Elucidating Methylamonium and Cesium based Hybrid Perovskites' Dielectric Properties using Ab Initio Calculations and Ellipsometry Measurements

Abstract : Methylamonium lead halide perovskite materials have emerged over the past five years as absorber layers for new high efficiency yet low cost solar cells that combine the advantages of organic and inorganic semiconductors. Similar to inorganic semiconductors, hybrid perovskites (mostly CH3NH3PbI3 and more recently cesium-based) have shown low exciton binding energies 1 (≈ 5-30 meV) indicating a potentially high value (≈ 70) at low frequencies for their relative dielectric constant. Others studies have even found giant values (≈ 1000 in the dark) for the dielectric constant 2 at very low frequencies (<1 Hz). Understanding the excitons' physics in these materials is crucial in order to increase the charge extraction and improve the solar cells' efficiency. Here, we study the phonon modes and dielectric properties of both methylamonium (CH3NH3PbI3) and cesium (CsPbI3) lead iodide perovskite structures using DFT (Density Functional Theory) calculations. Phonon frequencies for both the cubic (T>600K) and orthorhombic (T<530K) phases of CsPbI3 are derived using the linear response approach (DFPT). As for the orthorhombic phase (figure 1), we find that CsPbI3 shows a very flat energy profile around its equilibrium structure (figure 2). We derive the dielectric matrix in the high frequency regime (> 1 THz) from the linear response and aim to extract the low frequency dielectric constant from the phonon frequency splitting between normal and longitudinal modes. The results are expected to determine the possibility of a giant dielectric constant for these hybrid perovskites. The dielectric properties obtained for CH3NH3PbI3 are being compared to ellipsometry measurements performed in collaboration with Horiba Jobin Yvon company.