The Unique Roles of Organic Moiety in Organic-Inorganic Hybrid Perovskite Systems

DoctorOrganic-inorganic hybrid perovskite-based solar cells have revolutionized the photovoltaic landscape as they have demonstrated unprecedentedly high power conversion efficiencies (PCEs) and low cost. Their electrical PCE increases extremely rapidly and has reached ~19 % in 2013, up from ~3 % in 2009. The observed unusually high PCEs are currently attributed to several relevant physical factors that include low optical bandgaps, large absorption coefficients, and long carrier diffusion lengths. In addition to high PCEs, hybrid halide perovskites of RMX3-type show a remarkable capability of demonstrating diverse photovoltaic properties by suitable substitution or modification of organic molecules (R) or metal (M) ions in the hybrid perovskite lattice. According to the previous theoretical study, the organic cations of different sizes and hydrogen-bonding interactions [e.g., CH3NH3+ and (NH2)2CH+] are capable of affecting the optical bandgaps of RPbI3-based perovskites. Several other studies also indicate the important role of the MA+-ion orientation and, thus, the hydrogen-bonding interaction in controlling the core properties of the MAPbX3-based perovskite solar cells, which includes the enhanced carrier diffusion length, the ferroelectric photovoltaic effect, and the interplay of the MA-dipole orientation with the stability of perovskite structure. In spite of the key role of hydrogen bonding in CH3NH3PbI3 (MAPbI3) little progress has been made in our in-depth understanding of the hydrogen-bonding interaction between the MA+-ion and the iodide ions in the PbI6-octahedron network. The first part will discuss about two distinct type of hydrogen bonding mode in the tetragonal MAPbI3 on the basis of symmetry argument and density functional theory. The computed Kohn-Sham (K-S) energy difference between these two interaction modes is 45.14 meV per MA-site with the α-interaction mode being responsible for the stable hydrogen-bonding network. We have further estimated the individual bonding strength for the ten relevant hydrogen bonds having a bond critical point. The net difference in the total hydrogen-bonding energies between these two interaction modes is 43.87 meV per MA-site, which nearly coincides with the K-S energy difference of 45.14 meV. HB interaction in hybrid halide perovskites, little progress has been made in our systematic understanding of the atomic-scale role of HB interaction in (i) the octahedral tilting, (ii) the inversion-symmetry breaking, thus, the appearance of a polar PbI6 octahedron cage, and (iii) the degree of the Rashba splitting. The second part focuses on the the role of hydrogen-bonding (HB) in hybrid halide perovskites is a crucial issue in understanding the structural stabilization of polar octahedron cages and the observed enhanced carrier lifetimes. The octahedral network in CH3NH3PbI3 (abbreviated as MAPbI3) is stabilized by the HB interaction between the organic MA cations and the PbI6 octahedron cage. On the contrary, the Pb-I orbital hybridizations within the octahedron cage give rise to the overall destabilizing effect on the octahedral tilting. And Rashba zone-center splitting is determined primarily by the polar direction of the PbI6 cage which, in turn, is determined by the HB interaction