Measuring frontier orbital energy levels of OLED materials using cyclic voltammetry in solution

The operation of organic light emitting diodes (OLEDs) is governed by a range of material parameters, such as frontier orbital energy levels, charge carrier mobility and excitonic rate parameters. In state-of-the art numerical simulations of OLED devices, more than 30 parameters must be considered to describe the behavior of a multilayer device. Independent measurement techniques to reliably determine each material parameter individually are therefore highly desirable. While several techniques have been established in the OLED community to determine some of them, the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energy levels are not measured or reported on a regular basis, despite their significant influence on device performance. In this work, we show how cyclic voltammetry in solution can be used as a simple technique to measure the HOMO and LUMO energy levels of organic semiconductors. This easily performed experiment allows a fairly accurate estimation of the energy levels of the layers in a device stack. Cyclic voltammetry measurements of four typical OLED materials in solution are presented and their analysis is described in detail to encourage more such measurements in future OLED studies. Four distinctly different voltammograms were obtained, ranging from relatively ideal reversible behavior to a very non-ideal behavior, lacking electrochemical reverse reactions. Two methods for extracting the HOMO and LUMO energy levels from cyclic voltammetry are discussed and compared. The measured HOMO and LUMO levels compare well with reported values measured on thin films, showing that cyclic voltammetry in solution provides a viable means to determine this important, yet underinvestigated material property

Vacuum-annealing induces sub-surface redox-states in surfactant-structured α-Fe₂O₃ photoanodes prepared by ink-jet printing

Transparent nano-structured hematite (α-Fe2O3) films of approximately 550 nm thickness on tin-doped indium oxide (ITO) have been obtained conveniently by ink-jet printing of a Fe(NO3)3/Brij® O10 precursor ink and subsequent annealing at 500 °C in air. When illuminated with a blue LED (λ = 455 nm, ca. 100 mW cm−2), the hematite films exhibited photocurrents of up to 70 μA cm−2 at 0.4 V vs. SCE in 0.1 M NaOH electrolyte. Thermal annealing in vacuum at 500 °C for 2 h increased photocurrents more than three times to 230 μA cm−2 in agreement with previous literature reports for pure hematite materials. These results suggest that a simple ink-jetting process with surfactants is viable. The effects of vacuum-annealing on the photoelectrical properties of α-Fe2O3 films are discussed in terms of a sub-surface state templating hypothesis based on data gathered from photo-transients, field emission scanning electron microscopy, X-ray photoelectron spectroscopy analysis, X-ray diffraction, photocurrent spectra, and cyclic voltammetry

TRY plant trait database – enhanced coverage and open access

Plant traits - the morphological, anatomical, physiological, biochemical and phenological characteristics of plants - determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait‐based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits - almost complete coverage for ‘plant growth form’. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait–environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives

Photoelectrochemical Screening of Solar Cell Absorber Layers: Electron Transfer Kinetics and Surface Stabilization

edox electrolyte contacts offer a simple way of testing the photocurrent generation/collection efficiency in partially completed thin-film solar cells without the need to complete the entire fabrication process. However, the development of a reliable quantitative method can be complicated by the instability of the semiconductor/electrolyte interface. In the case of Cu(In,Ga)Se2 (CIGSe) solar cells, these problems can be overcome by using samples that have undergone the next processing step in solar cell fabrication, which involves chemical bath deposition of a thin (ca. 50 nm) CdS buffer layer. The choice of redox system is also critical. The frequently used Eu3+/2+ redox couple is not suitable for reliable performance predictions since it suffers from very slow electron transfer kinetics. This leads to the buildup of photogenerated electrons near the interface, resulting in electron–hole recombination. This effect, which can be seen in the transient photocurrent response, has been quantified using intensity-modulated photocurrent spectroscopy (IMPS). The study has demonstrated that the more oxidizing Fe(CN)63–/4– redox system can be used when a CdS buffer layer is deposited on the CIGSe absorber. The wide bandgap CdS acts as a barrier to hole injection, preventing decomposition of the CIGSe and formation of surface recombination centers. The IMPS response of this system shows that there is no recombination; i.e., electron scavenging is very rapid. It is shown that measurements of the external quantum efficiency made using the Fe(CN)63–/4– redox couple with CdS-coated CIGSe layers can provide reliable predictions of the short-circuit currents of the complete solar cells. Similar results have been obtained using CdS-coated GaAs layers, suggesting that the new approach may be widely applicable

Modulating the Reactivity of Electrode Surfaces by Electrostatic Assembly of Metal Nanoparticles and Quantum Dots

Charge transport phenomena in opto-electronic devices featuring functional polymers and nanostructured materials critically depend on the electronic communication between the building blocks and the metal contacts. The generation of ordered multilayer structures at electrode surfaces is often a key requirement to avoid electrically isolated (inactive) areas in the devices. This issue is particularly crucial in hybrid photovoltaic, light emitting and electrochromic systems. In the present contribution, the properties of electrode surfaces modified by electrostatic layer-by-layer methods are highlighted as a versatile approach for generating two- and three-dimensional assemblies of nanostructures. The connectivity between the nanoparticles and the electrode surface is probed by a variety of techniques including Kelvin probe, electrostatic force microscopy and electrochemical methods. The main characteristics of electrode surfaces modified by electrostatic self-assembly are illustrated with several examples involving metal nanostructures and CdTe quantum dots. ? Schweizerische Chemische Gesellschaft

Conductive-bridge memory cells based on a nano-porous electrodeposited GeSbTe alloy

International audienc

Structure and Band Edge Energy of Highly Luminescent CdSe_1–<i>x</i>Te_<i>x</i> Alloyed Quantum Dots

CdSe_1–<i>x</i>Te_<i>x</i> quantum dot (QD) alloys are characterized by high luminescence quantum yields and a strong band gap bowing as a function of the Se:Te ratio, featuring longer emission wavelengths than CdTe or CdSe dots of identical size. In this contribution, these properties are rationalized by examining the structure and band edge energy of CdSe_1–<i>x</i>Te_<i>x</i> as functions of <i>x</i>. The QDs were synthesized employing the “hot-injection” method, in the presence of either trioctylphosphine oxide (TOPO) or octadecene (ODE) as the Cd precursor solvent. Elementary analysis of the QDs indicated that TOPO plays a crucial role in tuning the content of Se in the alloys, as only traces of this element were found when using ODE. Detailed studies based on X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) revealed a high degree of complexity in the structure of the alloyed dots. The analysis concluded that the structure of the QDs was essentially wurtzite, although features associated with zinc blende can be seen due to the presence of stacking faults and to a small population of nanocrystals with cubic structure. More importantly, these studies reveal a nonlinear expansion of the effective lattice constant with increasing Te content. The valence band edge energy of the alloys in solution was estimated from the first oxidation potential measured by linear sweep voltammetry at Au microelectrodes. The results show that the valence band edge exhibits a very weak dependence on <i>x</i> for values below 0.5, indicating that the decrease in the optical band gap is mainly linked to a decrease in the conduction band edge energy. For <i>x</i> > 0.5, the conduction and valence band edges shift to higher values with an overall increase in the band gap. The experimental trends show, for the first time, that the characteristic red shift of the band gap with low to intermediate Te content is determined by relaxation of the lattice constant, whereas the contribution arising from the change in anion electronegativity becomes predominant for <i>x</i> > 0.5

Optoelectronic and spectroscopic characterization of vapour-transport grown Cu2ZnSnS4 single crystals

Single crystals of Cu2ZnSnS4 (CZTS) have been grown by iodine vapor transport with and without addition of NaI. Crystals with tin-rich copper-poor and with zinc-rich copper-poor stoichiometries were obtained. The crystals were characterized by single crystal X-ray diffraction, energy-dispersive X-ray spectroscopy, photocurrent spectroscopy and electroreflectance spectroscopy using electrolyte contacts as well as by spectroscopic ellipsometry, Raman spectroscopy and photoluminescence spectroscopy (PL)/decay. Near-resonance Raman spectra indicate that the CZTS crystals adopt the kesterite structure with near-equilibrium residual disorder. The corrected external quantum efficiency of the p-type crystals measured by photocurrent spectroscopy approaches 100% close to the bandgap energy, indicating efficient carrier collection. The bandgap of the CZTS crystals estimated from the external quantum efficiency spectrum measured using an electrolyte contact was found to be 1.64–1.68 eV. An additional sub-bandgap photocurrent response (Urbach tail) was attributed to sub bandgap defect states. The room temperature PL of the crystals was attributed to radiative recombination via tail states, with lifetimes in the nanosecond range. At high excitation intensities, the PL spectrum also showed evidence of direct band to band transitions at ∼1.6 eV with a shorter decay time. Electrolyte electroreflectance spectra and spectra of the third derivative of the optical dielectric constant in the bandgap region were fitted to two optical transitions at 1.71 and 1.81 eV suggesting a larger valence band splitting than predicted theoretically. The high values of the EER broadening parameters (192 meV) indicate residual disorder consistent with the existence of tail states