Understanding the Influence of Lattice Composition on the Photocatalytic Activity of Defect‐Pyrochlore‐Structured Semiconductor Mixed Oxides

The defect‐pyrochlore‐structured photocatalyst CsTaWO₆ is an ideal starting material for anion doping from the gas phase, and is known to be highly active for solar hydrogen generation under simulated sunlight without co‐catalysts. To investigate the active site of CsTaWO₆ for hydrogen generation and to understand the effects of the two d⁰ elements in the compound, systematic and successive element substitution of tantalum and tungsten on the crystallographic 16c sites of the starting material has been performed. Substituting lattice tantalum with niobium hardly changes the band gap of the resulting compounds CsTa_{(1 ?x)}Nb _x WO₆, but the photocatalytic activity for hydrogen generation and oxidation reactions is strongly influenced. By investigating the surface reactivity toward adsorption, surface effects altering the activity are identified. In contrast, substituting lattice tungsten with molybdenum reduces the band gap of CsTaWO₆ into the visible‐light range. Materials containing Mo are however not able to generate hydrogen anymore, due to the altered conduction band positions proven by density functional theory calculations. CsTaMoO₆ exhibits a band gap of 2.9 eV and evolves oxygen efficiently under UV light irradiation after CoPi co‐catalyst deposition, and even under visible light small amounts of oxygen.     

Corrosion effects in thin‐film photovoltaic modules

Electrochemical corrosion effects can occur in thin‐film photovoltaic (PV) modules that are fabricated on tin‐oxide‐coated glass when operating at high voltages and at elevated temperatures in a humid climate. The current study shows that this corrosion is associated with a delamination of the tin oxide layer from the glass, which is caused by sodium accumulation near the interface between the tin oxide and the glass and by the ingression of moisture into the PV module from the edges. This corrosion in thin‐film PV modules can be significantly reduced by altering the growth conditions of the tin oxide or by using zinc oxide as a transparent conductive oxide electrode. Copyright © 2003 John Wiley & Sons, Ltd.     

Probing Individual Layers in Functional Oxide Multilayers by Wavelength‐Dependent Raman Scattering

The integration of functional oxides on silicon requires the use of complex heterostructures involving oxides of which the structure and properties strongly depend on the strain state and strain‐mediated interface coupling. The experimental observation of strain‐related effects of the individual components remains challenging. Here, a Raman scattering investigation of complex multilayer BaTiO₃/LaNiO₃/CeO₂/YSZ thin‐film structures on silicon is reported. It is shown that the Raman signature of the multilayers differs significantly for three different laser wavelengths (633, 442, and 325 nm). The results demonstrate that Raman scattering at various wavelengths allows both the identification of the individual layers of functional oxide multilayers and monitoring of their strain state. It is shown that all of the layers are strained with respect to the bulk reference samples, and that strain induces a new crystal structure in the embedded LaNiO₃. Based on this, it is demonstrated that Raman scattering at various wavelengths offers a well‐adapted, non‐destructive probe for the investigation of strain and structure changes, even in complex thin‐film heterostructures.     

Mobility Enhancement in Solution‐Processed Transparent Conductive Oxide TFTs due to Electron Donation from Traps in High‐k Gate Dielectrics

High‐mobility ZnO thin films are deposited onto solution‐processed ZrO₂ dielectrics in order to investigate the large differences between experimental field‐effect mobility values obtained when transparent conductive oxide (TCO) materials are deposited onto high‐k dielectrics as opposed to thermally grown SiO₂. Through detailed electrical characterization, the mobility enhancement in ZnO is correlated to the presence of electron traps in ZrO₂ serving to provide an additional source of electrons to the ZnO. Furthermore, as a consequence of the general tendency for solution‐processed high‐k dielectrics to exhibit similar behavior, the broad applicability is suggested to other TCO/high‐k material combinations in agreement with experimental observations.     

Shift of the Branching Point of the Side‐Chain in Naphthalenediimide (NDI)‐Based Polymer for Enhanced Electron Mobility and All‐Polymer Solar Cell Performance

The branching point of the side‐chain of naphthalenediimide (NDI)‐based conjugated polymers is systematically controlled by incorporating four different side‐chains, i.e., 2‐hexyloctyl (P(NDI1‐T)), 3‐hexylnonyl (P(NDI2‐T)), 4‐hexyldecyl (P(NDI3‐T)), and 5‐hexylundecyl (P(NDI4‐T)). When the branching point is located farther away from the conjugated backbones, steric hindrance around the backbone is relaxed and the intermolecular interactions between the polymer chains become stronger, which promotes the formation of crystalline structures in thin film state. In particular, thermally annealed films of P(NDI3‐T) and P(NDI4‐T), which have branching points far away from the backbone, possess more‐developed bimodal structure along both the face‐on and edge‐on orientations. Consequently, the field‐effect electron mobilities of P(NDIm‐T) polymers are monotonically increased from 0.03 cm² V⁻¹ s⁻¹ in P(NDI1‐T) to 0.22 cm² V⁻¹ s⁻¹ in P(NDI4‐T), accompanied by reduced activation energy and contact resistance of the thin films. In addition, when the series of P(NDIm‐T) polymers is applied in all‐polymer solar cells (all‐PSCs) as electron acceptor, remarkably high‐power conversion efficiency of 7.1% is achieved along with enhanced current density in P(NDI3‐T)‐based all‐PSCs, which is mainly attributed to red‐shifted light absorption and enhanced electron‐transporting ability.     

Exploring the Leidenfrost Effect for the Deposition of High‐Quality In2O3 Layers via Spray Pyrolysis at Low Temperatures and Their Application in High Electron Mobility Transistors

The growth mechanism of indium oxide (In₂O₃) layers processed via spray pyrolysis of an aqueous precursor solution in the temperature range of 100–300 °C and the impact on their electron transporting properties are studied. Analysis of the droplet impingement sites on the substrate's surface as a function of its temperature reveals that Leidenfrost effect dominated boiling plays a crucial role in the growth of smooth, continuous, and highly crystalline In₂O₃ layers via a vapor phase‐like process. By careful optimization of the precursor formulation, deposition conditions, and choice of substrate, this effect is exploited and ultrathin and exceptionally smooth layers of In₂O₃ are grown over large area substrates at temperatures as low as 252 °C. Thin‐film transistors (TFTs) fabricated using these optimized In₂O₃ layers exhibit superior electron transport characteristics with the electron mobility reaching up to 40 cm² V^?1 s^?1, a value amongst the highest reported to date for solution‐processed In₂O₃ TFTs. The present work contributes enormously to the basic understanding of spray pyrolysis and highlights its tremendous potential for large‐volume manufacturing of high‐performance metal oxide thin‐film transistor electronics.     

Side Chain Optimization of Naphthalenediimide–Bithiophene‐Based Polymers to Enhance the Electron Mobility and the Performance in All‐Polymer Solar Cells

Tuning the side chains of conjugated polymers is a simple, yet effective strategy for modulating their structural and electrical properties, but their impact on n‐type conjugated polymers has not been studied extensively, particularly in the area of all‐polymer solar cells (all‐PSCs). Herein, the effects of side chain engineering of P(NDI2OD‐T2) polymer (also known as Polyera Activink N2200) are investigated, which is the most widely used n‐type polymer in all‐PSCs and organic field‐effect transistors (OFETs), on their structural and electronic properties. A series of naphthalenediimide‐bithiophene‐based copolymers (P(NDIR‐T2)) is synthesized, with different side chains (R) of 2‐hexyldecyl (2‐HD), 2‐octyldodecyl (2‐OD), and 2‐decyltetradecyl (2‐DT). The P(NDI2HD‐T2) exhibits more noticeable crystalline behaviors than P(NDI2OD‐T2) and P(NDI2DT‐T2), thereby facilitating superior 3D charge transport. For example, the P(NDI2HD‐T2) shows the highest OFET electron mobility (1.90 cm² V^?1 s^?1). Also, a series of all‐PSCs is produced using different electron donors of PTB7‐Th, PTB7, and PPDT2FBT. The P(NDI2HD‐T2) based all‐PSCs produce much higher power conversion efficiency (PCE) irrespective of the electron donors. In particular, the PTB7‐Th:P(NDI2HD‐T2) forms highly ordered, strong face‐on interchain stackings, and has better intermixed bulk‐heterojunction morphology, producing the highest PCE of 6.11% that has been obtained by P(NDIR‐T2) based all‐PSCs to date.