Electropolymerization of an EDOT-Quinoxaline Monomer for Green Electrochromic Thin Films and Devices.

In this study, we present a 5,8-bis(3,4-ethylenedioxythiophene)quinoxaline monomer with two 4-(octyloxy)phenyl side chains (EDOTPQ) that can be electropolymerized on ITO glass in standard electrolytes containing lithium salts and propylene carbonate as solvent. The electrochemically deposited PEDOTPQ layers show very good adhesion and homogeneity on ITO. The green-colored polymer thin films exhibit promising electrochromic (EC) properties and are interesting for applications such as adaptive camouflage, as well as smart displays, labels, and sensors. Novel organic-inorganic (hybrid) EC cell configurations were realized with Prussian blue (PB) or titanium-vanadium oxide (TiVOx) as ion storage electrodes, showing a highly reversible and fast color change from green to light yellow.

Enhanced Electroluminescence via a Nanohybrid Material Consisting of Aromatic Ligand-Modified InP Quantum Dots and an Electron-Blocking Polymer as the Single Active Layer in Quantum Dot-LEDs.

Electron overcharge causes rapid luminescence quenching in the quantum dot (QD) emission layer in QD light-emitting diodes (QD-LEDs), resulting in low device performance. In this paper we describe the application of different aromatic thiol ligands and their influence on device performance as well as their behavior in combination with an electron blocking material (EBM). The three different ligands, 1-octanethiol (OcSH), thiophenol (TP), and phenylbutan-1-thiol (PBSH), were introduced on to InP/ZnSe/ZnS QDs referred to as QD-OcSH, QD-TP, and QD-PBSH. PBSH is in particular applied as a ligand to improve QD solubility and to enhance the charge transport properties synergistically with EBM probably via π-π interaction. We synthesized poly-[N,N-bis[4-(carbazolyl)phenyl]-4-vinylaniline] (PBCTA) and utilized it as an EBM to alleviate excess electrons in the active layer in QD-LEDs. The comparison of the three QD systems in an inverted device structure without the application of PBCTA as an EBM shows the highest efficiency for QD-PBSH. Moreover, when PBCTA is introduced as an EBM in the active layer in combination with QD-PBSH in a conventional device structure, the current efficiency shows a twofold increase compared to the reference device without EBM. These results strongly confirm the role of PBCTA as an EBM that effectively alleviates excess electrons in the active layer, leading to higher device efficiency.

Probing the pathways of free charge generation in organic bulk heterojunction solar cells.

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Orthogonal Solution-Processable Electron Transport Layers Based on Phenylpyridine Side-Chain Polystyrenes.

This article reports the synthesis and characterization of a series of polystyrenes containing phenylpyridine moieties as side chains. Methanol solubility of these polymers is induced if the relative pyridine content of the overall aromatic units of the side chains is larger than 0.5. This allows for orthogonal processing of multilayered organic light emitting diode (OLED) stacks fabricated from solutions. The polymers show high thermal stability due to their glass-transition temperatures ranging from 136 up to 247 °C. High triplet energies of up to 2.8 eV are obtained by combination of the side-chain aromatic rings in the meta position. The use of the methanol soluble side-chain polymers as an electron transport layer (ETL) is demonstrated in an orthogonally processed three-layer green-emitting OLED stack. When depositing the ETL from methanol, redissolution of the underlying emission layer does not occur.

Fabrication and Characterization of Surfaces Modified with Carboxymethylthio Ligands for Chelate-Assisted Trapping of Copper.

The metal ion chelating property was conferred onto silicon (Si) and gold (Au) surfaces by direct electrografting of the 4-[(carboxymethyl)thio]benzenediazonium cation (4-CMTBD). Infrared spectroscopic ellipsometry showed the presence of characteristic phenyl and carbonyl vibrational bands on the functionalized surfaces as a proof of existence of surface-bound organic units of 4-[(carboxymethyl)thio]benzene, (4-CMTB). The loss of diazonium group (N≡N+) upon electrografting of 4-CMTBD was investigated using IR spectroscopy. A Faradaic efficiency of about 18.8-20.0% was realized in mass deposition experiments for grafting 4-CMTB on the Au surface using an electrochemical quartz crystal microbalance technique. Raman spectroscopy performed on the Si-(4-CMTB) surface after treatment with copper (Cu) ion solution provided evidence of metal ion chelation based on an observed v(Cu-O) peak at about 487 cm-1 and a v(Cu-S) signal at about 267 cm-1. The binding of Cu ions by the chelating ligands also caused a red shift of about 10 cm-1 in the Raman spectrum of the Si-(4-CMTB)-Cu surface within the spectral region, characteristic of the v(C-O) signal. X-ray photoelectron spectroscopy investigations showed indications of the Cu(II) ion species chelated by the surface-bound carboxymethylthio ligands. The functionalized surface, Si-(4-CMTB), constitutes an alternative metal ion chelating surface that may potentially be developed for applications in trace-level trapping of Cu ions.

Quantifying Charge Extraction in Organic Solar Cells: The Case of Fluorinated PCPDTBT.

We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.

Inkjet printed solar cell active layers based on a novel, amorphous polymer.

Organic solar cells are a favorable alternative to their inorganic counterparts because the functional layers of these devices can be processed with printing or coating on a large scale. In this study, a novel polymer was synthesized, blended with fullerene and deposited with inkjet printing for solar cell applications. Devices with printed layers were compared to those with spin coated films in order to evaluate inkjet printing as a thin film deposition method. Efficiency values of 3.7% were found for devices with inkjet printed or spin coated layers. Inkjet printing can be used to successfully process the active layers of organic solar cells consisting of novel polymers without sacrificing device performance.

Fluorinated copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells.

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC(70)BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC(70)BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.

Acrylate functionalized tetraalkylammonium salts with ionic liquid properties.

Acrylate functionalized ionic liquids based on tetraalkylammonium salts with terminal acrylates- and methylacrylates were synthesized. Melting points and ionic conductivity of twenty compounds in six groups were determined. Within one group the effect of three different counterions was investigated and discussed. The groups differ in cationic structure elements because of their functional groups such as acrylate and methacrylate, alkyl residues at the nitrogen and number of quaternary ammonium atoms within the organic cation. The effect of these cationic structure elements has been examined concerning the compiled parameters with a view to qualifying them as components for solid state electrolytes. The newly synthesized ionic liquids were characterized by NMR and FTIR analysis. The exchange of halide ions like bromide as counter ions to weakly coordinating [PF6]⁻, [OTf]⁻ or [TFSI]⁻ reduces the melting points significantly and leads to an ion conductivity of about 10⁻4 S/cm at room temperature. In the case of the dicationic ionic liquid, an ion conductivity of about 10⁻³ S/cm was observed.

Infrared spectroscopic study of the amidation reaction of aminophenyl modified Au surfaces and p-nitrobenzoic acid as model system.

We have investigated the fundamental amidation reaction by a model system consisting of an electrochemically functionalised Au surface by aminophenyl and 4-nitrobenzoic acid activated by EEDQ. The development of the NO(2) related stretching vibrations with time reveals that the amidation process is very slow at Au surfaces and is completed after about 2 days.