(704b) Toward Better Solar Cells: First Principles Understanding of the Water Instability of Hybrid Perovskites

Organometal halide perovskites (OMHPs), composed of organic molecular cations encaged in inorganic framework (anions), are a center of attention in the renewable energy community as photovoltaic (PV) materials, LEDs, and high-gain photodetectors. Low-cost and low-temperature production methods, high optical adsorption over a broad solar spectrum, tunable bandgap, and long charge carrier lifetime and diffusion length have made this class of materials exceptionally attractive. Since 2009, their power conversion efficiency (PCE) has grown from 3.8%, to above 23%. Methylammonium lead iodide (MAPbI₃) is one OMHP that has been widely studied. Moisture can significantly catalyze and accelerate the degradation of MAPbI₃ creating a prominent obstacle to the commercialization of MAPbI₃. Here, using first-principles density functional theory (DFT) calculations we reveal how water incorporation in MAPbI₃, catalyzes the phase transition to the (MAPbI₃.H₂O edge-sharing) monohydrate (colorless) phase, eliminating its favorable photovoltaic properties [1]. First, using extensive electronic structure analyses the fundamental chemical and electrostatic interactions between water and each component of MAPbI₃ (organic MA molecule and Inorganic PbI₃ octahedron) are investigated, demonstrating the nature of the bonding and its dependence on water concentration. Second, the energetics and kinetics 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 including the acceleration of octahedron breaking and rearrangement by the high water density are analyzed. We elucidate for the first time that the degradation is a bulk rather than surface process, initiated at the super-hydrous regions. We discuss how this bulk super-hydrous degradation model can explain disparate recent experimental observations concerning the water-induced transition from (black) perovskite to edge-sharing PbI₂ (yellow) phase, and allows one to design schemes to alleviate the water instability of not only MAPbI₃, but the bigger class of hybrid organic-inorganic perovskites, paving the way for their commercialization as very efficient solar cells.

In addition to the practical applications that such first principles understanding can have for this class of perovskite solar-cells; our findings can have broader impact: the fundamental knowledge gained on the nature of water (lone-pair) interaction with other molecular groups or surface/bulk binding sites [1,2] can be generally used for understanding and prediction of water--materials interactions, wetting or materials' moisture stability.

[1] Arvin Kakekhani, Radhika N. Katti, Andrew M. Rappe "Water in hybrid perovskites: Bulk MAPbI₃ degradation via super-hydrous state." APL Materials 7, 041112 (2019)

[2] Arvin Kakekhani, Luke T. Roling, Ambarish Kulkarni, Allegra A. Latimer, Hadi Abroshan, Julia Schumann, Hassan AlJama, Samira Siahrostami, Sohrab Ismail-Beigi, Frank Abild-Pedersen, Jens K. Nørskov. "Nature of Lone-Pair–Surface Bonds and Their Scaling Relations." Inorganic chemistry 57, no. 12 (2018): 7222-7238.