(559h) Understanding the Structural Stability of Perovskites for Solar Energy Harvesting Using Molecular Dynamics Simulations in Comparison to Experiments

Perovskites have outstanding promise as light harvesting materials in solar cells due to their great optoelectronic properties and easy fabrication methods [1]. Power conversion efficiency (PCE) of perovskite solar cells (PSCs) has improved from 2.2% to approximately 25.2% in the last 15 years [2]. Thus, perovskites have become one of the most rapidly evolving and promising candidates for next-generation photovoltaics. However, there are currently major issues in the development and commercialization of high efficiency solar cells as multiple environmental factors (e.g., temperature, light, humidity) affect their stability and performance [3]. The perovskite’s crystal lattice dynamics in the form of octahedral distortions, phase transitions, exciton delocalization, and halide ion migration of perovskites directly impacts the efficiency and long-term stability of PSCs [4]. These challenges can be addressed by making specific chemical modifications to the system and simulating the lattice dynamics using molecular dynamics (MD) simulations and combinations with quantum mechanical methods. However, to-date no validated force fields are available. Some reactive force fields (ReaxFF) have been explored for perovskites; however, the parameters are not interpretable, have low accuracy, and do not contain validated parameters for lead or tin [5,6].

The INTERFACE Force Field (IFF) is designed to calculate intermolecular potentials and dynamics for organic and inorganic systems up to the large nanometer scale in high accuracy. IFF’s parameter development builds on reproducible descriptions of chemical bonding via atomic charges, interpretability, and consistency of parameters for compounds across the periodic table. The parameters employ the same harmonic energy expressions as AMBER, CHARMM, OPLS-AA, CVFF, and PCFF/COMPASS force fields (both 12-6 and 9-6 LJ potentials) and standard combination rules [7]. IFF adds-on to these existing methods by increasing the parametrization for a vast range of chemical compounds and including bonding and nonbonding interactions [8]. Parameters for macroscale physical and chemical properties have been individually validated in comparison to experimental measurements, therefore eliminating large percent errors and discrepancies between computed and measured results of up to 2 orders of magnitude [7].

In this work, we introduce IFF parameters for several perovskites using a simple and consistent parameter set. The compounds include CsPbI₃, CsSnI₃, and various ammonium perovskites. Extensions to other metals, halides, and cations are straightforward. Details of the parameters, the derivation, and performance in reproducing lattice parameters (typically <0.5% deviation from experiment), surface energies (in comparison to high-level QM data), vibration, and mechanical properties (typically <10% deviation from experiment) will be shared. Structural and surface properties can be studied for realistic nanometer scale structures and electrode interfaces, including oxides, metals, and electrolytes, including the entire compound space of IFF and that of biomolecular and organic force fields (e.g., CHARMM, AMBER, OPLS, PCFF).

The femtosecond resolved dynamics up to microseconds allows to examine the challenge of octahedral distortions in the lattice and its impacts on band gaps and photovoltaic performance. In addition, it is easy to analyze the effects of chemical modifications steric effects, and changes in temperature on the lattice dynamics and phase stability. Orbital interactions and electronic structure effects may also be represented going forward through incorporation of additional details of the electronic structure. The MD simulations are a million times faster than QM based calculations and can be combined at a local scale to study exciton delocalization and the operating mechanisms in perovskite solar cells.

Simulations using the force field parameters (IFF) and surface models for perovskites help bridging the gap between perovskite theory, computational tools, experimental design, and applications. Exciting opportunities also include the application of in-silico methods to discover chiral hybrid halide perovskites. Furthermore, we plan to add the IFF parameters to CHARMM-GUI and make them available in OpenKIM for cross-platform benchmarking. Overall, this aims to benefit the development of novel technologies for light-harvesting materials as well as tailored perovskite materials with piezoelectric, ferroelectric, and spintronic properties for renewable energy solutions.

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