Water in hybrid perovskites: Bulk MAPbI3 degradation via super-hydrous state

Arvin Kakekhani,Radhika N Katti,Andrew M Rappe

doi:10.1063/1.5087290

Abstract

Here, first-principles density functional theory calculations are presented which reveal how water incorporation in hybrid halide perovskite [CH3NH3]PbI3 (MAPbI3) catalyzes the phase transition to the ([CH3NH3]PbI3.H2O edge-sharing) monohydrate (colorless) phase, eliminating its favorable photovoltaic properties. First, fundamental chemical and electrostatic interactions between water and each component of MAPbI3 are analyzed, demonstrating their dependence on water concentration. Second, the energetics of incorporated water is explored, leading to the discovery of spontaneous phase segregation into dry regions and regions with more than one water per formula unit—termed the “super-hydrous state.” Third, the properties of the super-hydrous state are analyzed, including the acceleration of octahedron breaking and rearrangement by the high water density. This reveals the phase transformation to be a bulk process, initiated at the super-hydrous regions. This paper concludes with a discussion of how this super-hydrous model explains disparate recent experimental observations concerning the water-induced transition from (black) perovskite to edge-sharing PbI2 (yellow) phase.

Highlights

Despite the advances in understanding the instability of perovskite MAPbI3, there are still important unsolved questions. We address four such questions: (i) What is the nature of bonding between water and MAPbI3, and what are the fundamental forces driving it? To answer this, we perform density functional theory (DFT)-based electronic structure analysis along with appropriately designed numerical experiments. (ii) How does the energetics of water bonding in MAPbI3 help us to determine the arrangement of water in the solid? To answer this, we compute the concentration dependence of water bonding geometries and energetics, varying the (H2O:MA)
We delve into the nature of water bonding in MAPbI3
Here we are not concerned with the semantics, the bonding between MA+ and water is fundamentally different from a canonical hydrogen bond, e.g., O–H⋯O bonds in a water network

Summary

Introduction

Organometal halide perovskites (OMHPs) have been a center of attention in the renewable energy community due to their rapid increase in efficiency as photovoltaic (PV) materials,[1,2,3,4,5,6,7,8,9,10] LEDs, and high-gain photodetectors.[11,12,13,14,15,16,17,18,19] Low-cost and low-temperature production methods, high optical adsorption over a broad solar spectrum, sharp band edge, tunable bandgap, and long chargecarrier lifetime and diffusion length have made this class of materials exceptionally attractive.[20,21,22,23,24,25,26,27,28,29] Since 2009, when the top reported power conversion efficiency (PCE) was 3.8%, further development of hybrid perovskite solar cells (PSCs) has enabled efficiencies higher than 23%.30–37 Methylammonium lead iodide (MAPbI3) is one OMHP that has been widely studied.[38,39,40,41,42,43] Due to optimal bandgap and long carrier diffusion length, MAPbI3 is a promising material for PV technology.[29,44,45,46,47,48] It consists of a perovskite (ABX3) organic-inorganic hybrid lattice, where the methylammonium (MA) organic A cations sit in the cages formed by the inorganic (BX3) lead iodide corner-sharing octahedral lattice. Methylammonium lead iodide (MAPbI3) is one OMHP that has been widely studied.[38,39,40,41,42,43] Due to optimal bandgap and long carrier diffusion length, MAPbI3 is a promising material for PV technology.[29,44,45,46,47,48] It consists of a perovskite (ABX3) organic-inorganic hybrid lattice, where the methylammonium (MA) organic A cations sit in the cages formed by the inorganic (BX3) lead iodide corner-sharing octahedral lattice. The polarity of the methylammonium molecule and its interaction with the inorganic lattice contribute to the favorable optical and electronic properties of the material which, in turn, make MAPbI3 a more efficient PV.[38,49,50,51,52,53,54,55]

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Results

Conclusion