Activation Energy of Organic Cation Rotation in CH3NH3PbI3 and CD3NH3PbI3: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects.

Jingrui Li,Peter Reiss,Stéphanie Pouget,Cyril Aumaitre,Mathilde Bouchard,Patrick Rinke,Mariana Rossi,Renaud Demadrille,Dmitry Aldakov,Bernhard Frick,David Djurado

doi:10.1021/acs.jpclett.8b01321

Abstract

The motion of CH3NH3+ cations in the low-temperature phase of the promising photovoltaic material methylammonium lead triiodide (CH3NH3PbI3) is investigated experimentally as well as theoretically, with a particular focus on the activation energy. Inelastic and quasi-elastic neutron scattering measurements reveal an activation energy of ∼48 meV. Through a combination of experiments and first-principles calculations, we attribute this activation energy to the relative rotation of CH3 against an NH3 group that stays bound to the inorganic cage. The inclusion of nuclear quantum effects through path integral molecular dynamics gives an activation energy of ∼42 meV, in good agreement with the neutron scattering experiments. For deuterated samples (CD3NH3PbI3), both theory and experiment observe a higher activation energy for the rotation of CD3 against NH3, which results from the smaller nuclear quantum effects in CD3. The rotation of the NH3 group, which is bound to the inorganic cage via strong hydrogen bonding, is unlikely to occur at low temperatures due to its high energy barrier of ∼120 meV.

Highlights

Hybrid perovskite photovoltaic (HPPV) technology[1,2] has received rapidly increasing interest from the emerging solar-cell community due to its record increase in powerconversion efficiency (PCE) during the last 5 years
Hybrid perovskites can be synthesized in solution at low temperature from common materials
Recent experimental and theoretical studies focused on the device architecture and performance,[9−11] different hybrid-perovskite compositions,[5,12−14] the large diffusion-lengths and low recombination rate,[7,8,15,16] the current−voltage hysteresis,[5,17−19] and the stability of hybrid perovskites materials.[20−22] The microstructure of MAPbI3, which is crucial for the initial stages of the light-to-energy conversion process in particular the generation and separation of an electron−hole pair, has received little attention