Phase-Pure Quasi-2D Perovskite by Protonation of Neutral Amine

Phase control of low-dimensional metal-halide perovskites (LDPs) greatly affects their optoelectronic properties, and phase-pure LDPs are desirable to achieve efficient perovskite optoelectronic devices such as solar cells and light-emitting diodes. Herein, we introduce a method to obtain phase-pure LDP by using a neutral amine, cyclohexylmethyl amine (CHMA). The incorporation of CHMA into a formamidinium lead bromide (FAPbBr(3)) precursor solution leads to the protonation of the amine that allows the phase transition of 3D FAPbBr(3) to phase-pure quasi-2D perovskite (n = 2). For comparison, cyclohexylmethylammonium bromide (CHMABr), which is a conventional form of ammonium halide salt with the same organic moiety to the amine, is used, which resulted in a 2D perovskite (n = 1). The perovskite films fabricated by the two different methodologies are characterized. This study paves the way for further research on the realization of phase-pure perovskites and their relevant optoelectronic devices.

Fluorination of Organic Spacer Impacts on the Structural and Optical Response of 2D Perovskites

Pietro Caprioglio, Inés Garcia Benito, Giulia Grancini, Mohammad Khaja Nazeeruddin, Valentin Ianis Emmanuel Queloz

Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)(2)PbI4 and (Lf)(2)PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.

2020

Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites

Neha Arora, Ariadni Boziki, Mohammad Ibrahim Dar, Michael Graetzel, Gwénolé Jean Jacopin, Simone Meloni, Ursula Röthlisberger, Shaik Mohammed Zakeeruddin

Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2)(2)PbBr3, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (

2016

Molecular dynamics simulations of nucleation and phase transition in halide perovskites

Paramvir Ahlawat

Perovskite solar cells have emerged as the most promising cheaper photovoltaic technology. Besides solar cells, halide perovskites have a wide range of applications due to their remarkable optoelectronic properties. Starting from 3.8% in 2009, solar to the power conversion efficiency of perovskite solar cells is now >25%, exceeding market leader polycrystalline silicon. These advancements are mainly due to the improvement in their manufacturing process targeted to make better morphology. To this objective, tens of thousands of experimental studies are conducted where a wide range of additives, compositions, and processing conditions are optimized based on a trial and error methodology. Although this approach increased the efficiency, however, perovskite electronics suffer from the problem of stability and reproducibility. Therefore, to industrialize highly efficient perovskite electronics, one needs to understand better and rationalize their fabrication process, i.e., crystallization of halide perovskites. In this thesis, I perform molecular simulations to understand the formation process of halide perovskites. First, I try to lay out the atomic details of the nucleation process of perovskite from the solution. I find that nucleation proceeds in a two-step manner where an intermediate phase forms before the emergence of perovskite crystals from the solution. I observe that monovalent cations can play an essential role in the nucleation process. On this information, simulation-driven experiments are performed by experimental collaborators to demonstrate that the grain size and crystallinity can be tuned by only varying the concentration of monovalent cations in solution. I also attempted to study the effects of different solvents and pseudo-halide additives used to make high-efficiency solar cells. I calculated that by modulating the solute-solvent interactions, one could minimize raw material losses during the coating process. In comparison, pseudo-halide additives are found to passivate the ionic defects and may slow down the growth process that helps to improve the final morphology. Second, I study the solid-solid phase transition in the popular two-step process. I discover a new path to the low-temperature formation of highly efficient metastable cubic phase of formamidinium lead iodide perovskite. I first validate my simulations with in-situ experiments performed by experimental collaborators. Then simulations are employed to design a practical methodology to successfully make thin films of phase pure cubic formamidinium lead iodide at lower temperatures. Moreover, I study the intrinsic stability of this material. I calculate the energy barrier between the metastable photo-active phase and its photo-inactive thermodynamically stable hexagonal phase. I find that once formed; the photo-active phase might be kinetically stable. Additionally, I identify new polymorphs (perovskitoids) of halide perovskites and also study the structural insights of low-dimensional halide perovskites. Overall, I present a fundamental understanding of the crystallization of perovskites which is helpful to design better experiments to make high efficiency and stable perovskite electronics.