Praseodymium Silicate as a High‐k Dielectric Candidate: An Insight into the Pr2O3‐Film/Si‐Substrate Interface Fabricated Through a Metal–Organic Chemical Vapor Deposition Process

Praseodymium‐containing thin films have been deposited on Si(001) substrates by metal–organic chemical vapor deposition (MOCVD) from the Pr(tmhd)₃ (H‐tmhd = 2,2,6,6‐tetramethyl‐3,5‐heptanedione) precursor. The structural, compositional, and morphological film characterization has been investigated using X‐ray diffraction (XRD), angle‐resolved X‐ray photoelectron spectroscopy (AR‐XPS), and transmission electron microscopy (TEM). Detailed studies of the deposition parameters indicate that the MOCVD process is governed by a kinetic regime and that some reactive phenomena occur at the film/substrate interface, forming a praseodymium silicate layer. A possible explanation for interfacial interaction has been proposed, taking into account the diffusion of Si from the substrate towards the bulk and that of oxygen from the film surface toward the substrate/film interface. Finally, the electrical characterization of the praseodymium silicate layer has been carried out in order to evaluate its potential implementation as an alternative dielectric. Its dielectric constant has been evaluated to be ～ 8.     

Harnessing Structure–Property Relationshipsfor Poly(alkyl thiophene)–Fullerene Derivative Thin Filmsto Optimize Performance in Photovoltaic Devices

Nanoscale bulk heterojunction (BHJ) systems, consisting of fullerenes dispersed in conjugated polymers have been actively studied in order to produce high performance organic photovoltaics. How the BHJ morphology affects device efficiency, is currently ill‐understood. Neutron reflection together with grazing incidence X‐ray and neutron scattering and X‐ray photoelectron spectroscopy are utilized to gain understanding of the BHJ morphology in functional devices. For nine model systems, based on mixtures of three poly(3‐alkyl thiophenes, P3AT) (A = butyl, hexyl, octyl) blended with three different fullerene derivatives, the BHJ morphology through the film thickness is determined. It is shown that fullerene enrichment occurs at both the electrode interfaces after annealing. The degree of fullerene enrichment is found to strongly correlate with the short circuit current (J_SC ) and to a lesser degree with the fill factor. Based on these findings, it is demonstrated that by deliberately adding a fullerene layer at the electron transport layer interface, J_SC can be increased by up to 20%, resulting in an overall increase in power conversion efficiency of 5%.     

Evidence and Effect of Photogenerated Charge Transfer for Enhanced Photocatalysis in WO3/TiO2 Heterojunction Films: A Computational and Experimental Study

Semiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar‐to‐chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO₃/TiO₂ heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO₃ layer of vertically aligned nanorods that is then coated with a conformal layer of TiO₂. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO₃ layer into TiO₂. State‐of‐the‐art hybrid density functional theory and hard X‐ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO₃/TiO₂ interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.     

Solar‐Energy Conversion in TiO2/CuInS2 Nanocomposites

The search for low‐cost thin‐film solar cells, to replace silicon multi‐crystalline cells in due course, calls for new combinations of materials and new cell configurations. Here we report on a new approach, based on semiconductor nanocomposites, towards what we refer to as the three‐dimensional (3D) solar‐cell concept. Atomic layer chemical vapor deposition is employed for infiltration of CuInS₂ inside the pores of nanostructured TiO₂. In this way it is possible to obtain a nanometer‐scale interpenetrating network between n‐type TiO₂ and p‐type CuInS₂. X‐ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, scanning electron microscopy, transmission electron microscopy, and current–voltage measurements are used to characterize the nanostructured devices. The 3D solar cells obtained show photovoltaic activity with a maximum monochromatic incident photon‐to‐current conversion efficiency of 80 % and have an energy‐conversion efficiency of 4 %.     

Hybridized Nanowires and Cubes: A Novel Architecture of a Heterojunctioned TiO2/SrTiO3 Thin Film for Efficient Water Splitting

A unique morphology of SrTiO₃ nanocubes precipitated on TiO₂ nanowires is successfully synthesized in the form of a thin‐film heterojunctioned TiO₂/SrTiO₃ photocatalyst using facile hydrothermal techniques. The formation mechanisms of the synthesized photocatalysts are meticulously studied and described. Growth of SrTiO₃ single crystal nanocubes (≈50 nm in width) on anatase polycrystalline nanowires follows an in situ dissolution‐precipitation pathway. This is consonant with the classic LaMer model. By analyzing the results of field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), X‐ray diffraction (XRD), energy dispersive X‐ray (EDX) spectroscopy, X‐ray photoelectron spectroscopy (XPS), and UV‐vis spectrophotometry, a comprehensive structural and morphological characterization of the photocatalysts is established. FESEM images reveal that the anatase film comprises mainly of nanowires bristles while the tausonite film is primarily made up of nanocube aggregations. In comparison to the respective pristine semiconductor photocatalysts, the heterostructured photocatalyst demonstrates the highest efficiency in photocatalytic splitting of water to produce H₂, 4.9 times that of TiO₂ and 2.1 times that of SrTiO₃. The enhanced photocatalytic efficiency is largely attributed to the efficient separation of photogenerated charges at heterojunctions of the two dissimilar semiconductors, as well as a negative redox potential shift in the Fermi level.     

Interface‐Engineered Amorphous TiO2‐Based Resistive Memory Devices

Crossbar‐type bipolar resistive memory devices based on low‐temperature amorphous TiO₂ (a‐TiO₂) thin films are very promising devices for flexible nonvolatile memory applications. However, stable bipolar resistive switching from amorphous TiO₂ thin films has only been achieved for Al metal electrodes that can have severe problems like electromigration and breakdown in real applications and can be a limiting factor for novel applications like transparent electronics. Here, amorphous TiO₂‐based resistive random access memory devices are presented that universally work for any configuration of metal electrodes via engineering the top and bottom interface domains. Both by inserting an ultrathin metal layer in the top interface region and by incorporating a thin blocking layer in the bottom interface, more enhanced resistance switching and superior endurance performance can be realized. Using high‐resolution transmission electron microscopy, point energy dispersive spectroscopy, and energy‐filtering transmission electron microscopy, it is demonstrated that the stable bipolar resistive switching in metal/a‐TiO₂/metal RRAM devices is attributed to both interface domains: the top interface domain with mobile oxygen ions and the bottom interface domain for its protection against an electrical breakdown.     

NiOX/MoO3 Bi‐Layers as Efficient Hole Extraction Contacts in Organic Solar Cells

The electronic structure of a bi‐layer hole extraction contact consisting of nickel oxide (NiO_x) and molybdenum trioxide (MoO₃) is determined via ultraviolet and X‐ray photoemission spectroscopy. The bi‐layer presents ideal energetics for the extraction of holes and suppression of carrier recombination at the interface. The application of the NiO_x/MoO₃ bi‐layer as the anode of organic bulk heterojunction solar cells based on PCDTBT/PC₇₁BM leads to improved device performance, which is explained by an intricate charge transfer process across the interface.     

p‐CuO/n‐Si heterojunction solar cells with high open circuit voltage and photocurrent through interfacial engineering

Heterojunction solar cells of p‐type cupric oxide (CuO) and n‐type silicon (Si), p‐CuO/n‐Si, have been fabricated using conventional sputter and rapid thermal annealing techniques. Photovoltaic properties with an open‐circuit voltage (V_oc) of 380 mV, short circuit current (J_sc) of 1.2 mA/cm², and a photocurrent of 2.9 mA/cm² were observed for the solar cell annealed at 300 °C for 1 min. When the annealing duration was increased, the photocurrent increased, but the V_oc was found to reduce because of the degradation of interface quality. An improvement in the V_oc resulting to a record value of 509 mV and J_sc of 4 mA/cm² with a high photocurrent of ~12 mA/cm² was achieved through interface engineering and controlling the phase transformation of CuO film. X‐ray diffraction, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy analysis have been used to investigate the interface properties and crystal quality of sputter‐deposited CuO thin film. The improvement in V_oc is mainly due to the enhancement of crystal quality of CuO thin film and interface properties between p‐CuO and n‐Si substrate. The enhancement of photocurrent is found to be due to the reduction of carrier recombination rate as revealed by transient photovoltage spectroscopy analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Preferred orientation of Cu(In,Ga)Se2 thin film deposited on stainless steel substrate

The influences of process parameters and Fe diffusing into Cu(In,Ga)Se₂ (CIGS) films on the orientation of CIGS absorbers grown on the stainless steel (SS) foils are investigated. The structural properties, morphology, and elemental profiles are characterized using X‐ray diffraction, scanning electron microscopy, and second ion mass spectroscopy, respectively. The orientation of CIGS thin films on the SS substrates strongly depends on the texture of the (In,Ga)₂Se₃ precursor, determined by the substrate temperature at the first stage (Ts1) and the flux ratio of Se to (In + Ga). Among these factors, Ts1 is the prerequisite to achieve [300]‐oriented IGS layer, which will yield [200]‐oriented CIGS thin film in the later process. The results indicate that through the comparison of CIGS thin films on the Mo/SS substrates and on the Mo/ZnO/SS substrates and combined with simply calculation, Fe diffusing into the CIGS layer will hinder the growth of the CIGS grains along [112] orientation. The grazing‐incidence X‐ray diffraction results suggest that the surface of the [220]‐textured CIGS thin film on the SS substrate still has [220] predominance, whereas the surface texture of the [220]‐texture CIGS thin film on the Mo/soda‐lime glass substrate became [112] predominant, which is due to the different compensation ability between Fe and Na elements. Finally, the relations between the device parameters and the degrees of the preferred orientation of CIGS absorbers are investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals

Photoemission electron microscopy (PEEM) excited by X‐ray and UV sources is used to investigate epitaxial anatase thin films with embedded rutile nanocrystals, a model system for the study of heterocatalysis on mixed‐phase TiO₂. Both excitation sources show distinct contrast between the two TiO₂ phases; however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X‐ray PEEM images but brighter in UV‐PEEM images. We observe that topography‐induced contrast is dominant in X‐ray PEEM imaging, whereas work function and density‐of‐state‐based contrast, dominates in UV‐PEEM. This assertion is confirmed by UPS and conducting AFM data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density‐of‐states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed‐phase TiO₂ interfaces and grain boundaries.     

Electronic Structure of Low‐Temperature Solution‐Processed Amorphous Metal Oxide Semiconductors for Thin‐Film Transistor Applications

The electronic structure of low temperature, solution‐processed indium–zinc oxide thin‐film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm² V^?1 s^?1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution‐processed oxide thin films as a function of annealing temperature and environment using a combination of X‐ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop‐off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub‐bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution‐processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy‐induced donor levels.     

Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells

Cu₂ZnSnS₄ (CZTS) is a promising thin‐film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se₂. We introduce a two‐step process for fabrication of CZTS films, involving reactive sputtering of a Cu‐Zn‐Sn‐S precursor followed by rapid annealing. X‐ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high‐temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with J_sc and V_oc both appearing to be limited by interface recombination. Copyright © 2012 John Wiley & Sons, Ltd.     

3D Hexagonal (R‐3m) Mesostructured Nanocrystalline Titania Thin Films: Synthesis and Characterization

A straightforward and reproducible synthesis of crack‐free large‐area thin films of 3D hexagonal (R‐3m) mesostructured nanocrystalline titania (meso‐nc‐TiO₂) using a Pluronic triblock copolymer (P123)/1‐butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high‐resolution scanning electron microscopy and scanning transmission electron microscopy), grazing‐incidence small‐angle X‐ray scattering, in situ high‐temperature X‐ray diffraction, and variable‐angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 °C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 °C. The accessible surface area of 3D hexagonal meso‐nc‐TiO₂ estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso‐nc‐TiO₂ at the same annealing temperature.     

Colossal X‐Ray‐Induced Persistent Photoconductivity in Current‐Perpendicular‐to‐Plane Ferroelectric/Semiconductor Junctions

Persistent photoconductivity (PPC) is an intriguing physical phenomenon, where electric conduction is retained after the termination of electromagnetic radiation, which makes it appealing for applications in a wide range of optoelectronic devices. So far, PPC has been observed in bulk materials and thin‐film structures, where the current flows in the plane, limiting the magnitude of the effect. Here using epitaxial Nb:SrTiO₃/Sm_0.1Bi_0.9FeO₃/Pt junctions with a current‐perpendicular‐to‐plane geometry, a colossal X‐ray‐induced PPC (XPPC) is achieved with a magnitude of six orders. This PPC persists for days with negligible decay. Furthermore, the pristine insulating state could be fully recovered by thermal annealing for a few minutes. Based on the electric transport and microstructure analysis, this colossal XPPC effect is attributed to the X‐ray‐induced formation and ionization of oxygen vacancies, which drives nonvolatile modification of atomic configurations and results in the reduction of interfacial Schottky barriers. This mechanism differs from the conventional mechanism of photon‐enhanced carrier density/mobility in the current‐in‐plane structures. With their persistent nature, such ferroelectric/semiconductor heterojunctions open a new route toward X‐ray sensing and imaging applications.     

X‐Ray Writing of Metallic Conductivity and Oxygen Vacancies at Silicon/SrTiO3 Interfaces

Tunable electronic properties of transition metal oxides and their interfaces offer remarkable functionalities for future devices. The interest in these materials has been boosted with the discovery of a 2D electron gas (2DEG) at SrTiO₃ (STO)‐based interfaces. For the majority of these systems, oxygen vacancies play a crucial role in the emergence of interface conductivity, ferromagnetism, and high electron mobility. Despite its great importance, controlling the density and spatial distribution of oxygen vacancies in a dynamic way remains extremely challenging. Here, lithography‐like writing of a metallic state at the interface between SrTiO₃ and amorphous Si using X‐ray irradiation is reported. Using a combination of transport techniques and in operando photoemission spectroscopy, it is revealed in real time that the X‐ray radiation induces transfer of oxygen across the interface leading to the on‐demand formation of oxygen vacancies and a 2DEG in STO. The formed 2DEG stays stable in ambient conditions as the interface oxygen vacancies are stabilized by the capping of Si. The study provides a fundamental understanding of X‐ray‐induced redox reactions at the SrTiO₃‐based interfaces and in addition shows the potential of X‐ray radiation for patterning stabile conductive pathways for future oxide‐based electronic devices.     

Reduction of Tungsten Oxide: A Path Towards Dual Functionality Utilization for Efficient Anode and Cathode Interfacial Layers in Organic Light‐Emitting Diodes

Here, we report on the dual functionality of tungsten oxide for application as an efficient electron and hole injection/transport layer in organic light‐emitting diodes (OLEDs). We demonstrate hybrid polymer light‐emitting diodes (Hy‐PLEDs), based on a polyfluorene copolymer, by inserting a very thin layer of a partially reduced tungsten oxide, WO_2.5, at the polymer/Al cathode interface to serve as an electron injection and transport layer. Significantly improved current densities, luminances, and luminous efficiencies were achieved, primarily as a result of improved electron injection at the interface with Al and transport to the lowest unoccupied molecular orbital (LUMO) of the polymer, with a corresponding lowering of the device driving voltage. Using a combination of optical absorption, ultraviolet spectoscopy, X‐ray photoelectron spectroscopy, and photovoltaic open circuit voltage measurements, we demonstrate that partial reduction of the WO₃ to WO_2.5 results in the appearance of new gap states just below the conduction band edge in the previously forbidden gap. The new gap states are proposed to act as a reservoir of donor electrons for enhanced injection and transport to the polymer LUMO and decrease the effective cathode workfunction. Moreover, when a thin tungsten oxide film in its fully oxidized state (WO₃) is inserted at the ITO anode/polymer interface, further improvement in device characteristics was achieved. Since both fully oxidized and partially reduced tungsten oxide layers can be deposited in the same chamber with well controlled morphology, this work paves the way for the facile fabrication of efficient and stable Hy‐OLEDs with excellent reproducibility.     

Determination of activation barriers for the diffusion of sodium through CIGS thin‐film solar cells

We have determined the activation energies of sodium diffusion from the soda‐lime glass substrate through the Mo back‐contact layer, as well as through copper indium gallium diselenide (CIGS) deposited on the Mo back‐contact layer of CIGS thin‐film solar cells. The activation energies were determined by X‐ray photoelectron spectroscopy (XPS) to measure surface sodium concentrations before and after thermally induced diffusion. The activation energies were found to be similar for the diffusion of Na through the Mo/glass and CIGS/Mo/glass thin films, approximately 8·6 and 9·6 kcal/mol, respectively. Furthermore, the sodium diffusion was found to occur by annealing in an environment of 1·0×10⁻⁵ Torr of air, oxygen, or water vapor, but not in vacuum of less than 1×10⁻⁸ Torr. In addition, the diffusion of Na was found to occur faster in the presence of oxygen than in water under identical annealing conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

Surface engineering in Cu(In,Ga)Se2 solar cells

Surface modifications of three‐stage co‐evaporated Cu(In,Ga)Se₂ (CIGS) thin films are investigated by finishing the evaporation with gallium‐free (CuInSe₂, CIS) stages of various lengths. Secondary‐ion mass spectrometry shows substantial interdiffusion of indium and gallium, smearing out the Ga/(Ga + In) profile so that the addition of a CIS layer merely lowers the gallium content at the surface. For the thinnest top layer, equivalent to 20 nm of pure CIS, X‐ray photoelectron spectroscopy does not detect any compositional difference compared with the reference device. The modifications are evaluated electrically both by temperature‐dependent characterisation of actual solar‐cell devices and by modelling, using the latest version of scaps‐1d (Electronics and Information Systems, Ghent University, Belgium). The best solar‐cell device from this series is obtained for the 20 nm top layer, with an efficiency of 16.6% after antireflective coating. However, we observe a trend of decreasing open‐circuit voltage for increasingly thick top layers, and we do not find direct evidence that the lowering of the gallium concentration at the CIGS surface should generally be expected to improve the device performance. A simulated device with reduced bulk and interface defect levels achieves nearly 20% efficiency, but the trends concerning the CIS top layer remain the same. Copyright © 2011 John Wiley & Sons, Ltd.     

Evidence for Chemicals Intermingling at Silicon/Titanium Oxide (TiOx) Interface and Existence of Multiple Bonding States in Monolithic TiOx

Metal oxides (MOs) are used in photovoltaics and microelectronics as surface passivating layers and gate dielectrics, respectively. The effectiveness of MOs predominantly depends on their structure and the nature of the semiconductor/MO (S/MO) interface. While some efforts are made to analyze interface behavior of a few MOs, greater fundamental understanding on the interface and structural behaviors of emerging MOs is yet to be established for enhanced scientific and technological developments. Here, the structure of atomic layer deposited titanium oxide (TiO_x) and the nature of the c‐Si/TiO_x interface on the atomic‐ to nanoscale are probed. A new breed of mixed oxide (SiO_x+TiO_x) interfacial layer with a thickness of ≈1.3 nm at the c‐Si/TiO_x interface is discovered, and its thickness further increases to ≈1.5 nm after postdeposition annealing. It is observed that both as‐deposited and annealed monolithic TiO_x films comprise multiple bonding states at varying film thickness, with an oxygen‐deficient TiO_x layer located close to the mixed oxide/TiO_x interface. The stoichiometry of this layer improves when reaching the middle and near surface regions of the TiO_x layer, respectively. This work uncovers several critical structural and interface aspects of TiO_x, and thus creates opportunities to control and design improved photovoltaic and electronic devices for future development.     

Tuning Thermal Transport Through Atomically Thin Ti3C2Tz MXene by Current Annealing in Vacuum

Heat transport across vertical interfaces of heterogeneous 2D materials is usually governed by the weak Van der Waals interactions of the surface‐terminating atoms. Such interactions play a significant role in thermal transport across transition metal carbide and nitride (MXene) atomic layers due to their hydrophilic nature and variations in surface terminations. Here, the metallicity of atomically thin Ti₃C₂T_z MXene, which is also verified by scanning tunneling spectroscopy for the first time, is exploited to develop a self‐heating/self‐sensing platform to carry out direct‐current annealing experiments in high (<10^?8 bar) vacuum, while simultaneously evaluating the interfacial heat transport across a Ti₃C₂T_z/SiO₂ interface. At room temperature, the thermal boundary conductance (TBC) of this interface is found, on average, to increase from 10 to 27 MW m^?2 K^?1 upon current annealing up to the breakdown limit. In situ heating X‐ray diffraction and X‐ray photo‐electron spectroscopy reveal that the TBC values are mainly affected by interlayer and interface spacing due to the removal of absorbents, while the effect of surface termination is negligible. This study provides key insights into understanding energy transport in MXene nanostructures and other 2D material systems.