We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)$_2$PbI$_4$) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the structure of the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.
Solution-processed quantum dots (QDs) have a high potential for fabricating low cost, flexible and large-scale solar energy harvesting devices. It has recently been demonstrated that hybrid devices employing a single monovalent cation perovskite solution for PbS QD surface passivation exhibit enhanced photovoltaic performance when compared to standard ligand passivation. Herein we demonstrate that the use of a triple cation Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3 perovskite composition for surface passivation of the quantum dots results in highly efficient solar cells, which maintain 96 % of th…
Previous theoretical calculations show azetidinium has the right radial size to form a 3D perovskite with lead halides [1], and has been shown to impart, as the A-site cation of ABX3 unit, beneficial properties to ferroelectric perovskites [2]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature where…
The effect of 14 organic cations on the optical properties of the lead iodine perovskite is analyzed. The electronic and optical properties are obtained using first principles. The absorption coefficients are split into inter-atomic species components in order to quantify all of the contributions. For energies close to the bandgaps, the main contribution is from the Pb-Pb intra-species transitions. For higher energy this contribution is still important in addition to I-I and Pb-I contributions, and to the 3 and 4 species term. Almost all absorption properties are qualitatively similar. Further…
We demonstrate the critical role of surface recombination in mixed-cation, mixed-halide perovskite, FA0.83Cs0.17Pb(I0.85Br0.15)3. By passivating non-radiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS) we transform these thin films. We demonstrate average minority carrier lifetimes > 4 μs, nearly single exponential monomolecular PL decays, and concomitantly high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and …
The scalability of highly efficient organic-inorganic perovskite solar cells (PSCs) is one of the remaining challenges of solar module manufacturing. Various scalable methods have been explored to strive for uniform perovskite films of high crystal quality on large-area substrates. However, each of these methods have individual drawbacks, limiting the successful commercialization of perovskite photovoltaics. Here, we report a fully scalable hybrid process, which combines vapor- and solution-based techniques to deposit high quality uniform perovskite films on large-area substrates. This two-ste…
Ionic migration in organo-halide perovskites plays an important role in operation of perovskite based solar cells and light emitting diodes. Despite the ionic migration being a reversible process, it often leads to worsening of perovskite based device performance, hysteresis in current-voltage characteristics, and phase segregation in mixed halide perovskites being as the most harmful effect. The reason is in dynamical band structure changes, which controllable engineering would solve one of the biggest challenges for development of light-emitting solar cells. Here we demonstrate controllable …
Nanoscale investigations by scanning probe microscopy have provided major contributions to the rapid development of organic-inorganic halide perovskites (OIHP) as optoelectronic devices. Further improvement of device level properties requires a deeper understanding of the performance-limiting mechanisms such as ion migration, phase segregation and their effects on charge extraction both at the nano- and macroscale. Here, we have studied the dynamic electrical response of Cs0.05(FA0.83-MA0.17)0.95PbI3-xBrx perovskite structures by employing conventional and microsecond time-resolved Kelvin prob…
Mixed-cation Cesium-Methylammonium lead halide perovskite (CsxMA1-xPbI3-xBrx) thin-films have been used to demonstrate stable and efficient perovskite devices. However, a systematic study of the Cs incorporation on the properties of the perovskite films has not been reported. In this report, Impact of Cesium incorporation on the minority carrier recombination lifetime of Cesium-Methylammonium lead halide perovskite thin-films is studied. The lifetime for the as-deposited perovskite films decreases with increasing concentration of cesium. However, mixed cation perovskite film is more stable, sh…
Exciton-polaritons, elementary excitations arising from the strong coupling regime between photons and excitons in insulators or semiconductors, represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Among different materials for room temperature polaritonic devices, two-dimensional (2D) layered perovskites have recently emerged as one of the promising candidates thanks to their prominent excitonic features at room temperature. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metas…
Adding a large A-site cation, such as dimethylammonium (DMA), to the perovskite growth solution has been shown to improve performance and long-term operational stability of halide perovskite solar cells. To better understand the origins of these improvements, we explore the changes in film structure, composition, and optical properties of a formamidinium (FA), Cs, Pb, mixed halide perovskite following addition of DMA to the perovskite growth solution in the ratio DMA0.1FA0.6Cs0.3Pb(I0.8Br0.2)3. Using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) we show that DMA is indeed incorpora…
Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr3 and organic-inorganic CH3NH3PbBr3 (MAPbBr3) perovskite sing…
Enhanced Stability and Linearly Polarized Emission from CsPbI$_3$ Perovskite Nanoplatelets through A-site Cation Engineering
Sungkyunkwan University（成均馆大学）；University of Oxford（牛津大学）；Ulsan National Institute of Science and Technology (UNIST)（蔚山科学技术院）；Sun Yat-sen University（中山大学）
