Pyrrolo[1,2‑<i>a</i>]quinoxalines: Novel Synthesis via Annulation of 2‑Alkylquinoxalines

In an attempt to synthesize a novel homoleptic complex <b>3</b> from 2-methyl-3-phenylquinoxaline <b>1</b> and Ir(acac)₃ for application as a triplet emitter in OLEDs (organic light-emitting diodes) no cyclometalation was observed. Instead, an annulation to 1-methyl-4-phenylpyrrolo[1,2-<i>a</i>]quinoxaline <b>2</b> was observed. Since pyrroloquinoxalines are potentially bioactive and few paths for their synthesis are known, selected reactions and conditions were investigated, suggesting Ir(acac)₃ as catalyst and proving glycerol to be a reactant

Pyrrolo[1,2‑<i>a</i>]quinoxalines: Novel Synthesis via Annulation of 2‑Alkylquinoxalines

In an attempt to synthesize a novel homoleptic complex <b>3</b> from 2-methyl-3-phenylquinoxaline <b>1</b> and Ir(acac)₃ for application as a triplet emitter in OLEDs (organic light-emitting diodes) no cyclometalation was observed. Instead, an annulation to 1-methyl-4-phenylpyrrolo[1,2-<i>a</i>]quinoxaline <b>2</b> was observed. Since pyrroloquinoxalines are potentially bioactive and few paths for their synthesis are known, selected reactions and conditions were investigated, suggesting Ir(acac)₃ as catalyst and proving glycerol to be a reactant

A New Framework of a Heteroleptic Iridium(III)–Carbene Complex as a Triplet Emitting Material

A new framework for a dark red emitting heteroleptic failed cyclometalated iridium­(III) carbene complex as an intermediate with a short triplet lifetime has been synthesized. A crystal structure has been determined by X-ray structure analysis. The photophysical behavior of this complex has been studied experimentally through UV–vis spectroscopy and photoluminescence studies with a view toward organic light-emitting diode (OLED) applications. The determination of HOMO/LUMO levels has been carried out by differential pulse voltammetry

A New Framework of a Heteroleptic Iridium(III)–Carbene Complex as a Triplet Emitting Material

A new framework for a dark red emitting heteroleptic failed cyclometalated iridium­(III) carbene complex as an intermediate with a short triplet lifetime has been synthesized. A crystal structure has been determined by X-ray structure analysis. The photophysical behavior of this complex has been studied experimentally through UV–vis spectroscopy and photoluminescence studies with a view toward organic light-emitting diode (OLED) applications. The determination of HOMO/LUMO levels has been carried out by differential pulse voltammetry

Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide

Solution-processed nickel oxide (sNiO) is known to be an excellent charge-selective interlayer in optoelectronic devices. Its beneficial properties can be further enhanced by an oxygen plasma (OP) treatment. In order to elucidate the mechanism behind this improvement, we use infrared transmission and X-ray photoelectron spectroscopy to probe the bulk and surface properties of the sNiO. We find that increasing the annealing temperature of the sNiO not only increases the structural order of the material but also reduces the concentration of nickel hydroxide species present in the bulk and on the surface of the film. This results in a decrease of the work function, while an additional OP treatment raises the work function to between 5.5 and 5.6 eV. For all annealing temperatures investigated, the consequences of the OP treatment are identified as reactions of both NiO and β-Ni­(OH)₂ to form thin β-NiOOH phases in the first atomic layers. Our results emphasize the importance of understanding the correlation between the preparation and resulting properties of sNiO layers and provides further insight into the interpretation of interface properties of NiO

Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes

We study two types of water/alcohol-soluble aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE), for their suitability as electron injection layers in solution-processed blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy is used to determine the nominal thickness of the polymer layers while ultraviolet photoelectron spectroscopy is carried out to determine the induced work-function change of the silver cathode. The determined work-function shifts are as high as 1.5 eV for PEI and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal that homogeneous PEI and PEIE layers are present at nominal thicknesses of about 11 nm. Finally, we solution prepare blue emitting polymer-based OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous efficiency reaches 3 and 2.2 cd A^–1, whereas maximum luminance values are as high as 8000 and 3000 cd m^–2 for PEI and PEIE injection layers, respectively. The prepared devices show a comparable performance to Ca/Ag OLEDs and an improved shelf lifetime

Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites

The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high-quality infrared spectroscopic measurements of methylammonium lead halide perovskite (CH₃NH₃Pb­(I/Br/Cl)₃) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH₃⁺ stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites

Unraveling the Nanoscale Morphologies of Mesoporous Perovskite Solar Cells and Their Correlation to Device Performance

Hybrid solar cells based on organometal halide perovskite absorbers have recently emerged as promising class for cost- and energy-efficient photovoltaics. So far, unraveling the morphology of the different materials within the nanostructured absorber layer has not been accomplished. Here, we present the first visualization of the mesoporous absorber layer in a perovskite solar cell from analytical transmission electron microscopy studies. Material contrast is achieved by electron spectroscopic imaging. We found that infiltration of the hole transport material into the scaffold is low and inhomogeneous. Furthermore, our data suggest that the device performance is strongly affected by the morphology of the TiO₂ scaffold with a fine grained structure being disadvantageous

Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface

Conformal and Highly Luminescent Monolayers of Alq₃ Prepared by Gas-Phase Molecular Layer Deposition

The gas-phase molecular layer deposition (MLD) of conformal and highly luminescent monolayers of tris­(8-hydroxyquinolinato)­aluminum (Alq₃) is reported. The controlled formation of Alq₃ monolayers is achieved for the first time by functionalization of the substrate with amino groups, which serve as initial docking sites for trimethyl aluminum (TMA) molecules binding datively to the amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting formation of highly luminescent Alq₃ monolayers is afforded. The growth process and monolayer formation were studied and verified by in situ quartz crystal monitoring, optical emission and absorption spectroscopy, and X-ray photoelectron spectroscopy. The nature of the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces and porous structures with high surface areas, as demonstrated in this work for silica aerogels. The concept presented here paves the way to highly sensitive luminescent sensors and dye-sensitized metal oxides for future applications (e.g., in photocatalysis and solar cells)