion between Free Charges and Bound Excitons in 2D Hybrid Lead Halide Perovskites

The optoelectronic properties of hybrid perovskites can be easily tailored by varying their components. Specifically, mixing the common short organic cation (methylammonium (MA)) with a larger one (e.g., butyl ammonium (BA)) results in 2-dimensional perovskites with varying thicknesses of inorganic layers separated by the large organic cation. In both of these applications, a detailed understanding of the dissociation and recombination of electron-hole pairs is of prime importance. In this work, we give a clear experimental demonstration of the interconversion between bound excitons and free charges as a function of temperature by combining microwave conductivity techniques with photoluminescence measurements. We demonstrate that the exciton binding energy varies strongly (between 80 and 370 meV) with the thickness of the inorganic layers. Additionally, we show that the mobility of charges increases with the layer thickness, in agreement with calculated effective masses from electronic structure calculations.ChemE/Opto-electronic Material

Effect of cation rotation on charge dynamics in hybrid lead halide perovskites

Organic-inorganic hybrid halide perovskites are a promising class of materials for photovoltaic application with reported power efficiencies over ∼22%. However, not much is known about the influence of the organic dipole rotation and phase transitions on charge carrier dynamics. Here, we report substantial changes in mobility and lifetime of charge carriers in CH3NH3PbI3 after the low-temperature tetragonal (β) to orthorhombic (γ) phase transition. By using microwave conductivity measurements, we observed that the mobility and lifetime of ionized charge carriers increase as the temperature decreases and a sudden increment is seen after the β-γ phase transition. For CH3NH3PbI3, the mobility and the half-lifetime increase by a factor of 3-6 compared with the values before the β-γ phase transition. We attribute the considerable change in the dynamics at low temperature to the decrease of the inherent dynamic disorder of the organic cation (CH3NH3 +) inside the perovskite crystal structure.ChemE/Opto-electronic Material