Effect of humidity on the orientational ordering of CH 3 NH 3 + in methylammonium lead iodide
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Ó Indian Academy of Sciences Sadhana(0123456789().,-volV)FT3 ](0123456789().,-volV)
Effect of humidity on the orientational ordering of CH3NH+3 in methylammonium lead iodide SHIVANI GROVER1
and UMESH V WAGHMARE2,*
1
Chemistry and Physics of Materials Unit, School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India 2 Theoretical Sciences Unit, School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India *Author for correspondence ([email protected]) MS received 30 January 2020; accepted 27 July 2020 Abstract. With high energy conversion efficiency and low-cost production, hybrid organic–inorganic perovskite solar cells (PSCs) have the potential to be alternative to silicon-based technology. However, there are concerns about their longterm stability and environmental friendliness, which must necessarily be addressed to enable large-scale commercialization of PSCs. Here, we use first-principles theory to determine and understand the effects of humidity on the T-dependent tetragonal to cubic structural transition in CH3NH3PbI3, which can impact the long-term stability of its properties. We show that ferroelectric vs. antiferroelectric structural ordering in CH3NH3PbI3 is influenced by humidity. Within first-principles density functional theory, we determine the lowest energy configurations of dipolar ordering in CH3NH3PbI3xH2O and effects of their interaction with H2O molecules. Developing a simple effective Hamiltonian to model these configurations, we use Monte Carlo simulations to determine temperature-dependent structural phase transitions in CH3NH3PbI3. We establish ferroelectric ordering in MAPbI3 at low temperature, and demonstrate that it changes to antiferroelectric ordering of MA? cations at x [ 0.2 in CH3NH3PbI3xH2O. Keywords.
1.
CH3NH3PbI3; phase transitions; ferroelectricity; humidity.
Introduction
Organic–inorganic hybrid ABX3 perovskites (where A is an organic cation, B is a divalent metal and X is a halide anion) have been a subject of intense research, since the report of first hybrid-based perovskite solar cell in 2009 [1]. This is because of a remarkable combination of properties they exhibit: high power conversion efficiency (*20%) [2], useful optoelectronic properties, such as high carrier mobility, long carrier diffusion lengths [3,4], high absorption coefficients, widely tuneable band gap (from 1.5 to 2.3 eV) [5,6] and low exciton binding energy [7,8]. One of the most promising hybrid perovskites is methylammonium lead iodide (CH3NH3PbI3, MAPI), which led to remarkable enhancement in power conversion efficiency of perovskite solar cells from 3.8 to 20% in less than a decade [2]. They are easy to synthesize with solution-based methods [9] enabling cost-effective and scalable production with outstanding optoelectronic properties. þ CH3 NHþ 3 cation (MA ) occupies the A site in the ABX3 perovskite structure and PbI6 units constitute BX6
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