Colloidal AInSe (A = K, Rb, Cs) Nanocrystals with Tunable Crystal and Band Structures.

Wide band gap AInSe (A = K, Rb, Cs) is an important interlayer material for improving the efficiency of Cu(In,Ga)(S,Se) (CIGS) solar cells. Compared to high-vacuum deposition and solid-state synthesis, a less energy-intensive method is of interest for its fabrication. Herein, we present the rapid, low-temperature colloidal synthesis of AInSe nanocrystals that opens a pathway for convenient solution processing.

Counterion Loss from Charged Surface-Bound Complexes Drives the Formation of Loosely Packed Monolayers.

The functionality of multicomponent self-assembled monolayers (SAMs) can be severely diminished by the segregation of like components into nanoscale domains, a process that maximizes favorable short-range intermolecular interactions. Here, we explore the use of a modular family of sulfur-functionalized metal bis(terpyridine) complexes ([M(tpy-R)](PF)) to prepare mixed SAMs, considering that the comparable structure, dimensions, and ionic composition of these species should render them interchangeable within the adsorbed surface layer. While surface voltammetry experiments show that these SAMs do exhibit compositions representative of their assembly solutions, they also suggest, in line with previous reports, that adjacent complexes in the monolayer are separated by a gap of ∼ 1 nm.

Revisiting Sub-Band Gap Emission Mechanism in 2D Halide Perovskites: The Role of Defect States.

Understanding the sub-band gap luminescence in Ruddlesden-Popper 2D metal halide hybrid perovskites (2D HaPs) is essential for efficient charge injection and collection in optoelectronic devices. Still, its origins are still under debate with respect to the role of self-trapped excitons or radiative recombination via defect states. In this study, we characterized charge separation, recombination, and transport in single crystals, exfoliated layers, and polycrystalline thin films of butylammonium lead iodide (BAPbI), one of the most prominent 2D HaPs.

Superior Phonon-Limited Exciton Mobility in Lead-Free Two-Dimensional Perovskites.

Tin-based two-dimensional (2D) perovskites are emerging as lead-free alternatives in halide perovskite materials, yet their exciton dynamics and transport remain less understood due to defect scattering. Addressing this, we employed temperature-dependent transient photoluminescence (PL) microscopy to investigate intrinsic exciton transport in three structurally analogous Sn- and Pb-based 2D perovskites. Employing conjugated ligands, we synthesized high-quality crystals with enhanced phase stability at various temperatures.

Tunneling-Driven Marcus-Inverted Triplet Energy Transfer in a Two-Dimensional Perovskite.

Quantum tunneling, a phenomenon that allows particles to pass through potential barriers, can play a critical role in energy transfer processes. Here, we demonstrate that the proper design of organic-inorganic interfaces in two-dimensional (2D) hybrid perovskites allows for efficient triplet energy transfer (TET), where quantum tunneling of the excitons is the key driving force. By employing temperature-dependent and time-resolved photoluminescence and pump-probe spectroscopy techniques, we establish that triplet excitons can transfer from the inorganic lead-iodide sublattices to the pyrene ligands with rapid and weakly temperature-dependent characteristic times of approximately 50 ps.

Excitonic Quantum Coherence in Light Emission from CsPbBr Metal-Halide Perovskite Nanocrystals.

The decay of excited states via radiative and nonradiative paths is well understood in molecules and bulk semiconductors but less so in nanocrystals. Here, we perform time-resolved photoluminescence (t-PL) experiments on CsPbBr metal-halide perovskite nanocrystals, with a time resolution of 3 ps, sufficient to observe the decay of both excitons and biexcitons as a function of temperature. The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures.

Controlling Charge Carrier Dynamics in Porphyrin Nanorings by Optically Active Templates.

Understanding the dynamics of photogenerated charge carriers is essential for enhancing the performance of solar and optoelectronic devices. Using atomistic quantum dynamics simulations, we demonstrate that a short π-conjugated optically active template can be used to control hot carrier relaxation, charge carrier separation, and carrier recombination in light-harvesting porphyrin nanorings. Relaxation of hot holes is slowed by 60% with an optically active template compared to that with an analogous optically inactive template.

Breaking the Condon Approximation for Light Emission from Metal Halide Perovskite Nanocrystals.

The idea that the electronic transition dipole moment does not depend upon nuclear excursions is the Condon approximation and is central to most spectroscopy, especially in the solid state. We show a strong breakdown of the Condon approximation in the time-resolved photoluminescence from CsPbBr metal halide perovskite semiconductor nanocrystals. Experiments reveal that the electronic transition dipole moment increases on the 30 ps time scale due to structural dynamics in the lattice.

Phenol as a Tethering Group to Gold Surfaces: Stark Response and Comparison to Benzenethiol.

Understanding the adsorption of organic molecules on metals is important in numerous areas of surface science, including electrocatalysis, electrosynthesis, and biosensing. While thiols are commonly used to tether organic molecules on metals, it is desirable to broaden the range of anchoring groups. In this study, we use a combined spectroelectrochemical and computational approach to demonstrate the adsorption of 4-cyanophenols (CPs) on polycrystalline gold.

Halide Vacancies Create No Charge Traps on Lead Halide Perovskite Surfaces but Can Generate Deep Traps in the Bulk.

Metal halide perovskites (MHPs) have attracted attention because of their high optoelectronic performance that is fundamentally rooted in the unusual properties of MHP defects. By developing an ab initio-based machine-learning force field, we sample the structural dynamics of MHPs on a nanosecond time scale and show that halide vacancies create midgap trap states in the MHP bulk but not on a surface. Deep traps result from Pb-Pb dimers that can form across the vacancy in only the bulk.

Formation of a P Ink from Elemental Red Phosphorus in a Thiol-Amine Mixture.

A P polyphosphide dianion ink was produced by the reaction of red phosphorus with a binary thiol-amine mixture of ethanethiol (ET) and ethylenediamine (en). The polyphosphide was identified by solution P NMR spectroscopy and electrospray ionization mass spectrometry. This solute was compared to the reaction products of white phosphorus (P) and other elemental pnictides in the same solvent system.

Breaking Phonon Bottlenecks through Efficient Auger Processes in Perovskite Nanocrystals.

The hot phonon bottleneck has been under intense investigation in perovskites. In the case of perovskite nanocrystals, there may be hot phonon bottlenecks as well as quantum phonon bottlenecks. While they are widely assumed to exist, evidence is growing for the breaking of potential phonon bottlenecks of both forms.

Point Defects in Two-Dimensional Ruddlesden-Popper Perovskites Explored with Ab Initio Calculations.

Two-dimensional Ruddlesden-Popper (RP) halide perovskites stand out as excellent layered materials with favorable optoelectronic properties for efficient light-emitting, spintronic, and other spin-related applications. However, properties often determined by defects are not well understood in these perovskite systems. This work investigates the ground state electronic structure of commonly formed defects in a typical RP perovskite structure by density functional theory.

Ag-Bi Charge Redistribution Creates Deep Traps in Defective CsAgBiBr: Machine Learning Analysis of Density Functional Theory.

Lead-free double perovskites hold promise for stable and environmentally benign solar cells; however, they exhibit low efficiencies because defects act as charge recombination centers. Identifying trap-assisted loss mechanisms and developing defect passivation strategies constitute an urgent goal. Applying unsupervised machine learning to density functional theory and nonadiabatic molecular dynamics, we demonstrate that negatively charged Br vacancies in CsAgBiBr create deep hole traps through charge redistribution between the adjacent Ag and Bi atoms.

Excited-State Properties of Defected Halide Perovskite Quantum Dots: Insights from Computation.

CsPbBr quantum dots (QDs) have been recently suggested for their application as bright green light-emitting diodes (LEDs); however, their optical properties are yet to be fully understood and characterized. In this work, we utilize time-dependent density functional theory to analyze the ground and excited states of the CsPbBr clusters in the presence of various low formation energy vacancy defects. Our study finds that the QD perovskites retain their defect tolerance with limited perturbance to the simulated UV-vis spectra.

Pb dimerization greatly accelerates charge losses in MAPbI: Time-domain ab initio analysis.

Metal halide perovskites constitute a new type of semiconducting materials with long charge carrier lifetimes and efficient light-harvesting. The performance of perovskite solar cells and related devices is limited by nonradiative charge and energy losses, facilitated by defects. Combining nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that charge losses depend strongly on the defect chemical state.

Unlocking the electronic genome of halogenobenzenes.

The drive to develop new organic materials for use in optoelectronic devices has created the need to understand the fundamental role functionalization plays concerning the electronic properties of conjugated molecules. Here density functional theory (DFT) is used to investigate how the HOMO-LUMO gaps of halogenobenzenes are affected as a function of substituent size, position, electronegativity, ionization potential, and polarizability. A detailed molecular orbital analysis is also provided.