Role Of Organic Cations Research Articles

Role of organic cations in tuning the exciton binding energy of 2D lead iodide perovskite derivatives

Control over the exciton binding energy (Eb) in two-dimensional (2D) perovskites can enable their application in a broad range of optoelectronic technologies. To investigate the effects of organic cations on the excitonic properties of 2D perovskites, films of lead iodide hybrids basing on four structurally similar cations of phenylethylammonium (PEA), 4-florophenylethylammonium (FPEA), 4-chlorophenylethylammonium (ClPEA) and 4-bromophenylethylammonium (BrPEA) were prepared and their optical properties were characterized. Results show that both the perovskite layer distortion and exciton absorption/emission peak do not vary too much by the substitution of halogen atom in the para H of phenylethyl cation. Further examination of the temperature-dependent exciton emission properties reveals that the thermal activation energy Ea, which can also be regarded as exciton binding energy in our work, can be tuned over ca. 170 meV within the set of hybrids. It is found that exciton binding energy is sensitive to the identity of organic cations and to the resulting PbI6 octahedron distortion. Among the compounds considered, FPEA based perovskite has the biggest PbI6 octahedron distortion and the largest calculated Ea of 330 meV, while the Br-PEA based perovskite shows a smaller octahedron distortion and the lowest calculated Ea. We believe that an enhanced polarizability of organic cations may be responsible for the observed decrease in octahedra distortion and Ea for the p-halophenylethylammonium lead iodide perovskite derivatives.

Large Cation Engineering in Organic Antimony Halides for Low‐Loss Active Waveguide

AbstractActive waveguides have attracted growing attention as a basic component of miniaturized and integrated photonic devices. However, most optical waveguide materials suffer from severe self‐absorption, which greatly increases the optical loss. Therefore, developing highly efficient red‐emitting materials with large Stokes shift and negligible self‐absorption is of great interest. Herein, this work proposes a strategy to achieve both large Stokes shifts and high photoluminescence quantum yield (PLQY) in organic metal halide via large cation engineering. Two zero‐dimensional antimony(III) chlorides, (TMA)2SbCl5 and (TMAA)2SbCl5, are designed and synthesized by screening the ligands with different cationic volumes. Experimental and theoretical results reveal that the large cation can promote the self‐trapped excitons emission and improve stability. Benefiting from the large Stokes shift (291 nm) and high PLQY (98.3%), the as‐prepared (TMAA)2SbCl5 microplate exhibits efficient active waveguide with low loss coefficient of 6.07 × 10−3 dB µm−1. This work not only develops a novel active waveguide material, but also provides insights into the role of organic cation, which will provide new inspiration for the exploitation of excellent luminescent metal halides and photonic devices.

The role of organic cations as additives in photovoltaic perovskites.

The design of additives for perovskite-based solar cells seeks to improve the balance between stability and power conversion efficiency. Organic molecules such as theophylline, theobromine and caffeine (xanthines) have proved to be a good engineering solution. As an alternative, we present a first-principles study of the use of organic cations as additives. These cations are obtained when the free nitrogen of the imidazole unit of the aforementioned molecules is quaternized. We have found that the interaction between the organic cations and the MAPbI3 perovskite surface is stronger compared to the organic molecules. The Pb-O and I-H bonds of the interface dominated these interactions. In addition, organic cations showed higher charge transfer through the interface and shallow states that are harmless and could improve the charge carrier mobility. These characteristics show that quaternized xanthines should be a promising additive for perovskite materials in photovoltaic applications.

Crystalline Phases Regulate Electronic Trap States at Defective Surfaces of Lead Halide Perovskites.

The role of organic cations in A-sites of lead halide perovskites (LHPs) on carrier dynamics has been debated in an effort to understand the remarkable properties in these materials. However, the change of A-site species in LHPs often leads to the variation of crystalline phases at room temperature. Herein, we combine density functional theory (DFT) and time-dependent DFT methods to study electron traps in CH3NH3PbI3 which exhibits different structural phases with temperature and in APbBr3 [A = CH3NH3, CH(NH2)2, or Cs] with their crystalline phases at room temperature. Regardless of halide species, electron traps arising from halide vacancies at surfaces are spatially localized in tetragonal phase and turn to be rather delocalized in orthorhombic and cubic phases. The reason is revealed by analyzing the projected p orbitals of Pb atoms at conduction band edges, providing a novel strategy of healing surface defects to improving the performances of the LHP solar cells.

Rashba Splitting in Two Dimensional Hybrid Perovskite Materials for High Efficient Solar and Heat Energy Harvesting.

The physical properties of two-dimensional (2D) lead halide based hybrid perovskites are quite exciting and challenging. Further, the role of organic cations in 2D perovskites is still in a debate. We investigated layered (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 2D Ruddlesden-Popper (2DRP) phase (M1) and 2D derivative of CH3NH3PbI3 (M2) using density functional theory. The spin orbit coupling mediates the significantly large Rashba splitting energy of 328.5 meV for M2, which is higher than earlier 2D hybrid perovskites. At the picosecond time scale, the dynamical Rashba effect was observed due to organic and inorganic cation dynamics. Two step absorption suggests an indirect optical gap of 2.38 and 2.15 eV for M1 and M2, respectively and solar performance depicts excellent power conversion efficiency of 14.92% and 19.75% for M1 and M2, respectively. For the first time, we explored the thermoelectric properties of 2D hybrid perovskites and perceived high power factor for p-type doping in M2. Our findings suggest that these novel 2D perovskites have the potential to be used in solar and heat energy harvesting.

Role of organic cation in modern lead-based perovskites

Hybrid organic-inorganic metal perovskites (ABX3, A- organic cation, B-metal ion, X-halide ion) have shown significant potential as thin-film solar cells or Dye solar cells (DSC) materials. Using a time-dependent density functional theory (TDDFT) study, this work explores the role that the central organic cation plays in defining the optoelectronic and physical properties of metal perovskites. Cubic phases of CH3NH3PbI3, CH3NH3PbCl3, CH3NH3PbICl2, and CH3NH3PbI2Cl perovskite structures are investigated using TDDFT with Gaussian 16. Electronic and optical properties, including transition energy, band gap, IR and absorption spectra and vibrational frequencies are examined. It is observed that the organic cation reduces the unit cell volume of the outer metal halide structure from approximately 9–20% for different perovskite materials. The peak optical absorption also correspondingly decreases by a significant margin, ranging from about 40–60%.

Cocatalysis: Role of Organic Cations in Oxygen Evolution Reaction on Oxide Electrodes.

Cocatalysis is a promising approach toward enhanced electrocatalytic activity. We report such synergic catalysis involving organic xanthylium-based catalyst, Xan2+, and oxides formed on the electrode surface. The oxygen evolution reaction (OER) was observed on some working electrodes (gold, platinum, glassy carbon, boron-doped diamond), while others (titanium and fluorine-doped tin oxide) exhibited no OER activity. On the basis of experimental data and supported by calculations, we propose a mechanism in which oxidized Xan2+ activates electrode toward the rate-determining O-O bond formation. In light of our findings, efficient OER electrocatalysis can be achieved using materials that strongly bind oxygen species and electron-deficient organic cations.

Organic Cation‐Dependent Degradation Mechanism of Organotin Halide Perovskites

The applications of organotin halide perovskites are limited because of their chemical instability under ambient conditions. Upon air exposure, Sn2+ can be rapidly oxidized to Sn4+, causing a large variation in the electronic properties. Here, the role of organic cations in degradation is investigated by comparing methylammonium tin iodide (MASnI3) and formamidinium tin iodide (FASnI3). Through chemical analyses and theoretical calculations, it is found that the organic cation strongly influences the oxidation of Sn2+ and the binding of H2O molecules to the perovskite lattice. On the one hand, Sn2+ can be easily oxidized to Sn4+ in MASnI3, and replacing MA with FA reduces the extent of Sn oxidation; on the other hand, FA forms a stronger hydrogen bond with H2O than does MA, leading to partial expansion of the perovskite network. The two processes compete in determining the material's conductivity. It is noted that the oxidation is a difficult process to prevent, while the water effect can be largely suppressed by reducing the moisture level. As a result, FASnI3‐based conductors and photovoltaic cells exhibit much better reproducibility as compared to MASnI3‐based devices. This study sheds light on the development of stable Pb‐free perovskite optoelectronic devices through new material design.

ChemInform Abstract: Nanoscale Polyoxometalate‐based Inorganic/Organic Hybrids

AbstractReview: 107 refs.

Role of organic cation mediated transport and oxidative stress in the synephrine induced-toxicity in freshly isolated cardiomyocytes

Role of organic cation mediated transport and oxidative stress in the synephrine induced-toxicity in freshly isolated cardiomyocytes